Arsenic mobility in suboxic environments: the role of phosphate competition and green rust mineralization
The mechanisms controlling arsenic mobility, including competition with oxyanion ortho-phosphate, in the subsurface and in laboratory controlled experiments were investigated. Additionally, the solubility and meta-stability of GR as a sorbing surface for arsenic in sub-oxic environments was examined. A systems approach was employed to: 1) investigate a model system constrained by thermodynamic constants and a well defined field site, 2) examine a synthetic system to elucidated the effects of co-precipitating oxyanions of arsenic and phosphate with GR, a scenario expected as an environment transitions from oxic to suboxic conditions, and 3) to study the abiotic conditions where GR formation is favorable in suboxic conditions. A systems approach, defined here as the method of investigating a problem by looking at individual but inter-dependent components in the broader context of a larger interdisciplinary environment, was employed to investigate GR in natural and laboratory controlled settings. This research has investigated the mineralogical response, and subsequent surface complexation reactions, to a redox change from oxic to sub oxic conditions. A redox gradient is commonly encountered in nature, observed in wetlands and from flooding sediments. These findings can be applied to a contaminant management plan, and may be important when considering the anticipated precipitation pattern changes due to global climate change. The overall goal of this study was to characterize mineralogical structural changes and solubilities of the solid phase iron sorbate host and the dissolved constituents (incl. nutrients and contaminants) in suboxic environments. Generally, suboxic environments are found in wetlands and submerged soils. Because wetlands accumulate dissolved ions from a vast watershed, a geochemical response to a hydrologic perturbation will likely affect contaminant and nutrient cycling in the region, and should be included in any management response. Iron is one of the most abundant elements in soils and the most abundant redox active element in natural systems, is generally dissolved as a reduced species (FeII) and insoluble as an oxidized species (FeIII). Therefore, iron transitions are principally controlled by the availability of oxygen. The result is a variety of iron oxide and hydroxide mineral phases that are stable and meta-stable under different soil PO2 conditions. Due to the transient variability between the aqueous Fe(II) and solid phase Fe(III) redox states, iron plays a major role in controlling the nutrient and contaminant cycling in subsurface environment. This research has investigated green rusts that may form at the redox boundary between fully oxidized and sub oxic sediments (Root et al. 2007, 2009). A primary goal of this research was to increase the scientific knowledge base concerning arsenic clean up and remediation, removal from the drinking water supply, and sequestration in the solid phase thereby isolating arsenic from uptake into the biota.