Synergistic effect and mechanism analysis of biochar regulator on heavy metal passivation and microplastic degradation in sewage sludge compost

Heavy metal passivation and microplastic in sludge threaten environment and cause human health risks, and thus necessary to find effective remediation strategies. To solve this issue, the synergistic effect of biochar on pollutants remediation during sludge composting is still not well explored. In this study, different doses of cotton stalk biochar (0 %, 2.5 %, 5 %, 7.5 %, 10 % CSB; and labeled T1-T5) were applied to sludge composting to investigate the synergistic effect of CSB on pollutants (copper, zinc, and microplastics) and explore the influence mechanism. Results showed that CSB could effectively increase the yield of humic acid (15.85-22.08 g/kg) and reduce the content of extractable copper (59.37-81.10 %) and extractable zinc (27.07-51.45 %). Among them, T5 was superior in the passivation of heavy metals. In addition, CSB optimized the environmental factors to increase the degradation rate of microplastics by 16.23∼57.86 %, exhibiting dose-dependent improvement. The microbiological analysis showed that CSB could decrease the relative abundance of Firmicutes (29.16-59.20 %) and increase the relative abundance of Proteobacteria (14.10-33.48 %), Actinobacteriota (2.55-46.25 %) and Ascomycota (11.36-65.71 %) in the high temperature stage of compost. For correlation analysis, T4 and T5 could better enhance the positive correlation between environmental factors and microorganisms. In summary, T5 could minimize the content of heavy metals and microplastics in compost products, and it had the highest level of application value. Hence, this study is of great significance for reducing the pollutant risk of sludge composting.