The Impacts of Environmental Parameters on the Evolving Reactivity of Hematite Nanoparticles

Abstract

Human activities have had a significant impact on our global ecosystems. Unsustainable patterns of consumption and production of natural resources have impacted our environmental systems creating pollution, altering climates, and decreasing biodiversity. One of these environmental systems impacted is our natural groundwater sources and aquifers. One-half of the drinking water needs, globally, are met by groundwater sources. As such, it is of utmost importance to understand how pollution in these systems can be remediated. Iron oxides are capable of facilitating the reduction of different types of groundwater pollutants such as heavy metals and nitroaromatic compounds. Given the variety of confounding variables that are present in natural aquifer systems, it becomes necessary to understand the interactions of iron oxides particles with different soil and groundwater constituents to inform the use of iron oxides as a remediation technique. This work focuses on filling knowledge gaps between well-controlled laboratory scale experiments and environmental field conditions. Results highlight the role of evolving minerology and reactivity of hematite, α-Fe2O3, during reactions with a model nitroaromatic compound, 4-chloronitrobenzene, and hexavalent chromium. Groundwater flow conditions on the reduction of 4-chloronitrobenze by Fe(II) through hematite-coated sands was investigated to identify the influence of water fluxes and residence time on chemical reactions. The influence of natural organic matter fractionation on hematite was studied on the reduction of 4-chloronitrobenzene by Fe(II) and the reduction of Cr(VI) by Fe(II). In all cases, kinetics of model pollutant reduction was summarized and used in tandem with solid-state characterization to understand mineralogical evolution and impacts thereof towards reactivity. Materials characterization techniques included X-ray diffraction, electron microscopy, and magnetometry to identify changes in minerals present and characterize mineral growth. In the case of continuous flow columns, results emphasis the importance of initial particle surface loading towards overall conversion of pollutant molecules. Variations in reactivity based on localization of natural organic matter, in the case of 4-chloronitrobenzene, help to inform understanding of the mechanism through which natural organic matter hindered reductivity ability. Rapidly occurring reduction by aqueous Fe(II), in the case of hexavalent chromium, highlights the minimal impacts of mineral surfaces in certain chemical reactions. The results of all these studies highlight the variety of ways that iron oxides can evolve during exposure to contaminates and the dependence of environmental factors on that evolution. Furthering this knowledge bases lays the foundation for identifying ways in which iron oxides may be used as nature-based solutions in responding to anthropogenic impacts on environmental systems. Additionally, growing our knowledge of subsurface reactivity of iron oxide minerals informs ways that native iron oxides may be evolving unintentionally due to chemical pollutants. Understanding this evolution, as well, will allow for better modeling of pollutant fate and projections of pollutant transport in our natural environments.