Papers by Author: Irena Spasova

Abstract: Pyrometallurgical copper final slag was subjected to leaching by means microbial cultures of three different groups based on their optimum temperature for growth and activity: mesophilic and moderate thermophilic bacteria, and extreme thermophilic archaea. The leaching experiments were performed by the shake-flask technique and in agitated bioreactors under batch and continuous-flow conditions. The effect of the most essential factors (particle size, pulp density, pH, aeration) on this process was studied. The highest rates of extraction of the non-ferrous metals (Cu, Zn, Co) and iron were achieved by means of some archaea but at relatively low pulp densities (5 – 10%). Some moderate thermophilic bacteria were the most efficient at the higher pulp densities (15 – 20 %). The, leaching by some mesophiles at pH 3.0 – 3.5 was also very attractive since it was connected with high extractions of these metals (about 85 – 92% at 20% pulp density), much lower acid consumption and low solubiliation of fayalite which resulted in the production of pregnant solutions suitable for the recovery of the dissolved non-ferrous metals.

Abstract: A sulphide concentrate containing 31.4% total sulphur (in which 30.7% was sulphidic), 31.8% iron, 8.0% arsenic, 18.5 g/t gold and 32.9 g/t silver finely disseminated in pyrite and arsenopyrite as the most essential components was subjected to microbial pretreatment to expose the precious metals from the sulphide matrix. Three different types of microbial cultures were tested for their ability to oxidize the sulphides and to expose the precious metals for the subsequent chemical leaching: mesophilic chemolithotrophic bacteria with a temperature optimum for their growth and activity at about 32 – 37°C, moderate thermophilic bacteria with the relevant optimum at 53 – 59°C, and hyperthermophilic archaea at 75 – 84°C. The microbial pretreatment of the concentrate was carried out in reactors with mechanical stirring and enhanced aeration by air enriched in CO_2. It was found that sulphide oxidation of about 40 – 45 % was sufficient for achieving gold extraction higher than 90% during the subsequent leaching of the pretreated concentrate by different gold-solubilizing reagent (cyanide, thiourea, thiosulphate). The highest rate of sulphide oxidation was achieved by a mixed culture of hyperthermophilic archaea at 82°C but at relatively low pulp densities (within 7 – 9%). At higher pulp densities (within 10 – 25%) the highest rates were achieved by means of moderate thermophilic bacteria at 57 – 59°C.

Abstract: Samples of anaerobically digested sludges from municipal wastewater treatment installations containing several toxic heavy metals (Cu, Zn, Ni, Co, Cr, Mn, Fe), different organochemicals and biological pollutants (mainly bacteria of the genera Escherichia, Klebsiella, Streptococcus and Clostridium) were subjected to different procedures for detoxification to make possible their use in agriculture. It was found that the bioleaching of these sludges at 70 °C by mixed cultures of extremely thermophilic chemolithotrophic bacteria was the most efficient way to decrease below the relevant permissible levels all above-mentioned pollutants. The bioleaching was carried out in reactors with mechanical stirring and enhanced aeration (by CO₂ enriched air) under batch and continuous-flow conditions. The extraction of some metals (mainly of Cu and Zn) which were present in relatively high concentrations in some sludge samples was very efficient and exceeded 90 % within residence times of about 96 - 120 hours. The dissolved copper and zinc were recovered from the pregnant solutions after bioleaching by means of solvent extraction plus electrolysis. At the same time, the bioassimilable forms of N, P, K and the essential microelements were still present in sufficient concentrations in the pretreated sludges making them suitable for use in agriculture. This was confirmed by experiments in green-houses with plants intended for use in the bioremediation of post-mining landscapes.

Abstract: Experimental plots consisting of acidic and alkaline soils heavily contaminated with radionuclides (mainly U and Ra) and non-ferrous metals (mainly Cu, Zn, Cd, Pb) were treated in situ under real field conditions using the activity of the indigenous soil microflora. This activity was enhanced by suitable changes of some essential environmental factors such as pH and water, oxygen and nutrient contents of the soil. The treatment was connected with solubilization and removal of contaminants from the top soil layers (horizon A) due to the joint action of the soil microorganisms and leach solutions used to irrigate the soils (mainly acidophilic chemolothotrophic bacteria and diluted sulphuric acid in the acidic soil, and various heterotrophs and bicarbonate and soluble organics in the alkaline soil). The dissolved contaminants were removed from the soil profile through the drainage soil effluents or were transferred to the deeply located soil subhorizon B2 where they were precipitated as the relevant insoluble forms (uranium as uraninite, and the non-ferrous metals as the relevant sulphides) as a result of the activity of the sulphate-reducing bacteria inhabiting this soil subhorizon.

Abstract: A sample of black shale fraction of copper ore from the Lubin deposit, Poland, was subjected to pretreatment by different microorganisms to improve the copper recovery during the subsequent flotation. Chemolithotrophic bacteria grown on So and S2O3 2- were used to depress pyrite before flotation by means of ethyl xantate as collector. Sulphate-reducing bacteria were used to perform a prior sulphidization of the black shales before flotation by means of the same collector. Two-stage flotation experiments were carried out in which initially the gangue minerals of the ore were depressed by means of “silicate” bacteria and then, during the flotation of the non-floated product from the first stage, the pyrite was depressed by means of pretreatment with So- grown chemolithotrophs. In all these cases the microbial pretreatment had positive effect on the copper recovery.

Abstract: Acid drainage waters generated in the uranium deposit Curilo, Bulgaria, were treated by means of different passive systems such as natural and constructed wetlands, alkalizing limestone drains, permeable reactive multibarriers and a rock filter, used separately or in different combinations. The waters had a pH in the range of about 2 – 4 and contained radionuclides (uranium, radium), heavy metals (copper, zinc, cadmium, lead, nickel, cobalt, iron, manganese), arsenic and sulphates in concentrations usually much higher than the relevant permissible levels for waters intended for use in agriculture and/or industry. The water flow rate through the individual systems was different and not stable, and varied in the range approximately from 0.02 to 1.5 l/s. Efficient removal of pollutants was achieved by means of these systems during the different climatic seasons, even during the cold winter months at water and ambient temperatures close to 0 oC. The removal was due to different mechanisms but microbial sulphate reduction, biosorption by living and dead plant biomass and chemical neutralization played the main roles.