Fate of antibiotic resistance genes and roles of biochar in wastewater treated with biochar/persulfate

Advanced oxidation processes based on persulfate activation by biochar have been widely used to remove antibiotics and antibiotic resistance genes (ARGs) from wastewater. In this study, we used a common continuous fixed-bed reactor based on a biochar/persulfate system to treat wastewater. The average apparent ARG-removal efficiency was 82.38% in the biochar/persulfate reactor. The results of continuous reactor activity suggested the presence of ARG residues in the biochar (the abundance of ARG in the biochar increased 103-fold) and unstable removal of extracellular ARGs, raising concerns about a potential environmental burden. Kinetic experiments showed that the absolute abundance of intracellular ARGs (iARGs) rapidly decreased 98.3% within 30 min, but extracellular ARGs (eARGs) correspondingly increased 15-fold, suggesting that persulfate broke bacterial cells open and quickly released iARGs as eARGs. Moreover, the proportions of the three types of ARGs showed that ARG removal was attributed to about 70% degradation and 30% adsorption by the biochar/persulfate reactor. Further analysis revealed that biochar acts as a special shelter for ARGs. Release experiment of used biochar indicated that nearly half of absorbed ARGs could be released into new environment and causing potential risk. Overall, our findings provide a fundamental understanding of the fate of ARGs during treatment of antibiotic-contaminated wastewater and new insights into the multiple roles of biochar, which can potentially represent an additional burden on ecosystems and human health.