Sorption of oxytetracycline in particulate organic matter in soils and sediments: Roles of pH, ionic strength and temperature

Particulate organic matter (POM) is a fraction of organic matter with dissimilar properties in different soils. POM isolated from soils and sediments (wetland, oil waste field, farmlands and aquaculture pond sediment) was used to study its sorption behavior on the antibiotic oxytetracycline (OTC). Impacts of solution pH, ionic strength and temperature on the OTC sorption were studied. The sorption rates of OTC in POM from wetland (POM-w) and farmland (POM-f1, POM-f2) were rapid during the first 3 h and gradually decreased with reaction time until reaching the equilibrium. Linear sorption occurred from 3 to 12 h in POM from oil waste field land (POM-o) and aquaculture pond sediment (POM-a). The organic carbon normalized partition coefficient (k) varied from 215.0 to 4493.6 L kg, and it was nearly 10× higher for the POM-w, POM-f1 and POM-f2 than in the POM-o and POM-a. Sorption of OTC by POM exhibited strong pH dependence. Ionic factors affected OTC sorption in POM-f1, POM-f2 and POM-a. The sorption capacity declined >50% in a solution with Ca compared to other ions with similar ionic strength. Sorption thermodynamics showed an entropy increasing and endothermic progress during the OTC sorption in POM, implying a spontaneous sorption process. Several mechanisms were involved in OTC sorption in POM, including hydrogen bonding, cation exchange, hydrophobic partitioning and surface complexation.