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The potential role of groundwater microplastics on arsenic cycling in alluvial aquifers: insights from links of DOM characteristics and microbial community
Summary
Scientists studying groundwater in China found an unexpected pattern: areas with more microplastic pollution actually had lower levels of arsenic, a toxic contaminant linked to cancer and other health problems. The likely explanation is that microplastics break down into particles that interact with bacteria and minerals in ways that may trap arsenic underground rather than letting it dissolve into water. While this is an early, observational finding that needs more direct testing, it suggests microplastic pollution could have surprising and complicated effects on other contaminants in our drinking water sources.
Groundwater microplastics (MPs) exhibit complex surface properties and produce microplastic-derived dissolved organic matter (MP-DOM), which may potentially interfere with subsurface elemental cycles. DOM can regulate microbial metabolism and further affect arsenic (As) behaviors in aquifers; however, the impacts of MPs and MP-DOM on As migration and transformation in alluvial aquifers remains poorly understood. Herein, we investigated high-As alluvial aquifer systems in the middle Yangtze River basin, using Fluorescence excitation-emission matrices (EEMs) spectra, Ultra high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), 16S rRNA gene sequencing and hydrochemical analysis to reveal the co-existing relationship between As and MPs as well as the potential cycling behavior of As in alluvial aquifers under MPs-containing groundwater conditions. Results indicated that confined groundwater (CG) suffered severe As contamination (14.1-240 μg/L) with relatively low MPs abundance, while unconfined groundwater (UG) had much higher MP concentrations (3.94 × 10 to 1.71 × 10 items/m) but lower As values (0.300-11.2 μg/L). EEMs and FT-ICR MS analysis revealed abundant lignins and specific molecular formulas with higher H/C ratios (>1.5) and lower O/C ratios (<0.50), typical features of MP-DOM. Based on correlation data, we propose that the formation of As-Fe-DOM ternary complexes serves as one plausible pathway for As immobilization, and hydrogen bonding and/or surface complexation between MPs and As may also retain dissolved As. Additionally, MPs may alter microbial communities and enrich As-tolerant and As-metabolizing bacteria (e.g., Methylomonas, Azospira, Acinetobacter) in CG. These functional bacteria can degrade MPs into MP-DOM, which may act as an electron donor and facilitate the reductive dissolution of Fe(III) oxides/oxyhydroxides. It is noteworthy that redox-driven As mobilization remains the dominant geochemical process in the study area, and above MPs-related pathways are only tentative inferences based on field correlations without direct experiment verification. This study provides new observational insights into the co-occurrence of MPs and As in alluvial aquifers and puts forward potential interaction mechanisms for further validation.