Ammonia Oxidation Driven Iron Reduction And Its Effects On Arsenic Mobilization During The Incubation Of High As Aquifer Sediments

High arsenic(As)groundwater is a worldwide environmental geologic issue.Reductive dissolution of As bearing iron minerals plays a key role in leading to the groundwater As enrichment.Positive correlation can be widely found between dissolved As and NH₄⁺concentration in typical high As groundwater.Ammonia oxidation coupled with Fe(III)reduction(termed as Feammox),has attracted increasing due to its important part of the global nitrogen cycle.However,the occurrence of Feammox and its contribution to the As mobilization in high As aquifers still remains to be explored,which is of great significance to a comprehensive understanding of the arsenic enrichment mechanism in groundwater,and is critical ensuring the quality of local drinking water and the development of national economy and society.In this study,the Hetao Basin in Inner Mongolia,a typical inland basin hosting high arsenic groundwater,was selected as the research area.High As aquifer sediments in the study area were selected to conduct the anaerobic microcosms experiments with the artificial groundwater(configured with the characteristics of groundwater in the study area),¹⁵NH₄⁺and different organic carbon sources(glucose,lactate,acetate)The stoichiometry of ammonium consumption and Fe(?)production,contributions of Feammox,and the microorganisms associated with Feammox were evaluated.The following conclusions are noted.(1)The Feammox dose occur in high As aquifer system,and the addition of carbon source can promote the Feammox.In the sediments incubated with NH₄⁺source,the dissolved NH₄⁺decreased while the total Fe(?)production increased.The change of gaseous?¹⁵N in the sediments incubated with ¹⁵NH₄⁺source was consistent with the consumption of ammonium.Stoichiometric analysis of NH₄⁺oxidation and total Fe(?)production confirmed the occurrence of the Feammox occurs in the sediment system from high As aquifer,and the ammonia oxidation tends to induce the N₂product with minor products of NO₃^-and NO₂^-.Organic carbon source enhanced the Fe(?)production,NH₄⁺oxidation and the increase rate of gaseous?¹⁵N compared with the series without organic carbon source,especially glucose treated experiments.(2)The enhancement of As mobilization by Feammox was observed.In the sediments incubates with NH₄⁺source,the increased total dissolved As was found as the function of incubation time.The As release was positively correlated with Fe(III)reduction and NH₄⁺oxidation,indicating that Fe(III)reduction induced by ammonia oxidation would drive arsenic release.At the later stage of culture,about 50%of As release was resulted from the Feammox.(3)The quatative PCR analysis of 16S r RNA gene showed that the total copies of 16S r RNA gene obviously increased after incubation,inferring the Feammox as a biological process.Tricococcus and Clostridium enriched in the sediments incubated with NH₄⁺and organic carbon source,and the relative abundance of Tricococcus and Clostridium were positive correlated with ammonia oxidation,iron reduction and arsenic release,suggesting their potential role in catalyzing the Feammox.