pH evolution during water washing of incineration bottom ash and its effect on removal of heavy metals

Incineration bottom ash (IBA) of municipal solid waste is a potential construction material for civil engineering. However, the possible leaching of trace heavy metals from IBA is a concern. Water washing is a simple and economic method to remove heavy metals from IBA. In order to optimize the water washing process of IBA, this study investigated the pH evolution during washing and its effect on the removal of several heavy metals, including lead (Pb), zinc (Zn), nickel (Ni), cadmium (Cd), copper (Cu), and chromium (Cr), through a small-scale laboratory experiment. The results show that the pH of washing water increases quickly in the first 1-3 h mainly due to the dissolution of quicklime and portlandite, and then decreases with the increasing of washing time might be due to consumption of OH^- by precipitation of metal hydroxides. The concentrations of Pb, Zn, and Ni in the washing water show a similar trend as that of the pH with time, whilst the concentrations of Cd, Cu, and Cr increase with the increase of washing time. Hence, the optimum washing time should be determined accordingly based on the most concerned metal(s), as well as the pH evaluation during washing.