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Biological hydrogen production using Citrobacter amalonaticus Y19 to catalyze the water-gas shift reaction Robaire, Sandra
This research reports on investigating Citrobacter amalonaticus Y19, a chemoheterotrophic facultative bacterium, as a catalyst to produce hydrogen from a lowgrade synthesis gas stream. Several production strategies were considered, with a two stage batch reaction shown to be most effective. This strategy begins with aerobic growth of the organism, where the biomass is generated, followed by hydrogen production in an anaerobic environment, where production and activation of carbon monoxide dehydrogenase and the CO-induced hydrogenase enzymes is required to catalyze biological H₂ production. The anaerobic environment was created by purging the reactors with a gas mixture of 40% CO in helium (v/v). The rates of hydrogen production were analyzed by measuring the concentrations of H₂, CO, and CO₂ during the anaerobic stage by collecting gas samples from the headspace. Samples were also collected from the fermentation medium to monitor the concentration of Ci amalonaticus Y19, organic acids, and pH. Design of experiments was used to investigate the dependence of hydrogen productivity on various process parameters, including reactor pressure and various media components, such as the presence of CO during the growth phase, the presence of glucose, tryptone, and trace metals such as nickel and iron. The effect of re-suspension was also investigated since decoupling the two stages allows for a larger amount of freedom in selecting process conditions. The results indicate that increasing pressure has a negligible effect on hydrogen productivity, likely due to substrate inhibition compensating increased substrate availability. The results also show that altering certain media components can increase hydrogen productivity. Nickel, in particular, increased the H₂ productivity from 0.47 to 0.87 mmol H₂/ (L x h) when its concentration was increased from 0 to 125mg/L. Re-suspension between stages decreased the inhibition of enzyme production and activation by eliminating the inhibitory metabolites produced during the growth stage. This was manifested in the form of a reduced lag phase from approximately 18 hours to 5 hours with re-suspension, as well as an increased H₂ production rate. The maximum H₂ productivity attained was 1.5 mmol H₂/ (L x h) in buffered media when the cells were re-suspended between stages.
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