Jan-Eric Sundkvist

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.

A sequential two-step process using moderately and extremely thermophilic cultures for biooxidation of refractory gold concentrates

In many cases, the use of extreme thermophiles, like the archeon Sulfolobus metallicus, in a continuous bioleaching process of gold concentrates is limited by the arsenic content in the feed. In this work, a sequential two-step bioleaching process for gold-containing refractory pyrite/arsenopyrite concentrates has been investigated for the possibility of lowering the toxicity of arsenic with respect to the extremely thermophilic culture. In the first stage, a moderately thermophilic culture was used followed by the extremely thermophilic S. metallicus in the second stage. It was found that the S. metallicus culture survives higher arsenic concentrations than expected when the concentrate was pre-oxidized at a lower temperature. Thus, with this sequential two-step bioleaching process, it is possible to reduce the toxicity of the released arsenic. Therefore, the use of higher pulp densities of arsenic-containing minerals is enabled. When the leached mineral residues were subjected to cyanidation, cyanide consumption and thiocyanate formation were significantly lower after the second stage. In addition, a somewhat higher gold and silver grade was found in the residue from the concentrate ultimately oxidized by S. metallicus.

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 Fe2+ 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 Fe2+ limited chemostat were studied. Results from the chemostat modeling suggest that the chloride toxicity was attributed to affects on the Fe2+ oxidation system, pH homeostasis, and lowering of the proton motive force. Modeling showed a decrease in the maximum specific growth rate (µmax) and an increase in the substrate constant (Ks) with increasing chloride concentrations, indicating an effect on the Fe2+ 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 (Ytrue). This model helps to understand the influence of chloride on Fe2+ biooxidation kinetics. Biotechnol. Bioeng. 2010; 106: 422–431.

Low temperature removal of inorganic sulfur compounds from mining process waters

Process water and effluents from mining operations treating sulfide rich ores often contain considerable concentrations of metastable inorganic sulfur compounds such as thiosulfate and tetrathionate. These species may cause environmental problems if released to downstream recipients due to oxidation to sulfuric acid catalyzed by acidophilic microorganisms. Molecular phylogenic analysis of the tailings pond and recipient streams identified psychrotolerant and mesophilic inorganic sulfur compound oxidizing microorganisms. This suggested year round thiosalt oxidation occurs. Mining process waters may also contain inhibiting substances such as thiocyanate from cyanidation plants. However, toxicity experiments suggested their expected concentrations would not inhibit thiosalt oxidation by Acidithiobacillus ferrivorans SS3. A mixed culture from a permanently cold (4–6°C) low pH environment was tested for thiosalt removal in a reactor design including a biogenerator and a main reactor containing a biofilm carrier. The biogenerator and main reactors were successively reduced in temperature to 5–6°C when 43.8% of the chemical oxidation demand was removed. However, it was found that the oxidation of thiosulfate was not fully completed to sulfate since low residual concentrations of tetrathionate and trithionate were found in the discharge. This study has demonstrated the potential of using biotechnological solutions to remove inorganic sulfur compounds at 6°C and thus, reduce the impact of mining on the environment. Biotechnol. Bioeng. 2011; 108:1251–1259.