Date of Award
Doctor of Philosophy (PhD)
The biogeochemical cycling of iron and manganese involves the reductive dissolution and oxidative precipitation of Fe(III) and Mn(IV/III) oxides. Biogenic Fe(III) and Mn(IV/III) oxides are often characterized by high surface areas and therefore high sorptive capacities. As a result, these minerals can substantially alter the chemistry of natural waters and the availability of micronutrients in soils and sediments by scavenging trace metals. Recent research indicates that the adsorption of aqueous Fe(II) onto Fe(III) oxides involves oxidative adsorption, electron transfer, and subsequent reductive dissolution at another surface site [a process collectively referred to as `electron transfer-atom exchange' (ET-AE)]. Aqueous Mn(II) adsorption onto Mn(IV/III) oxides likely also involves oxidation, but because of the potential for Mn(II) Mn(IV) comproportionation reactions and the accessibility of nearly all atoms in Mn(IV/III) oxide sheets to reaction with aqueous solution, aqueous Mn(II)-solid Mn(IV/III) interactions are expected to differ substantially from the analogous Fe system. These complex interactions between reduced and oxidized forms of Fe (and Mn) occur at redox interfaces and can exert substantial effects on trace metal fate. These processes may, in turn, be affected by ions common in natural systems. The main objective of this dissertation is to determine how interactions between ions commonly present during biogeochemical Fe or Mn cycling in natural systems [e.g., phosphate, sulfate, Ni, Zn, Fe(II), or Mn(II)] alter one another's interactions with Fe and Mn oxide surfaces. This research specifically seeks to (1) identify the mechanisms through which the oxoanions phosphate and sulfate alter Fe(II) adsorption onto Fe oxides; (2) determine how oxoanion-Fe(II) interactions alter trace metal partitioning between the mineral surface, bulk mineral structure, and aqueous phase; (3) characterize the effect of Mn(II) on phyllomanganate sheet structures; and (4) examine the effect of Mn(II) on trace metal sorption on phyllomanganates.
Macroscopic adsorption edges show that Fe(II) cooperatively co-adsorbs with sulfate and phosphate on Fe(III) oxide surfaces. Both attenuated total reflectance Fourier transform infrared spectroscopy and surface complexation modeling indicate that this cooperative adsorption behavior arises from a combination of ternary complexation and electrostatic interactions. The formation Fe(II)-oxoanion ternary complexes suggests that processes associated with Fe(II) Fe(III) ET-AE reactions may be altered in the presence of oxoanions, depending on the stability and identity of the ternary complex that forms. The effect of these oxoanions on one such process, trace metal repartitioning, was investigated in detail. Sulfate and, to a larger degree, phosphate suppress Ni cycling through hematite during Fe(II)-catalyzed recrystallization by altering Ni adsorption, structural incorporation, and release back into solution. Conversely, Ni cycling through goethite is unaffected or enhanced by phosphate and sulfate.
This dissertation also investigated Mn(II) effects on phyllomanganate structure and the fate of associated trace metals. Powder X-ray diffraction and X-ray absorption fine structure spectroscopic measurements indicate that Mn(II) causes distortion of the sheet structure of Mn(IV/III) oxides and alters sheet stacking at low pH, but has a minimal effect on phyllomanganate structures at circumneutral pH. As a result, Ni and Zn adsorption mechanisms on phyllomanganates are altered in the presence of aqueous Mn(II) at pH 4, but exhibit few changes at pH 7. The Ni and Zn adsorption behaviors with aqueous Mn(II) suggests that Mn(II) alters phyllomanganate reactivities by decreasing phyllomanganate vacancy content.
These results emphasize the importance of understanding adsorbate interactions in systems with coexisting reduced and oxidized Fe or Mn, as under such conditions Fe and Mn oxide minerals undergo dynamic structural transformations. Trace metal uptake and partitioning between Fe oxide surfaces can be altered in systems with appreciable amounts of phosphate or sulfate (e.g., riparian zones, estuaries, or marine sediments). The complex interactions at iron oxide surfaces must be considered when evaluating trace metal fate at redox interfaces or interpreting trace metal proxies in the rock record to reconstruct ancient water compositions. The Mn(II)-induced phyllomanganate structural changes observed here suggest a relationship between water composition and the reactivity of Mn oxides as adsorbent materials. The identified phyllomanganate restructuring may also modify the capacity of Mn oxides to serve as oxidants of inorganic and organic compounds in aquatic systems. This dissertation highlights the complex structural and chemical processes that occur via cooperative and competitive interactions of ion at iron and manganese oxide surfaces.
Chair and Committee
Jeffrey G Catalano
Alex S Bradley, David E Fike, Daniel E Giammar, Jill D Pasteris
Hinkle, Margaret A. G., "Ion Interactions at the Mineral-Water Interface During Biogeochemical Iron and Manganese Cycling" (2015). Arts & Sciences Electronic Theses and Dissertations. 441.