Coupled effect of colloids and surface chemical heterogeneity on the transport of antibiotics in porous media

Release of antibiotics into the environment has caused ecological and human health concerns in recent years. However, little is known about their transport behaviors in chemically heterogeneous porous media. In this study, we investigated the coupled effects of surface chemistry and soil colloids on the transport of ciprofloxacin and tetracycline through sand under steady state saturated flow conditions. Both antibiotics had a much higher capacity of adsorption on soil colloids (17,500 mg/kg for ciprofloxacin and 8600 mg/kg for tetracycline) than on sand (5.11 mg/kg for ciprofloxacin and 2.80 mg/kg for tetracycline). However, ciprofloxacin adsorption increased to 8.91 mg/kg after the sand was coated with iron oxide and to 8.73 mg/kg after the sand was coated with humic acid. Tetracycline, adsorption increased to 7.99 mg/kg after sand was coated with iron oxide coated sand and to 8.35 mg/kg after the sand was coated with humic acid coated The high adsorption capacity of ciprofloxacin led to a recovery rate of <4% in the effluents of the columns containing 0%, 20% and 50% of iron oxide/humic acid coated sand. The surface coating decreased the recovery rates of tetracycline from 35.4% (in uncoated sand) to 12.0% (in column containing 50% iron oxide coated sand) and 0.010% (in column containing 50% humic acid coated sand), respectively. Once adsorbed to soil colloids, the recovery rate of ciprofloxacin increased by 26.7% in uncoated sand column, 21.1% in iron oxide coated sand column, and 32.7% in humic acid coated sand column. Similarly, the presence of the colloids increased the recovery rate of tetracycline from 13.8% to 33.2% after the sand was coated with humic acid. Colloids did not significantly influence the transport and recovery of tetracycline in the uncoated sand and iron oxide coated sand due likely to its lower adsorption affinity.