CAESIUM REMOVAL | WASTE MANAGEMENT metastable Ca2+ into the Sn-S matrix, where Ca2+


enough to reserve space for locating Cs+ The structural Ca2+


condition and their sites were replaced with Cs+ is large during adsorption.


ions were leached out in the acidic . As the


structure of KCaSnS is rather stable, even partial leaching of the constituent ions requires a certain level of acidity, i.e., pH 2 in this case. Upon the release of weakly bonded Ca2+


of Cs+


Because a proton is a very hard Lewis acid, it may not dominate the structural Ca2+


empty sites. Furthermore,


as previously mentioned, protons alone cannot leach off all of the structural Ca2+


. To increase the likelihood of Cs+


being accessible into the structure, the structural Ca2+ must be completely removed. This requires very high Cs+ concentrations, even higher than H+ commencement of Cs+


concentrations. The concentration of 62.08 mmol/L


for the complete leaching at pH 2 indicates a synergistic interaction of Cs+


and H+ , resulting in the maximum


adsorption capacity of 4.66 mmol/g. The coexistence of ample H+


and Cs+ completely leaching the structural Ca2+ is instrumental in from the Sn-S matrix.


The adsorbent design strategy that incorporates structural lattice points into interlayer space of typical layered materials overturns our previous understanding. Notably, the Cs+


adsorption capacity is greater in acidic solutions


than that in neutral solutions, which contradicts the general trend of current SnS-type adsorbents. At first, a conspicuous


increase in Cs+ adsorption capacity in acid was not noticed,


although the release of appreciable amount of the structural Ca2+


was observed at 500 mg/L Cs+ and pH 2, for


example. Then, we attempted a much higher concentration of Cs+ Cs+


, i.e., above 8250 mg/L and could spot a steep rise in adsorption capacity. The structure-damaging proton is


ions from the crystal structure, preferential adsorption would take place due to its high affinity toward S2-.


used to mediate ion exchange in this case. The uncommon ion-exchange possibility is accomplished by the synthesis of an intermediate metastable crystal structure that can resist transformation while permitting ion exchange. This approach provides meaningful insights for the design of


different types of novel adsorbents. 137


Cs, a hazardous radionuclide generated during spent


fuel reprocessing and decommissioning of nuclear power plants, poses a threat to human health. However, the Cs+ adsorption performance of most adsorbents in acidic conditions has an insurmountable limit due to the presence of copious protons. In this study, we turn the problematic proton into a functional agent by incorporating a large, weakly bonding ion, Ca2+ Ca2+


, into an Sn-S matrix. Leaching from the matrix creates additional adsorption sites in


potassium calcium thiostannate (KCaSnS) resulting in the greater Cs+


in neutral solutions. ■


This article is an abridged version of a research paper which was first published in the Journal of Hazardous Materials 455 (2023) 131648


adsorption capacity in acidic solutions than that


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