Microbial-Mineral-Trace metal interactions in acid rock drainage biofilms: Integrating macro-, micro-, and molecular-level techniques to understand metal behaviour

Abstract

<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>