Dissolution kinetics of Zn2SiO4 powders: Effects of polymorphs, temperature, particle size, and Fe2+ presence

Abstract

By monitoring the change of the solution pH value, the dissolution kinetics of gel-derived Zn2SiO4 powders (amorphous, β and α-phase nanometers in size) at various solution pH ranges and temperatures were studied. At 30°C, all the polymorphs showed a near-linear pH dependence on logarithmic dissolution rates in the range of pH = 1–4 and the slopes (i.e., −n value in the formula of rate = kaH+n) of logarithmic dissolution rate-pH plots are −0.53, −0.63, and −0.51 for α, β and amorphous phases, respectively. The similarity of the pH dependencies for the polymorphs is due to the same leading cation (i.e., Zn2+ as active site) for dissolution and similar structural linkage and hence, a similar protonation-detachment process of dissolution. The apparent activation energies for dissolution of all the powders at pH = 2 and 30–60°C indicated a diffusion-controlled process. Smaller particles of α-Zn2SiO4 caused a higher acidic dissolution rate, a lower activation energy, but nearly the same pH dependence of dissolution. At 30°C and a given pH, a critical concentration of Fe2+ in solution is required to significantly suppress the acidic dissolution of α-Zn2SiO4 powders. This has been attributed to the adsorption equilibrium of Fe2+, which depends on pH and surface structure.