Size matters: nano-biochar triggers decomposition and transformation inhibition of antibiotic resistance genes (ARGs) in aqueous environments.

Antibiotic resistance genes (ARGs) are considered to be a type of emerging contaminant, their interaction with biochar (BC) could affect their dissemination and fate in the environment. Although adsorption of ARGs onto bulk-BC has been reported, the interaction with nano-sized BC (nano-BC) is largely unknown. In this study, the interactions of a model extracellular DNA (eDNA, calf thymus DNA) and two typical ARGs (ampC and ermB) extracted from a natural river with bulk- and nano-BCs from two pyrolysis temperature (400 and 700 oC) were investigated. Only adsorption was observed on bulk-BCs, while not only adsorption but also fragmentation of these eDNA were found to occur on nano-BCs. Also, their replication was greatly inhibited by nano-BCs. The electron paramagnetic resonance results indicated that hydroxyl radicals produced from persistent free radicals (PFRs) on nano-BCs played a major role in the damage of eDNA. Moreover, the direct contact with nonradical reacting sites and PFRs on nano-BCs also contributed to the decay of eDNA. Comparatively, PFRs in bulk-BCs were difficult to be reached by eDNA due to steric hindrance and played a negligible role in destroying eDNA. These findings highlight the importance of size effect on evaluating the reactivity and related environmental risk of PFRs on BC and improve our understanding on the interaction between ARGs and BC.