Sorption of oxytetracycline to microsized colloids under concentrated salt solution: A perspective on terrestrial-to-ocean transfer of antibiotics.

The sorption of antibiotics on soil minerals and their cotransport have been widely studied for the past few years; however, these processes in concentrated salt solutions (estuary-like conditions) are not fully understood. This study aims to determine the possible sorption of oxytetracycline (OTC) on various natural and synthesized microsized minerals (including haematite, goethite, kaolinite, bentonite, lateritic, kaolinitic and illitic soil clays) under conditions mimicking pure, fresh, brackish and sea waters. The sorption of OTC was found to decrease in surface charge (herein zeta potential), hence altering the colloidal properties of the materials used. The sorption capacities of soil clays for OTC follow the inequality illitic soil clay > kaolinitic soil clay > lateritic soil clay, and the sorption capacities were found to decrease at higher salt concentrations. Seawater can intensify the release of the sorbed OTC from soil clay surfaces while favouring the coaggregation of the remaining OTC with soil clays. This implies that the long-range transport of OTC or other similar antibiotics can be governed by the mineralogical composition/properties of the suspended particles. More importantly, increasing salt concentrations in estuaries may form a chemical barrier at which limited amounts of OTC/antibiotics can pass through, while the remaining OTC/antibiotics can be favoured to aggregate simultaneously with suspended mineral particles.

Coaggregation of micro polystyrene particles and suspended minerals under concentrated salt solution: A perspective of terrestrial-to-ocean transfer of microplastics.

This study evaluates the colloidal stability of polystyrene microplastics (PSMPs) in the presence of various mineral colloids. Although PSMPs were highly dispersive, they were found to be involved in the aggregation of each mineral colloid. The efficiency of mineral colloids to stimulate the coaggregation of PSMPs follows the order bentonite > kaolinitic soil clay > illitic soil clay > kaolinite > goethite > haematite. Surface charge density is likely a crucial factor that determines the efficiency of mineral colloids. In concentrated salt solution, PSMPs together with mineral colloids can be involved in various continuous and simultaneous electrochemical processes such as charge neutralization, double electric layer compression, van der Waals attraction stimulation and heteroaggregation. These processes may also occur in the estuary environments, where suspended mineral colloids may play an ultimate role in reducing the transport of microplastics into oceans while also intensifying microplastic enrichment in coastal sediments.