MnOx-biochar substrates improve tetracycline removal and potentially restrain antibiotic resistance gene accumulation in constructed wetlands

Tetracycline poses significant environmental risks to ecosystems and human health. Although biochar and manganese oxides (MnOx) have been recognized as functional materials for wastewater treatment, the performance and mechanisms for tetracycline removal in constructed wetlands (CWs) incorporating both biochar and MnOx have not been elucidated, which impedes their application. This study evaluates the influences of MnOx-loaded biochar amendment on tetracycline removal in constructed wetlands throughout a 210-day operation period. Results indicated that the MnOx-loaded biochar group (MB) exhibited superior performance compared to biochar group (BC) and control group (CK), with the average removal rates of tetracycline were 84.37%, 76.88%, and 65.50% for the MB, BC, and CK groups, respectively. The qualitative and quantitative analyses of microbial extracellular polymeric substances, along with X-ray photoelectron spectroscopy and metagenomic analyses of the substrates indicated that sorption, biotic metabolism, resistance, and electron transfer processes synergistically contributed to the removal of tetracycline. Genes associated with aromatic degradation and the TCA cycle (e.g., fadB, boxB, pcaH and DLAT ) and extracellular electron transfer genes (e.g., CcoQ and pilA ), which predominantly hosted by genera such as Thauera, Ralstonia, Pseudomonas and Acidovorax , were significantly enriched in the bacterial communities of the BC and MB groups. Notably, the biochar enhanced the abundance of antibiotic resistance genes (ARGs) primarily associated with efflux and inactivation mechanisms to cope with tetracycline toxicity, whereas the MB group suppressed this biochar-induced ARG proliferation and maintained comparatively low levels of ARGs. Considering both pollutant removal performance and ecological safety, MB may serve as a potential promising alternative for treating tetracycline in CWs.