Synergistic effect of Sr ²⁺ and ReO ₄ ⁻ adsorption on hexadecyl pyridinium-modified montmorillonite

Simultaneous adsorption of Sr ²⁺ and ReO ₄ ⁻ , which are surrogates of ⁹⁰ Sr ²⁺ and ^99m TcO ₄ ⁻ , on hexadecyl pyridinium (HDPy ⁺ )-modified montmorillonite (HDPy/Mt) was investigated. When the amount of HDPy ⁺ corresponding to 0.92 times the cation exchange capacity (CEC) of Mt was added, the obtained composite (HDPy/Mt–0.92) showed considerable adsorption capacities for Sr ²⁺ and ReO ₄ ⁻ . Some HDPy ⁺ desorbed from the Mt layer in the presence of ReO ₄ ⁻ , providing negatively charged sites on the Mt surface for Sr ²⁺ adsorption. The desorbed HDPy ⁺ interacted with ReO ₄ ⁻ and was trapped in the composite in the form of HDPy–ReO ₄ , resulting in adsorption of ReO ₄ ⁻ . At high initial concentrations of ReO ₄ ⁻ , the HDPy configuration changed after adsorption because of the desorption–adsorption process, which was supported by X-ray diffraction. Based on the results of X-ray photoelectron spectroscopy, in the binary system (i.e., Sr ²⁺ and ReO ₄ ⁻ ), Sr ²⁺ uptake occurred in the interlayer space, while adsorption of ReO ₄ ⁻ occurred on the external surface and probably in the interlayer space. Desorption–adsorption and ion exchange account for Sr ²⁺ uptake, while adsorption of ReO ₄ ⁻ was mainly attributed to desorption–adsorption. Compared with the one-adsorbate system, a synergistic effect of simultaneous adsorption of Sr ²⁺ and ReO ₄ ⁻ was found in the two-adsorbate system.