Leaching characterisation and geochemical modelling of minor and trace elements released from recycled concrete aggregates

The pH dependent release of Cd, Cr, Cu, Mn, Mo, Ni, Pb, V and Zn from different recycled concrete aggregate samples was determined. Geochemical speciation modelling was applied on the concentrations of Cu, Cr, Mo and Ni in the leachates in order to predict the measured concentrations and the specific release mechanisms. The model was able to reproduce the characteristic pH dependent release patterns for these elements and reasonable to sometimes excellent matches between the predicted and measured concentrations were achieved. Binding of Mo and Cr as oxyanions (MoO42− and CrO42−) to ettringite was modelled with fair agreement for Cr only. For Cu and Ni, the predicted and measured concentrations agreed well for the partly carbonated sample at high alkaline pH (11–13). The importance of complexation to humic substances was also shown in samples derived from construction debris.