Influence of hydraulic loading rate on antibiotics removal and antibiotic resistance expression in soil layer of constructed wetlands

Behavior of veterinary antibiotics, the corresponding resistant genes in soil layer of constructed wetlands (red soil), and their response to different hydraulic loading rates (HLR) (2, 5, and 10 cm/d) were investigated. Results indicated that the soil layer had perfect performance for oxytetracycline and ciprofloxacin, yet sulfamethazine removal was unsatisfactory. Detection rates of oxytetracycline, ciprofloxacin and sulfamethazine in the effluent of simulation systems of soil layer were 8.33-36.36%, 8.33-47.83% and 100%, respectively. The model analysis of adsorption and hydrolysis indicated that physical adsorption, which was controlled by exchange reaction process based on diffusion, was the primary adsorption mechanism of target antibiotics in red soil, and the hydrolysis half-life values of antibiotics in the water of soil layer were shorter than them in wastewater. The removal response of oxytetracycline and ciprofloxacin to change of HLR was insignificant, yet the respective effluent concentrations of sulfamethazine at HLR of 2-10 cm/d were 41.90, 61.35 and 73.54 mu g/L during treating synthetic livestock wastewater, which revealed significant positive correlation (P < 0.05). The relative abundances of each target resistance genes in soil showed significant increase after treating wastewater (10(-5)-10(-6) to 10(-4)-10(-1)), and the total level of those at different HLRs (2, 5, and 10 cm/d) were 3.02 x 10(-2), 7.54 x 10(-2) and 8.65 x 10(-1), respectively. In summary, HLR could affect the removal efficiency of partial antibiotic in soil layer of constructed wetlands, and the expression of antibiotic resistance in the soil gradually increased with increase in the HLR. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The behavior of veterinary antibiotics and corresponding resistant genes in the soil layer of constructed wetlands, as well as their response to different hydraulic loading rates (HLR), were investigated in this study. Results showed that the soil layer had good performance in removing oxytetracycline and ciprofloxacin, but unsatisfactory results for sulfamethazine. The study also found that the removal efficiency of certain antibiotics in the soil layer could be affected by the hydraulic loading rate, and the expression of antibiotic resistance genes in the soil increased with higher hydraulic loading rates.