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ISSN : 1225-0112(Print)
ISSN : 2288-4505(Online)
Applied Chemistry for Engineering Vol.29 No.2 pp.127-137

Recent Advances in Adsorption Removal of Cesium from Aquatic Environment

Lalhmunsiama, Jae-Gyu Kim, Suk Soon Choi*, Seung-Mok Lee
Department of Environmental Engineering, Catholic Kwandong University, Gangneung 25601, Korea
*Biological and Environmental Engineering, Semyung University, Jecheon 27136, Korea
Corresponding Author: Catholic Kwandong University, Department of Environmental Engineering, Gangneung 25601, Korea Tel: +82-33-649-7535 e-mail:
February 22, 2018 ; March 5, 2018 ; March 7, 2018


Radioactive contamination has become an important environmental concern after the accident occurred in Fukushima Daiichi Nuclear Power Plants. 134Cs and 137Cs are the major fission products and they are main problems in radioactive contamination. Huge amounts of Cs were released during the Fukushima Daiichi Nuclear Power Plants accident and as a result of this incident, many researchers focused on the development of adsorbents for decontamination of radiotoxic cesium. This review will critically evaluate recent advances in the preparation of Prussian blue and its analogue compounds, which are promising materials for cesium removal. Furthermore, this review will discuss recent studies on the cesium adsorption using different types of clay and clay based adsorbents and summarize various types of newly developed Cs adsorbents reported in recent years.

수환경에서 세슘 흡착 제거의 최근 동향

랄 문시아마, 김 재규, 최 석순*, 이 승목
가톨릭관동대학교 환경공학과
*세명대학교 바이오환경공학과


후쿠시마 다이치 핵발전소의 사고 이후 방사능 오염이 중요한 환경 관심사가 되었다. 원자량 134와 137 세슘은 주요 핵분열 산물이며, 이물질들은 방사능 오염의 주된 문제들이다. 후쿠시마 다이치 핵 발전소 사고에서 다량의 세슘이 방출되었으며, 이 사고의 결과, 많은 연구자들이 방사능-독성 세슘 제거를 위한 흡착제 개발에 집중하였다. 본 총설에 서는 세슘 제거를 위하여 각광을 받는 물질로서 청색 안료와 이와 유사한 화합물 제조의 최근 발전 동향을 자세하게 검토하였다. 또한, 다양한 형태의 점토와 점토 기반 흡착제 및 새로 개발된 흡착제를 이용한 세슘 흡착의 최근 연구들 을 고찰하였다.

    1. Introduction

    Radioactive contamination has become an important environmental concern in many countries around the world as the large amount of radioactivity produced in the environment from nuclear weapon testing and the accidents occurred in nuclear power plants due to earthquake and tsunami[1]. Moreover, nuclear waste produced from nuclear power plant is a potential source of radioactivity in human environment[2]. 134Cs and 137Cs are the major fission products having long half-lives of 2.06 yrs and 30.2 yrs respectively[3]. 137Cs has received more concern due to its abundance, long half-life and gamma radiation emitter. Moreover, 137Cs is found to be a major source of heat and radiation in the high-level radioactive waste[4,5]. It was assumed the total amount of 137Cs released during the Fukushima Daiichi Nuclear Power Plants accidents range from 15-30 PBq[1]. Due to high water solubility, cesium can easily enter human body and caused an internal hazard such as cancer. Moreover, due to the similarity in chemical properties with sodium and potassium, it can be easily incorporated to terrestrial and aquatic organisms which make it a potential toxic contaminant in water[6].

    In order to abate the radioactive contamination in aquatic environment, different types of technique such as electrocoagulation process[7], reverse osmosis filtration[8], membrane distillation[9], ion exchange[10], adsorption[11], electrochemical separation[12,13], etc. were investigated for the removal of Cs from aqueous solutions. Among these techniques, adsorption is found to be economical, versatile and easy to operate. Therefore, this review summarized recent reports on the removal of cesium from aqueous media by adsorption technique.

    2. Prussian Blue

    Prussian blue (PB) is the most widely used material for cesium adsorption. PB is the active pharmaceutical ingredient of Radiogardase and it is an approved drug product for treatment of radiocesium poisoning by Food and Drug Administration (FDA) of United States[14]. PB crystal possessed a cage size similar to the hydration radius of Cs+ and this makes the PB as excellent trap for Cs+ ions[15]. Ishizaki and co-workers studied the fundamental mechanism of cesium adsorption into PB using two types of PB nanoparticles FeIII4[FeII(CN)6]3xH2O (x = 10-15) and (NH4)0.70FeIII1.10[FeII(CN)6]⋅1.7H2O. They have proposed that the specific Cs+ adsorption mainly occurred through proton exchange mechanism as shown in Figure 1. The hydrated Cs+ ions were selectively adsorbed via the lattice defect sites with hydrophilicity and a proton is eliminated from the coordination water of Fe(III)[16]. PB particles with interior hallow cavities was prepared by Torad and co-workers. SEM and TEM images of hallow PB nanoparticles are shown in Figure 2. The surface area (338 m2/g) of hallow PB nanoparticles is considerably higher than other PB particles. The Cs adsorption capacity is eight times larger than the commercial PB which is due to the presence of hallow cavity within the PB nanoparticles[17]. Chen and co-workers investigated the removal behaviour of Cs by PB from drinking water. The Cs-PB affinity was high and selective from competing ions and hypochlorite. The column tests and simulated rapid sand filter tests indicate that PB granules are highly efficient Cs adsorbents[18]. The feasibility of PB + anion exchange (AE) process was assessed for decontamination of Cs-polluted drinking water and the result suggested that PB+AE unit can be adequately use as an emergency response plan for typical drinking water purifier at nuclear accident site[15]. In addition to Prussian blue, the hexacyanoferrates containing transition metals have been widely studied as an effective adsorbent of Cs due to their selectivity towards Cs ions. The synthesis methods of these hexacyanoferrates compounds are found to be relatively simple and cost effective[19].

    2.1. Composite of Prussian blue or hexacyanoferrate compounds

    The preparation of composite containing PB nanoparticles or other hexacyanoferrate compounds is an alternative process to increase the efficiency of these materials towards cesium ions and it is a suitable approach for real water treatment applications. Mobtaker and co-work- ers synthesized the copper hexacyanoferrate @polyacrylonitrile @magnetite (CuHCNPAN) nanocomposite having the particles size of 20-60 nm in diameter. The material possessed high mechanical, chemical and thermal stability. The removal of cesium ions from waste solutions was assessed and the maximum sorption capacity obtained was about 260 mg/g. After the sorption process, the composite solids can be easily separated by using external magnet[20]. Iron hexacyanoferrate/ graphene/carbon fibers composite (PB/GN/CFs) prepared by electrochemical method and the recovery of Cs+ by PB/GN/CFs was investigated in adsorption and electrically switched ion exchange desorption method. Cs+ was selectively adsorbed by PB/GN/CFs from aqueous solution with an endothermic spontaneous reaction[21]. The Prussian blue functionalized microcapsule (PB-MCs) was developed and employed for the removal of cesium ions by flotation separation from aqueous solutions. The rapid separation rate of cesium ions from aqueous solutions due to the efficient combination of the advantages of poly(lactic acid) microcapsules (PLA MCs) and Prussian blue nanoparticles (PB NPs). The PB-MCs adsorbent was found to be environmentally friendly due to a good bio safety of Prussian blue and biodegradability of PLA. Moreover, the Prussian blue functionalized microcapsules could save energy consumption as a result of the flotation separation from wastewater after adsorption[22]. The feasibility of PB along with anion exchange (AE) granules was assessed for decontamination of Cs-polluted drinking water. The simulated rapid sand filter tests and field column tests revealed that the tested PB and AE configuration effectively removed Cs and total cyanide from water. It was suggested that the column packed with PB and anion exchange granule can be the core of emergency response plan for the typical treatment of radiocesium contaminated drinking water[15]. The Prussian blue (PB) nanoparticles that implement non-woven fabric was also used as an efficient adsorbent for Cs removal. The maximum adsorption capacities were estimated as 216, 241, 260 mg/g at 288, 298 and 308 K, respectively. The adsorption capacity was lower in acid solutions compared to the alkaline solutions and the adsorption process was found to be endothermic[23].

    Use of PB or other hexacyanoferrate compounds has a drawback because of the difficulty in recovery of the fine powder after the sorption is completed. Therefore, several researchers work on the incorporation of PB with magnetic particles and reported that these materials offer high potential for the treatment of water contaminated with radioactive Cs[24,25]. Magnetic Prussian blue nanocomposite was prepared by binding PB to a core of magnetite (Fe3O4) nanoparticles. The average size is 13.6 nm with the surface are of 322.19 m3/g[26]. Similarly, the magnetic Prussian blue/Fe3O4 core/shell nanoparticles were fabricated via a facile one-pot method and the final products have 20-40 nm diameter with a core/shell structures which consist of face-centered-cubic lattice Prussian blue and Fe3O4[27]. The magnetic PB nanocomposite can be retrieved easily after the adsorption process and detailed studies suggest the potential applicability in the treatment of radioactive wastewater containing cesium ions. In another study, the magnetic nanoclusters were incorporated with Prussian blue (PB) via a simple reaction with potassium hexacyanoferrate under acidic conditions. The resulting material have a good removal efficiency (> 99.7%) of radioactive cesium from contaminated water even in the presence of high concentration of competing ions such as K+, Na+, Ca2+ and Mg2+. The material possesses a good saturation magnetization value (27.5 emu/g) which is useful for rapid separation from the aqueous solution using an external magnetic field[28]. The Prussian blue coated magnetic nanoparticles termed as ‘Prussian blueberry’ was prepared by a layer to layer assembly method. The Prussian blueberry was found to be efficient in reducing the Cs concentration in sea water under the magnetic field while normal PB could not eliminate cesium. Furthermore, Prussian blueberry rapidly eliminates Cs from biological matrices such as serum and milk under the magnetic field[29].

    Naeimi and Faghihian prepared the magnetic metal-organic framework (MOF) and then modified by potassium nickel hexacyanoferrate. The magnetic nanocomposite possessed high adsorption affinity and selectivity for Cs+ in the presence of Na+ and K+ ions. The sorption process was spontaneous and more favorable at higher temperature and chemisorption was suggested as predominant mechanism[30]. Pectin-stabilized magnetic graphene oxide Prussian blue (PSMGPB) nanocomposites was prepared and employed for removal of cesium from wastewater. An enhanced adsorption capacity of PSMGPB nanocomposites was attributed to the pectin-stabilized separation of graphene oxide sheets and enhanced distribution of magnetites on the graphene oxide surface. A thermodynamic study indicated the spontaneous and the exothermic nature of cesium adsorption[31]. Furthermore, the magnetic prussian blue/graphene oxide (PB/Fe3O4/GO) nanocomposites encapsulated in calcium alginate microbeads (PFGM) was reported as highly selective to Cs+ and could extract it even in trace amounts. The maximum adsorption capacity was determined to be 43.52 mg/g. Cesium ions were absorbed primarily by both of chemisorption (K+/H+-exchange) and physisorption (ion trapping). The microbeads were stable in natural water, seawater, and pH value of solutions ranging from 4 to 10 without collapse of microbeads and leaching of prussian blue. Moreover, the microbeads could be separated effectively from aqueous solution (or soil suspensions) by an external magnetic field, which was convenient for large-scale treatment of cesium-contaminated water or soil[32]. The Prussian blue, i.e., iron(III) hexacyanoferrate( II) was immobilized in chitosan sponge and the composite was further re-acetylated to produce a chitin-PB sponge. The preliminary tests conducted with 137Cs(I) recovery showed that the chitin- PB sponge has high efficiency for the treatment of radioelement dumped waters[33]. The K2Zn3[Fe(CN)6]2 was used for decontamination of Cesium from wood ash-washing solution which have high pH and high K+-concentration. The efficiency of Prussian blue (PB), and zeolite were evalauted and compared with K2Zn3[Fe(CN)6]2. The distribution coefficient, Kd of KZnHCF, PB, and zeolite were 719, 4400, and 210 mL/g, respectively. The KZnHCF showed an enhanced adsorption even at pH 12, meanwhile, PB is effective only at pH below 9[34].

    2.2. Immobilization hexacyanoferrate compounds on substrate

    The Prussian blue or other metals hexacyanoferrate has very fine particle size which is not suitable for real water treatment. Therefore, many researchers focussed on the immobilization of the Prussian blue to other substrate prior to its application for cesium adsorption in aqueous solutions. The prussian blue immobilized on rayon fiber coated with chitosan was assessed for the removal of cesium ion (Cs+) from contaminated water. The adsorption capacity of this composite material was comparable to Prussian blue alone[35]. The Prussian blue was synthesized inside diatomite cavities by in situ and the carbon nanotubes (CNTs) were used for coating diatomite to seal in the Prussian blue particles. This ternary composite (CNT/diatomite/Prussian-blue) was mixed with polyurethane prepolymers to produce a quaternary (PU/ CNT/diatomite/Prussian-blue), a spongiform adsorbent with an in situ foaming procedure. The theoretical adsorption capacity of the as-prepared spongiform is 167 mg/g and the removal of cesium primarily occurred through an ion-exchange mechanism[36]. In another study, the reticular ceramic foam was prepared from natural zeolite powder and a thin film of Prussian blue analogue was firmly combined with the desilicated ceramic surface of this foam. The Prussian blue loaded ceramic foam showed a good performance in cesium adsorption which occurred through ion exchange between K and Cs ions[37]. The Prussian blue nanoparticles immobilized on cotton gauze also showed high efficiency for Cs removal in column adsorption system and the ion exchange between K+ and Cs+ was proposed for the adsorption mechanism[38].

    Potassium copper hexacyanoferrate (KCuHCF) immobilized in a cellulose- based hydrogel made of carboxymethyl cellulose (CMC) and hydroxyethyl cellulose (HEC) was used for the adsorption of cesium ions in aqueous solutions. It was observed that immobilization with the cellulose- based hydrogel facilitated the dispersion of nano-sized KCuHCF particles, showing the unprecedented adsorption capacity of the composite which was attributed to the presence of ion-exchangeable sites (COO-Na+) in the cellulose hydrogel. The HCF-gels also exhibited a rapid Cs+ removal and adsorbed Cs+ selectively (> 90%) in seawater containing 0.11 mmol/L of Cs+[19]. The hydroxyapatite was prepared and modified with cobalt(II) ferrocyanide to obtain hydroxyapatite cobalt( II) ferrocyanide (HAFC) and then, used for removal of cesium and strontium ions from aqueous solution. The adsorption capacity of hydroxyapatite and HAFC for cesium were found to be increased from 9.26 to 69.49 mg/g, while the adsorption capacity of strontium increased from 12.84 to 13.44 mg/g only[39]. The magnetite and Prussian blue nanoparticles were incorporated with sepiolite and the adsorption of cesium ions from aqueous media was assessed. An easy and quick recovery material showed an enhanced removal of cesium in the presence of a high concentration of sodium chloride[40]. The porous silica or glass-based nanocomposites containing CO2+/[Fe(CN)6]3-nanoparticles was used as an efficient and selective adsorbent for the separation of cesium from water, sea water and radioactive solution simulating effluents of the Fukushima reactors (137Cs, 29 kB/L). Compared to the amount of CO2+/[Fe(CN)6]3-nanoparticles, the sorption capacities of studied nanocomposites is three times higher than that observed for the respective bulk materials[41]. The core-shell-structured magnetic microsphere functionalized with potassium titanium ferrocyanide (KTiFC) is another adsorbent used for the efficient removal of radioactive cesium from seawater. This material showed high removal efficiency (≥ 97.7%) of radiocesium from 137Cs-spiked solutions (3,000- 35,000 Bq/L and contaminated seawater. The adsorption equilibrium was rapidly achieved within 30 min and the maximum adsorption capacity was found to be 43.09 mg/g[42].

    Sodium-copper hexacyanoferrate-modified magnetic nanoparticles were fabricated in such a way that the Fe3O4 nanoparticles were coated with polyethyleneimine to immobilize Cu ions through metal coordination to amine groups in the PEI. Sodium-copper hexacyanoferrate (NaCuHCF) was subsequently formed on the surfaces of the magnetic nanoparticles as Cu ions coordinated the hexacyanoferrate ions. The resulting NaCuHCF-modified magnetic nanoparticles showed a rapid adsorption of cesium and adsorption equilibrium was achieved within 10 min, and the maximum adsorption capacity of the adsorbent was 125 mg/g. A high removal efficiency exceeding 99.428% of the radioactive cesium was achieved using this adsorbent and 137Cs is selectively adsorbed even in the presence of competing cations[43]. Sangvanich and co-workers evaluated the copper(II) ferrocyanide functionalized mesoporous silica for removing cesium (Cs+) and thallium (Tl+) from natural waters and simulated acidic and alkaline wastes. The material showed an enhanced affinity at lower pH and the removal was less affected by competing cations and matrices. Further, the authors suggest that their material can be used as orally administered drug for limiting the absorption of radioactive toxic ions in gastrointestinal tract[44].

    Nickel (II) hexacyanoferrate (III) (NiHCF) was incorporated into crude and oxidised barks of Douglas fir to remove cesium ion (Cs+) from water. The maximum sorption capacity of crude bark was increased up to seven times by the oxidation along with NiHCF impregnation. The amount of nickel, iron and cyanide released during the preparation of bark in the water remained within the permissible limits defined by US EPA[45]. A highly porous discs of chitin was used as substrate to immobilize nickel-potassium ferrocyanide and the sorbent was found to be highly selective to Cs(I) even in the presence of high concentrations of Na(I), K(I), Rb(I) or NH4+. It was interesting that the change in solution pH between 2 and 8 hardly affect the cesium sorption[46]. Nickel hexacyanoferrate nanoparticles were uniformly incorporated onto the surface of multiwalled carbon nanotubes (MWCNTs) with a grafted poly(4-vinylpyridine) linker. It was report that the NiHCF/PV4P-g-MWCNTs composites show a great enhancement in capacity and stability of ion exchange as compared to pure NiHCF due to the special nanostructure and unique composition[47]. Nickel hexacyanoferrate was immobilized on the activated carbons obtained from rice hulls and areca nut waste materials. The SEM images of these activated carbons with and without nickel hexacyanoferrate are shown in Figure 3. The SEM images clearly showed the immobilized nickel hexacyanoferrate particles onto the surface of ACs and they were substantially immobilized as small-sized particles and evenly distributed. These activated carbons incorporated with nickel hexacyanoferrate showed an enhanced capacity for the removal of Cs from water under batch and fixed bed column experiments[48]. Moreover, nickel heaxacyanoferrate was successfully incorporated into coir pitch[49] and walnut shell[50]. These materials were successfully utilized for selective adsorption of cesium from aqueous solutions.

    2.3. Encapsulation of hexacyanoferrate compounds into beads

    The fine powder materials are sometimes not suitable to use as adsorbent in aqueous solutions since it is difficult to recover the fine particle from aqueous media after used. Therefore, preparation of beads is a suitable method to resolve the drawbacks and increased the feasibility of powder materials in wastewater treatment technology. Prussian blue was impregnated in alginate gel (AG) beads and the beads possessed very fine crystals of PB incorporated into the alginate gel beads. The interaction between composite beads and Cs ions was evaluated by adsorption experiments and the maximum adsorption capacities of the PB-AG beads were 0.557 mg per bead. The changes in solution pH as well as the presence of organic acids, potassium and sodium ions did not show significant affect on the removal of cesium from aqueous solutions[51]. The PB was encapsulated into poly(vinyl alcohol) (PVA) and alginate cross-linked matrix and use for cesium adsorption. The PVA-alginate granules were found to possessed high structural stability in both fresh water and seawater and the Cs adsorption capacity is higher in fresh water. Further, the PB fragments released from the tested granules were filtered using resin[52]. The Prussian blue encapsulated alginate/calcium beads was reinforced with highly dispersed CNTs and used for radioactive cesium removal from aqueous solutions. The maximum monolayer adsorption capacity of the beads was found to be 143 mg Cs/g-beads. The beads can be used effectively over the pH range from 4 to 10. Further, the study conducted with fixed bed adsorption suggests the possible use of these beads for large scale treatment of cesium contaminated water[53].

    The hydrogel composite beads of potassium nickel hexacyanoferrate (KNiHCF) was prepared using polyvinyl alcohol and cross-linked alginate as the binding matrix and the as-prepared beads were employed for removal of cesium ions from low level liquid nuclear waste in batch system using a 137Cs radiotracer. The sorption capacity of the beads is determined to be 7 mg/g of the swollen beads, which corresponds to approximately 64 mg/g of the dry beads. The removal mechanism involves the ion-exchange of cesium ions with potassium ions and also intraparticle diffusion and film diffusion were found to be involved[54]. Similarly, hydrogel composite bead was prepared for potassium cobalt hexacyanoferrate (KCoHCF) and then employed for cesium adsorption from waste generated in the nuclear industry. The beads possessed highly porous structure and hydrophilic nature. The sorption capacity of KCoHCF-gel beads was found to be 15 mg/g[55]. In another study, a continuous fixed-bed adsorption experiment was conducted for the removal of cesium from aqueous solutions using PVA-alginate encapsulated Prussian blue-graphene oxide (PB-GO) hydrogel beads. The maximum adsorption capacity of the PB-GO hydrogel beads was found to be 164.5 mg/g at an initial cesium concentration of 5 mM, bed height of 20 cm, and flow rate of 0.83 mL/min at pH 7[56]. The sericite powder impregnated into nickel hexacyanoferrate (NiHCF-sericite) was encapsulated into beads using sodium alginate and further successfully employed for Cs adsorption under batch and fixed bed column system[57].

    3. Clay and Clay-based Adsorbents

    Various types of clays are reported to remove cesium from aquatic environment due to their cation exchange property. As shown in Table 1, the adsorption efficiency of cesium by some natural clays are comparable to other adsorbents. Therefore, the adsorption of radioactive ions using clay particles is another treatment options due to naturally abundant, low cost and simple operation. During recent past, researcher focussed on the modification of natural clay and a number of clay and clay based materials were successfully employed for the removal of cesium from water. Bentonite, one of the most common natural clay was assessed for radiotoxic ions removal and the maximum adsorption capacity of 83.3 and 15.9 mg/g were obtained for Cs(I) and Co(II), respectively. The presence of humic acid did not show significant affect on the adsorption of Cs(I). Further, thermodynamic studies indicate the spontaneous nature of Cs(I) adsorption by natural bentonite[58]. Suzuki and co-workers studied the adsorption behaviour of cesium by vermiculite in both pure water and artificial seawater samples. The adsorption amount of Cs(I) was low in artificial sea water; however, the ion exchange capacity is almost same in both of the water samples. Vermiculite with various grain size were investigated and the 500 μm was found to be most suitable for general use[59]. The naturally occurring sericite clay was activated using hydrochloric acid and then both pristine sericite and activated sericite were used for assessing their suitability in the adsorption of Cs from the aquatic environment. The activated sericite possessed enhanced porosity, thereby producing a significant increase in the specific surface area. The activated sericite showed significantly higher uptake of Cs compared to the pristine sericite[60]. Akadama clay was modified with nickel and the pores present within a clay were reduced into smaller pores from a diameter of > 20 nm to < 12 nm after the modification. However, the maximum adsorption capacity was increased from 4.5 to 16.1 mg/g. Moreover, it was reported that the applicable pH range was notably extended from pH > 11 to pH ⩾ 5[61].

    The Cs+ adsorption by ethylamine-modified montmorillonite (Ethyl-Mt) and calcium-saturated montmorillonite (Ca-Mt) were evaluated by Long and co-workers. The presences of K+ and Ca2+ in the aqueous solution significantly inhibited the Cs+ removal while Na+ showed less effect. The ion exchange process plays a major role in Cs+ removal by Ca-Mt, while the adsorption by Ethyl-Mt involves ion exchange, coordination with -NH2 and surface complexation with hydroxyl group. The hypothetical simulation of Cs+ adsorption on Ethyl-Mt is shown in Figure 4[62]. The phosphate-modified montmorillonite (PMM) was employed for the removal of CO2+, Sr2+ and Cs+ from aqueous solution. The maximum sorption capacity as determined by Langmuir model was in the order of Cs+ > CO2+ > Sr2+. However, the sorption of Cs+ was suppressed by competition with H+ at low pH (pH < pHpzc). The sorption of Sr2+ indicated endothermic sorption processes; whereas the sorption of CO2+ and Cs+ were exothermic in nature[63]. The composite material was prepared by combining ammonium-pillared montmorillonite (MMT) and magnetic nanoparticles (Fe3O4), and the material was found to possessed high removal efficiency of cesium from contaminated water and soil samples. Moreover, the composite material exhibited high selectivity of Cs+ and the sorbent can be rapidly separated from the mixed solution using an external magnet. The presence of coexisting ions reduced the adsorption capacity in the order of Ca2+ > Mg2+ > K+ > Na+[64]. Chitosan-grafted magnetic bentonite was synthesized via a plasma-induced method. The material possessed good magnetic properties, low turbidity, high stability in seawater and aqueous solutions. An enhanced adsorption capacity for Cs+ ions was observed and the Cs+ removal was primarily controlled by the cation exchange mechanism[65].

    Yang and co-workers investigated the interactions of Cs+ with various materials such as bentonite, carbon nanotube, chitosan grafted bentonite and chitosan grafted carbon nanotube composites. They observed that the sorption of Cs+ mainly occurred through strong cation exchange with alkali and alkaline earth metal cations and this cation-exchange mechanism is more effective than the hydroxyl group exchange. This study infers that the spatial structure and the cation-exchange capacity of the material are important factors for selecting the sorbent for Cs+ removal from radioactive waste water[66]. Li and co-worker tested 13 different cost-effective sorbents including illite and organoclay for the adsorption of TcO4-, I-, and Cs+ from contaminated groundwater and sediments. It was observed that two organoclays sorbed large amounts of TcO4- , I-, and Cs+ and the binding of these radionuclides are largely irreversible[67]. Ethylamine modified vermiculite was also reported as a good adsorbent for Cs(I). The modification of vermiculite using ethylamine improved the adsorption capacity from 56.92 to 78.17 mg/g and the formation of micropores and mesopores as well as an increased surface area are the major factors of the enhancement of cesium uptake[68].

    4. Miscellaneous

    Recently, various types of materials were assessed for their ability to decontaminate radiotoxic cesium in aqueous solutions. A detailed study was conducted to explore the potential of bamboo charcoal (BC) and modified bamboo charcoal as an adsorbent for the removal cesium from aqueous solution. It was observed that the modified bamboo charcoal is enriched with oxygen-containing functional groups but its porous structure and surface area are decreased; however, the maximum cesium adsorption capacity was increased from 0.17 and 45.87 mg/g after modified using nitric acid. Further, it has been observed that almost 100% cesium could be removed from water even at 400 mg/L concentrated cesium solution using the modified biochar[11]. Similarly, the adsorption capacity of persimmon waste (0.11 mmol/g) for Cs+ was considerably increased after cross-linking with concentrated sulphuric acid (0.76 mmol/g)[69]. The pine cone was treated with toluene-ethanol mixture and an increased in hydrophilicity was observed. Further, the affinity towards cesium was considerably high due to increased surface area, organics extraction, and formation of carboxylate ions. However, the presence of Na+ and Ca2+ reduced uptake of Cs from aqueous media[70].

    Metal oxides nanoparticles are classified as the promising adsorbents for heavy metals removal from aqueous media due to enhance sorption capacity and selectivity. However, fewer studies are conducted for the removal of Cs using metal oxides. The two-dimensional correlation infra-red spectroscopy (2D-COS-IR) was employed to investigate the mechanism of interaction of nano manganese oxide (NMO) with Cs and Co ions. The adsorption of these ions mostly occurred through the inner-sphere complex formation and further suggested the different coordination of the two metal ions with oxygen[71]. In another study, zirconium oxide (ZrO2) based adsorbent was employed for removing phosphate and cesium in sequence. The adsorbed phosphate was converted into a zirconium phosphate over layer by thermal treatment and the material was further employed for Cs removal[72].

    Zeolites are also reported as an efficient adsorbents for the removal of Cs from aqueous solutions[73,74]. Lee and co-workers investigated the fundamental structure-property of zeolites for studying the origin of Cs selectivity in zeolites framework. Zeolite A (LTA) was used as a model system. The single-crystal synchrotron X-ray diffraction analysis showed the significant differences in the ion exchange capacity and the site selectivity of Cs+ ions. Moreover, it was observed that Cs+ ions are energetically preferred in the center of 8 oxygen rings[75]. The resorcinol- formaldehyde (RF) resin beads in spherical form were evaluated for removal of cesium from alkaline medium in batch experiments. The material is found to have high sorption capacity of 287 mg/g. However, the presence of sodium ion decreases Kd value for Cs+ ions. The adsorption mechanism was found to be complex, consisting of both surface adsorption and pore diffusion[76]. The removal of Cs+ from the aqueous solution was assessed by the maghemite nanoparticles embedded in PVA and alginate matrix. Cs+ was removed rapidly under sunlight and the beads can be reused for several times without losing its efficiency[77]. The poly(AAc-co-B18C6Am) hydrogels was synthesized by thermally initiated free-radical copolymerization and the hydrogel is specifically designed with a synergistic effect, in which the AAc units are designed to attract Cs+via electrostatic attraction and the B18C6Am units are designed to entrap the attracted Cs+ by forming stable complexes[78]. Kumar et al. synthesized bisglycolamide substituted calix-benzo-crown-6 (CBCBGA) ionophore and the newly synthesized material was observed to be highly selective towards Cs over Na+ as well as other metal ions in the simulated high level aqueous waste; which is due to the complexing properties of this ionophore towards Cs+[79].

    The water-insoluble poly-γ-glutamic acid (γ-PGA) and water-soluble sodium salt form poly-γ-L-glutamic acid (γ-PGANa) were employed as adsorbents for cesium removal from radioactive wastewaters. The adsorption of Cs by both γ-PGA and γ-PGANa were very fast and the equilibrium was attained within 0.5 min. The maximum adsorption capacities of γ-PGA and γ-PGANa were 446 and 333 mg/g, respectively. The binding mechanism was confirmed by the spectra of FT-IR and XPS before and after adsorption, and the adsorption occurred through electrostatic interaction with carboxylate anions[80]. Seaton and co-workers synthesized the mesoporous silica gel containing embedded phosphotungstic acid (PTA) by sol-gel co-condensation using tetraethoxysilane and PTA in acidic media. The synthesized material possessed high surface area and the FT-IR spectrum confirms the embedding of PTA in a sub-molecular level. The adsorbent contain two types of adsorption sites, i.e., heteropolyacid anions and silanol groups; however, a silanol group was very sensitive to the change in temperature[81].

    The conjugate adsorbent was prepared by direct immobilization of dibenzo-18-crown-6 ether onto mesoporous silica monoliths and then assessed for Cs adsorption from aqueous solutions systematically. The conjugate adsorbent selectivity adsorbs Cs even in the presence of high concentration of Na and K ions due to strong Cs-π interaction of benzene ring. The material can be reused for several cycles after desorption using 0.20 M HCI,-.Further, the material was applied for the treatment of low level radioactive aqueous solutions and it was observed that the radioactive 127Cs was efficiently reduced to third-fourth of its original radioactive concentration[6]. Similarly, dibenzo-24- crown-8 ether and the macrocyclic ligand of di-o-benzo-p-xylyl-28- crown-8-ether (DOBPX28C8) were anchored onto the mesoporous silica to obtain the mesoporous hybrid adsorbent (MHA) and successfully employed for the adsorption of Cs in aqueous solutions[82,83]. The plausible complexation mechanism for cesium adsorption onto DOBPX28C8 is shown in Figure 5. It is assumed that the benzene ring in para position was consolidated for π-electron orientation with expanding the ring size which resulted in higher selectivity towards Cs ions. Furthermore, the large ring size dibenzo-30-crown-10-ether (DB30C10) impregnated mesoporous adsorbent was used for cesium (Cs) adsorption. The material has high sorption capacity; however, the efficiency was low at acidic environment and the presence of high concentration of competing ions of Na and K slightly affected the Cs removal[84]. The adsorption capacity of these materials prepared by Awual and co-workers are shown in Table 1. In general, these materials can be considered as promising adsorbent for the treatment of Cs contaminated water due to high adsorption capacity, fast mass transfer rate, good selectivity and reusability.

    5. Conclusions

    Adsorption technique is the most frequently studied for the removal of radiotoxic cesium from aqueous solutions. Prussian blue and its analogue compounds are widely investigated as an efficient adsorbent for the removal of radiotoxic cesium from aqueous phase. Novel methods for the preparation of PB are reported by several researchers during the recent past and the mechanism of specific Cs+ adsorption were also investigated; however, there is a need for further research on the use of Prussian blue or its analogue compounds for the removal of radiotoxic cesium from aqueous media. Natural clays are found to be useful adsorbent for cesium due to their cation exchange property. The clay structure can be modified by organic or inorganic compounds to increase their adsorption capacity for Cs+ ions. Furthermore, various types of novel adsorbents with high adsorption capacity have been developed for cesium removal from aqueous media within the recent past years. Although various types of materials can be employed for the adsorption of cesium from aqueous media, the technical applicability of the adsorbent for treatment of radioactive effluents is necessary.


    Schematic PB structures consisting of (a) 3-D regular lattice spaces surrounded by the FeII-CN-FeIII bonds in the case of a unit cell of 3 × 3 × 3 structure of Fe atoms and (b) containing a defect site of an [FeII(CN)6]4- moiety from the regular structure of (a). FeIII-OH2 bonds in six empty nitrogen (CN) coordination moieties were formed by defect of [FeII(CN)6]4-. Cs+ ions are trapped by simple physical adsorption in the regular lattice spaces of (a) and by chemical adsorption via the lattice defect sites of (b) with proton-exchange mechanism (proton-elimination from a coordination water of FeIII-OH2) apart from strong electrostatic attraction due to their counter ions, where the unit cells of (a) and (b) consist of eight and seven Fe atoms, respectively, and six coordination water molecules appear in the lattice defect sites of (b). Ref. [16].
    SEM (a) and TEM (b) images of hollow PB nanoparticles with average particle size : 110 nm. SEM (c) and TEM (d) images of hollow PB nanoparticles with average particle size : 190 nm. Ref. [17].
    SEM images of activated carbon obtained from rice hull (a) and areca nut waste (b) and SEM images taken after the incorporation of nickel hexacyanoferrate into activated carbon obtained from rice hulls (c) and areca nut waste (d). Ref. [48].
    The hypothetical simulating of Cs+ adsorption on Ethyl-Mt : (a) stands for Ethyl-Mt, and (b) stands for Cs+ adsorption on Ethyl-Mt. Ref. [62].
    Possible stable complexation mechanism between Cs and DOBPX28C8 based on the Cs-π interaction and ring size for the Cs adsorption in the presence of alkali metal ions of K and Na. Ref. [83].


    Langmuir Adsorption Capacity of Cs(I) Obtained for Various Adsorbent


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