Removal of Co2+, Sr2+ and Cs+ from aqueous

Desalination 276 (2011) 336–346

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Removal of Co2+, Sr2+ and Cs+ from aqueous solution by phosphate-modified montmorillonite (PMM) Bin Ma, Sanghwa Oh, Won Sik Shin ⁎, Sang-June Choi Department of Environmental Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 16 December 2010 Received in revised form 25 March 2011 Accepted 28 March 2011 Available online 22 April 2011 Keywords: Cesium Cobalt Competitive sorption Strontium Phosphate-modified montmorillonite

a b s t r a c t Sorptive removal of Co2+, Sr2+ and Cs+ from aqueous solution by phosphate-modified montmorillonite (PMM) was studied considering the influencing factors (initial metal ion concentrations, initial solution pH and temperature). The Freundlich, Langmuir and Dubinin–Radushkevich (DR) models were used to fit singlesolute sorption data. The Freundlich model was the best, indicating heterogeneous surface property of PMM. The maximum sorption capacity (qmL) of Langmuir model was in the order of Cs+ N Co2+ N Sr2+. The mean sorption energy (E) values of DR model (b 8 kJ/mol) at pH 5 for all metals indicated physical sorption. However, Cs+ sorption mainly occurred by chemical sorption. In bi-solute competitive sorption, the sorption of one metal ion was suppressed by the presence of competing metal ion. The Sheindorf–Rebhun–Sheintuch (SRS) model, the extended Freundlich model (EFM), the modified extended Langmuir model (MELM) and the IAST-Freundlich model predicted the competitive sorption adequately. Sorptions of Co2+ and Sr2+ were strongly dependent on the initial solution pH but that of Cs+ was not. The calculated thermodynamic parameters such as ΔH, ΔS, and ΔG showed that sorptions of Co2+ and Sr2+ onto PMM were endothermic, whereas that of Cs+ was exothermic and that all sorption occurred spontaneously. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The low-level radioactive waste (LLRW) is generated from operation and maintenance of nuclear reactors, mining and milling of nuclear fuel, and recycling the spent nuclear fuel. In general, such radioactive waste contains a variety of radionuclides that are fission byproducts generated from above processes [1]. Among the byproducts, Co2+, Sr2+ and Cs+ are considered as the most dangerous radionuclides to human health due to their high transferability, high solubility, long half-lives and easy assimilation in living organisms. Several technologies such as chemical precipitations, conventional coagulation, reverse osmosis, ion-exchange and adsorption have been developed for the removal of these radionuclides present in the wastewater [2]. Out of these methods, sorption has been most widely used for the removal of these radionuclides because it is simple and cost effective with low cost sorbents. Montmorillonite is an excellent sorbent because of high specific surface area, chemical and mechanical stability, layered structure and high cation exchange capacity (CEC). Sparks [3] reported that the high CEC for montmorillonite is due to substantial isomorphic substitution and to the presence of fully expanded interlayers that promote exchange of cations. Montmorillonite is consisted of two tetrahedral sheets, one octahedral sheet, and exchangeable cations and water

⁎ Corresponding author. Tel.: + 82 53 950 7584; fax: + 82 53 950 6579. E-mail address: [email protected] (W.S. Shin). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.03.072

molecules located in interlayer space between sheets. Montmorillonite also has the net permanent negative charge on the surface caused by substitution of Si4+ by Al3+ in tetrahedral layer and of Al3+ by Fe2+ and Mg2+ in octahedral layer [4]. The net negative charge is balanced by exchangeable cations between the interlayers or around their edges [5]. Montmorillonite, therefore, has been applied for the removal of heavy metals such as Ni2+ and Cu2+ [6], Sr2+ [7], and Co2+ [8] from contaminated water. Recently, chemical modification of natural clays such as montmorillonite and kaolinite to increase the sorption capacity by improving the clay structure has received extensive attention [5,9–12]. Acid activation followed by thermal treatment is also considered as a potential modification method to increase the sorption capacity of clays [9]. Lin and Juang [10] applied montmorillonite modified with sodium dodecyl sulfate (SDS) to remove Cu2+ and Zn2+ from aqueous solution. Phosphate-modified kaolinite has been investigated for the sorption of heavy metals [11,12]. Unuabonah et al. [11] reported that the modification of kaolinite clay with phosphate enhanced the Pb2+ sorption on kaolinite, which was more endothermic and spontaneous. Adebowale et al. [11] found that modification of kaolinite with phosphate salts changed the kaolinite structure and improved the holding capacity for metal ions (Pb2+ and Cd2+). Modification with phosphate increases the cation exchange capacity (CEC) of kaolinite and induces negative surface charge, thereby increasing sorption of heavy metal ions. However, sorptions of heavy metals onto phosphatemodified montmorillonite (PMM) were not fully understood.

B. Ma et al. / Desalination 276 (2011) 336–346

Bhattacharyya and Gupta [13] reported that the CEC of montmorillonite (153.0 meq Cu2+/100 g) is higher than that of kaolinite (11.3 meq Cu2+/100 g) due to difference in structures, indicating that montmorillonite would be more appropriate than kaolinite for sorption. Sorption is considered as the most promising process for the treatment of LLRW [14]. Several sorbents such as silicotitanate [15] and ammonium molybdophosphate-polyacrylonitrile (AMP-PAN) [16] and several types of commercial inorganic ion-exchange resins [15,17] have been applied to treat the high- and low-level radioactive wastes; however, these sorbents are highly expensive. PMM has also a promising potential for sorption of heavy metals because of high surface area and cost-effectiveness compared to the above mentioned sorbents and ion-exchange resins, however no information is currently available on the sorptive removal of Co2+, Sr2+ and Cs+ from LLRW by PMM. In this study, sorption of Co2+, Sr2+ and Cs+ from aqueous solutions onto the montmorillonite modified with KH2PO4 (phosphate-modified montmorillonite, PMM) was investigated. The purpose of this study is to determine the effects of operating variables such as initial metal ion concentrations, initial solution pH, temperature and presence of competing metal ion. Sorption mechanisms were discussed in detail. 2. Materials and methods 2.1. Chemicals The montmorillonite-KSF, Co(NO3)2·6H2O (N98%), Sr(NO3)2 (N99%) and CsNO3 (N99.9%) were purchased from Sigma-Aldrich (Seoul, Korea). KH2PO4 (N98%) was provided by Yakuri Pure Chemicals Co. (Japan). CH3COONa (N98.5%), CH3COONH4 (N95%) and isopropyl alcohol (99.5%) were purchased from Duksan (Korea). 2.2. Preparation of PMM The impurities of montmorillonite-KSF were removed by a digestion method using H2O2 and by washing it several times with distilled water at 60 °C. The clay suspensions were filtered with a 0.22 μm membrane filter, and the filtrate was examined for impurities using a UV–Visible spectrophotometer (Hewlett Packard, 8452A, USA). The washed montmorillonite was allowed to settle, dried in an oven at 60 °C for 24 h, and stored in a brownish bottle. The PMM was prepared by mixing 15 g of the washed montmorillonite with 1 L of 2000 mg/L of PO3− (from KH2PO4) with a rotary agitator for 24 h at 4 200 rpm and room temperature. Thereafter, the modified clay was washed three times with 1 L of distilled and deionized water in order to remove excess H2PO− 4 ions, air-dried for at least 3 days and kept in a brownish bottle before use. The point of zero charge of PMM was determined by the potentiometric titration method [18]. Briefly, 4 g of air-dried PMM and 10 mL of electrolyte solution (0.1 M KCl) were added into 50 mL beakers. The solution pH was adjusted in the range of 2 to 10 using 0.1 M HCl or 0.1 M KOH and the beakers were then filled up to 25 mL with distilled and deionized water. The samples were covered and mixed in a rotary shaker for 7 days. After that, pH of the samples was measured and the amounts of H+ and OH− adsorbed by PMM were determined by subtracting the amount of HCl or KOH required to bring 10 mL of the blank electrolyte (0.1 M KCl) plus 15 mL of distilled and deionized water (without PMM) to the same pH. Brunauer– Emmett–Teller (BET) surface area was determined from N2 adsorption isotherm using specific surface area analyzer (Micromeritics, ASAP-2010). Cation exchange capacity (CEC) of montmorillonite and PMM was also analyzed by sodium acetate method [19]. The sample was mixed with an excess of 1 N sodium acetate solution, resulting in an exchange of the added cations for the matrix cations. Subsequently, the sample was washed with isopropyl alcohol. 1 N ammonium

337

acetate solution was then added to replace the sorbed sodium with ammonium. The concentration of displaced sodium was then determined by inductively coupled plasma-optical emission spectrometer (ICP-OES, Perkin Elmer, Optima 2100DV). X-ray diffraction (XRD) patterns were obtained using a Philips PW2273 diffractometer and Cu Kα radiation (40 kV, 25 mA) in a range of 5–40° with a step size 0.02° and a time per step of 1 s. Scanning electron microscopy (SEM, Hitachi S-4200) was also used to examine the morphology and particle size of montmorillonite before and after modification. The chemical composition of montmorillonite and PMM was characterized by EDS analysis (Horiba E-MAX EDS detector). 2.3. Sorption experiment For single-solute sorption experiments, metal stock solutions were prepared by dissolving Co(NO3)2·6H2O, Sr(NO3)2 and CsNO3 in distilled and deionized water, respectively. The pH of the solution was measured by a pH meter (Thermo, Orion, model 720A+, USA). Single-solute sorption experiments were conducted at 25 °C using 50 mL conical centrifuge tube (polyethylene, SPL Labware, Korea). 1 g of PMM was transferred into the tube and the pH of sorbent was adjusted to 5 by using 0.05 M MES buffer solution (heavy metal free) before the addition of metal stock solution. Thereafter, the tubes containing 1 g of PMM each were filled with approximately 50 mL of the stock solution with different initial metal ion concentrations ranging from 1 to 20 mM, in some case, to 30 mM to obtain sorption isotherms. The pH values of heavy metal solutions were also controlled at 5 ±0.05 by using 0.05 M MES buffer. The MES buffer was used because no detectable complexation reactions occur between Co, Sr and Cs and MES buffer [20,21]. The solution pH was controlled to prevent the formation of metal hydroxides and carbonates. The exact amount of added stock solution was determined gravimetrically. The mixture was then placed on a rotary shaker and shaken for 24 h at 200 rpm. After mixing, the tubes were centrifuged for 20 min at 3000 rpm, filtered through 0.2 μm syringe filter (Whatman, cellulose nitrate membrane filter, ϕ = 25 mm). The metal ion concentration in the filtrate was analyzed by ICP-OES (PerkinElmer Optima 2100DV). All experiments were conducted in duplicate. The sorbed amount of metal ion, q (mmol/g), was calculated using difference between initial and equilibrium metal concentrations.

q=

ðC0 −C ÞV W

ð1Þ

where C0 (mmol/L) and C (mmol/L) are initial and equilibrium metal ion concentrations in solution, respectively and V (L) is the volume of the solution and W (g) is the weight of sorbent. Bi-solute competitive systems (Co2+/Sr2+, Sr2+/Cs+ and Cs+/Co2+) were prepared by mixing each metal solution of the same molar concentration in a 1:1 volume ratio for each solute. Bi-solute competitive sorption experiments were conducted in the same manner as used in the single-solute sorption experiments. The equilibrium concentrations in the mixture were also determined using the ICP-OES. 2.4. Sorption models 2.4.1. Single-solute sorption models The Freundlich isotherm model is an empirical expression that encompasses the heterogeneity of the surface and an exponential distribution of the sites and their energies. The isotherm has been further extended by considering the influence of sorption sites and the competition between different ions for sorption on the available site [22]. N

q = KF Ce F

ð2Þ

338


where KF [(mmol/g)/(mmol/L)NF] and NF (−) are the Freundlich sorption coefficient and the Freundlich constant, respectively. The Langmuir isotherm model is based on a monolayer sorption on a surface containing a finite number of binding sites. It assumes uniform energies of sorption on the surface and no transmigration of sorbates in the plane of the surface [23]. This model is represented as: q=

qmL bL C 1 + bL C

coefficient for the sorption of solute i in the presence of solute j. The parameters Ki and ni are the Freundlich parameters obtained from a single-solute system. By definition, αi, j equals 1 when i = j. If there is no competition, that is, αi, j = 0 for all j ≠ i. The equilibrium sorption from binary mixtures can also be represented by the extended Freundlich model (EFM) as given below [28]:

ð3Þ

N + b11

q1 = where qmL (mmol/g) is the maximum sorption capacity and bL (L/ mmol) is the Langmuir constant related to the sorption energy. The isotherm type can be used to predict whether a sorption system is favorable or unfavorable [11]. The dimensionless constant separation factor (KR) represents the essential features of the Langmuir isotherm and is defined by 1 KR = 1 + bL C0

ð4Þ

where KR is a dimensionless separation factor, C0 (mmol/L) is initial metal ion concentration and bL (L/mmol) is the Langmuir constant. The KR values indicate the type of isotherm to be unfavorable (KR N 1), linear (KR = 1), favorable (0 b KR b 1) or irreversible (KR = 0). The Dubinin–Radushkevich (DR) model has been used to describe the sorption of metal ions onto clays [24]. 2 q = qmD exp −βε

ð5Þ

where qmD (mmol/g) is the maximum sorption capacity, β (mol2/J2) is the activity coefficient related to mean sorption energy and ε is the Polanyi potential, which is equal to ε = RT lnð1 + 1 = Ce Þ

ð6Þ

where R is the gas constant (8.314 J/mol/K), and T is the absolute temperature (K). From the Polanyi potential (ε) and the amount of the sorbed metal (q) the activity coefficient (β) can be obtained by DR model. The mean sorption energy, E (J/mol), can be calculated using the following relationship: 1 E = pffiffiffiffiffiffi : 2β

ð7Þ

KF;1 C1 1 b C111

ð9Þ

b

+ a12 C212 N + b22

q2 =

KF;2 C2 2 b

ð10Þ

b

C222 + a21 C121

where KF,1, KF,2, N1 and N2 can be estimated from the corresponding individual Freundlich isotherm equations and the other six parameters (b11; a12; b12 and b22; a21; b21) are the multi-solute Freundlich sorption constants of the first and the second solutes [29]. The SRS and EFM model parameters were determined by using a commercial software package, TableCurve 3D® (Version 4.0, SYSTAT Software, Inc.). The modified extended Langmuir model (MELM) was derived from the extended Langmuir model (ELM) for analysis of the multisolute sorption, where the synergistic or antagonistic efficiency (θ) of sorbates is introduced [30,31]. This efficiency is linearly related to the amount sorbed at equilibrium of the other solute. θi = ai qj + bi ði = 1 or 2Þ

ð11Þ

where ai and bi are the constant parameters of synergistic efficiency and estimated from bi-solute competitive sorption data by non-linear regression. If θi N 0, sorbed amount one solute (qi) linearly increases with the sorbed amount of the other solute (q j) (i.e., synergistic effect). If θi b 0, sorption of one solute decreases by the presence of the other solute (i.e., antagonistic effect). The crossover point of the two behaviors is q j, cross = bi/ai at θi = 0. The MELM is defined as:

qi =

bL;i qmL;i Ci N

ð1 + θi Þði = 1or2Þ

ð12Þ

1 + ∑ bL;i Cj j=1

The magnitude of E can be used to estimate the type of sorption process, and if it lies between 8 and 16 kJ/mol the sorption can be explained by ion-exchange. In the case of E b 8 kJ/mol, physical force may affect the sorption mechanism. For the values of E N 16 kJ/mol, sorption occurs via chemical sorption [25]. The single-solute sorption model parameters were determined by using a commercial software package, TableCurve 2D® (Version 5.0, SYSTAT Software, Inc.). 2.4.2. Bi-solute competitive sorption models The Sheindorf–Rebhun–Sheintuch (SRS) model was developed to describe competitive sorption assuming that the single-component sorption follows the Freundlich model [26,27]. The derivation of SRS equation is based on the assumption of an exponential distribution of sorption energies for each component. A general form of the SRS model can be written as qi =

Ki Ci 1−n i ∑ αi; j Cj N

ð8Þ

where qi denotes sorbed amount at bi-solute competitive sorption equilibrium. Ci donates phase concentrations at bi-solute competitive sorption equilibrium. If θi = 0, the MELM becomes extended Langmuir model (ELM). A FORTRAN program was written to estimate the MELM model parameters. The programs calls the IMSL subroutine ZXSSQ, which is based on a finite difference, Levenberg–Marquardt algorithm for solving nonlinear least squares problems. IAST (Ideal Adsorbed Solution Theory) combined with single-solute Freundlich model (IAST-Freundlich) was also employed to fit bi-solute competitive sorptions of Co/Sr, Co/Cs and Sr/Cs onto PMM. IAST, which was originally proposed by Radke and Prausnitz [32], is of descriptive nature and requires aqueous-phase concentrations to predict sorbedphase concentrations. To utilize the full predictive power of the IAST, Yen and Singer [33] included the material balance on each solute (i.e., the last equation in Eq. 15). We followed their modifications of the IAST. IAST is based on the equivalence of spreading pressure in a mixture under equilibrium. The equivalence of spreading pressure in a mixture containing N solutes leads to:

j=1

where subscripts i and j denote metal solutes i and j, N is the total number of the solutes, and αi, j is a dimensionless competition

q⁎

∫

0

1

q⁎ d logC2 q⁎ d logCN d logC1 dq = ∫ 2 dq2 = ⋯ = ∫ N dqN 0 d logq 0 d logq d logq1 1 2 N

ð13Þ

B. Ma et al. / Desalination 276 (2011) 336–346 Table 1 The physicochemical characteristics of the untreated montmorillonite and PMM.

Point of zero charge (PZC) CEC (meq/100 g) BET Surface Area (ABET, m2/g) Pore volume (cm3/g) Pore size (Å)

Montmorillonite

PMM

5.5 49.9 2.592 0.011 38.05

4.0 74.6 115.9 0.1 40.73

339

where ΔS0 (J/mol/K) and ΔH0 (J/mol) are the changes in the entropy and enthalpy for the sorption process, respectively, Kd (mL/g) is the distribution coefficient, T is the absolute temperature (K), and R is the gas constant (8.314 J/mol/K). The plot of ln Kd versus 1/T is linear with the slope and the intercept which give the values of ΔH0 and ΔS0, respectively. All these relations are valid when the enthalpy change remains constant in the temperature range of study [6]. 2.6. Effect of pH

Other equations involved in IAST calculation are: 1 N N z = ∑ i ; Cm;i = zi Ci⁎ ; ∑ zi = 1; q⁎i = f Ci⁎ ; qT i=1 i = 1 q⁎ i 0 V Cm;i −Cm;i 0 : qm;i = zi qT = qm;i + W

ð14Þ

In the above equations, Cm,i and qm,i are equilibrium concentrations in the liquid and sorbed phases of a solute i in a mixture, respectively. Superscript 0 in these variables represent initial concentration in N-solute sorption. zi represents the mole fraction of solute i in the sorbed phase, and Ci⁎ and qi⁎ refer to equilibrium concentrations in the liquid and solid phases of solute i that sorbs singly from solution at the same temperature and spreading pressure as those of the mixture, respectively. The function f in qi⁎ =f(Ci⁎) denotes a single-solute sorption model for solute i. qT is the total sorbed concentration of all solutes in the mixture. V and W represent the solution volume and sorbent weight, respectively. There are 5N+ 1 equations in total, while Cm,i,, qm,i, Ci⁎, q⁎i , zi, and qT comprise a set of 5N+ 1 unknowns. Therefore, we can predict the multisolute sorption data, qm,i vs. Cm,i,, by solving these equations simultaneously. Fortran programs were written to calculate competitive sorption equilibria. 2.5. Thermodynamics of sorption The thermodynamic parameters for the sorption process, ΔH0, ΔS0 and ΔG0 can be calculated using the equations [34]: Kd =

qe Ce

ð15Þ

0

ΔG = −RT lnKd 0

0

ð16Þ 0

ΔG = ΔH −TΔS :

ð17Þ

These three equations can be expressed as: lnKd =

ΔS0 ΔH0 − R RT

ð18Þ

a) Montmorillonite

The 10 mM of Co2+, Sr2+ and Cs+ solutions were prepared by dissolving each of nitrite salts in distilled and deionized water. 30 mL of metal ion solution was added into 50 mL of conical centrifuge tube (polyethylene, SPL Labware, Korea) containing 0.6 g of PMM. The pH of solution was adjusted to 3–12 using 0.1 N HNO3 or 0.1 N NaOH solution. The suspensions were shaken for 24 h at 25 °C and 200 rpm on a rotary shaker, centrifuged for 20 min at 3000 rpm, and then filtrated through the 0.2 μm syringe filter. After filtration, the final pH and metal ion concentration in the filtrate were analyzed. The amount of precipitated metal remaining on the filter was calculated by subtracting the metal concentration in the filtrate from the total metal concentration. 3. Results and discussions 3.1. Characteristics of sorbents The physicochemical properties of the raw montmorillonite and PMM such as PZC, CEC, BET surface area, pore volume and pore size were summarized in Table 1. All physicochemical properties, except PZC, of the montmorillonite increased after phosphate-modification. Especially, the BET surface area (ABET) and pore volume increased remarkably from 2.6 m2/g to 115.9 m2/g and from 0.011 cm3/g to 0.1 cm3/g, respectively. The increases of surface area and pore volume after phosphate-modification would be due to the strongly sorbed phosphate ions via an inner-sphere complex formation and also the formation of Al–O–P–OH surface precipitates during phosphate modification [35]. The CEC of PMM (74.6 meq/100 g) was also higher than that of raw montmorillonite (49.9 meq/100 g) due to the phosphate-modification. The PZC values of montmorillonite decreased from pH 5.5 to 4 after phosphate-modification due to displacement of H+. As shown in Fig. 1, EDS analyses showed that K+ was detected in PMM (Fig. 1b), but not in raw montmorillonite (Fig. 1a). The K+ can be displaced with H+ by ion exchange, causing the decrease in equilibrium pH of the phosphate solution (KH2PO4) from 5.48 to 2.56. It is analogous to the result reported by Adebowale et al. [23]. They reported that pH of the phosphate solution used for the modification of kaolinite decreased from 5.55 to 4.81 after modification due to displacement of H+ during

b) PMM

Fig. 1. EDS peaks for (a) raw montmorillonite and (b) PMM.

340


corresponding to a basal spacing of 15.33 Å. The increase of relative intensity and d-spacing suggests that the interlayer space was expanded and the intercalated nanostructure was formed after modification [36]. The SEM images of the raw montmorillonite and the PMM are presented in Fig. 3. Fig. 3a shows the aggregated structure of montmorillonite. In contrast, the particles of the PMM are more widely distributed and the particle size of the PMM is much smaller than that of the raw montmorillonite as shown in Fig. 3b. This indicates that the modification expanded the interlayer space and formed a disordered intercalated structure, which is in agreement with the results of XRD patterns. Fig. 2. X-ray patterns of raw montmorillonite and phosphate-modified montmorillonite (PMM).

phosphate-modification. As shown in Fig. 1, a small P peak was detected in the PMM only, indicating that the phosphate-modification of montmorillonite was successful. The X-ray diffraction (XRD) patterns of the raw montmorillonite and the PMM are illustrated in Fig. 2. The phosphate-modification introduces some changes in the structure of montmorillonite as indicated by the difference in the patterns between montmorillonite and PMM at 2θ b 10°. The XRD pattern of montmorillonite shows a typical diffraction peak at 7.31°, corresponding to a basal spacing of 12.07 Å. After modification, this peak shifts to lower angle at 5.75°,

3.2. Sorption study 3.2.1. Single-solute sorption Molar distributions of cobalt, strontium and cesium species at all pH ranges predicted using the program of MINEQL (version 4.5) for Windows (Environmental Research Software, USA) are presented in Fig. 4. Co(OH)2 was formed at pH N7 and SrOH+ was formed at pH N11. Co2+, Sr2+ and Cs+ ions are the predominating species at pH 5 which was higher than the pHpzc (=4.0), resulting in favorable sorption of the metal cations onto PMM.

a) Co

8

b) Sr

c) Cs

Fig. 3. SEM images of (a) montmorillonite and (b) PMM (´1000).

Fig. 4. The distributions of cobalt, strontium and cesium species as a function of pH predicted by MINEQL+4.5 software.


The single-solute sorption isotherms of Co2+, Sr2+ and Cs+ onto the raw montmorillonite and PMM are shown in Figs. 5 and 6, respectively, with the predictions by the Freundlich, Langmuir and DR models. After sorptions onto PMM, Co, Sr and Cs peaks appeared in the EDS analyses (Fig. 7) indicating that the metal ions sorbed successfully. The sorption of metal ions onto PMM increases with initial metal ion concentrations. This is due to increase in driving force (i.e., concentration gradient) [37]. The relative affinity of PMM on the basis of equilibrium sorption capacity was in the order of Cs+ N Co2+ N Sr2+. Since Cs+ has less hydration energy, the electrostatic attraction of Cs+ by PMM particle is stronger which makes Cs+ to be sorbed preferentially [38]. The single-solute sorption data were fitted by the Freundlich, Langmuir, and Dubinin–Radushkevich (DR) models and the model parameters for raw montmorillonite and PMM are listed in Tables 2 and 3, respectively. It was noticed that the maximum sorption amount (qmL) of Cs+ (0.7063 mmol/g) of PMM was much higher than that of raw montmorillonite (0.4292 mmol/g), but those of Co2+ and Sr2+ were not. This indicates that phosphate-modification gave the further sorption sites to Cs+ only. The KF values in the Freundlich, bL values in the Langmuir and E values in the DR models also increased after

341

a) Co2+

b) Sr2+

a) Co2+ c) Cs+

b) Sr2+ Fig. 6. Single-solute sorption isotherms of (a) Co2+, (b) Sr2+ and (c) Cs+ onto PMM.

c) Cs+

Fig. 5. Single-solute sorption isotherms of (a) Co2+, (b) Sr2+ and (c) Cs+ onto raw montmorillonite.

phosphate-modification. This indicates that the phosphate-modification of montmorillonite can enhance the sorption affinity and sorption energy of Cs+ to the sorption sites available on the PMM surface, but not for Co2+ and Sr2+. This is because the selectivity of Cs+ is higher than that of K+ [3] and thus the K+ in the interlayer of PMM (see Fig. 1b) was ion-exchanged by Cs+. The Freundlich isotherm considers the heterogeneity of sorbent surface and the exponential distribution of active sites and their energies. It has been widely used to describe sorption of heavy metal ions onto clay [25]. For PMM, the Freundlich model fitted well to the sorption data (0.91 b R2 b 0.99). This indicates that the sorption of Co2+, Sr2+ and Cs+ mainly occurred onto the heterogeneous surface active sites on PMM. The Freundlich sorption constant, KF, indicates the sorption capacity of the sorbent. The KF value [(mmol/g)/(mmol/ L)NF] was in the order of Cs+ (0.426) N Co2+ (0.100) N Sr2+ (0.096). The NF values were in the range of 0.19–0.27, representing the sorption was non-linear and favorable [39]. For PMM, the coefficient of determination (0.85 b R2 b 0.98) of the Langmuir model indicates that this model is also in good agreement with our experimental data. The maximum sorption capacity (qmL) of Langmuir model was in the order of Cs + (0.706 mmol/g) N Co2+ (0.202 mmol/g) N Sr2+ (0.143 mmol/g), which was the same order as KF of the Freundlich model. The bL values related to sorption energy indicated that PMM had high affinity for these metal ions. If bL

342


a) PMM-Co

b) PMM-Sr

c) PMM-Cs

Fig. 7. EDS peaks for PMMs after metal sorption: (a) PMM-Co, (b) PMM-Sr and (c) PMM-Cs.

is larger, the surface is covered with sorbate molecules bound more strongly with the surface [40]. As listed in Table 3, the bL values for Co2+, Sr2+ and Cs+ were 1.132, 5.030 and 2.940 L/mmol, respectively, indicating that the binding energy was in the order of Sr2+ N Cs+ N Co2+. Therefore, Cs+ was bound to PMM by chemical ion-exchange although the amount of Cs+ sorption was higher than the others. The plot of separation factor, KR, against initial metal ion concentration is shown in Fig. 8. In the figure, the sorptions of all metal ions onto PMM were favorable (KR b 1). Sorption of metal ions at higher initial concentration was more favored than that at lower initial concentration due to higher driving force (i.e., higher concentration difference). The sorption of Sr2+ was more favorable than Cs+ and Co2+ at all initial concentrations. The Dubinin–Radushkevich (DR) model is often used to predict sorption isotherms of metal ions onto various sorbents. The qmD value of the DR model was also in the order of Cs+ (0.669 mmol/g) N Co2+ (0.177 mmol/g) N Sr2+ (0.137 mmol/g), but less than the qmL values of Langmuir model. This discrepancy was attributed to the difference in the definition of qm values in the models. In the Langmuir model, the qmL represents the maximum sorption capacity of metal ions at monolayer coverage, while the qmD in the DR model represents the maximum sorption capacity of metal ions at the total specific micropore volume of the sorbent [41]. The R2 values (0.73–0.96) indicate that theoretical predictions are in good agreement with experimental data, especially for Cs+ (0.96). The sorption energy (E) values were 2.42 kJ/mol for Co2+, 4.31 kJ/mol for Sr2+ and 3.05 kJ/mol

for Cs+, respectively. For all metals, the E values were less than 8 kJ/ mol. This indicates that physical sorption was the primary mechanism in the sorption processes at pH 5. 3.2.2. Bi-solute competitive sorption Bi-solute competitive sorptions of the metal ions in three systems such as Co2+/Cs+, Sr2+/Cs+, and Cs+/Co2+ were investigated at pH 5 and 25 °C. The heavy metals in each system were also fitted to the Langmuir model to obtain their maximum sorption capacity (q⁎mL) and sorption affinity (b⁎L ) in the bi-solute system. The maximum sorption capacity of the metal ions (q⁎mL) in the bi-solute system was less than that in the single-solute system (Tables 3 and 4). The qmL of Cs+ in single system (0.706 mmol/g) was reduced to 0.446 mmol/g in Cs+/ Co2+ system. Therefore, the sum of the qmL values of Cs+ and Co2+ in the Cs+/Co2+ system (0.527 mmol/g) was also less than those in the single-solute system (0.908 mmol/g). For this reason, the sorption of Cs+ and Co2+ in the bi-solute competitive system onto PMM may be partially overlapped. It may also imply that there exist some specific surface sites with specific affinity for individual metal ion [42]. The sorption behaviors of metal ions onto PMM in Sr2+/Cs+ and Co2+/Sr2+ systems can be also explained in the same manner. As shown in Table 4, the q⁎mL values of Co2+ (0.133 mmol/g) and Sr2+ (0.126 mmol/ + g) in Co2+/Sr2+ system were close. However, the q⁎ mL values of Cs 2+ + + 2+ (0.409 mmol/g in Sr /Cs and 0.446 mmol/g in Cs /Co systems, respectively) were higher than those of Sr2+ (0.090 mmol/g) in Sr2+/ Cs+ and Co2+ (0.081 mmol/g) in Cs+/Co2+ systems.

Table 2 Single-solute sorption model parameters for sorption of Co2+, Sr2+, and Cs+ onto montmorillonite. Freundlich

2+

Co Sr2+ Cs+

Langmuir

DR

KF

NF

R2

SSE

qmL

bL

R2

SSE

qmD

β

E

R2

SSE

0.0806 ± 0.0067 0.0952 ± 0.0058 0.1017 ± 0.0032

0.3325 ± 0.0362 0.1967 ± 0.0280 0.3685 ± 0.0135

0.9421 0.8850 0.9935

0.0025 0.0023 0.0006

0.2143 ± 0.0264 0.1513 ± 0.0073 0.4292 ± 0.1316

0.4705 ± 0.2349 3.2269 ± 0.9948 0.3000 ± 0.0246

0.7935 0.8511 0.9222

0.0089 0.0030 0.0068

0.1622 ± 0.0151 0.1418 ± 0.0075 0.2547 ± 0.0223

1.001E-07 3.633E-08 7.656E-07

2.235 3.710 0.808

0.6159 0.7730 0.7536

0.0165 0.0046 0.0216

Units: KF = [(mmol/g)/(mmol/L)NF], NF = dimensionless, qmL = mmol/g, bL = L/mmol, qmD = mmol/g, β = mol2/J2 and E = kJ/mol.

Table 3 Single-solute sorption model parameters for sorption of Co2+, Sr2+, and Cs+ onto PMM. Freundlich

2+

Co Sr2+ Cs+

Langmuir 2

KF

NF

R

0.0997 ± 0.0025 0.0957 ± 0.0030 0.4259 ± 0.0273

0.2726 ± 0.0113 0.1919 ± 0.0149 0.2124 ± 0.0288

0.9911 0.9616 0.9146

DR 2

SSE

qmL

bL

R

SSE

qmD

β

E

R2

SSE

0.0004 0.0007 0.0797

0.2019 ± 0.0145 0.1434 ± 0.0060 0.7063 ± 0.0208

1.1322 ± 0.4479 5.0301 ± 1.3346 2.9395 ± 0.4520

0.8538 0.8764 0.9772

0.0062 0.0022 0.0213

0.1766 ± 0.0126 0.1366 ± 0.0062 0.6689 ± 0.0212

8.562E-08 2.698E-08 5.371E-08

2.417 4.305 3.051

0.7273 0.8229 0.9646

0.0116 0.0031 0.0330

Units: KF = [(mmol/g)/(mmol/L)NF], NF = dimensionless, qmL = mmol/g, bL = L/mmol, qmD = mmol/g, β = mol2/J2 and E = kJ/mol.


343

Table 5 Comparison of qmL and bL values of single- and bi-solute competitive sorption. Binary system (1)/(2)

⁎ 1/qmL, 1 qmL,

⁎ 2 /qmL, 2 qmL,

b⁎L, 1 /bL, 1

b⁎L, 2 /bL, 2

Co2+/Sr2+ Sr2+/Cs+ Cs+/Co2+

0.6567 0.6299 0.6320

0.8761 0.5792 0.3988

0.4359 2.5789 2.7837

0.2026 3.8256 6.3569

qmL and bL indicate Langmuir model parameters for single-solute adsorption. q⁎mL and bL⁎ indicate Langmuir model parameters for bi-solute adsorption. The metal ions in bisolute systems were labeled in the order of (1) and (2).

experimental data and the theoretically predicted points and the coefficient of determination (R2) were calculated to assess the validity of the predictive models as listed in Tables 6–9: Fig. 8. Separation factor (KR) for the adsorption of Co2+, Sr2+ and Cs+ onto PMM.

N

SSE = ∑

i=1

The effect of bi-solute competition on the sorption of the metal ions can be also analyzed by the ratio of sorption capacity of one metal ion in the presence of other metal ion, q⁎mL , to the sorption capacity of corresponding metal ion in single-solute solution, qmL, as summarized in Table 5. All q⁎mL /qmL ratios are less than 1, indicating that the sorption was hindered by the presence of other metal ions [43]. As the q⁎mL /qmL ratio of a metal ion decreased, the sorption of such metal ion was more affected by the presence of the other competing metal ion. For example, in Cs+/Co2+ system, the q⁎mL /qmL value for Co2+ (0.399) was lower than that for Cs+ (0.632), indicating that the sorption of Co2+ was less competitive than Cs+ in the bi-solute system. The bonding energy coefficient (bL for single-solute and b⁎L for bisolute solutions) varied with sorbent type and relative affinity of the metal compared to competitive metal. The bL value was in the order of Sr2+ N Cs+ N Co2+ (Table 3), but the order of b⁎L value was changed to Cs+ N Sr2+ N Co2+ (Table 4). In addition, as shown in Table 5, the b⁎L /bL ratios in all cases except for Co2+/Sr2+ were greater than unity (2.58– 6.36), although maximum sorption coefficients for bi-solute solutions (q⁎ mL) were less than that for single-solute solutions (qmL). This indicates that, in all cases except for Co2+/Sr2+, competition for sorption sites promoted the retention of the metals on more specific sorption sites, thereby the metals were held more strongly [43,44]. In Co2+/Sr2+ system, however, b⁎L values for bi-solute solutions were less than bL values for single-solute solutions, thereby b⁎L /bL b 1, indicating that competition for sorption sites decreases the retention of both metals on more specific sorption sites. The competitive sorption data of heavy metals in bi-solute systems Co2+/Sr2+ (Fig. 9), Sr2+/Cs+ (Fig. 10) and Cs+/Co2+ (Fig. 11) onto PMM were fitted to the multi-solute sorption models. The model parameters for bi-solute systems (Co2+/Sr2+, Sr2+/Cs+ and Cs+/Co2+) are summarized in Tables 6 (SRS), 7 (EFM), 8 (MELM) and 9 (IASTFreundlich), respectively. The competitive sorption data and the model predictions were depicted together in Figs. 9 to 11. The sum of square errors (SSE) and the root mean square error (RMSE) between the

2 qi; exp −qi;pred

ð19Þ

Fig. 9. Bi-solute competitive sorption of Co2+ and Sr2+1 onto PMM. The mesh surface represents prediction by extended Langmuir model.

Fig. 10. Bi-solute competitive sorption of Cs+ and Sr2+ onto PMM. The mesh surface represents prediction by extended Langmuir model.

Table 4 Langmuir model parameters for bi-solute competitive sorption of metal ions onto PMM. Binary system

Metal

q⁎mL

b⁎L

R2

SSE

Co2+/Sr2+

Co2+ Sr2+ Sr2+ Cs+ Cs+ Co2+

0.1326 ± 0.0100 0.1256 ± 0.0074 0.0903 ± 0.0029 0.4091 ± 0.0247 0.4464 ± 0.0442 0.0805 ± 0.0047

0.4936 ± 0.1503 1.0189 ± 0.3006 12.972 ± 3.5551 11.245 ± 4.8573 8.1825 ± 7.4662 7.1979 ± 3.2011

0.8857 0.8541 0.7811 0.8782 0.7204 0.6011

0.0016 0.0018 0.0007 0.0319 0.0921 0.0016

Sr2+/Cs+ Cs+/Co2+

q⁎mL (mmol/g) and b⁎L (L/mmol) denote Langmuir model parameters for bi-solute competitive sorptions.

Fig. 11. Bi-solute competitive sorption of Cs+ and Co2+ onto PMM. The mesh surface represents prediction by extended Langmuir model.

344


Table 6 SRS model parameters for bi-solute competitive sorption.

Table 9 R2, SSE and RMSE values for the predictions of bi-solute competitive sorption from IAST coupled to single-solute Freundlich model.

Binary system

a12

a21

R2

SSE

RMSE

Co2+/Sr2+ Sr2+/Cs+ Co2+/Cs+

0.6487 1.2224 5.5629

0.1536 0.0481 0.0268

0.9940/0.9749 0.8939/0.7924 0.9948/0.8392

0.0006/0.0025 0.0095/0.2286 0.0181/0.2027

0.0080/0.0168 0.0325/0.1594 0.0448/0.1501

2

2

R =

∑qi; exp −SSE

RMSE =

rffiffiffiffiffiffiffiffiffiffiffiffiffi rss Nd −P

R2

SSE

RMSE

Co2+/Sr2+ Sr2+/Cs+ Co2+/Cs+

0.9528/0.9816 0.1130/– 0.9942/–

0.0045/0.0020 0.0973/3.1066 0.0203/2.8577

0.0237/0.0159 0.1103/0.6232 0.0503/0.5977

and Co2+/Cs+ systems were positive at low qj but decreased to be negative with qj, indicating antagonistic effects (Table 8). IAST-Freundlich model was fitted to the experimental data and the R2, SSE, RMSE values are listed in Table 9. IAST-Freundlich model predicted well Co2+/Sr2+ system only. Compared to the other models, IAST-Freundlich was poor in predicting the bi-solute competitive sorption.

ð20Þ

∑qi;2 exp

Binary system

ð21Þ

where qi,exp and qi,pred represent experimental data and theoretically predicted points, respectively, and rss is the residual sum of squares, Nd is the number of data points, and P is the number of parameters. In Table 6, the competition coefficients, aij, of the SRS model explain the suppression in metal sorption due to competition. In Co2+/Sr2+ system, a21 values of both metals are less than 1, indicating that they only slightly affected each other in competitive sorption. In Sr2+/Cs+ system, Sr2+ sorption (a12 N 1) was relatively more affected than Cs+ by competition whereas Cs+ sorption was not affected (a21 b 0.1). In Co2+/ Cs+ system, a12 value was very high (N5), meaning Co2+ sorption was suppressed due to the presence of Cs+; but that of Cs+ was not affected by Co2+ (a21 b 0.1). The high R2 values, low SSE and low RMSE values indicate that this model fitted well the experimental data. As can be seen in Figs. 9 to 11, the predictions are satisfactory. The predictions of bi-solute competitive sorption by the extended Freundlich model (EFM) were also depicted in Figs. 9 to 11. It is observed that the model fitted well the experimental data. Table 7 lists the involved six correlative model parameters in each bi-solute system obtained by non-linear regression method. The R2, SSE and RMSE values indicate that this model provide a good agreement between predicted and experimental data except for Sr2+ in Sr2+/Cs+ system. MELM was also fitted to the experimental data for competitive sorption of the metals onto PMM (Figs. 9–11). The estimated ai and bi values are listed in Table 8. As indicated by R2, SSE, and RMSE, the MELM fitted well the experimental data. This indicates that the synergistic efficiency (θ) of each heavy metal was fitted well to the linear Eq. (11). The positive a and negative b values of Co2+ and Sr2+ in Co2+/Sr2+ system, Sr2+ in Sr2+/Cs+ system and Co2+ in Co2+/Cs+ system explain that the θi values were negative at low qj but increased to be positive with qj, indicating the synergistic effects at above the crossover point (qj,cross). In contrast, the θi values for Cs+ in Sr2+/Cs+

3.3. Thermodynamic studies The effect of temperature on the sorption of Co2+, Sr2+ and Cs+ onto PMM is presented in Fig. 12 and the thermodynamic parameters are listed in Table 10. The sorption capacity of Sr2+ increased as temperature increases from 283 K to 303 K indicating an endothermic sorption process. As indicated by the ΔH value (6.15 kJ/mol for Sr2+), the sorption of Sr2+ onto PMM is more endothermic than those of Co2+ (−11.99 kJ/mol) and Cs+ (−14.90 kJ/mol). Thus the sorption of Sr2+ has to overcome an activation barrier and increasing temperature favors this reaction [45]. However, qe values for Co2+ and Cs+ decreased with increasing temperature suggesting an exothermic sorption process. The changes in entropy (ΔS) for Co2+, Sr2+ and Cs+ were − 17.55, 47.03 and −4.75 kJ/mol/K, respectively. The negative values of Co2+ and Cs+ indicated that the reactions occurred with the decrease in the entropies. The positive value of Sr2+ indicated that the sorption process was accompanied with some structural changes in the sorbent and sorbate during the sorption reaction [11,46]. Unuabonah et al. [11] reported that the positive ΔS value for Sr2+ is due to some structural changes in the adsorbate and adsorbent from aqueous solution onto the adsorbents by the increasing randomness at the solid–liquid interface during the adsorption. The change of free energy (ΔG) for all metal ions were negative indicating that the sorption of Co2+, Sr2+ and Cs+ onto PMM was spontaneous in nature and the degree of spontaneity of the reaction increases with increasing temperature. However, increasing temperature does not seem to change ΔG significantly. Similar results were reported by Unuabonah et al. [11]. They found that sorption of Pb2+ onto phosphate-modified kaolinite was a spontaneous process and this sorption reaction becomes more favorable with increasing temperature.

Table 7 Extended Freundlich model (EFM) parameters for bi-solute competitive sorption. Binary system 2+

2+

Co /Sr Sr2+/Cs+ Co2+/Cs+

b11

b12

b22

b21

a12

a21

R2

SSE

RMSE

− 1.2995 − 0.2723 − 1.0598

− 1.2634 − 0.0922 − 0.4451

− 0.3777 0.2418 0.0882

− 0.4823 − 0.0537 − 0.3079

0.5144 − 0.3756 0.1567

0.2705 1.8469 1.8746

0.9891/0.9771 –/0.8598 0.9953/0.8306

0.0010/0.0025 0.5898/0.1545 0.0163/0.2135

0.0122/0.0189 0.2903/0.1485 0.0483/0.1746

Table 8 Modified extended Langmuir model (MELM) parameters for bi-solute competitive sorption. Binary system 2+

2+

Co /Sr Sr2+/Cs+ Co2+/Cs+

a

b

qj,cross

θi at qj N qj,cross

R2

SSE

RMSE

26.439/6.1366 0.8514/− 9.1520 1.2527/− 11.160

− 1.0051/− 0.6763 − 0.4554/2.4987 − 0.7068/1.2152

0.0380/0.1102 0.5349/0.2730 0.5642/0.1089

Synergistic/synergistic Synergistic/antagonistic Synergistic/antagonistic

0.9508/0.9866 0.9564/0.7325 0.8298/0.7770

0.0047/0.0015 0.0040/0.2947 0.0108/0.2811

0.0259/0.0145 0.0240/0.2052 0.0393/0.2004


345

a) Co2+

Fig. 12. Relationship between ln Kd and 1/T for Co2+, Sr2+, and Cs+ sorption onto PMM.

b) Sr2+

3.4. Effect of pH The effect of initial solution pH on the sorption and precipitation of Co2+, Sr2+ and Cs+ were plotted in Fig. 13. For Co2+ (Fig. 13a), a sharp increase in Co2+ sorption was observed from pH 4 to 8. The steep increase is a typical sorption behavior of hydrolysable transition metal ions on clay surface. CoOH+ is formed and the interaction with net negative charged surface on PMM was rapidly completed [47]. In addition, the Co2+ was also sorbed onto hydroxyl groups of the phyllosilicates sheet [48]. Besides, the surface of PMM may be negatively charged and thus provide sorption sites for Co2+. 2þ

Co

þ

þH2 O⇔CoOH þ H −

þ

ð22Þ

þ

≡S O þCoOH ⇔≡S OCoOH

ð23Þ þ

2þ

ð≡S OHÞ þ Co ⇔ð≡S OÞ2 −Co þ 2H

ð≡S O Þ2 + Co

2þ

c) Cs+

ð24Þ

⇔ ≡S OÞ2 −Co

ð25Þ

where S represents the bivalent or trivalent metal ions (Fe2+, Si2+, Mg2+ or Al3+). At pH b4 (below pHpzc), the dissolution of sorbent and deconstruction of sorption sites on sorbent surface in acidic environment hinder the sorption reaction [48]. In addition, the excessive protons compete with Co2+ for the available sorption sites. Above pH 9, the disappearance of Co2+ in solution was mainly due to precipitation of Co(OH)2; approximately 95% of Co2+ was precipitated at pH 9. Chen and Lu [7] reported that the sorption of Co2+ onto montmorillonite increases markedly from pH 5 to 8 which is in agreement with our result. The strongly pH-dependent sorption of Co2+ indicates that surface complexation is the main mechanism of Co2+ sorption onto PMM. The effect of initial solution pH on Sr2+ sorption was presented in Fig. 13b. The final pH and the precipitated amount of Sr were also depicted. The influence of Sr2+ precipitation on the total sorption was negligible except at pH N11. At low pH (pH b pHpzc), the sorption of Sr2+ was suppressed by H+ competed for limited sorption sites [49,50] or the

Fig. 13. The influence of initial solution pH on the sorption and precipitation of (a) Co2+, (b) Sr2+ and (c) Cs+.

groups `S–O− on the surface were protonated to form `S–OH which was unable to make complex with Sr2+. At the initial pH range from 6 to 8, the Sr2+ sorption and final solution pH are nearly constant. This indicates that the main sorption mechanism for Sr2+ in this range is probably ion-exchange. The effect of H+ and precipitation on the sorption was negligible because H+ concentration was low and the precipitation could not occur at this pH range (6 to 8) as shown in Fig. 4. At pH 8, some groups `S–OH may be deprotonated to `S–O− and sorb Sr2+ through following interactions [6]: 2 +

≡S OH þ Sr

2 +

Table 10 Thermodynamic parameters for the sorption of Co2+, Sr2+ and Cs+ onto PMM.

Co2+ Sr2+ Cs+

≡S OH þ Sr −

2 +

≡S O þSr

ΔH (kJ/mol)

ΔS (kJ/mol/K, × 103)

ΔG (kJ/mol) 283 K

288 K

293 K

303 K

− 11.99 6.154 − 14.90

− 17.55 47.03 − 4.751

− 7.020 − 7.157 − 13.56

− 6.932 − 7.392 − 13.53

− 6.844 − 7.627 − 13.51

− 6.669 − 8.098 − 13.46

+ H2 O⇔≡S OSrOH þ 2H þ

⇔≡S OSr + H

⇔≡S OSr

þ

þ

þ

ð26Þ ð27Þ ð28Þ

At pH N11, the obvious increase in sorption of Sr2+ is caused by the precipitation of Sr(OH)2. Another possible mechanism is that the formation of SrOH+ enhanced the sorption onto PMM. The similar pH

346


dependent sorption behaviors of Sr2+ on various sorbents were reported by other researchers [6,50]. The sorption of Cs+ onto PMM increased slightly with increasing initial solution pH as shown in Fig. 13c. The relatively low sorption of Cs+ at low pH (bpHpzc) is attributed to competition with H+ for available sorption sites [49]. As pH increases, the numbers of protons on the sorbent surface decrease and give rise to more net negative charge, allowing more Cs+ to be sorbed electrostatically. Wang et al. [51] found that at high pH Cs+ sorption onto bentonite was dominated by surface complexation. A decrease in sorption was observed at pH 12 due to formation of carbonate or hydroxide of Cs+ ions resulting in lower Cs+ available for sorption [52].

4. Conclusions After modification with KH2PO4, the structure of montmorillonite was changed as confirmed by XRD and EDS analyses. The results of single-solute sorption experiments indicated that PMM has potential to remove Co2+, Sr2+ and Cs+ from aqueous solution. The sorption data were well fitted with Freundlich, Langmuir, and Dubinin– Radushkevich models. For bi-solute competitive sorptions, the sorption of each metal ion was suppressed by the presence of other metal ion. The maximum sorption capacity (q⁎mL) values in bi-solute system were lower than those calculated from single-solute sorption (qmL). The bonding energy coefficient (b⁎L ) values in bi-solute system were higher than those in single-solute sorption (bL), except for Co2+/ Sr2+. The Sheindorf–Rebhun–Sheintuch (SRS), the extended Freundlich model (EFM), the modified extended Langmuir model (MELM) and the IAST-Freundlich model were fitted to the bi-solute competitive sorption. Among these models, SRS and EFM models fitted the experimental data better than MELM and IAST-Freundlich models. The positive ΔH value for Sr2+ indicated endothermic sorption processes. However, sorptions of Co2+ and Cs+ were exothermic (i.e., negative ΔH values). All the sorption processes are spontaneous in nature according to the negative values of ΔG. The sorption of Co2+ onto PMM was strongly pH dependent and surface complexation was considered as the dominant sorption mechanism. The initial pH had slight effect on the sorption of Cs+ onto PMM. The sorption of Cs+ was suppressed by competition with H+ at low pH (bpHpzc). In this study, the dominant sorption mechanism of Sr2+ onto PMM depended on the initial solution pH range; ion-exchange (pH b 8) and surface complexation (pH N 8).

Acknowledgments This work was supported by Nuclear Research & Development Program through the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) (Grant number: M20706000036-07M0600-03610). The authors also would like to acknowledge Korea Basic Science Institute, Daegu for assistance of BET, SEM, EDS and XRD analyses.

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