Chemosphere 134 (2015) 7–15

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TiO2 nanoparticle transport and retention through saturated limestone porous media under various ionic strength conditions Ali Esfandyari Bayat a,⇑, Radzuan Junin a, Mohd Nawi Derahman a, Adlina Abdul Samad b a b

Department of Petroleum Engineering, Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia Language Academy, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia

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

 Investigation of TiO2 nanoparticles

transport through saturated limestone porous media for the first time.  Evaluation of the effects of NaCl and MgCl2 on stability and mobility of TiO2-nanoparticles.  Using ionic strength range proportional to the subsurface aquatic environment and hydrocarbon reservoirs.  Detection of deposited nanoparticles along porous media via FESEM and EDX analyses.  Assessment of the experimental results by DLVO and filtration theories.

a r t i c l e

i n f o

Article history: Received 8 January 2015 Received in revised form 17 March 2015 Accepted 24 March 2015

Handling Editor: Keith Maruya Keywords: Titanium dioxide (TiO2) Nanoparticle Stability Transport Ionic strength Limestone porous media

a b s t r a c t The impact of ionic strength (from 0.003 to 500 mM) and salt type (NaCl vs MgCl2) on transport and retention of titanium dioxide (TiO2) nanoparticles (NPs) in saturated limestone porous media was systematically studied. Vertical columns were packed with limestone grains. The NPs were introduced as a pulse suspended in aqueous solutions and breakthrough curves in the column outlet were generated using an ultraviolent–visible spectrometry. Presence of NaCl and MgCl2 in the suspensions were found to have a significant influence on the electrokinetic properties of the NP aggregates and limestone grains. In NaCl and MgCl2 solutions, the deposition rates of the TiO2-NP aggregates were enhanced with the increase in ionic strength, a trend consistent with traditional Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. Furthermore, the NP aggregates retention increased in the porous media with ionic strength. The presence of salts also caused a considerable delay in the NPs breakthrough time. MgCl2 as compared to NaCl was found to be more effective agent for the deposition and retention of TiO2NPs. The experimental results followed closely the general trends predicted by the filtration and DLVO calculations. Overall, it was found that TiO2-NP mobility in the limestone porous media depends on ionic strength and salt type. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding author. HP.: +60 14 7152595. E-mail address: [email protected] (A. Esfandyari Bayat). http://dx.doi.org/10.1016/j.chemosphere.2015.03.052 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.

The annual production and use of engineered titanium dioxide (TiO2) nanoparticles (NPs) is increasing exponentially (Robichaud

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A. Esfandyari Bayat et al. / Chemosphere 134 (2015) 7–15

et al., 2009). TiO2-NPs are applied widely in a broad range of applications such as sunscreens, photocatalysts, photovoltaics, cosmetics, pigments, paints and coatings (Solovitch et al., 2010; Chen et al., 2011). The various applications have caused noticeable quantities of TiO2-NPs to be released into the environment daily. The toxicity of particles in nano size is much greater than the toxicity of bulk formulations with the same chemistry (Colvin, 2003; Nel et al., 2006). Recent reports showed that in municipal wastewater treatment plant effluents, 10–100 lg L1 of Ti is discharged into surface waters (Kiser et al., 2009). Release and accumulation of TiO2-NPs have adverse influences on the environment especially on aquatic organisms such as microbes, algae, fish, and invertebrates (Scown et al., 2010). Furthermore, TiO2-NPs have the potential to be transported in subsurface, reaching and contaminating groundwater resources (Chowdhury et al., 2011). However, issues such as how the released TiO2-NPs change the environmental properties and nature of their distribution into the subsurface alluvial zones are still challenges for the environmental engineering field (Handy and Shaw, 2007; Darlington et al., 2009). Furthermore, the application of TiO2-NPs for enhanced oil recovery purposes has been recently proven (Esfandyari Bayat et al., 2014a; Ehtesabi et al., 2014). The use of TiO2-NPs is a new task in petroleum engineering and it needs to be tested and validated before the NPs are applied in a full field scale. The NPs usage for the EOR purpose faces with a huge question that is how the NPs are transported in hydrocarbon reservoirs. The NPs have the potential to precipitate in reservoir pore-throats and clog them during transport along the reservoirs. As a result of pore-throats clogging, the permeability of porous media declines and results in the reduction of hydrocarbon reservoir productivity. Thus, the transport of TiO2-NPs through natural porous media for both environmental and petroleum engineering is a crucial issue. There are several worthwhile studies on transport and retention of TiO2-NPs through saturated sandy porous media. The results of these studies demonstrated that transport of TiO2-NPs through porous media are affected by TiO2-NP concentration (Choy et al., 2008), flow velocity (Lecoanet and Wiesner, 2004; Choy et al., 2008; Chowdhury et al., 2011), background solution chemistry (i.e., pH and ionic strength) (French et al., 2009; Joo et al., 2009; Ben-Moshe et al., 2010; Chen et al., 2011), presence of surfactants (Godinez and Darnault, 2011), and natural organic matter (NOM) (Keller et al., 2010; Thio et al., 2011; Aiken et al., 2011), and clay particles (Fang et al., 2009; Cai et al., 2014). Porous media physics and chemistry are parameters that also influence NPs transport (Bradford and Torkzaban, 2008). According to the literature, the quartz sands and soils are the only porous media which have been applied in TiO2-NPs transport tests. Besides sands and soils, the earth’s crust is composed of other sedimentary rocks such as carbonates (i.e. limestones and dolomites) which make up approximately 15% of the earth’s sedimentary crust (Freas et al., 2006). A considerable portion of groundwater sources (House et al., 1994) as well as 40% of the hydrocarbon resources are located in the carbonate rocks (Salehi et al., 2008). Due to the different resources found in these rocks, attempts should be made to experimentally investigate the transport of TiO2-NPs through carbonate rocks. Thus, this study was designed to experimentally and fundamentally determine the transport and retention of TiO2-NPs through limestone porous media under various ionic strength conditions. For this purpose, laboratory scale columns and different electrolyte solutions including NaCl and MgCl2 solutions were prepared. Different tests were also performed to characterize NPs and porous media before and after the transport tests. Furthermore, to evaluate and assess the experimental results, the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) and filtration theories were applied.

2. Materials and methods 2.1. TiO2-NP suspension preparation and characterization TiO2 anatase NP (10–30 nm, purity 99.5%, specific surface area of 50–100 m2 g1, and density of 3.8 g cm3), received from SkySpring Nanomaterials, Inc. (Houston, TX), were utilized in this study. To check the NP crystalline size, morphology, and composition, transmission electron microscopy (TEM, Model JEM-2100/ HR, JEOL, Acc.200.00 kV) and X-ray diffraction (XRD, model D5000, SIMENS) analyses were performed. TiO2-NP suspensions were prepared by adding 50 mg of TiO2 nanopowders to 1L of deionized water (DIW) or 1 L of the electrolyte solutions (NaCl or MgCl2) at pH 6.2 ± 0.1. Experiments were carried out at ionic strengths ranging from DIW to 500 mM for NaCl solutions and to 10 mM for MgCl2 solutions. The NPs suspensions were agitated for 1 h using an Orbital shaker at 220 rpm and then immediately ultrasonicated in an ultrasonic bath (Branson 3510R-DTH sonicator, 100 W, 42 kHz, Danbury, CT) for 1 h to obtain homogenous suspensions. These processes were carried out 15 min prior to each test. To measure the stability of TiO2-NPs against deposition in NaCl and MgCl2 solutions, sedimentation tests were performed (Godinez and Darnault, 2011). For this aim, the amounts of NPs deposition was recorded at 5 min intervals over a 180 min time span with the use of time-resolved optical absorbance. The absorbance of the samples was measured using a ultraviolet–visible (UV–VIS) spectrophotometer (Model 105, BUCK SCIENTIFIC., Inc.) over a wavelength range of 200–800 nm. Calibration was based on the maximum absorbance wavelength of 400 nm. The experiments were repeated twice and the presented data is the average of the obtained records. In addition, zeta potential (f-potential) values of the TiO2-NPs in the various electrolyte solutions were measured using a ZEECOM zeta potential analyzer instrument (Microtec Co., Ltd. Japan). The f-potential values were obtained by averaging three measurements for all the NPs suspensions. Besides that, the average radius of TiO2-NP aggregates in different solutions was measured using a DynaPro Titan Dynamic Light Scattering (DLS) probe from Wyatt Technology Corporation. DLS scattering analysis was performed twice (20 DLS reading per each run) for each NP suspension. To determine the radii of NP aggregates, the mean value of the measurements was applied. All the above mentioned characterization tests were conducted at 26 °C.

2.2. Column transport experiment A limestone sample from an outcrop in Ipoh, Malaysia was utilized as the porous medium in this study. The limestone sample was broken off into smaller pieces and then crushed to fine grains using a crusher machine (PULVERIZER Type from BICO, Inc.,). Limestone grains were sifted via 125 and 175 lm stainless steel sieves (USA Standard Testing Sieves, ATM Corp., New Berlin, WI) to achieve the average collector diameter (dc) of 150 lm. The limestone fractions were pre-treated using sequential water rinse, ultrasonication, and oven-drying (at 110 °C for 4 h) procedure to eliminate impurities. Scanning electron microscope (SEM, Philips XL40) and field emission scanning electron microscopy (FESEM, HITACHI, SU8020) images were prepared from the limestone grains to determine their exact morphologies before the transport tests and to detect of the TiO2-NPs deposited on the grains surfaces after the tests. The XRD and energy dispersive X-ray (EDX) analyses were also carried out to determine the limestone sample composition before and after the transport tests. Besides that, the surface charge of the limestone grains in the different electrolyte solutions

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g0 ¼ gD þ gI þ gG

1

1:675 0:125 g0 ¼ 2:4A3S N0:081 N0:715 N0:052 NA v dw þ 0:55AS N R R Pe 0:053 þ 0:22N0:24 N1:11 R G N v dw

Lmax ¼ 

  2dc C ln Co 3ð1  uÞLg0

2dc lnð0:01Þ 3ð1  uÞag0

kd ¼

3ð1  uÞ U ago 2dc u

where dc is the mean diameter of the collector or porous medium,

u is porosity of porous media, L is the length of the column, g0 is the single collector efficiency, and C/CO is the normalized NP concentration acquired from the experimental breakthrough curves. The average value of C/CO between PVs of 1.8 to 2.0 was applied in Eq. (1), where the initial (clean bed) phase of NP elution exists (Tufenkji and Elimelech, 2004b). Furthermore, three mechanisms including Brownian diffusion, interception, and gravitational sedimentation that affect colloids transport are considered to calculate g0 (Yao et al., 1971; Elimelech et al., 1998; Tufenkji and Elimelech, 2004b). To predict g0 for physicochemical filtration, a correlation was developed by Tufenkji and Elimelech (2004b) which is:

ð5Þ

where U is Darcy velocity. 2.3.2. DLVO theory DLVO theory is applied to qualitatively determine the interactions between NP and NP, and between NP and porous medium grain (collector) (Chen et al., 2011; Ben-Moshe et al., 2010;

10

a

+9.1

+8.3 +8.2

8

+7.2

+7.3

6

+5.8

4

2

ð1Þ

ð4Þ

In addition, particle deposition rate coefficient (kd) that defines the rate of physicochemical filtration is calculated as:

+5.3 +4.1

+3.7

TiO2-NPs in DIW -

+1.6

TiO2-NPs in TiO2-NPs in NaCl NaCl solutions solutions TiO 2-NPs in 2 solutions TiO2-NPs inMgCl MgCl2 solutions

0 0.001

0.01

0.1

1

10

100

1000

Ionic strength (mM) 330

NP aggregations diameter size (nm)

a¼

ð3Þ

where AS is the Happel correction factor, NR is the interception number, NPe is the Peclet number, Nvdw is the London-van der Waals attractive forces number, NA is the attraction number, and NG is the gravitational number. Besides that, He et al. (2009) proposed the following correlation to calculate maximum distance (Lmax) that colloids can travel in saturated porous media:

2.3. Colloid transport theories 2.3.1. Filtration theory Filtration theory is applied to quantitatively calculate NP deposition rate and attachment on the non-mobile surface collector (i.e. limestone grains) (Godinez and Darnault, 2011; Li et al., 2011; Jiang et al., 2012; Petosa et al., 2012; Esfandyari Bayat et al., 2014b). According to filtration theory, the attachment collision efficiency factor (a) which is a function of physicochemical parameters of the systems is calculated as:

ð2Þ

where gD, gI, and gG are transport due to diffusion, interception and gravity respectively which are calculated as:

ζ-potential (mV)

were measured using the method explained by Tufenkji and Elimelech (2004a). A stainless steel tube with an inner diameter (ID) of 0.9 cm and a length of 33 cm were utilized as the porous medium holder. To prevent grains migration during the tests, a filter cloth (pore size, 50 lm; thickness, 100 lm) was placed at both ends of the holder. The column was then wet packed uniformly with the cleaned grains using the method explained by Chen et al. (2011). Once the column was packed, it was saturated in a vertical upward direction using a syringe pump (Model PSK-01; NIKKISO Co., Ltd) at a constant Darcy velocity of 4.66  103 cm s1 (flow rate of 1 cm3 min1) with approximately 10 pore volumes (PVs) of DIW and or electrolyte solutions to ensure a homogenous saturation of the pack. It should be noted that the flow direction was selected to be vertically upward to equilibrate the influent solution and enhance packing homogeneity (Godinez and Darnault, 2011). Then, a pulse of TiO2-NP suspension (50 mg L1) with the same background electrolyte (Darcy velocity of 4.66  103 cm s1) was injected into the column for 2 PVs. Thereafter, a similar NPfree background electrolyte solution was injected into the column until no NP was observed in the outlet to check for mobilization of the trapped NPs in the porous medium. At the same time, the effluent samples were collected using a fraction collector (CF-2, Spectrum Chromatography, Houston, TX, USA) in 2 cm3 sample sizes. An average of 170 effluent samples were collected for each conducted experiment. The NPs concentration in the collected samples was measured using the UV–VIS spectrophotometer at a wavelength of 400 nm. Finally, the concentration of NPs dispersions entering the porous medium, CO and in the outlet, C were applied to generate breakthrough curves of C/CO as function of PVs passing through the porous media. All the column experiments were duplicated. Prior to the column experiments with the NP suspensions, nonreactive tracer tests were carried out by a solution of 50 mM KNO3 to determine water flow characteristics and column performance. The average porosity of the packed columns was measured to be 43%. Moreover, the permeability coefficient (k) of the columns according to the ASTM standard D 2434-68 (2006) method averaged 3.16  1012 m2. All column experiments were carried out at ambient temperature (26 °C).

b

TiO2-NPs TiO2-NPs in NaCl solutions

310

TiO2-NPs in MgCl TiO2-NPs MgCl2 solutions 2 solutions

310

TiO2-NPs in DIW TiO2-NPs

290

292 270

270

278

253

250

252 239

230

235

228 220

210 0.001

0.01

0.1

1

10

100

1000

Ionic strength (mM) Fig. 1. (a) Relationship between TiO2-NPs f-potential and ionic strength for different electrolyte solutions. (b) Relationship between diameter size of TiO2-NPs aggregations and ionic strength in different electrolyte solutions.

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Esfandyari Bayat et al., 2014). The total NP–NP and NP-collector interaction energies are calculated as the summation of electrostatic double layer (EDL) and van der Waals (VDW) forces. To calculate the NP–NP interaction energy, it was assumed that the TiO2-NP is spherical while for NP-collector interaction, it was assumed that the TiO2-NP is spherical and the collector is flat (Chen and Elimelech, 2007; Chowdhury et al., 2011; Esfandyari Bayat et al., 2014b). The VDW force for the NP–NP (EVDW-NN) and NP-collector (EVDW-NC) systems are calculated as:

" !# 2a2p 2a2p A131 Dð4ap þ DÞ ¼ þ ln þ 6 Dð4ap þ DÞ ð2ap þ DÞ2 ð2ap þ DÞ2

EVDW-NN

 1 A132 ap 14D 1þ k 6D

ð7Þ

where A131(6.0  1020 J) and A132(1.4  1020 J) are Hamaker constants for NP-water-NP and NP-water-collector, D is the separation distance, ap is the radius of the NP aggregate, and k is the characteristic wavelength of interaction that is assumed to be 100 nm. In addition, the EDL force for the NP–NP (EEDL-NN) and NP-collector (EEDL-NC) systems are calculated as:

EEDL-NN ¼

EEDL-NC

  2 64pe0 er kB T 2 a2p expðjDÞ 2 zeWp   tanh 2a þ D e 2 z2 4kB T

ð8Þ

(     2 1 þ expðjDÞ 2 2 ln ¼ pe0 er ap np þ nc 2 2 1  expðjDÞ ðnp þ nc Þ ) þ ln ð1  expð2jDÞÞ

valence of ion in bulk solution, and j is the Debye–Hückel reciprocal length and is calculated as:

j¼ ð6Þ

EVDW-NC ¼ 

where T is the absolute temperature of the system (298.15 °K), kB is the Boltzmann constant (1.3805  1023 J °K1), e0 is the permittivity of free space (8.85  1012 C V1 m1), er is the relative dielectric constant of water (78.5), np and nc are the electrical potentials of the NP aggregate and the collector, Wp is the reduced potential of NP   , e is the electron charge (1.602  1019 C), z is the Wp ¼ kzen BT

 0:5 2000e2 NA I e0 er kB T

ð10Þ

where NA is the Avogadro’s number (6.022  1023) and I is the solution ionic strength. 3. Results and discussion 3.1. TiO2-NP suspensions characterizations The TEM analysis from the TiO2 nanopowder revealed that the NPs are spherical and the geometric mean of TiO2-NP diameters is 15 nm (Fig. S1a). The XRD analysis result also demonstrates that the TiO2 sample has anatase composition and its structure is partially amorphous (semi-crystalline) (Fig. S1b). Furthermore, the results of measuring f-potential and hydraulic size of the TiO2NPs in the NaCl and MgCl2 solutions are demonstrated in Fig. 1. As demonstrated, the f-potential value of TiO2-NPs declined considerably as the ionic strength value was increased in both NaCl and MgCl2 solutions. Based on literature, the absolute f-potential

ð9Þ 0.08 0.07

1

a

0.06

0.8

C/CO

0.8

C/CO

0.05

0.6

0.6

0.04 0.4

0.03 TiO in DIW TiO2 in DIW 2-NPs

0.4

0.02

TiO2 in NaCl solution (I=5 mM) TiO in NaCl solution (I=5 mM) 2-NPs TiO2 in NaCl solution (I=10 mM) in NaCl solution (I=10 mM) TiO2-NPs TiO2-NPs in NaCl solution (I=100 mM) TiO2 in NaCl solution (I=100 mM)

0

TiO2-NPs in NaCl solution (I=500 mM) TiO2 in NaCl solution (I=500 mM)

0

0

30

60

90

0.2

0.01

TiO2-NPs in NaCl solution (I=50 mM) TiO2 in NaCl solution (I=50 mM)

0.2

120

150

0 0

5

10

15

20

0.08

30

35

b

0.06

0.8

1

in DIW (I=0.003 mM) in MgCl MgCl2 solution(I=0.5 (I=0.5 mM) mM) 2 solution in MgCl MgCl2 solution(I=1 (I=1 mM) mM) 2 solution MgCl2 solution in MgCl2 solution (I=5 (I=5 mM) mM) in MgCl2 MgCl2 solution solution (I=10 (I=10 mM) mM) Tracer test

b

0.07

1

25

Pore volume

180

Time (min)

0.8

C/CO

0.05

0.6

C/CO

1

in DIW (I=0.003) in NaCl solution (I=5 mM) in NaCl solution (I=10 mM) in NaCl solution (I=50 mM) in NaCl solution (I=100 mM) in NaCl solution (I=500 mM) Tracer test

a

0.6

0.04 0.4

0.03 0.4

0.02

in DIW TiO2-NPs TiO2 in DIW TiO in MgCl (I=0.5mM) mM) TiO2 in MgCl2 solution (I=0.5 2-NPs 2 solution

0.2

TiO2 in MgCl2 solution (I=1 in MgCl (I=1mM) mM) TiO 2-NPs 2 solution

0

in MgClsolution (I=5mM) mM) TiO TiO2 in MgCl2 (I=5 2-NPs 2 solution

0

in MgCl (I=10mM) mM) TiO TiO2 in MgCl2 solution (I=10 2-NPs 2 solution

0

30

60

90

0.2

0.01

120

150

180

Time (min) Fig. 2. TiO2-NP sedimentation tests results in the electrolyte solutions (a) NaCl solutions and (b) MgCl2 solutions (I: ionic strength).

0

5

10

15

20

25

30

35

0

Pore volume Fig. 3. TiO2-NPs breakthrough curves transported through limestone in the presence of different electrolyte solutions; (a) NaCl solutions, (b) MgCl2 solutions. Breakthrough curve for the tracer test is also demonstrated. Right y-axis for tracer (KNO3) C/CO and left y-axis is for NPs C/CO.

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value of different NPs (e.g. TiO2, C60, SiO2, ZnO, Al2O3) decreases with increasing ionic strength (Jiang et al., 2009; Solovitch et al., 2010; Yu et al., 2010; Chen et al., 2011; Metin et al., 2011; Jiang et al., 2012). Furthermore, it was observed that MgCl2 as compared to NaCl was more effective in the reduction of TiO2-NPs f-potential value. Generally, salts that include bivalent cations (i.e. Ca2+ and Mg2+) are more effective than salts that include monovalent cations (Na+ and K+) in reducing the NPs f-potential values and stabilities against deposition (Yu et al., 2010; Chen et al., 2011; Metin et al., 2011; Esfandyari Bayat et al., 2014). Besides that, the growth of hydraulic size of TiO2-NP aggregates with increase in the value of ionic strength was another phenomenon observed through the DLS tests (Fig. 1b). The results depict that TiO2-NPs aggregated in electrolyte solutions. Solovitch et al. (2010) also observed that the hydrodynamic diameter of TiO2-NPs increased with increasing NaCl concentration in the suspensions over time. We believe that the presence of salts (e.g. NaCl or MgCl2) in TiO2-NPs suspensions caused the Cl ions to surround the TiO2-NPs as the effect of the Coulomb force which resulted in the reduction of the NPs surface charges. Moreover, the ions formed salt bridges among TiO2-NPs causing the NPs to attach to each other which resulted in increasing the NPs aggregates sizes as shown in Fig. 1b. The schematic formation of salt bridges among TiO2-NPs is shown in Fig. S2.

11

Results of the sedimentation tests revealed the stability of TiO2NPs in the electrolyte solutions against deposition depends on ionic strength and salt type. Suspensions with lower amounts of NPs precipitation over time are more stable. As shown in Fig. 2, increase in the ionic strength value caused the amount of NP concentration (C/CO) to decline further in all the electrolyte solutions over a period of 180 min. Furthermore, under the same ionic strength condition (i.e. 10 mM) and time, the concentration of TiO2-NPs in MgCl2 solution (C/CO: 0.54) was lower than in NaCl solution (C/CO: 0.61). The highest stability of TiO2-NPs was obtained in DIW (Fig. 2). Investigation of the interaction between solid particles is one of the parameters which can explain qualitatively the NPs stabilities in a dispersion medium (Esfandyari Bayat et al., 2014b). In this regard, DLVO theory is applicable to qualitatively explain this interaction by considering the EDL and VDW forces (Hotze et al., 2010). The EDL is a force which drives particles apart while the VDW force drives particles toward each other. The summation of these forces clarifies whether the net interaction between two particles is attractive or repulsive. High positive values of the interaction energy imply that the EDL force is greater in magnitude and the dominant force or vice versa (Bian et al., 2011). As shown in Fig. S3, the interaction energy height significantly drops as ionic strength increases causing

Fig. 4. (a) SEM image from limestone grains before transportation experiments and (b) XRD and EDX analyses results from the limestone sample.

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VDW to become the dominant force. The maximum interaction energy height is for TiO2-NPs in DIW showing that the NPs are more stable in DIW as compared to the other electrolyte solutions. Consequently, DLVO theory supports the experimental results. 3.2. TiO2-NP transport in the presence of NaCl solutions The first transport test was carried out by injecting 2 PVs of the dispersed TiO2-NPs in DIW into the column. The TiO2-NPs appeared in the effluent after 1.2 PVs injection. As shown in Fig. 3a, the amount of TiO2-NPs in the outlet increased until 13th PVs. Then, this amount plateaued for the next 13 PVs. Finally, it declined to zero at 30th PVs and after that there was no NP in the effluent. The result reveals that 72.2% of the entered TiO2NPs was recovered from the column. The relatively high TiO2NPs recovery is attributed to the electric surface charges of TiO2 (+9.1) and limestone (+33.1 mV) which are both positive. Dunphy Guzman et al. (2006) declared that surface charge of TiO2-NPs is a primary factor in transport through soils. Choy et al. (2008) and Godinez and Darnault (2011) observed that TiO2-NPs adsorbed highly on the sand grains surfaces due to their opposite surface charge signs. Moreover, Chen et al. (2011) reported a successful

transport of TiO2-NPs through sand grains in their study. The authors reported that TiO2-NPs and sand grains had similar surface charge signs (both were reported negative). Based on DLVO theory, there is fairly high electrostatic barrier energy between TiO2-NPs dispersed in DIW and limestone grains indicating a repulsion force between NPs and limestone grains (Fig. S4a). As a result of this repulsion force, the affinity of TiO2NPs to adsorb on the limestone grains surfaces was low. However, 27.8% of the entered TiO2-NPs remained in the porous medium. The reason can be attributed to the roughness of the limestone grains surfaces. The SEM images from limestone grains demonstrated that the grains surfaces are full of irregular dents and bumps (Fig. 4a). Bradford and Torkzaban (2008) and Esfandyari Bayat et al. (2014b) declared that grains surface roughness is a parameter which affects colloidal transport. Thus, there is a high possibility that the NPs become trapped inside the dents and bumps. TiO2-NPs recovery was reduced to 57.7% when NaCl solution 5 mM was utilized and the NPs appeared in the outlet after 1.2 PVs of suspension injection. For NaCl solution 10 mM, the NPs recovery was reduced to 48.7% and they appeared in the outlet after 1.4 PVs. Increasing NaCl concentration of up to 50 mM

Fig. 5. (a) FESEM image from deposition of TiO2-NP aggregations on the limestone grains’ surface after transport experiment in the presence of NaCl 500 mM (this image was taken from a grain that located in the middle of column after the transport experiment), (b) EDX result.

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and increase NPs breakthrough time. It has been proven that stability has a direct relationship with NP mobility, where stable NPs suspensions can be easily transported over long distance through porous media with the least NP retention (Yu et al., 2010; Esfandyari Bayat et al., 2014b). To prove aggregation and deposition of the TiO2-NPs in the porous media, FESEM and EDX analyses were carried out from the columns after the transport tests. The first finding from these analyses was observation of the TiO2-NPs on the limestone grains surfaces (Fig. 5). As shown in Fig. 4b, the limestone sample is composed of pure calcite (CaCO3) which proves that the source of observed Ti in the EDX analyses (Fig. 5b) was due to the introduction of TiO2-NPs in the columns. The next finding was observation of big TiO2-NPs clusters (up to micron size) on limestone grains surfaces which reveals that TiO2-NPs in the presence of NaCl solutions made bigger clusters as shown in Figs. 5 and 6. It can be concluded that NPs straining is another reason for reduction of NPs recoveries in the presence of NaCl solutions. The transport tests results were also checked by filtration and DLVO theories. According to filtration theory, increasing NaCl concentration leads to increasing attachment efficiency and consequently enhances deposition rate coefficient (Table 1). The results show that the maximum transport distance is reduced due to the increasing ionic strength. According to DLVO theory, increasing NaCl concentration causes the interaction energy height to reduce significantly (Fig. S4a and Table 1). This evidence proves that the repulsion (EDL) force between NPs and limestone grains become weaker as a result of increasing ionic strength. Therefore, both DLVO and filtration theories support the experimental results. 3.3. TiO2-NP transport in the presence of MgCl2 solutions

Fig. 6. FESEM image from deposition of TiO2-NP aggregations on the limestone grains surfaces after the transport tests in the presence of (a) NaCl 50 mM; (b) NaCl 100 mM (images were taken from grains located in the middle of column after the transport experiments).

resulted in a noticeable reduction in the NP recovery by 29.1%. Moreover, a huge delay occurred in NPs breakthrough where the first NPs were observed after 1.6 PVs. For 100 and 500 mM NaCl solutions, the NPs recoveries achieved 23.3% and 17.6%, respectively. The NPs breakthrough for 100 and 500 mM NaCl solutions occurred after 1.8 PVs (Fig. 3a). As an overall trend, it can be concluded that increasing NaCl concentration in porous media led to reduction of NPs recovery

The amounts of NPs recoveries in the presence of MgCl2 solutions with ionic strength of 0.5, 1, 5, and 10 mM achieved 64.6%, 50.4%, 33.0%, and 26.1% respectively. The TiO2-NP breakthrough curves in the presence of different MgCl2 concentrations are shown in Fig. 3b. The overall results from TiO2-NPs transport in the presence of different MgCl2 solutions depicted the same trend as NaCl where increasing ionic strength resulted in reduction of NPs recoveries (Fig. S5). However, there are two main differences between MgCl2 and NaCl solutions. First, the same ionic strength for both NaCl and MgCl2 solutions showed that the influence of MgCl2 on NPs retention through porous media was higher than NaCl. Second, the NPs breakthrough time in presence of MgCl2 had more delay time as compared to NaCl. The results from the transport experiments were also checked by filtration and DLVO theories. As shown in Table 1, the

Table 1 Electrokinetic properties of TiO2-NPs and limestone grains, energy barrier heights as calculated by DLVO theory for NP-limestone and experimental parameters from NPs transportation tests calculated by filtration theory. Dispersion system

Ionicstrength (mM)

DIW NaCl

MgCl2

DLVO Theory

Filtration Theory

RNP, (%)

TiO2-NPs fpotential (mV)

Limestonegrains f-potential (mV)

Maximum value of NPLimestone interaction energy height

Single-collector contact efficiency, g0 = gD + gI + gG

Attachment efficiency, a

Maximum transport distance, Lmax (cm)

Deposition rate coefficient, kd (h1)

0.003

+9.1 ± 0.3

+33.1 ± 0.5

+18.4

1.26E02

0.141

44.6

4.14

72.2

5 10 50 100 500

+8.3 ± 0.3 +7.2 ± 0.3 +5.3 ± 0.3 +3.7 ± 0.3 +1.6 ± 0.3

+32.2 ± 0.5 +31.0 ± 0.5 +27.6 ± 0.5 +25.3 ± 0.5 +23.1 ± 0.5

+16.8 +15.1 +10.6 +6.6 +2.0

1.23E02 1.12E02 1.06E02 1.04E02 9.83E03

0.179 0.211 0.239 0.288 0.306

37.3 33.0 31.5 27.5 26.9

4.72 5.72 5.86 6.39 6.7

57.7 48.7 29.1 23.3 17.6

0.5 1 5 10

+8.2 ± 0.3 +7.3 ± 0.3 +5.8 ± 0.3 +4.1 ± 0.3

+32.5 ± 0.5 +31.9 ± 0.5 +30.3 ± 0.5 +29.0 ± 0.5

+17.7 +15.8 +14.6 +9.7

1.26E02 1.20E02 1.14E02 1.12E02

0.173 0.180 0.210 0.276

37.7 37.3 33.0 26.7

4.66 4.83 5.59 6.58

64.6 50.4 33.0 26.1

14

A. Esfandyari Bayat et al. / Chemosphere 134 (2015) 7–15

attachment efficiency and deposition rate coefficient increase with increasing MgCl2 concentration. Furthermore, in situations where there is the same ionic strength value for NaCl and MgCl2 solutions, the calculated values of attachment efficiency and deposition rate coefficient are higher for MgCl2 which confirm NPs possess higher retention rate in the presence of MgCl2. Moreover, according to DLVO, increasing MgCl2 concentration would to lead to the interaction energy height being reduced significantly (Fig. S4b). Furthermore, under the same ionic strength condition, the maximum value of interaction energy height is also lower in the presence of MgCl2 (Table 1). Thus, both filtration and DLVO theories confirm that the influence of MgCl2 solutions on TiO2-NP retention in the porous media is higher than in NaCl solutions. Since the typical concentrations of monovalent cations (e.g., Na+, K+) and divalent cations (e.g., Ca2+, Mg2+) in the aquatic subsurface are around 1–10 and 0.1–2 mM, respectively (Atekwana and Richardson, 2004), it can be concluded that a considerable portion of TiO2-NPs can be transferred to the drinking groundwater resources located in the limestone porous media. However, in limestone oil reservoirs with higher salts concentration (50–500 mM), a noticeable portion of the NPs would precipitate within the pores resulting in the reduction of the hydrocarbon reservoirs productivity. 4. Conclusion This study was designed for the environmental and petroleum engineering fields concerning the mechanisms governing the transport and retention of TiO2-NPs through saturated limestone porous media in the presence two common salts in the aquatic environment namely NaCl and MgCl2 were investigated. It was found that the surface roughness of limestone grains is one of parameters which affected TiO2-NP transport. Besides that, ionic strength value and salt type also highly affected the TiO2-NP transport in the porous media. Increase in ionic strength value enhanced the NP deposition and retention in the porous media. Presence of salts in the porous media also caused considerable delay in the NPs breakthrough times. Furthermore, MgCl2 as compared to NaCl was found to be more effective for deposition and retention. Results of the transport test results were supported by the filtration and DLVO calculations. Generally, TiO2-NPs in the presence of salts would start to flocculate, aggregate, and eventually deposit on the limestone grains surfaces, resulting in the reduction of NP mobility. Acknowledgments The authors gratefully acknowledge Universiti Teknologi Malaysia which provided materials and equipment. The authors also thank Mrs. Norhazalina, Mrs. Nurfahana, and Mr. Adnan for providing FESEM and XRD results. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2015.03.052. References Aiken, G.R., Hsu-Kim, H., Ryan, J.N., 2011. Influence of dissolved organic matter on the environmental fate of metals, nanoparticles, and colloids. Environ. Sci. Technol. 45, 3196–3201. ASTM Standard D 2434-68, 2006. Standard Test Method for Permeability of Granular Soils (Constant Head). ASTM International, West Conshohocken, PA.

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