Stoyan Groudev

Treatment of waters polluted with radioactive elements and heavy metals by means of a laboratory passive system

Waters polluted with toxic heavy metals, radioactive elements, arsenic and sulphates were efficiently treated by means of a laboratory-scale passive system consisting of an anaerobic cell and a constructed wetland. These waters were polluted in a stream receiving leachates from an operating copper tailings dam and other effluents from man-induced activities at the area of Vromos Bay in Bulgaria. The anaerobic cell contained a mixture of horse, cow and sheep manure, spent mushroom compost, wheat straw and sawdust which were used as sources of carbon and energy by different metabolically interdependent microorganisms inhabiting the cell. The microbial dissimilatory sulphate reduction and the sorption on the organic matter were the main processes connected with the removal of pollutants. The effluents from the anaerobic cell were enriched in soluble organic compounds which were degraded in the constructed wetland containing rich microbial and plant cenoses. The effluents from the constructed wetland were suitable for use in the agriculture and industry.

Bioremediation of a soil contaminated with radioactive elements

Abstract Some agricultural lands located in the Vromos Bay area, near the Black Sea coast, Southeastern Bulgaria, have been contaminated with radioactive elements (uranium, radium and thorium) and toxic heavy metals (copper, cadmium and lead) as a result of mining and mineral processing of polymetallic ores. Laboratory experiments carried out with soil samples from these lands revealed that an efficient remediation of the soils was achieved by an in situ treatment method based on the activity of the indigenous soil microflora. The treatment was connected with the dissolution of the contaminants in the upper soil horizons and their transfer into the deeply located soil horizons (mainly to the horizon B2) where they were immobilized as different insoluble compounds. The dissolution of contaminants was connected with the activity of both heterotrophic and chemolithotrophic aerobic microorganisms and the immobilization was due mainly to the anaerobic sulphate-reducing bacteria. The activity of these microorganisms was enhanced by suitable changes in the levels of some essential environmental factors such as water, oxygen and nutrient contents in the soil. On the basis of the above-mentioned laboratory results, the method was then applied under real field conditions in a heavily contaminated experimental plot of land located in the Vromos Bay area. Within 8 months of treatment, the contents of radioactive elements and toxic heavy metals in the soil were decreased below the relevant permissible levels.

Bioleaching of a Rich-In-Carbonates Copper Ore at Alkaline pH

A rich-in- carbonates copper ore was subjected to bioleaching under alkaline pH conditions by means of different microorganisms or their metabolites. The ore contained 1.40% copper, 1.94% sulphur, 3.25% iron and 20.3% carbonates, and had a pH of 8.6 and a highly positive net neutralization potential (325 kg CaCO3/t). Copper was present mainly as different sulphide minerals (bornite, covellite, chalcopyrite). The leaching was carried out by means of the shake-flask technique at 32 oC using finely ground (minus 100 μm) ore. The following microorganisms were used to leach the sample: ammonifying bacteria (related to the genera Bacillus, Acinetocater and Vibrio); heterotrophic bacteria (Acetobacter and Pseudomonas) and fungi (Asperillus niger and Penicillium chrysogenum) producing citric acid; heterotrophic bacteria (Micrococcus, Alcaligenes and Bacillus) producing amino acids (mainly alanine); basophilic chemolithotrophic bacteria (Thiobacillus thioparus, Hallothiobacillus neapolitanus, Starkeya novella; ”silicate bacteria” (Bacillus circulans) producing organic acids and exopolysaccharides; bacteria possessing urease enzymatic activity (Corynebacterium). The best results were achieved by means of a mixed culture of urease-possessing bacteria, which under certain conditions was able to solubilize 64.4% of the copper within 30 days of leaching.

Bioremediation of Acid Mine Drainage in an Uranium Deposit

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.

In Situ Bioremediation of Contaminated Soils in Uranium Deposits

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.

Bioleaching of a Copper Sulphide Ore at Different Temperatures

Copper sulphide ore containing 0.37% Cu, with chalcopyrite as the main copper-bearing mineral, was leached in percolation columns containing 20 kg of ore crushed to minus 20 mm at different temperatures 18, 50 and 70°C. The columns were inoculated with cultures of mesophilic, moderately thermophilic and extremely thermophilic chemolithotrophic bacteria, respectively. Different variants of leaching with respect to the composition of the leached solution and the rate of irrigation of the ore were applied at each of the temperatures above-mentioned. The best results were achieved at leaching carried out at 70°C, with periodic additions of ferric ions, nutrients and bacterial cells by leach solutions prepared in a separate BACFOX-type unit, and by application of enhanced aeration of the ore by air enriched in CO2. 82.1% of the copper was leached in this way within a period of 230 days, while the highest Cu extraction achieved at 18°C was 28.0% and that at 50°C was 55.4%, within the same period of time.

Treatment of acid drainage in a uranium deposit by means of a passive system

Acid mine drainage generated in the uranium deposit Curilo, Bulgaria, with a pH of about 2.5 - 4.0 and polluted with radionuclides (U, Ra), heavy metals (Cu, Zn, Cd, Pb, Ni, Co, Fe, Mn), arsenic and sulphates, were treated efficiently by means of a passive system consisting of a permeable reactive multibarrier and a constructed wetland. The multibarrier consisted of an alkalizing limestone drain and an anoxic section for microbial sulphate reduction, biosorption and additional chemical neutralization. The water flow rate trough the multibarrier varied in the range of 1 - 17 m3/24 h, reflecting water residence times of about 300 to 18 h.

Bioleaching of Valuable Components from a Pyrometallurgical Final Slag

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.

Comparative Variants of Microbial Pretreatment and Subsequent Chemical Leaching of a Gold-Bearing Sulphide Concentrate

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 CO2. 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.

Passive treatment of heavily polluted drainage waters in a uranium deposit

The treatment of acid mine drainage in the uranium deposit Curilo, Bulgaria, was very efficient for a long period of time. However, the treatment of such waters containing high concentrations of iron, including a portion in the ferrous state, caused serious problem on the efficiency of the cleaning process. These problems were avoided by the pretreatment of these waters by a system for iron removal consisting of a unit for bacterial oxidation of the ferrous ions to the ferric state and a unit for precipitation of the ferric iron as a result of chemical neutralization.