Formation and transformation of green rust and its influence on the mobility of trace elements in the environment

Green rust is an iron oxyhydroxide mineral phase which forms in natural soils under reducing conditions. In addition, this mineral is an important product of iron metal corrosion in permeable zero-valent iron barriers, which are a novel remediation technology being used to decontaminate groundwaters of radionuclide, toxic metal and organic contaminants. Green rust generally consists of minute particles - nanoparticles - that have a very high surface area which gives them the ability to absorb a high concentration of species from solution. The formation of green rust can occur via both abiotic and biotic pathways forming a mineral structure containing both the reduced and oxidised forms of iron i.e. Fe(II) and Fe(III). The high surface area and presence of reduced iron within its structure make green rust an important reducing agent of both inorganic (e.g. uranium) and organic (e.g. tetrachloroethene) species within reducing and sub-oxic environments. This is particularly important for contaminant species which can be immobilised during such a reduction process (e.g. chromium). However, despite the hypothesised importance of green rust in natural and contaminated systems the contribution of green rusts to the biogeochemical cycle of iron has so far not been quantified. This is primarily due to the highly reactive nature of green rust, which means that the mineral breaks down within minutes when in contact with air. The characterisation of this phase has therefore been problematic using conventional analytical techniques. The aim of this project is to obtain quantitative data on the kinetics and mechanisms of GR formation and oxidative transformation using state-of-the-art in situ synchrotron-based techniques. In conjunction with this we will examine how the speciation i.e. oxidation state and nature of binding to the mineral, of trace elements (e.g. U and Cr) changes as the mineral particles growth and then transform during oxidation. By application of novel synchrotron based techniques we will be able for the first time to monitor these reactions in situ. This will provide high quality novel data on the reactions and also minimise the need to prepare the material for off-line analysis, which may cause oxidation artefacts to occur. During the project we will answer the following questions: 1. How does green rust nucleate and grow? 2. What controls the transformation of green rust to Fe3+-oxyhydroxides during oxidation? 3. What determines the speciation of trace elements associated with green rust as it forms and transforms during oxidation? 4. How do biogenic processes affect green rust formation and trace element speciation? 5. Under what environmental conditions does green rust form and how does this effect trace element and contaminant mobility in the environment? The first 4 objectives will consist of extensive experimental studies examining green rust under a variety of conditions analogous to those found in the natural environment. To answer question 5, the data from the experimental programme will be incorporated into geochemical computer modelling packages which will allow us to predict how green rust behaves in both natural system and contaminated land scenarios. For example, it will allow us to perform modelling under the conditions that green rust will form within a simulated nuclear waste repository so we can quantify the affect this phase will have on the mobility and bioavailability of uranium.