Migration and transformation of trace elements during sewage sludge and coal slime Co-combustion

Co-combustion of sewage sludge (SS) and coal slime (CS) could improve the combustion properties of the two materials, however, high levels of trace elements (TEs) can be released from the two wastes, resulting in secondary pollution. The migration and transformation behavior of As, Cr, Pb, Zn, and Mn during co-combustion is explored in current research. The results showed co-combustion could inhibit the emission of Zn, As, Pb, and Mn, and the effect was more pronounced for Zn, As and Mn. Meanwhile, minerals like kaolinite and gypsum were found to generated in the ash from co-combustion but not solo-combustion. Model experiments demonstrated that kaolinite captured As, Pb and Mn, while gypsum captured Zn, As and Mn but facilitated the emission of Pb and Cr. This well explained the distinct TEs emission characteristics between co-combustion and solo combustion. As the temperature elevated, kaolinite in co-combustion ash decomposed and the generation of gypsum was promoted. In this way, the emission ratios of Zn, As, and Mn initially increased but subsequently decreased between 700 and 1300 °C, whereas Pb and Cr emission ratios increased by twofold within the same temperature range. Leaching characteristics and risk assessment code on co-combustion ashes were also conducted in this study. The results indicated a marginal elevation in the risk associated with trace elements (TEs) following co-combustion, provided that all five TEs remained within the limits of national standards.