Coupled effects of pH and kaolinite colloids on antibiotic transport in porous media

Antibiotics can interact with natural colloids and the surrounding media upon entry into soil and groundwater systems, which significantly alters their dynamic behavior and complicates our understanding of antibiotic fate and transport in porous media. In this study, co-transport of antibiotics and kaolinite colloids was systematically investigated using combined column experiments and numerical simulation under different pH conditions. Sulfadiazine (SDZ) transport was enhanced by kaolinite colloids under neutral and alkaline conditions, which was attributed to the higher mobility of colloids as SDZ carriers, as well as competitive sorption. However, most injected SDZ was transported in a dissolved form owing to the low sorption capacity of SDZ to kaolinite colloids and quartz sand. The colloid-facilitated transport model provided a good description of total SDZ transport, but underestimated colloidal SDZ transport using parameters from kinetic sorption experiments. Kaolinite colloids significantly promoted ciprofloxacin (CIP) transport at pH 4.0, but inhibited it at pH 7.0 and 9.0. Interestingly, enhanced CIP transport was due to the decreased number of effective sorption sites on quartz sand and the increased desorption of CIP from kaolinite colloids. Under neutral and alkaline conditions, deposited colloids provided additional sorption sites for CIP, which contributed to CIP retention. Moreover, CIP significantly inhibited the transport of kaolinite colloids owing to the increases in colloidal aggregate size and zeta potential. Overall, our results highlighted the different effects of mobile and immobile colloids on antibiotic transport, in addition to the implications of antibiotic speciation and clay colloids when predicting the transport behavior of these compounds.