Date of Award
Doctor of Philosophy (PhD)
Trace metals in anoxic aquatic systems are of great importance for iron oxide transformations and biogeochemical cycling. The presence of trace metal can alter iron oxide transformations under anoxic conditions. Trace metals are also essential for microbially-mediated biogeochemical processes occurring in anoxic wetland soils and stream bed sediments, but low availability of these elements may inhibit anaerobic element cycling and transformations. Trace metal speciation is also vital for understanding the environmental control on nutrients and contaminants. Three projects were conducted to explore trace metal speciation and its associated effect on iron oxide transformations and biogeochemical cycling. First, the Fe(II)-promoted conversion of ferrihydrite to more crystalline phases was investigated in the presence of Zn to examine how trace metals impact and are affected by iron oxide mineral transformation pathways. Second, trace metal speciation was investigated in three distinct subsurface aquatic systems to evaluate the natural variability in the controls on metal behavior in uncontaminated systems. Third, the influence of trace metal availability on CH4 production was investigated for two wetland soils.Ferrihydrite, a ubiquitous iron oxide in soils, sediments, and aquatic systems, transforms to other more thermodynamically stable iron oxides through aging, and this process can be catalyzed by the presence of dissolved Fe(II). However, the impact of trace metals on Fe(II)-catalyzed ferrihydrite phase transformations at ambient temperatures and the associated effects on trace metal speciation has seen limited study. In this dissertation, phase transformations of ferrihydrite containing the trace metal zinc in its structure were investigated during aging at ambient temperature in the presence of two different dissolved Fe(II) concentrations at pH 7. X ray diffraction reveals that low Fe(II) concentration (0.2 mM) generates hematite plus minor lepidocrocite, whereas high Fe(II) concentration (1.0 mM) produces a magnetite-lepidocrocite mixture. Control experiments demonstrate that Zn promotes hematite formation and inhibits goethite formation during ferrihydrite transformations. Extended X-ray absorption fine structure spectroscopy suggests that zinc partitions into the newly formed phases, with a shift from tetrahedral to a mixture of tetrahedral and octahedral coordination in the 0.2 mM Fe(II) system and taking on a spinel-like local structure in the 1.0 mM Fe(II) reaction products. This work indicates that substituting elements in ferrihydrite may play a key role in promoting the formation of hematite in low temperature systems, such as soils or sediments. In addition, the retention of zinc in the products of ferrihydrite phase transformations shows that trace metal micronutrients and contaminants may not be mobilized under circumneutral conditions despite the formation of more crystalline iron oxides. Solid-phase speciation is likely a critical control on trace metal availability but has seen limited study in anoxic systems having concentrations similar to geological background levels, where metal limitations may be most prevalent. We have investigated trace metal concentrations and solid-phase speciation in three freshwater subsurface aquatic systems: marsh wetland soils, riparian wetland soils, and the sediments of a streambed. These systems displayed low solid-phase trace metal concentrations, generally at or below geological background levels. All soils and sediments were dominated by quartz but varied in clay mineralogy as well as the organic matter, total sulfur, and total iron contents. Quantitative analysis of copper, nickel, and zinc X-ray absorption near-edge structure spectra revealed similar metal speciation across all sites: adsorbed species, substituents in clay mineral structures, and metal bound to inorganic and organic forms of reduced sulfur. Trace metals bound to reduced sulfur occurred at every site, but the fractional abundance of sulfur-bound species showed no relationship with soil or sediment total sulfur content, which varied by two orders of magnitude. More broadly, the observations in this study suggest that trace metal speciation in freshwater wetland soils and stream sediments is consistently dominated by a small set of recurring components which are distinct for each metal. This may represent a general geochemical phenomenon in anoxic soils and sediments containing trace metals at background concentrations (as low as 3 µg g-1) that was not predicted from systems that are contaminated with or naturally-enriched in copper, nickel, or zinc. Freshwater wetland soils are foci of biogeochemical cycling as they serve as key sources of methane to the atmosphere. Nickel is notably recognized as playing key roles in the enzymatic pathways of methanogenesis. Low availability of trace metals limits microbial element cycling in laboratory studies, but the occurrence of such limitations in natural subsurface aquatic systems is poorly understood. Microcosm incubation studies were carried out using two distinct wetland soils to explore the effect of dissolved nickel concentrations on methane production. Dissolved nickel concentrations were substantially lower than the optimal range for methanogenesis identified in pure culture studies but both wetland soils continuously produced methane over a 7 to 10 week incubation period. Methane production by the marsh wetland soils did not change when dissolved Ni concentrations were increased to 0.5 to 4 µM through amendment to parallel incubations, indicating a lack of Ni limitation. In contrast, amending the riparian wetland soil to increase dissolved Ni concentrations to 0.4 to 1.3 µM simulated up to a ~75% increase in methane production from 3 to 10 weeks of incubation time. The water at the marsh wetland contained substantially greater dissolved sulfate concentration than for the riparian wetland (576 µM versus 8 µM), which may have impacted apparent methane production through substrate competition or promotion of anaerobic methane oxidation. Incubations of marsh wetland soils in sulfate-free water also displayed no evidence for nickel limitations. This study indicates that methane production by freshwater wetland soils displays variable responses to changing nickel availability. This work improves our understanding of the role of trace metals in anoxic aquatic systems. In Fe(II)-induced ferrihydrite transformations, the trace metal Zn alters transformation pathways of ferrihydrite and partially incorporates into the resulting crystalline iron oxides. Such transformations may preserve chemical or isotopic signatures associated with Zn in modern soils or aquatic systems as well as in the sedimentary record. In freshwater wetland soils and stream sediments, trace metal speciation is consistently dominated by a small set of recurring components which are distinct for each metal. Trace metals also play key roles in carbon and nitrogen biogeochemical cycling. Low nickel availability is known to limit methane production in wetland soils, but such limitation occur to variable extents in freshwater wetlands despite the naturally low dissolved concentration of this trace metal in natural aquatic systems.
Chair and Committee
Yan, Jinshu, "Trace Metal Speciation and Impacts on Iron Oxide Transformations and Biogeochemical Cycling" (2022). Arts & Sciences Electronic Theses and Dissertations. 2695.
Available for download on Saturday, April 20, 2024