Hydrodynamic effects on sulfamethoxazole adsorption on river sediments: Insights from bench-scale and flume experiments

River sediments play a crucial role in controlling the adsorption of contaminants in aquatic environments and act as major sinks for a wide range of organic pollutants, thereby significantly influencing the environmental fate of contaminants. In natural river systems, contaminant-sediment interactions occur under dynamic hydrodynamic conditions, which can alter mass transfer and adsorption processes. However, most existing studies rely on static batch experiments and therefore fail to capture flow-induced effects on contaminant adsorption by sediments. This limitation restricts the understanding of adsorption behavior under realistic flow conditions. In this study, sulfamethoxazole (SMX) is selected as a representative emerging contaminant to investigate how sediment properties and flow regimes jointly regulate adsorption behavior by integrating bench-scale tests with flume experiments.

Bench-scale results revealed a discrepancy between predicted and observed adsorption effectiveness among four sediments (Rebolim, Figueira da Foz, Doñana, and Mira). Mineralogical assessments suggested superior performance of sediments rich in reactive minerals (e.g., smectites), particularly those from Figueira da Foz. However, experimental results identified the sediments from Rebolim as the most effective adsorbent. This discrepancy indicates that the presence and accessibility of organic matter (OM), rather than mineral abundance alone, can govern adsorption performance. Notably, the removal of OM significantly reduced adsorption capacity, confirming its dominant role in SMX uptake. Furthermore, the results highlight a distinction between adsorption kinetics and ultimate capacity, as some sediments exhibited rapid initial uptake but limited long-term adsorption potential.

Flume experiments further demonstrated that hydrodynamic conditions fundamentally reshape the spatiotemporal distribution of SMX. In low-flow regimes, transport follows a classical advective-dispersive model with clear longitudinal gradients. Conversely, high-flow regimes induce intense turbulence, leading to near-instantaneous vertical and longitudinal homogenization. Crucially, a non-monotonic relationship was observed between flow velocity and SMX attenuation: moderate turbulence enhances adsorption by increasing contact frequency at the sediment-water interface, whereas high velocities inhibit net adsorption due to hydrodynamic flushing and reduced residence time.

These findings provide a more comprehensive framework for understanding the transport and adsorption fate of emerging contaminants in riverine systems. Future work will extend the current steady-state flow conditions to unsteady flow regimes to better understand the adsorption behavior under dynamic hydraulic conditions.