Degradation kinetics and mechanism of sulfadiazine and sulfamethoxazole in an agricultural soil system with manure application

Recently, under the application of waste-water, manure and biosolids, antibiotics have been used massively in agriculture resulted in antibiotic resistance and potential environmental risks. In the present study, the removal of sulfadiazine (SDZ) and sulfamethoxazole (SMX) in an agricultural soil system was explored. All the experiments were conducted under different incubation conditions for 49 days. The experimental results indicated that all the degradation processes could effectively follow a first-order kinetic model. Based on the analyses of these two antibiotics, SDZ had a higher reaction rate and a shorter DT50 value. Additionally, there were no marked differences in DT50 values at varying initial concentrations under the same conditions (p > 0.05). Compared with the non-sterile soil, the degradation rates of SMX and SDZ were slower (< 70%), and the associated DT50 values (> 21 days) were higher in the sterile soil. Because the biodegradation played a major role, it may be effective for the removal of these contaminants from the soils. The processes of SDZ and SMX degradations were slightly accelerated by applying manure (< 20%). There were different accelerating effects on the removal of SDZ and SMX in soils by manure Single- and Repeated-application, which may be related to the amount of manure during the degradation processes, and different methods of adding manure could only affect the degradation rate. The major intermediate products were derived from the hydroxylation, sulfonamide SN bond cleavage and aniline moiety oxidation. Therefore, the present study inferred that possible degradation pathways of SDZ and SMX were hydroxylation of the benzene ring, oxidation of the amine group at the benzene ring, ring open and SN bond cleavage. Results revealed that more attention should be paid to the transformation products because they could be more toxic than the parent compounds.