Biochar as a sorbent of ethofumesate and sulfamethoxazole in agricultural soil: changes of its sorption capacity over time

An important issue regarding the presence of pesticides and other organic pollutants in agricultural soils relates to their mobility, which may imply surface and groundwater contamination risks. Soil application of sorbents that increase the retention capacity can be an excellent strategy to reduce the dispersion of organic contaminants in the environment. Biochar is a sustainable carbon-rich product prepared by pyrolysis of biomass that has been proposed as a soil amendment due to its great sorption capacity for organic compounds. Furthermore, biochar has been proven to enhance CO2 sequestration, increase water retention, and supply nutrients. However, once in the soil, biochar undergoes biogeochemical changes with residence time (aging) that may alter its physicochemical properties and modify its sorption capacity. Therefore, research on biochar aging in the field and its simulation in laboratory experiments is of great interest. In this work, the sorption of the pesticide ethofumesate (ETFM) and the antibiotic of veterinary use sulfamethoxazole (SFMX) in an agricultural soil with low retention capacity amended with a commercial wood-derived biochar was assessed. In addition, the possible change of biochar sorption capacity over time was studied, ssimulating aging in the field by incubating the unamended soil and the soil amended with biochar (5%) at 40 ºC and a soil water content of 30% for 48 days. At selected times, the incubated soils were sampled and the sorption of the two compounds was determined by the batch technique. To evaluate sorption over time, 24 hdistribution coefficients, Kd (l/kg), were determined at each sampling time from the expression Kd = Cs/Caq, where Cs (mg/kg) is the amount sorbed and Caq (mg/l) is the concentration remaining in the aqueous phase. The results showed that the addition of biochar to the soil markedly increased the sorption of both ETFM (Kd = 168 ± 40 l/kg) and SFMX (Kd = 3.89 ± 0.58 l/kg) in comparison with the unamended so