Sewage sludge incineration ash for coimmobilization of lead, zinc and copper: Mechanisms of metal incorporation and competition.

Heavy metals such as lead, zinc, and copper always coexist in industrial wastes and tend to be released if the wastes are not treated properly. With abundant contents of aluminum, iron and silicon, sewage sludge incineration ash can provide a ceramic matrix for potential heavy metal stabilization. By using ceramic sintering, this study explored the coimmobilization mechanisms of lead, zinc and copper, with detailed explications on phase transformation, metal distribution and the effect of metal content. PbAl2Si2O8 was identified as the major phase for lead immobilization in series with low heavy metal content, while most of the lead was incorporated into Pb9(PO4)6 in high metal series. The ZnxCu1-xFeyAl2-yO4 spinel solid solution was the predominant product phase for copper and zinc stabilization in both reaction series, but zinc was more competitively incorporated into the spinel structure. Moreover, the pattern of heavy metal distribution in the sintered products was largely affected by the metal type and elemental composition of the reaction system. Although different leaching behaviors were observed for the three heavy metals, their leachability was found to reach very low value after the thermal treatment processes. This study proposed a "waste-to-resource" strategy to largely alleviate the environmental burden of solid wastes and heavy metal pollution by using sewage sludge incineration ash as raw materials for low-temperature glass-ceramics, with a simultaneous effect on metal immobilization.