LAUSR

Abstract The pyrolytic conversion of sewage sludge to char is a promising way for its utilization. To explore their effects, inorganic salts containing Ca, Al, chlorine, sulfate and phosphate were added into sewage sludge prior to pyrolysis at 500 °C. The ash content, yield and pH changed depending on salt species. Salt-addition catalyzed the decomposition of organic matters and decreased the contents of C, H, N and S. Some of the intrinsic minerals were lost or converted while new minerals were formed involving the added and intrinsic ions. The retention of Cu, Mn, Zn and Pb was reduced at the presence of excessive chlorine. Sulfate moderately promoted volatilization, while phosphate offset the effect of chlorine. Chlorine transformed residual fraction of Pb to exchangeable fraction, while sulfate inhibited the transformation of heavy metals from mobile to immobile fractions. Phosphate increased the residual fraction. The mechanisms included direct reactions between the added anions and heavy metals, and altered affinity between heavy metals and the organic and mineral components. The presence of inorganic salts affects char characteristics and heavy metal risks, which should be considered in char production and application.