Enhanced desorption of Cs from clays by a polymeric cation-exchange agent

Accepted Manuscript Title: Enhanced desorption of Cs from clays by a polymeric cation-exchange agent Author: Chan Woo Park Bo Hyun Kim Hee-Man Yang Bum-Kyoung Seo Kune-Woo Lee PII: DOI: Reference:

S0304-3894(16)31178-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.12.037 HAZMAT 18269

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

11-8-2016 17-12-2016 20-12-2016

Please cite this article as: Chan Woo Park, Bo Hyun Kim, Hee-Man Yang, Bum-Kyoung Seo, Kune-Woo Lee, Enhanced desorption of Cs from clays by a polymeric cation-exchange agent, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.12.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced desorption of Cs from clays by a polymeric cation-exchange agent Chan Woo Park,a* Bo Hyun Kim, a,b Hee-Man Yang,a Bum-Kyoung Seoa and Kune-Woo Leea*

a

Decontamination & Decommissioning Research Division, Korea Atomic Energy Research

Institute, Daedeok-daero 989-111, Yuseong-gu, Daejeon, Republic of Korea b

Department of Chemical Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-

gu, Daejeon, Republic of Korea KEYWORDS

Polyelectrolyte; Polyethyleneimine; Cesium; Clay; Desorption

Corresponding Author *Chan Woo Park Decontamination & Decommissioning Research Division, Korea Atomic Energy Research Institute, Daedeok-daero 989-111, Yuseong-gu, Daejeon, Republic of Korea E-mail: [email protected] Telephone: +82-42-866-6160

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*Kune-Woo Lee Decontamination & Decommissioning Research Division, Korea Atomic Energy Research Institute, Daedeok-daero 989-111, Yuseong-gu, Daejeon, Republic of Korea E-mail: [email protected] Telephone: +82-42-868-8269

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Graphical Abstract

Highlights - A cationic polyelectrolyte has excellent ability to desorb Cs bound strongly to clay - The polycation desorbed significantly more Cs from the clay than did single cations - Additional NH4+ treatment following the polycation treatment enhanced desorption of Cs - The reaction yielded efficient desorption (95 %) of an extremely low concentration of Cs-137 in the clay

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ABSTRACT

We report on a new approach to increase the removal of cesium from contaminated clays based on the intercalation of a cationic polyelectrolyte into the clay interlayers. A highly charged cationic polyelectrolyte, polyethyleneimine (PEI), was shown to intercalate into the negatively charged interlayers and readily replaced Cs ions adsorbed on the interlayers of montmorillonite. The polycation desorbed significantly more Cs strongly bound to the clay than did single cations. Moreover, additional NH4+ treatment following the PEI treatment enhanced desorption of Cs ions that were less accessible by the bulky polyelectrolyte. This synergistic effect of PEI with NH4+ yielded efficient desorption (95%) of an extremely low concentration of radioactive

137

Cs in the

clay, which is very difficult to remove by simple cation-exchange methods due to the increased stability of the binding of Cs to the clay at low Cs concentrations.

Keywords: Polyelectrolyte; Polyethyleneimine; Cesium; Clay; Desorption

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1. INTRODUCTION

Serious environmental contamination with radionuclides resulted from the Fukushima Daiichi nuclear accident in Japan, and a very large amount of soil (~28 million m3) in a wide area became contaminated [1–3]. In addition to this critical accident, unintended leakages of radioactive substances from various nuclear facilities have also contaminated the environment, and most of the radio-contaminated soils are subject to low-level wastes [4]. Remediation of radiocontaminated soil has been attempted with various techniques, such as soil washing, phytoremediation, and electroremediation, to name a few [5–9]. Although considerable progress has been made in the development of soil remediation techniques for some radionuclides such as U and Co, removal of radioactive Cs from soil has been relatively inefficient or has required high energy consumption owing to the strong and irreversible interaction of Cs with 2:1 clay minerals in the soil [2,5,7,8,10,11]. Expandable clay minerals, such as montmorillonite (MMT) and vermiculite, generally sorb much larger amounts of Cs than do non-expandable clay minerals such as illite and micas, and most Cs adsorption occurs in the clay interlayers [12–14]. Adsorption of Cs on clays occurs through not only outer-sphere complexation of hydrated Cs+ ions with the negatively charged clay surfaces but also direct coordination of partially or fully dehydrated Cs+ with siloxane groups of clays via inner-sphere complexation [14]. Cs bound to clays via inner-sphere complexes is believed to be very difficult to desorb due to the strong binding afforded by such complexes. Theoretical and experimental studies have suggested that MMTs partially form strong inner-sphere complexes with Cs ions [14–16]. Additionally, Chorover et al. observed an increasingly favourable formation of the inner-sphere complex as the concentration of Cs in MMT was reduced [14].

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Because of the high Cs sorption and retention capacity of MMT, engineered barriers made up of MMT are generally installed around nuclear facilities to reduce migration of radionuclides, and unintended leakages of radionuclides from nuclear facilities can generate a large amount of contaminated clay waste [17]. Desorption of Cs from MMT has been attempted with a simple ion-exchange reaction using solutions with high concentrations of a cation, such as NH 4+, Na+ or K+ [18–22]. However, such cations were only able to exchange less than 50~60% of the Cs due to the semipermanent adsorption of Cs on MMT. Similarly, only ~60% of the Cs was released from contaminated soil in Fukushima when 0.1 M K+ was applied over a course of 140 days [23]. In addition, divalent cations showed a rather poor Cs desorption efficiency due to agglomeration of MMTs even though these divalent cations have a higher ionic charge and selectivity than do the monovalent cations [18]. Therefore, a new method that can efficiently desorb Cs from clays must be developed in order to satisfactorily remediate a radiocesium-contaminated environment containing MMTs. Herein, we sought to enhance the removal of Cs from montmorillonite by carrying out an intercalation reaction using a polymeric cation-exchange agent (Figure 1). We hypothesized that cationic polyelectrolytes, which have many charged groups in a single polymer chain, would improve desorption of Cs through ion exchange because such polyelectrolytes form strong electrostatic interactions with negatively charged clays. Previous reports showed that cationic polyelectrolytes intercalate into clay interlayers, and the resulting strong electrostatic interactions sometimes exfoliated the clay layers [24–26]. Polyethyleneimine (PEI), containing various states of amine groups, was selected as the cationic polyelectrolyte, and the ability of this polyelectrolyte to replace Cs was investigated by comparing the results using the polyelectrolyte with the results using single cations at various reaction conditions. In addition, the possibility of a synergistic

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improvement of Cs desorption was investigated by following the intercalation of the polyelectrolyte with an ion-exchange reaction using single cations.

2. MATERIALS AND METHODS

2.1 Materials Cesium chloride, ammonium nitrate, diethylenetriamine (DETA), tetraethylenepentamine (TEPA), and branched polyethyleneimine (PEI) of various Mw values (i.e., ~800 g/mol, ~1300 g/mol, ~2000 g/mol, ~25,000 g/mol) were purchased from Sigma-Aldrich and were used as received. The branched polyethyleneimines are denoted as PEI 0.8k, PEI 1.3k, PEI 2k, and PEI 25k based on the molar mass. Ca-rich montmorillonite (Ca-MMT, SAz-1) was obtained from the Source Clay Repository (The Clay Minerals Society, USA) and was used as is, without purification. The SAz-1 is a high purity montmorillonite (>98%) collected from the Bidahochi formation in Apache County, Arizona, USA [27]. The cation-exchange capacity of the clay is 125 meq/kg [28].

2.2 Preparation of Cs-adsorbed montmorillonite Ca-MMT (10 g) was suspended in an aqueous solution of CsCl at 3 mM (100 mL) in a polypropylene bottle, and agitated at room temperature for seven days on a horizontal shaker. The Cs-adsorbed montmorillonites (Cs-MMTs) were then separated and washed twice with deionized water by centrifugation, followed by drying at 40 °C. The unbound cesium in the aqueous phase was quantified using inductively coupled plasma mass spectrometry (ICP-MS; ELAN DRC II,

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PerkinElmer, USA). The adsorbed amount of Cs on the Ca-MMT was 29.6 µmol/g clay, which is equivalent to 2.4% of the cationic exchange capacity (CEC) of SAz-1 [28].

2.3 Intercalation of cationic polyelectrolytes into Cs-MMT Cs-MMT (200 mg) was suspended in 20 mL of aqueous solutions containing various molar masses of cationic polyelectrolyte (DETA, TEPA and PEI). Various amounts of cationic polyelectrolyte were reacted with Cs-MMT in the range of 0 to 23 mmol/g clay, based on the moles of amine groups added per gram of clay. Each amine group of the cationic polyelectrolyte was considered as an ionizable group. The pH of the mixture was adjusted to be 3, 7 or 11 with 1 N HCl solution. Unless otherwise indicated, polyelectrolyte solutions directly dissolved in deionized water (~pH 11) were used without pH adjustment. Shaking of the mixture continued for one day at 20 °C or 80 °C using a temperature-controlled horizontal shaker. The clays were then separated by centrifugation and washed twice with deionized water, followed by drying in a convection oven. After the reaction of the Cs-MMT with PEI, an additional ion-exchange procedure with ammonium ions was carried out in some of the experiments. Ammonium nitrate (5 mmol/g clay) was added to the cooled clay suspension after the reaction with PEI at 80 °C for one day.

2.4 Characterizations The amounts of polyelectrolytes adsorbed on Cs-MMT were determined by quantitative analysis of unadsorbed polyelectrolyte in the supernatants using a total organic carbon (TOC) analyser (TOC-VWP, Shimadzu, Japan).

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X-ray diffraction analyses of the Cs-MMT with various polyelectrolyte loadings were performed on a Rigaku SmartLab diffractometer (Japan) using Cu Kα radiation (λ = 1.54 nm). Dried samples were prepared by applying sufficient drying at 70 °C. The desorbed amounts of non-radioactive Cs were quantified by carrying out ICP-MS (ELAN DRC II, Perkin-Elmer). After the Cs desorption experiment, the supernatant separated from the clays was filtered through a polyvinylidene fluoride (PVDF) membrane filter (pore size = 0.2 µm), and the filtrate (5 g) was digested in HNO3 (20 mL) at 110 °C for 6 h for the ICP-MS analysis. Zeta-potential measurements of the clay suspension in deionized water (10 mg/mL) was carried out using a Zetasizer Nano ZS (Malvern Instruments, UK) at 25 °C. The degree of ionization of the PEI was determined by potentiometric back-titration of the PEI solution in deionized water (5 mg in 10 mL). First, the polymer solution was titrated to pH 12 with 0.1 NaOH, and the solution was then back-titrated with 0.1 N HCl solution in various volume increments. The titrant was added to the polymer solution when the pH was stabilized within a change of ± 0.01 pH units over a monitoring period of 3 min. The titration was carried out at 25 °C using an Orion Versa Star Pro pH meter. The electrode was tree-point calibrated with standard buffer solutions (pH 4, 7, and 10) before measurement. The degree of ionization as a function of pH was estimated as follow, taking into account the volumes of HCl required to fully protonate amine groups from the neutralized state: (V-V1)/(V2-V1) where V1 and V2 represents the titrant volume (V) at the first and last inflection points of the back-titration curve, respectively[29,30].

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2.5 Adsorption and desorption of radioactive 137Cs Radioactive 137Cs-adsorbed MMT (137Cs-MMT) was prepared by mixing Ca-MMT (15 g) with 150 mL of a 137Cs solution (37 Bq/mL) for seven days at room temperature. After separation of the 137Cs-MMT by centrifugation and washing with deionized water, the clay was dried at 40 °C, and the radioactivity of the aqueous phase containing the unadsorbed 137Cs was analysed with a multi-channel analyser (MCA, CANBERRA Ind.) equipped with a high purity germanium detector to assess the 137Cs contamination level of the clay. The radioactivity of the obtained 137CsMMT was 369 Bq/g (= 15.8 nmol of Cs per gram clay). For the desorption experiments, 350 mg of 137Cs-MMT (369 Bq/g) was mixed with each PEI solution (12 mmol/g clay) at pH 3, 7 and 11 at 20 °C or 80 °C. After one day of reaction, ammonium nitrate (5 mmol/g clay) was added to the suspension, followed by mixing for one additional day at 20 °C. The radioactivity of the supernatant, filtered through a PVDF membrane filter with a pore size of 0.2 µm, was measured with the MCA to quantify the desorbed amount of 137

Cs.

3. RESULTS AND DISCUSSION 3.1 Effect of molar mass of the cationic polyelectrolyte on Cs desorption To find the cationic polyelectrolyte that best desorbs Cs from the clays, we tested cationic polyelectrolytes of various molar mass, such as PEI, with Mw values of 800, 1,300, 2,000, 2,500 g/mol, as well as DETA and TEPA, for their abilities to desorb Cs from Cs-MMT. We also

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investigated the adsorption of these polyelectrolytes on Cs-MMT. Cs-MMT (29.6 µmol/g clay, 2.4% of CEC) was mixed with 12 mmol/g clay of polyelectrolyte solutions, based on ionizable amine groups, and allowed to equilibrate at 80 °C for one day. As listed in Table 1, the percentage of the Cs desorbed from the clay tended to increase as the molar mass of the polyelectrolyte increased, and PEI 2k showed the highest Cs removal efficiency (91.3 ± 7.9%). However, the highest tested molar mass of PEI, 25k (i.e., 25,000 g/mol), desorbed less Cs than did PEI 2k. To account for the effect of molar mass on Cs desorption, the adsorption properties of polyelectrolytes were investigated. Changes in the interlayer distances of the Cs-MMT resulting from the polyelectrolyte adsorption were determined by XRD analysis to determine whether the polyelectrolytes were intercalated into the clay interlayers. After the reaction of Cs-MMT with the polyelectrolytes, the dried clays showed slightly increased interlayer distances (13~14 Å) compared to the pristine Cs-MMT (12.5 Å), indicating that the polyelectrolytes indeed intercalated into the clay interlayers. Moreover, the interlayer distance tended to slightly increase as the molar mass of the polyelectrolyte increased. The intercalated polyelectrolytes were derived to be 3~4 Å thick, by subtracting the thickness of the montmorillonite layer (9.7~10 Å) from the measured interlayer distance [31,32]. This 3~4 Å thickness corresponded to that of a single layer of extended PEI chains in the clay interlayers [33]. The adsorbed amount of the cationic polyelectrolyte also increased as its molar mass increased, and 4.6 mmol of PEI 25k were adsorbed per gram of clay, more than for the other polyelectrolytes tested. When the amount of adsorbed polyelectrolyte was converted to the number of adsorbed polymer chains based on their molar mass, however, it was found that fewer such chains were adsorbed on Cs-MMT as the molar mass increased. For example, only 8 µmol of PEI

11

25k chains were adsorbed per gram of clay, much less than that for any other polyelectrolyte tested. Considering the bulkiness of the high molar mass PEI 25k, the polymer has reduced access into the clay interlayers, and that may result in less homogeneous intercalation and adsorption of the PEI 25k into the interlayers rather than homogenous adsorption on the entire surfaces of the interlayers. We expect that this inhomogeneous adsorption of high molar mass PEI would cause reduced contact between the PEI and the Cs ions in the interlayers and result in reduced Cs desorption. The PEI 2k was the optimum molar mass for Cs desorption and therefore was used for the following experiments.

3.2 Desorption of Cs by PEI intercalation To verify the effect of PEI concentration on Cs desorption, solutions with various amounts of PEI 2k, in the range of 0 to 23 mmol/g clay, based on the moles of the amine groups, were reacted with Cs-MMT at 80 °C for one day. As the PEI 2k concentration increased, the amount of adsorbed PEI 2k increased up to 4.5 ± 0.5 mmol/g clay within the investigated range of PEI 2k concentrations (Figure 2a). Intercalation of PEI 2k into Cs-MMT with different PEI 2k loadings was determined by using XRD to measure the interlayer distances of the dried clays (Figure 2b). The interlayer distance increased from 12.5 Å to ~14 Å as the amount of PEI 2k added increased to 2.4 mmol/g clay, and this distance was maintained up to 23 mmol/g clay. The Cs removal efficiency rapidly increased as the concentration of PEI 2k increased, and 91.3 ± 7.9% and 94.1 ± 8.5% of the Cs ions were successfully removed from Cs-MMT as a result

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of adding 12 and 23 mmol/g clay of PEI 2k, respectively (Figure 2c). The amount of Cs desorbed from Cs-MMT increased proportionally with the amount of PEI 2k adsorbed onto the clay, and saturation of Cs desorption was observed above 12 mmol/g clay of PEI 2k added, similar to the saturation of PEI 2k adsorption. These results imply that, in our experiments, the adsorption of the cationic PEI 2k on the negatively charged MMT, stabilized by columbic interactions, led to the desorption of Cs ions by an ion-exchange process, and the increased intercalation of PEI 2k into the interlayers at high PEI 2k concentrations provided more efficient removal of Cs ions, which are mainly present in the interlayers of MMT [12]. To definitively determine whether the polyelectrolyte, which carries many cationic groups in a single polymer chain, can yield better Cs desorption than unlinked single cations, the Csdesorption efficiency of PEI 2k was compared with that of ammonium ions at similar moles of ionizable amine units at 80 °C (Figure 2c). Chaotropic ammonium ions and caesium ions have similar ionic radii, and these cations strongly interact with oppositely charged clays. Ammonium ions have therefore been widely investigated to exchange adsorbed Cs on clays [21,22,34]. As shown in Figure 2c, PEI 2k yielded better Cs desorption than did NH 4+. At 20 mmol/g clay, NH4+ ions desorbed only 21.3 ± 3.4% of the Cs while a similar amount of PEI 2k (23 mmol/g clay) desorbed 94.1 ± 8.5% of the Cs from Cs-MMT. Furthermore, excess ammonium ions (50 mmol/g), which is 40 times greater than the cationic exchange capacity of the Ca-MMT, desorbed only 34.1 ± 4.4% and 42.8 ± 5.1% of the Cs from Cs-MMT at 20 °C and 80 °C, respectively. On the other hand, the PEI 2k desorbed most of the Cs from MMT — including the Cs that was difficult for NH4+ to remove — because the adsorbed polyelectrolyte, unlike unlinked cations, can yield a significantly increased local concentration of cations on an interlayer. Upon adsorption of PEI on interlayer, cations of the polyelectrolyte not yet bound to the clay would continuously compete

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with Cs ions for desorption, and the expansion of the clay interlayer by PEI intercalation would enhance diffusion of the desorbed Cs from the interlayer.

3.3 Effect of additional NH4+ treatment on Cs desorption A further enhancement of Cs desorption was achieved by carrying out an additional NH4+ treatment at 20 °C for one day after the one-day PEI 2k intercalation treatment at 80 °C (Figure 2d). In the case of the PEI 2k-intercalated Cs-MMT at 2.3 mmol/g clay, the additional NH4+ (5 mmol/g clay) treatment desorbed ~23% additional Cs. This enhancement (Figure 2d) was in fact twice as high as the total amount of Cs desorbed (10.9 ± 2.2%) when treating Cs-MMT only with NH4+ (Figure 2c). Moreover, even though desorption of Cs by PEI 2k intercalation alone reached a near-saturation efficiency level of approximately 91% when treating the Cs-MMT with 12 mmol of PEI 2k per gram of clay (Figure 2c or 2d), additional desorption of Cs to an efficiency level of 96.4 ± 10.7% was achieved when this treatment with 12 mmol/g clay of PEI 2k was followed by the NH4+ treatment (Figure 2d). It seems that the small NH4+ ions can readily diffuse into the PEI 2k-intercalated interlayer and desorb Cs ions that were not accessible to the relatively bulky PEI. In addition, the maximum desorption efficiency of PEI 2k/NH4+ was slightly higher than the maximum Cs desorption (94.1 ± 8.5%) when using only PEI 2k. These results indicate that the PEI 2k intercalation and the additional exchange with NH4+ ions provided a complementary and synergetic effect on the Cs desorption.

3.4 Effect of pH on Cs desorption by PEI

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Because the extent of protonation of PEI depends on pH [35], the effect of pH on Cs desorption was investigated. The degree of ionization (α) of PEI 2k as a function of pH was characterized by potentiometric back-titration of PEI 2k solution with HCl solution. As shown in Figure 3a (crosshair symbol), the degree of ionization increased with decreasing pH, and the α values at pH 3, 7 and 11 were 0.8, 0.51 and 0.05, respectively. The zeta potential of branched PEI 2k was observed to be +3 mV at pH 11, and +43 mV at pH 3, indicating that PEI 2k became highly cationic due to protonation of its amines at the acidic condition. On the other hand, the negative net charge of Cs-MMT was significantly reduced as the pH decreased, and this is ascribed to the protonation of hydroxyl groups at clay edge faces (Figure 3a). Although zeta-potential measurements may not provide information on charge states in the clay interlayers, it is well known that the negative charge of the interlayers, which originates from the isomorphic substitution of the mineral structure, is pH-independent and can remain to provide an adsorption site for Cs [36,37]. The amount of PEI 2k adsorbed onto Cs-MMT, when the PEI 2k was added at a concentration of 12 mmol/g clay, was observed to slightly decrease as the solution pH decreased, and the differences in the amounts of PEI 2k adsorbed at 20 °C and 80 °C were negligible at each pH (Figure 3b). For instance, the amounts of PEI 2k adsorbed at pH 11 and pH 3 were 3.75 ± 0.31 and 3.27 ± 0.24 mmol/g clay, respectively, when the reaction temperature was 20 °C. We expect the reduced PEI 2k adsorption on Cs-MMT at acidic conditions can be ascribed to the reduced negative net charge of Cs-MMT (Figure 3a). In addition, the increased positive charge of PEI 2k at acidic conditions may increase the repulsive force between PEI chains. Similarly, reduction in the adsorption of the cationic polyelectrolyte on anionic surfaces at an acidic condition was theoretically predicted and experimentally demonstrated, and both the decreased negative surface

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charges and the increased repulsion between protonated polymers at acidic condition are believed to be key contributors [35,38,39]. Unlike the variable charges of the external edge planes, the clay interlayers possess a permanent negative charge even at acidic pH, and this suggests that the adsorption of PEI 2k may have been mostly occurring in the clay interlayers at pH 3. XRD analyses of Cs-MMTs treated with PEI 2k indicated that PEI 2k was successfully intercalated into the clay interlayers, as indicated by the expansion of the interlayer from 12.5 Å to ~14 Å, regardless of the reaction pH (i.e., at acidic, neutral and basic reaction pH conditions) and regardless of the reaction temperature (i.e., at 20 °C and 80 °C) (Figure 3c). However, changing the pH and temperature did significantly affect Cs desorption by PEI 2k. Decreasing the pH and increasing the reaction temperature significantly enhanced Cs desorption (Figure 3d). By decreasing the pH from 11 to 3, the Cs desorption efficiency of PEI 2k increased from 91.3 ± 7.9% to 97.9 ± 10.9% at 80 °C and from 59.2 ± 5.9% to 74.6 ± 8.5% at 20 °C. The control experiment of Cs desorption in the absence of PEI 2k at each pH showed that, at best, only 11.2% of the Cs was desorbed, which occurred at pH 3. X-ray fluorescence analyses of Cs-MMTs treated with acidic solutions at pH 3 and at 80 °C in the presence or absence of PEI 2k showed that the SiO2/Al2O3 ratios did not change significantly after the reaction (Table S1, see Supporting Information). This indicates that dissolution of the clay and consequent Cs releases at pH 3 were negligible. Ionic species having a high charge density are generally favoured to exchange like-charged ions adsorbed on oppositely charged surfaces due to their high columbic interactions, and the increased charge density of PEI 2k at pH 3 due to the protonation of its amine groups (α = 0.8) resulted in a strong ionic interaction with the negatively charged interlayers and consequently enhanced the exchange with Cs. In addition, a decrease in the hydration radius of the Cs cation at

16

higher temperatures may have resulted in a decreased binding stability of the clays for Cs and hence decreased adsorption of Cs by the clays [40]. Such a temperature dependence of the adsorption of Cs by the clay may explain why PEI 2k more readily desorbed Cs from Cs-MMT at 80 °C than at 20 °C (Figure 3d) even though similar amounts of PEI 2k were adsorbed on the clay at the two temperatures (Figure 3b).

3.5 Kinetics of Cs desorption by PEI

The kinetics of the PEI 2k adsorption by Cs-MMT and consequent Cs desorption from MMT were investigated after adding 12 mmol/g clay of PEI 2k at two different pH (pH 3 and 11) and at two different temperatures (20 °C and 80 °C). As shown in Figure 4a, the adsorption of PEI 2k on Cs-MMT rapidly reached equilibrium within one hour regardless of the reaction temperature and pH. The adsorption kinetic data were fit to pseudo-first-order and pseudo-second-order kinetic models. The fit of the data was better to the pseudo-second-order kinetic model (R2 = 0.99) than to the pseudo-first-order kinetic model (R2 = 0.65) at pH 3 and 20 °C (See Supplementary Information, ESI 3). An elevated reaction temperature increased the PEI 2k adsorption rate, and the second-order rate constant values for 20 °C and 80 °C at pH 3 were 4.93 and 6.02 mg/mg·min, respectively (Table S2 in ESI 3). Similarly, rapid desorption of Cs by PEI 2k occurred within 30 min, followed by slow desorption of Cs over one day (Figure 4b). For example, the PEI 2k treatment at pH 3 at 80 °C removed 88% of the Cs within one hour, and reached 97% Cs removal after one day.

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3.6 Removal of radioactive 137Cs in clay at extremely low concentrations by PEI intercalation We finally sought to demonstrate the ability of PEI 2k to remove

137

Cs from MMT at a

very low concentration of the Cs because most of the radio-contaminated soils are subject to lowlevel wastes [4]. The selectivity of clay minerals, such as Ca-MMT and illite, for Cs has been reported to increase for decreasing Cs concentrations, and the concentration-dependent selectivity of Ca-MMT is considered to be a result of inner-sphere complex formation [41]. Cs ions show an increasing tendency to form strong inner-sphere complexes with MMT as the Cs concentration in MMT decreases, and an increase in the percentage of the inner-sphere complex from 0% to 48% was previously observed by reducing the Cs concentration [14]. For this reason, desorption of Cs from MMT with a low Cs loading is expected to be relatively challenging. To investigate the desorption of Cs from Cs-MMT with an extremely low Cs loading, 137

Cs-contaminated MMT (137Cs-MMT) with a radioactivity of 369 Bq/g (= 15.8 nmol/g clay) was

obtained. This loading was ~1,874 times lower than that of the non-radioactive Cs-MMT sample described above. As observed with non-radioactive Cs, the desorbed amounts of 137Cs by PEI 2k intercalation at 12 mmol/g clay was observed to increase with decreasing pH and increasing reaction temperature. The desorption efficiency of this 137Cs from Cs-MMT was 8~27% less than that for the non-radioactive Cs, consistent with the greater stability of the binding of Cs to MMT at low Cs loading. Nevertheless, adding PEI 2k (12 mmol/g clay) at pH 3 desorbed significant amounts of

137

Cs, with 86.1 ± 6.5% and 66.0 ± 7.2% of

137

Cs removed at 80 °C and 20 °C,

respectively (Figure 5 and table S3 in ESI 4). These levels of desorption of 137Cs by PEI 2k were much greater than the levels of desorption of

137

Cs resulting from applying an NH4+ solution (5

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mmol/g clay), which were 9.5 ± 2.7% and 13.2 ± 3.9% at 20 °C and 80 °C, respectively, indicating the high effectiveness of PEI 2k intercalation at desorbing 137Cs from Cs-MMT. The effect of conducting ion exchange with NH4+ (5 mmol/g clay) after the PEI 2k treatment was also investigated as described above. Approximately 4~30% more Cs was desorbed by adding the NH4+ exchange step. The two-step reaction including the PEI 2k treatment at 80 °C and at pH 3 showed a maximum

137

Cs desorption of 95.2 ± 7.7%, and revealed the great Cs-

desorbing ability of PEI 2k/NH4+ even at an extremely low loading of Cs in MMT.

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CONCLUSIONS

In conclusion, we demonstrated the excellent ability of cationic polyelectrolyte polyethyleneimine to desorb Cs that is bound strongly to montmorillonite. Upon adsorption and intercalation of PEI into the clay interlayers, the polymer structure containing many cationic units in a single chain desorbed much more Cs than did the equivalent amount of ammonium cations. This improved desorption was likely due to the high charge density and high local concentration of cationic units in the polymer. The highly charged PEI more effectively desorbed Cs from MMT under acidic reaction conditions than did weakly charged PEI, and the cationic polyelectrolyte desorbed Cs readily at an elevated temperature. Moreover, adding an NH4+ treatment after the PEI treatment further enhanced the desorption of Cs, perhaps due to the NH4+ ions being able to exchange with Cs ions which are less accessible to the polyelectrolyte, and due to the small amount of PEI intercalation having promoted the Cs-desorption ability of the NH4+ ions. This synergistic effect of PEI with NH4+ yielded an efficient desorption (95.2 ± 7.7%) of an extremely low concentration of 137Cs in MMT, an impressive result since such desorption is usually difficult to accomplish by simple cation exchange due to the increased stability of the binding of Cs to MMT

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at low Cs concentrations via inner-sphere complexation. The cationic polyelectrolyte showed great promise as a desorption agent for Cs. The efficient metal-desorption ability of polyelectrolytes in general should open a variety of applications of polyelectrolytes for the remediation of toxic metalcontaminated environments and waste.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government, Ministry of Science, ICT and Future Planning (No. 2012M2A8A5025996). We thank Dr. S.-Y. Lee (KAERI) for helpful discussions and T. S. Song (KAERI) for MCA analysis.

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FIGURES AND TABLES

Figure 1. (a) Schematic of the Cs removal from clays based on intercalation of cationic polyelectrolytes. The additional ion-exchange step with ammonium ions affords a synergetic effect on the desorption of Cs resulting from cationic polyelectrolyte intercalation. (b) Chemical structures of cationic polyelectrolytes.

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Figure 2. Consequences of adding PEI 2k (Mw = 2000) for one day at 80 °C and/or NH4+ to CsMMT on the structure of the clay and the removal of Cs. (a) Adsorption of PEI 2k on Cs-MMT. (b) Interlayer distance of the PEI 2k-intercalated Cs-MMT as a function of the amount of PEI 2k added as measured using X-ray diffraction analysis. (c) Cs-removal efficiency as a function of the added amount of PEI 2k or NH4+. (d) Additional Cs removal by ion exchange with 5 mmol/g clay of NH4+ (○) at 20 °C for one day after the PEI-intercalation reaction (●). Each data point represents the average of at least 3 independent experiments and the error bar indicates standard deviation.

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Figure 3. (a) The dependence of the degree of ionization of PEI 2k (+) and zeta potentials of CsMMT (●) on pH. Effects of pH on (b) adsorption of PEI 2k on Cs-MMT, (c) interlayer distance changes resulting from PEI 2k intercalation and (d) desorption of Cs from Cs-MMT observed after adding PEI 2k (12 mmol/g clay) for one day at 20 °C (■) and 80 °C (●). Removal efficiency of Cs from Cs-MMT is also shown in the absence (empty symbols) of PEI 2k. Each data point represents the average of at least 3 independent experiments and the error bar indicates standard deviation.

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Figure 4. (a) PEI 2k adsorption on Cs-MMT and (b) Cs desorption as a function of time of reaction. Each reaction was carried out at 20 °C or 80 °C and at pH 3 or pH 11 with the 12 mmol/g clay of PEI 2k. Each data point represents the average of 3 independent experiments and the error bar indicates standard deviation.

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Figure 5. Removal efficiency of 137Cs from MMT containing an extremely small amount of 137Cs (15.8 nmol/g clay) by PEI 2k intercalation (12 mmol/g clay) with (empty symbols) or without (filled symbols) additional ion exchange with 5 mmol/g clay of NH4+ at various pH conditions at 20 °C for one day after the PEI 2k-intercalation reaction at (a) 20 °C or (b) 80 °C.

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Table 1. Effect of molar mass of the cationic polyelectrolyte on its adsorption on Cs-MMT and the consequent desorption of Cs at 80 °C and at pH 11. Compound

Molar mass (g/mol)

DETA TEPA PEI 0.8k PEI 1.3k PEI 2k PEI 25k

103 189 800 1,300 2,000 25,000

Adsorbed amine groups (mmol/g clay) 1.5 2.2 3.1 3.5 3.9 4.6

Adsorbed chains/clay (µmol/g clay) 497 433 169 113 84 8

Interlayer distance (Å) 13.3 13.3 13.8 13.9 14.1 14.1

removal Cs efficiency (%) 65.6 ± 12.4 71.3 ± 9.4 74.4 ± 13.3 81.3 ± 12.2 91.3 ± 7.9 79.4 ± 6.6

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