Competitive sorption and selective sequence of Cu(II) and Ni(II) on montmorillonite: Batch, modeling, EPR and XAS studies

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Competitive sorption and selective sequence of Cu(II) and Ni(II) on montmorillonite: Batch, modeling, EPR and XAS studies Shitong Yang a,b, Xuemei Ren c, Guixia Zhao c, Weiqun Shi d, Gilles Montavon e, Bernd Grambow e, Xiangke Wang a,b,c,f,⇑ a School for Radiological and Interdisciplinary Sciences, Soochow University, 215123 Suzhou, PR China Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, PR China c School of Environment and Chemical Engineering, North China Electric Power University, Beijing 102206, PR China d Institute of High Energy Physics, Chinese Academy of Sciences, 100049 Beijing, PR China e Laboratory SUBATECH, UMR 6457 Ecole des Mines de Nantes/IN2P3-CNRS/Universite´ de Nantes, 4 rue A. Kastler, BP 20722, 44307 Nantes cedex 03, France f NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia b

Received 6 July 2014; accepted in revised form 17 June 2015; Available online 24 June 2015

Abstract Heavy metal ions that leach from various industrial and agricultural processes are simultaneously present in the contaminated soil and water systems. The competitive sorption of these toxic metal ions on the natural soil components and sediments significantly influences their migration, bioavailability and ecotoxicity in the geochemical environment. In this study, the competitive sorption and selectivity order of Cu(II) and Ni(II) on montmorillonite are investigated by combining the batch experiments, X-ray diffraction (XRD), electron paramagnetic resonance (EPR), surface complexation modeling and X-ray Absorption Spectroscopy (XAS). The batch experimental data show that the coexisting Ni(II) exhibits a negligible influence on the sorption behavior of Cu(II), whereas the coexisting Cu(II) reduces the Ni(II) sorption percentage and changes the shape of the Ni(II) sorption isotherm. The sorption species of Cu(II) and Ni(II) on montmorillonite over the acidic and near-neutral pH range are well simulated by the surface complexation modeling. However, this model cannot identify the occurrence of surface nucleation and the co-precipitation processes at a highly alkaline pH. Based on the results of the EPR and XAS analyses, the microstructures of Cu(II) on montmorillonite are identified as the hydrated free Cu(II) ions at pH 5.0, inner-sphere surface complexes at pH 6.0 and the surface dimers/Cu(OH)2(s) precipitate at pH 8.0 in the single-solute and the binary-solute systems. For the Ni(II) sorption in the single-solute system, the formed microstructure varies from the hydrated free Ni(II) ions at the pH values of 5.0 and 6.0 to the inner-sphere surface complexes at pH 8.0. For the Ni(II) sorption in the binary-solute system, the coexisting Cu(II) induces the formation of the inner-sphere complexes at pH 6.0. In contrast, Ni(II) is adsorbed on montmorillonite via the formation of Ni phyllosilicate co-precipitate/a-Ni(OH)2(s) precipitate at pH 8.0. The selective sequence of Cu(II) > Ni(II) for binding on montmorillonite can be ascribed to the differences in the metal properties and the compatibility between the configurations of the montmorillonite binding sites and those of the

⇑ Corresponding author at: School for Radiological and Interdisciplinary Sciences, Soochow University, 215123 Suzhou, PR China.

Tel.: +86 551 65592788; fax: +86 551 65591310. E-mail addresses: [email protected], [email protected] (X. Wang). http://dx.doi.org/10.1016/j.gca.2015.06.020 0016-7037/Ó 2015 Elsevier Ltd. All rights reserved.

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Cu(II)O6/Ni(II)O6 polyhedra. The derived findings in this study could provide significant information for the evaluation of the competitive sorption behaviors at solid/water interfaces and the fate of the coexisting heavy metal ions in multicomponent environmental systems. Ó 2015 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Severe pollution is present in the natural geological environment due to the increasing discharge of contaminants. For example, Cu(II) and Ni(II) along with other toxic components, such as Cd(II), Cr(VI), phosphate and arsenate, are simultaneously distributed in the soil and water systems. The sorption, diffusion, deposition and precipitation of these trace metals onto reactive mineral surfaces play a significant role in regulating their migration, transformation, accumulation, bioavailability and potential ecological harm (Morton et al., 2001; Da¨hn et al., 2002, 2003; Peacock, 2009; Sherman and Peacock, 2010; Yang et al., 2011; Moon and Peacock, 2012; Schlegel and Manceau, 2013). Numerous studies have proven that the co-existence of Cu(II) and Ni(II) in water systems exhibits a synergistic effect on their toxicity to the aquatic organisms (Prˇibyl et al., 2008; Paraszkiewicz et al., 2009; Svecevicˇius et al., 2012). Therefore, it is crucial to gain a better understand of the competitive sorption behaviors, the chemical speciation and the underlying mechanisms of the coexisting Cu(II) and Ni(II) in the geochemical systems. The macroscopic experimental approach has been widely used to study the competitive sorption behaviors of metal ions on natural clays, organo-mineral complexes, biomass and sediments, among others. In addition, various theoretical models, such as the distribution coefficient model, the surface complexation model and the sorption isotherm equations, have been widely used to simulate the experimental data and to further predict the possible chemical species formed during the competitive sorption processes (Christl and Kretzschmar, 1999; Reddad et al., 2002a; Bradbury and Baeyens, 2005a,b; Akafia et al., 2011; Selim and Zhang, 2013). For example, Abolino et al. (2003) reported that the sorption of Co(II), Ni(II) and Cu(II) on K-montmorillonite followed the selective order of Cu(II) > Ni(II) > Co(II), which is negatively correlated with their ionic radii. The selectivity sequence of Pb(II) > Cu(II) > Zn(II) > Ni(II) > Cd(II) for their sorption on Egyptian soils agreed with the magnitude of the first hydrolysis constants (Usman, 2008). The simple correlation and the multiple regression analysis between the values of the solid/liquid distribution coefficients and the physicochemical properties of the soil suggested that the observed selectivity sequence was dependent on the type, the amount and the cation exchange capacity of the tested soils. The derived results in the mentioned studies can provide useful information for predicting the physicochemical behaviors and the fate of Cu(II) and Ni(II) in the aquatic systems. Specifically, the macroscopic experiments performed at different ionic strength can efficiently differentiate the inner-sphere versus outer-sphere complexation. However,

it is worth noting that the macroscopic data are not capable of explicitly differentiating the inner-sphere complexation and the nucleation processes. The surface complexation models cannot explicitly confirm the concrete driving force for the preferential binding in the competitive sorption systems because of the use of multiple adjustable parameters during the fitting procedures. Therefore, a valid method used to gain insight into the underlying mechanisms of the selective sequence of Cu(II) and Ni(II) on minerals is to combine the macroscopic approach with the spectral analysis, as has been proven in previous studies (Qin et al., 2006; Flogeac et al., 2007; Stathi et al., 2010; Wang et al., 2013). Clay minerals, with montmorillonite as the primary component, are commonly used as an effective leachate barrier in the landfills of toxic heavy metal ions and radionuclides (Rajesh and Viswanadham, 2011; Divya et al., 2012). Montmorillonite, an expandable 2:1 type phyllosilicate mineral, is the major clay component of sedimentary rock formations, weathered continental soils and aquatic sediments. Because of its good swelling property, high chemical/mechanical stability, high specific surface area and fine cation exchange capacity (60–120 mmol/100 g), montmorillonite plays a significant role in the migration and fate of environmental contaminants in natural soil and water systems. Therefore, a series of studies have been performed to investigate the sorption behaviors of Cu(II) and Ni(II) on montmorillonite. In addition, some spectral analysis techniques such as the electron paramagnetic resonance (EPR) and X-ray Absorption Spectroscopy (XAS) have also been applied to deduce the underlying sorption mechanisms (Hyun et al., 2000; Morton et al., 2001; Da¨hn et al., 2002, 2003; Undabeytia et al., 2002; Strawn et al., 2004; Nachtegaal et al., 2005; Tan et al., 2011; Schlegel and Manceau, 2013). Generally, Cu(II) and Ni(II) tend to be adsorbed via ion exchange reaction over the acidic pH range and surface complexation over the near-neutral pH range. However, the sorption mechanism for Cu(II) binding on montmorillonite over the alkaline pH range is significantly different from that for Ni(II). The adsorbed species for Cu(II) are identified as surface multinuclear complexes (Hyun et al., 2000; Morton et al., 2001; Strawn et al., 2004; Schlegel and Manceau, 2013), whereas that for Ni(II) are recognized as Ni phyllosilicate co-precipitate and/or nickel hydroxide precipitate (Nachtegaal et al., 2005; Tan et al., 2011). In addition, the sorption mechanisms of Cu(II) and Ni(II) on montmorillonite are significantly dependent on the ionic strength and the initial concentration. Specifically, the hydrated ions are the main sorption species of Cu(II) at low ionic strength, whereas the inner-sphere and/or surface multinuclear complexes are the species at high ionic strength

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(Strawn et al., 2004; Schlegel and Manceau, 2013). The sorption mechanism of Ni(II) on montmorillonite changes from the inner-sphere complexation to the neoformed Ni phyllosilicate co-precipitate with increasing Ni(II) concentration (Da¨hn et al., 2002, 2003). Although the sorption behaviors and mechanisms of the individual Cu(II) and Ni(II) on montmorillonite have been fully understood, no study has involved the competitive sorption mechanisms of Cu(II) and Ni(II) at the montmorillonite/water interfaces by combining the batch technique and the spectral analysis. Because of the heterogeneity and complexity of real aquatic systems, the sorption behaviors and the corresponding chemical species of Cu(II) and Ni(II) on montmorillonite when these two metal ions are simultaneously present in the system would be different from those in the ideal single-solute systems. Therefore, batch experiments were conducted herein to investigate the sorption behaviors of Cu(II) and Ni(II) on montmorillonite in single-solute and binary-solute systems. In addition, the XRD, EPR, surface complexation modeling and XAS techniques were adopted to verify the chemical species and the sorption mechanisms of Cu(II) and Ni(II) on montmorillonite at the molecular level. Moreover, the environmental and geochemical significances of the present findings to evaluate the fate and bioavailability of Cu(II) and Ni(II) in aquatic systems were proposed based on the derived interaction mechanisms. 2. EXPERIMENTAL SECTION 2.1. Materials The montmorillonite powder was purchased from Zhejiang Sanding Group Co. Ltd (Shaoxing, Zhejiang, China). The specific surface area was measured as 64.4 m2/g using the N2-BET method. The cation exchange capacity (CEC) was determined as 110 mmol/100 g by using the ammonium acetate method. The analytically pure Cu(NO3)23H2O and Ni(NO3)26H2O were dissolved in Milli-Q water to obtain the Cu(II) and Ni(II) stock solutions. 2.2. Batch experiments All of the batch experiments were carried out in controlled N2 atmosphere glove boxes (CO2  2 ppm, O2  2 ppm). Briefly, the montmorillonite stock suspension, the NaNO3 background electrolyte solution and the Cu(II) or Ni(II) stock solutions were added to the Teflon centrifuge tubes. For the binary-solute sorption systems, the Cu(II) and Ni(II) stock solutions were premixed and then added to the centrifuge tubes to ensure a simultaneous reaction with montmorillonite. Specifically, the concentration of the NaNO3 electrolyte solution in the sorption experiments was maintained at 0.01 mol/L. The single-solute sorption experiments were performed with 1.0 g/L of the montmorillonite suspension, 10 mg/L of Cu(II) (i.e., 1.56  104 mol/L) or 10 mg/L of Ni(II) (i.e., 1.70  104 mol/L). The binary-solute sorption experiments were carried out with 1.0 g/L of the montmorillonite suspension and a total metal concentration of

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3.26  104 mol/L (i.e., 1.56  104 mol/L of Cu(II) and 1.70  104 mol/L of Ni(II)). In this study, the pH value of the Cu(II) or Ni(II) stock solution in the single-solute systems was different from that of their mixed solution in the binary-solute systems. A certain volume of Milli-Q water with a given pH (adjusted with the HNO3 or NaOH solutions) was added to each centrifuge tube to ensure that the sorption experiments were performed at the desired pH values. This approach eliminated the interference of the pH variations during the addition on the actual sorption behaviors of Cu(II) and Ni(II) on montmorillonite. The suspensions were gently oscillated for 24 h and then centrifuged at 7788g for 20 min to separate the solid phase from the aqueous phase. Afterwards, the final concentrations of Cu(II) and Ni(II) in the supernatants were analyzed by using the atomic absorption spectrophotometry. The sorption percentage (S% = (C0  Ce)/C0  100%) and the sorption amount (qe = (C0  Ce)V/m) of Cu(II) and Ni(II) on montmorillonite were calculated from the initial Cu(II) and Ni(II) concentrations (C0), the equilibrium Cu(II) and Ni(II) concentrations (Ce), the montmorillonite mass (m) and the suspension volume (V). 2.3. Surface complexation modeling Montmorillonite possesses two types of binding sites, i.e., the planar sites (for example, the exchangeable cations located in the interlayer that neutralize the permanent negative charge due to an isomorphous substitution, denoted as XNa) and the edge sites (AlOH and SiOH). The AlOH sites are amphoteric and can be neutral, positively charged or negatively charged due to the protonation or deprotonation reactions (AlOH + H+ M AlOH+ 2 and AlOH M AlO + H+), whereas the SiOH sites can only be neutral or negatively charged due to the deprotonation reaction (SiOH M SiO + H+) (Brady, 1994; Tombacz et al., 2004; Tertre et al., 2006a). For the montmorillonite sample used in the present study, the surface site density and intrinsic surface complexation constants were calculated by using the constant capacitance model (CCM) in FITEQL 3.2 and the obtained values were listed in Table 1. The metal ions in solution can attach to the planar sites of montmorillonite via the cation exchange reaction, or they can directly bind on the edge sites via the surface complexation reaction. Based on the parameters listed in Table 1, the pH-dependent sorption data of Cu(II) and Ni(II) on montmorillonite were simulated with the CCM in FITEQL 3.2. According to the previous modeling of the Zn(II) and Ni(II) sorption on montmorillonite (Bradbury and Baeyens, 1997, 1999), three reactions were considered in the present modeling procedure, i.e., 2XNa + Me2+ M X2Me + 2Na+, AlOH + Me2+ M AlOMe+ + H+ and SiOH + Me2+ M SiOMe+ + H+. Herein, Me2+ represents the Cu2+ or Ni2+ ions, X2Me represents the adsorbed Cu(II)/Ni(II) species via cation exchange reaction, and AlOMe+/SiOMe+ represents the adsorbed Cu(II)/Ni(II) species via surface complexation reaction. The electrostatic terms of the edge sites (i.e., AlOH and SiOH) caused by the

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Table 1 Surface site density and intrinsic surface complexation constants of montmorillonite calculated by using the CCM in FITEQL 3.2. SBET = 64.4 m2/g, m/V = 1.0 g/L, Capacitance = 0.74 F/m2, I = 0.01 mol/L NaNO3. Sites

Densities (lmol/m2)

Surface reactions

log K

XNa (exchange) AlOH (complexation)

10.96 6.68

SiOH (complexation)

13.70

XNa + H+ M XH + Na+ AlOH + H+ M AlOH+ 2 AlOH M AlO + H+ SiOH M SiO + H+

2.9 5.0 8.5 9.7

protonation/deprotonation reactions are not considered due to their negligible contribution to the total surface charge of montmorillonite (Bradbury and Baeyens, 1999). A previous study showed that the CuOH+/NiOH+ species derived from the first hydrolysis reactions of the Cu2+/Ni2+ ions in solution had a negligible effect on the sorption of Cu(II)/Ni(II) on the planar and edge sites (Undabeytia et al., 2002). Hence, the surface complexation modeling can be reasonably performed by only considering the interactions of the simplest Cu(II)/Ni(II) ions with the active sites of the montmorillonite. Specifically, the stability constants (log K) for the three adsorbed species (i.e., X2Me, AlOMe+ and SiOMe+) were optimized to produce the best model fits. The quality of the modeling results was evaluated by the weighted sum of the squares divided by the degrees of freedom (WSOS/DF). A good modeling fit to the sorption data is indicated by a value of WSOS/DF over the range of 0.1–20 (Peacock and Sherman, 2004). 2.4. EPR and XAS analyses The sorption samples for the EPR and XAS analyses were prepared using 250 mL vessels with 1.0 g/L of the montmorillonite suspension, 0.01 mol/L NaNO3, 10 mg/L of Cu(II) (i.e., 1.56  104 mol/L) and/or 10 mg/L of Ni(II) (i.e., 1.70  104 mol/L) at different pH values. Briefly, the single or mixed stock solutions of Cu(II) and Ni(II) were slowly added to the montmorillonite suspension under constant stirring to avoid the aggregation of metal ions. This procedure can exclude the formation of bulk hydroxide precipitate during the metal addition. Hence, we can observe the actual sorption behaviors and chemical species of Cu(II) and Ni(II) on montmorillonite. The pH of the mixture was maintained at the desired value during the addition of the Cu(II) and/or Ni(II) stock solutions. The samples were shaken on a rotating oscillator for 24 h and then centrifuged at 7788g for 20 min. The collected wet pastes were then wrapped in a moist paper towel and sealed in a Ziploc bag. To minimize further interactions prior to the measurements, the Ziploc bag was stored in a sealed glass jar and placed in cold (3 °C) storage. Because the aging time has a significant influence on the types of sorption species, the EPR and XAS data collections were performed less than 16 h after centrifugation. The EPR spectra of the pure montmorillonite and Cu(II)-containing sorption samples were recorded at room temperature using an EMX X-band spectrometer (Bruker, Germany) with a microwave frequency of 9.3 GHz. The Cu and Ni K-edge XAS spectra were recorded at the BL14W1 of the Shanghai Synchrotron Radiation Facility

(SSRF, China). The electron beam energy was 3.5 GeV with a mean stored current of 300 mA. The X-ray energy was tuned using a double-crystal Si(1 1 1) monochromator. The high-order harmonics were suppressed by detuning the monochromator by 25%. The XAS signals of the samples were collected with a multi-element high purity Ge solid-state detector. Triplicate scans were collected and averaged to increase the signal-to-noise ratio of the XAS spectra. The Athena and Artemis software were used to perform the energy correction, the fluorescence dead time calibration and the data fitting. The theoretical scattering phases and their corresponding amplitudes were calculated by using the FEFF7 program based on the crystal parameters of Cu(NO3)2(aq), Cu(OH)2(s), Cu(CH3COO)24H2O, Ni(NO3)2(aq), Ni(OH)2(s) and Ni(CH3COO)24H2O (Ankudinov and Rehr, 1997). In this study, the Cu(OH)2(s) and Ni(OH)2(s) samples were freshly prepared precipitates through the reaction of CuSO45H2O/NiSO46H2O with the NaOH solutions followed by filtration. During the fitting, the amplitude reduction factor was fixed to 0.90 for the Cu(II)-containing samples and 0.85 for the Ni(II)-containing samples to reduce the number of adjustable parameters. These two values were obtained by fitting the spectra of the Cu(OH)2(s) and Ni(OH)2(s) reference samples to the theoretical scattering shells. The edge shift (E0) values for the first and the second coordination shells were constrained to be equal. The Debye–Waller factors (r2) for the first coordination shells were set as adjustable parameters, whereas those for the second coordination shells were fixed equal to that of ˚ 2) for the Cu(II)-containing samples Cu(OH)2(s) (0.008 A ˚ 2) and that of Ni(OH)2(s) (0.007 A for the Ni(II)-containing samples. These values were consistent with those used by other researchers to fit the second coordination shells of the Cu(II) and Ni(II) sorption samples (Farquhar et al., 1996; Cheah et al., 1998, 2000; Morton et al., 2001; Sheng et al., 2011; Yang et al., 2011). In addition, the restriction used in this study was rational because of the strong correlation of the atom backscattering in the second shells (Mandaliev et al., 2010). The accuracies of the R and CN values for the first coordination shells were ˚ and 20%, respectively, and those for the second 0.02 A ˚ and 40%, respectively. coordination shells were 0.03 A 3. RESULTS 3.1. Macroscopic data Fig. 1 shows the sorption edges of Cu(II) and Ni(II) over the pH range of 2–10 for the single-solute and binary-solute systems. No clear difference can be observed between the

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Fig. 1. Sorption of Cu(II) and Ni(II) on montmorillonite for single-solute and binary-solute systems in the pH range of 2–10. T = 298 ± 1 K, m/V = 1.0 g/L, CCu(II)initial = 1.56  104 mol/L, CNi(II)initial = 1.70  104 mol/L, I = 0.01 mol/L NaNO3. Note that all the experimental data are the average of triplicate measurements and the error bars are much smaller than the data points.

sorption curves of Cu(II) in the binary-solute and the single-solute systems (Fig. 1A), suggesting the weak competition effect of the coexisting Ni(II) on the Cu(II) sorption. In contrast, the sorption curve of Ni(II) in the binary-solute system is clearly lower than that of Ni(II) in the single-solute system at pH < 7.0 (Fig. 1B), suggesting the significant competition effect of the coexisting Cu(II) on the Ni(II) sorption behaviors under acidic and near-neutral conditions. The sorption curves of Ni(II) in the binary-solute system overlap well with those in the single-solute system at pH > 7.0. This phenomenon suggests that the coexisting Cu(II) has no influence on the macroscopic sorption behavior of Ni(II) on montmorillonite under alkaline conditions. The pH1/2 values (at which 50% of the initial metal ion is retained) are 4.9 for Cu(II) and 6.8 for Ni(II). These values indicate that Cu(II) has a higher affinity than Ni(II) for sorption on montmorillonite (Boudesocque et al., 2007; Flogeac et al., 2007). The sorption isotherms of Cu(II) and Ni(II) in the single-solute and binary-solute systems are shown in Fig. 2. It is clear that the sorption isotherms of Cu(II)

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Fig. 2. Sorption isotherms and the Langmuir model fitting for Cu(II) and Ni(II) on montmorillonite in single-solute and binarysolute systems. Symbols denote experimental data, solid lines represent the fitting curves of single-solute systems and dash lines represent the fitting curves of binary-solute systems. T = 298 ± 1 K, pH = 6.0 ± 0.1, m/V = 1.0 g/L, I = 0.01 mol/L NaNO3. Note that all the experimental data are the average of triplicate measurements and the error bars are much smaller than the data points.

and Ni(II) in the single-solute systems reveal the typical L shape with an asymptote at a higher equilibrium concentration. In this study, the L-type sorption isotherms exclude the occurrence of the surface co-precipitation, which is expected to induce a continuous increase in the sorption amount with an increasing equilibrium concentration. Note that the sorption isotherm of Cu(II) in the binary-solute system nearly overlaps with that in the single-solute system (Fig. 2A). This phenomenon indicates that the simultaneous presence of Ni(II) has a negligible influence on the Cu(II) sorption behavior. However, the sorption isotherm of Ni(II) in the binary-solute system shows a stepped shape with two consecutive L-type curves (denoted as phase 1 and phase 2, respectively) (Fig. 2B), which is different from the typical L shape for the single-solute system. This variation suggests that the simultaneous presence of Cu(II) significantly changes the sorption behavior of Ni(II) on montmorillonite. max C e The Langmuir model (qe ¼ bq1þbC ) is adopted to simulate e the sorption isotherms of Cu(II) and Ni(II) in the single-solute and binary-solute systems (Fig. 2). In this

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equation, Ce is the residual concentration of Cu(II)/Ni(II) in solution (mol/L), qe is the sorption amount of Cu(II)/Ni(II) on montmorillonite after equilibrium (mol/g), qmax is the maximum sorption amount of Cu(II)/Ni(II) on montmorillonite at the complete monolayer coverage (mg/g) and b (L/mol) is a parameter related to the sorption heat. Specifically, the two phases of the Ni(II) sorption isotherms in the binary-solute system were separately simulated with the Langmuir model (Fig. 2). It can be concluded from the high correlation coefficient values listed in Table S1 (Electronic Annex) that the Langmuir model simulates the sorption experimental data well. This phenomenon suggests a monolayer coverage of Cu(II) and Ni(II) on montmorillonite. Therefore, the adsorbed Cu(II) and Ni(II) do not interact, excluding the occurrence of surface polymerization and co-precipitation at pH 6.0. In this study, the agreement of the experimental data with the Langmuir model also indicates that chemisorption is the primary driving force for the Cu(II) and Ni(II) sorption on montmorillonite (Zhou et al., 2009a). The analysis of the derived qmax values (Electronic Annex, Table S1) confirms the higher selectivity of montmorillonite towards the Cu(II) species in solution. Detailed information for the qmax analysis procedure is described in the Electronic Annex (see section EA1).

Fig. 4. EPR spectra of pure montmorillonite, Cu(NO3)2(aq) reference sample and Cu(II)-containing sorption samples prepared in single-solute and binary-solute systems. T = 298 ± 1 K, pH = 6.0 ± 0.1, m/V = 1.0 g/L, CCu(II)initial = 1.56  104 mol/L, CNi(II)initial = 1.70  104 mol/L, I = 0.01 mol/L NaNO3.

3.2. XRD patterns To help deduce the sorption mechanisms of Cu(II) and Ni(II) on montmorillonite, the XRD patterns of pure montmorillonite and the sorption samples prepared in the single-solute systems (denoted as montmorillonite/Cu(II) and montmorillonite/Ni(II)) and the binary-solute systems (denoted as montmorillonite/Cu(II)/Ni(II)) at pH 6.0 were collected and compared in detail. As shown in Fig. 3, the XRD pattern of pure montmorillonite exhibits a characteristic (0 0 1) diffraction plane at 2h = 6.22° with a corre˚ . The presence of the sponding basal spacing of 14.21 A (0 6 0) diffraction plane at 2h = 61.9° indicates a

dioctahedral structure. The (0 0 1) diffraction plane of the montmorillonite/Cu(II), montmorillonite/Ni(II) and montmorillonite/Cu(II)/Ni(II) sorption samples appears at a lower diffraction angle of 2h = 6.04°, corresponding ˚ . Herein, the slight to a broader basal spacing of 14.62 A broadening of the basal spacing after Cu(II) and Ni(II) sorption may be attributed to the greater hydrated ionic ˚ ) and Ni(II) (4.04 A ˚ ) ions relative radius of Cu(II) (4.19 A ˚ ) (Volkov et al., to that of the original Na+ ions (3.58 A 1997). 3.3. EPR spectra

Fig. 3. XRD patterns of pure montmorillonite, montmorillonite/ Ni(II), montmorillonite/Cu(II) and montmorillonite/Cu(II)/Ni(II) samples. T = 298 ± 1 K, pH = 6.0 ± 0.1, m/V = 1.0 g/L, CCu(II)initial = 1.56  104 mol/L, CNi(II)initial = 1.70  104 mol/L, I = 0.01 mol/L NaNO3.

Fig. 4 illustrates the EPR spectra of pure montmorillonite, the Cu(NO3)2(aq) reference sample and the Cu(II)-containing sorption samples. The spectrum of pure montmorillonite exhibits a single resonance feature at a g value of 2.004 (denoted as ge, see the dashed line), which corresponds to the free-radical resonance commonly present in clay minerals (Strawn et al., 2004). In contrast, a single isotropic absorption line at a g value of 2.185 (see the dotted line) appears in the EPR spectrum of the Cu(NO3)2(aq) reference sample. In this study, the EPR spectra show that the adsorbed Cu(II) species on montmorillonite clearly change in the coordination environment with increasing pH values. For the single-solute system, the EPR spectrum of the montmorillonite/Cu(II) sample prepared at pH 5.0 appears similar to that of the Cu(NO3)2(aq) reference sample. The montmorillonite/Cu(II) sample prepared at pH 6.0 exhibits a reduced intensity for the isotropic resonance peak at g  2.185 compared with that prepared at pH 5.0. In addition, the spectrum shows a distinct anisotropic

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rigid-limit at g values of 2.07 and 2.35. The spectrum of the montmorillonite/Cu(II) sample prepared at pH 8.0 has none of the Cu(II)-induced resonance features of the montmorillonite/Cu(II) samples prepared at pH 5.0 and 6.0. Alternatively, the spectrum appears nearly the same as that of pure montmorillonite with a single resonance feature at g  2.004. No obvious difference can be observed between the spectra of the montmorillonite/Cu(II)/Ni(II) samples prepared in the binary-solute systems at pH values of 5.0, 6.0 and 8.0 and those of the montmorillonite/Cu(II) samples prepared at the corresponding pH values in the single-solute systems. 3.4. Surface complexation modeling The sorption speciation of Cu(II) and Ni(II) on montmorillonite can be determined from the surface complexation modeling. As observed from Figs. 5 and 6, the specific sorption species are highly dependent on the solution pH. For the sorption of Cu(II) in the single-solute system (Fig. 5A), the X2Cu species is dominant at pH < 5.5, suggesting that Cu(II) enters the interlayer location of montmorillonite via a cation exchange reaction over this pH range. The SiOCu+ and AlOCu+ species become

Fig. 6. Model fitting for the sorption of Ni(II) in the single-solute (A) and binary-solute (B) systems. T = 298 ± 1 K, m/V = 1.0 g/L, CCu(II)initial = 1.56  104 mol/L, CNi(II)initial = 1.70  104 mol/L, I = 0.01 mol/L NaNO3.

Fig. 5. Model fitting for the sorption of Cu(II) in the single-solute (A) and binary-solute (B) systems. T = 298 ± 1 K, m/V = 1.0 g/L, CCu(II)initial = 1.56  104 mol/L, CNi(II)initial = 1.70  104 mol/L, I = 0.01 mol/L NaNO3.

dominant at pH > 5.5, reflecting the occurrence of inner-sphere surface complexation. Specifically at pH 6.0, the adsorbed Cu(II) (86%) is composed of 41% of the SiOCu+ species, 28% of the AlOCu+ species and 17% of the X2Cu species. The calculated Cu(II) sorption species and their relative proportions in the binary-solute system (Fig. 5B) are relatively similar to those in the single-solute system (Fig. 5A). This comparability suggests that the presence of Ni(II) has no obvious influence on the sorption species of Cu(II) on montmorillonite. For the sorption of Ni(II) in the single-solute system (Fig. 6A), the X2Ni species is dominant at pH < 6.5, suggesting that the cation exchange reaction is the primary sorption mechanism over this pH range. Specifically at pH 6.0, the adsorbed Ni(II) (36%) is composed of 31% of the X2Ni species and 5% of the AlONi+ species. This modeling result shows that Ni(II) is primarily adsorbed via the cation exchange/outer-sphere complexation (86%) with a small contribution of inner-sphere complexation (14%). In contrast, inner-sphere surface complexation becomes the predominant driving force for the Ni(II) sorption on montmorillonite at pH > 6.5, as reflected by the calculated SiONi+ and AlONi+ species. For the sorption of Ni(II) in the binary-solute system (Fig. 6B), the experimental data can also be well simulated

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using the combination of the X2Ni, AlONi+ and SiONi+ species. However, the relative proportions of the sorption species are different from those in the single-solute system (Fig. 6A). Specifically at pH 6.0, the adsorbed Ni(II) (23%) is composed of 11% of the X2Ni species, 9% of the AlONi+ species and 3% of the SiONi+ species. This fitting result indicates that 50% of the adsorbed Ni(II) enters the interlayer of montmorillonite, whereas the remaining 50% directly forms inner-sphere surface complexes with the edge sites. The differences between the single-solute system and the binary-solute system suggest that the coexisting Cu(II) partly altered the sorption species of Ni(II) on montmorillonite. As observed from Figs. 5 and 6, the relative contribution of the SiOCu+/SiONi+ species to the Cu(II)/Ni(II) sorption is higher than that of the AlOCu+/AlONi+ species. This phenomenon is attributed to the higher density of the SiOH edge sites on the montmorillonite surfaces (Tertre et al., 2006a). However, the AlOH edge sites are reported as more reactive than the SiOH edge sites (Tertre et al., 2006b), which corresponds to the higher affinity of the former towards the Cu(II)/Ni(II) binding (Table 2). The derived WSOS/DF values are over the range of 0.1–20 (Table 2), suggesting that the CCM can well simulate the experimental data of the Cu(II) and Ni(II) sorption on montmorillonite (Sun et al., 2014). 3.5. XANES spectra The normalized XANES spectra can provide direct evidence for the identification of the oxidation state of the metal ions. In this study, the spectral characteristics of the Cu(II)-containing samples (Electronic Annex, Figure S1A) suggest that Cu is present in the +2 oxidation state (Palomino et al., 2000; Furnare et al., 2005a; Strawn and Baker, 2009; Tian et al., 2009). To further identify the detailed microstructures of Cu(II) in the sorption samples, the normalized XANES spectra were processed to produce their first derivative curves (Electronic Annex, Figure S1B). Specifically, the first derivative curves show two peaks (a and b) over the range of 8985–8996 eV (Electronic Annex, Figure S1B). The splitting of the a and b peaks (denoted as the energy separation, DE) reveals a tetragonal distortion of the Cu(II)O6 octahedra (Frenkel et al., 2000; Dupont et al., 2002; Boudesocque et al., 2007). The DE value is qualitatively related to the discrepancy between the equatorial Cu–O (RCu–Oeq) and the axial Cu–O bond distances (RCu–Oax) in the tetragonal-distorted Cu(II)O6 octahedra (Frenkel et al., 2000). According to the data presented in the previous studies (Dupont et al., 2002; Boudesocque et al., 2007), the RCu–Oax values for the sorption samples prepared at pH 5.0, 6.0 and 8.0 are ˚ , 2.36 A ˚ and 2.53 A ˚ , respectively. calculated as 2.26 A Detailed information on the calculation procedure of these RCu–Oax values is described in the Electronic Annex (see section EA2). The normalized K-edge XANES spectra (Electronic Annex, Figure S2A) and the corresponding first derivative curves (Electronic Annex, Figure S2B) of the

Ni(II)-containing samples suggest that Ni is present in the +2 oxidation state (Xia et al., 1997). Specifically, the adsorbed Ni(II) is present in an octahedral coordination environment. Given that no obvious differences can be observed from the XANES spectra, the specific binding type and the corresponding microstructure were identified in the EXAFS analysis procedures to further determine the underlying sorption mechanisms. 3.6. EXAFS spectra of the Cu(II)-containing samples Fig. 7A shows the k3-weighted EXAFS spectra of the Cu(II) reference and sorption samples at different pH values. For the binary-solute systems, the spectra of the montmorillonite/Cu(II)/Ni(II) samples prepared at the pH values of 5.0, 6.0 and 8.0 are exactly similar to those of the montmorillonite/Cu(II) samples prepared at the corresponding pH values in the single-solute systems (Fig. 7A). This phenomenon indicates that the coexisting Ni(II) has no influence on the coordination environment of Cu(II) on montmorillonite over a wide pH range. Specifically, the spectrum of the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 5.0 is similar to that of the Cu(NO3)2(aq) reference sample. As shown in Fig. 7A, the EXAFS spectral features undergo obvious changes with increasing pH val˚ 1 ues. The oscillation at 3.9 A for the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 5.0 ˚ 1 for the slightly shifts to a higher k position of 4.2 A samples prepared at pH 6.0 and 8.0 (see the dashed line). ˚ 1 appear In addition, two beat features at 5.3 and 7.5 A in the spectra of the montmorillonite/Cu(II)/Ni(II) samples prepared at pH 6.0 and 8.0 (marked by the gray and red circles1), suggesting the presence of heavy backscattering atoms from higher coordination shells. Specifically, the spectrum of the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 8.0 is relatively similar to that of the Cu(OH)2(s) reference sample, indicating the potential formation of the Cu(OH)2(s) precipitate. Additionally, the ˚ 1 (marked appearance of a new oscillation peak at 4.4 A by the green circle) implies the occurrence of an additional coordination mode. Fourier transformation was performed to gain the corresponding radial structural functions (RSFs) for the typical oscillation features in the k3-weighted EXAFS spectra. As shown in Fig. 7B, the RSF for the sorption sample prepared at pH 5.0 exhibits a single peak with a high intensity at ˚ (uncorrected for the phase shift), which arises from 1.50 A the signal of the O atoms in the first shell. In contrast, this ˚ main peak slightly shifts to a lower R position of 1.43 A for the samples prepared at the higher pH values of 6.0 and 8.0 (see the dashed line in Fig. 7B). This variation suggests a shorter Cu–O bond distance and a potential change in the Cu(II) binding modes on montmorillonite. An addi˚ , implytional peak appears over the R range of 2.20–3.00 A ing the presence of higher coordination shells (e.g., Cu–Al, Cu–Si and/or Cu–Cu). More specifically, the position of the mentioned peak exhibits a slight shift towards a higher R 1 For interpretation of color in Fig. 7A, the reader is referred to the web version of this article.

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Table 2 Surface complexation modeling parameters for Cu(II) and Ni(II) sorption in the single-solute and binary-solute systems by using the CCM in FITEQL 3.2. Capacitance = 0.74 F/m2, I = 0.01 mol/L NaNO3. Sorption conditions

Surface reactions

Sorption species

Cu(II) Single-solute systems

2XNa + Cu2+ M X2Cu + 2Na+ AlOH + Cu2+ M AlOCu+ + H+ SiOH + Cu2+ M SiOCu+ + H+

X2Cu AlOCu+ SiOCu+

6.351 0.521 7.209

5.624

Cu(II) Binary-solute systems

2XNa + Cu2+ M X2Cu + 2Na+ AlOH + Cu2+ M AlOCu+ + H+ SiOH + Cu2+ M SiOCu+ + H+

X2Cu AlOCu+ SiOCu+

6.343 0.504 7.448

4.576

Ni(II) Single-solute systems

2XNa + Ni2+ M X2Ni + 2Na+ AlOH + Ni2+ M AlONi+ + H+ SiOH + Ni2+ M SiONi+ + H+

X2Ni AlONi+ SiONi+

5.925 1.082 10.13

4.925

Ni(II) Binary-solute systems

2XNa + Ni2+ M X2Ni + 2Na+ AlOH + Ni2+ M AlONi+ + H+ SiOH + Ni2+ M SiONi+ + H+

X2Ni AlONi+ SiONi+

5.304 2.045 10.22

4.479

value as the pH increases from 6.0 to 8.0. This phenomenon can be regarded as direct evidence for the pH-dependent variation of the Cu(II) sorption process. The RSFs for the Cu(II)-containing samples were seriatim fitted with the least-square approach, and the obtained parameters are listed in Table 3. The central Cu atom of the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 5.0 is coordinated with 4.1 O atoms at an equatorial bond dis˚ and 1.8 O atoms at an axial tance (RCu–Oeq) of 1.97 A ˚ in the first shell. bond distance (RCu–Oax) of 2.23 A These structural parameters are similar to those of the Cu(NO3)2(aq) reference sample (Table 3). Herein, the deter˚ is relatively close to that mined RCu–Oax value of 2.23 A ˚ ). The RSF deduced from the XANES analysis (2.26 A of the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 6.0 can be properly fitted using the combination of two Cu–O and two Cu–Al/Si backscattering paths, i.e., ˚ , 1.8 O at RCu–Oax  2.38 A ˚, 4.0 O at RCu–Oeq  1.95 A ˚ 1.8 Al/Si at RCu–Al/Si1  2.93 A and 1.9 Al/Si at ˚ . The presence of neighboring Al/Si RCu–Al/Si2  3.15 A atoms in the second shell implies the conceivable formation of inner-sphere complexes. For the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 8.0, an overall good fit can be achieved by combining two Cu–O, a Cu–Cu and a Cu–Al/Si backscattering paths, ˚ , 2.0 O at RCu– i.e., 3.8 O at RCu–Oeq  1.94 A ˚ ˚ Oax  2.52 A, 1.3 Cu at RCu–Cu  2.98 A and 1.2 Al/Si ˚. at RCu–Al/Si  3.14 A 3.7. EXAFS spectra of the Ni(II)-containing samples Fig. 8A shows the k3-weighted EXAFS spectra of the Ni(II)-containing reference and sorption samples at different pH values. For the binary-solute systems, the spectrum of the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 5.0 is similar to that of the montmorillonite/Ni(II) sample prepared at pH 5.0. This phenomenon suggests that the central Ni atoms in the two samples are located in a similar local environment. The spectrum of the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 6.0 exhibits the oscillation features of the heavy backscattering

log K

WSOS/DF

˚ 1 marked by the gray and red atoms (i.e., 5.3 and 7.5 A 2 circles ), which are not present in that of the corresponding montmorillonite/Ni(II) sample. For the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 8.0, ˚ 1 the oscillation features over the k range of 7.5–9.0 A are significantly different from those for the montmorillonite/Ni(II) sample prepared at pH 8.0. Alternatively, the spectrum is analogous to that for the Ni phyllosilicate co-precipitate (Ren et al., 2013), whereas it is distinctly different from that for the Ni–Al LDH co-precipitate (Da¨hn et al., 2002). The differences presented in this study suggest that the coexisting Cu(II) significantly changes the coordination environment and microstructure of Ni(II) on montmorillonite under near-neutral (e.g., pH 6.0) and alkalescent (e.g., pH 8.0) conditions. Fourier transformation was performed to gain the corresponding RSFs for the typical oscillation features in the k3-weighted EXAFS spectra. As shown in Fig. 8B, the RSFs for all of the sorption samples exhibit a single peak ˚ (uncorrected for the with a high intensity at 1.60 A phase shift), which arises from the signal of the O atoms in the first coordination shell. An additional peak ˚ for the appears over the R range of 2.20–3.30 A montmorillonite/Cu(II)/Ni(II) samples prepared at pH 6.0 and 8.0, which indicates the presence of higher coordination shells (e.g., Ni–Al/Si/Ni). More specifically, the intensity of the second peak over the range of 2.20– ˚ for the sample prepared at pH 8.0 is higher than 3.30 A that for the sample prepared at pH 6.0, suggesting a more significant contribution from the heavy backscattering atoms at the higher pH value. The RSFs were fitted in a series with the least-square approach, and the obtained parameters are listed in Table 3. For the binary-solute systems, the structural ˚ , CNNi–O  5.9) for the parameters (RNi–O  2.05 A montmorillonite/Cu(II)/Ni(II) sample prepared at pH 5.0 are similar to those for the montmorillonite/Ni(II) sample prepared at pH 5.0. In contrast, the structural parameters 2 For interpretation of color in Fig. 8A, the reader is referred to the web version of this article.

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Fig. 7. The k3-weighted EXAFS spectra and the corresponding RSF magnitudes (uncorrected for phase shift) of Cu(II)-containing reference and sorption samples prepared in single-solute and binary-solute systems. (A) Solid lines represent the experimental k3-weighted EXAFS spectra; (B) Symbols represent the experimental RSF magnitudes and solid lines represent the spectral fits. T = 298 ± 1 K, m/V = 1.0 g/L, CCu(II)initial = 1.56  104 mol/L, CNi(II)initial = 1.70  104 mol/L, I = 0.01 mol/L NaNO3.

for the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 6.0 and 8.0 are significantly different from those for the montmorillonite/Ni(II) samples prepared in the single-solute systems. Specifically, the montmorillonite/ Cu(II)/Ni(II) sample prepared at pH 6.0 contains 0.7 ˚ , 0.8 Al/Si at RNi– Al/Si at RNi–Al/Si1  3.00 A ˚ ˚ . The  3.11 A and 1.1 Al/Si at RNi–Al/Si3  3.26 A Al/Si2 second coordination shell of the montmorillonite/ Cu(II)/Ni(II) sample prepared at pH 8.0 includes 2.4 Ni ˚ and 2.8 Al/Si at RNi–Al/Si  3.26 A ˚. at RNi–Ni  3.05 A 4. DISCUSSION 4.1. Sorption behaviors of Cu(II) at the montmorillonite/ water interfaces The macroscopic sorption edges indicate that the coexisting Ni(II) has a negligible influence on the Cu(II) sorption behavior (Fig. 2A). The microscopic results provide powerful evidence for the macroscopic sorption behaviors (Figs. 1A and 2A). Specifically, the EPR analysis (Fig. 4), the surface complexation modeling (Fig. 5) and the EXAFS spectra (Fig. 7A) as well as the derived structural parameters (Table 3) indicate that the coexisting Ni(II) has no interference on the microstructure of Cu(II) at the montmorillonite/water interfaces over a wide pH range. The reasons for this phenomenon have been analyzed and discussed in detail for the three pH values applied in this study (i.e., 5.0, 6.0 and 8.0). At pH 5.0, the XRD analysis (Fig. 3), the EPR spectrum (Fig. 4) and the surface complexation modeling (Fig. 5) indicate that the Cu(II) ions are adsorbed by entering the

montmorillonite interlayer. Specifically, this sorption behavior is driven by the ion exchange reaction between the hydrated Cu2+ ions and the exchangeable Na+ ions located in the interlayer (Fig. 9A). The conclusion herein is supported by the comparability between the EXAFS-derived structural parameters of the sorption sample and those of the Cu(NO3)2(aq) reference sample (Table 3). In a previous study, Auboiroux et al. (1998) have reported that the Vanselow selectivity coefficient for the Cu2+/Ca2+ exchange reaction (2.42) is clearly higher than that for the Ni2+/Ca2+ exchange reaction (1.48). In view of this, the Cu(II) ions are expected to more preferentially enter the montmorillonite interlayer compared with the Ni(II) ions. At pH 6.0, the slight broadening of the basal spacing (see the XRD patterns shown in Fig. 3) indicates that a certain amount of Cu(II) ions have entered the interlayer of the montmorillonite crystal structure. This deduction is further supported by the EPR spectral analysis (Fig. 4) and the derived X2Cu species from the surface complexation modeling (Fig. 5). However, the decrease in the EPR signal intensity at g  2.185 and the appearance of a distinct anisotropic rigid-limit (g  2.07 and 2.35) (Fig. 4) provides evidence for the occurrence of the inner-sphere complexation at pH 6.0 (Hyun et al., 2000; Martı´nez and Mcbride, 2000). In addition, the hyperfine lines with the g value of 2.35 indicate that a portion of the Cu(II) ions may bind on the vacant sites of the montmorillonite octahedron sheet (He et al., 2000). Furthermore, the surface complexation modeling (Fig. 5) and the EXAFS analysis (Fig. 7 and Table 3) also suggest that the inner-sphere complexation is the driving force for Cu(II) binding on montmorillonite

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Table 3 Structural parameters derived from EXAFS analysis for Cu(II) and Ni(II) reference and sorption samples prepared in single-solute and binary-solute systems. Sample conditions

Cu(II) Reference samples

First shell (M–O) ˚) ˚ 2) R (A CN r2 (A Cu(NO3)2(aq)

4.1 1.8 3.9 1.9

0.003 0.004 0.004 0.005

1.96 2.24 1.94 2.39 1.93 2.51

4.0 1.9 4.1 1.7 3.9 1.9

0.004 0.003 0.005 0.004 0.003 0.005

1.97 2.23 1.95 2.38 1.94 2.52

4.1 1.8 4.0 1.8 3.8 2.0

0.004 0.003 0.005 0.003 0.004 0.005

Cu–Al/Si1 Cu–Al/Si2 Cu–Cu Cu–Al/Si

2.93 3.15 2.98 3.14

1.8 1.9 1.3 1.2

0.008 0.008 0.008 0.008

Ni(NO3)2(aq) Ni(OH)2(s)

2.05 2.04

5.9 5.8

0.004 0.005

Ni–Ni

3.11

6.0

0.007

pH 5.0 pH 6.0 pH 8.0

2.04 2.05 2.03

6.1 5.9 5.7

0.004 0.003 0.004

pH 5.0 pH 6.0

2.05 2.04

5.9 5.7

0.005 0.003

pH 8.0

2.03

5.8

0.005

pH 5.0 pH 6.0 pH 8.0

Cu(II) Binary-solute systems

pH 5.0 pH 6.0 pH 8.0

Ni(II) Reference samples Ni(II) Single-solute systems

Ni(II) Binary-solute systems

% Res ˚ 2) r2 (A

1.97 2.25 1.94 2.62

Cu(OH)2(s) Cu(II) Single-solute systems

Second shells (M–Al/Si/M) ˚) Bond R (A CN

6.4 Cu–Cu

2.97

2.4

0.008

5.7 4.8

Cu–Al/Si1 Cu–Al/Si2 Cu–Cu Cu–Al/Si

2.92 3.14 2.98 3.15

1.7 2.1 1.4 1.3

0.008 0.008 0.008 0.008

6.8 5.2 6.7

Ni–Al/Si1 Ni–Al/Si2 Ni–Al/Si3

3.00 3.10 3.27

1.6 2.1 2.4

0.007 0.007 0.007

Ni–Al/Si1 Ni–Al/Si2 Ni–Al/Si3 Ni–Ni Ni–Al/Si

3.00 3.11 3.26 3.05 3.26

0.7 0.8 1.1 2.4 2.8

0.007 0.007 0.007 0.007 0.007

5.8 5.5 4.2 6.8 7.7 6.2 8.1

5.8 4.7

6.3

Cu(NO3)2(aq), Cu(OH)2(s), Ni(NO3)2(aq) and Ni(OH)2(s) are named as reference samples, and the other samples are named as sorption samples. R – Bond distance, CN – Coordination number, r2 – Debye–Waller factor, Res – a measure of the agreement between experimental and theoretical EXAFS curves. The r2 values in italic were constrained to remain constant during the data analysis.

at pH 6.0. Specifically, the shorter Cu–Al/Si bond at ˚ (Table 3) corresponds with Cu(II) RCu–Al/Si1  2.93 A binding on the Al(O,OH)6 octahedra in an edged-shared mode (Fig. 9B) (Schlegel and Manceau, 2013). The longer ˚ correspond with Cu–Al/Si bond at RCu–Al/Si  3.15 A Cu(II) binding on the (Al/Si)(O,OH)4 tetrahedra in a corner-shared mode (Fig. 9B) (Bargar et al., 1997; Cheah et al., 1998; Kumar et al., 2012). Detailed information on the identification of Cu(II) microstructure is described in the Electronic Annex (see section EA3). At pH 8.0, the complete loss of the Cu(II)-induced resonance features in the EPR spectrum (Fig. 4) points to the formation of the multinuclear Cu(II) complexes (Weesner and Bleam, 1997; Hyun et al., 2000; Strawn et al., 2004). However, the surface complexation modeling suggests the formation of the mononuclear SiOCu+ and AlOCu+ species at pH 8.0 (Fig. 5). The discrepancy herein arises from the fact that the surface complexation modeling cannot explicitly distinguish between the inner-sphere surface complexes, the multinuclear complexes and the

co-precipitate. Therefore, the specific sorption species and the corresponding microstructure of Cu(II) on montmorillonite can be further identified from the EXAFS analysis. Specifically, the presence of the neighboring Cu atoms at ˚ and the neighboring Al/Si atoms at RCu– RCu–Cu  2.98 A ˚ Al/Si value of 3.14 A (Table 3) indicates that Cu(II) tends to form surface polynuclear complexes (Fig. 9C) and/or the hydroxide precipitates at pH 8.0. A detailed discussion on the formation of these two phases is provided in the Electronic Annex (see section EA3). The formation of the Cu(II) dimers and/or the Cu(OH)2(s) precipitate does not agree with the inner-sphere complexes (i.e., the AlOCu+ and SiOCu+ species), as indicated by the surface complexation modeling (Fig. 5A). As previously mentioned, the surface complexation modeling cannot provide an estimate of the surface nucleation and co-precipitation processes. Therefore, the sorption species derived from the EXAFS analysis can provide an important supplement and a correction for the surface complexation modeling under alkaline conditions.

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Fig. 8. The k3-weighted EXAFS spectra and the corresponding RSF magnitudes (uncorrected for phase shift) of Ni(II)-containing reference and sorption samples prepared in single-solute and binary-solute systems. (A) Solid lines represent the experimental k3-weighted EXAFS spectra; (B) Symbols represent the experimental RSF magnitudes and solid lines represent the spectral fits. T = 298 ± 1 K, m/V = 1.0 g/L, CCu(II)initial = 1.56  104 mol/L, CNi(II)initial = 1.70  104 mol/L, I = 0.01 mol/L NaNO3.

4.2. Sorption behaviors of Ni(II) at the montmorillonite/water interfaces Similar to that of Cu(II), the sorption behaviors of Ni(II) on montmorillonite are also closely related to the solution pH (Fig. 1B). In addition, the sorption isotherms (Fig. 2B), the surface complexation modeling (Fig. 6) and the EXAFS analysis (Fig. 8) show that the coexisting Cu(II) significantly changes the sorption behaviors of Ni(II). For the binary-solute systems, the comparability between the structural parameters of the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 5.0 and those of the montmorillonite/Ni(II) sample prepared at pH 5.0 indicates that the binding mechanism of Ni(II) on montmorillonite under acidic conditions is not influenced by the coexisting Cu(II) in solution. However, the sorption percentage of Ni(II) in the binary-solute system is relatively reduced compared with that in the single-solute system at pH 5.0 (see Fig. 1B). The Ni(II) and the Cu(II) are retained via the cation exchange reaction between the Ni2+/Cu2+ ions (the main species of Ni(II)/Cu(II) at a low pH value) and the exchangeable Na+ ions located in the montmorillonite interlayer at pH 5.0 (Fig. 9A). The Vanselow selectivity coefficient for the Ni2+/Ca2+ exchange reaction (1.48) is reported to be lower than that for the Cu2+/Ca2+ exchange reaction (2.42) (Auboiroux et al., 1998). Hence, the exchange reaction between Ni(II) ions and the exchangeable Na+ ions is inhibited by the coexisting Cu(II) ions. The simultaneous presence of Cu(II) significantly changes the species of Ni(II) on montmorillonite at pH

6.0 and 8.0. For the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 6.0, the EXAFS-derived RNi–Al/Si values suggest the formation of edge-shared and corner-shared (Fig. 9B) inner-sphere complexes (Da¨hn et al., 2003; Sheng et al., 2011; Yang et al., 2011; Ren et al., 2013). Herein, the inner-sphere complexation occurs in the binary-solute system is obviously disparate from the cation exchange reaction that occurs in the single-solute system (see the detained analysis results in the Section EA4). The surface complexation modeling shows that the coexisting Cu(II) decreases the contribution of the X2Ni species for a relative proportion of 20% but increases the contribution of the AlONi+ and SiONi+ species for a relative proportion of 7% (Fig. 6A and B). As illustrated in Fig. 5B, 17% of the Cu(II) in the binary-solute system is present as the X2Cu species. We can infer that the Cu(II) ions located in the interlayer would prevent the Ni(II) ions from entering the interlayer or would partly displace the Ni(II) ions from the exchangeable sites. It appears that a portion of the released Ni(II) ions tend to bind on the montmorillonite edge sites in the presence of Cu(II), forming inner-sphere surface complexes. Similar phenomena have been reported for the competitive sorption of Pb(II) and Cd(II) on soils and sediments (Serrano et al., 2005; Oh et al., 2009). The relative affinity and selective order of Cd(II) for binding on soils and sediments are lower than those of Pb(II) on the same minerals. However, Cd(II) is induced by the competitive Pb(II) sorption to bind on high-affinity surface sites with a higher bonding strength. The abnormal phenomena in the present study and the previous studies may arise from the heterogeneity of the

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Fig. 9. Schematic illustrations of the possible modes for Cu(II) and Ni(II) binding on montmorillonite: (A) cation exchange; (B) inner-sphere complexation; (C) surface polynucleation; (D) phyllosilicate co(precipitation).

binding sites and the sorption reversibility. However, this tentative interpretation remains ambiguous, and further studies are required to verify the underlying mechanisms. For the montmorillonite/Cu(II)/Ni(II) sample prepared at pH 8.0, the EXAFS-derived structural parameters indicate that Ni phyllosilicate (Fig. 9D) and/or the Ni(OH)2(s) precipitate tend to form in the presence of Cu(II). A detailed discussion on the formation of these two phases is provided in the Electronic Annex (see section EA4). Although the sorption percentage of Ni(II) on montmorillonite with the coexisting Cu(II) is equal to that without the coexisting Cu(II) at pH 8.0 (Fig. 1B), the binding

mechanism of Ni(II) on montmorillonite changes from the inner-sphere complexation in the single-solute system (see the detained analysis results in the Section EA4) to the surface co-precipitation in the binary-solute system. This changing trend can be tentatively explained by the interference of the surface-attached Cu(II) species. Based on the foregoing EXAFS analysis, Cu(II) tends to form surface dimers and/or the hydroxide precipitate at pH 8.0 in the binary-solute system. Under this circumstance, a portion of the surface sites on the (Al/Si)(O,OH)4 tetrahedra would be preferentially occupied by the binding of the Cu(II) dimers, which competitively diminishes the binding

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of Ni(II) on these surface sites. As a result, Ni(II) tends to form the Ni(OH)2(s) precipitate and/or the Ni phyllosilicate co-precipitate with the dissolved Si in solution rather than directly binding on the surface edge sites. The formation of these two precipitate phases have been reported for the sorption of Ni(II) on various mineral surfaces, such as laterite, silicates and the gibbsite/silica mixture (Manceau, 1990; Charlet and Manceau, 1994; Scheckel and Sparks, 2000; Peltier et al., 2006; Fan and Gerson, 2011). 4.3. Selective order and the competitive mechanisms Because they are neighboring divalent metals in the periodic table, Cu(II) and Ni(II) are expected to compete for the same type of montmorillonite sites, leading to the formation of similar sorption complexes. However, the macroscopic data (Figs. 1A and 2A), the surface complexation modeling (Fig. 5) and the spectral analysis results (Figs. 4 and 7) clearly show that Cu(II) is preferentially retained on montmorillonite with no obvious interference from the coexisting Ni(II). In contrast, the sorption behavior (Figs. 1B and 2B) and microstructures (Figs. 6 and 8) of Ni(II) on montmorillonite are significantly influenced by the coexisting Cu(II). This result suggests that Cu(II) has a higher affinity for binding on montmorillonite. The selective order of Cu(II) > Ni(II) in this study is consistent with that reported in the previous studies (Reddad et al., 2002b; Kalmykova et al., 2008; de Pablo et al., 2011). An in-depth discussion is required to verify the influencing factors for the selective order of Cu(II) > Ni(II) for binding on montmorillonite. In this study, the more preferential retention of Cu(II) than Ni(II) on montmorillonite can be interpreted by their differences in electronegativity, the first hydrolysis constant, the Misono softness parameter and the electron configurations. Theoretically, the metal ion with the greater electronegativity is expected to possess a higher affinity for solid surfaces (Flogeac et al., 2007; Liu et al., 2008). The higher electronegativity of Cu(II) (2.00) than Ni(II) (1.91) is responsible for its preferential sequestration on montmorillonite (Kalmykova et al., 2008). The first hydrolysis constant is regarded as an important influencing factor for the selective retention of different metal ions by montmorillonite. Generally, it appears easier for the metal ion with lower first hydrolysis constant to be adsorbed on the solid surfaces (Saha et al., 2002). The first hydrolysis constant for Cu(II) is 8.0, whereas that for Ni(II) is 9.6 (McKenzie, 1980). Consequently, Cu(II) is more preferably sequestrated on montmorillonite due to its lower first hydrolysis constant. The Misono softness parameter, calculated from the ionic charge and the ionization potential, can reflect the affinity for a metal ion to form covalent bonds with colloid surfaces (Sposito, 1989; Flogeac et al., 2007). In this study, the higher Misono softness value of Cu(II) (i.e., 0.284) than that of Ni(II) (i.e., 0.252) also contributes to the preferential retention of Cu(II) on montmorillonite (Sposito, 1989). Furthermore, the electron configurations of the different metals can influence their affinity towards the solid surface sites and can change the thermodynamic stability of the formed complexes. In a cubic, octahedral

or tetrahedral system, the 3d9 configuration induces the Cu(II) ions subject to the Jahn–Teller distortions (Webster et al., 2007; Zhou et al., 2009b). This phenomenon reduces the molecular symmetry and orbital degeneracy, which correspondingly leads to the decline in the system energy and the enhancement of the sorption trend. In contrast, Ni(II) is relatively unreactive and is stable in a divalent ionic spin state with s = 0 (Ghaee et al., 2012). In addition to the differences of the metal properties, the physicochemical properties of montmorillonite play an important role in controlling the selective order of Cu(II) and Ni(II). Montmorillonite, with a dioctahedral structure, possesses certain amounts of large and distorted cavities in the octahedral sheet (Da¨hn et al., 2003). This configuration results in the formation of a corrugated and asymmetric basal plane, which permits a favorable “guest-host” consistency for the tetragonal-distorted Cu(II)O6 octahedra with a high asymmetry (Schlegel and Manceau, 2013; Kwon et al., 2013). The adsorbed Cu(II) species at the vacant sites may be gradually incorporated and steadily retained in the internal location of the montmorillonite crystal structure. This irreversible sorption mode provides a rationale to explain the higher affinity of Cu(II) towards the montmorillonite surfaces. In contrast, the regular Ni(II)O6 octahedra cannot enter the distorted vacancies in the octahedral sheet due to the incompatibility in size and symmetry. 5. CONCLUSIONS The macroscopic data, surface complexation modeling and microscopic findings in the present study can provide a crucial scientific basis for understanding the solid-water behaviors and potential risk of toxic heavy metal ions in the aquatic ecosystem. In this study, the solution pH variations were proven to significantly affect the competitive behaviors and the corresponding microstructures of Cu(II) and Ni(II) on montmorillonite. Specifically at pH 5.0, the sorption percentage of Ni(II) on montmorillonite is significantly reduced by the simultaneous presence of Cu(II). Therefore, more Ni(II) ions would be present in solution as free hydrated ions with a high mobility. This phenomenon clearly implies that the Cu(II) competition would cause a greater extent of Ni(II) contamination in an acidic water environment, e.g., the aquatic system polluted by acid mine drainage. At pH 6.0, the coexisting Cu(II) induces Ni(II) to bind on more specific surface sites, forming inner-sphere complexes with higher thermodynamic stability and lower chemical mobility. Under these circumstances, Ni(II) is retained more tightly on montmorillonite. Compared with the single-solute system, more Ni(II) ions are remaining in the solution due to the competitive effect of the coexisting Cu(II) in the binary-solute system. In other words, the coexisting Cu(II) would cause a greater degree of Ni(II) pollution in a near-neutral environment. At pH 8.0, Ni(II) tends to be adsorbed on montmorillonite via the formation of the Ni phyllosilicate co-precipitate in the presence of Cu(II). This species is thermodynamically more stable than the inner-sphere complexes formed in the absence of Cu(II). Over a prolonged aging time, the combined Ni phyllosilicate

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co-precipitate/montmorillonite colloids would gradually sink into the sediments, which would correspondingly reduce the bioavailability and potential risk of Ni(II) towards aquatic organisms in the alkaline water environment, e.g., the marine system with a pH value of 8.0. ACKNOWLEDGMENTS Financial supports from the National Natural Science Foundation of China (41203086; 21225730; 91326202), 973 Projects (2011CB933700), the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, and MCTL Visiting Fellowship Program from Key Laboratory of Marine Chemistry Theory and Technology (Ocean University of China), Ministry of Education are acknowledged. We thank Dr. Wei Tong at High Magnetic Field Laboratory, Chinese Academy of Sciences for the technical assistance in EPR spectral collection. We also acknowledge Prof. Yuying Huang, Zheng Jiang, Xiangjun Wei and Dr. Xing Gao of SSRF for their helpful technical assistance in the XAS spectra collection and their constructive discussion on the XAS data analysis.

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