Salt-tolerant microorganisms potentially useful for bioleaching operations where fresh water is scarce

• Salt tolerant acidophilic microbes found in South West Western Australia.
• Mesophilic cultures tolerating 70 g/L sea salts and 350 g/L MgSO4⋅7H2O found.
• Salt concentrations exceed tolerance levels previously reported.
• Iron oxidation inhibited more than sulfur oxidation at high salt concentrations.
• Sulfate salts may be tolerated better if Na or K sulfates used rather than Mg.

Anions and cations can accumulate in process waters due to the source water, to evaporation and because of gangue mineral dissolution. Common salts that can accumulate from mineral gangues are the anions, chloride and sulfate, both of which impact on microorganisms used in bioleaching processes. The search for salt-tolerant acidophilic microorganisms able to tolerate high concentrations of these salts, as well as high concentrations of metals, has been underway for at least 20 years because their application would considerably improve bioleaching process efficiency in areas where fresh water is scarce.A thorough search of microorganisms from saline and acidic drains, lakes and sediments in the South West of Western Australia and a CSIRO culture collection was undertaken to bio-prospect for salt-tolerant bioleaching cultures. Pure strains of common bioleaching acidophiles did not tolerate seawater salinities, however, enrichment cultures of mesophilic acidophilic microorganisms that could tolerate up to 70 g/L sea salts and 350 g/L MgSO4⋅7H2O were established. The salt tolerance of acidophiles was less in thermophilic temperature ranges, compared with mesophilic and moderately thermophilic temperature ranges. Tolerance to sulfate salts was greater than chloride with magnesium ions likely limiting maximum sulfate tolerance. Iron oxidising cultures were more sensitive than sulfur oxidising cultures to higher chloride concentrations. The addition of pyrite to enrichment cultures increased salt tolerance. The efficacy of the salt tolerant cultures to extract copper will be determined in bioleaching experiments with chalcopyrite ore and salty process water.