Source-dependent pyrolysis temperature optimization for sewage sludge biochar: balancing nutrient retention and heavy metal passivation.

Addressing the conflict between rising sludge production and its resource utilization, this study evaluated environmental risks and resource potential of sludge-derived biochar. The nutrients and heavy metals in sludge and biochar were systematically analyzed based on mixed industrial-domestic sludge and purely domestic sludge, produced at pyrolysis temperatures from 350°C to 800°C. Results indicated that sludge source critically determined its resource utilization pathway. For mixed sludge, pyrolysis at 550-650°C concentrated total nutrients by 77-82% (peaking at 13-14.5%), whereas domestic sludge retained available nitrogen best at < 450°C, limiting nitrogen loss to < 50%. Pyrolysis reduced environmental risks by stabilizing heavy metals. For mixed sludge at 650°C, residual fractions of Cu, Cr, and Pb exceeded 80%, reducing potential ecological risks (RI) to mild pollution levels (38.98 and 21.14). For domestic sludge at 450°C, Cd and Zn leaching toxicity was effectively passivated to below 0.001 mg/kg and 2 mg/kg, respectively, meeting safety standards. This study confirms that a temperature-based strategy based on source differentiation can simultaneously optimize the nutrient efficiency and environmental safety of sludge-derived biochar, providing important insights and theoretical framework for fertilizer use in future studies.