Ageing of added copper in bentonite without and with humic acid

Abstract

The ageing of metals in soils plays an important role in assessing ecological risk and soil quality criteria, it therefore is necessary to understand the ageing processes. Using a sequential extraction procedure and a parabolic diffusion equation, the ageing of added copper(Cu) in bentonite without and with humic acid (HA) was investigated as a function of incubation time over one year. At the same time, the effect of pH on Cu ageing in bentonite with HA was also studied. The most labile Cu fractions (water-soluble Cu, Cu extracted by NH4NO3 and EDTA) gradually transformed into less extractable forms. The modelled diffusion equation suggested that the ageing processes were controlled by surface nucleation/precipitation and diffusion. In the presence of HA, the contribution from surface nucleation/precipitation increased significantly, whereas that from diffusion decreased, this indicated the addition of HA significantly enhanced the short-term ageing by complexation with Cu ions, but strongly inhibited the further diffusion of Cu ions into interlayer of bentonite during long-term ageing owing to the more stable complexes formed on solid surface. With increasing pH in bentonite with HA, the apparent diffusion rate coefficient (D/r2) increased greatly, which showed that during long-term ageing pH played an important role in reducing the extractability as a result of the significant increase in residual Cu (RES-Cu), the most inert fraction, due to micropore diffusion.