Insight into size-dependent bio-removal of sulfonamide antibiotics by coated activated carbon particles on suspended biocarriers

The widespread use of sulfonamide antibiotics (SAs) causes environmental risks due to inefficient wastewater treatment. In this study, biocarriers coated with small (50–100 μm) and large (400–500 μm) activated carbon (AC) particles (as AC-SP and AC-LP) were applied to improve removal of SAs. The results showed that biofilm reached ∼60–150 μm and completely covered AC-SP in 45 d, leading to identical performance as raw carriers, whereas most of AC-LP on the moving carriers remained exposed during long-term operation. After 318 d, the biomass-normalized rate constants of sulfamethoxazole, sulfadiazine, sulfamonomethoxine by AC-LP were 0.0147, 0.0151 and 0.0153 L g −1  h −1 , respectively, which were ∼ 1.4–10 times higher than AC-SP, raw carrier and previous activated sludge and biofilm reactors. Remarkably, the rapid biotransformation contributed 47 %–50 % of overall SAs removal by AC-LP, likely driven by: 1) The adsorption and desorption might dynamically enrich SAs within biofilm-AC microenvironment, facilitating biotransformation on amino group of SAs; 2) Functional groups on AC might serve as redox mediators to accelerate electron transfer during biotransformation. Overall, this study presents AC-LP biocarrier as an efficient solution for SAs removal in wastewater treatment, maintaining stable performance over a year-long operation. For the first time, this work demonstrated the critical role of coated AC particle size in regulating adsorption-biotransformation balance, revealing AC size optimization on biocarriers as a novel strategy for antibiotic removal.