Removal trends of sulfonamides and their ARGs during soil aquifer treatment and subsequent chlorination: Effect of aerobic and anaerobic biodegradation
Sulfonamides (SAs), the most widely used antibiotics, are frequently detected in wastewater treatment plants effluent. In this study, the removal trends of four typical SAs (sulfadiazine (SDZ), sulfamethoxazole (SMX), sulfapyridine (SPD) and sulfamethazine (SMT)) in lab-scale soil aquifer treatment (SAT) systems, as well as their chlorination behaviors, were evaluated. As much as 68.2%-88.9% of SAs within the synthetic water samples (10 μg/L initial concentration) were efficiently removed during steady-state operated SAT, which was the highest for sulfadiazine and lowest for sulfamethazine. Specifically, aerobic biodegradation within the soil played a great role in SAs removal and contributed 55.7%, 57.8%, 61.1% and 74.0% to SMT, SMX, SPD and SDZ removal, respectively. Cleavage of aniline groups and desulfation of the SAs were the main pathways for their degradation during soil degradation. Subsequent chlorination could partially eliminate SAs within the SAT effluent but not entirely, with removal rate of 8.21% for SDZ, 7.45% for SMX, 14.21% for SPD, 16.52% for SMT, respectively. Electrophilic substitution to amino groups and reaction with desulfation products produced kinds of chlorinated byproducts. The intermediates of SAs originating from the biodegradation of SAT contributed to the formation of chlorinated byproducts and antibiotic resistance genes (ARGs), especially for organic compounds generated under aerobic biodegradation. Taken together, our findings demonstrated that the abundant existence of SAs within the recharging samples would lead to health concerns due to the high concentrations of ARGs and precursors of chlorination byproducts, especially with traditional chlorination processes.