Application of sludge biochar combined with peroxydisulfate to degrade fluoroquinolones: Efficiency, mechanisms and implication for ISCO

Peroxydisulfate (PDS) is increasingly used for in situ chemical oxidation (ISCO) of organic pollutants in groundwater, but the efficient and applicable activator is still scarce. In this study, sludge-derived biochar (SDBC) was prepared by pyrolysis to activate PDS, which could effectively degrade the fluoroquinolone antibiotics (FQs, levofloxacin, enrofloxacin, norfloxacin and ciprofloxacin). Compared with pig manure and corn straw derived biochar, SDBC showed higher efficiency in PDS activation. Singlet oxygen (1O2) was identified as the major reactive species, and the surface-bonded radicals also contributed to the FQs degradation. The selective oxidation of FQs by 1O2 was first reported, which followed the trend of enrofloxacin ~ levofloxacin > norfloxacin ~ ciprofloxacin. The CO and Fe2+ on SDBC were the dominant reactive sites for PDS activating. Products analysis revealed that FQs degradation proceeds via the cleavage of the piperazine ring, breaking of the quinolone ring, decarboxylation, and defluorination. Moreover, the tertiary amine of N (4) on enrofloxacin was more reactive towards singlet oxygen than the secondary amine of N (4) on ciprofloxacin, inducing the faster degradation and de-toxicity of enrofloxacin in the reaction system. SDBC showed high reusability in PDS activation and negligible metals leachates were detected. The column study proved the efficiency of PDS/SDBC in groundwater remediation. [Display omitted] •SDBC showed higher efficiency in PDS activation than manure and straw biochars.•FQs degraded through nonradical process dominated by 1O2.•1O2 was more selective to react with the tertiary amine than the secondary amine.•SDBC was stable and reusable for application in column experiments.