Molecular Evidence of Arsenic Mobility Linked to Biodegradable Organic Matter

Abstract

Molecular characteristics of natural organic matter (NOM) and their potential connections to arsenic enrichment processes remain poorly understood. Here, we examine dissolved organic matter (DOM) in groundwater and water-soluble organic matter (WSOM) in aquifer sediments being depth-matched with groundwater samples from a typical arid–semiarid basin (Hetao Basin, China) hosting high arsenic groundwater. We used Fourier transform ion cyclotron resonance mass spectrometry to determine molecular characteristics of DOM and WSOM and evaluate potential roles of biodegradable compounds in microbially mediated arsenic mobility at the molecular level. High-arsenic groundwater DOM was generally enriched in recalcitrant molecules (including lignins and aromatic structures). Although potential contribution of recalcitrant compounds to arsenic enrichment cannot be ruled out, preferential degradation of the labile molecules coupled with reduction of Fe­(III) (oxyhydr)­oxides seemed to dominate arsenic mobilization. Both the number and the intensity of biodegradable compounds (including aliphatic/proteins and carbohydrates) were higher in WSOM than those in DOM in depth-matched high-arsenic groundwater (arsenic >0.67 μmol/L or 50 μg/L). Groundwater arsenic concentration generally increased with the increase in the number and the intensity of unique biodegradable compounds (especially N-containing compounds) in WSOM at matched depths. Anoxic incubations of sediments and deionized water show that more arsenic and Fe­(II) were released from aquifer sediments with greater numbers and intensities of consumed biodegradable compounds in WSOM (especially N-containing compounds), with a higher proportion of microbially derived compounds produced. These observations indicate that the biodegradation of aliphatic/proteins and carbohydrates (especially CHON formulas) in WSOM fueling the reductive dissolution of Fe­(III) (oxyhydr)­oxides predominantly promotes arsenic release from aquifer solids. Our unique data present a better understanding of arsenic mobilization shaped by microbial degradation of labile organic compounds in anoxic aquifers at the molecular level.

Understanding Microbial Arsenic-Mobilization in Multiple Aquifers: Insight from DNA and RNA Analyses

“…(14.5 ± 3.2 mg/kg) in the sediments could be produced via organic N-containing molecule mineralization and via the interplay between denitrification and ammonium oxidation. A previous study showed that more arsenic and Fe­(II) release was found from the anoxic incubations of sediments with greater numbers and intensities of N-containing compounds in water-soluble organic matter . SEM-EDS images showed the presence of Fe–S containing mineral particles (∼1 μm in diameter) (Figure S3), consistent with the fact that the monosulfide fraction was generally the highest reduced S fraction (most around 50%) in the study area .…”

“…(14.5 ± 3.2 mg/kg) in the sediments could be produced via organic N-containing molecule mineralization and via the interplay between denitrification and ammonium oxidation. A positive correlation between organic N-containing molecules and groundwater As was recently reported in the studied area . The undetectable nitrate and high molar ratios of ammonium/nitrite (46–4600) highlighted the potential importance of ammonium oxidation.…”

“…The N-species (ammonium, nitrite, and nitrate) could be produced via organic N-containing molecule mineralization and via the interplay between denitrification and ammonium oxidation in the studied aquifers. More As and Fe­(II) release was found from the anoxic incubations of sediments with greater numbers and intensities of N-containing compounds in water-soluble organic matter . Taken together, internal natural nitrogen (e.g., N-containing organic matter buried in clay layers) and associated biogeochemical processes were vital in supporting the intense nitrogen cycling.…”

Impact of sedimentation history for As distribution in Late Pleistocene-Holocene sediments in the Hetao Basin, China

“…Therefore, the reactive organic carbon and peat produced within paleolakes provide an easily available electron source for microbial reduction of Fe oxy(hydr)oxides (McArthur et al 2001;Fendorf et al 2010;Guo et al 2019). This is consistent with the higher Fe 2+ concentration in the groundwater that is high in dissolved As (Zhang et al 2020;Qiao et al 2020).…”

Arsenic dynamics in paddy soil under traditional manuring practices in Bangladesh

“…As mobilization between the soil depths followed the same trend as observed for DOC; higher As mobilization at 0 -5 compared to 5 -15 cm depth in FYM treated plots whereas higher As mobilization at 5 -15 cm depth compared to 0 -5 cm depth in the rice straw treated plots. Qiao et al (2020) also reported that type of biodegradable OM has good correlations with As mobilization in porewater. Nakamura and Katou (2011) also reported higher As concentrations in porewater at 12.5 cm depth compared to 2.5 cm depth even under prolonged submergence in paddy soil.…”