Synergistic effect of Sr2+ and ReO4− adsorption on hexadecyl pyridinium-modified montmorillonite

Abstract

Simultaneous adsorption of Sr2+ and ReO4−, which are surrogates of 90Sr2+ and 99mTcO4−, 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 Sr2+ and ReO4−. Some HDPy+ desorbed from the Mt layer in the presence of ReO4−, providing negatively charged sites on the Mt surface for Sr2+ adsorption. The desorbed HDPy+ interacted with ReO4− and was trapped in the composite in the form of HDPy–ReO4, resulting in adsorption of ReO4−. At high initial concentrations of ReO4−, 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., Sr2+ and ReO4−), Sr2+ uptake occurred in the interlayer space, while adsorption of ReO4− occurred on the external surface and probably in the interlayer space. Desorption–adsorption and ion exchange account for Sr2+ uptake, while adsorption of ReO4− was mainly attributed to desorption–adsorption. Compared with the one-adsorbate system, a synergistic effect of simultaneous adsorption of Sr2+ and ReO4− was found in the two-adsorbate system.