Effect of chloride on ferrous iron oxidation by a Leptospirillum ferriphilum-dominated chemostat culture.

Biomining is the use of microorganisms to catalyze metal extraction from sulfide ores. However, the available water in some biomining environments has high chloride concentrations and therefore, chloride toxicity to ferrous oxidizing microorganisms has been investigated. Batch biooxidation of Fe(2+) by a Leptospirillum ferriphilum-dominated culture was completely inhibited by 12 g L(-1) chloride. In addition, the effects of chloride on oxidation kinetics in a Fe(2+) limited chemostat were studied. Results from the chemostat modeling suggest that the chloride toxicity was attributed to affects on the Fe(2+) oxidation system, pH homeostasis, and lowering of the proton motive force. Modeling showed a decrease in the maximum specific growth rate (micro(max)) and an increase in the substrate constant (K(s)) with increasing chloride concentrations, indicating an effect on the Fe(2+) oxidation system. The model proposes a lowered maintenance activity when the media was fed with 2-3 g L(-1) chloride with a concomitant drastic decrease in the true yield (Y(true)). This model helps to understand the influence of chloride on Fe(2+) biooxidation kinetics.

A study on the toxic effects of chloride on the biooxidation efficiency of pyrite.

Bioleaching operations in areas with limited chloride-free water and use of ashes and dust as neutralizing agents have motivated to study the chloride toxicity and tolerance level of the microorganisms. Biooxidation of pyrite using chloride containing waste ash compared with Ca(OH)(2)+NaCl as neutralizing agent was investigated to evaluate the causes of low pyrite oxidation. Both precipitation of jarosite as well as the toxic effect of chloride on the microorganisms were responsible for lower pyrite recoveries. Another study with sudden exposure of chloride during pyrite biooxidation, addition of 4 g/L was lethal for the microorganisms. Addition of 2g/L chloride resulted in precipitation of jarosite with slightly lower pyrite recovery whereas the addition of 3g/L chloride temporarily chocked the microorganisms but activity was regained after a short period of adaptation. Population dynamics study conducted on the experiment with 3g/L chloride surprisingly showed that Leptospirillum ferriphilum, which was dominating in the inoculum, completely disappeared from the culture already before chloride was added. Sulphobacillus sp. was responsible for iron oxidation in the experiment. Both Acidithiobacillus caldus and Sulphobacillus sp. were adaptive and robust in nature and their numbers were slightly affected after chloride addition. Therefore, it was concluded that the microbial species involved in the biooxidation of pyrite vary in population during the different stages of biooxidation.

Modeling of ferrous iron oxidation by a Leptospirillum ferrooxidans-dominated chemostat culture.

The objective of this study was to evaluate a direct classical bioengineering approach to model data generated from continuous bio-oxidation of Fe(2+) by a Leptospirillum ferrooxidans-dominated culture fed with either 9 g or 18 g Fe(2+) L(-1) under chemostat conditions (dilution rates were between 0.051 and 0.094 h(-1)). The basic Monod and Pirt equations have successfully been integrated in an overall mass balance procedure, which has not been previously presented in this detail for Fe(2+) oxidation. To ensure chemostat conditions, it was found that the range of the dilution rates had to be limited. A too long retention time might cause starvation or non-negligible death rate whereas, a too short retention time may cause a significant alteration in solution chemistry and culture composition. Modeling of the experimental data suggested that the kinetic- and yield parameters changed with the overall solution composition. However, for respective feed solutions only minor changes of ionic strength and chemical speciation can be expected within the studied range of dilution rates, which was confirmed by thermodynamic calculations and conductivity measurements. The presented model also suggests that the apparent Fe(3+) inhibition on specific Fe(2+) utilization rate was a direct consequence of the declining biomass yield on Fe(2+) due to growth uncoupled Fe(2+) oxidation when the dilution rate was decreased. The model suggested that the maintenance activities contributed up to 90% of the maximum specific Fe(2+) utilization rate, which appears close to the critical dilution rate. Biotechnol. Bioeng. 2008;99: 378-389. (c) 2007 Wiley Periodicals, Inc.