Reversible reduction-oxidation processes during the transformation of iron-containing minerals in rhizosphere soils

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The proposed research will lead to the discovery of the mechanisms used by microorganisms and plants to influence the movement of electrons in and out of minerals in the soil system. This knowledge is important for the management of the dynamic properties of soils for agriculture/forestry and environment. Our recent findings show that new clays arise from the alteration of iron-containing biotite and palygorskite soil minerals from the combined actions of plant roots and soil microorganisms in rhizosphere (or soil sticking on the roots). New minerals are synthesized when electrons are lost from biotite while palygorskite will lead to the crystallization of another mineral when the iron in its structure accepts electrons. Objectives – In general, the purpose is to understand the transformation of soil minerals and to discover means to apply this knowledge to conserve soil nutrients for plant growth and restoration of degraded lands arising from the extraction of ores, oil, gas and other natural resources. We want to determine the roles of bacteria, alone or in combination with fungus and plants, in moving electrons within the soil minerals, as well as in the detoxification of chromium-polluted soils. Toxic chromium-VI is rendered non-harmful as chromium-III after the addition of electrons into its structure. Soils are highly reactive materials and contain several sources/sinks of electrons including sulfur and iron-containing minerals. Scientific approach - We will use zero valent iron materials and the following minerals as sources/sinks of electrons in our experiments: biotite, palygorskite, smectite, chlorite to evaluate the role of fungus (e.g., Piloderma fallax) and bacterial strains (e.g., Acidothiobacillus ferrooxidans) in combination with agricultural crops to study the flow of electrons within the soil system. Inductively-coupled plasma mass spectroscopy will be used to characterize the inorganic and organic constituents of rhizosphere samples while molecular DNA sequencing techniques will be used to describe the soil organisms. X-ray and electron microscopy will be used to collect chemical and crystal information about the new minerals. Synchrotron radiation – a very powerful energy generated by bending the direction of very fast (accelerated) electrons will be used to cause changes in the structure of iron in minerals before and after the gain or loss of electrons. Some benefits of using synchrotron radiation include the very minimal sample preparation and its ability to work on very small amounts of sample (few milligrams). Our access to the Very Sensitive Elemental and Structural Probe Employing Radiation from a Synchrotron (VESPERS) beamline at the Canadian Light Source, the only synchrotron facility in Canada, has been continuous in the last four years, and is expected to continue for at least the next five years. Collaborators from Canada and abroad (e.g., École Normale Supérieure, France) will conduct specialized analysis such as quantification of various clay minerals and identification of soil organisms. Expected Significance – New information will advance knowledge about the biological and geochemical mechanisms involved in the loss/gain of electrons during the alterations of iron-containing minerals. Some of the benefits from the newly-formed expanding clays in rhizosphere soils include increases in essential nutrients (e.g., potassium) and cation exchange capacity (or the ability of soil to store nutrients). These will be useful to agriculture and forestry in Canada and elsewhere. In environmental applications, the detoxification of chromium-VI to chromium-III by gain of electrons from iron-rich materials will have practical applications for the remediation of chromium-contaminated soils.