Ammonium-Enhanced Arsenic Mobilization from Aquifer Sediments

Area of Science:

• Environmental Microbiology
• Geochemistry
• Biogeochemistry

Background:

• Ammonium influences groundwater arsenic (As) enrichment, particularly through microbial iron(III) reduction coupled with anaerobic ammonium oxidation.
• The specific pathways and microorganisms driving ammonium-induced iron(III) reduction and subsequent arsenic mobilization remain largely unknown.

Purpose of the Study:

• To investigate the key microbial pathways and microorganisms responsible for ammonium-induced iron(III) reduction and arsenic mobilization in high arsenic groundwater.
• To quantify the contribution of ammonium to arsenic release in aquifer sediments.

Main Methods:

• Incubation of aquifer sediments with 15N-labeled ammonium and organic carbon sources (glucose, lactate, lactate/acetate).
• Measurement of ammonium, iron(II)tot, and arsenic concentrations.
• Stable isotope analysis (δ15N) of headspace N2.
• Genome-resolved metagenomic analysis to identify microbial players.

Main Results:

• Decreased ammonium concentrations correlated with increased iron(II)tot and released arsenic.
• Molar ratios of Fe(II)tot to oxidized ammonium ranged from 3.1 to 3.7.
• 15N-labeling confirmed N2 as the primary product of ammonium oxidation.
• Ammonium addition increased arsenic release by 16.1%–49.6%, enhanced by organic electron donors.
• Metagenomics indicated syntrophic interactions between bacterial iron(III) reducers and archaeal ammonium oxidizers.

Conclusions:

• Syntrophic ammonium-stimulated iron(III) reduction is a significant, underestimated driver of arsenic mobilization in high arsenic groundwater.
• This process involves a metabolic partnership between bacterial iron(III) reduction and archaeal ammonium oxidation.