Nano- and Macroscale Physico-chemical Processes Impacting Arsenic Mobilization

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Managed aquifer recharge (MAR) is one water reuse technique with the potential to meet growing water demands. In MAR, reclaimed wastewater is injected into aquifer formations for later use. Although filtration and adsorption in the vadose zone and underlying aquifer can remove some contaminants from reclaimed water, unfavorable soil-water interactions can mobilize arsenic from arsenic-bearing aquifer formations. This work will investigate how the interactions between water and arsenic-bearing pyrite, such as arsenopyrite/arsenian pyrite, impact arsenic mobility during MAR, and investigators will provide new quantitative and qualitative fundamental information on the redox-promoted dissolution mechanisms of arsenic-bearing pyrite and the consequent nucleation, growth, and phase transformation of iron (hydr)oxide nanoparticles, which are closely linked with arsenic mobilization and attenuation. By combining novel multidisciplinary approaches and in situ observations, including atomic force microscopy using an electrochemical control, small angle X-ray scattering, and batch reactor experimental measurements, they will obtain quantitative parameters and clearer qualitative descriptions of the thermodynamics and kinetics of initial iron (hydr)oxide nuclei evolution at the mineral-water interface. For the first time, new information on nucleation will be incorporated into geochemical reactive transport models to improve the prediction accuracy. The experimental results will also be compared with available data from pilot-scale column experiments at the U.S. EPA's Test & Evaluation Facility in Cincinnati, OH, and from MAR field sites. The results will help determine pretreatment requirements for reclaimed water sources and will provide a basis for developing more sustainable MAR operation guidelines. Beyond this application, the knowledge gained can be applied to related geochemical systems, including regions struggling with pervasive arsenic contamination of groundwater and environments where quantification of arsenic sorption onto nanoscale iron (hydr)oxide precipitates is a source of great uncertainty. The proposed outreach plan will provide educational, research, public engagement, and professional development opportunities for middle school, high school, undergraduate, and graduate students. It will have far-reaching societal impacts. In addition, the participation of traditionally underrepresented students will be encouraged. To achieve this goal, investigators will develop a "Hot Topics" website and related workshops focused on water quality concepts, in collaboration with Washington University's Institute for School Partnership and teachers from St. Louis' middle and high schools. To encourage high school and undergraduate students' early involvement in science and engineering, they will provide scientific research projects and offer public lectures on water quality.