pH-dependent transport of tetracycline in saturated porous media: Single and combined effects of surfactants and iron oxide colloids.

Herein, sodium dodecyl sulfate (SDS) and rhamnolipid (Rha) were employed to investigate their influences on TC mobility and ferrihydrite colloid-mediated transport of TC at variable pH values (5.0-9.0). In the binary system, surfactants suppressed TC transport because of surfactants' bridging effects; similarly, ferrihydrite colloids also restrained TC mobility stemming from the colloid-associated TC retention. Interestingly, the degree of the inhibitory effects of colloids/surfactants increased with decreasing pH values. Surprisingly, the mutual influences of surfactants and colloids on TC movement displayed a strong pH dependence. Concretely, surfactants strengthened the repressive impacts of ferrihydrite colloids on TC mobility at pH 5.0 caused by the enhanced TC deposition on colloids attached to sand surfaces through the linking effects of surfactants. Nevertheless, at pH 7.0, adding surfactants reduced the repressive effects due to increased TC-colloid mobility and enhanced electrostatic repulsion. Unexpectedly, colloids accelerated the transport of TC with surfactants at pH 9.0 owing to colloids acting as TC carriers, the enhanced TC/TC species mobility, and competitive retention. Notably, SDS exhibited a greater effect on individual TC mobility or colloid-mediated TC transport than Rha at a certain pH, which was related to the different surfactant-binding abilities of sand grains/ferrihydrite colloids.