Understanding the Nano- and Macroscale Processes Impacting Arsenic Mobilization during Managed Aquifer Recharge using Reclaimed Wastewater
Date of Award
Doctor of Philosophy (PhD)
Managed aquifer recharge (MAR) utilizing reclaimed wastewater is a vital means of replenishing natural freshwater supplies in order to meet growing demands. Unfortunately, this process has been found to induce unfavorable geochemical reactions which mobilize arsenic from aquifer sediments. This process is further complicated by the simultaneous formation of iron(III) (hydr)oxide secondary minerals, which can sorb aqueous arsenic. It is crucial to fully elucidate these physico-chemical processes in order to establish safe MAR operations which minimize arsenic release. Thus, the mechanisms controlling arsenic release from arsenopyrite during MAR were investigated from the nano- to macroscale.
First, nanoscale iron(III) (hydr)oxide nucleation and growth was investigated in situ for aqueous systems relevant to MAR, including systems with arsenate, phosphate, and natural organic matter (NOM) present. It was found that oxyanions increased the growth of precipitates, while NOM induced large fractal aggregate formation. When arsenate and NOM existed together in solution with precipitating iron(III) (hydr)oxides, smaller aggregates and larger-sized particles both formed. We observed in situ that iron(III) (hydr)oxides formed in the presence of these constituents will have altered sizes and aggregation states. These changes will significantly affect their reactive surface area, subsequently impacting their capacity for arsenic attenuation in natural and engineered aquatic systems.
Next, arsenopyrite dissolution and secondary mineral precipitation and phase transformation were investigated at the microscale for wastewater and model wastewater systems. The effects of chloride, NOM, and Fe3+ ions were tested for model wastewaters. For the chloride system, faster aging of secondary mineral precipitates and higher arsenic mobility under aerobic conditions were observed. For NOM-containing systems, precipitation was inhibited. For Fe3+ systems, arsenic mobilization, secondary precipitation extent, and phase transformation were all accelerated. Newly reported information on secondary mineral phase transformation in the presence of different wastewater constituents gives important insight into how these minerals will interact with arsenic, potentially mitigating the risk of arsenic contamination.
Finally, arsenic mobilization from arsenopyrite at the macroscale was studied over a longer time frame in soil column reactors. After reaction, arsenic and iron solid phase speciation were determined using sequential extraction. Empirical dissolution rates were incorporated into CrunchFlow, a reactive transport simulator, to model arsenic mobilization and secondary mineral precipitation. Model calculations for aqueous arsenic concentrations and secondary mineral phase formation quantities were compared with experimental results, and recommendations were made to improve the model. Through this study, we demonstrated the importance of using quantitative arsenopyrite dissolution rates measured under MAR conditions in order to accurately predict arsenic mobilization. The development of better reactive transport models for arsenic mobilization will help to predict how site-specific mineralogy and MAR operating parameters can influence the degree of arsenic mobilization and transport in groundwater.
Outcomes from this study address critical knowledge gaps in our understanding of the geochemical conditions which mobilize naturally-occurring arsenic from sediments. Results are applicable not just to MAR operation, but also to acid mine drainage sites and locations with pervasive arsenic contamination of groundwater resources.
Daniel Giammar, John Fortner, Yinjie Tang, David Fike, Y. Jeffrey Yang