Effect of organic carbon and pH on soil sorption of sulfamethazine

Batch sorption of sulfamethazine was conducted using five soils with organic carbon (OC) contents ranging from 0.1% to 3.8% and solution pHs ranging from 5.5 to 9. Sorption of sulfamethazine was found to be impacted by OC, soil surface area and soil solution pH, with higher K d values for soils with higher OC and lower K d values as the pH increased. However, OC was found to be the more dominant parameter. Linear partition coefficients at pH 5.5 were found to be 0.58 ± 0.17 L kg −1 for soil with 0.1% OC and 3.91 ± 0.36 L kg −1 for soil with 3.8% OC. At pH 9, the K d values were found to decrease by more than 50% to 0.23 ± 0.06 L kg −1 (soil with 0.1% OC) and 1.16 ± 0.05 L kg −1 (soil with 3.8% OC). Hydrophobic sorption was probably involved for pH < 7.4 (p K a,2 = 7.4 for sulfamethazine) due to the non-ionized form of sulfamethazine while surface sorption was probably involved for pH > 7.4 due to the ionized form of sulfamethazine. This was confirmed by regressing the estimated sorption coefficients of cationic, uncharged, and anionic species against the soil properties. A stepwise linear regression model incorporating the anionic fraction of sulfamethazine ionization and soil properties were developed and were found to estimate the K d values of other studies using soils of different pH and soil properties.