Identifying nickel precipitation at the montmorillonite-water interface and its lower solubility products than in a pure water system

Abstract

Although the existence of surface precipitation has been reported in the literature, its solubility products (K_sp) and formation mechanism at the interface have not been widely studied. In the present study, the precipitation of nickel ions (Ni²⁺) on the surface of montmorillonite (Mt) in the form of Ni(OH)₂ was determined by a series of adsorption/precipitation experiments, extended X-ray absorption fine structure, X-ray powder diffraction, and high-resolution transmission electron microscopy, which analytical techniques can verify mutually. Meanwhile, the differences in K_sp between the Mt-water interface (heterogeneous) and pure water (homogeneous phase) precipitation were quantitatively analyzed. A newly developed adsorption-precipitation model was constructed, and the concept of apparent solubility product (K_sp') applicable to the heterogeneous systems was proposed. The Ni(OH)₂ K_sp' at the Mt-water interface (pH=7.0) decreased by 10–100 fold compared with K_sp in pure water, and K_sp' increased with the continuous accumulation of Ni²⁺ layer by layer on the surface of Mt until it returned to the value of the pure water system of K_sp. These findings can not only explain the bias of pure water chemistry theory used in a multi-media environment but also predict whether Ni(OH)₂ precipitates at the Mt-water interface.