Mechanisms of copper stabilization by mineral constituents in sewage sludge biochar

Sewage sludge has gained increasing attention as one of the most intractable environmental concerns. Pyrolysis is a promising technology that can convert the sewage sludge into biochar and the produced biochar has been increasingly used as adsorbents for heavy metal remediation. However, further environmental problems will be generated from the accumulation of spent biochar after metal adsorption. This work involved a series of experiments to study the adsorption of hazardous copper on the sewage sludge-derived biochar. A novel method was then proposed to stabilize the adsorbed copper in the biochar matrices. The results showed a growth in copper adsorption capacity with an increase in the initial pH of the solutions, and positive effects on copper adsorption were detected from the mineral constituents (e.g., silicon, aluminium, iron and calcium) of the sludge biochar. Furthermore, after copper adsorption, the spent biochar was thermally sintered, and the hazardous copper was found to be incorporated into a CuAl2O4 spinel phase through thermal reactions between copper and the mineral constituents. The stabilization effect on copper was further evaluated in the sintered ceramic products, and copper leachability was found to decrease by a factor of approximately 65 when samples were subjected to a leaching fluid with a pH of 2.9. A series of copper-enriched biochars was prepared to further explain the reaction mechanisms, which showed that aluminium-containing minerals and iron oxide were the most important constituents for copper stabilization. Through this study, a dual effect of the mineral constituents in the biochar was reported on both copper removal and further stabilization, which suggested a strategy to achieve sustainable waste management in an effective and environmentally friendly way. (C) 2018 Published by Elsevier Ltd.