Nanoscale zerovalent iron (nZVI) impact on soil microbial communities

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The release of particles of nanometre scale to the environment for the clean up of pollution is an emerging technology. At the forefront is the development of nanoscale zerovalent iron (nZVI) to clean up environments that have become polluted with chlorinated organic compounds. The nZVI works by removal of chlorine atoms from the pollutant, usually resulting in the production of a less toxic and less persistent compound. nZVI has been most commonly used for clean up of ground water, however, application to contaminated soil is now showing promise with the development of ways to deploy nZVI into the soil matrix without it sticking to the soil, or itself. Because of their dimensions, nZVI particles have the potential to interact at a scale relevant to microbes living in the soil, either directly through contact with microbial cells, or, indirectly through altering the chemical environment in which soil microbes live. It is important to assess whether nZVI has detrimental impacts on soil microbes because of the important functions the microbes carry out; these include biogeochemical cycling with resulting provision of nutrients to plants and the breakdown of organic pollutants. Thus, this proposal will test the impacts of nZVI on two important microbial groups; one involved in the decomposition of a major class of pollutants, the polychlorinated biphenyls (PCBs), and the other responsible for symbiotic improvement of plant growth and thus for enhancing and stabilising soils. The context of the study will be a two-step soil clean up scenario. The first step involves the use of nZVI to reduce the number of chlorines in the PCBs, which reduces their toxicity. The second, biological, step involves use of native soil microbes to decompose the nZVI-produced mono or dechlorinated products with subsequent planting of the soil to stabilise the site and stimulate beneficial pollutant decomposing microbes. For laboratory experiments, we will use commercially produced nZVI of different sizes and formulation. In consequence, we will address nZVI commercial applications whilst defining the environmental impact for a range of nZVI reactivities. In the course of our research we will answer the following questions: 1. Does nZVI impact on the numbers, activity and diversity of soil microbial communities responsible for the breakdown of pollutants? 2. Does nZVI impact on plant-microbe symbioses? Specifically: (i) legume symbiosis with symbiotic rhizobia bacteria responsible for formation of nitrogen-fixing nodules on plant roots; and, (ii) symbiotic arbuscular mycorrhizal fungi which form a 'fungus root' and benefit the plant host in many ways, including improvement of phosphorus nutrition. 3. Does the nZVI impact on soil microbial communities depend on the reactivity of nZVI and its environmental behaviour? We will assess the diversity of microbial communities using a nucleic-acid based fingerprinting method and measure their activity by quantifying the rate at which they decompose an added pollutant chemical. The impact on symbiosis will be determined by: (i) counting numbers of root nodules and the rate at which they fix nitrogen; and, (ii) the extent of root colonisation by arbuscular mycorrhizal fungi and plant phosphorus levels. We will manipulate nZVI reactivity by varying the size of the particles (bigger surface area = higher reactivity) and formulation (addition of a surfactant to stop them from agglomerating). We will see how the nZVI particles behave in soil by using an electron microscope. Also, we will apply to use the synchrotron 'super-microscope', a new facility in Oxfordshire, to assess its potential to look at nZVI agglomeration in an undisturbed soil situation.