Co-transport of U(VI), humic acid and colloidal gibbsite in water-saturated porous media

Chemosphere 231 (2019) 405e414

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Co-transport of U(VI), humic acid and colloidal gibbsite in watersaturated porous media Junwei Yang a, b, Mengtuan Ge a, b, Qiang Jin a, b, Zongyuan Chen a, b, **, Zhijun Guo a, b, * a b

Radiochemistry Lab, School of Nuclear Science and Technology, Lanzhou University, Lanzhou, 730000, China The Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, 730000, Lanzhou, China

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


Gibbsite colloids impeded U(VI) transport at relatively high uranium concentration.
HA of 5 mg L1 did not change the impediment of gibbsite colloids to U(VI) transport.
HA of 20 mg L1 facilitated the transport of gibbsite and U(VI).
Gibbsite colloids and HA transported together in the coexistence system.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 January 2019 Received in revised form 1 May 2019 Accepted 12 May 2019 Available online 21 May 2019

The release of uranyl from uranium tailing sites is a widely concerned environmental issue, with limited investigations on the effect of coexistence of various colloids. Gibbsite colloids extensively exist, together with ubiquitous humic substances, in uranium polluted waters at tailing sites, due to high concentration of dissolved Al in acid mine drainage. In this context, we investigated the co-transport of U(VI), gibbsite colloids and humic acid (HA) as a function of pH and ionic strength at a U(VI) concentration (5.0  105 M) relevant within mine tailings and related waste. It was found that, owing to electrostatic attraction, gibbsite colloids and HA associated with each other and transported simultaneously regardless of U(VI) presence. Besides the impact of pH and ionic strength, whether gibbsite colloids facilitated U(VI) transport depended on HA concentration. Gibbsite colloids impeded U(VI) transport at relatively low HA concentration (5 mg L1), because associated colloids loaded with U(VI) were positively charged which favored colloid retention on negatively charged quartz sand in the column. U(VI) together with gibbsite colloids and low concentration HA was completely blocked at natural pH and/or high ionic strength. At relatively high HA concentration (20 mg L1), however, the associated colloids showed negative zeta potential which facilitated U(VI) transport because of repulsion between negatively charged colloids and quartz sand. Meanwhile, high concentration of HA dramatically accelerated the transport of gibbsite colloids. These results implied that gibbsite colloids might imped U(VI) migration at uranium tailing sites unless the aquifers are enriched with abundant humic substances. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Martine Leermakers Keywords: Transport U(VI) Gibbsite colloids Humic acid

* Corresponding author. Radiochemistry Lab, School of Nuclear Science and Technology, Lanzhou University, Lanzhou, 730000, China. ** Corresponding author. Radiochemistry Lab, School of Nuclear Science and Technology, Lanzhou University, Lanzhou, 730000, China. E-mail addresses: [email protected] (Z. Chen), [email protected] (Z. Guo). https://doi.org/10.1016/j.chemosphere.2019.05.091 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

1. Introduction Uranium mining and milling activities commonly result in the release of excess level of uranium (Babu et al., 2008). The release of

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uranium at high concentrations possibly leads to a large-scale contamination and poses a great threat to human health. Therefore, uranium immobilization around its tailing sites becomes a worldwide interest. The released uranium typically occurs as hexavalent uranyl. The behavior and mobility of uranium at mill tailings is dependent on the dissolution, complex formation, and desorption-sorption interaction at the water-rock interface and affected by environmental conditions such as ionic strength, pH and redox conditions et al. (Liu et al., 2017; Tricca et al., 2000; Othmane et al., 2013; Liu et al., 2009). In this context, it is necessary to understand U(VI) migration behavior under in-situ conditions at uranium tailing sites. Colloids are among the most important factors which control environmental behaviors of polyvalent cations. Various dispersed colloids coexist in the environment, including natural organic matters (NOM), inorganic colloids (clay minerals, Fe/Al oxide/hydroxide nanoparticles et al.) and bio colloids (viruses and bacteria) (McCarthy and McKay, 2004; Zaenker and Hennig, 2014; Syngouna et al., 2017; Syngouna et al., 2016). Colloids have strong affinities to polyvalent cations due to their highly reactive functional groups and large specific surface areas (Andersson et al., 2001; Artinger et al., 2002; Crancon et al., 2010; Geckeis et al., 2004). Mobile colloids usually compete for loading pollutants with stationary matrix in the environment and act as vehicles to carry the pollutants transporting further and farer than the expected without considering the effect of colloids. This has been demonstrated by both field and laboratory studies (Kersting et al., 1999; Novikov et al., 2006; Wolfsberg et al., 2017; Yang et al., 2012). The effects of inorganic and NOM colloids on U(VI) transport have been investigated for decades (Artinger et al., 2002; Yang et al., 2012; Mibus et al., 2007a; Sachs et al., 2006). Previous studies mainly focused on individual colloids with the same chemical component, and found that individual colloids affected significantly U(VI) transport. For example, U(VI) transport in water-saturated quartz columns was facilitated as fast as a conservative tracer in the presence of humic acid (HA) at pH 7.2 (Artinger et al., 2002), whereas the presence of fulvic acid significantly increased U(VI) adsorption on silica column and thus retarded U(VI) transport at pH 3.7 (Zhang et al., 2009). Colloidal clay minerals, such as attapulgite and bentonite colloids, also presented a colloid-facilitated effect on re et al., 2009; U(VI) transport (Du et al., 2019; Claveranne-Lamole Tran et al., 2018). On the other hand, our previous study found that whether colloidal iron hydroxide facilitated U(VI) transport depended on U(VI) concentration. At relatively high concentrations, U(VI) transport was impeded by colloidal iron hydroxide because the colloids were more prone to aggregate and more sensitive to the changes in pH and ionic strength (Ge et al., 2018). Although various colloids co-exist in the environment, studies on the combined effect of different kinds of colloids on U(VI) transport are still scarce (Wang et al., 2014; Chen et al., 2018). The environmental behavior of colloids is primarily determined by the chemistry of surface functional groups, particle size and zeta potential. Different kinds of colloids may interact with each other through electrostatic interactions, ligand exchange reactions and weak interactions (e.g. van der Waals interactions) (Yang et al., 2013), which consequently regulates the environmental behavior of the colloids. The association of different kinds of colloids alters their aggregation state and surface property as well as their affinities to various contaminants (Yang et al., 2019; Tang and Cheng, 2018), and thus present different effects on pollutant migration as compared with their individual forms (Ma et al., 2018a). Some recent studies on U(VI) co-transport with HA and clay mineral colloids (illite and kaolinite) indicated that the transport of U(VI) and clay mineral colloids was enhanced due to the formation of ternary complexes of HAeU(VI)- colloidal clay mineral (Wang et al.,

2014; Chen et al., 2018). For uranium tailing sites, acid mine drainage usually contains high concentration of dissolved Al which enables colloidal Al hydroxide formed with the changes in Eh/pH values (Peng et al., 2009). From the perspective of predicting potential mobility of uranium contaminated tailing sites, it is of significance to investigate the contribution of colloidal Al hydroxide to uranium migration and the combined effects of colloidal Al hydroxide with other coexistent colloids on U(VI) transport. However, the subject has not been sufficiently studied yet. The objectives of this study were to elucidate the key factors that control U(VI) transport in the presence of colloidal Al hydroxide and humic acid, examine the combined effects of the two kinds of colloids, and to evaluate the contribution of the colloids to U(VI) migration at uranium concentration relevant to contaminated uranium tailing sites. HA was used as a representative of colloidal NOM and quartz sand as a model porous medium in this study. A series of column experiments were performed to determine individual and combined effects of colloidal Al hydroxide and HA on U(VI) transport. The evolutions of zeta potential and particle size of effluent colloids were analyzed to better understand the transport behaviors of U(VI) and the colloids in the ternary transport system. This study provided valuable insights into understanding the role of colloidal Al hydroxides in U(VI) migration at uranium tailing sites. 2. Materials and methods 2.1. Porous media To remove surface impurities, commercial quartz sand (SiO2 >99%, 20e40 meshes) was subsequently washed with NaOH (0.1 M), HCl (0.1 M) solutions and pure water, and then dried at 105  C. The specific surface area of the purified quartz sand, determined by N2 B.E.T. method, was found to be 0.163 m2 g1. Isoelectric point (IEP) of crushed quartz grains, obtained by laser Doppler electrophoresis using a zeta potential analyzer (Malvern Co.), was found to be at pH 3.1. 2.2. Humic acid and gibbsite colloids Commercial HA (Sigma Aldrich co.) was purified according to a method recommended by International Humic Substance Society (Aiken, 1985). HA suspensions were diluted with deionized water to desired concentrations (5 or 20 mg L1). The suspensions with desired pH and ionic strength were equilibrated for at least 24 h before using. Gibbsite colloids were prepared according to the method reported by Cesteros et al. (2001). In brief, ammonia solution (0.25%) was added dropwise into aluminum nitrate solution (0.5 M) under stirring at 75  C until the pH value of the mixture became 9.2. The obtained white gel was separated by filtration and washed with deionized water. The purified white gel was dispersed in deionized water and nitric acid was added under stirring at 80  C until the pH value of the suspension was around 3.7. After aging for 24 h at 85  C, a colloidal gibbsite stock suspension was obtained. For transport experiments, the colloidal gibbsite stock suspension was diluted to a desired concentration of 9.0 mg L1. The prepared colloid sample was confirmed to be gibbsite (aAl(OH)3) by powder X-ray diffraction (XRD) measurement on freeze-dried powders of the colloids (Fig. S1). Images obtained by scanning electron microscope showed that gibbsite particles were irregular, and their mean size was in nanometer range (Fig. S2). Further characterization indicated that the mean size was 130 nm (Fig. S3) and the isoelectric point was pH 10.5 (Fig. S4). The isoelectric point was consistent with the reported results (Cesteros

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et al., 2001; Addai-Mensah et al., 1998). Characterization details for the gibbsite colloids can be find in Supplementary data (S1). 2.3. Influents of U(VI) associated colloids Uranium (VI) stock solution was prepared from UO2(NO3)2$6H2O (A.R. grade). All other chemicals used in this study were of analytical grade. NaNO3 was used as background electrolyte to maintain ionic strength. pH was adjusted by adding small amounts of HCl or NaOH solutions. The initial concentration of uranium(VI) used in all transport experiments was 5.0  105 M, which is in the range of uranium concentration in waters at uranium contaminated tailing sites (IAEA, 2004). For the influents simultaneously containing U(VI) and colloids, the suspensions were prepared by mixing desired amounts of gibbsite/HA colloids and U(VI) solution, adjusted to desired pH values (5.9, 7.0 and 7.5), and then shaken for 24 h to reach a steady state before injection. 2.4. Column experiments A glass column with Teflon film end-caps on both ends was used for transport experiments. The column was vertically placed and wet-packed with 55.0 g quartz sand using sequential 1 cm lifts. After each lift, the columns were slightly tapped to eliminate air bubbles. The parameters of the packed quartz sand column was listed in Table .1. Chloride anion (NaCl) was used as a conservative tracer to determine the hydraulic properties of the column in transport experiments. The concentration of Cl was determined by an ion chromatography (IC861, METROHM, Switzerland). To ensure column saturated and sufficiently conditioned, more than 10 pore volumes (PVs) of deionized water followed by at least 10 PVs of the background electrolyte solution at desired pH were sequentially pumped through the column. In this study, injections were carried out in an up-flow direction by using a peristaltic pump at a constant flow rate (Q ¼ 0.189 mL min1) in order to minimize air entrapment and avoid gravity effects on colloid deposition (Chrysikopoulos and Syngouna, 2014). After conditioning the column with background electrolyte at required pH and ionic strength, two steps of injections were performed for each transport study. In the first step, 16 pore volumes (PVs) of influent were injected. In the second step, several PVs of background electrolyte solution at the same pH and ionic strength were pumped into the column until nearly no U(VI), gibbsite or HA colloids could be detected in effluents. During transport experiment, effluents were collected using a fraction collector at a regular time interval. The concentrations of U(VI) in effluents were analyzed at 652 nm by UV/VIS absorption spectroscopy (UV-1800, Mapada Co., Shanghai, China) using uranyl Arsenazo(III) complex (Guo et al., 2009a). The concentration of gibbsite colloids was evaluated by determining aluminum contents. Gibbsite colloids were dissolved in sulfuric acid and the obtained solution was measured by UV/VIS absorption spectroscopy at l ¼ 510 nm using Table 1 Parameters of the packed quartz sand column. Parameters

Values

Column length (L) cm

20.0

Column inner diameter (D) cm Averaged collector diameter (dc) mm Porosity (G) cm3 cm3 Pore volume (PV) mL Volumetric water content (q) cm3 cm3 Sand bulk density (rb) g cm3 Specific surface area of quartz sand (A) m2 g1

1.6 0.505 0.44 ± 0.01 17.6 0.438 1.368 0.163

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xylenol orange complexes (Zolgharnein et al., 2009). The concentration of HA was determined at 254 nm using a PerkinElmer Lambda UV/VIS spectrometer (Chen et al., 2012). The particle size and zeta potential of colloids in the effluents were also monitored to obtain insight into the interactions between colloids as well as the deposition behavior of the colloids in the porous media. All transport experiments were carried out at room temperature (22 ± 2  C) using degassed solutions. Each experimental item was performed in a separate column to avoid mutual interferences. Further details of the experimental conditions and the mass recoveries for individual, binary and ternary transport systems were listed in Tables 2 and 3, respectively. 3. Results and discussion In this study, breakthrough curves (BTCs) were expressed as relative concentration (C/C0) against the number of pore volumes (PVs), where C and C0 were concentrations of U(VI) or the colloids in effluents and influents, respectively. In co-transport experiments, C/C0 for both U(VI) and colloids was detected simultaneously in the same outflow. BTCs of Cl exhibited a high degree of symmetry (Fig. S5) and the corresponding parameters were listed in Table S1. The effect of colloids on U(VI) migration varies with the kind of colloids suspended in aqueous phase and depends on physicochemical properties, such as the chemistry of surface sites, zeta potential and particle size of the colloids. Colloidal gibbsite and HA may have different effects on U(VI) transport because the properties of colloidal gibbsite and HA are distinct from one another. Therefore, it is necessary to elucidate the effects of two colloids on U(VI) transport, respectively, to better understand their individual and combined effects in the co-transport system. 3.1. BTCs of individual U(VI), HA and colloidal gibbsite Compared with the transport of conservative tracer, individual transport of U(VI), and gibbsite colloids was retarded by the watersaturated quartz column (see Fig. S6, S7a-b), whereas no significant retardation was observed for HA (Figs. S7ced). The retardation profiles of individual U(VI), gibbsite colloids and HA as a function of pH and ionic strength were also distinct from each other. The BTCs of gibbsite colloids were more sensitive to the changes in pH and ionic strength. With increasing pH and ionic strength, BTCs were shifted backwards and more PVs were needed for gibbsite colloids to break through the column. U(VI) retention in the quartz sand column was due to U(VI) adsorption on quartz surfaces and showed only slight changes over the pH range of 6.0e7.5 and ionic strength from 8.0  104 to 1  102 M. The relative insensitivity of U(VI) transport to pH and ionic strength under the experimental conditions was consistent with the results of batch experiment (Fig. S8). Details for batch experiment was included in Supplementary data. U(VI) distribution coefficient (Kd) decreased slightly with pH at pH 6.0e7.5 and was not affected by ionic strength. Carbonate complexes of U(VI) might have an important influence on U(VI) adsorption over the pH range because the system was in equilibrium with atmospheric CO2 (Guo et al., 2009b). In contrast, the retardation of colloidal gibbsite in the column increased obviously with increasing pH and ionic strength (see Figs. S7aeb). The transport of colloids in water-saturated porous media is primarily influenced by colloid deposition on media grains (Bin et al., 2011). According to classical DerjaguineLandaueVerweyeOverbeek (DLVO) theory, colloid deposition rate is dependent on the overall interactive forces including van der Waals forces, electrostatic interactions, hydration forces, and steric repulsions between colloids and media grains (Kretzschmar et al., 1999). The Londonevan der

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Table 2 Properties of influents and mass recoveries for individual transport systems. Influent

pH

IS (M)

z a (mV)

Size

U(VI)

5.9 7.0 7.5 5.9 5.9 5.9 7.0 7.5 5.9 5.9 5.9 7.0 7.5 5.9 5.9

8  104 8  104 8  104 4  10-3 0.01 8  104 8  104 8  104 4  103 0.01 8  104 8  104 8  104 4  103 0.01

NA f NA NA NA NA 43 41 37 41 40 33 36 39 35 31

NA NA NA NA NA 140 145 150 141 154 277 246 193 281 290

Gibbsite colloids

HA colloids

a b c d e f

b

(nm)

U(VI) recovery (%)

c

82 79 77 79 78 NA NA NA NA NA NA NA NA NA NA

gibbsite recovery (%)

d

HA recovery (%)e

NA NA NA NA NA 82 71 64 77 75 NA NA NA NA NA

NA NA NA NA NA NA NA NA NA NA 94 99 98 98 93

Initial zeta potentials of colloid influents. Initial size of colloid influents. Mass recovery of effluent U(VI). Mass recovery of effluent gibbsite colloids. Mass recovery of HA colloids. Not applicable.

Table 3 Properties of influents and mass recoveries for the binary and ternary co-transport systems; [U(VI)]0 ¼ 5.0  105 M; gibbsite colloids, 9 mg L1. Influent

pH

NaNO3 (M)

HA (mg L-1)

z a (mV)

Size

U(VI)-gibbsite colloids

5.9 7.0 7.5 5.9 5.9 5.9 7.0 7.5 5.9 5.9 7.5 7.5 7.5 7.5

8  104 8  104 8  104 4  103 0.01 8  104 8  104 8  104 4  103 0.01 8  104 8  104 4  103 0.01

0 0 0 0 0 20 20 20 20 20 5 20 20 20

42 20 18 30 21 31 35 36 31 28 8 30 29 26

153 7000 9378 3922 11234 230 228 224 270 330 9400 270 740 1140

U(VI)-HA colloids

U(VI)-gibbsite-HA colloids

a b c d e f

b

(nm)

U(VI) recovery (%)

69 0 0 67 0 71 82 84 78 82 0 86 86 34

c

HA recovery (%)e

gibbsite recovery (%) d 77 0 0 67 0 NA NA NA NA NA 0 97 94 39

NA NA NA NA NA 94 80 99 98 94 0 99 94 39

f

initial zeta potentials of colloid influents. Initial size of colloid influents. Mass recovery of effluent U(VI). Mass recovery of effluent gibbsite colloids. Mass recovery of HA colloids. Not applicable.

Waals interaction is attractive and independent of solution chemistry (Liu et al., 1995), whereas the electrostatic interactions vary with pH and ionic strength. Increasing pH led to the decrease of the zeta potential of gibbsite colloids (Table S1), while increasing ionic strength resulted in decrease in the thickness of diffuse double layers (Yu et al., 2013). Both situations weakened the electrostatic repulsions between colloid particles and brought about decreasing stability, increasing deposition and severe retardation of gibbsite colloids in the column. In contrast, HA showed only a very slight retardation to the conservative tracer. As pH increased, HA transport became slightly easier to break through the column (Figs. S7ced) and the BTCs shifted forwards to some extent. This could be illustrated by the fact that the negative charges of both HA and quartz surfaces increased with increasing pH. In addition, ionic strength had few apparent effects on HA transport, which obviously differed from the response of many inorganic colloids to the change in ionic strength.

3.2. U(VI) transport in the presence of gibbsite colloids Gibbsite has a high affinity to U(VI) and may affect significantly its migration in the environment. Similar to many polyvalent transition metals, the adsorption of U(VI) on gibbsite (Chang et al., 2006; Gueckel et al., 2012; Hattori et al., 2009; Zhang et al., 2005) and quartz sand increased with pH in acidic pH range and was almost independent of ionic strength (Fig. S8). In U(VI) transport system in the presence of colloidal gibbsite, the changes in pH and ionic strength also affected the stability of colloidal gibbsite suspension. Both effects of pH and ionic strength on adsorption and colloid stability were vital to the role of gibbsite colloids in U(VI) migration. Fig. 1 shows BTCs of U(VI) and gibbsite colloids in the binary cotransport system as a function of pH and ionic strength. The transport of U(VI) and gibbsite colloids was obviously retarded as compared with that in the individual transport systems. The

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Fig. 1. Observed BTCs of U(VI) (red points) and gibbsite colloids (blue points) in the co-transport system at various ionic strengths and pH values. For comparison, BTCs of individual U(VI) (red lines) and gibbsite colloids (blue lines) under the same conditions are also presented as lines, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

retardation of both U(VI) and gibbsite colloids was improved with increasing ionic strength and pH, which was evidenced by the recoveries of U(VI) and gibbsite (Table S2) and by the PVs of beginning breakthrough for U(VI) and gibbsite as well (see Fig. 1aeb). When ionic strength increased to 1  102 M at pH 5.9 (Fig. 1c), or the pH increased to pH 7.0e7.5 at ionic strength of 8  104 M (Fig. 1dee), no gibbsite colloids and U(VI) could be detected in column effluents. Both gibbsite colloids and U(VI) were almost completely blocked in the column. Compared with the individual system (Figs. S7aeb), the transport of gibbsite colloids in the presence of U(VI) (5.0  105 M) was dramatically impeded, especially at high ionic strength (Fig. 1c) and high pH values (Fig. 1dee). The impediment effect of gibbsite colloids obviously had advantages for the remediation of uranium contaminated tailing sites.

As mention previously, many field and laboratory studies demonstrated that colloids could act as vehicles and thus promote radionuclide migration (Kersting et al., 1999; Novikov et al., 2006; Sen and Khilar, 2006). The impediment effect of gibbsite colloids in this study was obviously contrary to the reported facilitating effect of colloids. Our previous study revealed that whether colloids facilitate radionuclide migration depended on the concentration of radionuclide (Ge et al., 2018). The concentration of concerned contaminant could account for the discrepancy. When the concentration was low enough and the adsorption of radionuclide did not affect apparently the physicochemical properties of colloids, the colloids might carry the radionuclide to migrate faster and farer compared with the situation without colloids. When the concentration of radionuclide was relatively high, however, the surface

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characteristics of the colloids would be largely changed by radionuclide adsorption. The concentration of U(VI) around uranium tailing sites is usually relatively high (IAEA, 2004) and thus the existence of gibbsite colloids may be a favored factor to inhibit uranium migration. This could be evidenced by the changes in size and zeta potential of gibbsite colloids (see Table S2). The zeta potential of gibbsite colloids in the presence of U(VI) (5.0  105 M) decreased compared to that without U(VI), meanwhile the size significantly increased especially at around neutral pH or high ionic strength. Due to the increasing size and decreasing zeta potential, decreasing stability and severe deposition of colloids occurred in the column, which even blocked the breakthrough of gibbsite colloids and the adsorbed U(VI) at high pH and ionic strength under the experimental conditions (Fig. 1cee).

3.3. U(VI) transport in the presence of humic acid Humic substances (HSs) are ubiquitous and have at least two dominant features which affect the fate of heavy metals in the environment. First, HSs have massive binding sites including mainly carboxylate and phenolic groups, and can form various complexes with most of metal cations (Joseph et al., 2013). Second, HSs modify the surfaces of solid matrix in the environment (Mal'tseva and Yudina, 2014; Pitois et al., 2008). The two aspects influence U(VI) transport in the presence of HA. Fig. 2 shows the BTCs of U(VI) and HA in the binary co-transport system. In general, HA breakthrough was postponed to some extent by the presence of U(VI), whereas the breakthrough of U(VI) was promoted by HA, compared with that in the individual transport systems (Fig. S6 and S7c-d). U(VI) might act as a bridge between HA

Fig. 2. BTCs of U(VI) (red point) and HA colloids (blue point) in the co-transport system. BTCs of individual U(VI) (red line) and HA (blue line) at corresponding conditions are also presented as lines for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 3. BTCs of U(VI) (red point), gibbsite colloids (blue point) and HA (olive point) at pH ¼ 7.5. BTCs of individual U(VI) (red line), gibbsite colloid (blue line) and HA (olive line) are presented as lines for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

and quartz surfaces via ternary complexes (Bryan et al., 2012; Krepelova et al., 2007). In addition, the complexation between HA and U(VI) decreased negative charges on HA and thus decreased electrostatic repulsion between HA and quartz surfaces. Both effects improved HA loading on quartz. Meanwhile, from the point view of U(VI), HA in aqueous phase competed U(VI) with quartz surfaces and led to less U(VI) adsorption especially in high pH range (Yang et al., 2012; Lenhart et al., 2000). Thereby, even in a relatively high concentration of U(VI) at 5.0  105 M, HA enabled U(VI) breakthrough faster with lower PVs as compared with that in U(VI) individual transport system. Similar findings were reported in the literature (Artinger et al., 2002; Mibus et al., 2007b; Stockdale and Bryan, 2013).

3.4. U(VI) transport in the presence of mixed humic acid and gibbsite colloids According to the results in Figs. 1 and 2, it is clear that individual gibbsite colloids and HA had different effects on U(VI) transport. The difference could be ascribed to distinct characteristics of the two kinds of colloids, i.e., the variations in the chemistry of binding sites, particle size and zeta potential. HA possesses high capacity of acidic functional groups on its flexible organic skeleton (Kimuro et al., 2015). Thereby, HA is usually viewed as polyelectrolyte, develops negative charges and always shows negative zeta potential over wide pH range (Fig. S4). As one of common metal hydroxides in the environment, however, colloidal gibbsite contains only amphoteric hydroxyl groups on its surfaces (Adekola et al., 2011)

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and has positive zeta potential up to pH 10.4 (Fig. S4). Fig. 3 shows the BTCs of U(VI), gibbsite colloids and HA in the ternary co-transport system as a function of ionic strength and HA concentration. For comparison, BTCs of gibbsite and HA in binary co-transport system in the absence of U(VI) is also shown in Fig. 3a. There were two outstanding points shown in Fig. 3. First, it was notable that HA concentration was a vital factor controlling transport profiles in the ternary transport system. The presence of 5 mg L1 HA had few apparent effects on BTCs of both gibbsite colloids and U(VI) (see Figs. 1e and 3b). Gibbsite colloids and U(VI) together with HA were completely blocked in the column in the presence of 5 mg L1 HA at pH 7.5 and ionic strength 8.0  104 M. The zeta potential of the colloids in the presence of 5 mg L1 HA became small but still positive, and the mean particle size increased to around 9.4 mm. U(VI) was almost completely loaded on the deposited particles and did not break through the column. When HA amount increased to 20 mg L1 (Fig. 3c), however, the transport of three components in the ternary transport system became unhindered with high recoveries (see Table 3). The transport of U(VI) was promoted and the BTCs of U(VI) shifted forwards (Fig. 3c). The recoveries of two kinds of colloids decreased with increasing ionic strength due to increasing mean particle size (see Table 3). The maximum C/C0 of the colloids decreased to around 40% at ionic strength of 0.01 M. Second, both colloidal gibbsite and HA transported simultaneously in all cases, indicated that gibbsite colloids and HA were associated with each other in the coexistence system no matter the presence/absence of U(VI). The association of colloidal gibbsite and HA were straightforward considering their opposite charges and zeta potential under the experimental conditions (see Tables 2 and 3). Besides the electrostatic attraction between gibbsite colloids and HA, chemical binding between them via ligand exchange reactions were complementary mechanism (Arnarson and Keil, 2000; Liu and Gonzalez, 1999). In the ternary system with relatively high concentration of HA (20 mg L1) (Fig. 3cee), the transport of gibbsite colloids was impressively promoted, while HA transport was slowed down to some extent, compared with that in individual transport systems. The transport behaviors of gibbsite colloids and HA (20 mg L1) in the ternary system could be illustrated by the negative zeta potential of the associated colloids (Table 3). The value of negative zeta potential of the colloids for the ternary system was comparable to that for the binary system of HA and U(VI). Due to electrostatic repulsion of negatively charged quartz surfaces and the colloids, sever colloid deposition in the column was avoid (Ma et al., 2018b). The increase in ionic strength made the size of the associated colloids increase, and thus both the recoveries of the colloids and their maximum C/ C0 decreased. 3.5. Evolutions of zeta potential and particle size in the effluents Non-equilibrium is an important feature for colloid transport system not only because colloid suspension is actually a nonequilibrium system, but also because column experiment is kinetics-dependent. Zeta potential and particle size of colloids in effluents usually vary with PVs and the variations provide some insight into interaction mechanism of the transport system. Fig. 4 shows the evolutions of zeta potential and particle size of colloids in effluents as a function of PVs. In general, the absolute value of zeta potential increased within the first a few PVs, which were corresponding to the breakthrough period of BTCs, and then plateaued off. Meanwhile, the mean size of colloids decreased with the change in zeta potential within the first a few PVs and then maintained constant. The variation in particle size was in accordance with the evolution of colloid zeta potential in the effluents. Additionally, size exclusion effect might also contribute to the

Fig. 4. Evolutions of zeta potential and particle size in the effluents for the systems of gibbsite colloids, HA, gibbsite-HA (20 mg L1), and gibbsite-U(VI)-HA (20 mg L1); pH ¼ 7.5, I ¼ 8.0  104 M.

larger particle size of the colloids in effluents at earlier breakthrough time (Sirivithayapakorn and Keller, 2003).

4. Conclusions In this study, we comparatively investigated individual and cotransport behaviors of U(VI), HA and gibbsite colloids as a function of pH and ionic strength. We found that gibbsite colloids (9 mg L1) impeded U(VI) (5  105 M) transport at relatively low pH and ionic strength, and even blocked U(VI) migration at neutral pH and/or high ionic strength. The presence of low concentration of HA (5 mg L1) did change apparently the impediment of gibbsite to U(VI) transport, whereas the presence of relatively high concentration of HA (20 mg L1) facilitated dramatically the transport of gibbsite and consequently the transport of U(VI) as well. Gibbsite colloids and HA always transported together in the coexistence system irrespective of U(VI) presence. The results imply that colloidal gibbsite formed from dissolved Al in acid mine drainage at uranium mining sites may benefit the remediation of uranium contamination unless the environment is enriched with humic substances.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11675070, U1730245, 21806064), the Natural Science Foundation of Gansu Province, China (No. 17JR5RA195), and Fundamental Research Funds for the Central Universities (No. lzujbky-2018-kb06).

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