The bioleaching of metals from organic-rich black shale by Pseudomonas fluorescens

Biohydrometallurgy is known as one of techniques of beneficiation of metal ores, utilizing microorganism activity to enhance the extraction of metal ions from ore minerals. The method is widely known from its economic and environmental advantages in comparison to other beneficiation methods, i.e. pyrometallurgy. The development of biohydrometallurgy applications for the mining industry has continued since the eighties when several bioleach operations for low-grade copper ores and refractory gold concentrates have been commissioned. Although significant extraction rate, biological-induced leaching does not constitute main processing technique due to economic reasons, what drive researchers to searching novel ways to enhance the efficiency of metal recovery.

 

This research came with the idea of improving the efficiency of metal bioleaching by presence of dispersed organic matter naturally occurring in some rocks. Main hypothesis of the research assumes using dispersed organic matter as a source of organic carbon by heterotrophic bacteria that could further enhance dissolution of ore minerals. The idea came from both facts: consumption and metabolic utilization of dispersed organic matter by heterotrophic microorganisms and influence of consumed organic compounds on microbial activity e.g. secretion of siderophores (metal chelating compounds, crucial for solubilization metal ions and thus acceleration the metal leaching).

 

The examined rock is metalliferous-bearing shale enriched in dispersed organic matter (with average TOC parameter from few to even 30 wt.%). Two shale samples different in their metal and organic matter quantity were chosen for testing the hypothesis. The potential of black shale for bioleaching was examined through series of incubation experiments. Different experiment conditions were applied, involving both autotrophic (Acidithiobacillus thiooxidans) and heterotrophic (Pseudomonas fluorescens) bacteria and different medium compositions (with presence and absence of organic nutrient in particular).

 

The incubation experiment took place in a shaker incubator at a controlled temperatures for a period of five weeks. After the incubation experiment the leachates were collected and analyzed for the concentrations of eight metals: copper, lead, zinc, molybdenum, arsenic, nickel, cobalt and vanadium. The metal recovery percent features variation depending on shale sample, specific metal, and incubation condition. The highest metal recovery was achieved for heterotrophic bacterium in case of copper and molybdenum and for autotrophic one in case of arsenic, while rest of metals showed insignificant recovery. Lack of organic nutrients weakened the activity of P. fluorescens compared to bacterium supplied with organic nutrient, however metal leaching was still maintained.