A review of multi-contaminant risks in textile dyeing sludge pyrolysis: Transformation mechanisms and mitigation strategies.

Textile dyeing sludge (TDS), an industrial byproduct containing various pollutants like heavy metals, polycyclic aromatic hydrocarbons (PAHs), microplastics, per- and poly-fluoroalkyl substances (PFAS), necessitates efficient treatment to mitigate environmental risks. Pyrolysis has become an effective alternative for treating TDS due to its advantages in carbon mitigation and resource utilization compared to incineration and landfilling. However, a comprehensive understanding of the generation and transformation mechanisms of multi-contaminants during pyrolysis is still lacking, hindering its large-scale application. This review systematically analyzes the behavior of multi-contaminants during pyrolysis, with a special concern on the emerging contaminants, including PAH derivatives, microplastics, and PFAS. The potential application and environmental risks of TDS-derived biochar are also outlined, followed by a comprehensive investigation on the pollution mitigation of pyrolysis regulation strategies. The evaluation of risks posed by emerging contaminants and long-term application of biochar, as well as the source control of multi-contaminants is recognized as the dilemma, which stems from the limitations in quantifying method of emerging pollutants, the variability of biochar properties, complicated environmental influences over long-term application, and the tradeoffs among multi-contaminants during pyrolysis regulation. Future research is proposed to prioritize (1) quantitative risk assessment of emerging contaminants and long-term application of biochar, (2) elucidating pollutant formation and transformation pathways under pyrolysis regulation strategies for targeted control, and (3) multi-objective optimization to balance product valorization and integrated risk of multi-contaminants. This review aims to provide guidance for the research on pollution risk evaluation and control in the pyrolysis process of TDS.