Nickel(II) sorption on hydrous silica: A kinetic and thermodynamic study

Summary Sorption of nickel, Ni²⁺, on SiO₂ ^. xH₂O (silica gel) has been studied as a function of time conditions: amount of silica gel: 0.10-1.00 g, nickel concentration: 5.00 ^. 10^-5-1.20 ^. 10^-3M, ionic strength: 0.20-1.40M NaClO₄, pH 6.50 to 8.50, and temperature 273-318 K. From the kinetic data, the diffusion coefficient of Ni²⁺ ion was calculated to be 1.28(±0.07) ^. 10^-11 m^{2 .} s^-1 under particle diffusion-controlled conditions. The sorption rate was determined as 3.79(±0.35) ^. 10^-3 s^-1 at 298 K, pH 8.40 in 0.20M (NaClO₄). The sorption data were fitted to Freundlich, Langmuir and Dubinin-Radushkevich (D-R) isotherms. Nickel sorption on 0.20 g silica gel decreased with ionic strength from 77.70±0.70% (0.20M NaClO₄) to 16.12±0.37% (1.40M NaClO₄) at intial pH of 8.50±0.05. A gradual decrease in pH with increasing ionic strength suggests an ion exchange mechanism and the sorption of Ni²⁺ on silica gel increasing with temperature indicates an endothermic enthalpy. The effect of different ligands such as fluoride, carbonate, phosphate and oxalate on Ni²⁺ sorption on silica gel was studied.