THE BIOREDUCTION OF COBALT AND NICKEL BEARING MANGANESE MINERALS

Abstract

Cobalt is a critical metal of the Anthropocene. Regarded a precious commodity in developed countries, the supply of cobalt remains strongly dependant on the Democratic Republic of Congo (DRC), a politically strained region. The economic demand for cobalt is a result of both industrial and military uses, most notably a central role in rechargeable batteries for portable devices and hybrid/electric vehicles. Owing to increased use, in conjunction with concerns regarding the continued supply from DRC and political/environmental incentives for increased use of hybrid/electric vehicles to reduce climate change, as stipulated by the 2015 Paris agreement; the trend of increasing demand for cobalt is expected to continue. Typically, cobalt is mined from sediment-hosted stratiform Cu deposits. However, cobalt can also be found within the structure of manganese minerals, such as asbolane [(Ni,Co)xMn(IV)(O,OH)4•nH2O], and ferrihydrite [Fe(III)10O14(OH)2] within lateritic ores and deep sea nodules, providing potential new sources for this critical metal. Geobacter sulfurreducens, a model dissimilatory metal-reducing bacterium that can use Mn(IV) or Fe(III)-bearing minerals as a terminal electron acceptor, coupled to the oxidation of organic matter, was used to investigate the liberation and natural biogeochemical cycling of cobalt-substituted Fe/Mn-oxides. Furthermore, the interactions and electron transfer processes that occur between microbes and metals at the cell-mineral interface during enzymatic metal reduction are poorly understood. Synchrotron-based scanning X-ray microscopy (SXM) was used as the key technique to elucidate the nature of these processes and improve our understanding of the fate of cobalt in redox horizons. The enzymatic reduction of synthetic cobalt-doped asbolane and synthetic cobalt-doped ferrihydrite by G. sulfurreducens, of relevance to Co-rich lateritic ores and polymetallic nodules deposits, respectively, were studied to investigate the microbial reduction of cobalt enriched manganese and iron oxides. Using a multi-technique approach combining nanoscale spectroscopy with laboratory analyses, it was found that biotic Mn(IV) reduction occurs through a successive two single step electron pathway facilitating the reduction of Mn(IV) to Mn(II) via a solid phase Mn(III) intermediate; confirmed to be the rarely reported mineral feitknechtite [Mn(III)O(OH)]. While metal solubilisation occurred in both systems, cobalt largely remained in the solid phase with only 10 % solublised over 72 hours in the asbolane and 11 % in the ferrihydrite, thus improving our understanding of retention mechanisms and the reordering of cobalt bearing manganese and iron oxides during redox cycling of cobalt, thus helping to inform future strategies for cobalt bioprocessing techniques. SXM data at the nano scale, coupled with imaging, demonstrated that metal reduction is focused in regions co-located with the bacterial cells, facilitated by electron transfer at the cell surface, likely using c-type cytochromes. This is an intricacy not highlighted in previous studies, by bulk L2,3 edge data or by the spatially constrained aggregate SXM data analyses in this thesis. Therefore, here, I demonstrated the capacity for SXM to provide high resolution, spatially defined mechanistic information to comment on the nuances of the electron transfer pathways occurring at the nano scale at the cell-mineral interface. Following the successes of these synthetic systems, G. sulfurreducens was used to investigate the bioreduction of Co rich Mn phases: lithiophorite [(Al,Li)(Mn(IV),Mn(III))O2(OH)2] and cryptomelane [K(Mn(IV)7Mn(III))O16] isolated from the Nkamouna laterite, south-east Cameroon. As expected, more metal solubilisation occurred in the pure culture experiment samples, compared to the abiotic control. When the natural microbial profile was stimulated using artificial ground water and glucose, which served not only as an electron donor but also ge

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jonathan Lloyd (Supervisor) & Victoria Coker (Supervisor)