Comparative study for adsorptive removal of radioactive Cs+, Sr2+ and Co2+ ions from aqueous solution by acid-activated white clay

Daeik Kim; Sun-Jin Hwang; Su Ho Bae; Keon Sang Ryoo

doi:10.4491/eer.2024.039

Environ Eng Res > Volume 29(5); 2024 > Article

Kim, Hwang, Bae, and Ryoo: Comparative study for adsorptive removal of radioactive Cs+, Sr2+ and Co2+ ions from aqueous solution by acid-activated white clay

Research

Environmental Engineering Research 2024; 29(5): 240039.

Published online: February 29, 2024

DOI: https://doi.org/10.4491/eer.2024.039

Comparative study for adsorptive removal of radioactive Cs⁺, Sr²⁺ and Co²⁺ ions from aqueous solution by acid-activated white clay

Daeik Kim¹, Sun-Jin Hwang², Su Ho Bae³, Keon Sang Ryoo^4,^†

¹Department of Electronic Communication Engineering, Chonnam National University, Yeosu, 59626, Korea

²Environmental Science & Environmental Engineering, College of Engineering, Kyung Hee University, Yongin 17104, Korea

³Department of Civil Engineering, Andong National University, Andong 36729, Korea

⁴Department of Chemical and Biological Engineering, Andong National University, Andong 36729, Korea

^†Corresponding author: E-mail: ksr@andong.ac.kr, Tel: +82-54-820-5453

Received January 19, 2024 Revised February 25, 2024 Accepted February 28, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Raw white clay was acidified with H₂SO₄ (2 M) and then activated in a furnace for 2 h at 150°C. The resulting acid-activated clay product was used as an adsorbent for the removal of radioactive substances such as Cs⁺, Sr²⁺ and Co²⁺ ions in aqueous solution. The specific surface area and the pore volume of acid-activated clay were three times higher compared to raw white clay. At 100 mg L⁻ of initial concentration and 1.0 g of dosage, removal efficiencies of Cs⁺, Sr²⁺ and Co²⁺ ions showed 60.0%, 27.3%, and 88.1%, respectively. It was found that the Langmuir isotherm was described well to the adsorption behavior of Cs⁺, Sr²⁺ and Co²⁺ ions on acid-activated clay rather than Freundlich isotherm. The Langmuir isotherm constant (Q) for Cs⁺, Sr²⁺ and Co²⁺ ions was found to be 8.81, 3.78 and 10.20 mg g⁻¹, respectively. Compared to the pseudo-first-order kinetic model, the pseudo-second-order kinetic model was more suitable for adsorption of Cs⁺, Sr²⁺ and Co²⁺ ions in water/acid-activated clay medium owing to the higher correlation coefficient (R²) and the more proximity value of the experimental value q_e,exp and the calculated value q_e,cal.

Keywords: Acid-activated clay, Adsorption isotherm, Adsorption kinetic, Co²⁺, Cs⁺, Sr²⁺

Graphical Abstract

Keywords: Acid-activated clay, Adsorption isotherm, Adsorption kinetic, Co²⁺, Cs⁺, Sr²⁺

1. Introduction

Nuclear power plants provide economically and stably producing optimal energy among the current electricity supply technologies. However, when they are operated, repaired and disposed, or damaged by an external shock, harmful radioactive pollutants can be inevitably generated. As an example, the Fukushima Daiichi Nuclear Power Plant disaster in Japan in 2011 released a large amount of radioactive materials such as Cs, Sr and Co, which rarely exist in the nature, into the environment [1]. Due to this incident, the surrounding atmosphere, soil, and seawater were significantly contaminated with these radioactive substances. The half-lives of ¹³⁷Cs, ⁹⁰Sr and ⁶⁰Co are known to be 30.2 years, 28.7 years and 5.27 years, respectively, and these long half-lives leads to severe diseases when exposed to humans for a long time [2]. In particular, Cs⁺ and Sr²⁺ ions have similar chemical properties as K⁺ and Ca²⁺ ions, which are essential elements of life [3]. For this reason, they are easily absorbed and concentrated into the biological organization. It is generally well known that Cs⁺ ions bring about various cancers, general paralysis, and infertility in the human body [4]. When Sr²⁺ ions are ingested into the body, it accumulates in the bones and is difficult to be excreted from the body, so it arouses bone marrow cancer and leukemia, and adversely affects animals and plants such as genetic mutations [5]. Long-term exposure to Co²⁺ ions has also been known to cause heartburn, dizziness, and local skin necrosis [6].

To date, methods for treating liquid radioactive waste have been introduced in many literatures. The available methods include ionic exchange, coagulative precipitation, reverse osmosis filtration, membrane distillation, adsorption, electrochemical separation, etc., and these methods were treated alone or in combination of several methods [7, 8]. Among the methods for removing radioactive substances from liquid radioactive waste, adsorption has turned out to be diverse, economical, and convenient to operate. Therefore, a plenty of reviews have been reported concerning the materials of adsorbent that can be removed radioactive substances from liquid radioactive waste [9–11]. The adsorption materials that have been developed and applied for practical use so far are Prussian blue, which is used as a paint or dye in a core material (acrylic acid polymerization) [12, 13], crown ether and calixarene derivatives [14], metal (Fe³⁺/Cu²⁺) ferrocyanide [15], carbonated cement slurry powder [16], crystalline silicotitanate [17], iron hexacyanoferrate/graphene/carbon fiber composite [18], metal hexacyanoferrates (MHCF) [19, 20], hybrid porous magnetic bentonite-chitosan beads [21] and etc. However, although these adsorbents are very effective in removing radioactive substances, they have limitations in terms of cost and flexibility of process for producing the adsorbent.

As an alternative, it is necessary to introduce an adsorbent that can be supplied cheaply and stably, and is environmentally friendly. Raw clay has the potential capability as core or supporting material to eliminate metal ions from wastewater by providing a large amount of active sites in its layer structure [22]. However, in general, raw clay itself is known to be significantly less capable of adsorbing various adsorbates [23. 24]. Therefore, in order to enhance this, there needs to increase the adsorption capacity of raw clay through activation. One of the most appropriate activation techniques is an inorganic acid treatment process. It showed that acid-activated clay improves the adsorption capability by increasing the specific surface area and porosity as well as active sites as compared to raw clay [25, 26]. In the process of treating raw clay with acid, the most commonly used acids are hydrochloric acid and sulfuric acid. In industry, H₂SO₄ is relatively chosen more than HCl because it is less expensive and is not as harsh as HCl [27]. This makes the acid-activated clay more economical and effective when remove radioactive substances from aqueous solution.

In this study, authors used H₂SO₄ as an acid modifier to activate raw white clay, and compared the change of physicochemical properties for raw white clay and acidic activated product. After that, adsorption experiments were conducted to evaluate the possibility of removing radioactive substances such as Cs⁺, Sr²⁺ and Co²⁺ ions from aqueous solution using acid-activated clay manufactured in this study. Various experimental conditions such as contact time, dosage of acid-activated clay, and initial concentrations of radioactive substances were investigated in detail to determine optimal conditions. Aside from these, the adsorption isotherm and kinetic models were used to establish the adsorption rate and the maximum adsorption capacity as well as the mechanism for adsorption of radioactive substances.

2. Materials and Methods

2.1. Materials

2.1.1. Preparation of acid (H2SO4)-activated clay

Raw white clay was obtained from the ocean shore in Pohang, Gyeongsangbuk-do. The obtained white clay contains a montmorillonite ((MgAl)₄Si₈O₂₀(OH)₄·nH₂O) as the main component, which are member of 2 : 1 layer silicate family which share the common feature that two tetrahedral sheets consisting of Al sandwich a sheet of octahedrally coordinated Si. The raw white clay was firstly calcined in a muffle furnace at 600°C for 2 h. The calcined raw white clay was then heated at 80°C for 6 h under mechanical stirring after adding H₂SO₄ (2 M) solution. In succession, the resulting acidified white clay was washed several times with distilled water for the purpose of removing sulfate ions and impurities. Then, acidified white clay was dried in an oven at 105°C overnight. Following drying in an oven, the acidified clay was activated in a furnace for 2 h at 150°C using a heating rate of 10°C per min and cooled in a desiccator at room temperature prior to experiment. Acid-activated white clay in the particle size range of 100 μm to 200 μm was employed as adsorbent for the adsorption of Cs⁺, Sr²⁺ and Co²⁺ ions.

2.1.2. Reagents

Cesium chloride (CsCl), Strontium chloride hexahydrate (SrCl₂· 6H₂O), and Cobalt chloride hexahydrate (CoCl₂·6H₂O) were supplied by Daejeong Chemicals & Metals, Co, Ltd. Siheung-si, Korea. All reagents were used as received without further purification. A stock solution containing 1000 mg L⁻¹ of Cs⁺, Sr²⁺ and Co²⁺ ions was provided by dissolving above-mentioned reagents with distilled water. The desired concentrations in this experiment were prepared from a stock solution through continuous dilutions. The cation exchange capacity (CEC) of raw white clay and acid-activated clay was measured according to the Agricultural Environmental Resource Analysis Method.

2.2. Methods

2.2.1. Instrumental analysis

The specific surface area, BJH (Barret-Joyner-halenda) pore size, and BJH pore volume were investigated with a gravimetric nitrogen BET (Brunauer-Emmett-Teller) surface area analyzer (BET, 3 Flex, Micrometrics, Atlanta, USA). The mineral oxides were measured by X-ray fluorescence spectroscopy (XRF, ZSX Primus II, Rigaku, Japan). The elemental composition was determined using energy dispersive X-ray spectroscopy (EDX, X-MaxN, Oxford, UK). The pH was observed using a pH meter (Radiometer, PHM 250 ion analyzer, Woonsocket, USA). Cs⁺, Sr²⁺ and Co²⁺ ions were analyzed using an inductively coupled plasma-atomic emission spectrometer (ICP/AES, Flame Modula S, Spectro, Kleve, Germany). CECs were quantified by ion chromatography with a conductivity detector (940 professional IC Vario, Metrohm, Swiss).

2.2.2. Adsorption study

The adsorption experiments were conducted in a batch type. Acid-activated white clay (0.5 – 5.0 g) was added into 500 mL of solution (25 – 100 mg L⁻) containing Cs⁺, Sr²⁺ and Co²⁺ ions and shaken on a laboratory shaker at the ambient temperature without adjusting pH. A certain amount of solution was collected at a designated time of 5, 10, 30, 60, 120 mins and filtered with a GF/C (0.45 μm) filter paper. The supernatant was stored in the refrigerator prior to analysis.

Removal efficiency (RE) and adsorption capacity (q_e) were calculated by Eqs. (1) and (2).

(1)

R E (%) = (\frac{C_{0} - C_{e}}{C_{0}}) \times 100 %

(2)

q_{e} (mg / g) = \frac{(C_{0} - C_{e}) V}{W}

where, V (L) is the volume of the solution and W (g) is the weight of adsorbent. C₀ (mg/L) and C_e (mg/L) are initial and equilibrium concentration of the adsorbate, respectively. q_e (mg/g) is the amount of adsorbate adsorbed per g of adsorbent at equilibrium.

3. Results and Discussion

3.1. Characterization of Acid-Activated Clay

3.1.1. XRF analysis

Table 1 shows the constituents of oxide present in raw white clay and acid-activated clay obtained by XRF. As shown in Table 1, major oxides consisting of raw white clay and acid-activated clay were Al₂O₃, SiO₂, SO₃, CaO and Fe₂O₃, along with other minor constituents. It was found that the content of Al₂O₃, SO₃, CaO and Fe₂O₃ in acid-activated clay was reduced compared to those in raw white clay. On the other hand, the content of SiO₂ was significantly increased. During acid treatment of raw white clay, most of oxides including calcium, magnesium, aluminum, and iron are partially dissolved from the lattice except for silicone that stay in solid state. For instance, the weight percentage of MgO, Al₂O₃ and Fe₂O₃ dropped by 51.9%, 30.2% and 75.8%, respectively. This confirms that the leaching of interlayer cations gives rise to the structural changes in the raw white clay, which leads to opening of the lattice and an increase in internal surface area. Consequentially, acid-activated clay is almost saturated with hydrogen ions through the replacement of cations such as Mg²⁺, Al³⁺, Ca²⁺ and Fe³⁺ located at the surface of the lattice with H⁺ from acid and exhibits acidic property (Fig. 1). It suggests that acid-activated clay can have better adsorption capacity than raw white clay.

3.1.2. EDX analysis

EDX was used to indentify the elemental constituents of raw white clay and acid-activated clay, and its analysis results were listed in Table 2. The main elements constituting raw white clay and acid-activated clay were O, Al, Si, S and Fe. The wt.% of Al, S, Ca, Mg and Fe of acid-activated clay showed lower than raw white clay, whereas Si showed the opposite trend and TiO₂ remained unchanged. These results are almost completely consistent with data by XRF. When raw white clay is treated with sulfuric acid, above-mentioned main elements are more easily eluted, causing deformation of the the lattice of raw white clay. This phenopenon results in the further formation of pore, which in turn leads to an increase in surface acidity.

3.1.3. Textural analysis

Table 3 represents the analysis results for the textural properties of raw white clay and acid-activated clay. The values of the BET surface area and BJH pore volume of acid-activated clay were about 3 times higher in comparison to those of the raw white clay, while BJH pore size of acid-activated clay was slightly reduced. All of these changes are attributed to a consequence of the leaching of the several cations such as Fe³⁺, Al³⁺ and Mg²⁺ during acid activation, which could develop not only more porous structure and internal surface area but also smaller particles. The pH value of acid-activated clay was 2.9, indicating it can be regarded as an acidic adsorbent.

3.1.4. Analysis of cation exchange capacity (CEC)

The CEC by raw white clay and acid-activated clay is shown in Fig. S1. As seen, when raw white clay was treated with sulfuric acid, there was a significant change in the CEC. The CEC of the acid-activated clay was largely decreased compared to raw white. The reduction of CEC may be due to the reason that main cations existing interlayers of raw white clay are eluted by acid and substituted with hydrogen ions as stated earlier.

3.2. Effects of Initial Concentration

Fig. S2 (a, b, c) show a comparison of the amount of Cs⁺, Sr²⁺ and Co²⁺ ions adsorbed by acid-activated clay as a function of contact time (5–120 min). Where, the dosage of acid-activated clay was 1.0 g and the initial concentrations of Cs⁺, Sr²⁺ and Co²⁺ ions were 10, 25, 50 and 100 mg L⁻, respectively. In the case of Cs⁺ and Sr²⁺ ions, a rapid adsorption appeared with in 5 min of contact time. Thereafter, the adsorbed amount remained constant after 10 min of contact time, implying that equilibrium was reached. On the contrary of this, the equilibrium of Co²⁺ ions appeared later than that of Cs⁺ and Sr²⁺ ions. The removal efficiencies for the initial concentrations of Cs⁺, Sr²⁺ and Co²⁺ ions at equilibrium were depicted in Fig. 2. When the initial concentration of Cs⁺ ions increased from 10 mg L⁻ to 100 mg L⁻, removal efficiency of Cs⁺ ions steadily decreased from 81% to 60%. This trend was similar for Sr²⁺ and Co²⁺ ions. This is because the gradual increase in initial concentration gradually saturates the active adsorption sites at fixed adsorbent amount. For this reason, it can be seen that even if the initial concentration of Cs⁺, Sr²⁺ and Co²⁺ ions were increased further, it does not have any effect on the improvement of adsorption efficiency of the acid-activated clay. It was found that the higher removal efficiency by acid-activated clay regardless of the initial concentrations was in order of Co²⁺ > Cs⁺ > Sr²⁺.

3.3. Effects of Dosage

The effect of adsorbent dosage on the adsorption of Cs⁺, Sr²⁺ and Co²⁺ ions in aqueous solution were studied by varying the amount of the acid-activated clay according to contact time. In this study, the dosage of acid-activated clay was 0.5 g to 5.0 g and the initial concentration of Cs⁺, Sr²⁺ and Co²⁺ ions was 50 mg L⁻. The degree of adsorption obtained were plotted versus the dosage of acid-activated clay for Cs⁺, Sr²⁺ and Co²⁺ ions (Fig. S3). As seen in Fig. 5(a, b, c), adsorption took place quickly in the few minutes and then the adsorption rate became slow. Fig. 3 shows that the adsorption capacity of Cs⁺, Sr²⁺ and Co²⁺ ions gradually increased as the dosage of acid-activated clay was increased from 0.5 g to 5.0 g. The removal efficiency of Cs⁺ ions showed 56.1%, 72.1%, 78.8% and 82.1% at 0.5 g, 1.0 g, 2.5 g and 5.0 g of acid-activated clay, respectively. Unexpectedly, removal efficiency of Cs⁺ ions did not improve proportionally even if the dosage was increased. Effects of further increase in adsorbent dosage were only slight. At higher adsorbent dosage, anyway, the increase in removal percentage of Cs⁺ ions was suppressed. This trend was similar in the cases of Sr²⁺ and Co²⁺ ions. It is assumed that the reduction of available adsorption sites of acid-activated clay is caused by agglomeration of acid-activated clay at higher dosages. Therefore, it is thought that it is preferable to use an appropriate amount of acid-activated clay rather than excessive when trying to adsorb radioactive substances in aqueous solution.

3.4. Adsorption Isotherms

In this study, the Freundlich and Langmuir adsorption isotherm models were employed to investigate the isothermal adsorption characteristics of Cs⁺, Sr²⁺ and Co²⁺ ions in aqueous solution using acid-activated clay. The adsorption isothermal model shows detailed information for the maximum amount of adsorbate removed by the adsorbent and the adsorption affinity when the adsorption reachs the equilibrium at a certain temperature. The Freundlich adsorption isotherm is an empirical model representing the adsorbate that forms a multilayer on the heterogeneous surface of the adsorbent. This isotherm equation is defined by the following q_e = K_F ·C_e^1/n and can be transformed into a linear equation by taking the logarithm on both sides, as shown in Eq. (3).

(3)

log q_{e} = log K_{F} + \frac{1}{n} log C_{e}

The Langmuir adsorption isotherm is a model based on the assumption that adsorption occurs at specific homogeneous adsorption sites of the adsorbent without mutual attraction between neighboring active sites. The Langmuir adsorption isotherm is expressed as the Eq. (4) as follows:

(4)

\frac{C_{e}}{q_{e}} = \frac{1}{Q K_{L}} + \frac{C_{e}}{q}

where, K_F and Q (mg/g) are the maximum adsorption amount of the adsorbent per unit mass of adsorbate at equilibrium, and n and K_L represent the adsorption affinity of the adsorbate for the adsorbent.

The obtained correlation coefficient R² of Freundlich and Langmuir adsorption isotherms for Cs⁺, Sr²⁺ and Co²⁺ ions were depicted in Figs. S4 and S5. The R² is a measure of the fit of the regression linear equation, and a value closer to 1 indicates that the adsorption isotherm model is more suitable. The R² of Freundlich and Langmuir adsorption isotherms for Cs⁺, Sr²⁺ and Co²⁺ ions were 0.9702, 0.9766 and 0.9714 and 0.9907, 0.9986, and 0.9903, respectively. It was found that, regarding adsorption of Cs⁺, Sr²⁺ and Co²⁺ ions on acid-activated clay, the fit of experimental data with Langmuir adsorption isotherm was rather better than that with Freundlich adsorption isotherm.

The values of Freundlich and Langmuir constants were given in Table 4. The maximum adsorption capacity of acid-activated clay for Cs⁺, Sr²⁺ and Co²⁺ ions was 8.91, 3.78, and 10.20 mg/g, respectively. Freundlich and Langmuir constants 1/n and K_L are related with the affinity between adsorbent and adsorbate in adsorption process. In general, when the constants mentioned above are in the range of 0.1 to 0.5 or less than, adsorption is known to be effective. Whereas, when they are 2 or more, it shows poor adsorption. Considering the obtained constant values, it was found that acid-activated clay is a relatively good adsorbent for adsorption of Cs⁺, Sr²⁺ and Co²⁺ ions in aqueous solution.

Langmuir constant K_L is related to Gibbs free energy as shown in Eq. (5). Where, R is the ideal gas constant (8.314 J/mol·K) and T is the Kelvin temperature. If the calculated value of Gibbs free energy is negative, adsorption process proceeds as a spontaneous reaction. Commonly, the change in Gibbs free energy for physisorption is between −20 and 0 kJ/mol, but chemisorption is in the range of −80 and −400 kJ/mol.

(5)

Δ G = - R T ln (1000 K_{L})

The values of ΔG obtained for Cs⁺, Sr²⁺ and Co²⁺ ions were − 9.697, −9.144, and −9.890 kJ/mol, respectively. Since ΔG is negative value of about −10 kJ/mol, it can be seen that the adsorption process of Cs⁺, Sr²⁺ and Co²⁺ ions in the water/acid-activated clay medium is spontaneous and are favored by physisorption.

3.5. Adsorption Kinetics

The adsorption kinetics of Cs⁺, Sr²⁺ and Co²⁺ ions on acid-activated clay were investigated by applying pseudo-first-order and pseudo-second-order kinetic models. The obtained kinetic parameters provide useful information for determining adsorption rate and the amounts of adsorbate adsorbed per unit mass. The pseudo-first-order kinetic equation is given as Eq. (6).

(6)

\frac{d q t}{d t} = k_{1} (q_{e} - q_{t})

Eq. (6) can be expressed as Eq. (7) which is linear equation of logarithm form.

(7)

log (q_{e} - q_{t}) = log q_{e} - \frac{k_{1} t}{2.303}

where, k₁ (min⁻¹) is the rate constant of the pseudo-first-order kinetics. q_e (mg g⁻¹) and q_t (mg g⁻¹) are the amounts of adsorbate adsorbed per unit mass of adsorbent at the equilibrium and at any time t (min), respectively.

The pseudo-second-order kinetic equation is expressed in the following Eq. (8).

(8)

\frac{d q_{t}}{d t} = k_{2} {(q_{e} - q_{t})}^{2}

Eq. (8) can be converted into a linear equation as shown in Eq. (9).

(9)

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{1}{q_{e}} t

where k₂ (g mg⁻¹ min⁻¹) is the rate constant of pseudo-second-order kinetics.

Fig. S6 shows the pseudo-first-order and the psedo second-order kinetic plots for adsorption of Cs⁺, Sr²⁺ and Co²⁺ ions on acid-activated clay. The obtained R² values of the pseuso-second-order kinetic are found to be greater than those of the pseudo-first-order kinetic and nearly equal to 1, which indicates that the kinetic experimental data were better fitted by the pseuso-second-order kinetic model. The derived kinetic parameters of the pseudo-first-order and the pseudo-second-order kinetic models are shown in Table 5. The calculated values q_e,cal derived by fitting the second-order kinetic equation are quite similar to q_e,exp obtained the experimentally, indicating that adsorption of Cs⁺, Sr²⁺ and Co²⁺ ions follows the the pseudo-second-order kinetic. In addition, it was investigated that when the dosage of acid-activated clay was increased, the reaction rate value (k₂) was also increased.

4. Conclusions

In this study, we explored the possibility of removing Cs⁺, Sr²⁺ and Co²⁺ ions in aqueous solution using acid-activated clay. While activating raw white clay with acid, most of oxides such as calcium, magnesium, aluminum, and iron were partially dissolved from the lattice except for silica that remains in solid state, resulting in the opening of the lattice and an increase in internal surface area. The values of the BET surface area and BJH pore volume of acid-activated clay were approximately 3 times higher compared with those of the raw white clay. Applicability of acid-activated clay for Cs⁺, Sr²⁺ and Co²⁺ ions removal was carried out in a batch mode. It was observed that the rapid adsorption of Cs⁺ and Sr²⁺ relative to Co²⁺ occured within the first few minutes and equilibrium was reached in about 10 minutes. The effects of initial concentrations and dosage on adsorption capacity were tested according to designated contact time. In the case of Cs⁺ ions, the removal efficiency of Cs⁺ ions steadily decreased from 81% to 60% when the initial concentration of Cs increased from 10 mg L⁻ to 100 mg L⁻ at fixed 1 g of acid-activated clay. It is thought that the gradual increase in initial concentration is due to saturating the available adsorption active sites of acid-activated clay. On the contrary of this, when the dosage of acid-activated clay increased from 0.5 g to 5.0 g at a 50 mg L⁻ of concentration, the removal efficiency of Cs⁺ ionas efficiency gradually increased from 56.1% to 82.1%. However, the effect of further increase in dosage was inadequate than expected. It is presumed that this is because the agglomeration of acid-activated clay at higher dosages reduces available adsorption sites of acid-activated clay. The experimental adsorption data were applied by adsorption isotherm models in order to know the adsorprion behavior of Cs⁺, Sr²⁺ and Co²⁺ ions on acid-activated clay. When considering correlation coefficient (R²), it turned out that the Langmuir adsorption isotherm is more appropriate than the Freundlich adsorption isotherm. The Langmuir maximum adsorption capacity (Q) of acid-activated clay for Cs⁺, Sr²⁺ and Co²⁺ ions was found to be 8.91, 3.78 and 10.20 mg/g, respectively. Furthermore, since the Langmuir constant (K_L) is in the range of 0.1 to 1.0, acid-activated clay can be considered as a good adsorbent for the adsorption of Cs⁺, Sr²⁺ and Co²⁺ ions. Aside from these, the experimental results obtained were fitted by the pseudo-first-order kinetic and pseudo-second-order kinetic models to elucidate the rate and mechanism of adsorption. The pseudo-second-order kinetic was revealed to the better fitting model for Cs⁺, Sr²⁺ and Co²⁺ ions removal, where R² value was almost 1.0 and the calculated value q_e,cal showed similar value to the experimental value q_e,exp.

Supplementary Information

eer-2024-039-supplementary.pdf

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2022R1F1A1060823).

Notes

Conflict of Interest Statement

The authors declare that they have no conflict of interest.

Author Contributions

K.S.R (professor) established the concept of the experiment, wrote a draft thesis, and revised the draft thesis. D.K (professor) gave a lot of advice in conducting the experiment. S.J.H (professor) collected data from the experiment and prepared pictures and tables in the paper. S.H.B (professor) mainly conducted experiments and achieved the tangible results.

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Fig. 1

Structural changes of raw white clay by acid attack.

Fig. 2

Removal efficiency of Cs⁺, Sr²⁺ and Co²⁺ ions on acid-activated clay.

Fig. 3

Removal efficiency of Cs⁺, Sr²⁺ and Co²⁺ ions by acid-activated clay.

Table 1

Oxide Constituents of raw white clay and acid-activated clay obtained by XRF

Oxide	Raw white clay (wt.%)	Acid-activated clay (wt.%)
Na₂O	2.88	1.16
MgO	1.66	0.76
Al₂O₃	19.69	13.74
SiO₂	31.58	64.75
P₂O₅	‹ 0.1	‹ 0.1
SO₃	14.62	7.21
K₂O	1.74	1.31
CaO	4.59	3.59
TiO₂	1.64	1.83
Cr₂O₃	‹ 0.1	‹ 0.1
MnO	0.40	‹ 0.1
Fe₂O₃	20.26	4.91

Table 2

Elemental constituents of raw white clay and acid-activated clay obtained by EDX

	O (wt.%)	Na	Mg	Al	Si	S	K	Ca	Ti	Fe
Raw white clay	54.48	2.54	1.22	9.23	11.37	4.60	0.94	2.37	0.76	11.50
Acid-activated clay	58.90	1.25	-	5.45	28.90	0.8	0.81	1.33	0.88	1.87

Table 3

Textural properties of raw white clay and acid-activated clay

Adsorbent	BET surface area (m²g⁻¹)	BJH pore volume (cm²g⁻¹)	BJH pore size (Ǻ)	pH
Raw white clay	88.6028	0.0993	86.287	6.4
Acid-activated clay	252.6905	0.2788	62.607	2.9

Table 4

Freiundlich and Langmuir constants for adsorption of Cs⁺, Sr²⁺ and Co²⁺ ions on acid-activated clay

Adsorbent	Adsorbate	Freundlich Constants		Langmuir Constants
Adsorbent	Adsorbate	K_F (mg/g)	1/n	Q (mg/g)	K_L
Acid-activated clay	Cs⁺	0.53	0.66	8.81	0.05
	Sr²⁺	0.22	0.59	3.78	0.04
	Co²⁺	2.34	0.54	10.20	0.53

Table 5

Adsorption kinetic parameters for adsorption of Cs⁺, Co²⁺ and Sr²⁺ ions on acid activated clay

Adsorbent	Initial ⁻ concentration (50 mg L⁻¹)	Dosage (g)	q_e_,exp (mg g⁻¹)	Pseudo-first-order kinetic model		Pseudo-second-order kinetic model

				q_e_,cal (mg g⁻¹)	k₁ (min⁻¹)	q_e_,cal (mg g⁻¹)	k₂ (g mg⁻¹ min⁻¹)
Acid-activated clay	Cs⁺	0.5	5.61	0.55	0.013	5.55	0.09
		1.0	3.61	3.70	0.10	3.60	0.39
		2.5	1.58	5.34	0.01	1.52	1.09
		5.0	0.82	0.08	0.14	0.82	5.34
	Co²⁺	0.5	8.24	9.71	0.04	6.81	0.02
		1.0	4.74	7.80	0.03	4.44	0.13
		2.5	1.94	2.02	0.10	1.93	1.54
		5.0	0.96	1.41	0.04	0.98	5.70
	Sr²⁺	0.5	2.71	9.20	0.01	2.54	0.27
		1.0	2.08	3.45	0.03	1.91	0.19
		2.5	1.05	1.02	0.13	1.06	0.63
		5.0	0.60	0.62	0.10	0.59	2.63