Aging mechanism of copper added to bentonite

The extractability, bioavailability and toxicity of metals in soils decline with time owing to aging processes. The processes have important impact on the ecological risk assessment of metals in soils. In this study, a sequential extraction procedure was used to investigate the aging processes of copper added to bentonite and the effects of temperature and pH on the aging processes; X-ray diffraction technique was used to study the changes of basal spacing of bentonite saturated with Cu at different pH. The results from the sequential extraction demonstrated that the proportions of the most labile Cu fractions (viz. water-soluble, NH4NO3 extractable and EDTA extractable Cu) to total added Cu decreased with increasing incubation time, whereas the potentially labile fraction extracted by 0.5 mol L− 1 HCl and inert fractions (6 mol L− 1 HCl extractable Cu and residual Cu) exhibited the opposite trend. Also the aging of added Cu was a slow process (more than one year) where the easily extractable Cu species on soil solid surfaces gradually transformed into less extractable Cu forms which possibly diffused into interlayer of bentonite. The aging experimental data were well fitted using a parabolic diffusion equation (R2 = 0.690–0.943, p < 0.0001) with apparent diffusion rate coefficients (D/r2) of 6.43–20.0 × 10− 6 d− 1 and activation energy of diffusion (Ea) of 38.3–56.1 kJ mol− 1, indicating that micropore diffusion could be the main mechanism of aging processes of Cu added to bentonite. The results of X-ray diffraction showed that the basal spacing of bentonite decreased with increasing pH, which was from 1.51 nm at pH 4.42 and 5.51, 1.37 nm at pH 6.40, to 1.26 nm at pH 7.68. It was proposed that Cu2+ was dominant diffusive ions at low pH (< 5.5), while CuOH+ became dominant at relatively high pH. During long-term aging, copper ions in interlayer of bentonite might diffuse into hexagonal cavities of clay mineral by dehydration.