Sorption-desorption and transport of trimethoprim and sulfonamide antibiotics in agricultural soil: effect of soil type, dissolved organic matter, and pH.

Use of animal manure is a main source of veterinary pharmaceuticals (VPs) in soil and groundwater through a series of migration processes. The sorption-desorption and transport of four commonly used VPs including trimethoprim (TMP), sulfapyridine, sulfameter, and sulfadimethoxine were investigated in three soil layers taken from an agricultural field in Chongming Island China and two types of aqueous solution (0.01 M CaCl2 solution and wastewater treatment plant effluent). Results from sorption-desorption experiments showed that the sorption behavior of selected VPs conformed to the Freundlich isotherm equation. TMP exhibited higher distribution coefficients (K d = 6.73-9.21) than other sulfonamides (K d = 0.03-0.47), indicating a much stronger adsorption capacity of TMP. The percentage of desorption for TMP in a range of 8-12 % is not so high to be considered significant. Low pH (<pK a of tested VPs) and rich soil organic matter (e.g., 0-20 cm soil sample) had a positive impact on sorption of VPs. Slightly lower distribution coefficients were obtained for VPs in wastewater treatment plant (WWTP) effluent, which suggested that dissolved organic matter might affect their sorption behavior. Column studies indicated that the transport of VPs in the soil column was mainly influenced by sorption capacity. The weakly adsorbed sulfonamides had a high recovery rate (63.6-98.0 %) in the leachate, while the recovery rate of TMP was only 4.2-10.4 %. The sulfonamides and TMP exhibited stronger retaining capacity in 20-80 cm and 0-20 cm soil samples, respectively. The transport of VPs was slightly higher in the columns leached by WWTP effluent than by CaCl2 solution (0.01 M) due to their sorption interactions.