Transport of tetracycline in saturated porous media: combined functions of inorganic ligands and solution pH

Abstract

To date, there is still very little knowledge about the combined effects of typical inorganic ligands and solution pH values on mobility characteristics of tetracycline (TC) through saturated aquifer media. In this work, three typical inorganic ligands (i.e., phosphate, silicate, and iodate) were employed in the transport experiments. Generally, all the ligands promoted TC mobility over the pH range of 5.0–9.0 owing to the enhanced electrostatic repulsion between sand grains and TC anionic forms (i.e., TC⁻ and TC²⁻) as well as the competitive deposition between ligands and antibiotic molecules for attachment sites. Furthermore, the transport-enhancement effects of ligands on TC intensively depended on ligand type and followed the sequence of phosphate > silicate > iodate. This phenomenon was ascribed to their different molecular sizes and binding abilities to sand grains. Interestingly, the differences in extents of enhanced effects of various inorganic ligands on TC transport varied with background solution pH due to pH-induced different extents of deposition site competition effects. Moreover, the two-site nonequilibrium model (which accounts for an equilibrium site and a kinetic site) as well as adsorption and kinetic studies were performed to help interpret the controlling mechanisms for the synergistic effects of inorganic ligands and solution pH on TC transport in saturated quartz sand. The findings of our study clearly demonstrate that inorganic ligands may be critical factors in assessing the fate and transport of antibiotics in groundwater systems.