Degradation of antibiotics in activated sludge by ionizing radiation: Effect of adsorption affinity of antibiotics

The degradation of emerging pollutants in activated sludge is more difficult than that in aqueous solution, which have received increasing attention in recent years because it is crucial for the safe disposal of sludge. In present work, the degradation of various antibiotics in activated sludge was investigated by ionizing radiation, focusing on the effect of adsorption affinity of antibiotics onto sludge. Among the four antibiotics selected, penicillin (PEG, β-lactam) and sulfamethoxazole (SMX, sulfonamide) displayed the low adsorption affinity to sludge with the solid-water partition coefficient (Kd) of 8.0 and 12.7 L/kg, while norfloxacin (NOF, quinoline) and oxytetracycline (OTC, tetracycline) displayed the high sorption affinities with Kd values as high as 1223 and 833 L/kg respectively. The higher the adsorption affinity of antibiotics to sludge, the lower the degradation rate of antibiotics. With the absorbed dose of 10 kGy, more than 97 % of SMX and PEG in the sludge was removed, while the removal of NOF and OTC was 40 % and 52 %. The G value for SMX and PEG was 3.6–5.3 times higher than that for OTC and NOF. The antibacterial activity of PEG-containing sludge abated sharply and disappeared at 10 kGy, while the antibacterial activity of OTC-containing sludge abated gradually and disappeared at 25 kGy. Since OTC and NOF can be decomposed efficiently by ionizing irradiation in different water matrices, including deionized water and supernatant of sludge with the degradation rate even higher than the degradation rate of SMX, the low removal of OTC and NOF was attributed to the high adsorption affinity onto sludge. During ionizing irradiation, ·OH played the major role in antibiotic degradation, especially for the antibiotics with the low adsorption capacity onto sludge like PEG and SMX. The competition of antibiotic degradation for radicals also abated the radicals used to disintegrate the microbial cells in sludge. As exposed to ionizing irradiation, the content of soluble compounds released from antibiotic-containing sludge, such as protein, polysaccharides and nucleic acids, was lower than that from sludge alone by 27 %–50 %.