Enhanced removal of sulfamethoxazole and tetracycline in bioretention cells amended with activated carbon and zero-valent iron: System performance and microbial community

Antibiotics, heavily used as medicine, enter the environment inevitably and raise concerns of the risk to the ecosystems. In this study, we explored the removal efficiency and mechanism of sulfamethoxazole (SMX) and tetracycline (TC) in activated carbon (AC) and AC-zero-valent iron amended bioretention cells (AC-BRC and AC-Fe-BRC) compared with a conventional bioretention cell (BRC). Moreover, the system performance of BRCs, the shifts of the microbial community, as well as the fate of corresponding antibiotic resistance genes (ARGs) were comprehensively investigated. The results showed that, exposed to antibiotics notwithstanding, AC-BRC and AC-Fe-BRC significantly outperformed BRC on total nitrogen (TN) removal (BRC: 70.36 +/- 13.61%; AC-BRC: 91.43 +/- 6.41%; AC-Fe-BRC: 83.44 +/- 12.13%). Greater than 97% of the total phosphorous (TP) was removed in ACFe-BRC, remaining unimpacted despite of the selective pressure from SMX/TC. Excellent removals of antibiotics (above 99%) were achieved in AC-BRC and AC-Fe-BRC regardless of the types and initial concentrations (0.8 mg/L, 1.2 mg/L and 1.6 mg/L) of antibiotics, dwarfing the removal performance of BRC (12.2 +/- 4.4%-64.2 +/- 5.5%). The illumina high throughput sequencing analysis demonstrated the concomitant variations of microbial communities as SMX/TC was loaded. AC layers tended to alleviate the adverse effect of SMX/TC on microbial biodiversity. Proteobacteria (34.55-68.47%), Chloroflexi (7.13-33.54%), and Bacteroidetes (6.20-21.03%) were the top three dominant phyla in the anaerobic zone of the BRCs. The abundance of antibiotic resistance genes (ARGs) sulI, sulII and tetA genes were dramatically higher in AC-BRC and AC-Fe-BRC when exposed to 0.8 mg/L SMX/TC, which indicated that relatively low concentrations of SMX/TC induced the production of these three ARGs in the presence of AC. Although the amendment of AC led to highly efficient SMX/TC removals, further investigation is still required to improve the retention of ARGs in BRCs. (c) 2018 Elsevier B.V. All rights reserved.

Automated summary

This study found that activated carbon modified bioretention cells showed significantly higher removal efficiency of antibiotics compared to traditional bioretention cells, especially in terms of nitrogen and phosphorus removal. Antibiotics were removed above 99% regardless of types and initial concentrations, outperforming the conventional bioretention cells. Changes in microbial community composition were observed when exposed to antibiotics, with activated carbon layers alleviating negative impacts on microbial biodiversity. Further research is needed to improve the retention of antibiotic resistance genes in bioretention cells.