Advanced Materials Research Vol. 825

Abstract: The gol aim of this study is to optimize the ability to produce hydrogen sulphide (H₂S) in pure and mixed cultures of sulfate reducing bacteria (SRB) at pH variations from 9 to 5. Hydrogen sulphide produced by SRB reacts with dissolved metals in water or tailings generating highly insoluble metal sulfides and therefore the selective immobilization of different metals. Three strains of SRB were isolated from Orcopampa mine tailings and from the Pantanos de Villa wetlands, both located in Peru. Cultures were identified by microscopy, cultural characteristics and biochemical tests as production of desulfoviridin and growth in different substrates. The production of H₂S by pure and mixed cultures was evaluated at: acid pH (5), neutral pH (7) and alkaline pH (9). The mixed culture consisted of all three isolated species: Desulfobacter sp. from mine tailings and Desulfovibrio desulfuricans and Desulfovibrio sapovorans from wetland sludges. Pure cultures of these three strains grew and produced H₂S at both neutral or alkaline pH. At low pH no pure culture was able to grow and no production of H₂S was detected. A mixed culture formed by the three isolated SRB was the only culture that grew and produced sulphide at the three different pH tested in shorter time (24 hours). The improvement of bacterial activity can be based in the metabolic diversity of the mixed culture able to use lactate and acetate as a result of the joint activity of these species. Energy obtained from the substrate is thus used more efficiently.

Abstract: The bioleaching of sulfide minerals by iron oxidizing bacteria are implemented by direct and indirect mechanisms. The direct dissolution of minerals is caused by the attack of sulfide ions by enzymatic system of bacteria. In the indirect mechanism the ferric ion from oxidation of ferrous iron serves as a leaching agent that reacts chemically with the minerals. The increase of ferrous ion oxidation contributes to the intensification of bioleaching process. On this purpose the immobilization of iron oxidizing bacteria on different organic and inorganic carriers (calcium alginate, carragiran, ceramic support, activated carbon, porous glass, etc.) has been implemented which allows to increase cell concentration. In the present work for the first time the native shungit, zeolit and their chemically modified forms have been used for immobilization of new isolated iron oxidizing bacteria of the genera Leptospirillum and Sulfobacillus. Efficient physico-chemical conditions for immobilization on the mentioned carriers have been developed. The ferrous ion oxidation by immobilized cells has been studied both in shake-flasks experiments and in the glass column using air-lift process. It has been shown that in both cases the rate of iron oxidation considerably enhances in comparison with free cells.

Abstract: Treatment of acidic Fe (II)- and sulfate-rich mine waters represents a major problem in many areas of the world. Therefore, a process was developed which utilises naturally occurring sulfate-reducing microorganisms for the elimination of sulfate and of part of the acidity from the acidic mine water. In order to improve the performance of this biological sulfate reduction process an in-depth analysis of the microbial diversity and activity in dependence of the hydraulic retention time (HRT) and other process parameters used to run the bioreactors was undertaken. This comparison demonstrated a positive correlation between shorter HRT and increasing sulfate reduction rates. The improvement in performance with decreasing HRT was paralleled by an increase of the total enzymatic activity (measured as hydrolase activity) of the microbial community and of the biomass (measured as protein concentration) in the bioreactors. A partial taxonomic identification of the microbial community in the bioreactors was achieved via nucleotide sequence analysis of a clone library of PCR-amplified 16S rRNA gene fragments prepared from a sample of the microbial biofilm in the bioreactor. Additionally, the genetic fingerprint technique T-RFLP was used to assess temporal changes of the microbial community in the biofilm within the reactor.

Abstract: Shake flask bioleaching of a low-grade nickel-cobalt-copper sulfide ore from Lao was investigated to test the technical feasibility to recovery metals from this ore. The mineralogical results indicated that the valuable minerals were disseminated finely and the ore had a low content of sulfur and high content of iron and magnesium, which was very harmful for bioleaching. The effects of different parameters such as the amount of acid addition, inoculation volume, initial pH, temperature and leaching time on the bioleaching performance of the ore were evaluated after pre-leaching with dilute acid. The results showed that nickel, cobalt, copper leaching rates reached 81.61%, 80.75%, 70.97% at 33 °C and 83.40%, 82.13%, 70.34% at 45 °C. The mineralogical analysis of the leaching residues revealed that the leaching rate of nickel sulfide was more than 92% and the unleached nickel was mainly present as nickel silicate.

Abstract: Bioleaching of low-grade sulphide minerals is now an established process, with much interest in chalcopyrite. However few studies have been carried out on ores containing silicates gangue materials. Chalcopyrite has been reported to be refractory at ambient temperature. Several factors that influence bioleaching kinetics are well documented such as particle size, pH, temperature, galvanic interaction and microbial activity. The purpose of this research was to investigated the effect of pH as well as pre-leaching on bioleaching of silicate rich and low-grade chalcopyrite using mixed thermophilic cultures, with a view to maximize copper solubilization rate in a column reactor operated at 70°C. The column was packed with low-grade chalcopyrite of the size range -20+15 mm. Leaching was monitored at specific time intervals (3 days) by measuring the pH, the redox potential, the copper and iron concentration in the solution. The results of the investigation have shown that copper extracted was highest at pH 1.3 and at moderately low redox potential (410 – 430 mV) using Ag/AgCl electrode, and that pre-leaching contributed insignificantly to the leaching rate. At pH 2.5, the copper extraction was low due to the jarosite. Furthermore, the analysis XRD of leached residues has indicated that the main passivating products were gypsum, jarosite, hexahydrite, and silica. However, although low pH resulted to high copper recovery, the results also showed that the pregnant leach solution (PLS) contained high concentrations of dissolved ions which might have inhibited the microbial activities.

Abstract: Mining industry is a source of wealth but also of environmental pollution in Peru. In this study 12 colonies of actinomycetes were isolated in acidic cultures from mineral ores and concentrates from mines of the Peruvian highlands. The isolates were characterized phenotypically by microscopy and growth at different conditions as pH tolerance, temperature, and sodium chloride, heavy metals resistance; ferrous iron and thiosulfate oxidation. All isolates were identified as actinomycetes based on their cultural and spore characteristics. Most of the isolates were able to grow at 8 - 45°C and pH 4 - 11. 60% of isolates grew at 10% NaCl but none of them growth at 13%. Iron oxidation was shown by 60% of isolates at pH 4, but only 25% were able to oxidize iron at pH 2. Thiosulfate oxidation was not detected in any isolate. Most of the isolates showed capacity to grow in medium with 200 ppm of Pb, Fe, Zn and 100 ppm of Cu. All of the physiological characteristics found in this work indicate the potential of these isolates as source for bioremediation and bioleaching.

Abstract: Although the biological techniques application into sulfide minerals leaching to obtain valuable metals ​​is an important technological advance, the disadvantage of relatively slow kinetics bio-oxidation still limits its commercial application. However, it stimulates the research for constant improvement. Microorganisms are capable of performing their intended role with great efficiency under optimal conditions. Bioreactor design seeks to maintain certain environmental conditions that favour efficient microbial growth as pH, temperature, oxygen and carbon dioxide dissolution, stirring speed, etc. In an aerobic process such as bioleaching the optimum oxygen transference is extremely difficult to be achieved, because little oxygen dissolves in water. Furthermore the oxygen transference usually is facilitated by agitation, which is also required to mix the nutrients and maintain homogeneous solution, this transference is limited by the agitation speed because of the high power consumption, and the damage that undergoes the microorganisms submitted to excessive shear. In order to improve bio oxidation kinetics, in this paper we designed a new bioreactor, which combines elements of the stirred tank and airlift reactor, and introduces new elements, to their effective use in biohydrometallurgy. A new bio-reactor called RELBA was designed and build-up in the UNAM laboratories. It controls dissolved O₂ among other parameters and it prevents shear. Bioleaching tests of the ore, containing 0.135% Cu and 3.07% Fe, were performed in the orbital incubator and in the RELBA bio-reactor to 35°C using a mixed strain of Acidithiobacillus caldus, Acidihiobacillus thiooxidans and Leptospirillum ferriphilum, which confirm the advantages of the new bioleaching reactor.

Abstract: The volumetric oxygen transfer coefficient (k_La) was used to define the conditions necessary for minimum aeration and to eliminate potential oxygen limitation in bioleaching cultures of Acidithiobacillus ferrooxidans. The Michaelis constants for oxygen were 1.07 and 0.71 μmol O₂ l^-1 for the oxidation of ferrous iron and elemental sulphur, respectively. The critical oxygen concentration, below which oxygen limitation occurred, was determined to be 6.25 and 3.125 μmol O₂ l^-1 for the oxidation of ferrous iron and elemental sulphur, respectively. The (k_La)_crit values required to maintain oxygen-unlimited substrate oxidation for ferrous iron and elemental sulphur were 7.70 and 4.88 h^-1, respectively.

Abstract: The biological technologies application in sulfide minerals leaching to the recovery of valuables metal is a very important technological advance. They are environmental friendly and favor the sustainability of metals production in the mining and metals industry. Nowadays there is not enough basic information on molybdenite bioleaching, as well as there isn`t equip capable to obtain efficient molybdenum extractions. The difficulty depends in part on microorganisms tolerance to the molybdenum. An 85% molybdenum extraction in 6 months has been reported. This work assesses the molybdenite bioleaching from a concentrate (MoS₂) with thermophilic microorganisms, at a temperature of 65 °C, on a new reactor designed at the Faculty of Chemistry of the Universidad Nacional Autónoma de México, to obtain higher extractions of molybdenum. Since bioleaching an aerobic process, the oxygen transference is a difficult condition to achieve because its solubility in water is very low and in the air 20.9 %. In addition to homogenize and maintain constant the nutrients concentration, in this new bioreactor could be established the physical, chemical and biological conditions that lead to the microorganisms optimal growth, due to these are environmental sensitive. This new equip allows us mix the nutrients in a homogeneous way. Also is able maintain constant the nutrients concentration, and all controlled physical, biological and chemical parameters. Finally does not cause any microorganisms damage. The RELVA-ARBP bioreactor allows us the control of necessary conditions for a growth more efficient of the bacterial strains. This will allow leach more rapidly the ore, increasing the molybdenum extraction kinetics. The tests results of bioleaching molybdenite with a mixed strain of extreme thermophiles carried out at 65 °C in an orbital incubator and in the RELVA-ARBP bioreactor, show a better efficiency of the bioreactor used.

Abstract: Owing to lack of soluble potassium resource in some countries, it will be the competitive alternative to obtain potassium from insoluble K-bearing minerals (e.g. K-feldspar, biotite) in order to satisfy the requirements of potassium fertilizers for the agricultural sustainable development. In this work, seven fungal species including Penicillium oxalicum, P. ochrochloron, P. simplicissimum, P. spinulosum, Aspergillus niger, A. fumigatus, and A. oryzae were investigated for potassium extraction from biotite (9.64% K₂O, weight fraction). Experiments were carried out in a shaker (30 °C, 180 rpm) with biotite directly dispersed in the leaching medium or enclosed in a dialysis bag. The concentrations of potassium, silicon and aluminum in the bioleaching solution were measured by Inductive Coupled Plasma (ICP), and K release amounts of biotite by seven fungal species were compared. It was found that all the strains had the abilities to enhance the dissolution of biotite, and P. oxalicum showed the highest K release amount among seven tested strains. Furthermore, the metabolites of fungi in the bioleaching process were analyzed by high performance liquid chromatography (HPLC), and the surface morphologies of biotite before and after bioleaching were characterized by Scanning Electron Microscope (SEM). The mechanism of microbial release of potassium from biotite by Penicillium and Aspergillus was discussed.