Factors affecting surface precipitate solubility products of nickel at the montmorillonite-water interface

Surface precipitates of metals at soil-water interfaces were identified. Recently, the surface precipitate apparent solubility product (K_sp’) of nickel (Ni) at the montmorillonite-water interface was first determined using the Langmuir-precipitation model, enabling quantitative prediction of Ni partitioning. Factors affecting interface surface precipitation are critical for thermodynamic and mechanistic understanding but remain unstudied. In the present study, the nickel hydroxide's K_sp’ at the montmorillonite-water interface under varying pH, temperature (T), and montmorillonite concentrations were investigated. It was found that K_sp’ decreased significantly with increases in the pH and montmorillonite concentration: pK_sp’ increased by 0.03 per 0.1 pH increase and by 0.04 per 1 g L⁻¹ montmorillonite concentration increase. The values of K_sp’ ranged from 2.19 × 10⁻¹⁷ to 5.25 × 10⁻¹⁷, which were 39 to 91 times lower than the solubility product (K_sp, 2.00 × 10⁻¹⁵) of Ni(OH)₂ in a pure water system. Thermodynamic parameters (enthalpy change, ΔH⁰ = 36.62 kJ mol⁻¹; entropy change, ΔS⁰ = 0.438 kJ/(mol·K); Gibbs free energy change, ΔG⁰ < 0) indicate that surface precipitation is an endothermic reaction overall with an increase in entropy, contrary to precipitation in pure water, and is spontaneous (T > 84K). These findings can effectively predict Ni retention at soil-water interfaces, addressing critical gaps in conventional environmental risk assessments of surface precipitation mechanisms.