Tracking the transformation pathway of dissolved organic matters (DOMs) in biochars under sludge pyrolysis via reactomics and molecular network analysis

This work examined the transformation pathways of sludge biochar-derived dissolved organic matters (SBC-derived DOMs) under sludge pyrolysis via FT-ICR-MS-based reactomics and molecular network analysis. Lignin/carboxylic-rich alicyclic molecules, proteins/aliphatic, and lipids of SBC-derived DOMs did not contribute equally to the overall pyrolytic reactions. Reactomics suggested that the pyrolysis reactions of SBC-derived DOMs consist of multiple cascade reactions involving the elimination of assemblages of reactive fragments during each pyrolysis reaction region, and the overall pyrolysis process was divided into three stages according to cascade reaction variations. Especially, cascade reactions at 400-500 °C produced potential environmental risk substances of N-containing, carbonyl-containing, and phenolic compounds. Besides, network analysis unraveled the complexity and number of molecular reaction pairs of SBC-derived DOMs decreased with the increase in pyrolytic temperatures. Keystone molecules and pathways results indicated that the pyrolytic temperature of the sludge pyrolysis process should be controlled at temperatures above 500 °C according to the harmful substances generation pattern in reaction products. Overall, the possible transformation pathways of SBC-derived DOMs during sludge pyrolysis treatment were proposed. This study elucidated the underlying mechanisms in generating SBC-derived DOMs and provided theoretical support for process optimization and harmful substances control of sludge pyrolysis.