Abstract

Iron oxides play an important role in controlling the behavior of inorganic species in both natural and engineered environments. The research presented in this dissertation examined the adsorption and redox abilities of iron oxides for the immobilization of critical elements and contaminants in aquatic systems. Specifically, the adsorption of rare earth elements (REEs) to goethite was investigated to understand the role of iron oxides in accumulating REEs in potential resource deposits. The generation and use of iron oxides as tailored adsorbents and redox-active materials were also evaluated in water treatment using continuous flow-through iron electrocoagulation (EC), with a focus on the removal of selenium (Se), uranium (U), chromium (Cr), and arsenic (As). The role of iron oxides in regulating REEs occurrence was evaluated under aqueous conditions relevant to natural environments. Batch experiments were used to examine the adsorption of Nd, Dy, and Yb to goethite as a function of pH, electrolyte (type and concentration), and dissolved inorganic carbon and citrate concentrations. Adsorption of REEs was strongly pH dependent, increasing from essentially no adsorption at pH 3.0 to nearly complete adsorption at pH 6.5 and above. Citrate enhanced adsorption at low pH (<5.0), likely through formation of goethite-REE-citrate ternary surface complexes, but inhibited adsorption at higher pH (>5.0) by promoting aqueous REE-citrate complexation. Ionic strength had only a minor effect, and dissolved inorganic carbon had no discernible influence on REE adsorption. Equilibrium adsorption was interpreted with a triple-layer surface complexation model guided by results from extended X-ray absorption fine structure spectra. One bidentate inner-sphere surface complex was able to describe Nd adsorption, whereas Dy and Yb adsorption required two inner-sphere complexes, one monodentate and one bidentate. Describing the effects of citrate required the addition of up to two ternary REE-citrate-goethite surface complexes. Reactions at water-iron oxide interfaces were also investigated in water treatment systems using continuous flow-through iron EC followed by flocculation and sedimentation. The removal of Se(VI), U(VI), Cr(VI), As(III), and As(V) was evaluated individually under varying pH, dissolved oxygen, and operating conditions. The removal processes were also examined in the presence of co-occurring water constituents and in environmentally relevant water matrices. To elucidate the removal mechanisms, the solid components and redox speciation of immobilized contaminants in EC-generated solids were characterized. The reactivity of EC-generated solids was compared with that of chemical coagulation products and preformed iron solids. The stability of immobilized contaminants in residual solids was evaluated by the toxicity characteristic leaching procedure (TCLP), air exposure, and desorption experiments. Iron EC achieved >98% Se(VI) removal under anoxic pH 8 conditions with an 11-s reactor residence time followed by 1-h settling. Removal was driven by the formation of reactive mixed-valence Fe(II)/Fe(III) solids (green rust and magnetite). Se(VI) was removed by adsorption to these solids, followed by reduction to less soluble Se(0) or FeSe. Although immobilized Se was oxidized during air exposure, Se release from the residual solids remained minimal, and the solids would be classified as nonhazardous based on the TCLP. Bicarbonate inhibited Se(VI) removal by promoting the formation of less reactive carbonate-containing green rust. Sulfate limited Se(VI) removal by suppressing iron solid transformation and reactivity. In complex matrices representing mining, agricultural, and flue-gas desulfurization wastewaters, sulfate and ionic strength were the primary factors that limited treatment performance. Over 90% of U(VI) was removed by flow-through EC under both oxic pH 7 and anoxic pH 8 conditions. Under oxic conditions, U(VI) was removed by adsorption to EC-generated Fe(III) oxides. Under anoxic conditions, U(VI) was removed through adsorption and subsequent reduction by green rust and magnetite, which produced less soluble U(IV) and U(V) species. Dissolved inorganic carbon and Ca2+ inhibited U(VI) removal under both redox conditions by forming aqueous complexes with U(VI) that were less favorable for adsorption and reduction. EC outperformed chemical coagulation and preformed iron solids with respect to both removal rate and extent. Uranium immobilized by anoxic EC remained stable during air exposure, with only ~10% released after 24 h. In simple water matrices, EC removed >98% Cr(VI) with an 11-s reactor residence time under both oxic and anoxic conditions at pH 8. Removal proceeded through reduction of Cr(VI) to Cr(III) by Fe(II), followed by formation of Cr(III)-Fe(III) coprecipitates. Dissolved silica and bicarbonate did not prevent Cr(VI) reduction, but they inhibited particle aggregation and stabilized small Cr(III)-Fe(III) particles that were difficult to separate by microfiltration. Higher ionic strength and Ca2+ mitigated the inhibitory effects of dissolved silica and bicarbonate by promoting particle aggregation. In an artificial groundwater used as a challenge water for Cr(VI) treatment, near-complete Cr(VI) removal was achieved within 11 s at pH 8 with an iron dose of 5 mg/L. Iron EC could achieve >90% removal of both As(III) and As(V) under oxic conditions at pH 4. Both species were removed primarily by adsorption to EC-generated ferrihydrite and lepidocrocite. During treatment, As(III) was also oxidized to As(V), which is more readily removed. Immobilized arsenic remained largely stable in the residual solids when exposed to phosphate and high pH (11). Freshly generated solids from EC outperformed preformed iron adsorbents in the removal of arsenic because of their higher surface area and reactivity. Phosphate inhibited the removal of both As(III) and As(V) through competitive adsorption. Dissolved silica suppressed As(III) removal but had little effect on As(V) removal. In addition to removal extents, co-occurring species and pH also influenced removal rates by affecting iron oxidation, nucleation, and aggregation. This research advanced the understanding of interfacial processes involving iron oxides in both natural and engineered environments. Mechanistic insights into adsorption, redox reactions, and solid phase transformation established in this work provide a framework for interpreting the occurrence, fate, transport, removal, recovery, and stability of metals and metalloids in aquatic systems. These findings provide new insights into the role of iron oxides in the accumulation of critical elements and may provide a mechanistic basis for their extraction from natural resources. In engineered systems, they can support the optimization and design of iron EC and related treatment processes for effective and reliable water treatment.

Committee Chair

Daniel Giammar

Committee Members

Jeffrey Catalano; Xinhua Liang; Young-Shin Jun; Zhen He

Degree

Doctor of Philosophy (PhD)

Author's Department

Energy, Environmental & Chemical Engineering

Author's School

McKelvey School of Engineering

Document Type

Dissertation

Date of Award

6-9-2026

Language

English (en)

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Available for download on Tuesday, December 08, 2026

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