Temperature-dependent co-transport behavior of goethite, Fe2+, and antibiotic in the hyporheic zone

The hyporheic zone is a crucial ecohydrological interface that plays a substantial role in the biogeochemical activity of iron and its mediated pollutant conversion. It is significantly influenced by dissolved oxygen and temperature fluctuations, but the combined effects and mechanisms are unknown. In this study, the co-transport behavior of goethite colloid (Goe), aqueous Fe 2+ and oxytetracycline (OTC) in groundwater discharge was simulated by column experiments. Our findings reveal that compared with room temperature (25 °C), the penetration rates of these compounds were generally promoted (0.1–7.0 % Goe, 14.1–43.1 % Fe 2+ , 0–19.6 % OTC) at low temperature (10 °C) but inhibited (0–5.0 % Goe, 0–51.0 % Fe 2+ , 0–3.8 % OTC) at high temperature (35 °C). At room temperature (25 °C), only 5 % of the Goe can penetrate the triadic transport system, where the Fe-OTC complex decreased the Zeta potential of Goe, hence improving its transport capacity. Compared with the penetration of individual Fe 2+ , the Fe 2+ transport was increased by 13.2 % due to the promoting effect of OTC on Fe redox cycling, whereas the electron transfer effect between Goe and Fe 2+ inhibited the transport by 46.6 %. The impact of μg/L OTC on the migration of Fe and Goe was dramatically diminished compared to the mg/L level. OTC was eliminated mainly by complex internal oxidation with Fe, weak adsorption, chemisorption, and hydroxyl degradation effects, but these were diminished at low temperatures while intensified at high temperatures. This study provides a deeper understanding of the intricate mechanisms of Fe and antibiotic transport in hyporheic zones, highlighting the significant roles of temperature and chemical interactions, particularly during seasonal changes.