Mobility of tetracycline in saturated porous media: Single and combined functions of ligands and ferrihydrite colloids

Considering that the ubiquitous organic/inorganic ligands and iron oxide minerals play significant roles in antibiotic fate in the aquifers. In this study, the influences of two ligands (i.e., citric acid and phosphate) on the transport of tetracycline (TC, a broad-spectrum antibiotic) with or without ferrihydrite colloids in quartz sand were explored. In the absence of ferrihydrite colloids, column studies indicated that the two ligands promoted TC transport and citric acid exhibited a greater transport-enhancement effect than phosphate. The observation was mainly ascribed to a stronger competition effect, steric hindrance, and electrostatic repulsion in the addition of citric acid compared to phosphate. Moreover, ferrihydrite colloids inhibited TC transport, which was mainly caused by contaminant retention associated with colloid deposition and the additional active sites for TC provided by the deposited iron oxide colloids with positive charges. Another interesting finding was that both ligands weakened the inhibitory influences of ferrihydrite colloids on the mobility of TC. This observation stemmed from the enhanced transport of free TC molecules (i.e., TC unassociated with colloids), colloids acting as carriers of TC, as well as the competitive deposition of TC − species and ferrihydrite colloids. Meanwhile, the extent of ferrihydrite colloid-mediated mobility of TC affected by citric acid was much greater than phosphate. Because citric acid exhibited a much greater influence on the interaction of colloids, TC molecules, and sand grains during the transport process. These results reveal the importance of accounting for the impacts of natural ligands on colloid-mediated transport of antibiotics in the subsurface systems.