Effect Of Indigenous Arsenic-reducing Bacteria On Arsenic Biogeochemistry: A Case Study In The Hetao Basin, Inner Mongolia

Arsenic contamination in groundwater has become a worldwide environmentalissue, which is one of the hottest topics in the field of environmental study. The Hetaobasin is one of the typical regions with high arsenic groundwater in China.Groundwater has been used as drinking water and irrigation water. This has led tomany cases of chronic arsenic poisoning in this area. Previous studies haveinvestigated distribution, migration and transformation of groundwater As, andchemical and isotopic characteristics of high arsenic groundwater. More and morestudies have shown that microbes play an important role in the release, migration andtransformation of As in the aquifer sediments. However, few studies are conducted toinvestigate influence of microbiological processes on As speciation in the Hetao basin,Inner Mongolia. Therefore, it is important to understand the effect of biogeochemicalon arsenic migration and transformation.In order to obtain arsenic-resistant bacteria in high arsenic aquifers, aquifersediments were collected from the Hetao basin. Two strains of aerobic arsenic-resistant bacteria B and P (Bacillus sp. and Pseudomonas sp.) and one strain ofanaerobic arsenic-resistant bacteria C (Clostridium sp.) were isolated from the aquifersediments. Those strains grew in culture medium with high arsenic concentrations.Dissolved As(V), goethite-adsorbed As(V), scorodite and sediment As(V) were usedas the source of arsenic to reveal As(V) reduction and As mobilization in microcosmsystems with the presence of bacterial strains.Results showed that two strains of aerobic arsenic-resistant bacteria B and P had ahigh tolerance on dissolved As. In comparison with the As free treatment, themaximum biomass were identical for high As treatments (7.5mg/L As(V)/As(III)).Low nutrient condition was needed for their growth, with only sodium acetate as anorganic carbon source. While anaerobic strain had a lower tolerance on dissolved As.In comparison with the As free treatment, the maximum biomass reduced to21%inthe medium which had8mg/L As(V)/As(III). A higher nutrient condition was neededfor its growth, with beef extract and peptone as organic carbon sources.All strains used As(V) as electron acceptors. Both strain B and strain P led toreduction of dissolved As(V), goethite-adsorbed As(V), scorodite As(V) and sedimentAs(V), but they did not lead to Fe(III) reduction. However, strain C not only reduced dissolved As(V), but also Fe(III). In addition, strain C led to reduction of goethiteFe(III), scorodite Fe(III), sediment As(V) and Fe(III).After reduction of solid As(V), As(III) was transferred from the solids to solutions.The rate of As(V) reduction and the release of As in the sediments by strains aredifferent, with strain B> strain P> strain C. Studies indicated that these microbesplayed an important role in migration and transformation of As. With the presence ofstrains, As(V) in the sediment was reduced to mobile As(III). Then, As was releasedinto groundwater from sediments. The processes affected the transform and thetransportation of As in groundwater systems, and promoted As enrichment ingroundwaters.