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Chemical speciation of silver (Ag) in soils under aerobic and anaerobic conditions: Ag nanoparticles vs. ionic Ag Yohey Hashimoto a,∗ , Satoshi Takeuchi a , Satoshi Mitsunobu b , Yong-Sik Ok c a b c
Department of Bioapplications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology, 2-24-16 Koganai, Tokyo 184-8588, Japan Institute for Environmental Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan Korea Biochar Research Center & Department of Biological Environment, Kangwon National University, Chuncheon 200-701, South Korea
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• We investigated how AgNP and ionic Ag undergo phase-transformations in soils. • 88% of AgNP remained persistent after 30 day of aerobic soil. • In the anaerobic soil, 83% of the spiked AgNP was transformed into Ag2 S. • Ionic Ag was sorbed with soil colloids or reduced to metallic Ag.
a r t i c l e
i n f o
Article history: Received 10 June 2015 Received in revised form 29 August 2015 Accepted 1 September 2015 Available online xxx Keywords: Chemical speciation Nanomaterials Redox XAFS
a b s t r a c t This study investigated how silver nanoparticles (AgNP) and ionic silver (AgNO3 ) undergo phasetransformations in soils under aerobic and anaerobic conditions using extended X-ray absorption fine structure (EXAFS) spectroscopy. After 30 days of aerobic incubation, 88% of AgNP added to the soil remained persistent, whereas AgNO3 was completely transformed into Ag associated with humus and clay minerals. In the anaerobic soil, 83% of the spiked AgNP was transformed into Ag2 S, accompanied by significant decrease in water- and acid-extractable Ag fractions. About 50% of AgNO3 spiked to the anaerobic soil underwent transformations into metallic Ag and associations with clay minerals. Oxide (Ag2 O) and carbonate (Ag2 CO3 ) forms of Ag were not predominant in aerobic and anaerobic soils. The redox potential of soil had a profound effect on determination of the phase-transformation pathways for AgNP and ionic Ag. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. E-mail address: [email protected] (Y. Hashimoto).
The increased use of silver (Ag)- and silvernanoparticle (AgNP)containing products raises concerns for the environment. Taking advantage of their bacteriocidal properties, Ag and increasingly
http://dx.doi.org/10.1016/j.jhazmat.2015.09.001 0304-3894/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Hashimoto, et al., Chemical speciation of silver (Ag) in soils under aerobic and anaerobic conditions: Ag nanoparticles vs. ionic Ag, J. Hazard. Mater. (2015), http://dx.doi.org/10.1016/j.jhazmat.2015.09.001
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AgNP are being employed in a wide range of commercial products, but the fate and behavior of Ag released into the environment are poorly understood. Silver is a minor component in soils with concentrations between < 0.01 and 5 mg kg−1 [1] and cited therein. The Ag concentration in soils near mining areas is generally elevated to 100 times above the background levels [2]. Asami et al. [3] found that in soils and river sediments adjacent to mining and refinery areas, Ag accumulated at up to 20-fold and 157-fold magnitude greater than those in the background levels, respectively. Apart from mining activities, the use of Ag and AgNP in various industries results in new exposure pathways for Ag into the environment. The release of Ag from AgNP-containing plastics, textiles [4,5] and socks [6] has been reported. As a result of these exposure pathways, one of the largest sinks of Ag generated in municipal areas can be wastewater treatment plants. The soil environment is also a potential sink for Ag via land applications of biosolids with levels of Ag as high as 856 mg kg−1 [7]. Bioavailability of metals is related to their mineral forms that control solubility and chemical speciation in solution. Thermodynamic solubility products and stability constants of Ag bound with inorganic and organic compounds can be useful to predict the Ag species and their solubility in the soil environment. According to a review by Levard et al. [8] and information cited therein, the predominant Ag species in the freshwater and soil environment are most likely to be AgCl, Ag2 S and Ag0 (or metallic Ag), depending chiefly on the redox potential of the system. Because of strong affinity for inorganic and organic sulfur groups, Ag can outcompete with other metal ions (e.g., Pb2+ and Zn2+ ) to bind with sulfides [9]. It has been reported that Ag associated with organic thiol or humus is a thermodynamically favored species in the soil. Hou et al. [10] performed Ag fractionation in uncontaminated soils using a chemical extraction method and found that organically-associated Ag was one of the major fractions accounting for 48% of total Ag. Jones et al. [2] reported a similar result showing that Ag was mostly abundant in humus-enriched surface soils and was present mainly in the oxidizable fraction (i.e., organic/sulfide-bound forms). In contrast, few studies have investigated about how Ag dissolves from AgNP in soils. Coutris et al. [11] reported that soil Ag derived from AgNP was extracted in readily reducible, oxidizable, and acid digestible fractions, depending on soil mineral types. Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy is a technique to provide information on the oxidation state and coordination environment of metals in soil solid phases. Information gained from XAFS provides direct evidences for metal speciation and is different from the results from chemical extraction and thermodynamic equilibrium model, which allow prediction of possible metal species. Studies using XAFS spectroscopy (mainly by X-ray absorption near-edge structure: XANES) to determine Ag speciation have been conducted for biosolid samples for the scenario of wastewater treatment plants [12–15]. In the soil environment, contrarily, few studies are available on Ag speciation with regard to how AgNP and Ag salts undergo phasetransformations using the XAFS spectroscopy technique. Settimio et al. [16] found that AgNO3 added to various soils underwent transformation into metallic Ag (Ag0 ), AgCl, and Ag associated with Fe-oxyhydroxides and reduced S compounds. Shoults-Wilson et al. [17] found that over 74% of AgNP added to soils remained unchanged after 28 days. Few studies have reported the phase-transformation (e.g., sulfidation, complexation and adsorption) of AgNP in anoxic soil conditions [18]. The primary pathway for phase-transformation of AgNP is still poorly understood in reducing soils as a function of Eh variation. Sulfidation in reducing soils significantly decreases the solubility of Ag and other trace elements [8,19], potentially limiting their environmental impact. The objective of this study was to determine how AgNP and ionic Ag (i.e., AgNO3 ) added to the soil
undergo transformations in aerobic (oxic) and anaerobic (anoxic) conditions. The purpose of employing aerobic and anaerobic soil conditions was to allow comparisons of phase transformations of AgNP and ionic Ag in different redox gradients. We employed extended X-ray absorption fine structure (EXAFS) spectroscopy to determine Ag speciation in the soil and related such information to Ag solubility and soil’s redox potential. 2. Materials and methods 2.1. Soil characterization and experiment setup A typical Andisols (Typic Hapludands) representative of Japanese upland soils was collected from an agricultural experiment field of Tokyo University of Agriculture and Technology (FM Fuchu, Tokyo). The soil was passed through a 2-mm sieve and used for the following experiment. The soil consisted of 45% sand, 24% silt and 31% clay and was classified as clay-loam texture (USDA system). The soil contained relatively high levels of humus (10.9% C) and Fe minerals. Detailed soil properties and their analytical procedures are shown in Table S1, Supporting information (SI). Soil incubation studies were conducted using AgNP (99.5% purity; 50 nm nominal particle size with PVP coating; purchased from Sigma–Aldrich), and AgNO3 (99.5% purity, purchased from Wako Pure Chemical Industries). The soils were spiked with AgNP suspension (deionized water with 15 min sonication) or AgNO3 solution to obtain a final concentration of 1000 mg Ag kg−1 soil. Based on our preliminary studies, the water extractable Ag fraction, which is one of the official standard soil tests in Japan [20], was extremely low in the soil containing AgNP. Since a similar problem was also reported elsewhere [11], we set up relatively high Ag conditions in order to ensure detection of Ag in the water fraction. The Ag concentrations used in this study are one or two orders of magnitude higher than the background levels or mining areas of soil Ag [2,3], but were close to the concentrations in biosolids up to 856 mg kg−1 , [7]. Previous studies also added similar levels of AgNP to soils in order to examine the phase-transformation and detrimental biological effects of Ag in soils [17,21,22]. To compare the solubility and speciation of Ag in different redox gradients, the soils were incubated for up to 30 days at 25 ◦ C under anaerobic and aerobic conditions by adding different amounts of water. For the aerobic treatment, the soils (20 g) were incubated in polypropylene tubes and rewetted periodically with 85% water content. For the anaerobic treatment, the soil (400 g) was filled in a 1 L container with water content of 150%. A preliminary study confirmed that these water contents can reproduce oxic and anoxic conditions of this soil. The pH and Eh of the soil for the anaerobic treatment were measured periodically using electrodes (F-73, HORIBA; IM-32P, TOA-DKK). The soils were periodically collected, and Ag was extracted with deionized water. After 30 days of incubation, the soil samples were collected and stored in vacuum at −80 ◦ C until the XAFS analysis. 2.2. Chemical extraction and thermodynamic equilibrium model Soil samples were collected at 2 h and 1, 4, 7, 15, 20, and 30 days after starting the incubation to determine water-extractable Ag concentrations using a 1:10 soil-water ratio for a 6-h extraction on a reciprocal shaker at 25 ◦ C. Soil samples collected on day 30 were used for a two-step fractionation analysis for Ag at a 1:20 soilsolution ratio for 20 h at 25 ◦ C [23]. For the first extraction step, 1.0 g of soils was extracted with 0.05 M Ca(NO3 )2 solution. The filtrate (fraction 1) was considered to include exchangeable metals. Soil Ag was next extracted with a 0.42 M CH3 COOH solution. The filtrate at this fraction (fraction 2) includes acid-extractable metals. The supernatant was separated from the solid by centrifugation and
Please cite this article in press as: Y. Hashimoto, et al., Chemical speciation of silver (Ag) in soils under aerobic and anaerobic conditions: Ag nanoparticles vs. ionic Ag, J. Hazard. Mater. (2015), http://dx.doi.org/10.1016/j.jhazmat.2015.09.001
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-1
Ag concentration (mg L )
0.08
0.06
AgNP (a)
30
aerobic
20
AgNO3 (b)
anaerobic 10
0.04 2 0.02
1
0.00 0
10
20
0.04
0 30 0 Time (day) (c)
AgNP 2 R = 0.90*
-1
Ag concentration (mg L )
3
10
20
30
3.0 (d)
AgNO3 2.5
2
R = 0.66*
0.03 2.0 0.02
1.5 1.0
0.01 0.5 0.00 -200
0
200
400
0.0 600 -200 Eh (mV)
0
200
400
600
Fig. 1. Water extractable Ag in aerobic and anaerobic soils spiked with AgNP (a) and AgNO3 (b), and linear regression between water extractable Ag and soil Eh values in the anoxic soils spiked with AgNP (c) and AgNO3 (d). Error bars are standard deviation of mean value (n = 3). *Significant at p < 0.05 probability levels.
analyzed for Ag by atomic absorption spectroscopy (Hitachi HighTechnologies Corp., Japan). An Eh-pH diagram for the Ag–Cl–S system at 25 ◦ C was constructed to predict the predominant Ag species in the anaerobic soil. The diagram was constructed with the expected Ag phases in the soil systems, which include AgCl(s) , Ag0 (s) , and AgS(s) . Equilibrium reactions and constants for potential Ag phases used in the Eh-pH diagram are listed in Table S2. The plotted lines in the Eh-pH diagram assumed activity of 10−6 M for Ag+ , 10−2 M for Cl− and 10−3 M for HS− . To determine the time-course change in Ag species during the incubation period, the values of pH and Eh in the anaerobic soils treated with AgNP and AgNO3 were plotted on the diagram. 2.3. Silver K-edge XAFS spectroscopy The K-edge Ag XAFS spectroscopy was performed on the soil samples at the beamline BL01B1 at SPring-8 in Hyogo, Japan. We prepared the following Ag references: Ag2 CO3 , AgNO3 , AgCl, Ag2 SO4 , Ag2 S (>99.9% purity, purchased from Kanto Kagaku Ltd., Japan), Ag nanoparticle (Sigma–Aldrich), metallic Ag (Ag foil), and Ag sorbed on ferrihydrite, gibbsite, birnessite, humus and phyllosilicate (kaolinite). The detailed method and procedure for the preparation of standard references are described in SI. The purchased Ag compounds were passed through a 106-m sieve, diluted with boron nitride to the Ag concentration of 10% w/w, and pressed into pellets with 7-mm diameter and 1-mm thick. The XAFS data were collected in transmission mode for BN diluted samples or in fluorescent mode for synthesized references and soil samples with a 19-element Ge semiconductor detector at ambient temperature across the Ag K absorption edge at 25,516 eV using Si(311) monochromator crystals. The soils in a 3 × 3 cm vacuumed plastic bag were thawed at ambient temperature before being placed on
the beamline stage. Two or three scans were collected for the soil samples and then averaged. We confirmed that the chemical speciation of AgNP used for this study was virtually identical to that of the metallic Ag reference (see Fig. 4). Background correction, normalization and conversion to kspace (1–10.5 Å−1 ) were performed on the data for Ag K-edge EXAFS using programs, Athena and Artemis software suite [24]. To identify which Ag species were predominant in the soil, principal component analysis (PCA) and target transformation were performed on normalized k3 -weighted EXAFS spectra. These procedures were performed to compute a SPOIL value that is an indicator to which the reference consists of principal component for EXAFS spectra of soil samples. Based on their SPOIL value (Table S3), the reference spectra were grouped according to the scores of “excellent” (0–1.5), “good” (1.5–3.0), “fair” (3.0–4.5) and “poor” (>4.5) [e.g.,25]. Then, the relative proportions of Ag species in the multicomponent EXAFS spectra of the soil samples were obtained by the liner-combination fitting (LCF) procedure using selected Ag references previously determined by the SPOIL value. The quality of LCF result was quantified as a residual (R) value defined by the following equation:
R=
exp − model
exp
2
2
where is normalized absorption. The LCF procedure was initiated using the reference that yielded the best one component fit, which is defined as the one with the smallest R value. Adding a second component species in the fitting procedure was allowed when addition of a new species improved the fit (i.e., smaller R value). This LCF procedure, referring to Hashimoto et al. [26], can avoid
Please cite this article in press as: Y. Hashimoto, et al., Chemical speciation of silver (Ag) in soils under aerobic and anaerobic conditions: Ag nanoparticles vs. ionic Ag, J. Hazard. Mater. (2015), http://dx.doi.org/10.1016/j.jhazmat.2015.09.001
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4
0.3
-1
Ag concentration (mg kg )
300
AgNO3 day 1
250 0.2
0.1
0.0
100
0
exch
50 aero
anaero AgNP
aero
anaero
(K P9
150
AgCl(s)
acid extractable
200
AgNP day 1
day 30
Ago(s)
day 30
Ag2S(s)
H2
AgNO 3
Fig. 2. Two-step sequential fractionation of Ag in the aerobic (aero) and anaerobic (anaero) soils added with AgNP and AgNO3 at day 30. The data with a magnified y-axis were shown in a box. exch: Ag extracted with with Ca(NO3 )2 solution; acid extractable: Ag extracted with CH3 COOH solution.
non-unique fitting solution since the use of a large number of references can inappropriately improve the R value. The second-best fit results, which were similar to those of the best fit, are listed in Table S4. The LCF was performed in k3 -space between 2 and 9 or 9.5 Å−1 , depending on spectrum quality. The EXAFS data were also analyzed by shell fits on k3 -weighted Fourier-transformed spectra using the Artemis code. The methods and procedures of EXAFS shell fit are from Mitsunobu et al. [27] and are given in SI. 3. Results and discussion 3.1. Ag solubility in anaerobic and aerobic soils Fig. 1 illustrates water extractable Ag from anaerobic and aerobic soils containing AgNP and AgNO3 during the 30-day incubation study. In the soil containing AgNP (hereafter AgNP soil), waterextractable Ag was smaller in the anaerobic conditions than the aerobic conditions throughout the incubation period (Fig. 1a). In the soil containing AgNO3 (hereafter AgNO3 soil), water extractable Ag in the anaerobic condition was one to two orders of magnitude less than that in the aerobic condition (Fig. 1b). Regardless of Ag types, Ag concentrations in the anaerobic soils decreased rapidly in the first 15 days and thereafter remained unchanged in the AgNP soil or decreased gradually in the AgNO3 soil. Such decrease in Ag solubility in the anaerobic soils can be illustrated as a strong linear relationship between Eh values and water-extractable Ag concentrations (Fig. 1c and d). The time-series Eh data during the incubation period are shown in Fig. S1. After 30 days of incubation, soil Ag was extracted sequentially using two different fluids, Ca(NO3 )2 (exchangeable), and CH3 COOH (acid extractable). Regardless of Ag types, the concentrations of exchangeable and acid-extractable Ag were notably smaller in the anaerobic soils, compared with the aerobic soils (Fig. 2). In the anaerobic condition, the labile fractions were considerably smaller in the AgNP soil than in the AgNO3 soil. In the AgNP soils, these Ag fractions were two orders of magnitude less in the anaerobic than in the aerobic soils. Lower Ag solubility in the anaerobic soil was consistent with the results from the time course changes for water-extractable Ag (Fig. 1a and b). 3.2. Eh-pH diagram for predicting Ag phases We evaluated the potential Ag transformation pathways as a consequence of the redox potential change using a constructed
S+ Fig. 3. Redox speciation of Ag in the soils added with AgNP (filled circles) and AgNO3 (open circles). Arrows indicate time sequence of Ag speciation in the soils. The diagram does not take into account changes in stability based on variable ion concentrations.
Eh-pH diagram (Fig. 3). According to the diagram, the formations of AgCl(s) , Ag2 S(s) and Ag0 (s) are thermodynamically favorable in a wide range of soil pH. Other potential reactions including the formation of AgSO4(s) , AgCO3(s) and Ag2 O(s) are not thermodynamically predicted since these products are highly labile in the normal soil pH range [28]. In Fig. 3, Eh and pH data are plotted for the AgNP and AgNO3 soils during the 30-day incubation under anaerobic conditions. For the AgNP soil, the Eh values decreased throughout the incubation period, and Ag2 S was subsequently predicted to be the controlling phase of Ag solubility at day 30. In contrast, the Eh values of AgNO3 soil hovered around 300 mV, and even after 30 days, the soil Eh did not reach favorable conditions for Ag2 S formation. Rick VandeVoort et al. [18] also reported the retardation of soil reductive reactions in AgNP and Ag-salt spiked soils. It is therefore plausible that a high concentration of dissolved Ag+ in the AgNO3 soil (Figs. 1 and 2) retarded microbial-mediated redox reactions, slowing soil reduction processes. 3.3. Ag K-edge EXAFS spectroscopy Silver K-edge EXAFS k3 (k)-spectra of selected references and soil samples are shown in Fig. 4. The selected reference Ag compounds possess differences in their EXAFS spectra, and therefore can be used to determine the Ag species in the soils. For the aerobic AgNP soil, the EXAFS spectrum was very similar with that of metallic Ag. In contrast, the EXAFS spectrum for the anaerobic AgNP soil was not similar to the metallic Ag reference, but was rather similar to Ag2 S in the second (∼4.0 Å−1 ), fourth (∼7.5 Å−1 ) and fifth (∼8.8 Å−1 ) oscillations. In contrast, the EXAFS spectrum for both aerobic and anaerobic AgNO3 soils was dissimilar to that of the AgNO3 reference. Such features in k3 -weighted EXAFS spectra were also confirmed on Fourier transformed EXAFS spectra (Fig. 5), which contained peaks reflecting the relative radial distance uncorrected for phase shift between the central Ag and neighboring atoms. The second peak at 2.5–3.0 Å (phase shift uncorrected) for the aerobic AgNP soil corresponded to the metallic Ag reference. For the anaerobic AgNP soil sample, the first peak at 1.5–2.4 Å, which reflects the Ag-S bond for the Ag2 S reference, was pronounced and so was the second peak (2.5–3.0 Å), corresponding to the metallic Ag reference. For the anaerobic AgNO3 soil sample, the second pronounced peak at 2.5–3.0 Å for the anaerobic AgNO3 soil sample was matched with that of the metallic Ag reference whereas the first peak was much less intense and did not exactly correspond to the Ag2 S reference. For more quantitative alternative to visual inspection and comparisons for differentiating Ag species in the soils, PCA and
Please cite this article in press as: Y. Hashimoto, et al., Chemical speciation of silver (Ag) in soils under aerobic and anaerobic conditions: Ag nanoparticles vs. ionic Ag, J. Hazard. Mater. (2015), http://dx.doi.org/10.1016/j.jhazmat.2015.09.001
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5
Ag2S
Ag phyllo
Ag humus
Metallic Ag (solid line) AgNP (dash)
AgNP (aerobic): 88% metallic Ag R = 0.048
AgNP (anaerobic): 17% metallic Ag, 83% Ag2S R = 0.165
AgNO3 (aerobic): 26% Ag-phyllo, 63% Ag-humus R = 0.327
AgNO3 (anaerobic): 41% metallic Ag, 58% Ag-phyllo R = 0.180
2
3
4
5
6
7
8
9
10
o -1
k (A ) Fig. 4. Ag K-edge EXAFS spectra of selected reference compounds, and soil samples (dotted line) and their best component fit by LCF (solid line). Ag phyllo and Ag humus were the reference Ag sorbed with kaolinite and humus, respectively. Metallic Ag was an Ag foil reference. The spectrum for “AgNP (dash)” was the intact AgNP sample used for the soil incubation study. The results of the second-best LCF were listed in Table S4.
computation of SPOIL values were performed on soil EXAFS spectra. Principal component analysis and the target transformation showed that the reference spectra with high-scored SPOIL values (
ARTICLE IN PRESS
HAZMAT-17074; No. of Pages 7
Journal of Hazardous Materials xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Chemical speciation of silver (Ag) in soils under aerobic and anaerobic conditions: Ag nanoparticles vs. ionic Ag Yohey Hashimoto a,∗ , Satoshi Takeuchi a , Satoshi Mitsunobu b , Yong-Sik Ok c a b c
Department of Bioapplications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology, 2-24-16 Koganai, Tokyo 184-8588, Japan Institute for Environmental Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan Korea Biochar Research Center & Department of Biological Environment, Kangwon National University, Chuncheon 200-701, South Korea
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• We investigated how AgNP and ionic Ag undergo phase-transformations in soils. • 88% of AgNP remained persistent after 30 day of aerobic soil. • In the anaerobic soil, 83% of the spiked AgNP was transformed into Ag2 S. • Ionic Ag was sorbed with soil colloids or reduced to metallic Ag.
a r t i c l e
i n f o
Article history: Received 10 June 2015 Received in revised form 29 August 2015 Accepted 1 September 2015 Available online xxx Keywords: Chemical speciation Nanomaterials Redox XAFS
a b s t r a c t This study investigated how silver nanoparticles (AgNP) and ionic silver (AgNO3 ) undergo phasetransformations in soils under aerobic and anaerobic conditions using extended X-ray absorption fine structure (EXAFS) spectroscopy. After 30 days of aerobic incubation, 88% of AgNP added to the soil remained persistent, whereas AgNO3 was completely transformed into Ag associated with humus and clay minerals. In the anaerobic soil, 83% of the spiked AgNP was transformed into Ag2 S, accompanied by significant decrease in water- and acid-extractable Ag fractions. About 50% of AgNO3 spiked to the anaerobic soil underwent transformations into metallic Ag and associations with clay minerals. Oxide (Ag2 O) and carbonate (Ag2 CO3 ) forms of Ag were not predominant in aerobic and anaerobic soils. The redox potential of soil had a profound effect on determination of the phase-transformation pathways for AgNP and ionic Ag. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. E-mail address: [email protected] (Y. Hashimoto).
The increased use of silver (Ag)- and silvernanoparticle (AgNP)containing products raises concerns for the environment. Taking advantage of their bacteriocidal properties, Ag and increasingly
http://dx.doi.org/10.1016/j.jhazmat.2015.09.001 0304-3894/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Hashimoto, et al., Chemical speciation of silver (Ag) in soils under aerobic and anaerobic conditions: Ag nanoparticles vs. ionic Ag, J. Hazard. Mater. (2015), http://dx.doi.org/10.1016/j.jhazmat.2015.09.001
G Model HAZMAT-17074; No. of Pages 7 2
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AgNP are being employed in a wide range of commercial products, but the fate and behavior of Ag released into the environment are poorly understood. Silver is a minor component in soils with concentrations between < 0.01 and 5 mg kg−1 [1] and cited therein. The Ag concentration in soils near mining areas is generally elevated to 100 times above the background levels [2]. Asami et al. [3] found that in soils and river sediments adjacent to mining and refinery areas, Ag accumulated at up to 20-fold and 157-fold magnitude greater than those in the background levels, respectively. Apart from mining activities, the use of Ag and AgNP in various industries results in new exposure pathways for Ag into the environment. The release of Ag from AgNP-containing plastics, textiles [4,5] and socks [6] has been reported. As a result of these exposure pathways, one of the largest sinks of Ag generated in municipal areas can be wastewater treatment plants. The soil environment is also a potential sink for Ag via land applications of biosolids with levels of Ag as high as 856 mg kg−1 [7]. Bioavailability of metals is related to their mineral forms that control solubility and chemical speciation in solution. Thermodynamic solubility products and stability constants of Ag bound with inorganic and organic compounds can be useful to predict the Ag species and their solubility in the soil environment. According to a review by Levard et al. [8] and information cited therein, the predominant Ag species in the freshwater and soil environment are most likely to be AgCl, Ag2 S and Ag0 (or metallic Ag), depending chiefly on the redox potential of the system. Because of strong affinity for inorganic and organic sulfur groups, Ag can outcompete with other metal ions (e.g., Pb2+ and Zn2+ ) to bind with sulfides [9]. It has been reported that Ag associated with organic thiol or humus is a thermodynamically favored species in the soil. Hou et al. [10] performed Ag fractionation in uncontaminated soils using a chemical extraction method and found that organically-associated Ag was one of the major fractions accounting for 48% of total Ag. Jones et al. [2] reported a similar result showing that Ag was mostly abundant in humus-enriched surface soils and was present mainly in the oxidizable fraction (i.e., organic/sulfide-bound forms). In contrast, few studies have investigated about how Ag dissolves from AgNP in soils. Coutris et al. [11] reported that soil Ag derived from AgNP was extracted in readily reducible, oxidizable, and acid digestible fractions, depending on soil mineral types. Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy is a technique to provide information on the oxidation state and coordination environment of metals in soil solid phases. Information gained from XAFS provides direct evidences for metal speciation and is different from the results from chemical extraction and thermodynamic equilibrium model, which allow prediction of possible metal species. Studies using XAFS spectroscopy (mainly by X-ray absorption near-edge structure: XANES) to determine Ag speciation have been conducted for biosolid samples for the scenario of wastewater treatment plants [12–15]. In the soil environment, contrarily, few studies are available on Ag speciation with regard to how AgNP and Ag salts undergo phasetransformations using the XAFS spectroscopy technique. Settimio et al. [16] found that AgNO3 added to various soils underwent transformation into metallic Ag (Ag0 ), AgCl, and Ag associated with Fe-oxyhydroxides and reduced S compounds. Shoults-Wilson et al. [17] found that over 74% of AgNP added to soils remained unchanged after 28 days. Few studies have reported the phase-transformation (e.g., sulfidation, complexation and adsorption) of AgNP in anoxic soil conditions [18]. The primary pathway for phase-transformation of AgNP is still poorly understood in reducing soils as a function of Eh variation. Sulfidation in reducing soils significantly decreases the solubility of Ag and other trace elements [8,19], potentially limiting their environmental impact. The objective of this study was to determine how AgNP and ionic Ag (i.e., AgNO3 ) added to the soil
undergo transformations in aerobic (oxic) and anaerobic (anoxic) conditions. The purpose of employing aerobic and anaerobic soil conditions was to allow comparisons of phase transformations of AgNP and ionic Ag in different redox gradients. We employed extended X-ray absorption fine structure (EXAFS) spectroscopy to determine Ag speciation in the soil and related such information to Ag solubility and soil’s redox potential. 2. Materials and methods 2.1. Soil characterization and experiment setup A typical Andisols (Typic Hapludands) representative of Japanese upland soils was collected from an agricultural experiment field of Tokyo University of Agriculture and Technology (FM Fuchu, Tokyo). The soil was passed through a 2-mm sieve and used for the following experiment. The soil consisted of 45% sand, 24% silt and 31% clay and was classified as clay-loam texture (USDA system). The soil contained relatively high levels of humus (10.9% C) and Fe minerals. Detailed soil properties and their analytical procedures are shown in Table S1, Supporting information (SI). Soil incubation studies were conducted using AgNP (99.5% purity; 50 nm nominal particle size with PVP coating; purchased from Sigma–Aldrich), and AgNO3 (99.5% purity, purchased from Wako Pure Chemical Industries). The soils were spiked with AgNP suspension (deionized water with 15 min sonication) or AgNO3 solution to obtain a final concentration of 1000 mg Ag kg−1 soil. Based on our preliminary studies, the water extractable Ag fraction, which is one of the official standard soil tests in Japan [20], was extremely low in the soil containing AgNP. Since a similar problem was also reported elsewhere [11], we set up relatively high Ag conditions in order to ensure detection of Ag in the water fraction. The Ag concentrations used in this study are one or two orders of magnitude higher than the background levels or mining areas of soil Ag [2,3], but were close to the concentrations in biosolids up to 856 mg kg−1 , [7]. Previous studies also added similar levels of AgNP to soils in order to examine the phase-transformation and detrimental biological effects of Ag in soils [17,21,22]. To compare the solubility and speciation of Ag in different redox gradients, the soils were incubated for up to 30 days at 25 ◦ C under anaerobic and aerobic conditions by adding different amounts of water. For the aerobic treatment, the soils (20 g) were incubated in polypropylene tubes and rewetted periodically with 85% water content. For the anaerobic treatment, the soil (400 g) was filled in a 1 L container with water content of 150%. A preliminary study confirmed that these water contents can reproduce oxic and anoxic conditions of this soil. The pH and Eh of the soil for the anaerobic treatment were measured periodically using electrodes (F-73, HORIBA; IM-32P, TOA-DKK). The soils were periodically collected, and Ag was extracted with deionized water. After 30 days of incubation, the soil samples were collected and stored in vacuum at −80 ◦ C until the XAFS analysis. 2.2. Chemical extraction and thermodynamic equilibrium model Soil samples were collected at 2 h and 1, 4, 7, 15, 20, and 30 days after starting the incubation to determine water-extractable Ag concentrations using a 1:10 soil-water ratio for a 6-h extraction on a reciprocal shaker at 25 ◦ C. Soil samples collected on day 30 were used for a two-step fractionation analysis for Ag at a 1:20 soilsolution ratio for 20 h at 25 ◦ C [23]. For the first extraction step, 1.0 g of soils was extracted with 0.05 M Ca(NO3 )2 solution. The filtrate (fraction 1) was considered to include exchangeable metals. Soil Ag was next extracted with a 0.42 M CH3 COOH solution. The filtrate at this fraction (fraction 2) includes acid-extractable metals. The supernatant was separated from the solid by centrifugation and
Please cite this article in press as: Y. Hashimoto, et al., Chemical speciation of silver (Ag) in soils under aerobic and anaerobic conditions: Ag nanoparticles vs. ionic Ag, J. Hazard. Mater. (2015), http://dx.doi.org/10.1016/j.jhazmat.2015.09.001
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0.04 2 0.02
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AgNP 2 R = 0.90*
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10
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AgNO3 2.5
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0.01 0.5 0.00 -200
0
200
400
0.0 600 -200 Eh (mV)
0
200
400
600
Fig. 1. Water extractable Ag in aerobic and anaerobic soils spiked with AgNP (a) and AgNO3 (b), and linear regression between water extractable Ag and soil Eh values in the anoxic soils spiked with AgNP (c) and AgNO3 (d). Error bars are standard deviation of mean value (n = 3). *Significant at p < 0.05 probability levels.
analyzed for Ag by atomic absorption spectroscopy (Hitachi HighTechnologies Corp., Japan). An Eh-pH diagram for the Ag–Cl–S system at 25 ◦ C was constructed to predict the predominant Ag species in the anaerobic soil. The diagram was constructed with the expected Ag phases in the soil systems, which include AgCl(s) , Ag0 (s) , and AgS(s) . Equilibrium reactions and constants for potential Ag phases used in the Eh-pH diagram are listed in Table S2. The plotted lines in the Eh-pH diagram assumed activity of 10−6 M for Ag+ , 10−2 M for Cl− and 10−3 M for HS− . To determine the time-course change in Ag species during the incubation period, the values of pH and Eh in the anaerobic soils treated with AgNP and AgNO3 were plotted on the diagram. 2.3. Silver K-edge XAFS spectroscopy The K-edge Ag XAFS spectroscopy was performed on the soil samples at the beamline BL01B1 at SPring-8 in Hyogo, Japan. We prepared the following Ag references: Ag2 CO3 , AgNO3 , AgCl, Ag2 SO4 , Ag2 S (>99.9% purity, purchased from Kanto Kagaku Ltd., Japan), Ag nanoparticle (Sigma–Aldrich), metallic Ag (Ag foil), and Ag sorbed on ferrihydrite, gibbsite, birnessite, humus and phyllosilicate (kaolinite). The detailed method and procedure for the preparation of standard references are described in SI. The purchased Ag compounds were passed through a 106-m sieve, diluted with boron nitride to the Ag concentration of 10% w/w, and pressed into pellets with 7-mm diameter and 1-mm thick. The XAFS data were collected in transmission mode for BN diluted samples or in fluorescent mode for synthesized references and soil samples with a 19-element Ge semiconductor detector at ambient temperature across the Ag K absorption edge at 25,516 eV using Si(311) monochromator crystals. The soils in a 3 × 3 cm vacuumed plastic bag were thawed at ambient temperature before being placed on
the beamline stage. Two or three scans were collected for the soil samples and then averaged. We confirmed that the chemical speciation of AgNP used for this study was virtually identical to that of the metallic Ag reference (see Fig. 4). Background correction, normalization and conversion to kspace (1–10.5 Å−1 ) were performed on the data for Ag K-edge EXAFS using programs, Athena and Artemis software suite [24]. To identify which Ag species were predominant in the soil, principal component analysis (PCA) and target transformation were performed on normalized k3 -weighted EXAFS spectra. These procedures were performed to compute a SPOIL value that is an indicator to which the reference consists of principal component for EXAFS spectra of soil samples. Based on their SPOIL value (Table S3), the reference spectra were grouped according to the scores of “excellent” (0–1.5), “good” (1.5–3.0), “fair” (3.0–4.5) and “poor” (>4.5) [e.g.,25]. Then, the relative proportions of Ag species in the multicomponent EXAFS spectra of the soil samples were obtained by the liner-combination fitting (LCF) procedure using selected Ag references previously determined by the SPOIL value. The quality of LCF result was quantified as a residual (R) value defined by the following equation:
R=
exp − model
exp
2
2
where is normalized absorption. The LCF procedure was initiated using the reference that yielded the best one component fit, which is defined as the one with the smallest R value. Adding a second component species in the fitting procedure was allowed when addition of a new species improved the fit (i.e., smaller R value). This LCF procedure, referring to Hashimoto et al. [26], can avoid
Please cite this article in press as: Y. Hashimoto, et al., Chemical speciation of silver (Ag) in soils under aerobic and anaerobic conditions: Ag nanoparticles vs. ionic Ag, J. Hazard. Mater. (2015), http://dx.doi.org/10.1016/j.jhazmat.2015.09.001
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(K P9
150
AgCl(s)
acid extractable
200
AgNP day 1
day 30
Ago(s)
day 30
Ag2S(s)
H2
AgNO 3
Fig. 2. Two-step sequential fractionation of Ag in the aerobic (aero) and anaerobic (anaero) soils added with AgNP and AgNO3 at day 30. The data with a magnified y-axis were shown in a box. exch: Ag extracted with with Ca(NO3 )2 solution; acid extractable: Ag extracted with CH3 COOH solution.
non-unique fitting solution since the use of a large number of references can inappropriately improve the R value. The second-best fit results, which were similar to those of the best fit, are listed in Table S4. The LCF was performed in k3 -space between 2 and 9 or 9.5 Å−1 , depending on spectrum quality. The EXAFS data were also analyzed by shell fits on k3 -weighted Fourier-transformed spectra using the Artemis code. The methods and procedures of EXAFS shell fit are from Mitsunobu et al. [27] and are given in SI. 3. Results and discussion 3.1. Ag solubility in anaerobic and aerobic soils Fig. 1 illustrates water extractable Ag from anaerobic and aerobic soils containing AgNP and AgNO3 during the 30-day incubation study. In the soil containing AgNP (hereafter AgNP soil), waterextractable Ag was smaller in the anaerobic conditions than the aerobic conditions throughout the incubation period (Fig. 1a). In the soil containing AgNO3 (hereafter AgNO3 soil), water extractable Ag in the anaerobic condition was one to two orders of magnitude less than that in the aerobic condition (Fig. 1b). Regardless of Ag types, Ag concentrations in the anaerobic soils decreased rapidly in the first 15 days and thereafter remained unchanged in the AgNP soil or decreased gradually in the AgNO3 soil. Such decrease in Ag solubility in the anaerobic soils can be illustrated as a strong linear relationship between Eh values and water-extractable Ag concentrations (Fig. 1c and d). The time-series Eh data during the incubation period are shown in Fig. S1. After 30 days of incubation, soil Ag was extracted sequentially using two different fluids, Ca(NO3 )2 (exchangeable), and CH3 COOH (acid extractable). Regardless of Ag types, the concentrations of exchangeable and acid-extractable Ag were notably smaller in the anaerobic soils, compared with the aerobic soils (Fig. 2). In the anaerobic condition, the labile fractions were considerably smaller in the AgNP soil than in the AgNO3 soil. In the AgNP soils, these Ag fractions were two orders of magnitude less in the anaerobic than in the aerobic soils. Lower Ag solubility in the anaerobic soil was consistent with the results from the time course changes for water-extractable Ag (Fig. 1a and b). 3.2. Eh-pH diagram for predicting Ag phases We evaluated the potential Ag transformation pathways as a consequence of the redox potential change using a constructed
S+ Fig. 3. Redox speciation of Ag in the soils added with AgNP (filled circles) and AgNO3 (open circles). Arrows indicate time sequence of Ag speciation in the soils. The diagram does not take into account changes in stability based on variable ion concentrations.
Eh-pH diagram (Fig. 3). According to the diagram, the formations of AgCl(s) , Ag2 S(s) and Ag0 (s) are thermodynamically favorable in a wide range of soil pH. Other potential reactions including the formation of AgSO4(s) , AgCO3(s) and Ag2 O(s) are not thermodynamically predicted since these products are highly labile in the normal soil pH range [28]. In Fig. 3, Eh and pH data are plotted for the AgNP and AgNO3 soils during the 30-day incubation under anaerobic conditions. For the AgNP soil, the Eh values decreased throughout the incubation period, and Ag2 S was subsequently predicted to be the controlling phase of Ag solubility at day 30. In contrast, the Eh values of AgNO3 soil hovered around 300 mV, and even after 30 days, the soil Eh did not reach favorable conditions for Ag2 S formation. Rick VandeVoort et al. [18] also reported the retardation of soil reductive reactions in AgNP and Ag-salt spiked soils. It is therefore plausible that a high concentration of dissolved Ag+ in the AgNO3 soil (Figs. 1 and 2) retarded microbial-mediated redox reactions, slowing soil reduction processes. 3.3. Ag K-edge EXAFS spectroscopy Silver K-edge EXAFS k3 (k)-spectra of selected references and soil samples are shown in Fig. 4. The selected reference Ag compounds possess differences in their EXAFS spectra, and therefore can be used to determine the Ag species in the soils. For the aerobic AgNP soil, the EXAFS spectrum was very similar with that of metallic Ag. In contrast, the EXAFS spectrum for the anaerobic AgNP soil was not similar to the metallic Ag reference, but was rather similar to Ag2 S in the second (∼4.0 Å−1 ), fourth (∼7.5 Å−1 ) and fifth (∼8.8 Å−1 ) oscillations. In contrast, the EXAFS spectrum for both aerobic and anaerobic AgNO3 soils was dissimilar to that of the AgNO3 reference. Such features in k3 -weighted EXAFS spectra were also confirmed on Fourier transformed EXAFS spectra (Fig. 5), which contained peaks reflecting the relative radial distance uncorrected for phase shift between the central Ag and neighboring atoms. The second peak at 2.5–3.0 Å (phase shift uncorrected) for the aerobic AgNP soil corresponded to the metallic Ag reference. For the anaerobic AgNP soil sample, the first peak at 1.5–2.4 Å, which reflects the Ag-S bond for the Ag2 S reference, was pronounced and so was the second peak (2.5–3.0 Å), corresponding to the metallic Ag reference. For the anaerobic AgNO3 soil sample, the second pronounced peak at 2.5–3.0 Å for the anaerobic AgNO3 soil sample was matched with that of the metallic Ag reference whereas the first peak was much less intense and did not exactly correspond to the Ag2 S reference. For more quantitative alternative to visual inspection and comparisons for differentiating Ag species in the soils, PCA and
Please cite this article in press as: Y. Hashimoto, et al., Chemical speciation of silver (Ag) in soils under aerobic and anaerobic conditions: Ag nanoparticles vs. ionic Ag, J. Hazard. Mater. (2015), http://dx.doi.org/10.1016/j.jhazmat.2015.09.001
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Ag2S
Ag phyllo
Ag humus
Metallic Ag (solid line) AgNP (dash)
AgNP (aerobic): 88% metallic Ag R = 0.048
AgNP (anaerobic): 17% metallic Ag, 83% Ag2S R = 0.165
AgNO3 (aerobic): 26% Ag-phyllo, 63% Ag-humus R = 0.327
AgNO3 (anaerobic): 41% metallic Ag, 58% Ag-phyllo R = 0.180
2
3
4
5
6
7
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10
o -1
k (A ) Fig. 4. Ag K-edge EXAFS spectra of selected reference compounds, and soil samples (dotted line) and their best component fit by LCF (solid line). Ag phyllo and Ag humus were the reference Ag sorbed with kaolinite and humus, respectively. Metallic Ag was an Ag foil reference. The spectrum for “AgNP (dash)” was the intact AgNP sample used for the soil incubation study. The results of the second-best LCF were listed in Table S4.
computation of SPOIL values were performed on soil EXAFS spectra. Principal component analysis and the target transformation showed that the reference spectra with high-scored SPOIL values (
