Isolation Of Sulfur-oxidizing Acidophilic Bacteria And Bioleaching Of Chalcopyrite By Its Mixed Culture With Iron-oxidizing Acidophiles

Bioleaching is an economical method for the recovery of metals fromminerals, especially from low grade ores, overburden and waste from currentmining operations, which requires moderate capital investment with lowoperating cost. Furthermore, bioleaching are generally more environmentallyfriendly than conventional metal recovery processes such as concentration andsmelting. In the present study, four strains of acidophilic sulfur-oxidizingbacteria was isolated and characterized. To understand the role sulfur-oxidizingbacteria plays in mineral bioleaching and whether they enhance the dissolutionof metal sulfides by oxidizing S⁰ formed on the surface during the leachingprocess, we focused on the effects of two pure iron-oxidizing cultures(Leptospirillum ferriphilum or Acidithiobacillus ferrooxidans) and theircombination with the sulfur-oxidizing bacteria on copper dissolution fromchalcopyrite. The role of jarosite precipitation in the bioleaching ofchalcopyrite was also investigated, and the bioleaching mechanisms ofchalcopyrite by the above four bacterial species were discussed.Four strains of acidophilic sulfur-oxidizing bacteria named as S2, AA011,AA012 and DMC were isolated from coal heap drainage. A moderatelythermophilic bacterium named S₂ is motile, Gram-negative, and rod-shaped,measures 0.4 to 0.6 by 1 to 2μm, and grows optimally at 42-45℃and aninitial pH 2.5. Three strains of mesophilic bacteria AA011, AA012 and DMCare motile, Gram-negative, rod-shaped, and measures 0.4 to 0.7 by 1 to 2μm.They grow optimally at 30℃and initial pH 2.0-2.5. The four strains growautotrophically by using elemental sulfur, sodium thiosulfate and potassiumtetrathionate as energy sources. The strains can not use organic matter andinorganic minerals including ferrous sulfate, pyrite and chalcopyrite as energysources. The morphological, biochemical and physiological characterizationand analysis based on 16S rRNA gene sequence indicated that the strain S₂ ismost closely related to Acidithiobacillus caldus and that the strains AA011,AA012 and DMC are most closely related to Acidithiobacillus thiooxidans(＞99% similarity in gene sequence).The combination of the strain S₂ with L. ferriphilum or A. ferrooxidans inchalcopyrite bioleaching improved the copper leaching efficiency. Compared with the pure iron-oxidizing culture, percentage copper release of the mixedculture increased 50.6% (L. ferriphilum) and 10% (A. ferrooxidans)respectively. Scanning electron microscope(SEM) analysis revealed that thechalcopyrite surface in a mixed culture of L. ferriphilum and A. caldus washeavily etched. The energy dispersive X-ray (EDX) analysis indicates that A.caldus has the potential role to enhance the recovery of copper fromchalcopyrite by oxidizing the sulfur formed during the bioleaching progress.The bioleaching of chalcopyrite in shake flasks was investigated withpure and mixed cultures of A. ferrooxidans, A. thiooxidans, A. caldus and L.ferriphilum. The mixed cultures containing both iron- and sulfur-oxidizingbacteria were more efficient than the pure culture alone. The presence ofsulfur-oxidizing bacteria positively increased the dissolution rate and thepercentage recovery of copper from chalcopyrite. An increase of 4.4% to13.9% in percentage recovery of copper was reached. Mixed culturesconsisting of moderately thermophilic L. ferriphilum and A. caldus leachedchalcopyrite more effectively than mesophilic A. ferrooxidans mixed cultures(8.6%-18% higher percentage recovery of copper). The decrease ofchalcopyrite dissolution rate in leaching systems containing A. ferrooxidansafter 12-16 days coincided with the formation of jarosite precipitation on themineral surface during the bioleaching as a passivation layer. Low pHsignificantly reduces jarosite formation in pure and mixed cultures of L.ferriphilum and A. caldus.