Synergistic effects of unsaturated flow and soil organic matter on retention and transport of PPCPs in soils

This study examines the effects of soil organic matter (SOM) and water content on the transport of five selected pharmaceutical and personal care products (PPCPs, ibuprofen, carbamazepine, bisphenol A, tetracycline, and ciprofloxacin) in four natural soils with different SOM contents. Batch isotherm experiment results showed that SOM effect was very significant for positively charged tetracycline and ciprofloxacin (>99% adsorption, no desorption), relatively significant for non-dissociated carbamazepine and bisphenol A (17–57% adsorption, 6–71% desorption) and insignificant for negatively charged ibuprofen (4–8% adsorption, 60–87% desorption) in the soils. Transport results showed that neither tetracycline nor ciprofloxacin moved through the saturated and unsaturated soil columns, demonstrating their very limited mobility in soils as a result of significant electrostatic attraction independent of SOM and water conditions. Overall, higher SOM content and lower water content were favorable to the retention of ibuprofen, carbamazepine and bisphenol A in the soils. Breakthrough of ibuprofen, carbamazepine and bisphenol A was 100% (both saturated and unsaturated), 94% (saturated)-97% (unsaturated) and 85% (saturated)-90% (unsaturated) in SOM-removed soils; however only 78% (saturated)-57% (unsaturated), 93% (saturated)-67% (unsaturated), 11% (saturated)-0% (unsaturated) in the SOM-high soils. The effect of water content was not significant in the SOM-removed soils. The SOM could increase the kinetic (type 2) adsorption of PPCPs at the solid-water interface (SWI), and the air phase could increase the instantaneous (type 1) adsorption of PPCPs at the air-water interface (AWI). This result suggests that lowering water content could greatly enhance the adsorption of PPCPs that had high affinities to soils and vice versa. This study provides an important implication that AWI and SWI might have a nonlinear relationship in promoting the adsorption and reducing the mobility of PPCPs under unsaturated flow conditions.