Effects of pyrolysis parameters on biochar derived from sewage sludge including environmental risk assessment of heavy metals

Pyrolysis is a promising thermochemical process for managing sewage sludge while simultaneously producing biochar, a valuable co-product. This study systematically investigated the effects of the pyrolysis parameters of temperature (200-800 °C), residence time (5-60 min), and inert gas flow rate (0.25-1.0 dm³/min) on the properties of biochars obtained from sewage sludge. Comprehensive characterization of the biochars was conducted, including composition analyses (ultimate, proximate, elemental, and molecular), acidity, specific surface area and pore size, and the assessment of eight heavy metals (As, Cd, Pb, Cr, Zn, Mn, Ni and Cu) in the biochars and sewage sludge. The results showed pyrolysis temperature and residence time were the most critical parameters affecting biochar quality, with negligible influence of inert gas flow rate. Higher pyrolysis temperatures (>500 °C) increased biochar pH to alkaline values (>10), ash content, and nutrient concentrations (Ca, K, Mg, P). Temperatures above 600 °C significantly increased biochar surface area, reduced pore size, and yielded H/C ratios below 0.57, improving suitability for soil remediation. Pyrolysis also facilitated volatilization of heavy metals, particularly As and Cd, which were reduced to safe levels, with Cd removal exceeding 90% at 700 °C. Metal analysis confirmed the immobilization of heavy metals in biochar, significantly reducing the environmental risk, from high (PERI=1158) in the sewage sludge feedstock to low (PERI<50) in biochar obtained at temperatures above 600 °C. Most heavy metals in biochar at these temperatures were concentrated in oxidizable and residual fractions. The results provide valuable new data to guide development of pyrolysis for the sustainable management of sewage sludge.