The objective of this study is to design and apply a small diagnostic flow-through column system that would enable the quantitative investigation of colonisation of mineral surfaces, using pyrite-containing concentrates and ores in this instance. Colonisation is studied across varying sulfide mineral grade, by mixed mesophilic microbial cultures, with the aim of enhancing leaching for metal recovery and, on the other hand, characterising and understanding uncontrolled leaching, typical of ARD generation, with the aim of preventing it. The objective is addressed through the following:
• Design a small-scale heap simulating system that has defined mineral surface area using mineral coated beads.
• Develop an IMC approach to measure quantitatively the activity of mineral associated microbial populations.
• Apply the developed IMC approach to the heap simulating flow-through system and progressively measure metabolic activity of microorganisms that colonise the surfaces of pyrite mineral with varying sulfide content under typical and high flow rates.
• Refine the detachment method and validate its efficacy through integration with IMC to account for total microbial populations that colonise mineral surfaces.
• Cultivate, extract and perform biochemical analysis of EPS produced by colonising populations, in a complete flow-through system.
• Integrate metabolic activity data of colonising populations with visualising tools and traditional analysing tools to better understand colonisation, particularly as a function of mineral surface area available.
• Develop insight into the impact of mineral grade and flow rate of irrigant on early stage microbial colonisation of the mineral.
• Demonstrate potential for application of the flow-through system in feasibility and characterisation studies through further refinement of the existing biokinetic method to characterise sulfide mineral-containing waste rock for their potential to form ARD. 2.8.1 Developed hypotheses and research questions
The following hypotheses and research questions were formulated to address the above objectives.
Hypothesis 1
Metabolic activity of mineral associated mixed mesophilic microbial communities that occupy available mineral surfaces, increases over time. The increase in metabolic activity is reflected by the growth in microbial cell numbers as well as activity per cell over the colonisation period on a basis of surface area. This is since microbial communities adapt well to the environment on the surface and become highly active over time until maximum activity is achieved, prior to limiting conditions occuring.
• Can isothermal microcalorimetry be used to investigate and assess the metabolic activity of microbial communities associated with pyrite mineral surface quantitatively as a function of surface area in a heap simulating system over time?
• Does microbial metabolic activity as well as microbial growth on the mineral surface and resultant surface coverage progress with time?
• Does growth in microbial numbers on the mineral surface result in increased microbial metabolic activity?
• Under what conditions is the highest microbial activity per unit surface area measured on mineral surfaces?
• Does an increased surface area load increase microbial activity (heat-flow)? • Does reagent limitation affect microbial activity on the mineral surface? Hypothesis 2
The current detachment method used to enable quantification of surface associated microbial populations is assumed to be completely efficient based on cell count limitations. IMC, as a more sensitive measure of microbial presence than cell counts, will account for, and reconcile for any population remaining on the surface post detachment, testing the complete efficiency assumption and providing an approach for its correction if not valid.
• Do detachment washes efficiently remove surface associated microbial populations and, if so, what number are required to achieve this?
• If not, what fraction of the surface associated population remains on the mineral surfaces?
• Using both IMC and the detachment protocol, can the residual population be accounted for? If yes, is it significant?
Hypothesis 3
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microbial-mineral interaction including growth and activity in complete flow-through bioleach environments.
• What effect does mineral grade and flow rate have on the ability of microorganisms to attach to and colonise mineral surfaces?
• What effect does mineral grade and flow rate have on the growth of surface associated microorganisms as well as their activity?
Hypothesis 4
In the developed unsaturated complete flow-through system developed, mixed mesophilic cultures attach to available mineral surface and produce EPS. The EPS is characterised by varying biochemical properties and results in the facilitation of accelerated mineral surface degradation.
• Do mixed mesophilic microorganisms produce EPS in complete flow-through systems?
• If yes, what is the biochemical make-up of that EPS? Hypothesis 5
The unsaturated flow-through mineral system with defined surface area provides a tool for use in feasibility and characterisation studies. For example, prior to their disposal, mine waste material needs to be characterised in terms of their potential to form ARD in the future. Development of a flow-through biokinetic configuration as a further refinement of the current batch biokinetic system, developed at UCT, allows for a truer representation of a waste rock dump and can account for the competing neutralising and acid forming reactions that take place, as well account for microbial-mineral interaction and mineral degradation.
• Are the initial neutralising reactions washed out in the flow-through configuration? • Is microbial growth and activity progressive on the mineral surface of the waste rocks? • Is the flow-through assay sensitive to the flow rate at which it is operated?
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