A hypoxic pyrolysis process to turn petrochemical sludge into magnetic biochar for cadmium-polluted soil remediation

Biochar has been considered as a promising material for soil remediation, particularly for its ability to reduce the bioavailability of cadmium (Cd) in soil through sorption. However, long-term remediation may cause Cd to be released from a fixed state, making the recovery of biochar as an adsorbent for Cd removal an area of increasing interest. The study aims to synthesize biochar with magnetic properties using petroleum sludge containing iron in one-step, and investigate their adsorption efficiency and passivation mechanism for Cd in liquid-solid phase, as well as ecological risks. The results indicate that the petrochemical sludge waste can be directly resourced into magnetic biochar (PSMBCs) using hypoxic pyrolysis, and that it exhibits good recycling performance in water/soil. Specifically, the obtained biochar showed strong sorption capacity for Cd (18.4 to 29.8 mg/g) due to surface mineralization and cation-π coordination, which played a critical role. After applying 1.5% of PSMBCs for 30 d in paddy soil, the bioavailable content of Cd was decreased by 85.0%. Importantly, the biochar leachates did not have any toxic effects on wheat root elongation. Therefore, this study presents a promising strategy for the benign disposal of petrochemical sludge and their utilization for the remediation of Cd-contaminated soil.