Pollutant Behavior at the Soil/Sediment–Water Interface: From Two to Multiple Phases

Public awareness and much research notwithstanding, pollution of aquatic systems with nutrients, heavy metals, organic contaminants, and microplastics remains a challenging problem. (1) A major difficulty in addressing water pollution is the complex biological and physicochemical behavior of the pollutants. Historically, studies of aquatic systems have focused on two distinct phases: the water and the soil/sediment. Having entered the water body, pollutants are subject to physical transport processes, mixing, dilution, and natural precipitation. Water flow or lack thereof is associated with changes in dissolved oxygen, driving redox reactions, such as the oxidation of heavy metal ions to form insoluble substances. Pollutants can also be degraded by microorganisms and adsorbed by colloidal particles in the water. (2) The adjacent soil/sediment interacts with this water through various processes such as ion exchange, sorption, and degradation (biotic and abiotic), thereby participating in the transport and transformation of pollutants. The frequent exchanges of mass and energy between the water and soil/sediment make the soil/sediment–water interface (SWI) a hot spot for the accumulation and transformation of pollutants. The SWI is also home to some of the most active and diverse biological components of the aquatic ecosystem and gives rise to the formation of diverse microbial aggregates (Figure 1a).