Interfacial Chemistry of Trace Elements at Mineral Surfaces in Engineered Water Systems.

摘要

This thesis research consists of two independent research projects that both studied interfacial chemical processes affecting trace elements at mineral surfaces.;The objectives of Project 1 were to 1) quantify the impact of water chemistry on As(III) adsorption on lepidocrocite, 2) develop a surface complexation model to describe equilibrium As(III) and As(V) adsorption to lepidocrocite and 3) elucidate the mechanism of Fe(II)-mediated As(III) oxidation at the lepidocrocite-water interface. Arsenic is a regulated element that can be found at high concentrations in groundwater resources that are used as drinking water sources. Iron (oxyhydr)oxides are one of the most abundant groups of minerals in soils and aquifers, and their presence can significantly affect the behavior of arsenic. Iron (oxyhydr)oxides are also commonly used as adsorbents in engineered system to remove arsenic from drinking water. In addition to adsorbing arsenic, Fe(III) minerals can participate in As(III) oxidation to As(V), which can reduce arsenic's mobility and enhance its adsorption. Advances in the understanding of the environmental chemistry of arsenic are important to the development of water treatment technologies.;The adsorption of arsenic to lepidocrocite strongly depends on water chemistry. Experiments that pursued objectives in Project 1 examined As(III) and As(V) adsorption to lepidocrocite as a function of pH, total As(III) concentration, iron loading, Fe(II) and competing adsorbate presence. For the arsenic concentrations and Fe loadings studied, As(V) adsorption decreases substantially with increasing pH, while As(III) adsorption is less sensitive to pH changes, characterized by a stable level of high adsorption between pH 6-9. For As(III), the presence of oxygen promoted the overall arsenic adsorption via partial As(III) oxidation. A surface complexation model, optimized for both adsorption isotherms and adsorption edges, was able to describe the adsorption of both As(III) and As(V) to lepidocrocite over a broad range of conditions.;The concentration and oxidation states of dissolved arsenic measured over the course of a reaction provided information on As(III) oxidation. When dissolved oxygen and Fe(II) were not present, As(III) was not oxidized by the Fe(III) in lepidocrocite. At both oxic and anoxic conditions, As(III) was oxidized to As(V) in systems that contained lepidocrocite together with Fe(II); this oxidation led to overall enhanced arsenic adsorption at near neutral pH. With oxygen, the pH-dependent generation of oxidants from the Fenton reaction drove the As(III) oxidation. In the absence of oxygen, the As(III) was probably oxidized by Fe(III) in lepidocrocite that had become more reactive upon reaction with Fe(II). The two reaction pathways could occur individually or in combination. Findings in Project 1 provide a deeper understanding of arsenic behavior in engineered water systems and are instrumental to manipulating the conditions under which arsenic is removed via adsorption.;The objectives of the second project were to 1) investigate the impact of water chemistry on trace element mobilization from shales during shale-fluid contact and 2) to identify the dominant mobilization pathways. The rapid development and expansion of hydraulic fracturing operations for enhanced energy recovery can affect water quality. The flowback and produced waters after injection of a fracking fluid could contain high total dissolved solids and trace elements mobilized from contact with shales. The concentrations of specific elements depend on the geochemistry of the formation, fluid composition, and time of shale-fluid contact. An understanding of shale-bound element mobilization will facilitate wastewater management associated with hydraulic fracturing practices.;Experiments in Project 2 were performed to evaluate trace element mobilization from shales over a range of fluid chemistries with core samples from the Eagle Ford and Bakken formations that are currently producing natural gas and oil via hydraulic fracturing. Samples were characterized with regard to their mineralogy, surface area and total carbon prior to experiments. The fluid chemistry was varied in pH, oxidant level, solid:water ratio, and temperature. Analytical results from experiments and chemical equilibrium modeling were integrated to identify dominant mobilization pathways.;The Eagle Ford samples used in this research were rich in carbonates and quartz with minor amounts of kaolinite, albite, pyrite and 5 wt % total organic carbon. The release of most elements strongly depended on pH, which was primarily controlled by carbonate dissolution. The introduction of oxygen and other oxidants (H2O2) significantly increased the amount of sulfate over time; the sulfate generated had a direct impact on Ba concentrations due to the formation of BaSO4 as a secondary phase. For these Eagle Ford samples, trace elements (such as As and U) mobilized from rock-fluid contact had low concentrations in all the conditions studied. (Abstract shortened by UMI.).