Investigating Some Chemical, Morphological Characteristics and Contamination Risk Assessment of Heavy Metals in Sewage Sludge and Its Biochar at Different Temperatures

Pyrolysis is important as an economical and environmentally friendly technology to facilitate recycling and reduce the environmental risk of sewage sludge. The aim of this research was to study changes in the structure, specific surface area, and pores of biochar, and to evaluate the risk of heavy metals contamination due to pyrolysis of sewage sludge. Sewage sludge biochar was prepared under limited oxygen conditions at temperatures of 300, 400, 500, 600 and 700 0C. Specific surface area and pore analysis were determined using the N2 adsorption isotherm, BET equation, and scanning electron microscopy (SEM) images. The risk of lead and zinc contamination was assessed using the BCR fractionation method and contamination factors. The results showed that the specific surface of biochar under the influence of temperatures of 400, 500, 600 and 700 0C was 2.65, 3.65, 8.38, and 5.57 times that of sludge, respectively. By comparing the total volume and the average pore diameter, biochar had more porosity than sewage sludge. The main pores in the biochar were meso (8.34-19.98 nm). The morphological structure of the sewage sludge was smooth and irregular, and, in contrast to the biochars, they had more porous and uneven surface. The presence of honeycomb-shaped structures was detectable at high temperatures, especially at 600 ° C. The toxic response factor (Er) for sewage sludge and biochar was low. By converting the sewage sludge to biochar, the contamination factor (Cf) of Zn and Pb was reduced by more than 50%. The conversion of sewage sludge to biochar increased the specific surface, increased porosity, and stabilized carbon in the form of aromatic compounds. Due to the reduction of metal contamination factor and the possibility of environmental risk in biochar, these materials can be used as adsorbents or modifiers in different environments.