Transport of an Antibiotic Resistance Plasmid through Iron-Oxide-Coated Sand: Influence of the Anionic Hofmeister Effect and Coexisting Kaolinite Colloids

The transport of antibiotic resistance plasmids (ARPs) in porous media is a process closely related to the spread of antibiotic resistance among pathogens in the environment, an emerging global challenge. In this study, the mobility of an ARP under the Hofmeister effect and in the presence of clay colloids was explored with column experiments. An engineered plasmid (pBR322), kaolinite colloids, and three anions (NO3-, Cl-, SO4 (2)(-)), representing chaotropic, indifferent, and kosmotropic ions in the Hofrneister series, respectively, were employed. The study results show that the monovalent anions NO3- and Cl- had comparable effects on the ARP mobility, whereas with chaotropic NO3-, the ARP attained a greater negative zeta potential and particle size. Even though it is kosmotropic, the bivalent anion SO42- was more effective than NO3- in promoting the negative zeta potential and mobility of the ARP. Generally, the magnitude of the zeta potential and plasmid mobility correlated positively, both of which increased with increasing ionic strengths (ISs) in all solutions. The mobility of the ARP was promoted by kaolinite colloids in all solutions and to comparable levels, resulting in a percentage mass recovery of 22-25% by a colloid-facilitated transport process. These findings imply that high-salinity groundwater systems with high levels of clay colloids and SO42-, that is, those impacted by organic fertilizers, may he sites with a high risk for spreading ARPs.