Preservation of Acidithiobacillus caldus: A moderately thermophilic bacterium and the effect on subsequent bioleaching of chalcopyrite

Four bacterial preservation methods: passage culture, sterile sand tube preservation, freezing preservation and freeze-drying preservation, were used to store Acidithiobacillus caldus strain S2, a moderately thermophilic bacterium. Among the four methods, the second is shown to be the best for short term preservation (6 months), and the fourth is the best for long term preservation (15 months). Specifically, the results show that using the second method, 32% cell viability was obtained after 6 months, and the cell survival rate reaches 17% using the fourth method after 15 months. In the bioleaching experiments, A. caldus strain S2 preserved via the second and the fourth methods, along with Leptospirillum ferriphilum strain YSK, were applied to bioleach chalcopyrite. Under the two preservation methods for A. caldus strain S2, the results show that 3.72 g/L and 1.68 g/L copper are extracted in the first 20 days and the maximum biomass delays to the 15th day and to the 18th day, respectively. When the bioleaching time was extended to the 40th day, copper extraction increased significantly to 5.02 g/L and 4.88 g/L, respectively. However, A. caldus strain S2 under no preservation, along with L. ferriphilum strain YSK, extract only 0.54 g/L Cu from the 20th day to the 40th day and finally release 5.14 g/L Cu. Therefore, the preserved culture shows almost the same total copper extraction as the unpreserved one in 40 days. As a result, sterile sand tube preservation and freeze-drying preservation greatly prolong the lag phase of cell growth without much decrease in the bioleaching ability of A. caldus strain S2.