Effects of residence time on characteristics of biochars prepared via co-pyrolysis of sewage sludge and cotton stalks

Conversion of sewage sludge and agriculture straws into biochars via co-pyrolysis is a promising strategy for the safe disposal and reuse of sewage sludge. In this study, biochars were prepared via the co-pyrolysis of sewage sludge and cotton stalks (1:1, w/w) with different residence times (30-150 min). The influence of residence time on the physicochemical properties and surface characteristics of the biochars were studied. Meanwhile, the chemical forms of six heavy metals (Pb, Cu, Zn, Cr, Ni, and Cd) in the biochars were investigated with BCR (Community Bureau of Reference)-recommended extraction procedures and the potential environmental risks of heavy metals were assessed. It was found that prolonged residence times increased the pH and ash contents of the biochars, but these conditions decreased the C, N, and H contents. The surface areas of the biochars increased in the range of 30 min to 90 min, while values decreased in the range of 90 min to 150 min. Although more heavy metals were volatilized or transferred into biogas with the increasing residence time, a considerable amount was retained in the biochars, and this resulted in an increase in the total concentrations of heavy metals. The BCR extraction experiments confirmed that the heavy metals became immobilized in the biochars following the transformation of heavy metals from active chemical forms to stable ones. The results of risk evaluation indicated that prolonged residence times decreased the potential environmental risks of heavy metals in the biochars.