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Investigation of U(VI) Adsorption in Quartz-Chlorite Mineral Mixtures Zheming Wang, John M Zachara, Jianying Shang, Choong Jeon, Juan Liu, and Chongxuan Liu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 14 Jun 2014 Downloaded from http://pubs.acs.org on June 19, 2014
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Investigation of U(VI) Adsorption in Quartz-Chlorite Mineral Mixtures Zheming Wang,* John M. Zachara, Jianying Shang, Choong Jeon†, Juan Liu and Chongxuan Liu
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Pacific Northwest National Laboratory, Richland, WA 99352
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
*
Corresponding author: Pacific Northwest National Laboratory, P.O. Box 999, MS K8-96, Richland, WA 99354; Phone: (509) 371-6349; Fax: (509) 371-6354; E-mail: [email protected] † Present Address: Department of Environmental & Applied Chemical Engineering, Gangneung-Wonju National University, Gangwon-do, Republic of Korea.
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ABSTRACT
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A batch and cryogenic laser-induced time-resolved luminescence spectroscopy
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investigation of U(VI) adsorbed on quartz-chlorite mixtures with variable mass ratios
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have been performed under field-relevant uranium concentrations (5x10-7 M and 5x10-6
43
M) in pH 8.1 synthetic groundwater. The U(VI) adsorption Kd values steadily increased
44
as the mass fraction of chlorite increased, indicating preferential sorption to chlorite. For
45
all mineral mixtures, U(VI) adsorption Kd values were lower than that calculated from the
46
assumption of component additivity possibly caused by surface modifications stemming
47
from chlorite dissolution; The largest deviation occurred when the mass fractions of the
48
two minerals were equal. U(VI) adsorbed on quartz and chlorite displayed characteristic
49
individual luminescence spectra that were not affected by mineral mixing. The spectra of
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U(VI) adsorbed within the mixtures could be simulated by one surface U(VI) species on
51
quartz and two on chlorite. The luminescence intensity decreased in a nonlinear manner
52
as the adsorbed U(VI) concentration increased with increasing chlorite mass fraction –
53
likely due to ill-defined luminescence quenching by both structural Fe/Cr in chlorite, and
54
trace amounts of solubilized and re-precipitated Fe/Cr in the aqueous phase. However,
55
the fractional spectral intensities of U(VI) adsorbed on quartz and chlorite followed the
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same trend of fractional adsorbed U(VI) concentration in each mineral phase with
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approximate linear correlations, offering a method to estimate of U(VI) concentration
58
distribution between the mineral components with luminescence spectroscopy.
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INTRODUCTION
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The adsorption of uranium to soil minerals and subsurface sediments has been
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extensively investigated to resolve issues of environmental contamination and to support
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site selection of nuclear waste repositories.1-4 Under most environmental conditions,
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uranium exists as the hexavalent uranyl ion, UO22+. Uranyl ion adsorbs to a broad range
64
of mineral phases including quartz,5, 6 feldspar,7 calcium carbonates,8, 9 iron oxides,10, 11
65
and phyllosilicates.12,
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strongly on solution pH and counter anions.
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precipitate in the form of uranyl phosphate, carbonates, and oxyhydroxides mineral
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phases.14-16
13
Adsorption affinities to these phases are variable, depending At higher concentrations, uranyl may
69
Metal ions may exhibit distributed adsorption in mineral assemblages and natural
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sediments. For example, Stubbs et al. identified six mineral hosts for sorbed uranium in
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Hanford vadose zone sediment based on electron microprobe and TEM analysis.17 The
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overall adsorptivity of a composite mineral material is influenced by the adsorption
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characteristics of the contributing mineral phases and their mass fractions, and by
74
interactions between the adsorbing mineral phases.18
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The description of metal ion adsorption on mineral mixtures and sediments relies
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heavily on surface complexation modeling to account for aqueous chemical and surface
77
effects. Two common surface complexation modeling approaches are the generalized
78
composite model (GCM) and the component additivity model (CAM).19-22 In the former,
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generic surface sites are assumed for the entire mineral assemblage. Site density and
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possible surface complexes with fitted stability constants are applied under different
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adsorption conditions to determine the likely adsorption mechanism(s) based on the
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goodness of fit. In the latter, adsorption is described as the summation of that occurring
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on individual mineral phases,19 as computed from individual surface complexation
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constants and the abundance of the mineral components in the assemblage.
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Regardless of the surface complexation modeling approach, understanding the
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adsorption process in natural materials is advanced by direct identification of surface
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complex types and their concentrations. These speciation measurements are challenging
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due to sensitivity issues and the multi-component nature of the surface complexation
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process. In a previous study, we applied time-resolved U(VI) luminescence spectroscopy
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and chemometric methods to determine the chemical forms of adsorbed U(VI) in a
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contaminated subsurface sediment.23 As U(VI) adsorbed to constituent mineral phases
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displayed characteristic luminescence spectra, we were able to determine that sediment
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adsorbed U(VI) existed as surface complexes on quartz and phyllosilicates. However, it
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was unclear if sorbent interactions influenced the luminescence spectral behavior, and if
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the spectral intensity could be used to estimate the concentrations of specific surface
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complex type. Here, we investigated U(VI) adsorption on mixtures of quartz and chlorite
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to simulate the reactive fraction of a subsurface sediment. Sorbent mass ratios were
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varied and the adsorption complex was examined by cryogenic luminescence
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spectroscopy to determine how luminescence spectral behavior evolved with changing
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mineral mass ratios in the mixtures. We assess whether it is possible to estimate the
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concentration of U(VI) on the individual phases in the mixtures based on the collective
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luminescence response.
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EXPERIMENTAL
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Materials
Quartz (Min-U-Sil 30, U.S. Silica Company, Mill Creek,
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Oklahoma) was pretreated to remove organic contaminants and reactive fines following
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the procedures of Kohler et al24 and rinsed with DI water until the supernatant
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conductivity was 18 MΩ cm or lower. The material was then oven-dried at 40 °C for 3−4
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days. Specific surface area was determined by the N2 BET method (Quantachrome
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Autosorb 6-B gas adsorption system) to be 1.63 m2/g.
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[(Mg2.95Al0.05)(Si2.99Al1.01)O10(OH)2.(Mg1.97Al0.66 Cr0.25Fe3+0.06Fe2+0.06) (OH)6;25] was
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hand ground to yield 20-53 µm fraction with a BET surface area of 2.6 m2/g. The
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properties of smaller size fractions of the same mineral specimen (2 to 5 µm and 5 – 20
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µm) have been previously reported.23 Calcite-saturated synthetic groundwater (SGW2;
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pH 8.1) was prepared according to Bond et al.26 A stock solution of uranium nitrate (1.06
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x 10-3 M, pH 1) was prepared by dissolving a weighed amount of uranyl nitrate (Fluka) in
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0.1 M nitric acid. All other chemicals were reagent grade.
California chlinochlore
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U(VI) Adsorption Two identical series of eleven quartz-chlorite mixtures were
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prepared by mixing weighed amounts of the solids in 15 ml polypropylene centrifuge
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tubes to quartz:chlorite mass ratios of 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9 and
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0:10. The total mineral weight was 0.2 g. The sorbents were twice equilibrated with
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SGW2 for 24 h and separated from the supernatants by centrifugation. The resulting
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solids were then re-suspended in 10 ml of SGW2 and spiked with the U(VI) stock
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solution to yield 5x10-6 M U(VI) in one series and 5x10-7 M U(VI) in the other. The
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suspension pH was maintained at pH 8.1 with dilute NaOH. The U(VI)-bearing mineral
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suspensions were placed on an orbital shaker and slowly agitated for 48 h to allow U(VI)
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adsorption equilibrium.
After 48 h, the solid and solution were separated by
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centrifugation. Both the solid paste and the supernatant were sampled into 2 mm x 4 mm
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x 25 mm (ID) quartz cuvettes for luminescence analysis, while the remainder of the
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supernatants was acidified for U(VI) (KPA), Mg, Al, Si and Fe analysis (ICP/MS).
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Aqueous U(VI) speciation calculation The speciation of U(VI) in SGW2 at pH
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8.1 under ambient conditions was calculated using the MINTEQA227 software with the
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most current, critically reviewed thermodynamic stability constants for the U(VI)
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complexes.28
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Cryogenic Laser Luminescence Spectroscopy
The instrumentation and
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experimental procedures for luminescence spectroscopic measurements at LHeT were
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described previously.15, 29 The quartz sample cuvettes were attached to the cold finger of
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a CRYO Industries RC152 cryostat with liquid helium vaporizing beneath the sample to
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reach a sample temperature of 8 ± 2 ̊K. The sample was excited with a Spectra-Physics
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Nd:YAG laser pumped Lasertechnik-GWU MOPO laser at 415 nm and the emitted light
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was collected at 85˚ to the excitation beam and detected with a thermoelectrically cooled
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Princeton Instruments PIMAX intensified CCD camera after spectral dispersion through
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an Acton SpectroPro 300i double monochromator spectrograph.
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analyzed using a commercial software, IGOR®, from Wavematrix, Inc. All spectra were
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corrected for the mass ratio-weighted background emission observed for quartz and
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chlorite prepared under the same conditions as those of the U(VI)-bearing samples except
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that no U(VI) was added.
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RESULTS AND DISCUSSION
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The spectra were
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Adsorption of U(VI) on quartz-chlorite mixtures The magnitude of U(VI)
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adsorption on the quartz-chlorite mixtures varied with the mineral mass ratio and U(VI)
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concentration (Table 1 and Figure 1). The highest adsorption was seen on pure chlorite
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and the lowest on pure quartz. At a U(VI) concentration of 5×10-6 M, U(VI) adsorption
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was 36.2% on quartz and 84.9% on chlorite, corresponding to adsorption Kd values of
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28.3 ml/g and 281 ml/g, respectively. Lowering the U(VI) concentration to 5×10-7 M
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increased U(VI) adsorption to 46.7% on quartz and 92.9% on chlorite, corresponding to
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Kd values of 43.7 and 870 ml/g, respectively. These measured Kd values were consistent
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with ones measured previously,23 when differences in surface areas are taken into
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account.
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As a result of the disparate U(VI) adsorption on the two phases, the total adsorbed
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U(VI) concentration, CU,Ads, increased with the mass fraction of chlorite (Table 1 and SI
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Figure 1). The concentration of adsorbed U(VI) contributed by quartz and chlorite in the
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solid mixtures at each initial U(VI) concentration were calculated through the following
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relationship, f Q × K d ,Q f Q × K d ,Q + f CC × K d , CC
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CU,Q = CU,Ads ×
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CU,CC = CU,Ads - CU,Q
(1)
(2)
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Where CU,Q and CU,CC were adsorbed U(VI) concentrations contributed by quartz and
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chlorite in the mineral mixture. Kd,Q and Kd,CC were the U(VI) adsorption Kd values on
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pure quartz and chlorite at the corresponding initial U(VI) concentration, and fQ and fCC
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were the respective mass fractions. The fractional concentration contribution to the
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adsorbed U(VI) by quartz (FU,Q) and chlorite (FU,CC) in the mixtures were defined as: 7 ACS Paragon Plus Environment
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FU,Q = CU,Q/CU,Ads and FU,CC = CU,CC/CU,Ads
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(3)
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Reflecting the lower affinity of quartz as compared to chlorite, the calculated U(VI)
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concentration contributions from quartz were consistently lower in all quartz-chlorite
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mixtures (SI Figure 2). For all mineral mixtures in which fCC > 0.5, the fractional
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concentration contribution of quartz to adsorbed U(VI) in the mineral mixtures was ~ 5%
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or less (Figure 2).
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With the component additivity model, the distribution coefficient for adsorbed
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U(VI) in the quartz-chlorite mixtures equals the mass-weighted sum of the adsorption Kd
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values on the individual sorbents,18, 30, 31
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Kd, mixture = fQ Kd, Q + fCC Kd, CC
(4)
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Accordingly, the calculated Kd values displayed straight lines as a function of chlorite
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mass fraction at both U(VI) concentrations (dashed lines in Figure 1). The calculated Kd
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values were all above the measured ones for the mineral mixtures, and the largest
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differences were observed when both minerals were at equal mass. Such deviations
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suggest that either there is interaction between the quartz and chlorite, or that surface
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modifications occurred as a result of mineral mixing and co-equilibration. Apparently,
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the component additivity approach is not valid for quantitatively modeling U(VI)
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adsorption in such mixtures without properly taking into account the causes for the
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deviation.
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Previous studies on the applicability of adsorption additivity for metal ion
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adsorption in binary mineral mixtures18, 32, 33 revealed dependence on the specific metal
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ion and sorbent identity. The adsorption of both Cs+ and Sr2+ followed the additivity rule
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on bentonite-sand and silica-montmorillonite mixtures.18 However, significant negative 8 ACS Paragon Plus Environment
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deviations from adsorption additivity were observed for Sr2+ adsorption on alumina-illite
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mixtures, while large positive deviations were observed for Sr2+ adsorption on alumina-
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montmorillonite mixtures.18 The dissolution of alumina and the formation of unidentified
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Sr-bearing precipitates were speculated to cause the noted effect.
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Anderson and Benjamin systematically investigated the adsorption of Ag+, Cd2+,
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Zn2+ and several inorganic anions on Fe(OH)3, Al(OH)3, and Fe(OH)3-Al(OH)3 mixtures
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and co-precipitates under various chemical conditions.32,
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caused small enhancements in Zn2+ adsorption by Fe(OH)3 but inhibited the adsorption of
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Ag+ and Cd2+. Constant-capacitance surface complexation modeling suggested that a
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majority of the Fe(OH)3 surface sites were covered and replaced by Al(OH)3 upon
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mixing, leading to reduced metal ion adsorption. Surface and bulk analyses clearly
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indicated that particle interactions in the mixtures, including dissolution and precipitation,
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resulting in changes in particle size distribution, surface charge, and surface site
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distribution.
33
The presence of Al(OH)3
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SEM images of the post-reaction quartz-chlorite mixture (SI Figure 3) revealed
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that the individual phases generally remained separate except that fine-grained quartz
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associated with larger chlorite platelets.
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aggregation of the sorbents was an unlikely cause of the observed lower Kd values, which
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in some samples was as much as 50% as compared to the pure phases (Figure 1). We
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hypothesize that the decrease of Kd values for the quartz-chlorite mixtures was due to
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trace amounts of the dissolved solids, particularly the brucite layer of chlorite, releasing
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Mg2+, Al3+, Cr3+, Fe3+ and Fe2+.
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concentration in the presence of chlorite was between 17 µM to 48 µM higher than that in
Thus, surface area loss due to physical
As indicated by the results of ICP analysis, Mg
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SGW2 and increases along with the mass fraction of chlorite (SI Figure 4), equivalent to
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between 0.01% to 0.04% chlorite dissolution.
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stoichiometric concentrations of other metal ions are also released into the solution.
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Some of the leached metal ions may occupy surface sites that would otherwise be
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available for uranyl adsorption, especially on quartz which is mostly negatively charged
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at the present pH. As will be shown in the next section, a major surface U(VI) complex
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on chlorite involved the negatively charged uranyl-tricarbonate anion complex,
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UO2(CO3)34-. Therefore, the loss of highly-charged cation sites on chlorite would also
227
reduce U(VI) sorption on chlorite. In addition, the attachment of negatively charged fine
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quartz particle might also adversely impact the adsorption U(VI) on some of the
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positively charged surface sites on chlorite.
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If the dissolution is congruent,
Luminescence spectroscopy of adsorbed U(VI) The luminescence spectra of
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U(VI) adsorbed on the quartz-chlorite mixtures displayed characteristic features (Figure
232
3). The vibronic band maxima were located at 499.7 nm, 520.4 nm and 541.3 nm for
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U(VI) adsorbed on quartz at an initial U(VI) concentration of 5x10-6 M, while those for
234
U(VI) on chlorite were at 480.3 nm, 500.8 nm, 521.4 nm and 543.2 nm (Figure 3A and SI
235
Table 1). These results were in good agreement with previously reported values.23 The
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uranyl ion formed inner-sphere surface complexes with deprotonated silanol sites on
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quartz, while the U(VI) spectra on chlorite resembled those of uranyl-
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tricarbonate/dicarbonate or calcium-uranyl-tricarbonate complexes.29, 34, 35 The spectral
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features of U(VI) adsorbed on quartz decreased and eventually disappeared as the mass
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fraction of chlorite (fCC) increased. The spectral pattern for chlorite, defined here as UCa,
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became dominant over fCC = 0.5.
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The spectrum of U(VI) adsorbed on quartz was unchanged at 5x10-7 M U(VI) as
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compared with that at 5x10-7 M U(VI) (Figure 3B). However, the spectra of U(VI)
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adsorbed on chlorite developed more than one spectral pattern at this concentration. The
245
spectrum was identical to UCa when fCC < 0.5 while a broad, poorly resolved spectra
246
with an emission maximum of 521.7 nm (SI Table 1), defined here as pattern UCb,
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became dominantt at fCC > 0.5 (Figure 3B lower traces), indicating the presence of
248
another U(VI) surface complex on chlorite.
249
U(VI) luminescence spectra were also recorded for the supernatants and for
250
SGW2 that was spiked with U(VI) nitrate (data not shown). Consistent with previous
251
measurements, U(VI) in the supernatants showed luminescence spectra that closely
252
resembled those of Ca2UO2(CO3)3(aq) and CaUO2(CO3)32- complexes as expected from
253
aqueous speciation calculation. The same type of spectra were displayed on the time-
254
resolved spectra of both of the wet paste samples of U(VI) adsorbed on quartz and
255
chlorite at 5x10-6 M U(VI) at long delay times, consistent with the presumption that the
256
wet paste samples entrained a small fraction of the supernatant. Such spectra were absent
257
in samples with large fCC values at 5x10-7 M U(VI) as a results of nearly complete U(VI)
258
adsorption.
259
Time-resolved luminescence spectra at delay times up to 4 ms showed that while
260
there was a single dominant species of U(VI) adsorbed on quartz (data not shown), both
261
U(VI) surface species on chlorite represented by the UCa and UCb spectral patterns were
262
present at 5x10-6 M U(VI), as expected, and UCb became more pronounced at longer
263
delay times (SI Figure 5). For all the samples with U(VI) adsorbed on quartz-chlorite
264
mixtures, the time-resolved spectra reflected the variation of the relative intensities of the
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three surface U(VI) species as governed by their luminescence lifetimes. For example,
266
for U(VI) adsorbed on quartz-chlorite mixture with fCC = 0.4 at 5x10-6 M U(VI), the
267
spectra were primarily composed of those of U(VI) adsorbed on quartz and on chlorite
268
with a spectral pattern UCa at shorter delay times (Figure 4A). As the delay time
269
increased, the relative intensity of UCa decreased gradually as indicated by the slight
270
drop of intensity in the 480.3 nm band where the spectra of U(VI) adsorbed on quartz
271
displayed only weak emission, pointing to increased character of U(VI) adsorbed on
272
quartz. At the same time, the relative intensity in the region from ~ 510 nm to 550 nm
273
was noticeably elevated, a clear indication of the growing contribution of the species
274
UCb.
275
spectral pattern UCa had a shorter luminescence lifetime than the U(VI) surface species
276
on quartz and the U(VI) species with a spectral pattern UCb had the longest luminescence
277
lifetime.
Such spectral changes implied that the surface U(VI) species corresponding to
278
As shown for the sample with fCC = 0.3, the time-resolved emission spectra at
279
5x10-7 M U(VI), again, indicated that at shorter delay times, the spectra represented both
280
U(VI) adsorbed on quartz and chlorite (UCa), and the latter decreased as delay time
281
increased to 0.7 ms based on the gradual drop of the intensity of the 480.3 nm band
282
(Figure 4B). As delay time further increased to 2.8 ms and 4.0 ms the spectral character
283
of U(VI) adsorbed on quartz also disappeared and the spectra became similar to those
284
observed at large fCC values (≥ 0.6) with dominant spectral pattern UCb at the same
285
U(VI) concentration, confirming the presence of two U(VI) surface complexes on
286
chlorite: one with spectral pattern UCa at lower fCC values and shorter delay times, and
287
the other with spectral pattern UCb that was present primarily at higher fCC values and
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longer delay times at low U(VI) concentration (i.e. 5x10-7 M)(Figures 3B, 4B). Thus the
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second surface U(VI) species on chlorite (UCb) likely formed on a subset of surface sites
290
with the highest U(VI) affinities. The site density for surface U(VI) complex with
291
spectral pattern UCa is much higher and thus this complex dominated at higher U(VI)
292
concentrations. The UCb spectrum was red-shifted by over 20 nm as compared to UCa,
293
which reflected the formation of strong, inner-sphere surface complex, consistent with
294
U(VI) adsorption at sites of higher U(VI) affinity.
295
Further spectral analyses verified that the luminescence spectra of adsorbed U(VI)
296
on all of the quartz-chlorite mixtures could be simulated by a linear combination of the
297
three species described above: one U(VI) surface complex on quartz and two on chlorite
298
with spectral profiles of UCa and UCb (Figure 5 as an example).
299
characteristics of U(VI) surface complexes on the individual phases were not affected by
300
mixing of the mineral components.
Therefore, the
301
Luminescence decay analysis indicated that the decay curves of U(VI) adsorbed
302
on chlorite could be fit by two exponential functions, resulting in luminescence lifetimes
303
of 155 ± 52 µs and 1328 ± 396 µs, respectively, consistent with the assignment to the
304
U(VI) species with spectral patterns of UCa and UCb. Unexpectedly, fit of the decay
305
curves of U(VI) adsorbed on quartz also required two exponential functions with
306
luminescence lifetime of 796 ± 142 µs and 233 ± 135 µs, respectively (SI Table 1). This
307
could be due to many reasons, such as two U(VI) surface complexes on quartz with
308
similar profiles, same surface species with different hydration waters or surface cite
309
inhomogeneity 2, 36, 37. The trend of the measured luminescence lifetimes were consistent
310
with that expected from the variations of the time-resolved spectra (Figure 4). However,
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it should be cautioned that large deviations were observed in the measured luminescence
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lifetimes data, possibly due to sample heterogeneity, and luminescence decay fitting with
313
multiple exponential functions are prone to errors.
314
Variation of U(VI) spectral intensities with mineral compositions Countering
315
the trend of the increasing Kd values (Table 1) and adsorbed U(VI) concentration with
316
increasing fCC values (SI Figure 1), the overall spectral intensity decreased with
317
increasing fCC (Figure 6). The sharpest intensity reduction occurred at fCC ≤ 0.2. The
318
spectral area for U(VI) adsorbed on quartz (fCC = 0) was 461 times larger than U(VI)
319
adsorbed on chlorite (fCC = 1) at 5x10-7 M U(VI), and 775 times larger at 5x10-6 M
320
U(VI). Given that the adsorbed U(VI) concentration on chlorite was more than twice that
321
on quartz, these intensity values indicated that that the luminescence quantum yield of
322
U(VI) adsorbed on quartz was about three orders of magnitude larger than that on
323
chlorite, assuming that the luminescence spectra of U(VI) adsorbed on quartz and chlorite
324
have similar molar absorptivity.
325
Evaluation of both the deconvoluted luminescence spectra and the corresponding
326
normalization factors indicated that the sharp intensity drop from fCC = 0 to fCC = 0.2 was
327
more significant for U(VI) adsorbed on quartz. The subsequent intensity drop at fCC > 0.2
328
applied to U(VI) adsorbed on both quartz and chlorite (SI Figure 6).
329
concentration of 5×10-7 M, the concentration of U(VI) adsorbed on quartz decreased from
330
1.17×10-8 to 0.32×10-8 mole/g from fCC =0 to fCC = 0.2. However, the corresponding
331
luminescence spectral area dropped from 1550 to 32 based on the results of spectral
332
deconvolution. Such large disproportionality between the emission intensity and the
333
concentration of adsorbed U(VI) indicated that there was pronounced luminescence
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quenching,38, 39 and/or self-absorption of the emitted light by Fe and Cr in chlorite.40, 41 If
335
self-absorption were the primary factor, it would be expected that adsorbed U(VI) on
336
quartz and chlorite would behave equally and that the decrease of luminescence intensity
337
would be proportional to fCC. Neither of these expectations was observed (Figure 6 and
338
SI Figure 6). Thus, the luminescence decrease with increasing fCC was likely caused by
339
luminescence quenching by Fe, Cr and other transition metal impurities in the chlorite.
340
The exact quenching mechanisms of adsorbed U(VI) luminescence by the mineral
341
mixtures was not determined. The chlorite contained both Fe (1.2%) and Cr (2.3%). The
342
total available Fe and Cr concentrations (3.8x10-3 M Fe and 8.7x10-3 M Cr) were three or
343
four orders of magnitude higher than that of U(VI) (5x10-6 M or 5x10-7 M, respectively)
344
at the given the solid-solution ratios. Assuming congruent dissolution of chlorite, the
345
dissolved Cr3+, Fe3+, and Fe2+ concentrations would be in the ranges of 0.86x10-6 M –
346
2.44x10-6 M (Cr3+) and 0.21x10-6 M – 0.59x10-6 M (both Fe2+ and Fe3+), respectively,
347
based on the measured Mg concentrations.
348
dissolved oxygen concentration could be as high as 1.4x10-3 M,42 and thus the solubilized
349
Fe2+ would be oxidized to Fe3+ by oxygen. The low Fe2+ concentration and the presence
350
of dissolved oxygen would also minimize any reduction of uranyl ion by Fe2+ in the
351
present system, such reductions had been observed with Fe-rich chlorite under anaerobic
352
conditions.43 Limited by low solubility, both Fe3+ and Cr3+ ions could exist as surface-
353
sorbed species, precipitated clusters, or ferrihydrite-like nano-particulates,44, 45 leading to
354
enhanced luminescence quenching. Indeed, trace Fe was observed in association with
355
both chlorite and quartz in the EDS elemental map of the U-bearing quartz-chlorite
Considering the aerobic conditions, the
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356
mixtures (SI Figure 3). It is also possible that exposed structural and surface-adsorbed Fe
357
were active U(VI) adsorption sites in the mineral mixtures.43, 44
358
Comparison of Figures 2 and 3 showed that the increasing spectral characters of
359
U(VI) adsorbed on chlorite with increasing fCC values on the normalized luminescence
360
spectra followed the same trend as that of the fractional concentration of adsorbed U(VI)
361
in the mineral mixtures. In order to quantify the correlations between the fractional
362
U(VI) concentration in each mineral phase in the solid mixture with the corresponding
363
fractional intensity in the measured spectra, the fractional spectral intensity of U(VI)
364
adsorbed on quartz and chlorite, fS,Q and fS,CC, were calculated at all fCC values as follows,
365
f S ,Q =
AQ AQ + AUCa + AUCb
and
f S ,CC =
AUCa + AUCb AQ + AUCa + AUCb
(5)
366
where AQ, AUCa and AUCb are the integrated areas under the corresponding deconvoluted
367
luminescence spectra of U(VI) associated with quartz and chlorite (both UCa and UCb).
368
Even though the calculations employed nontrivial approximations in the absence of
369
precise determination of the quantum yields of the surface complexes, plots of the
370
calculated fractional spectral intensity as a function of fCC (Figure 7) displayed
371
approximate resemblance to the fractional adsorbed U(VI) concentrations on quartz and
372
chlorite (Figure 2).
373
function of the spectral fractions of U(VI) adsorbed on quartz and chlorite (SI Figure 7)
374
only displayed qualitative linear correlations in the fCC value range of 0.2 – 1.0 at both
375
U(VI) concentrations, and the concentration fractions of U(VI) adsorbed on quartz were
376
underestimated while those on chlorite were overestimated in all the mixtures. The latter
377
inferred that the observed negative deviation of the measured Kd values from the
378
calculated ones based on component additivity (Figure 1) had a larger contribution from
However, correlation plots of the concentration fractions as a
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379
decreased U(VI) adsorption on chlorite in the mixtures. By assuming that the deviation
380
was solely due to decreased U(VI) adsorption on chlorite, the Kd contributions from
381
quartz and chlorite, Kd,Q’ and Kd,CC’ were calculated from their mass fractions as the
382
following,
383
Kd,Q’ = Kd,Q × fQ
(6)
384
Kd,CC’ = Kd - Kd,Q’
(7)
385
The resulting Kd,Q’ and Kd,CC’ values were used to calculate the concentration
386
contributions and fractional concentrations of U(VI) adsorbed on quartz and chlorite in
387
the mixtures using Eqs. (1) – (3).
388
Plots of the calculated concentration fractions as a function of fCC (SI Figiure 8)
389
displayed improved resemblance to those of the spectral fractions (Figure 7) and
390
correlation plots of the concentration fractions as a function of the spectral fractions
391
became linear across the entire fCC range with correlation coefficients of 0.95 and 0.92 at
392
U(VI) concentrations of 5 × 10-6 M and 5 × 10-7 M, respectively (SI Figure 9). These
393
results indicated that mixing quartz and chlorite primarily affected U(VI) adsorption on
394
chlorite with little impact on U(VI) adsorption on quartz even though larger U(VI)
395
luminescence quenching was observed on the latter. The approximate linear correlations
396
between the concentration fractions and the spectral fractions suggest that it is possible to
397
estimate U(VI) concentration in individual mineral phases based on the mineralogical
398
composition, total U(VI) concentration and the measured luminescence spectra, thus
399
offering credible empirical inputs for model simulation of U(VI) adsorption in mineral
400
mixtures when precise measurement is not feasible.
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401
Implications Quartz is the most abundant mineral phase among the fine size
402
fraction (< 125 µm) vadose zone sediments at both the US DOE Hanford Site and
403
Naturita Site.
404
abundance, possess high surface area and strong affinities to U(VI). Their common
405
association with larger crystallites as surface coatings further highlight their role in U(VI)
406
adsorption in subsurface sediments. The present results indicate that mixing quartz and
407
chlorite leads to deviation of U(VI) adsorption properties (e.g. Kd) from what is expected
408
from component additivity model, particularly for more reactive minerals such as
409
chlorite. This implies that for natural sediments, which are aged mineral mixtures,
410
adequate corrections taking into consideration of possible alteration of the mineral
411
constituents are required to simulate U(VI) adsorption in the mineral mixture based
412
adsorption parameters obtained on the individual mineralogical components.
413
invariant spectral profiles of the primary U(VI) surface species and approximate linear
414
correlations between the fractional U(VI) concentration and spectra in the binary mineral
415
mixtures, on the other hand, suggest that with precise knowledge of the mineralogical
416
composition and overall U(VI) concentration, it is possible to predict U(VI) distribution
417
empirically in such mineral mixtures using U(VI) luminescence spectroscopy
418
measurements.
Chlorite, along with other phyllosilicates, though present with low
The
419 420
ASSOCIATED CONTENT
421
Supporting Information
422
Additional information as noted in the text. This material is available free of charge via
423
the Internet at http://pubs.acs.org.
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ACKNOWLEDGEMENTS
426
The authors thank Dr. James Amonette for providing the chlorite mineral
427
specimen and Mr. Bruce Arey for SEM and EDS sample analysis. The authors are also
428
indebted to four anonymous reviewers whose insightful comments greatly helped to
429
improve the quality of the manuscript. This project was supported by the Hanford site
430
SFA Program managed by the U.S. DOE Office of Biological and Environmental
431
Research (OBER). Part of this research was performed at EMSL, a national scientific
432
user facility at PNNL managed by the Department of Energy’s Office of Biological and
433
Environmental Research. Pacific Northwest National Laboratory is operated for the U.S.
434
Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.
435
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436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462
Table 1. Experimental data for U(VI) adsorption on quartz (Qtz)-chlorite (Chl) mixtures in pH 8.1 synthetic groundwater at 25°C. Total mineral weight = 0.2 g. Solution volume: 10 ml. Sample Qtz:Chl Surface [U]total U UAds Kd U Adsorption No. Ratio Area Adsorbed Concentration Density (m2) (M) (%) (10-8 Mole/g) (ml/g) (Mole/m2·108) 36.2 9.04 28.3 5.55 1 10:0 0.326 5.0·10-6 2. 9:1 0.346 5.0·10-6 35.6 8.90 27.6 5.15 -6 41.0 10.3 34.8 5.62 3 8:2 0.364 5.0·10 4 7:3 0.384 5.0·10-6 53.2 13.3 56.9 6.93 57.5 14.4 67.8 7.13 5 6:4 0.404 5.0·10-6 6 5:5 0.424 5.0·10-6 62.2 15.6 82.3 7.35 7 4:6 0.442 5.0·10-6 72.5 18.1 132 8.19 -6 8 3:7 0.462 5.0·10 76.1 19.0 160 8.24 80.2 20.0 202 8.33 9 2:8 0.482 5.0·10-6 10 1:9 0.500 5.0·10-6 83.7 20.9 258 8.36 11 0:10 0.520 5.0·10-6 84.9 21.2 281 8.16 -7 12 10:0 0.326 5.0·10 46.7 1.17 43.7 0.72 13 9:1 0.346 5.0·10-7 69.5 1.74 114 1.01 75.1 1.88 150 1.03 14 8:2 0.364 5.0·10-7 15 7:3 0.384 5.0·10-7 82.6 2.07 237 1.08 -7 16 6:4 0.404 5.0·10 85.8 2.14 301 1.06 17 5:5 0.424 5.0·10-7 88.8 2.22 397 1.05 18 4:6 0.442 5.0·10-7 90.4 2.26 470 1.02 -7 91.8 2.30 562 0.99 19 3:7 0.462 5.0·10 20 2:8 0.482 5.0·10-7 93.0 2.32 663 0.97 -7 21 1:9 0.500 5.0·10 94.1 2.35 793 0.94 94.6 2.36 870 0.91 22 0:10 0.520 5.0·10-7
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463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508
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Figure Captions Figure 1.
Plot of measured Kd values of U(VI) adsorption in quartz-chlorite mixtures as a function of the mass fraction of chlorite in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (filled circle) and 5·10-7 M (open circle), respectively. Solid lines are polynomial fit of the measured Kd values. Dashed lines are calculated Kd values based on the component additivity model.
Figure 2.
Simulated fractional contribution to the concentration of adsorbed U(VI) by quartz (open circle) and chlorite (filled circle) in the mineral mixtures as a function of the mass fraction of chlorite in quartz:chlorite mixtures in pH 8.1 synthetic groundwater. Total U(VI) concentrations are 5·10-6 M (A) and 5·10-7 M (B), respectively.
Figure 3.
LHeT Luminescence spectra of U(VI) adsorbed on quartz:chlorite mixtures in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively. All spectra were normalized to the same maximum intensity and offset along the Y-axis for clarity. λex = 415 nm.
Figure 4.
Time-resolved LHeT Luminescence spectra of U(VI) adsorbed on quartz:chlorite mixtures in synthetic groundwater, pH 8.1 at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively. In A, fCC = 0.4. In B, fCC = 0.3. The delay times are: a) 0.0 ms; b) 0.2 ms; c) 0.7 ms; d) 1.1 ms; e) 2.8 ms; and f) 4.0 ms. All spectra were normalized to the same maximum intensity and were offset along the Y-axis for clarity. λex = 415 nm.
Figure 5.
Deconvoluted LHeT luminescence spectra (circles) of U(VI) adsorbed on quartz:chlorite mixtures in pH 8.1 synthetic groundwater, pH 8.1 at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively, as the combination of U(VI) adsorbed on quartz (blue) and chlorite (green and purple). In A: fCC = 0.6. In B: fCC = 0.3. Both measured spectra were normalized to the same maximum intensity. λex = 415 nm.
Figure 6.
Luminescence intensity of U(VI) adsorbed on quartz-chlorite mixtures as a function of the mass fraction of chlorite in the mineral mixtures in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (filled circle) and 5·10-7 M (open circle), respectively. The solid and dashed lines are fitted trend lines of the intensity data.
Figure 7.
Simulated spectral fractions of U(VI) adsorbed on quartz (filled circle) and chlorite (open circle) in the mineral mixtures as a function of the mass fraction of chlorite in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively.
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509
REFERENCES
510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552
(1) Davis, J. and Curtis, G., Application of surface complexation modeling to describe uranium (VI) adsorption and retardation at the uranium mill tailings site at Naturita, Colorado in NUREG Report CR-6820 2003, U. S. Nuclear Regulatory Commission: Washington, DC. (2) Morris, D.E., Allen, P.G., Berg, J.M., Chisholm-Brause, C.J., Conradson, S.D., Donohoe, R.J., Hess, N.J., Musgrave, J.A., and Tait, C.D. Speciation of uranium in Fernald soils by molecular. spectroscopic methods: Characterization of untreated soils. Environ. Sci. Technol. 1996, 30(7), 2322-2331. (3) Wan, J.M., Kim, Y.M., Tokunaga, T.K., Wang, Z.M., Dixit, S., Steefel, C.I., Saiz, E., Kunz, M., and Tamura, N. Spatially Resolved U(VI) Partitioning and Speciation: Implications for Plume Scale Behavior of Contaminant U in the Hanford Vadose Zone. Environ. Sci. Technol. 2009, 43(7), 2247-2253. (4) Zachara, J.M., Brown, C., Christensen, J., Dresel, E., Kelly, S., Liu, C., McKinley, J., and Um, W., A Site-Wide Perspective on Uranium Geochemistry at the Hanford Site. 2007, Pacific Northwest National Laboratory: Richland, WA. (5) Fox, P.M., Davis, J.A., and Zachara, J.M. The effect of calcium on aqueous uranium(VI) speciation and adsorption to ferrihydrite and quartz. Geochim. Cosmochim. Acta 2006, 70(6), 1379-1387. (6) Greathouse, J.A., O'Brien, R.J., Bemis, G., and Pabalan, R.T. Molecular dynamics study of aqueous uranyl interactions with quartz (010). J. Phys. Chem. B 2002, 106(7), 1646-1655. (7) Chardon, E.S., Bosbach, D., Bryan, N.D., Lyon, I.C., Marquardt, C., Römer, J., Schild, D., Vaughan, D.J., Wincott, P.L., Wogelius, R.A., and Livens, F.R. Reactions of the feldspar surface with metal ions: Sorption of Pb(II), U(VI) and Np(V), and surface analytical studies of reaction with Pb(II) and U(VI). Geochim. Cosmochim. Acta 2008, 72(2), 288-297. (8) Dong, W.M., Ball, W.P., Liu, C.X., Wang, Z.M., Stone, A.T., Bai, J., and Zachara, J.M. Influence of calcite and dissolved calcium on uranium(VI) sorption to a Hanford subsurface sediment. Environ. Sci. Technol. 2005, 39(20), 7949-7955. (9) Reeder, R., Nugent, M., Lamble, G., Tait, C.D., and Morris, D.E. Uranyl incorporation into calcite and aragonite: XAFS and luminescence studies. Environ. Sci. Technol. 2000, 34(4), 638-644. (10) Duff, M.C., Coughlin, J.U., and Hunter, D.B. Uranium co-precipitation with iron oxide minerals. Geochim. Cosmochim. Acta 2002, 66(20), 3533-3547. (11) Waite, T.D., Davis, J.A., Payne, T.E., Waychunas, G.A., and Xu, N. Uranium(VI) adsorption to ferrihydrite - application of a surface complexation model. Geochim. Cosmochim. Acta 1994, 58(24), 5465-5478. (12) Catalano, J.G. and Brown, J.G.E. Uranyl adsorption onto montmorillonite: Evaluation of binding sites and carbonate complexation. Geochim. Cosmochim. Acta. 2005, 69(12), 2995-3005. (13) Chisholm-Brause, C., Conradson, S.D., Buscher, C.T., Eller, P.G., and Morris, D.E. Speciation of uranyl sorbed at multiple binding-sites on montmorillonite. Geochim. Cosmochim. Acta 1994, 58(17), 3625-3631.
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(28) Guillaumount, R., Fanghänel, T., Neck, V., Fuger, J., Palmer, D.A., Grenthe, I., and Rand, M.H., Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium. 2003, Amsterdam: Elsevier. (29) Wang, Z., Zachara, J.M., Yantasee, W., Gassman, P.L., Liu, C.X., and Joly, A.G. Cryogenic laser induced fluorescence characterization of U(VI) in Hanford vadose zone pore waters. Environ. Sci. Technol. 2004, 38(21), 5591-5597. (30) Jacquier, P., Meier, P., and Ly, J. Adsorption of radioelements on mixtures of minerals - experimental study. Appl. Geochem. 2001, 16(1), 85-93. (31) Tao, Z.Y. and Dong, W.M. Additivity rule and its application to the sorption of radionuclides on soils. Radiochim. Acta 2003, 91(5), 299-303. (32) Anderson, P.R. and Benjamin, M.M. Modeling adsorption in aluminum-iron binary oxide suspensions. Environ. Sci. Technol. 1990, 24(10), 1586-1592. (33) Anderson, P.R. and Benjamin, M.M. Surface and bulk characteristics of binary oxide suspensions. Environ. Sci. Technol. 1990, 24(5), 692-698. (34) Ilton, E.S., Wang, Z., Boily, J.-F., Qafoku, O., Rosso, K.M., and Smith, S.C. The Effect of pH and Time on the Extractability and Speciation of Uranium(VI) Sorbed to SiO2. Environ. Sci. Technol. 2012, 46(12), 6604-6611. (35) Wang, Z., Zachara, J.M., Liu, C., Gassman, P.L., Felmy, A.R., and Clark, S.B. A cryogenic fluorescence spectroscopic study of uranyl carbonate, phosphate and oxyhydroxide minerals. Radiochim. Acta 2008, 96(9-11), 591-598. (36) Del Nero, M., Galindo, C., Barillon, R., and Made, B. TRLFS Evidence for Precipitation of Uranyl Phosphate on the Surface of Alumina: Environmental Implications. Environ. Sci. Technol. 2011, 45(9), 3982-3988. (37) Gabriel, U., Charlet, L., Schlapfer, C.W., Vial, J.C., Brachmann, A., and Gabriel, G. Uranyl surface speciation in silica particles studied by time-resolved laserinduced fluorescence spectroscopy. J. Colloid Interf. Sci. 2001, 239(2), 358-368. (38) Bodenant, B., Fages, F., and Delville, M.H. Metal-induced self-assembly of a pyrene-tethered hydroxamate ligand for the generation of multichromophoric supramolecular systems. The pyrene excimer as switch for iron(III)-driven intramolecular fluorescence quenching. J. Am. Chem. Soc. 1998, 120(30), 75117519. (39) Varnes, A.W., Dodson, R.B., and Wehry, E.L. Interactions of transition-metal ions with photoexcited states of flavines. Fluorescence quenching studies. J. Am. Chem. Soc. 1972, 94(3), 946-950. (40) Maksimovic, Z., White, J.L., and Logar, M. Chromium-bearing dickite and chromium-bearing kaolinite from Teslic, Yugoslavia. Clay Clay Miner. 1981, 29(3), 213-218. (41) White, W.B. and Keester, K.L. Optical absorption spectra of iron in rock-forming silicates. Am. Mineral. 1966, 51(5-6), 774-790. (42) Lide, D.R., ed. CRC Handbook of Chemistry and Physics. 72nd ed. 1991, CRC Press: Boston. (43) Singer, D.M., Maher, K., and Brown Jr, G.E. Uranyl-chlorite sorption/desorption: Evaluation of different U(VI) sequestration processes. Geochim. Cosmochim. Acta 2009, 73(20), 5989-6007.
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(44) Arnold, T., Zorn, T., Zanker, H., Bernhard, G., and Nitsche, H. Sorption behavior of U(VI) on phyllite: experiments and modeling. J. Contam. Hydrol. 2001, 47(2-4), 219-231. (45) Banfield, J.F. and Murakami, T. Atomic-resolution transmission electron microscope evidence for the mechanism by which chlorite weathers to 1 : 1 semiregular chlorite-vermiculite. Am. Mineral. 1998, 83(3-4), 348-357.
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1000
Kd(ml/g)
800
600
400
200
0 0.0
0.5
1.0
fCC
650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673
Figure 1
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674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715
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fSolid U Conc
Page 27 of 33
1.0
1.0
0.8
0.8
0.6
0.6
A
0.4 0.2
0.2
0.0
0.0 0.0
0.2
0.4
0.6
0.8
B
0.4
1.0
0.0
0.2
fCC
0.4
0.6
0.8
1.0
fCC
Figure 2
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6
A
6
B FCC =
FCC = 0.0
5
Relative Intensity
716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760
Page 28 of 33
0.0
5 0.2
0.2
4
4 0.4
0.4
3
3 0.6
0.6
2
2 0.8
0.8
1
1 1.0
1.0
0
0 500
550
600
Wavelength (nm)
500
550
600
Wavelength (nm)
Figure 3
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6
A
6
B a
a 5
Relative Intensity
761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805
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5
b
b 4
4
c
c 3
3
d
d 2
2
e
e 1
1
f
f 0
0 500
550
600
Wavelength (nm)
500
550
Wavelength (nm)
Figure 4
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600
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1.0
Intensity
806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851
A
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0 500
550
600
Wavelength (nm)
Page 30 of 33
B
500
550
Wavelength (nm)
Figure 5
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Log(Peak Area)
4
3
2
1
0 0.0
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Figure 6
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fNormalized Spectra
878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923
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Figure 7
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Article
Investigation of U(VI) Adsorption in Quartz-Chlorite Mineral Mixtures Zheming Wang, John M Zachara, Jianying Shang, Choong Jeon, Juan Liu, and Chongxuan Liu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 14 Jun 2014 Downloaded from http://pubs.acs.org on June 19, 2014
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Environmental Science & Technology
1 2 3 4 5
Investigation of U(VI) Adsorption in Quartz-Chlorite Mineral Mixtures Zheming Wang,* John M. Zachara, Jianying Shang, Choong Jeon†, Juan Liu and Chongxuan Liu
6
Pacific Northwest National Laboratory, Richland, WA 99352
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
*
Corresponding author: Pacific Northwest National Laboratory, P.O. Box 999, MS K8-96, Richland, WA 99354; Phone: (509) 371-6349; Fax: (509) 371-6354; E-mail: [email protected] † Present Address: Department of Environmental & Applied Chemical Engineering, Gangneung-Wonju National University, Gangwon-do, Republic of Korea.
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ABSTRACT
39 40
A batch and cryogenic laser-induced time-resolved luminescence spectroscopy
41
investigation of U(VI) adsorbed on quartz-chlorite mixtures with variable mass ratios
42
have been performed under field-relevant uranium concentrations (5x10-7 M and 5x10-6
43
M) in pH 8.1 synthetic groundwater. The U(VI) adsorption Kd values steadily increased
44
as the mass fraction of chlorite increased, indicating preferential sorption to chlorite. For
45
all mineral mixtures, U(VI) adsorption Kd values were lower than that calculated from the
46
assumption of component additivity possibly caused by surface modifications stemming
47
from chlorite dissolution; The largest deviation occurred when the mass fractions of the
48
two minerals were equal. U(VI) adsorbed on quartz and chlorite displayed characteristic
49
individual luminescence spectra that were not affected by mineral mixing. The spectra of
50
U(VI) adsorbed within the mixtures could be simulated by one surface U(VI) species on
51
quartz and two on chlorite. The luminescence intensity decreased in a nonlinear manner
52
as the adsorbed U(VI) concentration increased with increasing chlorite mass fraction –
53
likely due to ill-defined luminescence quenching by both structural Fe/Cr in chlorite, and
54
trace amounts of solubilized and re-precipitated Fe/Cr in the aqueous phase. However,
55
the fractional spectral intensities of U(VI) adsorbed on quartz and chlorite followed the
56
same trend of fractional adsorbed U(VI) concentration in each mineral phase with
57
approximate linear correlations, offering a method to estimate of U(VI) concentration
58
distribution between the mineral components with luminescence spectroscopy.
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INTRODUCTION
60
The adsorption of uranium to soil minerals and subsurface sediments has been
61
extensively investigated to resolve issues of environmental contamination and to support
62
site selection of nuclear waste repositories.1-4 Under most environmental conditions,
63
uranium exists as the hexavalent uranyl ion, UO22+. Uranyl ion adsorbs to a broad range
64
of mineral phases including quartz,5, 6 feldspar,7 calcium carbonates,8, 9 iron oxides,10, 11
65
and phyllosilicates.12,
66
strongly on solution pH and counter anions.
67
precipitate in the form of uranyl phosphate, carbonates, and oxyhydroxides mineral
68
phases.14-16
13
Adsorption affinities to these phases are variable, depending At higher concentrations, uranyl may
69
Metal ions may exhibit distributed adsorption in mineral assemblages and natural
70
sediments. For example, Stubbs et al. identified six mineral hosts for sorbed uranium in
71
Hanford vadose zone sediment based on electron microprobe and TEM analysis.17 The
72
overall adsorptivity of a composite mineral material is influenced by the adsorption
73
characteristics of the contributing mineral phases and their mass fractions, and by
74
interactions between the adsorbing mineral phases.18
75
The description of metal ion adsorption on mineral mixtures and sediments relies
76
heavily on surface complexation modeling to account for aqueous chemical and surface
77
effects. Two common surface complexation modeling approaches are the generalized
78
composite model (GCM) and the component additivity model (CAM).19-22 In the former,
79
generic surface sites are assumed for the entire mineral assemblage. Site density and
80
possible surface complexes with fitted stability constants are applied under different
81
adsorption conditions to determine the likely adsorption mechanism(s) based on the
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goodness of fit. In the latter, adsorption is described as the summation of that occurring
83
on individual mineral phases,19 as computed from individual surface complexation
84
constants and the abundance of the mineral components in the assemblage.
85
Regardless of the surface complexation modeling approach, understanding the
86
adsorption process in natural materials is advanced by direct identification of surface
87
complex types and their concentrations. These speciation measurements are challenging
88
due to sensitivity issues and the multi-component nature of the surface complexation
89
process. In a previous study, we applied time-resolved U(VI) luminescence spectroscopy
90
and chemometric methods to determine the chemical forms of adsorbed U(VI) in a
91
contaminated subsurface sediment.23 As U(VI) adsorbed to constituent mineral phases
92
displayed characteristic luminescence spectra, we were able to determine that sediment
93
adsorbed U(VI) existed as surface complexes on quartz and phyllosilicates. However, it
94
was unclear if sorbent interactions influenced the luminescence spectral behavior, and if
95
the spectral intensity could be used to estimate the concentrations of specific surface
96
complex type. Here, we investigated U(VI) adsorption on mixtures of quartz and chlorite
97
to simulate the reactive fraction of a subsurface sediment. Sorbent mass ratios were
98
varied and the adsorption complex was examined by cryogenic luminescence
99
spectroscopy to determine how luminescence spectral behavior evolved with changing
100
mineral mass ratios in the mixtures. We assess whether it is possible to estimate the
101
concentration of U(VI) on the individual phases in the mixtures based on the collective
102
luminescence response.
103 104 105
EXPERIMENTAL
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Materials
Quartz (Min-U-Sil 30, U.S. Silica Company, Mill Creek,
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Oklahoma) was pretreated to remove organic contaminants and reactive fines following
108
the procedures of Kohler et al24 and rinsed with DI water until the supernatant
109
conductivity was 18 MΩ cm or lower. The material was then oven-dried at 40 °C for 3−4
110
days. Specific surface area was determined by the N2 BET method (Quantachrome
111
Autosorb 6-B gas adsorption system) to be 1.63 m2/g.
112
[(Mg2.95Al0.05)(Si2.99Al1.01)O10(OH)2.(Mg1.97Al0.66 Cr0.25Fe3+0.06Fe2+0.06) (OH)6;25] was
113
hand ground to yield 20-53 µm fraction with a BET surface area of 2.6 m2/g. The
114
properties of smaller size fractions of the same mineral specimen (2 to 5 µm and 5 – 20
115
µm) have been previously reported.23 Calcite-saturated synthetic groundwater (SGW2;
116
pH 8.1) was prepared according to Bond et al.26 A stock solution of uranium nitrate (1.06
117
x 10-3 M, pH 1) was prepared by dissolving a weighed amount of uranyl nitrate (Fluka) in
118
0.1 M nitric acid. All other chemicals were reagent grade.
California chlinochlore
119
U(VI) Adsorption Two identical series of eleven quartz-chlorite mixtures were
120
prepared by mixing weighed amounts of the solids in 15 ml polypropylene centrifuge
121
tubes to quartz:chlorite mass ratios of 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9 and
122
0:10. The total mineral weight was 0.2 g. The sorbents were twice equilibrated with
123
SGW2 for 24 h and separated from the supernatants by centrifugation. The resulting
124
solids were then re-suspended in 10 ml of SGW2 and spiked with the U(VI) stock
125
solution to yield 5x10-6 M U(VI) in one series and 5x10-7 M U(VI) in the other. The
126
suspension pH was maintained at pH 8.1 with dilute NaOH. The U(VI)-bearing mineral
127
suspensions were placed on an orbital shaker and slowly agitated for 48 h to allow U(VI)
128
adsorption equilibrium.
After 48 h, the solid and solution were separated by
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129
centrifugation. Both the solid paste and the supernatant were sampled into 2 mm x 4 mm
130
x 25 mm (ID) quartz cuvettes for luminescence analysis, while the remainder of the
131
supernatants was acidified for U(VI) (KPA), Mg, Al, Si and Fe analysis (ICP/MS).
132
Aqueous U(VI) speciation calculation The speciation of U(VI) in SGW2 at pH
133
8.1 under ambient conditions was calculated using the MINTEQA227 software with the
134
most current, critically reviewed thermodynamic stability constants for the U(VI)
135
complexes.28
136
Cryogenic Laser Luminescence Spectroscopy
The instrumentation and
137
experimental procedures for luminescence spectroscopic measurements at LHeT were
138
described previously.15, 29 The quartz sample cuvettes were attached to the cold finger of
139
a CRYO Industries RC152 cryostat with liquid helium vaporizing beneath the sample to
140
reach a sample temperature of 8 ± 2 ̊K. The sample was excited with a Spectra-Physics
141
Nd:YAG laser pumped Lasertechnik-GWU MOPO laser at 415 nm and the emitted light
142
was collected at 85˚ to the excitation beam and detected with a thermoelectrically cooled
143
Princeton Instruments PIMAX intensified CCD camera after spectral dispersion through
144
an Acton SpectroPro 300i double monochromator spectrograph.
145
analyzed using a commercial software, IGOR®, from Wavematrix, Inc. All spectra were
146
corrected for the mass ratio-weighted background emission observed for quartz and
147
chlorite prepared under the same conditions as those of the U(VI)-bearing samples except
148
that no U(VI) was added.
149 150
RESULTS AND DISCUSSION
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The spectra were
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Adsorption of U(VI) on quartz-chlorite mixtures The magnitude of U(VI)
152
adsorption on the quartz-chlorite mixtures varied with the mineral mass ratio and U(VI)
153
concentration (Table 1 and Figure 1). The highest adsorption was seen on pure chlorite
154
and the lowest on pure quartz. At a U(VI) concentration of 5×10-6 M, U(VI) adsorption
155
was 36.2% on quartz and 84.9% on chlorite, corresponding to adsorption Kd values of
156
28.3 ml/g and 281 ml/g, respectively. Lowering the U(VI) concentration to 5×10-7 M
157
increased U(VI) adsorption to 46.7% on quartz and 92.9% on chlorite, corresponding to
158
Kd values of 43.7 and 870 ml/g, respectively. These measured Kd values were consistent
159
with ones measured previously,23 when differences in surface areas are taken into
160
account.
161
As a result of the disparate U(VI) adsorption on the two phases, the total adsorbed
162
U(VI) concentration, CU,Ads, increased with the mass fraction of chlorite (Table 1 and SI
163
Figure 1). The concentration of adsorbed U(VI) contributed by quartz and chlorite in the
164
solid mixtures at each initial U(VI) concentration were calculated through the following
165
relationship, f Q × K d ,Q f Q × K d ,Q + f CC × K d , CC
166
CU,Q = CU,Ads ×
167
CU,CC = CU,Ads - CU,Q
(1)
(2)
168
Where CU,Q and CU,CC were adsorbed U(VI) concentrations contributed by quartz and
169
chlorite in the mineral mixture. Kd,Q and Kd,CC were the U(VI) adsorption Kd values on
170
pure quartz and chlorite at the corresponding initial U(VI) concentration, and fQ and fCC
171
were the respective mass fractions. The fractional concentration contribution to the
172
adsorbed U(VI) by quartz (FU,Q) and chlorite (FU,CC) in the mixtures were defined as: 7 ACS Paragon Plus Environment
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FU,Q = CU,Q/CU,Ads and FU,CC = CU,CC/CU,Ads
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(3)
174
Reflecting the lower affinity of quartz as compared to chlorite, the calculated U(VI)
175
concentration contributions from quartz were consistently lower in all quartz-chlorite
176
mixtures (SI Figure 2). For all mineral mixtures in which fCC > 0.5, the fractional
177
concentration contribution of quartz to adsorbed U(VI) in the mineral mixtures was ~ 5%
178
or less (Figure 2).
179
With the component additivity model, the distribution coefficient for adsorbed
180
U(VI) in the quartz-chlorite mixtures equals the mass-weighted sum of the adsorption Kd
181
values on the individual sorbents,18, 30, 31
182
Kd, mixture = fQ Kd, Q + fCC Kd, CC
(4)
183
Accordingly, the calculated Kd values displayed straight lines as a function of chlorite
184
mass fraction at both U(VI) concentrations (dashed lines in Figure 1). The calculated Kd
185
values were all above the measured ones for the mineral mixtures, and the largest
186
differences were observed when both minerals were at equal mass. Such deviations
187
suggest that either there is interaction between the quartz and chlorite, or that surface
188
modifications occurred as a result of mineral mixing and co-equilibration. Apparently,
189
the component additivity approach is not valid for quantitatively modeling U(VI)
190
adsorption in such mixtures without properly taking into account the causes for the
191
deviation.
192
Previous studies on the applicability of adsorption additivity for metal ion
193
adsorption in binary mineral mixtures18, 32, 33 revealed dependence on the specific metal
194
ion and sorbent identity. The adsorption of both Cs+ and Sr2+ followed the additivity rule
195
on bentonite-sand and silica-montmorillonite mixtures.18 However, significant negative 8 ACS Paragon Plus Environment
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deviations from adsorption additivity were observed for Sr2+ adsorption on alumina-illite
197
mixtures, while large positive deviations were observed for Sr2+ adsorption on alumina-
198
montmorillonite mixtures.18 The dissolution of alumina and the formation of unidentified
199
Sr-bearing precipitates were speculated to cause the noted effect.
200
Anderson and Benjamin systematically investigated the adsorption of Ag+, Cd2+,
201
Zn2+ and several inorganic anions on Fe(OH)3, Al(OH)3, and Fe(OH)3-Al(OH)3 mixtures
202
and co-precipitates under various chemical conditions.32,
203
caused small enhancements in Zn2+ adsorption by Fe(OH)3 but inhibited the adsorption of
204
Ag+ and Cd2+. Constant-capacitance surface complexation modeling suggested that a
205
majority of the Fe(OH)3 surface sites were covered and replaced by Al(OH)3 upon
206
mixing, leading to reduced metal ion adsorption. Surface and bulk analyses clearly
207
indicated that particle interactions in the mixtures, including dissolution and precipitation,
208
resulting in changes in particle size distribution, surface charge, and surface site
209
distribution.
33
The presence of Al(OH)3
210
SEM images of the post-reaction quartz-chlorite mixture (SI Figure 3) revealed
211
that the individual phases generally remained separate except that fine-grained quartz
212
associated with larger chlorite platelets.
213
aggregation of the sorbents was an unlikely cause of the observed lower Kd values, which
214
in some samples was as much as 50% as compared to the pure phases (Figure 1). We
215
hypothesize that the decrease of Kd values for the quartz-chlorite mixtures was due to
216
trace amounts of the dissolved solids, particularly the brucite layer of chlorite, releasing
217
Mg2+, Al3+, Cr3+, Fe3+ and Fe2+.
218
concentration in the presence of chlorite was between 17 µM to 48 µM higher than that in
Thus, surface area loss due to physical
As indicated by the results of ICP analysis, Mg
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SGW2 and increases along with the mass fraction of chlorite (SI Figure 4), equivalent to
220
between 0.01% to 0.04% chlorite dissolution.
221
stoichiometric concentrations of other metal ions are also released into the solution.
222
Some of the leached metal ions may occupy surface sites that would otherwise be
223
available for uranyl adsorption, especially on quartz which is mostly negatively charged
224
at the present pH. As will be shown in the next section, a major surface U(VI) complex
225
on chlorite involved the negatively charged uranyl-tricarbonate anion complex,
226
UO2(CO3)34-. Therefore, the loss of highly-charged cation sites on chlorite would also
227
reduce U(VI) sorption on chlorite. In addition, the attachment of negatively charged fine
228
quartz particle might also adversely impact the adsorption U(VI) on some of the
229
positively charged surface sites on chlorite.
230
If the dissolution is congruent,
Luminescence spectroscopy of adsorbed U(VI) The luminescence spectra of
231
U(VI) adsorbed on the quartz-chlorite mixtures displayed characteristic features (Figure
232
3). The vibronic band maxima were located at 499.7 nm, 520.4 nm and 541.3 nm for
233
U(VI) adsorbed on quartz at an initial U(VI) concentration of 5x10-6 M, while those for
234
U(VI) on chlorite were at 480.3 nm, 500.8 nm, 521.4 nm and 543.2 nm (Figure 3A and SI
235
Table 1). These results were in good agreement with previously reported values.23 The
236
uranyl ion formed inner-sphere surface complexes with deprotonated silanol sites on
237
quartz, while the U(VI) spectra on chlorite resembled those of uranyl-
238
tricarbonate/dicarbonate or calcium-uranyl-tricarbonate complexes.29, 34, 35 The spectral
239
features of U(VI) adsorbed on quartz decreased and eventually disappeared as the mass
240
fraction of chlorite (fCC) increased. The spectral pattern for chlorite, defined here as UCa,
241
became dominant over fCC = 0.5.
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242
The spectrum of U(VI) adsorbed on quartz was unchanged at 5x10-7 M U(VI) as
243
compared with that at 5x10-7 M U(VI) (Figure 3B). However, the spectra of U(VI)
244
adsorbed on chlorite developed more than one spectral pattern at this concentration. The
245
spectrum was identical to UCa when fCC < 0.5 while a broad, poorly resolved spectra
246
with an emission maximum of 521.7 nm (SI Table 1), defined here as pattern UCb,
247
became dominantt at fCC > 0.5 (Figure 3B lower traces), indicating the presence of
248
another U(VI) surface complex on chlorite.
249
U(VI) luminescence spectra were also recorded for the supernatants and for
250
SGW2 that was spiked with U(VI) nitrate (data not shown). Consistent with previous
251
measurements, U(VI) in the supernatants showed luminescence spectra that closely
252
resembled those of Ca2UO2(CO3)3(aq) and CaUO2(CO3)32- complexes as expected from
253
aqueous speciation calculation. The same type of spectra were displayed on the time-
254
resolved spectra of both of the wet paste samples of U(VI) adsorbed on quartz and
255
chlorite at 5x10-6 M U(VI) at long delay times, consistent with the presumption that the
256
wet paste samples entrained a small fraction of the supernatant. Such spectra were absent
257
in samples with large fCC values at 5x10-7 M U(VI) as a results of nearly complete U(VI)
258
adsorption.
259
Time-resolved luminescence spectra at delay times up to 4 ms showed that while
260
there was a single dominant species of U(VI) adsorbed on quartz (data not shown), both
261
U(VI) surface species on chlorite represented by the UCa and UCb spectral patterns were
262
present at 5x10-6 M U(VI), as expected, and UCb became more pronounced at longer
263
delay times (SI Figure 5). For all the samples with U(VI) adsorbed on quartz-chlorite
264
mixtures, the time-resolved spectra reflected the variation of the relative intensities of the
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265
three surface U(VI) species as governed by their luminescence lifetimes. For example,
266
for U(VI) adsorbed on quartz-chlorite mixture with fCC = 0.4 at 5x10-6 M U(VI), the
267
spectra were primarily composed of those of U(VI) adsorbed on quartz and on chlorite
268
with a spectral pattern UCa at shorter delay times (Figure 4A). As the delay time
269
increased, the relative intensity of UCa decreased gradually as indicated by the slight
270
drop of intensity in the 480.3 nm band where the spectra of U(VI) adsorbed on quartz
271
displayed only weak emission, pointing to increased character of U(VI) adsorbed on
272
quartz. At the same time, the relative intensity in the region from ~ 510 nm to 550 nm
273
was noticeably elevated, a clear indication of the growing contribution of the species
274
UCb.
275
spectral pattern UCa had a shorter luminescence lifetime than the U(VI) surface species
276
on quartz and the U(VI) species with a spectral pattern UCb had the longest luminescence
277
lifetime.
Such spectral changes implied that the surface U(VI) species corresponding to
278
As shown for the sample with fCC = 0.3, the time-resolved emission spectra at
279
5x10-7 M U(VI), again, indicated that at shorter delay times, the spectra represented both
280
U(VI) adsorbed on quartz and chlorite (UCa), and the latter decreased as delay time
281
increased to 0.7 ms based on the gradual drop of the intensity of the 480.3 nm band
282
(Figure 4B). As delay time further increased to 2.8 ms and 4.0 ms the spectral character
283
of U(VI) adsorbed on quartz also disappeared and the spectra became similar to those
284
observed at large fCC values (≥ 0.6) with dominant spectral pattern UCb at the same
285
U(VI) concentration, confirming the presence of two U(VI) surface complexes on
286
chlorite: one with spectral pattern UCa at lower fCC values and shorter delay times, and
287
the other with spectral pattern UCb that was present primarily at higher fCC values and
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288
longer delay times at low U(VI) concentration (i.e. 5x10-7 M)(Figures 3B, 4B). Thus the
289
second surface U(VI) species on chlorite (UCb) likely formed on a subset of surface sites
290
with the highest U(VI) affinities. The site density for surface U(VI) complex with
291
spectral pattern UCa is much higher and thus this complex dominated at higher U(VI)
292
concentrations. The UCb spectrum was red-shifted by over 20 nm as compared to UCa,
293
which reflected the formation of strong, inner-sphere surface complex, consistent with
294
U(VI) adsorption at sites of higher U(VI) affinity.
295
Further spectral analyses verified that the luminescence spectra of adsorbed U(VI)
296
on all of the quartz-chlorite mixtures could be simulated by a linear combination of the
297
three species described above: one U(VI) surface complex on quartz and two on chlorite
298
with spectral profiles of UCa and UCb (Figure 5 as an example).
299
characteristics of U(VI) surface complexes on the individual phases were not affected by
300
mixing of the mineral components.
Therefore, the
301
Luminescence decay analysis indicated that the decay curves of U(VI) adsorbed
302
on chlorite could be fit by two exponential functions, resulting in luminescence lifetimes
303
of 155 ± 52 µs and 1328 ± 396 µs, respectively, consistent with the assignment to the
304
U(VI) species with spectral patterns of UCa and UCb. Unexpectedly, fit of the decay
305
curves of U(VI) adsorbed on quartz also required two exponential functions with
306
luminescence lifetime of 796 ± 142 µs and 233 ± 135 µs, respectively (SI Table 1). This
307
could be due to many reasons, such as two U(VI) surface complexes on quartz with
308
similar profiles, same surface species with different hydration waters or surface cite
309
inhomogeneity 2, 36, 37. The trend of the measured luminescence lifetimes were consistent
310
with that expected from the variations of the time-resolved spectra (Figure 4). However,
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311
it should be cautioned that large deviations were observed in the measured luminescence
312
lifetimes data, possibly due to sample heterogeneity, and luminescence decay fitting with
313
multiple exponential functions are prone to errors.
314
Variation of U(VI) spectral intensities with mineral compositions Countering
315
the trend of the increasing Kd values (Table 1) and adsorbed U(VI) concentration with
316
increasing fCC values (SI Figure 1), the overall spectral intensity decreased with
317
increasing fCC (Figure 6). The sharpest intensity reduction occurred at fCC ≤ 0.2. The
318
spectral area for U(VI) adsorbed on quartz (fCC = 0) was 461 times larger than U(VI)
319
adsorbed on chlorite (fCC = 1) at 5x10-7 M U(VI), and 775 times larger at 5x10-6 M
320
U(VI). Given that the adsorbed U(VI) concentration on chlorite was more than twice that
321
on quartz, these intensity values indicated that that the luminescence quantum yield of
322
U(VI) adsorbed on quartz was about three orders of magnitude larger than that on
323
chlorite, assuming that the luminescence spectra of U(VI) adsorbed on quartz and chlorite
324
have similar molar absorptivity.
325
Evaluation of both the deconvoluted luminescence spectra and the corresponding
326
normalization factors indicated that the sharp intensity drop from fCC = 0 to fCC = 0.2 was
327
more significant for U(VI) adsorbed on quartz. The subsequent intensity drop at fCC > 0.2
328
applied to U(VI) adsorbed on both quartz and chlorite (SI Figure 6).
329
concentration of 5×10-7 M, the concentration of U(VI) adsorbed on quartz decreased from
330
1.17×10-8 to 0.32×10-8 mole/g from fCC =0 to fCC = 0.2. However, the corresponding
331
luminescence spectral area dropped from 1550 to 32 based on the results of spectral
332
deconvolution. Such large disproportionality between the emission intensity and the
333
concentration of adsorbed U(VI) indicated that there was pronounced luminescence
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quenching,38, 39 and/or self-absorption of the emitted light by Fe and Cr in chlorite.40, 41 If
335
self-absorption were the primary factor, it would be expected that adsorbed U(VI) on
336
quartz and chlorite would behave equally and that the decrease of luminescence intensity
337
would be proportional to fCC. Neither of these expectations was observed (Figure 6 and
338
SI Figure 6). Thus, the luminescence decrease with increasing fCC was likely caused by
339
luminescence quenching by Fe, Cr and other transition metal impurities in the chlorite.
340
The exact quenching mechanisms of adsorbed U(VI) luminescence by the mineral
341
mixtures was not determined. The chlorite contained both Fe (1.2%) and Cr (2.3%). The
342
total available Fe and Cr concentrations (3.8x10-3 M Fe and 8.7x10-3 M Cr) were three or
343
four orders of magnitude higher than that of U(VI) (5x10-6 M or 5x10-7 M, respectively)
344
at the given the solid-solution ratios. Assuming congruent dissolution of chlorite, the
345
dissolved Cr3+, Fe3+, and Fe2+ concentrations would be in the ranges of 0.86x10-6 M –
346
2.44x10-6 M (Cr3+) and 0.21x10-6 M – 0.59x10-6 M (both Fe2+ and Fe3+), respectively,
347
based on the measured Mg concentrations.
348
dissolved oxygen concentration could be as high as 1.4x10-3 M,42 and thus the solubilized
349
Fe2+ would be oxidized to Fe3+ by oxygen. The low Fe2+ concentration and the presence
350
of dissolved oxygen would also minimize any reduction of uranyl ion by Fe2+ in the
351
present system, such reductions had been observed with Fe-rich chlorite under anaerobic
352
conditions.43 Limited by low solubility, both Fe3+ and Cr3+ ions could exist as surface-
353
sorbed species, precipitated clusters, or ferrihydrite-like nano-particulates,44, 45 leading to
354
enhanced luminescence quenching. Indeed, trace Fe was observed in association with
355
both chlorite and quartz in the EDS elemental map of the U-bearing quartz-chlorite
Considering the aerobic conditions, the
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356
mixtures (SI Figure 3). It is also possible that exposed structural and surface-adsorbed Fe
357
were active U(VI) adsorption sites in the mineral mixtures.43, 44
358
Comparison of Figures 2 and 3 showed that the increasing spectral characters of
359
U(VI) adsorbed on chlorite with increasing fCC values on the normalized luminescence
360
spectra followed the same trend as that of the fractional concentration of adsorbed U(VI)
361
in the mineral mixtures. In order to quantify the correlations between the fractional
362
U(VI) concentration in each mineral phase in the solid mixture with the corresponding
363
fractional intensity in the measured spectra, the fractional spectral intensity of U(VI)
364
adsorbed on quartz and chlorite, fS,Q and fS,CC, were calculated at all fCC values as follows,
365
f S ,Q =
AQ AQ + AUCa + AUCb
and
f S ,CC =
AUCa + AUCb AQ + AUCa + AUCb
(5)
366
where AQ, AUCa and AUCb are the integrated areas under the corresponding deconvoluted
367
luminescence spectra of U(VI) associated with quartz and chlorite (both UCa and UCb).
368
Even though the calculations employed nontrivial approximations in the absence of
369
precise determination of the quantum yields of the surface complexes, plots of the
370
calculated fractional spectral intensity as a function of fCC (Figure 7) displayed
371
approximate resemblance to the fractional adsorbed U(VI) concentrations on quartz and
372
chlorite (Figure 2).
373
function of the spectral fractions of U(VI) adsorbed on quartz and chlorite (SI Figure 7)
374
only displayed qualitative linear correlations in the fCC value range of 0.2 – 1.0 at both
375
U(VI) concentrations, and the concentration fractions of U(VI) adsorbed on quartz were
376
underestimated while those on chlorite were overestimated in all the mixtures. The latter
377
inferred that the observed negative deviation of the measured Kd values from the
378
calculated ones based on component additivity (Figure 1) had a larger contribution from
However, correlation plots of the concentration fractions as a
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379
decreased U(VI) adsorption on chlorite in the mixtures. By assuming that the deviation
380
was solely due to decreased U(VI) adsorption on chlorite, the Kd contributions from
381
quartz and chlorite, Kd,Q’ and Kd,CC’ were calculated from their mass fractions as the
382
following,
383
Kd,Q’ = Kd,Q × fQ
(6)
384
Kd,CC’ = Kd - Kd,Q’
(7)
385
The resulting Kd,Q’ and Kd,CC’ values were used to calculate the concentration
386
contributions and fractional concentrations of U(VI) adsorbed on quartz and chlorite in
387
the mixtures using Eqs. (1) – (3).
388
Plots of the calculated concentration fractions as a function of fCC (SI Figiure 8)
389
displayed improved resemblance to those of the spectral fractions (Figure 7) and
390
correlation plots of the concentration fractions as a function of the spectral fractions
391
became linear across the entire fCC range with correlation coefficients of 0.95 and 0.92 at
392
U(VI) concentrations of 5 × 10-6 M and 5 × 10-7 M, respectively (SI Figure 9). These
393
results indicated that mixing quartz and chlorite primarily affected U(VI) adsorption on
394
chlorite with little impact on U(VI) adsorption on quartz even though larger U(VI)
395
luminescence quenching was observed on the latter. The approximate linear correlations
396
between the concentration fractions and the spectral fractions suggest that it is possible to
397
estimate U(VI) concentration in individual mineral phases based on the mineralogical
398
composition, total U(VI) concentration and the measured luminescence spectra, thus
399
offering credible empirical inputs for model simulation of U(VI) adsorption in mineral
400
mixtures when precise measurement is not feasible.
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401
Implications Quartz is the most abundant mineral phase among the fine size
402
fraction (< 125 µm) vadose zone sediments at both the US DOE Hanford Site and
403
Naturita Site.
404
abundance, possess high surface area and strong affinities to U(VI). Their common
405
association with larger crystallites as surface coatings further highlight their role in U(VI)
406
adsorption in subsurface sediments. The present results indicate that mixing quartz and
407
chlorite leads to deviation of U(VI) adsorption properties (e.g. Kd) from what is expected
408
from component additivity model, particularly for more reactive minerals such as
409
chlorite. This implies that for natural sediments, which are aged mineral mixtures,
410
adequate corrections taking into consideration of possible alteration of the mineral
411
constituents are required to simulate U(VI) adsorption in the mineral mixture based
412
adsorption parameters obtained on the individual mineralogical components.
413
invariant spectral profiles of the primary U(VI) surface species and approximate linear
414
correlations between the fractional U(VI) concentration and spectra in the binary mineral
415
mixtures, on the other hand, suggest that with precise knowledge of the mineralogical
416
composition and overall U(VI) concentration, it is possible to predict U(VI) distribution
417
empirically in such mineral mixtures using U(VI) luminescence spectroscopy
418
measurements.
Chlorite, along with other phyllosilicates, though present with low
The
419 420
ASSOCIATED CONTENT
421
Supporting Information
422
Additional information as noted in the text. This material is available free of charge via
423
the Internet at http://pubs.acs.org.
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ACKNOWLEDGEMENTS
426
The authors thank Dr. James Amonette for providing the chlorite mineral
427
specimen and Mr. Bruce Arey for SEM and EDS sample analysis. The authors are also
428
indebted to four anonymous reviewers whose insightful comments greatly helped to
429
improve the quality of the manuscript. This project was supported by the Hanford site
430
SFA Program managed by the U.S. DOE Office of Biological and Environmental
431
Research (OBER). Part of this research was performed at EMSL, a national scientific
432
user facility at PNNL managed by the Department of Energy’s Office of Biological and
433
Environmental Research. Pacific Northwest National Laboratory is operated for the U.S.
434
Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.
435
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436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462
Table 1. Experimental data for U(VI) adsorption on quartz (Qtz)-chlorite (Chl) mixtures in pH 8.1 synthetic groundwater at 25°C. Total mineral weight = 0.2 g. Solution volume: 10 ml. Sample Qtz:Chl Surface [U]total U UAds Kd U Adsorption No. Ratio Area Adsorbed Concentration Density (m2) (M) (%) (10-8 Mole/g) (ml/g) (Mole/m2·108) 36.2 9.04 28.3 5.55 1 10:0 0.326 5.0·10-6 2. 9:1 0.346 5.0·10-6 35.6 8.90 27.6 5.15 -6 41.0 10.3 34.8 5.62 3 8:2 0.364 5.0·10 4 7:3 0.384 5.0·10-6 53.2 13.3 56.9 6.93 57.5 14.4 67.8 7.13 5 6:4 0.404 5.0·10-6 6 5:5 0.424 5.0·10-6 62.2 15.6 82.3 7.35 7 4:6 0.442 5.0·10-6 72.5 18.1 132 8.19 -6 8 3:7 0.462 5.0·10 76.1 19.0 160 8.24 80.2 20.0 202 8.33 9 2:8 0.482 5.0·10-6 10 1:9 0.500 5.0·10-6 83.7 20.9 258 8.36 11 0:10 0.520 5.0·10-6 84.9 21.2 281 8.16 -7 12 10:0 0.326 5.0·10 46.7 1.17 43.7 0.72 13 9:1 0.346 5.0·10-7 69.5 1.74 114 1.01 75.1 1.88 150 1.03 14 8:2 0.364 5.0·10-7 15 7:3 0.384 5.0·10-7 82.6 2.07 237 1.08 -7 16 6:4 0.404 5.0·10 85.8 2.14 301 1.06 17 5:5 0.424 5.0·10-7 88.8 2.22 397 1.05 18 4:6 0.442 5.0·10-7 90.4 2.26 470 1.02 -7 91.8 2.30 562 0.99 19 3:7 0.462 5.0·10 20 2:8 0.482 5.0·10-7 93.0 2.32 663 0.97 -7 21 1:9 0.500 5.0·10 94.1 2.35 793 0.94 94.6 2.36 870 0.91 22 0:10 0.520 5.0·10-7
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463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508
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Figure Captions Figure 1.
Plot of measured Kd values of U(VI) adsorption in quartz-chlorite mixtures as a function of the mass fraction of chlorite in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (filled circle) and 5·10-7 M (open circle), respectively. Solid lines are polynomial fit of the measured Kd values. Dashed lines are calculated Kd values based on the component additivity model.
Figure 2.
Simulated fractional contribution to the concentration of adsorbed U(VI) by quartz (open circle) and chlorite (filled circle) in the mineral mixtures as a function of the mass fraction of chlorite in quartz:chlorite mixtures in pH 8.1 synthetic groundwater. Total U(VI) concentrations are 5·10-6 M (A) and 5·10-7 M (B), respectively.
Figure 3.
LHeT Luminescence spectra of U(VI) adsorbed on quartz:chlorite mixtures in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively. All spectra were normalized to the same maximum intensity and offset along the Y-axis for clarity. λex = 415 nm.
Figure 4.
Time-resolved LHeT Luminescence spectra of U(VI) adsorbed on quartz:chlorite mixtures in synthetic groundwater, pH 8.1 at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively. In A, fCC = 0.4. In B, fCC = 0.3. The delay times are: a) 0.0 ms; b) 0.2 ms; c) 0.7 ms; d) 1.1 ms; e) 2.8 ms; and f) 4.0 ms. All spectra were normalized to the same maximum intensity and were offset along the Y-axis for clarity. λex = 415 nm.
Figure 5.
Deconvoluted LHeT luminescence spectra (circles) of U(VI) adsorbed on quartz:chlorite mixtures in pH 8.1 synthetic groundwater, pH 8.1 at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively, as the combination of U(VI) adsorbed on quartz (blue) and chlorite (green and purple). In A: fCC = 0.6. In B: fCC = 0.3. Both measured spectra were normalized to the same maximum intensity. λex = 415 nm.
Figure 6.
Luminescence intensity of U(VI) adsorbed on quartz-chlorite mixtures as a function of the mass fraction of chlorite in the mineral mixtures in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (filled circle) and 5·10-7 M (open circle), respectively. The solid and dashed lines are fitted trend lines of the intensity data.
Figure 7.
Simulated spectral fractions of U(VI) adsorbed on quartz (filled circle) and chlorite (open circle) in the mineral mixtures as a function of the mass fraction of chlorite in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively.
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509
REFERENCES
510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552
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(44) Arnold, T., Zorn, T., Zanker, H., Bernhard, G., and Nitsche, H. Sorption behavior of U(VI) on phyllite: experiments and modeling. J. Contam. Hydrol. 2001, 47(2-4), 219-231. (45) Banfield, J.F. and Murakami, T. Atomic-resolution transmission electron microscope evidence for the mechanism by which chlorite weathers to 1 : 1 semiregular chlorite-vermiculite. Am. Mineral. 1998, 83(3-4), 348-357.
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1000
Kd(ml/g)
800
600
400
200
0 0.0
0.5
1.0
fCC
650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673
Figure 1
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674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715
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fSolid U Conc
Page 27 of 33
1.0
1.0
0.8
0.8
0.6
0.6
A
0.4 0.2
0.2
0.0
0.0 0.0
0.2
0.4
0.6
0.8
B
0.4
1.0
0.0
0.2
fCC
0.4
0.6
0.8
1.0
fCC
Figure 2
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6
A
6
B FCC =
FCC = 0.0
5
Relative Intensity
716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760
Page 28 of 33
0.0
5 0.2
0.2
4
4 0.4
0.4
3
3 0.6
0.6
2
2 0.8
0.8
1
1 1.0
1.0
0
0 500
550
600
Wavelength (nm)
500
550
600
Wavelength (nm)
Figure 3
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ACS Paragon Plus Environment
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6
A
6
B a
a 5
Relative Intensity
761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805
Environmental Science & Technology
5
b
b 4
4
c
c 3
3
d
d 2
2
e
e 1
1
f
f 0
0 500
550
600
Wavelength (nm)
500
550
Wavelength (nm)
Figure 4
29
ACS Paragon Plus Environment
600
Environmental Science & Technology
1.0
Intensity
806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851
A
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0 500
550
600
Wavelength (nm)
Page 30 of 33
B
500
550
Wavelength (nm)
Figure 5
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ACS Paragon Plus Environment
600
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Environmental Science & Technology
Log(Peak Area)
4
3
2
1
0 0.0
852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877
0.2
0.4
0.6
0.8
1.0
fCC
Figure 6
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ACS Paragon Plus Environment
Environmental Science & Technology
1.0
fNormalized Spectra
878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923
1.0
A
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0 0.0
0.5
1.0
Page 32 of 33
B
0.0
0.2
fWeight of Chlorite
0.4
0.6
0.8
1.0
fWeight of Chlorite
Figure 7
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TOC Figure
1.0
0.8
U in Chlorite
0.6 0.4
U in Quartz
0.2 0.0
Fractional U(VI) Concentration
1.0
Fractional Normalized Spectra
924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943
Environmental Science & Technology
U in Chlorite 0.8 0.6 0.4 0.2
U in Quartz
0.0 0.0
0.5 1.0 Chlorite Mass Fraction
33
0.0
ACS Paragon Plus Environment
0.5 1.0 Chlorite Mass Fraction
