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ABSTRACT
At the Fukushima Daiichi Nuclear Power Station site, crystalline silicotitanate (CST) ion exchangers containing sodium ions (Na+) as exchange ions are used as an adsorbent to remove radioactive cesium (Cs) and strontium (Sr) from groundwater and contaminated water. The ion exchange reactions to remove Cs+ and Sr2+ compete with the reactions with other cations in the water sources. Since the groundwater and contaminated water have low concentrations of metal cations, the influence of competitive ion exchange reactions with protons (H+) is relatively large in the removal of Cs+ and Sr2+ from the water. In this paper, the H+/Na+ ion exchange property of the aqueous solution-CST system was evaluated. Results of batch tests were used to draw the isotherm and Kielland plot for the H+/Na+ ion exchange. The thermodynamic equilibrium constant K and the standard Gibbs energy ΔG0 of the H+/Na+ ion exchange reaction were evaluated from the Kielland plot. CST exhibited high H+ selectivity in the H+/Na+ ion exchange reaction. The values of K and ΔG0 were determined to be 7.3 × 104 and − 28 kJ mol−1, respectively. The selectivity of CST for H+ decreased with increasing H+ concentration on the ion exchange sites in CST.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Supplementary material
Supplemental data for this article can be accessed online at https://doi.org/10.1080/00223131.2024.2381561
Nomenclature
| A | = | coefficient in the Debye-Hückel theory (mol−1/2 dm3/2) |
| aH | = | parameter based on effective radius of a hydrated proton (nm) |
| aNa | = | parameter based on effective radius of a hydrated sodium ion (nm) |
| B | = | coefficient in the Debye-Hückel theory (nm−1 mol−1/2 dm3/2) |
| C0 | = | total concentration of cations in test solution (mol dm−3) |
| cH | = | concentration of protons in sample solution (mol dm−3) |
| cNa | = | concentration of sodium ions in sample solution (mol dm−3) |
| I | = | ionic strength (mol dm−3) |
| K | = | equilibrium constant of H+/Na+ ion exchange reaction between adsorbent and test solution (—) |
| mads | = | mass of adsorbent (g) |
| Q0 | = | cation exchange capacity of adsorbent (mol kg−1) |
| qH | = | concentration of protons on ion exchange sites in adsorbent (mol kg−1) |
| qNa | = | concentration of sodium ions on ion exchange sites in adsorbent (mol kg−1) |
| R | = | gas constant (J K−1 mol−1) |
| S | = | selectivity coefficient of H+/Na+ ion exchange between adsorbent and test solution (—) |
| S’ | = | corrected selectivity coefficient of H+/Na+ ion exchange between adsorbent and test solution (—) |
| T | = | absolute temperature (K) |
| Vsol | = | volume of test solution (cm3) |
| V/m | = | liquid – solid ratio (dm3 kg−1) |
| xH | = | mole fraction of protons in test solution (—) |
| xNa | = | mole fraction of sodium ions in test solution (—) |
| yH | = | mole fraction of protons on ion exchange sites in adsorbent (—) |
| yNa | = | mole fraction of sodium ions on ion exchange sites in adsorbent (—) |
| ΔG0 | = | standard Gibbs energy change by H+/Na+ ion exchange between adsorbent and test solution (kJ mol−1) |
| ΔGeleH | = | electrostatic potential energy for protons in adsorbent (kJ mol−1) |
| ΔGeleNa | = | electrostatic potential energy for sodium ions in adsorbent (kJ mol−1) |
| ΔGhydH | = | hydration energy for protons in test solution (kJ mol−1) |
| ΔGhydNa | = | hydration energy for sodium ions in test solution (kJ mol−1) |
| [H3-CST] | = | ratio of H3-form present in CST (—) |
| [NaH2-CST] | = | ratio of NaH2-form present in CST (—) |
| [Na2H-CST] | = | ratio of Na2H-form present in CST (—) |
| [Na3-CST] | = | ratio of Na3-form present in CST (—) |