Developing a Scalable System to Harvest Critical Materials from Industrial Waste with Gluconobacter Oxydans

Organic acid production by Gluconobacter oxydans has been proposed as part of a potential economical and environmentally friendly heterotrophic bioleaching process for recovery of rare earth elements from industrial waste. To provide organic acids in a quantity and at a rate useful for commercial bioleaching, continuous culture of the acid-producing organism will be necessary. The objective of this research project was to determine optimal conditions for continuous cultivation of G. oxydans for organic acid production for recovery of lanthanum and cerium from fluidized catalytic cracking catalyst. Cultures were subjected to dilution rates ranging from .05 hr^-1 to .38 hr^-1 to determine the optimum conditions for production of lixiviant agents conducive to leaching of inorganics. Previous studies have identified methods for maximizing production of gluconic acid and 2,5-diketogluconic acid from G. oxydans, yet the utilization of these and other unidentified G. oxydans metabolites for leaching of rare earth metals is largely unexplored. Continuous cultures exhibited sustained organic acid production for at least 110 hours, while batch fermentations showed slowed growth rates after 16 hours. Several parameters, namely optical density, dissolved oxygen, organic acid concentration and pH, were monitored to characterize the steady state conditions of the fermentation process. Leaching efficiency of organic acid solutions was quantified using inductively coupled plasma mass spectrometry. The observed trend was increasing acidity with decreasing dilution rates, as the lowest dilution rate of 0.05 hr^-1 yielded the highest gluconic acid concentration of 176 mM and lowest pH of 2.2. Recovery of 41.5% to 47.9% of the total lanthanum and cerium in the fluidized catalytic cracking catalyst was attained with the organic acid solutions produced by G. oxydans. Leaching efficiency of the organic acid solutions increased with decreasing dilution rates, suggesting the effectiveness of lanthanum and cerium leaching from fluidized catalytic cracking catalyst is dependent on acidity of the lixiviant solution.