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

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 (Ni 2+ ) 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 Ni 2+ 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.