Impact of environmental conditions on the sorption behavior of radiocobalt onto montmorillonite

Transport of oxytetracycline through saturated porous media: role of surface chemical heterogeneity

The current state of knowledge on the transport behaviors of oxytetracycline (OTC, a typical tetracycline antibiotic) in porous media with heterogeneous chemical surfaces is inadequate. In this work, the mobility properties of OTC through saturated porous media with different chemical heterogeneities (i.e., quartz sand, montmorillonite (MMT)-, humic acid (HA)-, and goethite (Goe)-coated sands) were investigated. In comparison with the mobility of OTC in the quartz sand, HA and goethite coatings inhibited the mobility of OTC, whereas montmorillonite coating enhanced OTC mobility. HA coating inhibited the transport of OTC that stemmed from the strong interactions between HA and OTC via complexation, π-π stacking, hydrogen bonding, and hydrophobic interaction. The positively charged iron oxide coating on Goe-coated sand provided favorable sites for OTC deposition through complexation and electrostatic attraction. The enhanced transport of OTC through MMT-coated sand was mainly due to the strong electrostatic repulsion between the anionic OTC species (i.e., OTC^-) and negatively charged porous media. Solution pH (5.0-9.0) posed a negligible effect on the trend of OTC mobility in different porous media. Furthermore, Ca²⁺ inhibited the transport of OTC mobility through various porous media via cation-bridging. The findings of this work contribute significantly to our understanding of the influence of aquifer surface chemical heterogeneities on OTC mobility behaviors in the subsurface environment.

Colloid-mediated transport of tetracycline in saturated porous media: Comparison between ferrihydrite and montmorillonite

Given the ubiquitous mineral (e.g., clays and iron oxides) playing critical roles in impacting the fate of antibiotics in the subsurface environment, the effects of two mineral colloids (i.e., ferrihydrite and montmorillonite) on tetracycline (TC, a representative of antibiotic) transport in sand columns were investigated in this study. Interestingly, the results clearly showed that ferrihydrite colloids inhibited TC transport, while montmorillonite colloids enhanced TC mobility under neutral conditions (pH 7.0). This phenomenon resulted from the positively charged ferrihydrite colloids with weak mobility, which assisted TC deposition; besides, providing additional deposition sites for TC by the deposited ferrihydrite colloids was another important mechanism. In contrast, the transport-enhancement effect of montmorillonite on TC was attributed to the strong binding affinity of TC to clay particles as well as the competition between colloids and TC for deposition sites on sand surfaces. Moreover, the transport-inhibition effect of ferrihydrite at pH 7.0 was greater than that at pH 5.0, mainly due to more colloid-associated TC under neutral conditions. Surprisingly, ferrihydrite colloids could act as carriers of antibiotics and enhanced TC transport at pH 9.0. Because the surface charge of colloids was altered to negative and could break through the column. Meanwhile, the transport-enhancement effect of montmorillonite decreased with increasing pH from 5.0 to 9.0, resulting from the decrease of colloid-adsorbed TC. Furthermore, colloid-mediated transport of TC was influenced by ionic strength, which affected the aggregation characteristics of colloids and the binding affinities of TC to minerals. These findings provide critical information for assessing the risks of antibiotics in aquatic ecosystems where abundant natural minerals are present.