Phosphoric acid modified biochar enhanced heavy metals passivation via accelerating humification and bioremediation in sewage sludge composting

This study investigated the effects of phosphoric acid modified coconut shell biochar (PMB) on humification parameters, heavy metals bioavailability, and the structure and metabolism of bacterial community during sewage sludge composting. The results indicated that PMB addition decreased the polyphenols content and increased polyphenol oxidase (PPO) and laccase activity. At the end of composting, the humic acid (HA) concentration of PMB treatment rose by 22.19%, and the degree of polymerization and humification index (HIX) also increased significantly, compared with control (CK) treatment. In addition, the bioavailable contents of Cu, Fe, Mn, Ni, Pb and Zn in PMB were significantly lower than CK, which decreased by 19.32%, 35.28%, 39.72%, 41.99%, 36.28% and 32.27%, respectively. The addition of PMB enhanced the richness and diversity of bacterial community. The abundances of sequences associated with carbohydrate metabolism, amino acid metabolism, energy metabolism and lipid metabolism in PMB were consistently higher than that in CK. Correlation analysis showed that Streptomyces , Bacillus , Pseudomonas and Taibaiella were significantly positively associated with heavy metals passivation and HA formation. These findings suggested that PMB promoted organic matter decomposition and HA formation, and decreased bioavailability of heavy metals by promoting bacterial bioremediation and HA absorption and complexation. Therefore, the addition of PMB is a suitable strategy to mitigate the environmental risk of heavy metals in sewage sludge. • PMB increased the activity of enzymes related to humic acid synthesis. • PMB enhanced the propagation and metabolism of bacteria related to humic acid formation. • Bacterial bioremediation and humic acid fixation facilitated heavy metals passivation.