No description available.

<p> In this study a combined field and laboratory approach was used to identify the bio-geochemical processes that control trace metal (Ni, Co, Cr) reactive transport within natural acid rock drainage (ARD) biofilms, over both diel and seasonal timescales. Results indicated that metal (Mn, Ni, Co and Cr) scavenging by these biological solids is stable on a seasonal time frame. Metal scavenging occurs within two key solids, the organic constituents of the biofilm (Ni, Co) and associated biogenic hydrous Mn oxyhydroxides (HMO; Ni, Co and Cr), and not in association with Fe-oxyhydroxysulphates which dominate the mineralogy of the biofilm samples by mass. On a diel basis, cycling of HMO and associated trace metal dynamics appear to be contingent on the vertical migration of the biofilm oxic-anoxic boundary, a microbially controlled process. </p> <p> The reactivity and sorptive capacities of synthetic HMO analogs for Ni were further examined under well-characterized laboratory conditions. Analysis of the local chemical environment of Ni sorbed to HMO by synchrotron-based X-ray absorption spectroscopy was integrated with a bulk geochemical model of the acid-base characteristics of HMO and a theoretical model of the HMO structure. The synergistic use of these techniques allowed unique insight into the structural reactivity of HMO for Ni and is the first study to mechanistically demonstrate why bulk surface complexation models (SCM) are not accurate for HMO metal uptake. </p> <p> Overall, the results of this thesis highlight the utility of combined field and laboratory investigation to characterize relevant processes for reactive metal transport and underscore the need to: (1) consider microscale microbial-geochemical linkages in geochemical behaviour; (2) use caution when applying results derived from synthetic analogs to interpret natural system behaviour; and (3) examine processes at the appropriate scale e.g. microscale, to evaluate the mechanisms involved in metal reactions with solids. </p> / Thesis / Doctor of Philosophy (PhD)

<p>This doctoral research comparatively assesses the biogeochemical properties of suspended aquatic flocs through a integrated field-laboratory approach; providing new insight into the linkages among floc associated bacteria, floc-reactive solid phases and trace metal uptake.</p> <p>Results show flocs to possess a distinct geochemistry, microbiology and composition from bed sedimentary materials in close proximity (III-oxyhydroxide minerals (FeOOH); resulting in localized floc-Fe-mineral precipitates and enhanced reactivity. Further, the Fe-enrichment of floc and of floc bio-mineral constituents in turn provides an important and novel lens through which to examine how environmental microbial communities, microbial metabolism and Fe^III/Fe^IIredox transformations interact. The results were the discovery of floc-hosted, Fe^III/II-redox cycling bacterial consortia across diverse oxygenated (O₂^Sat.=1-103%) aquatic systems, which were not predicted to sustain bacterial Fe-metabolism. Both environmental<em> </em>and experimentally-developed consortial aggregates constituted multiple genera of aero-intolerant Fe^III-reducing and Fe^II-oxidizing bacteria together with oxygen consuming organotrophic species. These findings highlight that the implementation of geochemical thermodynamic constraints alone as a guide to investigating and interpreting microbe-geosphere interactions may not accurately capture processes occurring <em>in situ.</em></p> <p><em> </em> Seasonal investigation of microbial Fe^III/II-redox transformations highlighted the interdependence of floc Fe-redox cycling consortia members, revealing that cold conditions and a turnover in putative Fe-reducing community membership extinguishes the potential for coupled Fe-redox cycling by wintertime floc bacteria. Further, the observed summer-winter seasonal turnover of <em>in situ</em> floc community membership corresponded with an overall shift from dominant Fe to S redox cycling bacterial communities. This significantly impacted observable floc Fe and TE (Cd, Pb) geochemistry, resulting in a shift in floc associated Fe-phases from dominantly Fe^(III)_(s) to Fe^(II)_(s), and, in turn, corresponded to a large decrease of TE uptake by flocs under ice.</p> / Doctor of Science (PhD)