The potential role of groundwater microplastics on arsenic cycling in alluvial aquifers: insights from links of DOM characteristics and microbial community.

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 × 103 to 1.71 × 104 items/m3) 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.