Microplastic-Facilitated Transport of Emerging Contaminants in Redox-Active Environments

Microplastics (MPs) are ubiquitous environmental contaminants that not only accumulate in sediments and biota but also act as vectors for toxic substances, facilitating the transport and dispersion of hazardous chemicals within ecosystems. Their environmental impact is exacerbated by their propensity to adsorb pollutants from their surroundings, a process influenced by the physicochemical properties of both the polymers and the contaminants. While MPs typically exhibit limited surface porosity, exposure to natural environmental factors, such as weathering, abrasion, and photo-oxidation, significantly alters their surface characteristics. These transformations result in structural modifications, including the incorporation of oxygen-containing functional groups, sulfhydryl groups, and persistent free radicals. Such alterations lead to the generation of negatively charged surfaces, which enhance the adsorption of metal cations and other pollutants, thereby amplifying their environmental persistence and toxicity.

This study aimed to investigate the distribution of MPs and their interactions between surface water (SW) and groundwater (GW) systems, with a particular focus on understanding how redox potential (ORP), and surface modifications influence their structural transformations, pollutant adsorption mechanisms, and role as carriers of hazardous substances within natural ecosystems. SW and GW samples were collected from various locations across Uttarakhand, India. In-situ parameters, including pH, conductivity, TDS, DO, temperature, salinity, pressure, ORP, turbidity, and alkalinity, were measured using a portable multiparameter probe (HANNA-HI9829-01201). The average concentration of MPs in the GW and SW samples was found to be 34 MPs/L and 29 MPs/L, respectively. In SW, the relationship between MPs and ORP appears less direct but is still influenced by ionic parameters such as conductivity and TDS, reflecting the potential for pollutant adsorption in regions with high redox activity. In GW, MPs exhibit a moderate correlation with ORP and alkalinity, suggesting that redox conditions may play a significant role in their behavior or interaction with other pollutants. Notably, pH and ORP were clustered together in both GW and SW, suggesting a link between acidity/alkalinity and redox conditions driven by shared environmental or geochemical processes. This research provides key insights into MPs' behavior, aiding strategies to combat water pollution and guide policies to protect ecosystems and public health.

Keywords: microplastics; redox; emerging contaminants; transport; interactions.