Insights into pharmaceuticals removal in an anaerobic sulfate-reducing bacteria sludge system

Abstract

In this study, we examined eight typical and widely detected pharmaceuticals (PhAs) removal in an anaerobic sulfate-reducing bacteria (SRB) sludge system (five antibiotics: sulfadiazine (SD), sulfamethoxazole (SMX), trimethoprim (TMP), ciprofloxacin (CIP) and enoxacin (ENO), and three nonsteroidal anti-inflammatory drugs (NSAIDs): ibuprofen (IBU), ketoprofen (KET) and diclofenac (DIC)). The results showed that the SRB sludge had the higher removal efficacy (20 to 90%) for antibiotics (SD, SMX, TMP, CIP and ENO) than NSAIDs (<20%) via adsorption and biodegradation under different operating conditions. Based on a series of batch studies, fluoroquinolone antibiotics (CIP and ENO) were instantly (<15 min) removed (∼98%) via adsorption on SRB sludge with adsorption coefficient (Kd) as high as 25.3 ± 1.8 L/g-suspended solids (SS). And thermodynamics results indicated that the adsorption of CIP and ENO on SRB sludge was spontaneous (Gibbs free energy change (ΔG°) <0 kJ/mol), exothermic (enthalpy change (ΔH°) <0 kJ/mol), and the adsorption process involved both physisorption and chemisorption (absolute value of ΔH° = 20 to 80 kJ/mol). Three widely prescribed antibiotics (SMX, TMP and CIP) were further investigated for their possible biodegradation pathways along with functional enzymes involved through a series of batch experiments. The biotransformation intermediates indicated that biotransformations of SMX and CIP in SRB sludge system could be initiated from the cleavage of isoxazole and piperazinyl rings catalyzed by sulfite reductase (SR) and cytochrome P450 (CYP450) enzymes, respectively. TMP was likely biotransformed via O-demethylation and N-acetylation coupled with hydroxylation reactions with CYP450 enzymes as the main functional enzymes. This study provided new insight into PhAs removal in SRB sludge system, and has significant potential of implementing sulfur-mediated biological process for the treatment of PhAs containing wastewater.