Journal of Contaminant Hydrology 170 (2014) 76–85

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Influence of mineral colloids and humic substances on uranium(VI) transport in water-saturated geologic porous media Qing Wang a, Tao Cheng b,⁎, Yang Wu b a b

Environmental Science Program, Faculty of Science, Memorial University, St. John's, Newfoundland and Labrador A1B 3X7, Canada Department of Earth Sciences, Memorial University, St. John's, Newfoundland and Labrador A1B 3X5, Canada

a r t i c l e

i n f o

Article history: Received 24 May 2014 Received in revised form 19 September 2014 Accepted 6 October 2014 Available online 12 October 2014 Keywords: Uranium transport Mineral colloids Humic acid Adsorption Column experiments

a b s t r a c t Mineral colloids and humic substances often co-exist in subsurface environment and substantially influence uranium (U) transport. However, the combined effects of mineral colloids and humic substances on U transport are not clear. This study is aimed at quantifying U transport and elucidating geochemical processes that control U transport when both mineral colloids and humic acid (HA) are present. U-spiked solutions/suspensions were injected into water-saturated sand columns, and U and colloid concentrations in column effluent were monitored. We found that HA promoted U transport via (i) formation of aqueous U–HA complexes, and (ii) competition against aqueous U for surface sites on transport media. Illite colloids had no influence on U transport at pH 5 in the absence of HA due to low mobility of the colloids. At pH 9, U desorbed from mobile illite and the presence of illite decreased U transport. At pH 5, high U transport occurred when both illite colloids and HA were present, which was attributed to enhanced U adsorption to illite colloids via formation of ternary illite–HA–U surface complexes, and enhanced illite transport due to HA attachment to illite and transport media. This study demonstrates that the combined effects of mineral colloids and HA on contaminant transport is different from simple addition of the individual effect. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Uranium (U) is a naturally occurring radionuclide that can be found in rock, water, and soil. U in groundwater is derived from natural interactions between U-bearing minerals and groundwater as well as processing of nuclear fuels and materials (Dresel et al., 2011; Jerden and Sinha, 2006). Once released to groundwater, U can migrate with groundwater flow and cause contamination in drinking water aquifers (Babu et al., 2008; Kronfeld et al., 2004). To assess the extent of U contamination in groundwater and the risk to human health from U exposure by consuming contaminated groundwater, it is important to understand how U transports in subsurface environment. ⁎ Corresponding author. Tel.: +1 709 864 8924; fax: +1 709 864 7437. E-mail address: [email protected] (T. Cheng).

http://dx.doi.org/10.1016/j.jconhyd.2014.10.007 0169-7722/© 2014 Elsevier B.V. All rights reserved.

Adsorption of dissolved uranium to aquifer materials such as iron oxides can significantly reduce U transport (Barnett et al., 2000; Cheng et al., 2007; Gabriel et al., 1998). In contrast, the ubiquitous presence of submicron-sized particles such as mineral colloids and natural organic matters (NOM) (e.g., humic substances) in groundwater can enhance U transport (Bekhit and Hassan, 2007; Crancon et al., 2010; Kaplan et al., 1994; Utsunomiya et al., 2009). Mineral colloids and NOM have large surface areas that contain highly reactive functional groups, therefore these small particles have high affinity to many dissolved contaminants (Aiken et al., 2011; Baek and Pitt, 1996; Grolimund et al., 1996; Kalbitz and Wennrich, 1998; Kretzschmar and Schafer, 2005; Schmitt et al., 2003). The ability of colloids and NOM to promote contaminant transport depends on the mobility of these small particles and their ability to retain contaminants. NOM and colloidal particles

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were found to strongly adsorb U (Chau et al., 2011; Crancon et al., 2010; Lenhart et al., 2000; Lienert et al., 1994) and promote U transport in groundwater (Bekhit and Hassan, 2007; Kaplan et al., 1994; Kim et al., 1994). Although extensive studies on colloid-facilitated U transport (e.g., Porcelli et al., 1997; Andersson et al., 2001) and NOM-facilitated U transport (Artinger et al., 2002; Mibus et al., 2007; Sachs et al., 2006; Yang et al., 2012) have been carried out, fewer studies investigated the combined effects of NOM and mineral colloids on U transport. In natural environment, NOM, mineral colloids and contaminants usually co-exist and interact with one another via a suite of geochemical processes (Yang et al., 2013). For instance, NOM can (i) form aqueous complexes with contaminants (Datta et al., 2001; Kostic et al., 2011; Metreveli et al., 2010; Schmitt et al., 2003; Zhao et al., 2011), and (ii) compete with contaminants for surface sites on aquifer materials (Antelo et al., 2007; Giasuddin et al., 2007; Wang et al., 2013). At the same time, by attaching to mineral colloids, NOM can (i) alter (either enhance or reduce) mineral colloids' ability to adsorb contaminants (Yang et al., 2013), and (ii) enhance the mobility of mineral colloids (Morales et al., 2011; Yoshida and Suzuki, 2008). The overall effects of NOM and mineral colloids on U transport due to these diverse and concomitant processes are difficult to predict, because these processes are additive under certain conditions, but become competitive under other conditions depending on water chemistry and chemical compositions of NOM, mineral colloids, and contaminants. Experimental studies of the combined effects of mineral colloids and NOM on U transport will provide valuable data for evaluating U transport in real subsurface environment. The objectives of this study were to determine the combined effects of mineral colloids and NOM on U transport, and elucidate the important geochemical processes that control U transport in the presence of mineral colloids and NOM. Illite was used as a representative mineral colloid due to its abundance in natural subsurface environment (Gradusov, 1974; Ransom and Helgeson, 1993). Humic acid (HA), a major component of NOM, was used as a proxy for NOM in our study. U and illite transport was investigated by injecting U-spiked solutions/suspensions into water-saturated columns packed with a mixture of natural sediment and quartz sand, and analyzing U and illite concentration in column effluent. Eight column experiments, distinguished based on the composition of the U-spiked solution/suspension (U only, U + illite, U + HA, and U + HA + illite) and solution pH (pH 5 and pH 9), were performed to determine the influence of illite colloids and HA on U transport.

2. Materials and methods 2.1. Preparation of influent solutions/suspensions Nitric acid (HNO3), sodium hydroxide (NaOH), sodium nitrate (NaNO3), sodium bromide (NaBr) (all certified ACS grade) and 1000 mg/L U stock solution (UO2(NO3)2∙6H2O, catalog # 82026-084) were purchased from VWR. Humic acid (HA) purchased from Alfa Aesar (catalog # 41747-14) was used without pre-treatment. Illite (IMt-2) was purchased from Clay Mineral Society and its major chemical composition (mass %) was: SiO2: 49.3, Al2O3: 24.25, TiO2: 0.55, Fe2O3: 7.32, FeO: 0.55,

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MnO: 0.03, MgO: 2.56, and CaO: 0.43. All the solutions and suspensions were prepared using nano-pure water. Illite colloid stock suspension was prepared following the method of Saiers and Hornberger (1999). Four grams (4.0 g) of illite powder was suspended in 1000 mL nano-pure water in a high-density polyethylene (HDPE) bottle. After vigorous shaking, the suspension was dispersed for 30 min in an ultrasonic bath. Then the suspension was transferred to an Erlenmeyer flask and let stand for 24 h before the supernatant was carefully transferred to another HDPE bottle. The supernatant was used as our illite stock suspension. Illite concentration in the stock suspension was measured gravimetrically by filtering 100 mL of the suspension through a 100 nm polyethersulfone membrane filter (Pall Life Sciences). The filter was oven dried at 60 °C before and after the filtration, and the difference between the weight of the filter was considered as the mass of the colloids. Four types of U-spiked influent solutions/suspensions (U only, U + illite, U + HA, and U + HA + illite) were prepared at both pH 5 and pH 9. To prepare an influent solution/ suspension, 1.700 g NaNO3 and was added to a 2 L volumetric flask and dissolved in a small amount of nano-pure water. For influents that required HA (i.e., U + HA, and U + illite + HA), 0.040 g humic acid was also added and dissolved in the flask. Following the dissolution of NaNO3 and humic acid, illite colloid stock suspension was added to the flask for influents that required illite (i.e., U + illite, U + illite + HA) and well mixed before adding U stock solution. The mixed suspension was filled to 2 L with nano-pure water, well mixed, transferred to a HDPE bottle, and adjusted to desired pH. Final concentrations in the U-spiked influent solution/suspension were: U = 1 mg/L, HA = 0 (for U only, U + illite treatment) or 20 mg/L (for U + HA, U + illite + HA treatments), and illite = 0 (for U only, U + HA treatments) or 100 mg/L (for U + illite, U + illite + HA treatments). NaNO3 concentration was 0.01 M in all influent solutions/suspensions. Freshly prepared U-spiked solutions/suspensions were let stand for overnight before used in subsequent experiments. Two types of U-free background electrolyte solution was prepared: HA-free background solution was prepared by dissolving 1.700 g NaNO3 in 2 L nano-pure water, and HAspiked background solution by dissolving 1.700 g NaNO3 and 0.040 g HA in 2 L nano-pure water. The background solutions had a NaNO3 concentration of 0.01 M and HA concentration of 0 or 20 mg/L, with pH adjusted to either 5 or 9 using 1 M HNO3 and 1 M NaOH.

2.2. Stability of U-spiked suspensions, and zeta potential and hydrodynamic diameter of illite and HA in the suspensions Stability of (U + illite), (U + HA), and (U + illite + HA) suspensions was tested by monitoring optical absorbance of the suspensions using a spectrophotometer at a wavelength of 350 nm. During the stability tests, illite suspensions (i.e., U +illite, U + illite + HA) were mixed using a magnetic stirrer to keep the illite colloids suspended. At the beginning (t = 0) and end (t = 240 min) of the stability test, hydrodynamic diameter and zeta potential of illite and HA in the suspensions was determined using a Zetasizer Nano-ZS (Malvern).

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2.3. HA and U adsorption to illite, and U adsorption to HA in influent suspensions To quantify HA adsorption to illite colloids, (U + illite), (U + HA), and (U + illite + HA) suspensions were centrifuged at 4950 rpm for 30 min. Light absorbance at wavelength of 350 nm of (i) well mixed suspensions before centrifugation, and (ii) supernatant after centrifugation was measured using a spectrophotometer. To quantify U adsorption to illite in (U + illite) suspensions, the suspensions were centrifuged at 4950 rpm for 30 min. Supernatant after centrifugation was collected and analyzed for U concentration by ICP-MS. U adsorbed to illite was calculated as the difference between U concentration in the suspension and the supernatant. To estimate dissolved U (i.e., U not associated with illite or HA) concentrations in the suspensions, (U + HA) suspensions and supernatant of (U + illite + HA) suspensions after centrifugation was filtered through 5 kDa ultrafiltration membrane, and the filtrate were analyzed for U concentration by ICP-MS. Light absorbance of the filtrate at wavelength of 350 nm was measured using a spectrophotometer to estimate HA concentration. 2.4. Transport media Columns were packed with a mixture of natural sediment and pure quartz sand. The natural sediment was collected from Avondale, a town located in Avalon Peninsula in Eastern Newfoundland, Canada. The sediment was air dried, gently ground to smaller particles, and passed a serial of sieves. The sieved fraction with a grain size range between 0.25 and 0.60 mm was collected. Mineral composition of the sieved sediment was determined using X-ray diffraction (XRD), and concentrations of Fe oxides and Al hydroxides in the sieved sediment were determined using a sequential extraction method (Tessier et al., 1979). The quartz sand (VWR, catalog # 71008-394) is mineralogically pure and contains N 99.7% of SiO2, b0.02% of Fe2O3, and b0.05% of Al2O3. The quartz sand was washed with nano-pure water, air dried, and sieved to grain sizes between 0.25 and 0.60 mm before used in our experiments. The above pretreated natural sediment and quartz sand were well mixed at a mass ratio of 4:1 (quartz sand to sediment) and used to pack the columns. 2.5. Column experiments Eight U transport column experiments, distinguished by the pH and different illite and HA composition of the U-spiked influent (described in Section 2.1), were conducted. Each of the eight experiments were conducted in duplicate. A Kontes ChromaFlex™ chromatography glass column (2.5 cm inner diameter, 15 cm length) was used in the column experiments. Sand columns were packed by slowly pouring small amount of well-mixed dry column materials (described in Section 2.4) into the glass column pre-filled with background solution at ~1 cm increments. After each increment of sand, the side of the column were tapped to maintain uniformity of packing and to remove air bubbles. Total dry mass of the column materials and volume of background solution used to pack each column (i.e., pore volume of the column) was 127 g and 23 cm3

respectively (Table S1, supporting information), corresponding to a bulk density of 1.73 g/cm3 and a porosity of 0.31. Three phases were run for each column experiment: (1) pre-condition phase with background solution injected into the column for ~18 h until the pH of effluent was stable, followed by (2) U injection phase with 10 pore volumes (Vp) of U-spiked influent solution/suspension injected, and (3) elution phase with 15 pore volume of background solution injected. A peristaltic pump (Masterflex, Cole-Parmer Instruments) was used for injection with upward flow at a specific discharge rate of 0.66 cm/min during all three phases. During U injection phase, influents containing illite (i.e., U + illite, U + illite + HA) were mixed using a magnetic stirrer to keep the illite colloids suspended. Effluent from the top of the column was collected with a fraction collector (CF-2, Spectrum Chromatography) for analyses of U and illite concentrations. For each column experiment, pH and HA concentration of the background solution used for column packing, pre-condition phase, and elution phase matched the pH (5 or 9) and HA concentration (0 or 20 mg/L) of the U-spiked solution/suspension used for that experiment. U and illite concentrations in water samples was determined by ICP-MS. One gram (1.000 g) of well mixed influent or effluent samples was transferred to a Teflon screw cap jar, mixed with 1 mL of 16 mol/L HNO3 and kept on a hot plate (70 °C) for 24 h to dissolve U and particles. The solution was then transferred to a clean tube and diluted to 20.00 g before sent to ICP-MS analysis. Our tests showed that this method was able to recover 104 ± 4% of U even in the presence of 100 mg/L illite colloid. ICP-MS measured Fe concentration was used to quantify illite colloid concentration with a calibration curve made with a series of suspensions containing known concentration of illite colloids (Grolimund et al., 1996) (Fig. S1, supporting information). Conservative tracer (Br−) experiments were performed in duplicates to check hydraulic characteristics of the columns and comparability of column packing. The same materials and protocols used for packing columns for U transport experiments were used for packing columns for bromide experiments, except that nano-pure water instead of background solution was used for packing. Bromide transport experiments were performed in the same manner as the U transport experiments except that nano-pure water instead of background solution was used during pre-condition and elution phases. During injection phase, 6.25 μmol/L Br− solution was introduced into the column as a step input for 10 pore volumes. Effluent bromide concentration was determined using a Dionex ion chromatography.

3. Results and discussion 3.1. Mineralogical and chemical properties of the natural sediment The natural sediment used to pack the columns mainly consists of albite (50.5% mass), quartz (34.9% mass) and muscovite (14.7% mass) according to our X-ray diffraction (XRD) results. Fe and Al (hydro)oxides were not detected by XRD, indicating their concentration was below the detection limit of XRD analysis (~5% of total mass). Sequential extraction and elemental analysis showed the concentration of Fe and Al

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(hydro)oxides are 3592 mg (as Fe)/kg and 1057 mg (as Al)/kg respectively. 3.2. Zeta potential, hydrodynamic diameter, and stability of illite, HA, and mixture of illite + HA suspensions At pH 5 and 9, illite colloids, HA, and mixture of illite and HA were all negatively charged, and their zeta potential was more nagative at pH 9 (Table 1). The above results were consistent with the point of zero charge of illite (pHpzc = 2.5) (Kosmulski, 2011), and dissociation of acidic groups of HA (pK1 = 3.7 ± 0.1, pK2 = 6.6 ± 0.1) (Kretzschmar et al., 1999; Tombacz et al., 2000). As pH increases, de-protonation of surface functional groups increases negative charges on illite and HA and resulted in more negative zeta potential. The measured zeta potentials were in the range of −28.3 to −51.9 mV, indicating electrostatic repulsive forces between particles were high and the suspensions were not susceptible to aggregation. pH has marked effects on size of illite colloids. Hydrodynamic diameter of illite colloids was 1194 and 487 nm respectively at pH 5 and 9 (Table 2). Size of HA was less susceptible to pH changes compared to illite. When pH increased from 5 to 9, hydrodynamic diameter of HA decreased from 296 to 211 nm. The above results showed that aggregates of illite and HA were easier to be broken down to smaller particles at higher pH. Particle size of illite + HA mixture was the least sensitve to pH changes among the three suspensions, and the measured average hydrodynamic diameter of the illite + HA mixture was lower than that of illite but higher than HA (354 nm at pH 5 and 339 nm at pH 9). Measurements of optical absorbance of the suspensions confirmed that illite, HA, and mixture of illite + HA suspensions were stable up to 240 min (Fig. S2, supporting information). Hydrodanamic diameter and zeta-potential of illite, HA, and mixture of illite + HA were essentially unchanged after 240 min (Tables 1 and 2). These results demonstrated that all the suspensions were stable during injection phase (230 min) of the column experiments under our experimental conditions. 3.3. HA adsorption to illite and U adsorption to illite and HA At pH 9, light absorbance of (U + illite) suspension after centrifugation reduced from 0.290 to 0.002 (Table 3), indicating N99% illite were removed. Light absorbance of (U + HA) suspension reduced from 0.261 to 0.232, indicating 90% of HA

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were still suspended. The much higher illite removal compared to HA is due to higher particle density of illite. For (U + illite + HA) suspension, light absorbance after centrifugation was 0.232, the same as that of (U + HA) suspension, indicating equal amount of HA was removed from (U + HA) and (U + illite + HA) suspensions. The above results suggest that HA did not adsorb to illite at pH 9, since those HA adsorbed to illite would be removed along with illite during centrifugation. At pH 9, lack of HA adsorotion to illite can be explained by high energy barrier due to strong electrostatic replusion forces between illite (zeta potential = −47.0 mV) and HA (zeta potential = −46.6 mV). At pH 5, changes in light absorbance of the (U + illite) suspension after centrifugation (Table 3) showed that N97% of illite was removed. Light absorbance of (U + HA) suspension reduced only 10% from 0.229 to 0.207. Light absorbance of (U + illite + HA) suspension after centrifugation was lower compared to that of (U + HA) suspension (0.181 vs. 0.207). The lower absorbance was attributable to removal of those HA adsorbed to illite. Based on the difference in light absorbance, the amount of HA adsorbed to illite was estimated as: (0.207 − (0.181 − 0.010))/0.229 = 16%. At pH 5, electrostatic replusion between illite (zeta potential = −28.3 mV) and HA (zeta potential = −35.6 mV) were weaker than that at pH 9, therefore some of the HA could overcome the relatively lower energy barrier and adsorb to illite. Higher HA adsorption at lower pH observed in this study is consistent with previous reports (e.g., Feng et al., 2005; Liu and Gonzalez, 1999). HA adsorbs to clay minerals through association with poly-valent metals (e.g., Al(III) on clay surface) (Greenland, 1971), van der Waals interactions, bridging by aqueous polyvalent metal cations (e.g., Ca2+, Pb2+), and ion/ligand exchange (Arnarson and Keil, 2000; Liu and Gonzalez, 1999). In (U + illite) suspensions, 11% and 18% of U was associated with illite at pH 5 and pH 9 respectively (Table 3). At low pH, U adsorbs to fixed-charge sites on the basal planes of clay minerals. While at high pH, U also adsorbs to metal-oxide like edge sites (e.g. Al–OH, Si–OH) (Bachmaf and Merkel, 2011; Chisholm-Brause et al., 2004; McKinley et al., 1995), resulting in higher adsorption. In the presence of HA, U adsorption to illite could not be determined based on U concentration in the supernatant, since a substantially fraction of HA (10% at pH 5 and 11% at pH 9) was removed from supernatant during centrifugation. Measurements on U concentration and light absorbance of the filtrate of

Table 1 ζ-potential of illite and HA in U-spiked solutions/suspensions at the beginning (t = 0) and end (t = 240 min) of the stability test. “Standard deviation of ζ-potential” given by zetasizer indicates the width of ζ-potential distribution of the sample. The reported values of “ζ-potential” and “standard deviation of ζ-potential” represent mean ± standard deviation associated with triplicate measurements. pH

Influent

ζ-potential (mV) t=0

Standard deviation of ζ-potential (mV) t=0

ζ-potential (mV) t = 240 min

Standard deviation of ζ-potential (mV) t = 240 min

5

U U U U U U U U

NAa −28.3 ± 1.6 −35.6 ± 2.0 −49.0 ± 1.0 NAa −47.0 ± 2.7 −46.6 ± 4.5 −51.9 ± 2.5

NAa 4.5 ± 0.1 11.0 ± 7.9 9.2 ± 8.7 NAa 9.9 ± 1.1 9.9 ± 7.4 9.0 ± 0.1

NAa −30.2 −34.3 −47.9 NAa −40.7 −45.4 −52.2

NAa 5.4 ± 0.2 7.5 ± 2.6 12.4 ± 0.7 NAa 8.3 ± 1.0 12.0 ± 7.7 11.5 ± 4.2

9

a

Not applicable.

+ illite + HA + HA + illite + illite + HA + HA + illite

± 2.0 ± 2.2 ± 2.5 ± 1.4 ± 4.1 ± 2.8

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Table 2 Hydrodynamic diameter and polydispersity index of illite and HA in U-spiked solutions/suspensions at the beginning (t = 0) and end (t = 240 min) of the stability test. The reported values of “hydrodynamic diameter” and “polydispersity index” represent mean ± standard deviation associated with triplicate measurements. pH

Influent

Hydrodynamic diameter (nm) t=0

Polydispersity index t=0

Hydrodynamic diameter (nm) t = 240 min

Polydispersity index t = 240 min

5

U U U U U U U U

NAa 1194 ± 44 296 ± 58 354 ± 14 NAa 487 ± 165 211 ± 6 339 ± 7

NAa 0.41 0.45 0.33 NAa 0.38 0.45 0.30

NAa 1187 ± 82 233 ± 35 323 ± 16 NAa 476 ± 44 173 ± 7 349 ± 18

NAa 0.42 0.42 0.33 NAa 0.32 0.62 0.33

9

a

+ illite + HA + HA + illite + illite + HA + HA + illite

± 0.07 ± 0.08 ± 0.01 ± 0.02 ± 0.08 ± 0.02

± 0.09 ± 0.06 ± 0.04 ± 0.06 ± 0.17 ± 0.01

Not applicable.

the (U + HA) and (U + illite + HA) suspensions showed that 14% to 46% of U, and 27% to 57% of HA passed through the 5 kDa ultrafiltration membrance, indicating substantial fraction of the HA used in this study were of low molecular weight and could not be separated by ultrafiltration. As a result, dissolved U in the presecen of HA and U adsorbed to HA was not determined in this study. 3.4. Bromide and illite transport Dry mass of the solid materials and volume of solution used to pack a column was practically the same for different columns (Table S1, supporting information), and bromide breakthrough curves for the two independently packed columns reproduced well (Fig. S3, supporting information). These results indicated that the procedure we used to pack the columns resulted in similar column properties and that hydraulic characteristics of the packed columns were comparable. Fig. 1 illustrates illite breakthrough curves at pH 5 and pH 9 and the influecne of HA on illite transport at each pH. Both pH and HA had strong influence on illite transport: at pH 5, illite breakthrough barely occurred in the (U + illite) experiments, indicating nearly all the influent illite colloids were retained in the column. In contrast, high illite breakthrough was observed in the (U + illite) experiment at pH 9, and nearly complete breakthrough (C/C0 = 0.98 ± 0.09) occurred at 6 pore volume (Vp). The presence of HA in the influent significantly enhanced illite transport at pH 5: maximum effluent illite concentration (C/C0) increased from 0 to N 0.8 and the overall illite breakthrough increased from 0 to 77.8% (Table 4). At pH 9,

illite transport also increased due to the presence of HA, with the overall illite recovery increased from 88.9% to 100.8% (Table 4). Colloid deposition is a major process that influences colloid transport in porous media (Gao et al., 2011). Colloid deposition rate depends on the overall interactive forces (i.e., van der Waals forces, electric double layer forces, hydration forces, and steric repulsion) between colloid and media grains (Kretzschmar et al., 1999). pH influences illite deposition and transport by affecting surface charge of illite colloids and media grains. The dominant minerals (i.e., quartz, albite, and muscovite) in our

Table 3 Light absorbance at wavelength of 350 nm of well mixed suspension and supernatant after centrifugation measured by spectrophotometer, and percentage of U adsorbed to illite in the influent suspensions. The reported values represent mean ± standard deviation associated with duplicate samples. pH Suspension

Absorbance of well mixed suspension

%U Absorbance of supernatant after adsorbed to illite centrifugation

5

0.276 0.229 0.518 0.290 0.261 0.549

0.010 0.207 0.181 0.002 0.232 0.232

9

U U U U U U

+ + + + + +

illite HA illite + HA illite HA illite + HA

± ± ± ± ± ±

0.000 0.004 0.004 0.006 0.003 0.014

± ± ± ± ± ±

0.001 0.001 0.001 0.003 0.008 0.002

(11 ± 1) % a

(18 ± 3) %a

a U adsorbed to illite was estimated as the difference between total U concentration in the suspension and U concentration in the supernatant after centrifugation.

Fig. 1. Breakthrough curves of illite through columns filled with mixture of natural sediment and quartz sand in the absence and presence of HA. Breakthrough curve for bromide is included for comparison (C: concentration in effluent samples; C0: concentration in influent; C/C0: normalized effluent concentration; V: total volume of effluent; Vp: pore volume of the packed column; V/Vp: normalized volume of effluent).

Q. Wang et al. / Journal of Contaminant Hydrology 170 (2014) 76–85

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Table 4 U, illite, and bromide recovery in column experiments. Recovery was calculated based on U, illite, and bromide breakthrough curves shown in Figs. 1 and 2 using trapezoidal rule. The reported recovery represents mean ± deviation associated with duplicate column experiments. Experiment U transport pH = 5

U transport pH = 9

Bromide transport a

Influent U U+ U+ U+ U U+ U+ U+ Br−

illite HA HA + illite illite HA HA + illite

U recovery in the effluent 0 0 54.3 ± 1.3% 58.0 ± 1.8% 24.5 ± 0.8% 9.0 ± 0.6% 60.5 ± 3.0% 61.8 ± 2.4% NAa

Illite recovery in the effluent a

NA 0 NAa 77.8 ± 3.0% NAa 88.9 ± 10.2% NAa 100.8 ± 15.6% NAa

Br− recovery in the effluent NAa NAa NAa NAa NAa NAa NAa NAa 102.1 ± 4.9%

Not applicable.

column were not expected to provide favorable deposition sites for illite colloids because point of zero charge (pHpzc) of these minerals are low (pHpzc of quartz b 3, albite b 2, muscovite b 4) (Kosmulski, 2011), and therefore these minerals were negatively charged at pH 5 and 9. At these pH, illite colloids (pHpzc = 2.5) (Kosmulski, 2011) were also negatively charged (Table 1). As such, illite deposition was under unfavorable conditions and high illite transport was expected if illite deposition was only controlled by those minerals. The low illite transport observed at pH 5 in the absence of HA was attributable to illite deposition onto Fe and Al (hydro)oxides. In our porous media, concentration of Fe and Al (hydro)oxides were 718 mg (as Fe) /kg porous media and 211 mg (as Al) / kg porous media respectively (calculated based on the amount of Fe and Al (hydro)oxides determined by sequential extraction). Although Fe oxides are usually minor components in natural sediments, they substantially influence colloid deposition and transport (Johnson et al., 1996). While many natural mineral colloids carry negative charges at ambient pH, Fe/Al oxides can carry positive charges. pHzpc ranges from 7.8 to 8.5 for Fe oxides (Darland and Inskeep, 1997) and from 6.5 to 7 for Al hydroxides (Xu et al., 1991). At pH 5, Fe/Al oxides in our column carried positive charges and provided favorable deposition sites for negatively charged illite colloids, leading to near complete illite retention in the column. At pH 9, Fe/Al oxides carried negative charges and illite became more negatively charged compared to that at pH 5, electrostatic repulsion between Fe/Al oxides and illite colloids resulted in high illite breakthrough. Illite transport was enhanced in the presence of HA. At pH 5, maximum effluent illite concentration increased from near zero to N0.8 when HA was added to the influent (Fig. 1). This drastic increase is attributed to HA adsorption to column materials, especially to Fe/Al oxides. HA adsorbs to Fe/Al oxides by electrostatic interactions, association via aliphatic/aromatic carbon, and ligand exchange or H-bonding with carboxyl/ hydroxyl functional groups (Borggaard et al., 2005; Fein et al., 1999; Gu et al., 1994; Tipping, 1981; Yang et al., 2013). pH controls HA adsorption by influencing electrostatic interactions between HA and Fe/Al oxides and the number of adsorption sites on Fe/Al oxides (Gu et al., 1994; Tipping, 1981). In our column experiments, at pH 5, negatively charged HA strongly adsorbed to positively charged Fe/Al oxides, which diminished or even reversed the positive charges on these metal oxides and therefore hindered deposition of negatively charged illite colloids, leading to increased illite transport. At pH 9, both Fe/Al oxides and HA were negatively charged, therefore HA adsorption

to Fe/Al oxides was weak and only slightly increased illite transport. Besides Fe/Al oxides, HA also adsorbs to clay minerals (Lippold and Lippmann-Pipke, 2009; Liu and Gonzalez, 1999). At pH 5, 16% of HA in the (U + illite + HA) influent suspension was adsorbed to illite (discussed in Section 3.3). HA adsorption to illite could increase steric repulsions between illite and media grains, which further enhanced illite transport. In the (U + illite + HA) experiments, effluent illite concentration at 6 pore volume were abnormally high (C/C0 N 1.0) at both pH 5 and pH 9 (Fig. 1). This is presumably due to incidental elution of Fe-rich mineral colloids contained in the natural sediment used to pack the columns. These mineral colloids in the natural sediment were immobile under the steady flow conditions in our experiments. However, the presence of HA could substantially enhance their mobility and even a small perturbation in water flow could mobilize some of these particles. Effluent illite concentration in this study was calculated based on Fe concentration of the digested water samples, however, we could not distinguish between Fe originating from injected illite and Fe from mineral colloids in the natural sediment. At pH 9, effluent illite concentration in the (U + illite) experiments decreased after 6 pore volume (Fig. 1). This decrease was unlikely due to aggregation or coagulation of illite colloids in the influent suspension, since the illite colloids were confirmed to be stable by our stability test. Illite aggregation on sand surface (i.e., ripening) is more likely. 3.5. Uranium transport Experimentally measured U breakthough curves are shown in Fig. 2. Effluent U concentrations of duplicate experiments reproduced well, as evidenced by small error bars. These results demonstrated that hydraulic characteristics of different columns were similar, therefore differences in U and illite transport between different experiments (i.e., U only, U + illite, U + HA, and U + iilite + HA) are attributable to the composition in the influent, rather than hydraulic characteristics of the columns. U recovered in the effluent for each experiment were calculated based on the U breakthrough curves and shown in Table 4. 3.5.1. U transport in the absence of HA and illite colloids U(VI) transport in the absence of HA and illite (U only experiments) was strongly influenced by pH. At pH 5, effluent U concentration was practically zero (Fig. 2), indicating all the

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showed very similar pH-dependent U adsorption in the pH range of 2.5 to 10. U adsorption to minerals is pH dependent. At neutral pH range (i.e., pH = 5.5–7.5), considerable amount of U could adsorb to albite, quartz, and muscovite; at either low or high pH (pH b 5 or pH N 8), U adsorption is low (Arnold et al., 1998; Prikryl et al., 2001). U adsorption to various Fe oxides such as goethite, hematite, ferrihydrite, and amorphous Fe hydroxide showed similar pH dependence: adsorption increases from near zero at pH 3 to near 100% around pH 5 to 8, then decreases to near zero at pH N 9 (Barnett et al., 2000; Hsi and Langmuir, 1985; Shuibo et al., 2009; Waite et al., 1994). pH influences U adsorption by controlling mineral surface properties and aqueous U species. When pH increases from acidic to near netural, U adsorption increases due to deprotonation of surface sites (Waite et al., 1994). Additionally, strongly sorbing species such as UO2+ and UO2(OH)+ dominate at near netural pH 2 (Prikryl et al., 2001; Walter, et al., 2005), resulting in high U adsorption. When pH increases further to alkaline range, weakly sorbing U species such as UO2(CO3)3 4− and UO2(OH)− 3 dominate, resulting in low U adsorption (Barnett et al., 2002, 2000). Changes in aquesous U species with changing pH and the difference in these aqueous U species' affinity to the major minerals and Fe/Al oxides in our column experiments implies high U adsorption at pH 5 but low U adsorption at pH 9, which explains low U transport at pH 5 but high U transport at pH 9. Fig. 2. Breakthrough curves of 1 mg/L U(VI) through columns filled with mixture of natural sediment and quartz sand in the absence and presence of HA and illite colloids. Breakthrough curve for bromide is included for comparison (C: concentration in the effluent samples; C0: concentration in the influent; C/C0: normalized effluent concentration; V: total volume of effluent; Vp: pore volume of the packed column; V/Vp: normalized volume of effluent).

injected U was retained in the column. At pH 9, U breakthrough was obvious: effluent U concentration increased during injection of the U-spiked solution, and peaked at 0.26 at 10 pore volume (Fig. 2). Overall U recovered in the effluent was 24.5% at pH 9 (Table 4). In the absence of HA and illite colloids, U transport in porous media is controlled by U adsorption to transport media. All the major mineral constituents (i.e., albite, quartz, and muscovite) in our column material adsorb U (Arnold et al., 1998; Moyes et al., 2000; Prikryl et al., 2001). Quartz (Fox et al., 2006; Prikryl et al., 2001) and albite (Walter et al., 2005) adsorb U through surface complexation, muscovite through surface complexation at aluminol sites on the edge-surface (Arnold et al., 2006) and surface precipitation on basal plane (Moyes et al., 2000). The minor component of Fe oxides (718 mg (as Fe) / kg) in transport media also strongly absorbs U. U adsorbs to Fe oxides via formation of inner-sphere surface complexes between UO2+ (uranyl ion) and Fe oxides (Dodge et al., 2002; Reich 2 et al., 1998; Walter et al., 2003). Arnold et al. (1998) reported that in the pH range of 3.5 to 9.5, U adsorption to phyllite, a metamorphic rock mainly consists of muscovite, quartz, chlorite and albite, was dominated by U adsorption to the minor component of Fe oxides in that rock. Barnett et al. (2000) argued that low concentration of Fe oxides (25 g/kg as Fe) in heterogeneous soils dominated U adsorption, based on their observation that soil samples with different physical, chemical, and mineralogical properties but similar Fe oxides content

3.5.2. Effects of illite colloids Effects of illite on U transport varied at different pH. At pH 5, no U was detected in the effluent in our (U + illite) experiments (Fig. 2), indicating U transport was not enhanced by illite. At pH 9, U was detected in column effluent in our (U + illite) experiment with an overall effluent U recovery of 9.0% (Table 4). The ability of colloids to facilitate contaminant transport depends on the ability of the colloid to retain contaminant and mobility of the colloid. U adsorption to clay minerals was widely observed (Bachmaf and Merkel, 2011; Missana et al., 2004). In our (U + illite) experiments, 11% of the influent U was associated with illite at pH 5 (Table 3). However, U transport was not enhanced by illite at this pH due to the low mobility of illite (Fig. 1). At pH 9, 18% of the influence U was associated with illite, and illite recovery was high (88.9%, Table 4). Therefore, it was expected that illite colloid would increase U transport. However, effluent U recovery in our (U + illite) experiment was surprisingly lower than that in U only experiment (9.0% vs. 24.5%, Table 4). Although illite transport was quite high at pH 9 in our (U + illite) experiment, some of the injected illite (100%–88.9% = 11.1%) were retained in the column. Retention of those illite colloids immobilized the U adsorbed to them, reducing U transport. However, this process alone cannot fully explain the low U transport observed in our (U + illite) experiments, since this mechanism alone can only immobilize 11.1% of the illite-associated U in the influent, i.e., 11.1% × 18% = 2.0% of the total U in the influent, which was much lower than the experimentally measured U immobolization (100%–9.0% = 91.0%). The additional U immobilization was attributed to adsorption of aqueous U (which accounted for 82% of U in the influent) to transport media, and U desorption from illite during illite transport. In our (U + illite) experiments, 88.9% of the injected illite were recovered in the effluent, and these illite could carry 88.9% × 18% ≈ 16.0% of

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the total U in the influent suspension with them. However, only 9.0% of the total U in the influent suspension was recovered in the effluent, suggesting U must have desorbed from these illite colloids during colloid transport through the column. In the influent suspension, aqueous U and illite-associated U were at adsorption equilibrium, and the ratio of illite-associated U concentration to aqueous U concentration was constrained by U distribution coefficient. When (U + illite) suspension was injected into the column, aqueous U concentration decreased due to its adsorption to Fe/Al oxides and other minerals (quartz, albite, and muscovite). The decrease in aqueous U concentration induced U desorption from illite. The extent of desorption depends on the equilibrium and kinetics of desorption and adsorption reactions. The higher affinity of column materials for U, and the faster U desorbs and adsorbs, the greater amount of U will desorb. The substantial desorption observed in our experiments demonstrates that the column materials had high affinity for U, and that both U desorption and adsorption were fast. In addition, this result shows the ability of colloid to facilitate contaminant transport is influenced by competition for contaminants between colloid and aquifer minerals. The presence of strongly sorbing minerals (e.g., Fe oxides) could render the colloids an ineffective “shield” for contaminant transport. At pH 9, effluent U concentration decreased after 6 pore volume in the (U + illite) experiments (Fig. 2). The same trend was also observed for effluent illite concentration in these experiments (discussed in Section 3.4). The similarity in illite and U breakthrough confirmed that transport of these injected illite indeed decreased after 6 pore volume. 3.5.3. Effects of HA The presence of HA substantially enhanced U transport. At pH 5, overall effluent U recovery reached 54.3% in the (U + HA) experiment (Table 4), compared to the near zero U breakthrough in U only experiment. At pH 9, elevated U breakthrough was observed as well, with the overall effluent U recovery increased from 24.5% for U only experiment to 60.5% for (U + HA) experiment (Table 4). Enhanced U transport by HA has been reported (Mibus et al., 2007; Sachs et al., 2006; Yang et al., 2012). One mechanism for the increased U transport in the presence of HA is association of aqueous U with HA. Aqueous U can form U– HA complexes via reacting with hydrophilic functional groups (carboxylic and phenolic) (Lenhart et al., 2000; Yang et al., 2012) as well as hydrophobic (alkyl and aromatic) carbon in HA (Yang et al., 2012). The formation of U–HA complexes “shielded” U from adsorption to column materials and increased U transport. The other mechanism for increased U transport in the presence of HA is competition between HA and aqueous U for surface sites on column materials. HA could adsorb to Fe/Al oxides (Gu et al., 1994; Tipping, 1981; Yang et al., 2013), as well as to the main minerals in our column (i.e. albite, quartz, and muscovite) (Pitois et al., 2008; Schmeide et al., 2000). The competition between HA and U made U less adsorbing and more mobile. U transport at pH 9 in our (U + HA) experiment was slightly higher than that at pH 5, as evidenced by the higher effluent U recovery at pH 9 (60.5% at pH 9 vs. 54.3% at pH 5). The higher U transport was due to higher HA mobility and higher U adsorption to HA. At pH 5, negatively charged HA adsorption to

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the positively charged Fe/Al oxides and main minerals was relatively strong. While at pH 9, metal oxides became negatively charged and main minerals and HA became more negatively charged, resulting in less HA adsorption and higher transport of HA and HA-associated U. Additionally, binding of U by HA is stronger at higher pH (Lenhart et al., 2000; Li et al., 1980), which also increased the amount of U transported by HA. 3.5.4. U transport in the presence of HA and illite colloids At both pH 5 and 9, U transport in the (U + HA + illite) experiments was the highest, and was much higher than those in U only and (U + illite) experiments (Table 4). At pH 5, effluent U recovery in the (U + HA + illite) experiments reached 58.0%, slightly higher compared to that in the (U + HA) experiments (54.3%). At pH 9, effluent U recovery in the (U + HA + illite) experiments was 61.8%, practically the same as that in the (U + HA) experiments (60.5%). High U transport in the (U + illite + HA) experiments was primary due to HA-facilitated U transport. The slightly higher U transport in (U + illite + HA) experiments compared to (U + HA) experiments at pH 5 indicated that illite further enhanced U transport. The ability of illite to promote U transport depends on the mobility of illite and the ability of illite to retain U. At pH 5, mobility of illite enhanced substantially (77.7% vs. 0) in the presence of HA mainly due to HA adsorption to column materials (discussed in Section 3.4). In addition, adsorption of HA to illite (discussed in Section 3.3) provided additional binding sites for U, which could increase U adsorption to illite by forming ternary illite–HA–U surface complexes. Formation of ternary clay–HA–U surface complexes at acidic pH was previously reported (Kornilovich et al., 2000; Krepelova et al., 2006; Sachs and Bernhard, 2008). At pH 9, the increase in illite transport (100.8% vs. 88.9%) in the presence of HA was lower than that at pH 5 (77.8% vs. 0), and ternary illite–HA–U complexes was not formed (since HA did not adsorb to illite), therefore influence of illite on U transport was negligibly small. Although a spike was observed in illite breakthrough curves at 6 pore volume for the (U + illite + HA) experiments (discussed in Section 3.4), the corresponding U breakthrough curves showed no such spike (Fig. 2), and that effluent U concentration continued to increase after 6 pore volume until termination of U injection at 10 pore volume. The disparity between illite and U breakthrough curves showed that U transport in the (U + illite + HA) experiments was mainly controlled by HA, not mineral colloids. 4. Conclusions Both illite colloids and HA influences U transport via a suite of interfacial geochemical reactions that include: (i) HA adsorption to transport media and illite, (ii) U adsorption to transport media and illite, (iii) formation of HA–U complexes, and (iv) formation of ternary illite–HA–U surface complexes. Illite colloids had no observable influence on U transport in the absence of HA at low pH due to the low mobility of illite. At high pH, U desorbed from mobile illite and the presence of illite decreased U transport. HA not only increased illite transport by adsorbing to surface sites on transport media, but also increased U transport via (i) formation of aqueous U–HA complexes and (ii) competition against aqueous U for surface sites on transport media. At pH 5, when both illite colloids and

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HA were present, formation of ternary illite–HA–U surface complexes as well as increased illite mobility enhanced U transport. HA plays a significant and complicated role in mediating U transport both in the absence and presence of mineral colloids. The ubiquitous co-existence of mineral colloids (e.g., illite) and HA in subsurface environment implies that the combined effects of mineral colloids and HA must be considered when estimating U transport in real subsurface environments.

Acknowledgments This work was supported by Research & Development Corporation of Newfoundland and Labrador's Ignite R&D Program (Project No.: 5404.1354.101), and Natural Sciences and Engineering Research Council of Canada's Discovery Grant (Application No.: 402815-2012). We thank Dr. Valerie Booth and Ms. Donna Jackman for the use of zetasizer, and Dr. Baiyu Zhang for the use of spectrophotometer and centrifuge machine. The constructive comments and suggestions of two anonymous reviewers led to great improvements of this manuscript.

Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jconhyd.2014.10.007.

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Influence of mineral colloids and humic substances on uranium(VI) transport in water-saturated geologic porous media.

Mineral colloids and humic substances often co-exist in subsurface environment and substantially influence uranium (U) transport. However, the combine...
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