Influence of coal slime on migration behavior and ecological risk of heavy metals during hydrothermal carbonation of sewage sludge

The heavy metals (HMs) presented in sewage sludge (SS) limit the application of SS hydrochar. Compared to SS, coal slime (CS) contains higher carbon, silicates and aluminates. Addition of CS during the hydrothermal carbonization (HTC) of SS can improve the quality of hydrochar, and facilitates better solidification of HMs. In this paper, the influence of CS on migration behavior and ecological risk of HMs during HTC of SS was investigated, and the environmental risks associated with HMs in co-hydrochar were assessed. The results showed that adding CS during SS-HTC could lower the content of HMs in co-hydrochar, and compared to the total content of Cd, Cr, Cu and Zn in SS-240-1, the total contents of these HMs in S5C5-240-1 decreased by approximately 43.8, 51.6, 36.5, 42.8 %, respectively. The addition of CS also could facilitate the transformation of HMs from bioavailable fractions to relatively stable fractions. The leaching concentration and bioavailability of HMs in hydrochar decreased substantially with the increase of reaction temperature and residence time, and the leaching concentration of Ni in S5C5-280-1 decreased by 83.7 %, and the bioavailability concentration decreased by 89.5 %. Interestingly, the results also showed that co-HTC of CS and SS could effectively reduce the ecological risk and the potential contamination level associated with HMs in hydrochar, and then reduced the potential environmental risk. The results of this study may provide theoretical basis for the solidification of HMs in hydrochar, as well as for the clean and efficient utilization of SS and CS.