Aging process of hydrochar and microplastics in aerobic composting: Effects on environmentally persistent free radicals and antibiotic degradation

The aging of biochar by microorganisms was a long-term process. During this process, microbial growth and metabolism inevitably influenced the removal of organic pollutants from the biochar. Aging biochar through aerobic composting more closely simulated the natural microbial processes. The addition of hydrochar (HBC) and microplastics (MPs) had minimal impact on temperature, humidity, moisture content, and organic matter levels during the composting process. Antibiotic degradation by HBC/peroxymonosulfate (PMS) and (preparation of hydrochar from sludge and microplastics, HBC + MPs)/PMS initially decreased but later increased during compost aging, with the highest degradation efficiencies observed on day 30 of compost aging at 95.93 % and 96.30 %, respectively. The concentration changes in environmentally persistent free radicals (EPFRs) in hydrochar were consistent with the antibiotic degradation process. The concentrations of HBC-30 and HBC + MPs-30 were 1.52 x 1017 and 1.66 x 1017 spins/g, respectively. The degradation of antibiotics by HBC/PMS mainly occurred through a free radical pathway, and sulfate radicals (center dot SO4 center dot-) and hydroxyl radicals (center dot OH) played key roles in the degradation process. Microbial diversity analysis revealed that the dominant bacteria were Firmicutes, Proteobacteria, and Bacteroidota, collectively accounting for over 90 % of the total population. The biotoxicity test demonstrated that the degradation of antibiotics by hydrochar/PMS significantly reduced the biotoxicity of antibiotics in water. Moreover, the MPs were non-toxic and served as carriers for the growth of Scenedesmus obliquus. The aging process of aerobic compost enhanced the hydrochar/PMS degradation of antibiotics, which suggested that hydrochar could be effectively used over the long term.