Evolution of the bacterial population during the batch bioleaching of a cobaltiferous pyrite in a suspended-solids bubble column and comparison with a mechanically agitated reactor

Abstract

The aim of the present work was to compare the bioleaching kinetics and the evolution of bacterial populations between the classical mechanically agitated bioreactor and a suspended-solids column reactor. For this purpose, batch experiments were performed in a 20-L stirred reactor and a 26-L bubble column, respectively. The substrate was a cobaltiferous pyrite. At 20% solids concentration (w/w), the cobalt leaching rates in both reactors were similar. In the bubble column, bioleaching rate increased with gas velocity and at low air-flow rate, dissolved oxygen concentration could decrease down to 0.5 mg/L, oxygen mass transfer being a limiting factor in that situation. These results suggest that bioleaching in the bubble column could be at least as efficient as in the stirred tank. The composition of the bacterial population was determined by sequencing 16S rDNA from biopulp samples: Leptospirillum ferrooxidans, an organism close to Thiobacillus caldus, and a bacterium related to Sulfobacillus thermosulfidooxidans were found. The single-strand conformation polymorphism (SSCP) technique was used to monitor the population. Proportions of bacteria attached to the solids or suspended in the liquid were evaluated. In the liquid, T. caldus is dominant during the first phase of the experiments, then supplanted by L. ferrooxidans. L. ferrooxidans is always in the majority on the solids. S. thermosulfidooxidans is less constant. The growth of this bacterium seems to be favored in the bubble column. The SSCP technique proved to be an interesting and low-cost way to use molecular biology, which is worth developing in the field of bioleaching.