From single- to binary-sorbate systems: Zeolite selectivity for Cs⁺ and Sr²⁺ in Na⁺- and Ca²⁺-rich waters

This study investigated Cs⁺ and Sr²⁺ sorption-individually and in binary mixtures-by aluminous (4A and 13X) and siliceous (SSZ-13 and MOR) zeolites across Na⁺ and Ca²⁺ backgrounds, using kinetic modeling, extended Langmuir isotherms, multivariable regression, and SEM-EDS/XRD analyses. In single-sorbate systems, siliceous zeolites achieved superior Cs⁺ removal via accessible 8-membered-ring (8MR) sites accommodating partially dehydrated Cs⁺, while aluminous zeolites preferentially removed Sr²⁺ through high-charge-density sites, though Ca²⁺ competition sharply suppressed Sr²⁺ uptake. Unexpectedly, Ca²⁺ conditioning enhanced Sr²⁺ uptake by H⁺-form siliceous zeolites, requiring a modified Langmuir model incorporating a synergistic conditioning term. Regression identified accessible 8MR density and Si/Al ratio as principal descriptors governing Cs⁺ and Sr²⁺ distribution coefficients, respectively. In binary systems, co-sorbate effects were condition-dependent: Sr²⁺ strongly inhibited Cs⁺ uptake by aluminous zeolites in Na⁺ media but not in siliceous zeolites, whereas Cs⁺ produced framework-dependent inhibition or synergistic enhancement of Sr²⁺ uptake by 13X, SSZ-13, and MOR, likely reflecting steric relief from displacement of bulkier hydrated cations. A 4A-SSZ-13 composite achieved simultaneous Cs⁺/Sr²⁺ removal in sodic waters, though pure 4 A was preferable in Ca²⁺-rich waters. These results establish a mechanistic and practical framework for selecting zeolite sorbents according to contaminant mixtures and water chemistry in nuclear waste remediation.