Interactions of microplastics, dissolved organic matter, and coexisting pollutants: Mechanisms, environmental implications, and knowledge gaps

Microplastics (MPs) are an important type of emerging pollutants, which can affect the behavior and effects of dissolved organic matter (DOM) and other pollutants. However, a comprehensive review of the effects of MP-DOM interactions on coexisting pollutants is lacking, in particular for the dual roles of environmental DOM and MPs-derived DOM in regulating the pollutant behavior. This would significantly limit understanding of the behavior, fate, and risk of environmental pollutants. Therefore, this paper summarizes the adsorption behaviors of DOM on MPs, the influence of DOM on the interactions between MPs and coexisting pollutants, as well as the characteristics and effects of MPs-derived DOM (MPs-DOM). On one hand, DOM can be adsorbed by MPs through several mechanisms, such as hydrophobic partitioning, π-π interactions, and hydrogen bonding. This can be enhanced by the aging of MPs, which changes the surface properties of MPs. The influence of surrounding DOM on the interactions between MPs and coexisting pollutants is determined by the net effect of competitive adsorption/surface coverage (inhibition) and bridging effect (promotion). On the other hand, MPs can leach organic compounds into the DOM pool. The MPs-DOM is mainly composed of the CHO molecules (containing C, H, and O) and the unsaturated lignin-like compounds, along with protein-like and phenol-like fluorescent components. MPs-DOM has higher bioavailability (9.5 %-85 %) than natural organic matter, which can result in more greenhouse gas emissions. MPs-DOM can also affect the speciation, behavior, and fate of environmental pollutants by either complexing with them or affecting their degradation. However, several knowledge gaps remain unclear, and future research is required to reveal the different roles of biodegradable and conventional MPs in changing the DOM cycle, especially to identify the MPs-absorbed molecules, the biologically labile molecules of MPs-DOM, and the molecular-level coupling between photochemical and microbial degradation of MPs-DOM.