Influence of adsorption parameters on cesium uptake from aqueous solutions- a brief review

Abstract

Due to rapid population growth, technological advancement and industrial revolution, the rate of generated waste effluents has become a grave concern. Cesium which possesses high fission yield is generally transferred to liquid wastes especially those emanated from the nuclear power plants, reprocessing of spent fuels, nuclear weapon testing and radionuclides production facilities for medical applications etc. Radiocesium (¹³⁷Cs) is one of the hazardous radionuclides creating adverse effects on human health and environment. Due to its physical (T_1/2 = 30.17y) and chemical characteristics (alkalinity, solubility etc.), it can be easily assimilated by the living organisms. As a result, the removal of cesium from wastewater is imperative from the health point of view. Several techniques are implemented but in recent time, adsorption has been gaining increasing attention to the scientific community owing to a number of reasons. Hence, this paper presents an overview on sorption of cesium from wastewaters. Consequently, several critical parameters such as sorption capacity, percentage efficiency and the influence of several factors on cesium uptake by various adsorbents have been reviewed in details.

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Michael Adekunle Olatunji

Michael Adekunle Olatunji obtained his M.Sc. degree in Physics (Radiation and Health Physics option) in 2010 from the University of Ibadan, Nigeria. He is currently pursuing his Ph.D. degree in Physics at the Department of Physics, Faculty of Science, University of Malaya, Malaysia. Michael is interested in the development of low cost adsorbents for sequestering radionuclides from real waste effluents. His research interests also include synthesis of conducting polymers for various applications, and radiation measurement in environment.

Mayeen Uddin Khandaker

Associate Prof. Dr. Mayeen Uddin Khandaker obtained his Ph.D. from the Department of Physics, Kyungpook National University, South Korea. Previously, he served in numerous institutions like the Korea Atomic Energy Research Institute, the International Atomic Energy Agency and American International University – Bangladesh, etc. Currently (2010 – onwards), he is working in the Department of Physics, University of Malaya, Malaysia. His areas of expertise include: synthesis and characterization of nanostructured materials, production of radionuclides for medical and industrial applications, measurement of natural radioactivity in food and environment and development of optical fibre based TLDs for dosimetry applications.

H. N. M. Ekramul Mahmud

H.N.M. Ekramul Mahmud, Ph.D. has received his BSc and MSc in Applied Chemistry and Chemical Technology from the University of Dhaka, Bangladesh in 1986 and 1987, respectively. He received his M. Eng in Chemical Engineering in 1998 from the Universiti Teknologi Malaysia (UTM) and his PhD from Universiti Putra Malaysia (UPM) in 2006. His research interests include synthesis and characterization of conducting polymers for its various applications and also catalytic conversion of biomass to energy. He is now working as an associate professor in the Department of Chemistry, University of Malaya (UM), Malaysia.

Yusoff Mohd Amin

Prof. Dr Yusoff Mohd Amin obtained his Ph.D. degree in Physics from Birmingham, UK. Currently, he is working as a Professor at the Department of Physics, University of Malaya, Malaysia. His areas of expertise include: production, detection and applications of radionuclides in nuclear medicine and industry, radiation detection and measurement.

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1.0 Introduction

The treatments of low-, intermediate- and high-level contaminated liquid wastes are gaining increasing attention worldwide owing to a number of imperative reasons: the generation of wastewater is increasing at a rate proportional to the rapid technological advancement and population growth.^1,2 The environmental pollution resulted in the discharge of the wastes in water-bodies and soil which inadvertently results into a prolonged health problem in both human and animals via enhancing radiation burden of the entire ecosystem. Most importantly, the needs to recover limited precious and expensive substances which often get lost through the generated wastes and the demand for wastewater reuse³ have always been the concern of the scientists and engineers. For safer and effective management, wastewaters containing radioactive materials and heavy metals are of particular concern due to their behaviours within the environment and associated health implications upon exposure to human and animals. The sources of liquid radioactive wastes have always been through nuclear power generation from nuclear plants, reprocessing or recycle of spent fuel, mining and milling of mineral ores, nuclear application involving radioisotopes in medical and agricultural practices, exploration of natural gas and oil, nuclear tests and accidents and so on. From these sources, potential health challenging radionuclides which are always parts of the wastes are being generated and released into the ecosystem. Among the radionuclides of environmental impacts and hazardous effects are uranium, radium, thorium, strontium, cesium, europium, polonium and cobalt. These radionuclides are classified as natural (e.g. uranium, radium, thorium, etc.) and artificial (e.g. cesium, cobalt, strontium, etc.). Apart from the natural radionuclides that take their origins from the formation of the earth, the artificial radionuclides emanated as a part of the wastes from the aforementioned processes and streams. As regards to accidents, the most prominent of such were the Chernobyl in 1986 and the Fukushima accidents in 2011.

This review is focussed on the removal of caesium (Cs) due to its significant health effect to human and environment. As an alkali metal, caesium has similar characteristics as potassium and rubidium, and belongs among the few metals that appear as liquids at near the room temperature. It is a very reactive metal and reacts with water explosively than other alkali metals in the same group even at low temperature.^4,5 Caesium has a higher atomic mass and more electropositive than other non-radioactive alkali metals and it is the most stable chemical element ever known.^6–8 Because of its high reactivity, it is classified as a hazardous material. Cs metal, which behaves similar to potassium, can easily be adsorbed to the body and distributed into the soft tissues of the whole body.⁹ Thyroid cancer is one of the terrible consequences of this metal adsorption.^10,11 Its acute poisoning causes medullar dystrophy, asthma, allergy, heart problems, disorders in reproductive function and bone mineralization and damages of thyroid and liver and mutagenic disorders.¹² Cesium is mined mostly from pollucite, while as a by-product of uranium fission,¹³ many radioactive isotopes of cesium (¹³⁴Cs and ¹³⁷Cs are of significant health concern) are released into the environment. In particular, ¹³⁷Cs a gamma-emitter, is part of generated wastes from extractive industries, and due to its physical (long half-life, T_1/2 = 30.17y) and chemical (high solubility) characteristics, it could easily transferred to the food chain.^14–17

Moreover, the toxicity, non-biodegradability, ease of migration from underground to surface water as well as soil-to-plants transfer via root uptake leading to external and internal exposures to man have made it as the most important issues to consider. Hence, several separation techniques are being employed to remove cesium from low-, intermediate- and high-level solid and liquid radioactive wastes before disposal. But, the method could vary from conventional to more sophisticated approach as the case may be to treat a particular waste for the total removal of radioactive materials present in a particular waste effluent. Methods such as chemical precipitation, coagulation/co-precipitation, membrane process, reverse osmosis, chemical reduction, solvent extraction and foam flotation have been used to remove radioactive materials, heavy metals and decolouration from liquid processes (Fig. 1).^18–31 Several inorganic, organic, biological, agricultural materials and magnetite or their mixtures have also been used as adsorbents to separate solid radioactive materials from liquid wastes before disposal.^14–37 A host of other novel methods and materials are still being developed and used for treatment of radioactive liquid wastes.³⁸ Normally, radioactive wastes are classified for effective treatment and management. The classification depends on the characteristics of the radioactive materials present in the wastes, application of radioisotopes, the rate of waste generation and the regulatory frameworks which bound on the disposal of radioactive wastes in a particular place or country.³⁹

Fig. 1 Schematic diagram showing various adsorption materials and adsorption factors.

Fig. 2 Effect of initial pH on cesium adsorption using Ni^IIHCF^III-WS.^49,106

In this review, several factors affecting the removal of cesium from solutions are examined based on the available literature (Fig. 1). Sorption capacities and efficiencies of each adsorbent were reported and compared under various conditions.

2.0 Adsorption/ion exchange of cesium ions onto synthesized and natural adsorbents

This is a method by which a sorbate is removed or accumulated on the immobile solid from its liquid or gaseous surroundings, and usually referred to as liquid–solid interaction. Normally, this process occurs at the phase of the sorbent and so, it is a surface-based phenomenon.⁴⁰ Adsorption can take different dimensions, if the process is initiated as a result of weak intermolecular forces (van der Waals interactions) then it is specifically called physisorption and this force being weak in nature results into reversible adsorption, but if occurs due to the formation of chemical bonds between the molecules of adsorbate and the adsorbent it is known as chemosorption. Ion exchange on the other hand is a reversible chemical process in which an ion from solution exchanges with similarly charged ion attached to the adsorbent, the solid particle.⁴⁰ Due to their common features, adsorption and ion exchange are grouped together as sorption processes for effective sequester of toxins from waste/drinking waters.^41,42 Adsorption is recognized as an effective and economic method for the removal of radionuclides compared to the other approaches owing to a number of reasons such as; the ease of design and operation, adsorbate-specificity, better treatment with no/little secondary waste generation, possibility of using at low concentration, high efficient performance, ease of regeneration and reuse, suitability for both batch and column studies, and the availability of the adsorbents.⁴³ Normally, a good adsorbent must have a large surface area, good macroporous structure and be stable under chemical, radiation and thermal reaction conditions. However, the behaviour and efficiency of adsorbents can also be influenced by the condition of the solution.

2.1 Various adsorbents used in recent time for cesium adsorption

The treatments of waste-water and drinking water for radionuclide decontamination have involved several sorbents in recent time. However, this review focuses on various adsorbents as reported in the literature which are classified as inorganic, inorganic-polymer composites and bio-adsorbents.

2.1.1 Inorganic adsorbents. Several metal oxides, hexacyanoferrates and ferrocyanides of transition metals as well as clay minerals have been reported for adsorption of radioactive materials^{44–47,61–67} as mentioned in the Table 1. For radioactive waste treatments, certain characteristics of inorganic adsorbents such as high selectivity, good mechanical, radiation and thermal stability in the solution containing the adsorbate, high porosity, cost effectiveness, environmental friendliness which does not result into generation of secondary contaminants and the possibility of regeneration are usually considered.^44,45 However, the hexacyanoferrate of transition metals is generally too difficult to be separated from the solution due to its fine particulate nature which often create further problems of contamination and hence, needed to be incorporated into supporting materials,⁴⁸ but it is a highly selective agent with special cubic structure and channel diameter that allows only small hydrated ions such as cesium ions to permeate through and block the larger ones that compete for the sorption sites.⁴⁹ On the other hand, metal ferrocyanide sorbents were reported to have good structure, ion exchange ability for cesium from different types of solution such as natural waters and radioactive wastewater, stable in acidic and moderate alkaline medium, stable in radiation and high temperature.^50–57 However, adsorption by ferrocyanides sorbents have been reported to be strongly affected by preparation, difficult to handle for column application due to its fine nature^58,59 and results into large sludge production.⁶⁰ In case of clay minerals which are generally less expensive and of larger surface area and layered structure for adsorption have some disadvantages due to competitiveness of other monovalent cations such as K and Na ions with cesium adsorption which may block sorption sites.⁴⁹ Some of the inorganic adsorbents that have been commonly used are presented in Table 1 along with their adsorption capacity.

Table 1 Cesium ion adsorption capacities of some inorganic adsorbents

Inorganic adsorbents	Adsorption parameters and adsorbent characterization	Adsorption capacity, cesium (efficiency, %)	Comment	Ref.
Self-assembled mesoporous silica	Adsorbent amount: 0.05 g. Initial concentration: 10 ml of 2 ppm Cs solution. Cs solution: acidic and alkaline media. Contact time: 2 h. Interfering metal ions: presence of sodium and potassium. Characterization: XPS	179 mg g^−1 (∼99%)	Sorption kinetic: fast. Interfering ions: no effect (Na^+ and K^+ ions). Data fitted by: Langmuir model	8
Copper ferrocyanide mesoporous silica	Adsorbent amount: 0.1 g initial concentration: 100 ml of 0.5–500 ppb of both natural and simulated acidic/alkaline solutions. Adsorption parameters studied: pH (0.1–7.3) and competing ions	21.7 mg g^−1 (95%)	Adsorption kinetic: rapid. pH effect: increases. Interfering ions effect: not significant effect. Stability: little leaching of component elements over time. Data fitted by: Langmuir model	11
Prussian blue	Adsorbent amount: 0.1 g initial concentration: 100 ml of 0.5–500 ppb of both natural and simulated acidic/alkaline adsorption parameters studied: pH (0.1–7.3) and competing ions	2.6 mg g^−1 (75%)	Adsorption kinetic: less rapid pH increase effect: favourable. Interfering ions: no effect. Stability: leaching of component elements. Data fitted by: Langmuir model	11
Activated silico-antimonate	Adsorbent amount: 0.1 g. Initial concentration: 5 ml of 10^−4 M. Contact time: 6 h. Adsorption parameters: acidity, temperature (303–333 K) and silicon/antimony ratios. Characterization: FT-IR, XRD, XRF and DTA-TG	0.220–0.520 mg g^−1 (30 to >90%)	Adsorption kinetic: relatively rapid. Results: temperature increase, Si : Sb ratios (SiSb 1 : 2) and acidity favoured Cs uptake. Adsorption mechanism: chemisorption which is spontaneous and endothermic in nature. Data fitted by: pseudo-second kinetic model	46
Hydrous mixed metal oxides	Solution volume: exchanger ratio: 100–400 cm^3 g^−1. Contact time: 3 days. Studied parameters: temperature and pH characterization: XRD, TGA and elemental analysis	2.2–2.9 mmol g^−1 (exchanger capacity)	Effect of temperature increased the crystallinity of the material. Doping with tungsten or niobium increases separation capacity	47
Nickel hexacyanoferrate incorporated into walnut shell	Adsorbent amount: 4 g. Initial concentration: 200 ml of 5–400 mg L^−1. Studied parameters: contact time and temperature (298–313 K). Characterization: FESEM and TG-DTA	4.94 mg g^−1	Adsorption kinetic: rapid within 2 h. Results: temperature increase and K^+ ion favoured Cs uptake. The sorbent was thermally stable. Data fitted by: Feundlich isotherm and pseudo-second order kinetic models. Mechanism: spontaneous and endothermic in nature. Stability: adsorbent thermally stable	49
Potassium copper ferrocyanide	Initial concentration: 0.015 M. Studied parameters: pH and interfering ions	2.25 mmol g^−1	The sorbent is not stable in acidic pH (<3) and sorption capacity is influenced by magnesium nitrate of about 0.82 M. Desorption is possible with nitric acid	50
Copper(II) ferrocyanide incorporated vermiculite	—	179 mg g^−1	—	57
Manganese-oxide	Studied parameters: pH and interfering ions	172 mg g^−1	Cesium uptake favourable over pH 2–10 and the presence of Na^+, Ca^2+, Mg^2+ and K^+	61
Hydrous titanium oxide	Adsorbent amount: 0.1 g. Studied parameters: temperature, contact time and pH. Characterization: IR, XRD and DTA	0.6189–0.4303 mmol g^−1	pH effect: pH < 2 inhibited cesium uptake and pH 6 favourable. Equilibration time: 5 h. Temperature increase: favoured Cs uptake and the process was spontaneous and endothermic in nature. Data fitted by: D-R isotherm model	62
Zirconium iodomolybdate	Adsorbent amount: 100 mg. Initial concentration: 10 ml of 10^−4 M. Studied parameters: pH (1–8), temperature (25–45 °C) and competing ions (Na^+). Characterization: XRD and TGA	7.94 to 9.62 mmol g^−1 ion exchange capacity (80% sorption)	Adsorption kinetic: rapid. Equilibration time: 2 h. Optimum pH: pH 3 and % decrease at higher pH. Effects of ionic strength and organic acids: decreased uptake capacity. Data fitted by: pseudo-second order and Elovich models. Sorption mechanism: chemisorption. Sorbent thermal stability: up to 350 °C	63
Nickel potassium ferrocyanide immobilized in chitin	Adsorbent amount: 100 mg. Initial concentration: 50 ml of 10–400 mg Cs L^−1. Contact time: 24 h. Studied parameters: pH and interfering ions. Characterization: SEM-EDX and XRD	80 mg g^−1	Parameters effects: pH (2.5–6.4) and the presence of monovalent ions (0.01–1 M) have no much effect on sorption capacity	64
Potassium copper nickel hexacyanoferrate	Adsorbent amount: 100 mg. Initial conc.: 10 cm^3 of 3.7 mmol g^−1. Studied parameters: Temperature (≤353 K) and initial concentration (3.7–75 mmol g^−1)	2.215–2.257 mmol g^−1	Temperature increase: favourable for adsorption. Thermodynamic studies: endothermic and spontaneous. Data fitted by: Langmuir model and D-R isotherm model	65
Potassium nickel hexacyanoferrate loaded-silica gels and chabazite	Adsorbent amount: 0.07 g. Initial concentration: 7 cm^3 of 10 ppm. Contact time: 7d. Studied parameters: neutral pH and the presence of 5 M NaNO3. Column experiments were used to estimate dynamic sorption capacity. Characterization. XRD, SEM and IR	0.123 mg g^−1 (breakthrough capacity) and 0.179 mg g^−1 (T. capacity) (69% column utilization)	Equilibration time: 2d. Influence of sorption parameters: favourable	66
Bentonite	Studied parameters: effects of pH, ionic strength and contact time. Characterization: N2-BET, SEM, XPS and XRD	1.334 mmol g^−1 (89%)	Optimum pH: pH 7.0. Equilibration time: 7 h. Influence of alkali and alkaline earth metal ions: negative. Data fitted by: Langmuir model	67
Local Taiwan laterite (LTL)	Adsorbent amount: 15 g. Initial concentration: 450 ml of 1 mM–0.1 μM. Contact time: 7d. Studied parameters: effects of time, sorbent dosage and aqueous temperature characterization: XRD, SEM/EDS and BET	0.3 mmol g^−1	Adsorption kinetic: rapid. Adsorption mechanism: physisorption. Equilibration time and temperature: 1 h and low temperature (25 °C). Desorption is possible at higher temperature (55 °C). Thermodynamic studies: spontaneous and exothermic in nature. Data fitted by: Freundlich and Langmuir isotherm models	68
Zeolite A	Adsorbent amount: 10 mg. Initial concentration: 10 ml of 100 mg L^−1. Treatment conditions: pH 6.0 for 3 h. Studied parameters: effects of pH (2–8), initial concentration (50–150 mg L^−1), time and temperature (298–333 K). Characterization: XRD, XRF and thermal analysis	207.47–211.41 mg g^−1	Adsorption kinetic: rapid within 30 min. Equilibration time: 90–120 min. Effects of parameters: increase in pH (6–8), temperature, initial concentration and contact time favoured the adsorption. Zeolite structure disruption at acidic pH range. Data fitted by: pseudo-second order kinetic and Langmuir isotherm models. Mechanism: chemical sorption process	69
Clinoptilolites (CLI)	Adsorbent amount: 1 g. Initial concentration: 50 cm^3 of 10^−6–10^−1 mol dm^−3. Contact time: 48 h. Studied parameters: effects of temperature and cesium loading. Characterization: XRD, SEM and BET	1.31–3.00 mg g^−1 (75–92%)	Equilibration time: 4 h. NH^4+-CLI form was found more favourable for Cs below 60 °C temperature. Temperature of <60 °C and low Cs loading: favourable for Cs adsorption. Data fitted by: D-R isotherm model	70
Sericite	Initial concentration: 100 ml of 10–100 mg L Cs. Studied parameters: effects of pH, initial concentration, temperature (15–45 °C) and sorbent loading. Characterization: XRD, SEM/EDX and FT-IR	6.68 mg g^−1 (∼80%)	Equilibration time: 120 min. Influence of parameters: increase in pH 2–5 and 6.0 g L^−1 sorbent concentration favourable for Cs uptake but decreased at higher temperature. Thermodynamics: exothermic and spontaneous. Data fitted by: Freundlich isotherm and pseudo-second order kinetic models	71
Ethylamine-modified montmorillonite	Adsorbent amount: 0.05 g. Initial concentration: 25 ml of 20–340 mg L^−1. Treatment conditions: 30 °C and pH 7.5 for 240 min. Studied parameters: the effects of pH (1–10), contact time, initial concentration, temperature and ionic strength. Characterization: FT-IR, BET, SEM and EDS	80.27 mg g^−1	Influence of the parameters: increase in pH favoured the adsorption but temperature increase and presence of alkali and alkaline earth metal ions decrease uptake and hence, sorption process was exothermic and spontaneous in nature. Data fitted by: Langmuir isotherm and pseudo-second order kinetic models	72

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2.1.2 Inorganic-polymer composites. Enhancement of the adsorption properties of inorganic compounds as sorbents for cesium ion in solution has been realized by the incorporation of polymers. Inorganic compounds of nanosizes with high selectivity, high surface area and high speed of chemical reactions have been incorporated in fibrous and synthetic polymers with ion exchange groups.⁴⁸ Technological properties of fibrous polymer matrix (flexibility, chemical stability, high specific surface, low hydraulic permeability, and ease utilization in dynamic sorption regimes) make the synthesized composites of good adsorption materials with high sorption capacity. Some of the polymers such as polyacrylonitrile, polyaniline and chitosan have been investigated for cesium ion removal.^67,73–78 Table 2 shows some common inorganic-polymer composites used in the recent time.

Table 2 Adsorption capacities of inorganic-polymer composites for cesium ion removal from solution

Inorganic-polymer composites	Adsorption parameters and adsorbent characterization	Adsorption capacity	Comment	Ref.
Chitosan-grafted bentonite	Studied parameters: effects of pH, ionic strength and time. Characterization: N2-BET, SEM, XPS XRD and TGA	1.164 mmol g^−1	Equilibration time: 10 h. Influence of parameters: pH 7.0 was favourable but the presence of alkali and alkaline earth metal ions and partial exfoliation of bentonite layer and lower ion-exchange properties of hydroxyl groups decreased the uptake of Cs	67
Chitosan-grafted carbon nanotubes	Studied parameters: effects of pH, ionic strength and contact. Characterization: N2-BET, SEM, XPS and XRD	0.333 mmol g^−1	Low cation exchange capacity of CNT with high hydroxyl groups lower sorption capacity of the sorbent for Cs. pH and competing monovalent group 1 and divalent group 2 ions greatly influenced sorption capacity	67
Ammonium molybdophosphate incorporated into polyacrylonitrile (AMP-PAN)	Adsorbent amount: 0.2 g initial concentration: 15 ml of 10 mM radioactive laundry wastewater. Treatment conditions: pH 5.0, 20 °C and 200 rpm for 24 h. Studied parameters: effects of pH (≤14), three kinds of surfactants and co-existing metal ions. Characterization: BET, EDS and FT-IR	0.61 mmol g^−1	Effect of pH: constant uptake. Single- and bi-solute competitive adsorption: Cs uptake favourable over Co and Sr. Influence of other parameters: the presence of Na^+ and Ca^2+ ions suppressed Cs^+ uptake. The presence of cationic surfactants (OTMA and HDTMA) and anionic surfactants (SDBS and SOBS) decreased adsorption of Cs but non-ionic surfactants (Tween 80 and Triton X-100) has no effect. Data fitted by: Langmuir, Freundlich and D-R isotherm models. Mechanisms: ion-exchange and physisorption	75
Ammonium molybdophosphate incorporated into polyacrylonitrile	Initial Cs solution concentration: 10 ml of acidic tank waste containing 250 Bq ml^−1 137Cs. Treatment conditions: 24 h At 23 ± 5 °C. Column test at flow rates of 5, 10 and 20 bed volume per hour. Influencing parameters: effects of nitric acid, K^+ and Na^+ up to 2 M. Characterization: TGA	22.5–19.6 mg g^−1	Effects of parameters: K^+ ion effect is significant on Cs uptake, the effects of both acid and Na^+ is less significant. Data fitted by: Langmuir isotherm model. Dynamic sorption capacity decreased as flow rates increased. Stability: the sorbent thermally stable up to 400 °C with 10% weight loss due to water of hydration	76
Tungstate/polyacrylonitrile composite bead	Adsorbent amount: 230 mg. Initial concentration: 20 ml of 0.075 mM traceable in 1 M HNO3. Treatment conditions: 25 °C temperature and contact time of 480 min. Characterization: SEM	∼8.89–8.91 mg g^−1	Maximum sorption capacity achieved between 30 and 60 min. Data fitted by: Langmuir isotherm and pseudo-second order kinetic models. The bead size affects the rate of uptake and optimization of Cs uptake possible with wet beads after coagulation	73
Polyaniline titanotungstate (PATiW)	Adsorbent amount: 50 mg. Initial concentration: 5 ml of 0.1 M Cs. Treatment conditions: 25 °C temperature and contact time of 24 h. Studied parameters on Cs uptake: elemental composition, chemical solubility, ion-exchange capacity and pH-titration curve. Characterization: IR, XRD and TGA-DTA	217 mg g^−1	Equilibration time: 24 h. Sorption distribution coefficient increased with pH (2–9) and temperature (25–60 °C). The pH titration curve showed surface precipitation rather than conventional ion exchange or surface adsorption. Effects of metal ions: no effect. Data fitted by: Freundlich isotherm model	74
Crystalline manganese dioxide polyacrylonitrile	Adsorbent amount: 0.1 g. Initial concentration: 10 ml of 10^−4 mol L^−1. Treatment conditions: 25 °C temperature and pH 4.0. Studied parameters: effects of contact time, temperature (298–338 K), interfering ions and pH. Characterization: XRD, FT-IR, SEM, CHN, TGA-DSC and BET	0.007 mmol g^−1	Adsorption kinetic: rapid. Equilibration time: 35 min. Parameters effects: adsorption favourable within pH 4–9. Temperature increase but the presence of mono and divalents ions is negative on Cs sorption. Data fitted by: Freundlich isotherm model. Thermodynamic studies: endothermic and spontaneous in nature. Stability tests: sorbent stable up to 200 kGy radiation dose, 310 °C heat, water, dilute acid, ethanol and alkaline solutions but decomposed in concentrated acids. Desorption: difficult due to chemisorption and irreversible process	77
Potassium nickel hexacyanoferrate loaded polypropylene fabric	0.1 g sorbent mixed with 20 ml Cs solution. Studied parameters: effects of contact time, pH and sodium ion concentration. Characterization: XRD, FT-IR-ATR and SEM	78 mg g^−1 (>95%)	Equilibration time and kinetics: 30 min and rapid. Effects of parameters: constant uptake within pH 6–12, neutral and basic solutions but decreased with increase in sodium concentration. Sorbent structure: face-centered cubic crystalline	48
Sodium titanosilicate polyacrylonitrile composite	Adsorbent amount: 0.1 g. Initial concentration: 10 ml of 10^−4 mol L^−1 treatment condition: pH 6.0 and 25 °C temperature. Studied parameters: effects of pH (1–9), temperature (25–65 °C), contact time (5–120 min) and interfering competing ions. Column test was performed to estimate the dynamic sorption capacity at 5 and 100% breakthroughs. Characterization: XRD, FT-IR, SEM, BET, CHN and TGA-DSC	9.80 to 22.06 mg g^−1 (44.42%)	Equilibration time: 130 min parameters effects: pH 6.0 and increase in temperature favoured the uptake but mono and divalent metal ions hindered favourable Cs uptake and hence, the sorption process is endothermic. Data fitted by: Langmuir isotherm model. Stability: thermal and gamma irradiation stability were 275 °C and 200 KGy	79
Copper hexacyanoferrate–polyacrylonitrile composite	Adsorbent amount: 0.1 g. Initial concentration: 10 ml of 10^−4 mol L^−1 Cs. Treatment conditions: temperature 25 °C, pH 9.0 and 120 min contact time. Studied parameters: influence of pH, contact time, temperature and interfering cations. Column studies were performed to fit the dynamic sorption capacity at 5 and 100% breakthroughs. Characterization: XRD, FT-IR, TG-DSC, BET, SEM and XRF	7.31–11.46 mg g^−1 (63.78%)	Equilibration time: 280 min. Parameters effects: pH increase (optimum pH 9.0), favoured adsorption process. Effect of interfering cations (Na^+, K^+, Ca^2+ and Mg^2+) negative on Cs adsorption. Thermodynamic studies: Cs uptake is endothermic and spontaneous ion exchange reaction. Data fitted by: Freundlich isotherm model. Stability: the sorbent is thermally stable up to 200 °C	9
Whisker-supported ion-imprinted polymer	Adsorbent amount: 0.4 g. Initial concentration: 50 ml of 10 mg L^−1 Cs. Treatment conditions: temperature 25 °C and contact time of 2 h. Studied parameters: effects of pH, sorption rate and sorbent loading. Characterization: FT-IR and XRD	32.9 mg g^−1	Equilibration time and kinetics: 2 h and rapid sorption. Parameters effects: acidic pH and increase in temperature (from 25 to 55 °C) adversely affected Cs sorption until optimum pH 6.0 and 0.4 g sorbent loading sufficient for maximum sorption. Competitive ions have no significant effect on Cs selectivity onto the sorbent. Data fitted by: pseudo-second order kinetic and Langmuir isotherm models. Desorption: possible by 99% using acid at 50 °C and 6 cycles reusability of the sorbent	80
Potassium copper nickel hexacyanoferrate–polyacrylonitrile	Adsorbent amount: 0.01 g. Initial concentration: 10 ml acidic solution of 7.5 × 10^−5 M Cs. Treatment conditions: contact time 3 h at 25 °C temperature. Studied parameters: effect of shaking time, pH, acid concentration and drying temperature on the ion-exchange capacity of the sample for Cs. Characterization: FT-IR, XRD and BET	2.85 mmol g^−1 ion-exchange capacity for Cs	Equilibration time: 2 h. Parameters: pH (2–12) did not have much influence on the sorption. Increase in drying temperature of the samples increases the exchange capacity. EDTA has a decreasing effect on the distribution coefficient of Cs. Stability: adsorbent stable to 100 KGy gamma-ray dose, thermal (up to 110 °C) and in dilute acid, water and alkaline solutions but decomposed in concentrated acid	81

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2.1.3 Bio-adsorbents. The characteristics of dead and living biomass have equally been explored in biosorption and bioaccumulation of heavy metals and radionuclides from soils, sediments, surface and groundwater from a long time.^82–87 The fact that these biological/agricultural materials are abundant and affordable in the terrestrial environment and in particular, they have good stability, minimise disposable sludge volume, large porosity, high surface area, natural capacity to accumulate heavy and radioactive metals and degrade organic compounds, and hence, increase their usage for environmental pollution remediation (Table 3). In a nutshell, biosorbents have limitedly been used for treatment in real industrial waste effluents and in few cases where they are used,^{12,14,85–93} a number of factors have been reported to hamper effective binding abilities of biosorbents among which are extreme characteristics such as pH, conductivity and hardness.^{12,14,85–93}

Table 3 Various bio-adsorbents and their adsorption capacities as reported in literature

Bio-adsorbents	Adsorption parameters and adsorbent characterization	Adsorption capacity	Comment	Ref.
Raw pine cone	Adsorbent amount: 1 g initial concentration: 100 ml of 50–250 mg L^−1 Cs solution. Treatment conditions: contact time of 30 min at pH 8 and room temperature. Studied parameters: the effect of pH (1–10), interfering metal ions. Characterization: FT-IR, BET and XRD	2.28 mg g^−1	Parameters effects: increase in pH increases Cs uptake but decrease as initial Cs concentration increases. The presence of Na^+ did not affect Cs uptake much as Ca^2+. Data fitted by: pseudo-second order kinetic model	12
Chemically treated pine cone	Adsorbent amount: 1 g. Initial concentration: 100 ml of 50–250 mg L^−1 Cs solution. Treatment conditions: contact time of 30 min at pH 8 and room temperature. Studied parameters: the effect of pH (1–10) and interfering metal ions. Characterization: FT-IR, BET and XRD	3.58 mg g^−1	Parameters effects: increase in pH increases Cs uptake but decrease as initial Cs concentration increases. The presence of alkali metals reduced sorption of Cs as well as changed the rate-limiting kinetics. Data fitted by: diffusion–chemisorption model	12
Arca shell	Adsorbent amount: 0.5 g. Initial concentration: 100 ml of known stable solution spiked with 260 Bq ^137Cs. Treatment condition: temperature of 25 °C. Studied parameters: effects of pH (1–7), contact time, dosage (0.1–15 g L^−1), initial concentration (10 to 500 ppm) and alkali/alkaline earth metals	3.93 mg g^−1/0.03 mol kg^−1 (98.2%)	Adsorption kinetics: rapid within 60 min. Equilibration conditions: pH 5.5, 3 h time and adsorbent dosage of 5 g L^−1. Parameters effects: cesium uptake was increased with initial concentration (beyond 100 μg mL^−1) but the presence of alkali/alkaline earth metals to concentration of about 500 μg mL^−1 adversely affected sorption percentage. Acidic pH ≤ 3 hindered Cs uptake. Data fitted by: Langmuir model thermodynamic studies: spontaneous and exothermic in nature	14
Polyphenols crosslinked persimmon tannin	Adsorbent amount: 0.01 g. Initial concentration: 10 cm^3 of 0.1 mM Cs solution. Treatment conditions: temperature of 303 K and 24 h time. Studied parameters: effects of pH, Na^+ ions in solution, initial concentration and temperature. Characterization: FT-IR and BET	1.34 mol kg^−1	Adsorption kinetics: fast in 5 min. Equilibration time: 8 h. Parameters effects: increase in pH positively affects the % uptake of Cs up to about neutral pH. Adsorption not affected by the presence of Na^+. Both initial concentration and temperature (up to 323 K) increased sorption capacity of Cs onto the sorbent. Data fitted by: pseudo-second order kinetic and Langmuir isotherm models. Regeneration: sorbent could be reused for about 4 cycles retaining its ion-exchange capacity	78
Polyphenols crosslinked tea leaves	Adsorbent amount: 0.01 g. Initial concentration: 10 cm^3 of 0.1 mM. Treatment condition: temperature of 303 K for 24 h contact time. Studied parameters: effects of pH, Na^+ ions, initial concentration and temperature. Characterization: FT-IR and BET	1.22 mol kg^−1	Adsorption kinetic: fast in 10 min. Equilibration time: 8 h. Parameters effects: increase in pH positively affects the % uptake of Cs. Uptake amount not affected by the presence of Na^+. Increase in initial Cs concentration and temperature (up to 323 K) increased Cs sorption capacity. Data fitted by: pseudo-second order kinetic and Langmuir isotherm models. Regeneration and reusability: elution possible with acid and reused for about 4 cycles showing undiminished capacity	78
Microalgal waste	Adsorbent amount: 10 mg. Initial concentration: 10 ml of 0.1 mM each of Cs and Na^+ solution. Treatment conditions: temperature of 303 K and desired pH for 24 h contact time. Studied parameters: the effects of pH and contact time	1.36 kmol kg^−1 (>85%)	Equilibration time: 60 min. Adsorption kinetic: rapid adsorption of Cs over Na^+. Optimum pH: pH 6.5. Parameters effects: increase in pH favoured adsorption of Cs. Data fitted by: Langmuir isotherm model. Regeneration and elution studies: possible elution with acid but simple incineration is proposed as an alternative due to the adsorbent combustible nature	85
Funaria hygrometrica	Adsorbent amount: 50 mg. Initial concentration: 4 ml of unreported Cs concentration at room temperature. Studied parameters: effects of pH (1–13), sorbent dosage (5–150 mg), time (5–180 min) and other cations. Characterization: FT-IR	∼6 mg g^−1 (94%)	Equilibration time: 30 min. Parameters effects: pH increased favoured adsorption and attained maximum between pH 6–10. Increase in sorbent to volume ratio increased % sorption. The presence of competitive metal ions affected adsorption of Cs at higher concentrations	86
NaOH treated Funaria hygrometrica	Adsorbent amount: 50 mg. Initial concentration: 4 ml of unreported Cs concentration at room temperature. Studied parameters: effects of pH (1–13), sorbent dosage (5–150 mg), time (5–180 min) and other cations. Characterization: FT-IR	∼17 mg g^−1	Equilibration time: 30 min. Parameters effects: pH increased favoured adsorption and attained maximum between pH 6–10. Increase in sorbent to volume ratio increased % sorption. The presence of competitive metal ions affected adsorption of Cs at higher concentrations. Leaching of exchangeable metal ions and the surface modification by NaOH favoured higher Cs uptake	86
Ocimum basilicum	Adsorbent amount: 0.5 g. Initial concentration: 20 ml of 100 μl of Cs tracer. Treatment conditions: temperature of 28 °C and contact time of 60 min. Studied parameters: the effects of pH, contact time and interfering ions. Characterization: SEM	160 mg g^−1 (48.14%)	Equilibration time: 30 min. Parameters effects: maximum sorption achieved at optimum pH 7. Increase in concentration of treatment acid decreases the uptake of the ions. The presence of divalent ions has no effect Cs sorption but monovalent ions did. Large number of carboxylic groups in mucilage polysaccharide facilitated Cs sorption onto the sorbent	88
Brewery's waste	Adsorbent amount: 0.1 g. Initial concentration: 50 ml of 1 mmol L^−1. Treatment conditions: temperature of 30 °C and pH 4.0. Studied parameters: effects of contact time and increase in initial concentration	0.076 mmol g^−1 (90%)	Adsorption kinetics: rapid within 30 min. Equilibration time: 3 h. Parameters effects: increase in initial concentration reduced Cs efficiency. Data fitted by: pseudo-second order kinetic and Langmuir isotherm models	89
P. australis	Adsorbent amount: 100 mg. Initial concentration: 50 ml unreported Cs concentration treatment condition: temperature of 30 °C, pH 5.5 and contact time of 3 h. Studied parameters: effects of contact time, pH (1–10), particle size, interfering ions and desorption/reusability	0.122 mmol g^−1	Adsorption kinetics: rapid. Equilibration time: 30 min. Parameters effects: highest uptake amount of Cs was at pH 4 and no decrease in the presence of alkali metal ions. Particle size of the sorbent affects the uptake with the big sizes showing highest uptake. Chemical treatment of the biomass decrease sorption capacity. Desorption: high concentration of NaOH and KOH suggested for desorption of Cs from the sorbent but has some damages to the capacity	90
Azolla filiculoides	Adsorbent amount: 60 mg. Initial concentration: 30 ml of 25–600 mg L^−1 Cs solution. Treatment conditions: temperature of 30 °C and contact time of 3 h. Studied parameters: effects of pH (2–10), equilibration time, particle size and desorption. Characterization: FT-IR	195 mg g^−1	Adsorption kinetics: rapid within 30 min. Equilibration time: 60 min. Parameters effects: increase in pH favoured Cs uptake and the pH with highest sorption was in the range pH 8–9. Bigger particle size favoured the adsorption data fitted by: Freundlich isotherm model	91
Coconut shell	Adsorbent amount: 20–100 mg. Initial concentration: 10 and 30 mg L^−1 Cs solution. Treatment condition: 24 h contact time. Studied parameters: effect of pH. Characterization: SEM	0.76 mg g^−1	Adsorption kinetics: poor and low affinity. Parameters effects: adsorption not affected by the pH change. pHpzc (point of zero charge) measurement revealed the alkaline nature of the sorbent with pHpzc = 10.22.	92
Almond shells	Adsorbent amount: 0.1 g. Initial concentration: 4 cm^3 of 10^−7 M Cs solution. Treatment conditions: pH 6 and temperature of 298 K. Studied parameters: effect of contact time, pH and sorbent dosage	12.63 mg g^−1	Adsorption kinetics: rapid within 20 min. Equilibration time: 60 min. Parameters effects: change in pH influences uptake amount and efficiency of Cs. Increase in adsorbent dosage increases sorption percentage	93
Almond shell with EDTA	Adsorbent amount: 0.1 g. Initial concentration: 4 cm^3 of 10^−7 M Cs solution. Treatment conditions: pH 6 and temperature of 298 K. Studied parameters: effect of contact time, pH and sorbent dosage	19 mg g^−1	Adsorption kinetics: rapid within 20 min. Equilibration time: 60 min. Parameters effects: change in pH influences uptake amount and efficiency of Cs. Increase in adsorbent dosage increases sorption percentage. The presence of EDTA increases the density of negative charges on the sorbent surface and its capacity leading to higher sorption capacity of Cs	93

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2.1.4 Advantages and disadvantages of various adsorbents. Owing to a large volume of waste in the environment, different adsorbents are applicable for removal of the contaminants for large volume reduction and safer disposal. In such a case, discrimination of the adsorbents is extremely difficult, however; the comparison can be made considering the benefits of using each material for particular waste decontamination. Apart from this, they can be classified in terms of basic advantages such as cost implication of adsorbents materials, operational requirements, mechanical strength, chemical, thermal and radiation stability and so on. For instance, decontamination of aqueous solution can be done using any adsorbents having surface functional groups for binding the adsorbates but when it involves recovering valuable adsorbates such as cesium, only polymeric adsorbents are most appropriate due to the difficulty in desorption using other sorbents.⁹⁴

In case of inorganic composite adsorbents, a number of available natural materials that do not need much reprocess before use due to their cation exchange capacity (such as naturally occurring clay minerals like zeolite, montmorillonite, bentonite, and coal) have been reported which make them economically feasible in real application at commercial scale.^68,69 The major issue with these materials is the presence of a number of cations that can block active sorption sites and hence, requires pre-treatment with chemicals before use. It was reported that the number, types and locations of cations in the zeolite greatly influenced the selectivity and rate of ion-exchange of zeolite sorbents.⁶⁹ Apart from these, titanosilicate materials,⁹⁵ fly ash, metal transition ferrocyanides^8,11,58 and hexacyanoferrates⁶⁴ have also been credited with excellent surface area for adsorption, high cation exchange capacity, compatibility with final waste forms, high swelling, high mechanical strength and radiation stability (up to about 200 KGy). However, apart from the naturally occurring inorganic materials, most of the inorganic material sorbents are quiet expensive to use especially in developing and underdeveloped countries, they are difficult to separate from solution due to their fine microcrystalline nature resulting into secondary waste disposal problems. As regard to liquid radioactive waste, most inorganic sorbents do not withstand high level radiation; they have low chemical stability making difficulty in handling and hence, suffer irreversible structural changes under extreme environmental conditions.⁹⁵ They pose limitation due to slow mass-transfer rate in column operation as a result of the fine particle size.⁷⁹

On the other hand, due to its excellent binding ability, high porous structure, good mechanical strength for longer use, strong adhesive forces and stability to thermal, chemical and radiation, polymers have been found as good supporting materials to solve most of the aforementioned challenges of inorganic material sorbents.⁷⁹ The composites of inorganic-polymer sorbents have better sorption capacity, high selectivity and improved rapid kinetics of adsorption compared to inorganic materials.⁶⁴ Contrary to the inorganic materials that generate huge sludge, their composites produce less sludge and are good in preventing the release of the sorbed radionuclides after disposal. The challenge with polymer composites is the limitation to the long-term use due to the polymer biodegradability nature. However, synthetic polymers are more resistible to decomposition or biodegradability over a long-term use compared to the natural polymers.⁹⁴ More information on the properties of polymer-inorganic composites that make them useful in environmental remediation, regeneration and reusability has been reviewed by Zhao et al.⁹⁶ and Hua et al.⁹⁷

As regard to bio-adsorbents, they are the best alternative for inorganic/organic sorbents and are the most low cost effective materials known. This is due to the fact that they are naturally abound and free in the environment as biomass wastes from dead algae, moss, bacteria or fungi and demonstrate good adsorption for cesium from the radioactive waste effluents. Apart from the low procurement and operational cost, they offer excellent means of minimizing the volume of chemical to be disposed of. The major setbacks with bio-adsorbents are low sorption capacity, high chemical and biological oxygen demand due to dissolution of organic compounds contained in the plant materials and the weakening of active surface functional groups under extreme environmental conditions.^98–100 Comprehensive information on the sorption capacity, mechanism of adsorption and issues regarding regeneration and reusability of bio-adsorbents are found in the review work of Kratochvil and Volesky.¹⁰¹ To use biological/agricultural waste materials, it demands chemical pre-treatment or modification. Review on the advantages and disadvantages of untreated and chemically treated biomass as bio-sorbents is reported by Ingole and Patil, 2013.¹⁰⁰ Biomass materials are also incorporated into the polymer matrix to enhance its sorption capacity and mechanical strength, but reports are still limited in this area for cesium uptake from its radioactive waste solution.⁴⁹ Regeneration and reusability studies are still very limited in bio-sorbents to optimize their usage.

3.0 Factors affecting adsorption of cesium ion from solutions

Decontamination of waste streams by adsorption technique is commonly influenced by a number of factors which require optimization for better adsorption process. Some of these factors are physico-chemical properties of the adsorption materials, solution pH, temperature, influence of co-existing ions, initial cesium concentration, contact time, surface character of the sorbent, sorbent dosage, etc. These factors are discussed briefly as follows:

3.1 Effect of solution pH

A major factor influencing sorption capacity of different sorbents is solution pH. Different sorbents have different behaviours in solution media and therefore, their efficiency in removing sorbate from such solutions.¹⁰² A change in the pH of a solution affects not only the degree of protonation but also the metal chemistry such as speciation and precipitation^103,104 and the surface properties (charge and functional groups) of the sorbents.¹⁰⁴

The equilibrium uptake of different radionuclides including cesium in different acidic media using activated and non-activated silico-antimonate (SiSb) has been reported.⁴⁶ According to that report, there is a strong uptake and high affinity of the radionuclides to antimonite matrix at low acid concentration (0.1 M) but decreased as the concentration increased to 5 M. The uptake followed selectivity order of Cs⁺ > Eu³⁺ >> Co²⁺. Adsorption of cesium on CHCF–PAN was carried out at pH values ranging from 1–9 to determine the optimum condition.⁹ The result showed that the uptake of cesium was continuously improved from the acidic to alkaline pH (9.0) following other similar reports using copper ferrocyanide functionalized mesoporous silica¹¹ and aluminum-pillared montmo-rillonite on the removal of cesium and copper from aqueous solutions.¹⁰⁵ Suppression of cesium sorption at acidic conditions was attributed to competition of H₃O⁺. In contrast, suppression of adsorption was reported to be due to the electrostatic repulsion of negatively charged calcium hydroxide as pH values varied from 2–11 using nickel(II) hexacyanoferrate(III) functionalized walnut shell (Ni^IIHCF^III-WS).^49,106 Fig. 2 shows the sorption percentage of cesium based on solution pH. Chitrakar et al. reported 172 mg g⁻¹ cesium adsorption capacity for layered manganese oxide at pH 2–4 and 132 mg g⁻¹ when pH increased to 10.⁶¹ Crystalline manganese dioxide polyacrylonitrile composite was evaluated for sorptive removal of cesium from mineral acid and weak alkaline solutions within the pH range of 4–9.⁷⁷ The adsorption capacity of 0.007 mmol g⁻¹ was estimated by Freundlich isotherm model. The sorbent was thermally stable up to 300 °C and 200 kGy gamma radiation. Desorption of sorbed cesium was very difficult with the eluent. Synthesized KNiHCF was noticed to adsorb cesium in a wide range of pH values (6–12)⁴⁸ but only at neutral solution KNiFC-loaded silica gel was reported to remove trace amount of cesium.⁶⁶ Maximum uptake of 0.4 to 1.05 mol mol⁻¹ Cs/Fe was reported using copper–potassium hexacyanoferrate(II) at pH 5–8,^50,107,108 and 27.40 and 50.23 mg g⁻¹ were reported for a pH range of 1.0–9.0 (but high sorption values obtained at pH 5.5 and 7) using mesoporous silica (IA) and ligand immobilized mesoporous silica (CA), respectively.¹⁰⁹ Zirconium iodomolybdate was also used to remove cesium ions from aqueous solution at pH 1–7, but at pH < 2 the adsorption process was characterised by competition of H⁺ and Cs⁺ ions on the negatively charged anionic functional groups or dissociated edge of hydroxyl groups on ZIM surface.^63,110 Table 4 shows different adsorbents, solution pH and adsorption percentage as reported in the literature.

Table 4 Comparison of various sorbents, sorption capacities and efficiencies for cesium ions

Types of adsorbent	Material	Sorption medium/pH range	Sorption capacity (mg g^−1)	Sorption efficiency (%)	Ref.
Inorganic and its composites	SiSb	Acidic	0.170–0.540	30 to >90	46
	Activated SiSb (with phosphoric acid)	Acidic	0.220–0.520	30 to >90 in 6 h	46
	Zeolite A	2.0–8.0	60.5	86.4	111
	Zeolite A	6.0	76.69–78.25	90	69
	Ceiling tiles	4.95	0.5	—	112
	Stannic phosphate	∼2.4	0.371 × 10^−3 mol g^−1	37.1–74.1	113
	Sericite	2.0–8.0	6.68	75	71
	Bentonite	3–10	1.334 mmol g^−1	—	67
	Natural clay (bentonite)	2–12	4.10 mmol kg^−1	90	114
	Ferrite	2.4–11	108.58	82	115
	Natural magnetite	2.4–11	70.77	61	115
	Mesoporous silica (IA)	1.0–9.0	27.40	70	109
	Immobilized mesoporous silica (CA)	1.0–9.0	50.23	85	109
	Zirconium iodomolybdate	1.0–8.0	—	∼90	63
	Zirconium phosphate	∼2.4	0.915 × 10^−3 mol g^−1	91.5–98.4	113
	Raw montmorillonite	3–12	0.4292 mmol g^−1	—	116
	Phosphate-modified montmorillonite (ppm)	3–12	0.7063 mmol g^−1	93.87	116
	Ethylamine-modified montmorillonite	1.0–10.0	80.27	—	72
	Calcium-saturated montmorillonite	1.0–10.0	60.03	—	72
	Chinese weathered coal	5.01	—	45–60	38
	KNiFC-loaded chabazite	3.7–5.83	1.44–1.97 (mmol g^−1)	95	117
	Copper ferrocyanide functionalized mesoporous silica	7.7	17.1	95	11
	Copper ferrocyanide functionalized mesoporous silica	1.1	21.7	—	11
	Prussian blue	1.1	2.6	—	11
	Prussian blue	7.7	12.5	75	11
	Prussian blue	4.0–10.0	110.5	42	118
	KNiFC-loaded silica gel	High neutral solution	0.305 mmol g^−1		66
	Natural clinoptilonite	6.5	0.37 mmol g^−1	—	120
	Sulfuric acid crosslinked Pseudochoricystis ellipsoidea	6.5	1.36 mmol g^−1	—	85
	Chabazite and activated carbon mix	5.6–8.5	8.19	—	16
	K2CuFe(CN)6	Acidic	1.3 mol mol^−1	—	107
	K2CuFe(CN)6	8	0.4 mol mol^−1	—	107
	Potassium nickel ferrocyanide	Acidic	390	72	111
	Cu2^IIFe^II(CN)6	5–8	0.99–1.05 mol mol^−1	∼100	108
	Cu3^II[Fe^III(CN)6]2	Acidic	0.073 mol mol^−1	∼100	108
Inorganic-polymer composites	Manganese oxide–polyacrylonitrile	4.0–9.0	0.007 mmol g^−1	—	77
	CHCF–PAN	1.0–9.0	7.31–11.46 (0.084 mmol g^−1)	63.78	9
	Ni^IIHCF^III-WS	2–11	6 ± 4.3	∼100	49 and 106
	KNiHCF-loaded PP fabric	2.0–12	78	>95	48
	ZrP–AMP	Acidic	0.058 mmol g^−1	96	120
	AMP-PAN	2–10	0.610 mmol g^−1, 81 mg g^−1	—	75 and 76
	Whisker-supported ion-imprinted polymer	6.0	32.9	—	80
	STS–PAN	1.0–9.0	22.06	44.42	79
	KCNF–PAN	2–14	2.85 mmol g^−1	—	81
	PB-encapsulated alginate/calcium beads	4.0–10.0	144.72	45	118
	Nickel–potassium ferrocyanide immoblized chitin	1.0–9.0	80.7	—	64
	Chitosan-grafted-bentonite and CNT	3–10	0.333–1.164 mmol g^−1	—	67
	Coal and chitosan	3.0–6.0	3	—	121
Bio-sorbents	O. basilicum seeds	1–7	160	48.14	88
	Brewery's waste	4	0.076 mol g^−1	90	89
	Moss immobilized silica matrix	1–13	8.5	>94	87
	Coconut shell activated carbon	5.7–8.15	0.76	—	92
	Almond shells	1.5–4.5	12.63–19* (* with addition of EDTA)	90	93
	Azolla filiculoides	2.0–10.0	70.5–195	85.2	91
	Ferrocyanide modified algal sorbents	1–10	24.5–198.7	—	90
	Native biomass sorbents	1–10	14.5–71.9	—	90
	P. australis biomass	5.5	0.122 mmol g^−1	—	90
	Arca shell	1–7	3.93	98.2	14
	CTL and CPT gel	6.5	1.22–1.34 mmol g^−1	95.2–97.3	78
	Raw and modified pine cone	1–10	2.45–2.83	—	12

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3.1.1 Effect of temperature on cesium adsorption. Temperature is other important parameter for adsorption studies, and it tells more about the mechanism involved in the sorption process.¹⁰² Basically, there are two major effects of temperature on adsorption process. When temperature is increased, the diffusion rate of the adsorbate molecules is increased across the external boundary layer and in the internal pores of the adsorbent particle, due to the decrease in the velocity of the solution.¹²² In the same vein, variation in the temperature of the adsorption system results into variation in the equilibrium capacity of the adsorbent for a particular adsorbate.¹²³

Therefore, if the rate of adsorption is increased with temperature, the mechanism controlling the process is endothermic but if the rate is decreased with temperature, it is exothermic. The spontaneity of the process depends on whether the change in entropy and the free energy of adsorption system is positive or negative.⁷⁴ Positive values of entropy change and negative values of free energy change indicate the solution interface is in random increase and spontaneous sorption process, but negative entropy change and positive free energy change show that the solution interface is slow and hence non-spontaneous sorption process.³⁰ So, it could be said that temperature is responsible for the behaviour/nature of sorbate in solution and the availability of active sorption sites on the adsorbent surface as temperature varies.¹⁰² This means that if solution temperature increases, it weakens the electrostatic interactions of the ions in solution and hence, increases their mobility towards sorbent and vice versa. Besides, it is generally known that if sorption is governed by physical phenomenon, an increase in temperature will cause a reduction in sorption capacity.⁴⁶ In other words, increase in sorption capacity with temperature is as a result of chemical process involved in the adsorption. For instance, the effect of temperature on sorption of Cs⁺, Eu³⁺ and Co²⁺ onto non-activated and activated-SiSb (1 : 2) from different acidic media was reported to involve chemisorption process and that the equilibrium sorption capacity of the metal ions was increased with temperature (from 303 to 333 K). In 3 M H₂SO₄, sorption capacity for Cs at 303, 318 and 333 K are 0.25, 0.24, 0.23 mg g⁻¹ on non-activated SiSb and 0.36, 0.38 and 0.41 mg g⁻¹ on phosphoric acid activated-SiSb.⁴⁶ Nilchi et al.⁹ and El-Naggar et al.⁷⁴ also reported similar results with solution temperature ranging from 298 to 338 K.⁹ The increase in adsorption coefficient as a result of temperature increase was attributed to the faster migration of ions and stronger electrostatic interactions of adsorbate–adsorbent. In contrast, other phenomena such as surface precipitation of metal oxides or ternary processes were ascribed to influence the adsorption efficiency of Cs and some heavy metals from the solution than temperature (increase of which caused about 50% reduction in efficiency) using manganese oxide–Anfezh mixture as chemisorbent.¹²⁴ Table 5 and Fig. 3 show the effect of temperature on cesium sorption by various sorbents as reported in the literature. The compiled literature generally showed increased distribution coefficient or better sorption capacity of cesium as temperature increased.

Table 5 Effect of temperature on cesium sorption by different sorbents

Type of adsorbent	Material	Temperature range (K)	Type of process	Distribution coefficient (mL g^−1)	Adsorption capacity (mg g^−1)	Ref.
Inorganic adsorbents	Hydrous titanium oxide	298–325	Endothermic	—	0.6189–0.4303 mmol g^−1	62
	Clinoptilolites	298–353	Exothermic	92	1.31–3.00	70
	Zirconia powder	298–333	Endothermic	—	7.01–9.25 mmol g^−1	125
	KCNF	293–353	Endothermic	2.215–2.257	—	65
	Sericite	288–318	Exothermic	0.227 L mg^−1	6.68	71
	PPM	283–303	Exothermic	—	0.7063 mmol g^−1	116
	Zirconium phosphate–ammonium molybdophosphate	301–323	Endothermic	—	Increase	126
	Stannic phosphate	301–333	Endothermic	188.8–386.5	0.653–0.741 × 10^−6 mol g^−1	113
	Zirconium phosphate	301–333	Endothermic	—	0.984–0.985 × 10^−6 mol g^−1	113
	Zeolite A	298–333	Endothermic	212.5–225.73	—	122
	Zeolite A	298–333	Endothermic	—	76.69–78.25	69
	Crushed granite	298–328	Exothermic	—	0.83–0.01 mmol g^−1	127
	Local Taiwan laterite	298–328	Exothermic	25–11	0.3–0.2 mmol g^−1	68
	KNiFC-impregnated zeolite	298–333	Endothermic	4200 cm^3 g^−1	—	139
	Ethylamine-modified montmorillonite	303–333	Exothermic	—	80.27	72
	Calcium-saturated montmorillonite	303–333	Exothermic	—	60.03	72
	Titanotungstate	298–333	Endothermic	—	19.79–20.82	128
	Activated silico-antimonate	303–333	Endothermic	—	0.220–0.520	46
Inorganic-polymer composite	STS–PAN	298–338	Endothermic	8406–10 362	19.6–22.9	79
	Polyaniline titanotungstate	298–333	Endothermic	—	32.08–33.5	128
	Polyaniline titanotungstate	298–333	Endothermic	—	217	74
	CHCF–PAN	298–338	Endothermic	1673–2109	—	9
	Manganese oxide–PAN	298–338	Endothermic	944–1058.5	—	77
	KCNF–PAN	298	—	9.9 × 10^4 cm^3 g^−1	2.85 mmol g^−1	81
	Nickel hexacyanoferrate incorporated walnut shell	298–318	Endothermic	171.4–2264.3	—	49

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Fig. 3 Effect of temperature on removal efficiency of cesium ions using sericite⁷¹.

3.1.2 Effect of contact time on distribution coefficient of cesium. Adsorption time is an important factor that determines if an appropriate technology has been designed. If the treatment technologies implemented for separation of any adsorbates from their solutions are appropriate and considered economically viable then, optimum equilibrium and adsorption time must be short with higher adsorption rate.⁶³ Variation of contact time results into change in the optimum sorption efficiency of cesium from wastewaters. However, the equilibration time for adsorption of cesium onto different adsorbents also depends on a number of factors such as: the electrostatic attraction of adsorbate–adsorbent, complexation reaction on the outside surfaces of adsorbents, distribution of pores on adsorbents, availability of sorption sites or pores or composition of the adsorbents, interparticle diffusion of cesium on the pores of the adsorbents and other environmental conditions.^71,72 Most of the earlier works reported that initial rapid adsorption of cesium within short contact time before equilibrium was achieved, and it attributes to the availability of large number of sorption sites for cesium from the bulk solution. For instance, Han et al. used copper ferrocyanide to sorb cesium from raw water and found that rapid adsorption occurred in the first 20 min and then became slowly until equilibrium was attained in 90 min.¹²⁹ Ali synthesized silico-antimonate and modified with phosphoric acid to sorb cesium and other radionuclides from acidic waste-effluent.⁴⁶ Rapid equilibrium was attained between 1 and 6 h with >90% uptake of Cs⁺ and Eu³⁺ but only 60% of Co²⁺ could be sorbed onto activated silico-antimonate. Copper(II) ferrocyanide incorporated into vermiculite was also prepared and used to adsorb cesium, showing high affinity for the metal ion with sorption capacity of 0.05 to 0.13 mmol Cs g⁻¹ (equivalent of 179 mg g⁻¹) of the sorbent and the equilibrium was reached within 1–2 h.⁵⁷ The compilations of earlier studies based on contact time and adsorption rate are listed in Table 6 while Fig. 4 shows the effect of contact time on cesium removal by ethylamine-modified montmorillonite.

Table 6 Variation of contact time on the adsorption equilibrium of cesium ions by various adsorbents

Type of adsorbent	Material	Equilibration time (min)	Nature of adsorption rate	Ref.
Inorganic adsorbents	Sericite	120	Rapid	71
	Ethyl-Mt & Ca-Mt	45	Rapid	72
	FC–Cu–EDA–SAMMS	5	Rapid	11
	Prussian blue	60	Slow	11
	Cu–FC–EDA–SAMMS	≤2 h	Very rapid	8
	Mesoporous and ligand immobilized silica	1–1.5 h	Very rapid	109
	Natural clay (bentonite)	20	Fairly rapid	114
	Tin(IV) phosphate	5 days	Slow	130
	Calix[4]arene impregnated zeolite	6 h	Rapid	131
	Activated silico-antimonate	6 h	Fairly rapid	46
	Non-activated silico-antimonate	6 h	Slow	46
	Copper hexacyanoferrate	72 h in 6 months	Slow	108
	Zeolite A	90–120	Moderately rapid	69
	Metals phosphate	60	Rapid	113
	Chinese weathered coal	400	Rapid	38
	Zirconium iodomolybdate	80	Rapid	63
	PB-coated magnetic nanoparticle	24 h	Less rapid	132
	PB-caged in diatomite–CNT	10	Rapid	133
	Zirconyl molybdopyrophosphate	30	Rapid	134
	Hydrous titanium oxide	5 h	Fairly rapid	62
	Ferrite and natural magnetite	60	Rapid	115
	Crushed granite	8 h	Fairly rapid	127
	Clinoptilolites	4 h	Rapid	70
	Ceiling tiles	240 h	Slow	112
Inorganic-polymer composites	CHCF–PAN	280	Rapid	9
	STS–PAN	130	Rapid	79
	KNiHCF-loaded PP fabric	30	Rapid	48
	Polyaniline titanotungstate	240	Rapid	128
	MoHTB–PAN and AMP–PAN-85	100	Fairly rapid	73
	MnO2–PAN	<35	Rapid	77
	Polyaniline titanotungstate	2 h	Rapid	74
	CTL and CPT gel	8 h	Rapid	78
	Ni^IIHCF^III-WS	2 h	Rapid	106
	PB-alginate/calcium beads	20	Rapid	118
	KCNF–PAN	2	Rapid	81
	STS–PAN	130	Rapid	79
	Imprinted polymer	2 h	Rapid	80
Bio-adsorbents	Arca shell	1 h	Rapid	14
	Coconut shell activated carbon	24 h	Very poor	92
	O. Basilicum seed	30	Varied	88
	Brewery's waste	3 h	Rapid	89
	Ferrocyanide algal sorbent	30	Very rapidly	90
	A. filiculoides	60	Very rapid	91
	Immobilized moss	30	Rapid	87
	Modified and unmodified algal sorbents	30	Rapid	90
	Activated carbon from almond shell	60	Fairly rapid	135

---

Fig. 4 Effect of contact time on Cs adsorption using ethylamine-modified montmorillonite.⁷²

3.1.3 Effect of initial concentration of cesium on adsorption. This is another important parameter that determines the amount of cesium sorbed onto a particular sorbent. The percentage removal of cesium from the bulk and the sorption capacity of every adsorbent depend strongly on the possible existing relation between the adsorbate and the adsorbent. This, in-turn, depends on the initial concentration of the adsorbate in solution and the available sites or the sorption pores in the adsorbent surface.¹⁰² Most of the adsorbents reported in the literature have their sorption capacity increased as initial cesium concentration increased, probably as a result of increase of driving force for mass transfer at high initial concentration but the percentage removal is decreased which can be attributed to sorbent surface being saturated.^{11,49,89,102,113,115}

Long et al. used ethylamine-modified montmorillonite and calcium montmorillonite to sorb cesium ions from the initial concentration of 20–230 mg L⁻¹ and showed that the sorption capacity increased to 80.27 and 60.03 mg g⁻¹, respectively.⁷² In case of using copper–potassium hexacyanoferrate(II), increasing the cesium concentrations does not influence the sorption capacity of cesium sorbed onto the sorbent.¹⁰⁷ Murthy et al. studied the effects of increasing the initial cesium concentration on sorption by zirconium phosphate–ammonium molybdophosphate and reported that it increases both the amount and percentage removal of cesium which was attributed to the larger sorbent surface sites, but influenced by variation in the acidity of solution.¹²⁶ Similar investigation was reported by El-Kamash using zeolite A but the higher uptake capacity was attributed to the higher probability of collision between cesium ion and the zeolite particles.⁶⁹ Table 7 shows the range of initial bulk concentration and percentage removal of cesium using various adsorbents.

Table 7 The effect of initial concentration and percentage removal range on cesium adsorption by various adsorbents

Type of adsorbent	Material	Initial concentration (ppm)	Uptake percentage (%)	Ref.
Inorganic adsorbents	Stannic phosphate	1.0 × 10^−8–1.0 × 10^−2 M	74.1–34.1	113
	Zirconium phosphate	1.0 × 10^−5–1.0 × 10^−2 M	98.4–91.5	113
	PB	150–280	42	118
	FC–Cu–EDA–SAMMS	500–50 ppb	Decrease	11
	PB	500–50	Decrease	11
	PUP/CNT/DM/PB	10	95.96	134
	Ferrite	0.001–0.1 mol L^−1	Decrease	115
	Crushed granite	10^−3–10^−7M	0.83–0.01 mmol g^−1	127
	Clinoptilolites	10^−6–10^−1 mol dm^−3	92	70
	K2Ni[Fe(CN)6]	10–400	Decrease
	Ethylamine montmorillonite	20–340	Increase	72
	LTL	10^−3–10^−7M	Increase	68
	Ceiling tiles	0.114–23.9	Increase	112
Inorganic-polymer composites	ZrP–AMP	3.76.0 × 10^−5–7.52 × 10^−3 M	4–96	126
	PB-coated MNP	50–2780	96 mg g^−1	132
	Nickel–potassium ferrocyanide loaded chitin	41 000 Bq L^−1	>97.7	64
	NiHCF-WS	1–100	99.1–70	106
	Imprinted polymer	2–500	Increase	80
	Silico-titanate-loaded membrane sheets	25.30	99.98	136
	PATiW	660–6600	Increase	128
	PATiW	13–13 290	Increase	74
	PB-alginate/calcium beads	150–280	45	118
Bio-adsorbents	Coconut shell activated carbon	10–30 mg L^−1	No affinity	92
	Brewery's waste	0.157–6.189 mmol l^−1	6.3–2.5	89
	Azolla filiculoides	25–600 mg L^−1	68	91
	Raw and modified pine cone	50–250 mg dm^−3	0.89–2.76 mg g^−1 capacity	12
	Walnut shell	5–400	Decrease	49
	Arca shell	10–500	Increase	14
	Crosslinked persimmon tannin (CPT) and crosslinked tea leaves (CTL)	0.2–8.0 mM	Increase	78

---

3.1.4 Effect of competing ions on the adsorption of cesium. Adsorption of cesium ions from solution onto sorbents could be greatly influenced by the presence of competing ions for the binding sites on adsorbent surfaces. The selectivity of sorbents for cesium depends largely on the similar behaviours of cesium and other ions in solution and towards the sorbent. Alkali and alkaline earth metal ions are the major competing metal ions reported to influence cesium selectivity onto the sorbents. According to Ding et al.,¹⁰⁶ the influence of competitive ions showed that the distribution coefficient was strongly decreased in the presence of Na⁺ and K⁺ ions as their concentration increase from 100–1000 mg L⁻¹. A compilation of different investigations using various sorbents and condition of competitive ions on distribution coefficient of cesium uptake is presented in Table 8 and Fig. 5. Meanwhile, the effects of competing ions on adsorption depend on the intrinsic nature of the adsorbent surface, the pH of the solution, the chemical properties of the cesium, etc.⁶³

Table 8 Adsorbents and conditions of competitive ions on distribution coefficient of cesium uptake

Type of adsorbent	Material	Dosage (g L^−1)	Competing ions/solution condition	Distribution coefficient, Kd (L g^−1)	Sorption capacity (mg g^−1)	Percentage reduction (%)	Ref.
Inorganic adsorbents	Raw bentonite	66.7	Synthetic groundwater	1.9	—	—	137
	Activated bentonite	66.7	Synthetic groundwater	8.9	—	—	137
	Cu–FC–EDA–SAMMS	0.05 g	3 M Na^+	10^5	—	—	8
	Cu–FC–EDA–SAMMS	0.05 g	1 M K^+	111	—	—	8
	ZIM		1 × 10^−6 to 1 M Na^+	—	—	88–34	63
	IA and CA	—	0.1–3.5 mM Na^+, K^+	—	—	Decrease	109
	KNiFC-loaded chabazite	—	10^−3–5 M Na^+	10^4 cm^3 g^−1	—	—	117
	KNiFC-loaded silica gel	—	10^−3–5 M Na^+	10^4 cm^3 g^−1	—	—	66
	NaSM zeolite	10	0.0119 M K^+	4.65	—	—	131
	ISM–25 mg Calix[4]arene	10	0.0119 M K^+	27.63		99.64	131
	Aluminum-pillared montmorillonite	—	1.95 g per L K^+	0.4	—	—	105
	FC–Cu–EDA–SAMMS	1	Sequim Bay seawater	240	—	—	11
	FC–Cu–EDA–SAMMS	1	Hanford groundwater	1400	—	—	11
	Na-illite	—	0.01–1 M Na^+	1.75–6.95	—	—	138
	Natural clinoptilolite	—	0–3 M Na^+	2000 to ∼300 cm^3 g^−1	—	—	139
	KNiFC-impregnated zeolite	—	0–3 M Na^+	2000 to ∼500 cm^3 g^−1	—	—	139
	Antimony silicate	5	0.23 g per L Na^+, 3.9 g L^−1	1	—	—	140
	Iron fericite	—	0	∼6.5	108.58	∼82.5	115
	Iron fericite	—	0.05–0.4 M Na^+, Mg^2+, Al^3+	—	—	∼30 to ∼15	115
	Natural magnetite	—	0	∼0.2	70.77	∼60	115
	Natural magnetite	—	0.05–0.4 M Na^+, Mg^2+, Al^3+	—	—	∼30 to ∼15	115
	Crushed granite	15 g	0.001–1 M Na^+, K^+, Ca^2+ and Mg^2+	Decrease	Decrease	Decrease	127
	CoHCF-doped sol–gel	—	0.5–100 M Na^+	—	0.60–0.61 mmol g^−1	Decrease	141
	CoHCF-doped sol–gel	—	10–100 M Ca^2+	—	0.60–0.46 mmol g^−1	Decrease	141
	Vermiculite	50 mg	10^−5 M Na^+	—	—	80	142
Inorganic-polymer composites	Bentonite and CNT- based composites	—	0.001–0.4 M K^+	Decrease	Decrease	Decrease	67
	CHCF–PAN	—	0	1.67	7.31–11.46	63.78	9
	CHCF–PAN	—	10^−4 M K^+	0.17	—	—	9
	CHCF–PAN	—	10^−4 M Na^+	0.23	—	—	9
	CHCF–PAN	—	10^−4 M Ca^2+	0.54	—	—	9
	CHCF–PAN	—	10^−4 M Mg^2+	0.32	—	—	9
	NiHCF-WS	5	0	10.8	—	—	106
	NiHCF-WS	5	1 L per g K^+	3.5	—	—	106
	NiHCF-WS	5	0.1 L per g Na^+	4.7	—	—	106
	KNiHCF-loaded PP fabric	36 mg L^−1	0.1–1 M per L Na^+	—	—	Decrease	48
	Nickel–potassium ferrocyanide immobilized chitin	—	0–1 M Na^+	—	68.7	88.2	64
			0.01–0.1 M K^+	—	51.4	96.2
			0.01–0.5 M NH4^+	—	52.8	70.6
			0.01–0.5 M Rb^+	—	62.7	80.8
	AMP–PAN		0.2–20 mM Na^+, Ca^2+	0.46–0.38	—	85–89	75
	AMP–PAN	0.108	1 M Na^+	76.2	—	—	76
	MnO2–PAN	—	0	0.944	—	—	77
	MnO2–PAN	—	10^−4 M Na^+	0.412	—	—	77
	MnO2–PAN	—	10^−4 M K^+	0.257	—	—	77
	STS–PAN	—	0	8.41	—	—	79
	STS–PAN	—	10^−4 M Na^+, K^+	1.84, 1.29	—	—	79
	STS–PAN	—	10^−4 M Ca^2+, Mg^2+	2.41, 2.72	—	—	79
Bio-adsorbents	O. Basilicum seed	—	Li^+, Na^+, K^+	—	160	48.14	88
	Immobilized moss	0.2–1.2 g	150 mg L^−1 Na^+ & K^+	—	—	99 & 94	87
	Raw pine cone	—	0	—	0.89	—	12
	Raw pine cone	—	0.5 M Na^+ and Ca^2+	—	0.48 and 0.27	46 and 70	12
	Modified pine cone	—	0	—	1.31	—	12
	Modified pine cone	—	0.5 M Na^+ and Ca^2+	—	0.89 and 0.27	37 and 63	12

---

Fig. 5 Effect of competing ions on the adsorption of cesium.¹¹⁵

3.1.5 Effects of adsorbent particle size and surface area. Apart from the properties of the adsorbates and solution, the intrinsic nature of adsorbents plays an important role in adsorption.⁸⁹ The size of particles of adsorbents has been found to influence the sorption capacity of the sorbent.^91,128 This is because the bigger the particle size, the smaller the sorbent surface area and vice versa.¹⁴³ An excellent sorbent for sorption must have larger surface area i.e., more sorption sites or higher pore volume and reduced effective density for the adsorbate. Mashkani et al. found that Azolla filiculoides of 2–4 mm particle size could be used in large-scale application owing to its higher sorption capacity compared to others of different sizes.⁹¹ Ofomaja et al. reported that the bulk density of raw pine cone reduced from 0.499 to 0.427 g cm⁻³ which is attributed to the opening of pore spaces after chemical treatment and hence, the increase in sorption capacity as a result of enhanced surface areas and reduced point of zero charge pH_PZC.¹² The stability and ease of separation of Prussian blue were enhanced when coated with magnetic nanoparticle (MNP), providing longer contact time with adsorbate in wastewater and better sorption capacity of up to 96 mg g⁻¹.¹³²

Sangvanich et al.¹¹ and Lin et al.⁸ synthesised and characterised copper ferrocyanide functionalized mesoporous silica and found to have an excellent porous structure with 900 m² g⁻¹ and ∼1000 m² g⁻¹, respectively for surface area, 3.5 nm pore size and ligand loading capacity of 3.6–4.9 silane per nm² elemental and 3.7 silane per nm² gravimetric. The sorbent was reported to outperform Prussian blue in acidic waste stimulant with maximum capacity of 21.7 mg g⁻¹ against 2.6 mg g⁻¹ and 95% as against 75% removal in seawater.¹¹ Similar investigations were reported for spongiform Prussian blue based adsorbent¹³³ and Prussian blue caged in alginate/calcium beads¹¹⁸ used for cesium removal. Anhydrous titanium oxide was characterised to have surface area 216 m² g⁻¹ and it was discovered that as the surface area increases, the capacity of the sorbent increases.⁶² Raw montmorillonite and phosphate-modified montmorillonite were used to sorb cesium from aqueous solution.¹¹⁶ The surface area was found to increase from 2.6 to 115.9 m² g⁻¹ and the pore volume increased from 0.011 to 0.1 cm³ g⁻¹ after the modification. The sorption amount equally increased as a result of modification from 0.4292 mmol g⁻¹ for the raw montmorillonite and to 0.7073 mmol g⁻¹ for the modified montmorillonite. Ethylamine-modified montmorillonite was described by characterisation to be favourable for positively charged Cs⁺ sorption through electrostatic interactions and more negative charge due to surface hydroxyl groups of the sorbent.⁷² The surface area was increased after modification from 71.15 to 154.17 m² g⁻¹ and microporous volume from 0.00844 to 0.04846 cm³ g⁻¹, hence, providing larger sorption sites for the adsorption with sorption capacity increasing from 60.03 to 80.27 mg g⁻¹.

The SEM/EDS studying of Cs adsorption on crushed granite revealed it has extensive sorption sites for Cs adsorption but the affinity is reduced as temperature increases due to the enhancement of Cs desorption.¹²⁷ The synthesized sol–gel encapsulated cobalt hexacyanoferrate was used to extract cesium from water by solid phase extraction (SPE).¹⁴¹ The characterisation of the sorbent material shows that increase in pore size does not cause a corresponding increase in sorption capacity. The pore volume and surface area were increased after modification under different conventions for an improved uptake capacity (from 0.43 to 0.61 mmol g⁻¹) by controlling the amount of HCF in the silica sol–gel solid sorbent.¹⁴¹ The surface area was increased from 408 to 457 m² g⁻¹ while the pore volume was 0.194 to 0.217 cm³ g⁻¹. Loos-Neskovic et al. found that the prepared copper–potassium hexacyanoferrate sorbent has surface area and pore volume of 46 m² g⁻¹ and 0.042 m³ g⁻¹, respectively.¹⁰⁷ Hanafi prepared activated carbon from almond shell and characterised it for cesium and other radionuclides sorption from solution.¹³⁵ He reported that the surface area of the activated carbon increased as activation time increased, with the highest area obtained being 1288 m² g⁻¹ and micropore volume 0.35 cm³ g⁻¹ having ash content of 0.21%. The uptake efficiency could reach about 90% as reported. Similarly, activated carbon and chabazite zeolite were used for Cs-137 and I-129 removal from aqueous solution.¹⁶ The sorbents micropore areas were reported as 2.72 × 10⁵ m² g⁻¹ for chabazite, 7.68 × 10⁵ m² g⁻¹ for activated carbon and 6.17 × 10⁵ m² g⁻¹ for mixed sorbent. However, for simultaneous removal of the ¹³⁷Cs and ¹²⁹I from aqueous solution, the mixed sorbent was prepared at 7 : 3 ratio given adsorbed amount to be 0.062 and 0.00058 mol kg⁻¹, respectively for Cs and I ions.¹⁶ Approximately 8 nm diameter, length of 100–200 nm and interlayer spacing of 0.72 nm were reported for titanate nanofiber and nanotube used to sorb Cs and I ions from water by Yang and colleagues.¹⁴⁴ The sorbents can remove up to 80% Cs⁺ within the Cs concentration of 80 ppm but reduced to 36% as concentration increased to 125 ppm. Abusafa and Yücel used different cationic forms of natural zeolite (clinoptilolite) to sorb cesium and the important physical properties of the sorbent for adsorption were reported as apparent density (1.3889 g cm⁻³), pore volume (0.2216 cm³ g⁻¹), pore diameter (0.050 μm) and BET surface area (17.5 m² g⁻¹).⁷⁰ The distribution coefficient of cesium was reported to reduce as the initial concentrations reduced, which is attributed to site heterogeneity. Ceiling tiles have also been used with the intra- and inter-fiber pore diameters ranging between 1 and 2 μm, pore volume 0.72 cm³ g⁻¹ and density of 0.21 g cm⁻³.¹¹² Other researchers have also used this sorbent and characterised with similar results reported such as Levit and Teather¹⁴⁵ and Baig¹⁴⁶ reported the pore volume of their ceiling tiles ranged from 0.5–0.7 cm³ g⁻¹, fiber pore diameters of 1.5 nm and 20 mm (ref. 145) and 0.1–1 mm.¹⁴⁶ Czech smectite-rich clay material mixed with sand was previously used and the BET surface area and exchangeable cation capacity were analysed.¹³⁷ The highest surface area was obtained in sedimentary clay (153 m² g⁻¹) and least was in raw-bentonite (97 m² g⁻¹) and distribution ratio of cesium for all the selected clays increased as clay fraction in clay/sand mixture. Kim et al. characterised their sericite as having BET surface area of 0.021 m² g⁻¹ and cation exchange capacity of 3.25 meq per 100 g.⁷¹ The sericite material has a low adsorption capacity of 6.68 mg g⁻¹ much lower than 32.3 mg g⁻¹ reported for ion-imprinted polymer⁸⁰ but better than 0.5 mg g⁻¹ achievable using ceiling tiles.¹¹² The surface areas of 71.29, 78.82, 18.52 and 27.92 m² g⁻¹ were reported for carbon nanotube, chitosan-grafted carbon nanotube, bentonite and chitosan-grafted bentonite composite, respectively.⁶⁷ The morphology and surface diameter of the nanoplate-like CS nanostructure were also reported.

Silico-antimonate materials are also good sorbents for sorption due to better surface area and could be enhanced by activation or by incorporation of organic ion exchanger. It has been reported that the surface of phosphoric acid activated silico-antimonate contains hydroxyl and phosphate groups which may enhance the ion exchange affinity because an increasing amount of water content increases the porosity of sorbent material and localization of protons.⁴⁶ Besides, sodium titanosilicate–polyacrylonitrile composite was analysed and found to have a BET surface area of 96.66 m² g⁻¹, larger inner particulate pore size compared to near surface and wider dispersion of STS powder throughout the binding matrix, ensuring fast and high adsorption level.⁷⁹ The sorbent is reported to have high gamma radiation stability up to 200 KGy which is an important factor for the removal and immobilization of fission product from radioactive waste solution. Stannic and zirconium phosphates have been reported to display such resistant or stability to radiation of up to 300 mCi Ra–Be neutron and 1.72 Gy h⁻¹ gamma-dose rate.¹¹³

The synthesised ammonium molybdophosphate–polyacryonitrile bead was reported after adjustment for large-scale application to have surface area of 32.69 m² g⁻¹, pore volume of 0.17 cm³ g⁻¹ and bead diameter of 1–2 mm.⁷⁵ Griffith et al. reported the surface area of microporous tungstate/polyacrylonitrile composites used to sorb cesium and strontium from acidic radioactive waste stimulant as ranged between 31 and 36 m² g⁻¹ with granular particle of mesh size less than 0.3 mm giving optimal adsorption in column operation.⁷³ Polyacrylonitrile-based manganese dioxide composite was characterised and used for cesium removal with BET surface area reported to be 53.03 m² g⁻¹ by Nilchi et al.⁷⁷ Colloid stable sorbents prepared from latex particles functionalized with transition metal ferrocyanides was reported to have large surface area of 960 m² g⁻¹.⁵⁸ The effect of ferrocyanide composition and content in polymeric matrix was evaluated on selectivity and sorption capacity of the sorbents and it was shown that unmodified latexes has only about 10% Cs retention whereas, the modified latexes could remove as much as 99% Cs (0.053–0.084 meq per g) from the solution over a wide pH range. Nilchi et al. reported BET surface area of 73.58 m² g⁻¹ for the prepared copper hexacyanoferrate–polyacrylonitrile composite used to sorb cesium with adsorption dynamic capacity of 7.31 and 11.46 mg Cs g⁻¹ at 5% and 100% breakthrough.⁹ The effects of contact time, temperature and initial cesium concentration were investigated on the cesium adsorption.

Chinese weathered coal was used to sorb Am(III), Eu(III) and Cs(I) and was reported to have surface area between 1.319 and 19.533 cm² g⁻¹, total pore volume 0.007 and 0.061 cm³ g⁻¹ and pore size of 9.080 and 16.420 nm at point of zero charge (5.030–6.650).³⁸ The sorbent has functional oxidized groups (such as carboxyl, hydroxyl, phenol, etc.) with surface charge controlled by pH change. Ethylenediaminetetracetic acid (EDTA) and its other degradation products were reported to have BET surface area of 54.30 cm³ g⁻¹ and cation exchange capacity of 26.42 meq/100 g.¹⁴⁷

4.0 Future direction and recommendations for Cs adsorption

In adsorption studies, rapid kinetics, high sorption capacity, selectivity, volume reduction and compatibility of the adsorbates with the sorbents for safe disposal are priorities. As such, the choice of adsorption materials, methods of preparation and immobilization are normally considered for a particular adsorbate. A survey through the literature suggests that extensive research is needed on sorbents selection for higher adsorption of Cs from the environment.

Cost involvement of the adsorbents' precursors and/or the adsorbents are crucial issues in adsorption technique to access the feasibility of implementation of the process in real wastewater treatment technologies. Virtually, till now, no report is available in the literature on this and this makes it impossible to be certain if these materials as reported could be used in developing nations in particular. Several factors are responsible for high cost of adsorbents which include availability, the form of the precursors and the adsorbents, processes involved before its usable form, treatment conditions, production period, location, etc.^148,149. These factors are important to be considered before any materials are regarded as low cost. Particular interests should be given to research on bio-sorbents and naturally occurring inorganic sorbents like clay minerals since the materials involved are mostly free in the natural environment and they appear as the main hope for less economically buoyant nations from the materials procurement cost point of view. In addition, studies should focus on the chemical modifications of these waste materials to enhance their sorption capacity, mechanical stability and surface functional groups maintenance under different radioactive waste solution conditions. This is because the feasibility and efficiency of sorption process do not lie only on the physico-chemical properties of the adsorbents but as well as on the composition of the wastewaters and other surrounding influencing factors. Until now, progress in wastewater managements and technologies is mostly depended on pilot investigations carried out with specific waste effluents which are often simulated and as a result, serious attentions should be given to real industrial waste effluents for extensive studies.¹⁰¹ Information from various investigations available in the literature suggests several factors affecting sequestering of cesium from radioactive wastewaters are still less considered and this makes drawing general conclusion difficult. Every system should be considered independently for better understanding of the mechanism of the adsorption. In addition, special care should be given to the choice of modification agents for bio-sorbent materials. This is because certain chemical agents and conditions inhibit activation of binding sites in them such as acetone, detergent, high temperature, autoclaving, etc.¹⁵⁰ Characterization of bio-sorbent materials surface characteristics and pore sizes should be given particular attention as this will no doubt promote improvement on the sorbent performance which is less considered. Regeneration studies as mentioned before are still limited for reuse of all the adsorbents to ascertain their lifetime in practical application. This is an issue that needs to be urgently addressed, as there is no point in acquiring high cost materials that have low operational cycles. Importantly, research should focus on possibility of recovering of adsorbed cesium without disrupting the active sorption sites of the sorbents by the eluents. So far, few reports have suggested polymeric composites as the best adsorbents for metal recovery due to high desorption properties of the adsorbents.⁹⁴ Finally, efforts should be taken to consider the health implications of these adsorbents before application.

5.0 Conclusions

The review presents comprehensive information on the removal of cesium from industrial effluents by adsorption technique. Effects of important factors associated with the adsorption of cesium from solutions are compiled and discussed accordingly. Several sorbents used in recent time to remove cesium from various solutions are highlighted. Specific advantages and disadvantages of each sorbent were discussed. The effects of the factors on adsorption capacity and efficiency of the adsorbents are discussed. Future direction on adsorption technique is discussed; particular attention is advocated for bio-sorbents and naturally occurring inorganic sorbents, regeneration, reusability and cost involvement of the precursors and the adsorbents. Issues related to the health implications of these adsorbents are advocated for general public safety. Since the success of wastewater technologies mostly hinges on pilot studies and the previous studies are virtually conducted under batch conditions hence, more column experiments should be given serious attention to make steps forward real industrial realization. Finally, as the rate of waste effluents has not been reduced by previous efforts, more studies are advocated into various novel sorbents that could reduce or eliminate the cesium from the waste solution before disposal.

6.0 Disclosure

The authors declare no conflict of financial interest or any other.

Acknowledgements

The review was carried out under the University of Malaya Grant no. PG027-2014A.

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