Long-term Wastewater Irrigation Reduces Sulfamethoxazole Sorption, but Not Ciprofloxacin Binding, in Mexican Soils

As a consequence of population growth and urbanization, arable fields are increasingly irrigated with wastewater, but the related environmental and health risks (e. g., pollution with antibiotics) are poorly understood. We performed batch sorption experiments with sulfamethoxazole (SMX) and ciprofloxacin (CIP) and soils that had been irrigated with untreated wastewater for 0, 14, 35, and 100 yr. Sorption of CIP was always strong and largely irreversible irrespective of the duration of wastewater irrigation and the content and quality of soil organic matter (SOM) (Freundlich sorption coefficient, K-F: 346-979 mg(1-1/n) L-1/n kg(-1); 1/n: 0.62-0.76) but decreased with increasing soil pH due to a decreasing fraction of the cationic species. Sorption of SMX and sorption hysteresis were stronger in the nonirrigated soil (K-F: 4.14 mg(1-1/n) L-1/n kg(-1) +/- 0.02; 1/n: 0.69 =/- 0.02) than in the irrigated soils (K-F: 0.65-1.38 mg(1-1/n) L-1/n kg(-1); 1/n: 0.68-0.75). Irrigation (e. g., competition with SMX accumulated in soil or with other organic compounds contained in wastewater) and SOM quality (i.e., increase of carboxylic moieties with increasing time of irrigation) had a stronger effect on SMX sorption and its hysteresis than soil organic carbon content. Whereas sorption of SMX can be reduced by long-term irrigation with wastewater, sorption of CIP is intense also after prolonged irrigation.