Читать книгу X-Ray Fluorescence in Biological Sciences - Группа авторов - Страница 69

Micro‐organisms are key‐drivers of carbon‐, nitrogen‐, sulfur‐, and metal‐cycling on Earth, and they directly and indirectly drive element cycling [10]. Micro‐organisms can influence the cycling process of heavy metal elements through dissolution, adsorption, transformation, and other processes. XRF and XAS analysis techniques play important roles in revealing the mechanism of heavy metals migration between water, soils, plants, and gases. The toxicity of heavy metals is mainly influenced by its chemical species and mobility.

XANES analysis, used to detect the species of As, can help to understand the redox mechanism of arsenic, the circulation of arsenic between solid and liquid phases, and the release mechanism of arsenic from sediments into water. Under reducing conditions, 74–85% of the arsenic exists as As (III), which has a high ecological risk. After switching to a toxic condition, the native micro‐organism can oxidize As (III) to As (V) and adsorb it to the solid phase, indicating that oxidation conditions can reduce the solubility and toxicity of As [11].

Iron‐bearing minerals have strong adsorption capacity for heavy metals. The interaction between micro‐organisms and iron minerals is of great significance to the cycle of heavy metals in the environment. Arsenate is strongly associated with soil minerals, particularly iron(hydr)oxides [12]. The catalytic oxidation of iron has an important effect on the adsorption and chemical species of arsenate. Analysis of XRD and XANES showed that Shewanella putrefaciens could oxidize Fe (II) to feroxyhyte (δ^′‐FeOOH) and goethite (α‐FeOOH), while Sb(V) was reduced to Sb (III) and adsorbed on the surface of these secondary Fe(III) oxides, which can effectively reduce the toxic effect of Sb in aqueous solutions [13]. The mine water contained high concentrations of iron and other heavy metals, and the formation of iron‐containing biofilms may affect the cycling of heavy metals. 16S rDNA, TEM, XRD, XANES, and scanning‐particle induced X‐ray emission analysis (S‐PIXE) results revealed that under the action of Gallionella sp., the concentration of Fe and As decreased significantly, accompanied by the formation of a yellowish biofilm precipitate. The precipitate was confirmed to be schwertmannite containing Fe, As, and S, and consequently As was deposited in the mineral. These results indicated that Gallionella Sp. could promote the formation of iron‐bearing minerals, which improved the adsorption capacity to arsenic [14].

In the reduction condition, some micro‐organisms may promote the release of arsenic. In anoxic sediments, Geobacter sp. one can use Fe (III) as the electron acceptor to reduce Fe (III) to Fe (II) to dissolve iron‐bearing minerals, accompanied by reduction of As (V) to As (III), so the As originally adsorbed on the surface of iron‐bearing minerals was dissolved and released into the aquatic environment, increasing the ecological risk of As entering the food chain [15].