Insight into the effect of phosphate on ferrihydrite colloid-mediated transport of tetracycline in saturated porous media

Colloid-mediated contaminant mobility is absolutely critical for the environmental behavior of contaminants such as antibiotics in water resources. In this study, the influences of phosphate (a commonly inorganic ligand in the environment) on the ferrihydrite colloid-mediated transport of tetracycline (TC, a typical antibiotic) in porous media were investigated. In the absence of colloids, phosphate promoted TC mobility due to the competitive deposition of phosphate and TC on the sand surface as well as the electrostatic repulsion. Interestingly, ferrihydrite colloids could inhibit TC transport; however, the inhibitory effect of the colloids was weakened by the addition of phosphate. This phenomenon stemmed from colloid-associated TC mobility, the increased electrostatic repulsion induced by adsorbed phosphate, and deposition site competition effect. Another interesting finding was that the impacts of phosphate on the colloid-mediated mobility of TC were pH-dependent. That is, phosphate exhibited a weaker effect on the inhibitory role of ferrihydrite colloids in TC mobility at pH 5.0 than that at pH 7.0; specially, ferrihydrite colloids acted as possible carriers of TC and facilitated antibiotic transport at pH 9.0. The observations were ascribed to different influences of phosphate on the binding affinity of ferrihydrite toward TC and the mobility of free TC under different pH conditions. Therefore, the findings of this study provide useful information about the fate and co-transport of antibiotics and natural mineral colloids in the presence of inorganic ligands in the aquatic environment.