Microbial l e a c h i n g o f u r a n i u m f r o m flotation tailings in alkaline m e d i a V.I.Groudeva a and S.N.Groudev b a Department of Microbiology, Faculty of Biology, University of Sofia, 8 Dragan Tsankov Boul., Sofia 1421, Bulgaria. b Department of Engineering Geoecology, University of Mining and Geology, Studentski grad-Durvenitza, Sofia 1100, Bulgaria.
Different heterotrophic microorganisms were able to leach uranium from flotation tailings in alkaline solutions. The solubilization of uranium was carried out by at least two different mechanisms. The first mechanism was connected with the microbial production of peroxide compounds which turned the tetravalent uranium to the hexavalent state. The U 6+ was then solubilized as uranyl carbonate or as complexes with some organic compounds present in the solutions. The second mechanism was connected with the solubilization of uranium as complexes with some organic compounds, mainly organic acids, produced by the microorganisms.
1. INTRODUCTION Pollution of aqueous and terrestrial ecosystems with uranium and other radioactive elements is a persistent environmental problem at many active and abandoned mine sites. In most cases this pollution occurs in rich-in-pyrite uranium deposits and is connected with the oxidation of pyrite as a result of which acidic waters containing sulphuric acid, dissolved radionuclides and toxic heavy metals and solid iron precipitates are released to the environment. The oxidation is carried out mainly by the indigenous acidophilic chemolithotrophic bacteria, which oxidize also the tetravalent uranium to the hexavalent state (1,2). The U 6+ is then solubilized as uranyl sulphate. At the same time, pollution by uranium is observed also in rich-in-carbonates deposits, in waters with a neutral or a slightly alkaline pH. The role played by some microorganisms in the leaching of uranium in such medis has been documented (3-6) but the information on this subject is scare. Little is known about the mechanisms of leaching as well as about the conditions which are favorable for this process to proceed. This paper contain data from a study on the ability of different microorganisms to leach uranium in the media with an alkaline pH.
M A T E R I A L S
A N D
M E T H O D S
Tailings from a plant for processing polymetallic ores were used as an uranium-bearing mineral raw material in the experimental work. Data about the chemical composition, radiological characteristics and some essential parameters of the tailings are shown in Tables 1 and 2. Uranite, nasturanium and pitchblende were the main uranium minerals in the tailings. Quartz and feldspar were the main minerals of the host rock. Pyrite, chalcopyrite and galena were also presented in the tailings.
Table 1. Data about the chemical composition and some geotechnical parameters of the flotation tailings Index Chemical composition: - SiO2 - Fe2 03 - A1203 - CaO - MgO - S total - S sulphidic -U - Ca - Pb pH (H20) pH ( HC1 Net neutralization potential Bulk density Specific density Particle size: + 0.3 mm - 0.3 mm + 0.08 mm -0.08 mm
51.55 % 14.53 % 10.32 % 6.84 % 3.28 % 0.90 % 0.17 % 107 g/t 1883 g/t 427 g/t 7.8 7.1 79 kg CaCO3/t 1.61 g/cm3 2.91 g/cm 3 3.5 % 36.1% 60.4 %
A sample of drainage waters from the operating tailings pond in the deposit was used as a leach solution containing indigenous uranium-solubilizing microorganisms. The sample contained 820 mg/l total dissolved solids (mainly hydrocarbonates, sulphates, chlorides, Ca, Mg and Na), 23 mg/l of dissolved organic carbon, 0.53 mg/1 of uranium and had a pH of 7.5. Different synthetic leach solutions were also used in the experimental work. The leaching of flotation tailings was carried out in 2 1 Erlenmeyer flacks containing 100 g tailings and 500 ml solution on a rotary shaker (160 rpm), at 20~ for a different period of
321 time. The process was followed by periodic chemical analysis of aliquot samples taken from the leach solutions. After the end of leaching, the solid residues were washed with distilled water and were subjected to both chemical and radiological analysis. The dissolved uranium concentration was measured photometrically using the arsenazo III reagent. The dissolved metal concentrations were determined by ICP spectrophotometry. The radioactivity of the samples was measured by means of a low background gammaspectrophotometer ORTEC ( HpGe-detector with a high distinguishing ability). The specific activity of 226Ra was measured using a 10-litter ionization chamber. The isolation, identification and enumeration of microorganisms were carried out by methods described else where (7).
Table 2. Radiological characteristics of the flotation tailings Component
238 U 226 Ra
214pb / Bi 21o Pb 232 Th 4~
Nuclide specific activity, Bq/kg
1360 -+ 5 % 1500_+ 7 % 1370 _+6% 800 _+25% 16 + 6% 700 + 4 %
3. RESULTS AND DISCUSSION
The experiments for chemical leaching of the flotation tailings revealed that the sulphuric and hydrochloric acids were not very effective with respect to the uranium leaching (Table 3). The addition of oxidants such as the ferric ions or oxone (KHSO5) to the above-mentioned acid solutions increased considerably the uranium extraction. These results are an indication that most of the uranium in the tailings is in the tetravalent state and its solubilization is connected with a prior oxidation to the hexavalent state. Radium was also solubilized under such conditions but the effect of the oxidants was not so pronounced. The leaching of uranium and radium with oxalic acid was less efficient than the leaching with the above-mentioned strong mineral acids. Leaching with a high concentration of KC1 (3M) was efficient, especially with respect to the radium. The chloride ions in the drainage waters in the deposit probably contribute to the solubilization but their concentrations are relatively low for obtaining such high degrees of extraction of the radioactive elements. The carbonate leaching of uranium and radium was also efficient. The molecular oxygen obviously plays the role of uranium oxidant under such conditions. It must be noted, however that the addition of an alternative oxidant such as hydrogen peroxide increased markedly the extraction of both uranium and radium.
322 Table 3. Data about the chemical leaching of uranium and radium from the flotation tailings Leach solution
H2SO4 H2S04 4- Fe2(SO4)3
H2SO4 + Oxone (KHSOs) HC1 HC1 + FeCI3 HC1 + Oxone (KHSOs) Oxalic acid KC1 Na2CO3 + NaHCO3 Na2CO3 + NaHCO3 + H202 Na2CO3 + NaHCO3 + Cu 2+
Metal extraction, % U
80 16.9 15.0 10.4 23 1 20.3 7.9 31.4 32.5 42.4 33.2
62.0 59.4 11.3 56.3 51.9 6.8 15.0 35.0 50.5 37.4
Notes: The leaching was carried out in Erlenmeyer flasks on a rotary shaker (160 rpm) with 20% pulp density, at 20~ for 48 h. The above-mentioned reagents were used in the following concentrations 9 H2SO4- 0.4 M; Fe3+ -3 g/l (as Fe2(SO4)3 or FeCI3 ); oxone - 3 g/l; HC1 - 0.2 M; oxalic acid - 0.1M; KC1 - 3M; NaHCO3- 15 g/l ; H202 - 3 g/1 ; Cu 2+ ( as CuSO4) - 50 mg/1.
The leaching experiments with different microorganisms revealed that the solubilization of radioactive elements from the tailings was connected mainly with some microbially catalyzed processes ( Table 4). Uranium leaching in alkaline media markedly depended on the presence of both uranium oxidants and uranium-complexing agents secreted by the microorganisms. The most efficient leaching was carried out by microorganisms producing peroxide compounds and possessing a high catalase activity as well as by microorganisms producing organic acids. It was found that different heterotrophic bacteria were able to leach uranium by at least two different mechanisms. The first mechanism was connected with the microbial production of peroxide compounds which turned the tertavalent uranium to the hexavalent state. The U 6+ was then solubilized as uranyl carbonate or as complexes with some organic compounds present in the solutions. The second mechanism was connected with the solubilization of uranium as complexes with some organic compounds, mainly organic acids, produced by the microorganisms.
323 Table 4. Data about the microbial leaching of uranium and radium from the flotation tailings Microorganisms and leach solutions
Metal Extraction, % U
Sample of the above-mentioned water with the microflora + 0.5 g/1 (NH4)2HPO4
Sample of the above-mentioned water + 0.5 g/1 (NH4)2HPO4 + H2SO, (to pH 2.0)
Sample of the above-mentioned water + H2SO4 (to pH 2.0)
Mixed enrichment culture on nutrient medium with 2 % glucose (pH 7.5)
Mixed enrichment culture on nutrient medium with 2% peptone (pH 7.5)
Mixed enrichment culture on nutrient medium with 0.1% glucose + 0.1% peptone (pH 7.5)
Mixed enrichment culture of acidophilic chemolithotrophic bacteria grown on flotation tailings in the water from the deposit with 0.5 g/1 ( NH4)2 HPO4 (pH 2.0 with H2SO4)
The above-mentioned culture and medium with 3 g/l Fe2+ (as FeSO4)
Mixed enrichment culture of acidophilic chemolithotrofic bacteria grown on flotation tailings in the 9K (pH 2.0)
Drainage water from the deposit with natural microflora (pH 7.5)
Notes: The leaching was carried out in Erlenmeyer flasks on a rotary shaker (160 rpm) with 20 % pulp density, at 20~ for 14 days. The 9K nutrient medium (8) was prepared with distilled water ; all other media were prepared with drainage water from the deposit.
Data about the natural microflora of the drainage water from the polymetallic deposit are shown in Table 5. Different heterotrophic bacteria, mainly such related to genera Pseudomonas, Alcaligenes and Bacillus, were the prevalent microorganisms in the microbial community. It was demonstrated that this microflora was able to perform uranium leaching by the above-
324 mentioned mechanisms under laboratory conditions. Such leaching is probably carried out also under natural conditions. In the natural ecosystems, however, the number and activity of the relevant microorganisms are limited by the shortage of soluble organic substrates and some essential nutrients (mainly such used as sources of N and P). This was demonstrated by the increase of metal extraction in the experiments in which the leach solutions, i.e. the natural drainage water, were supplemented with an organic substrate (glucose and/or peptone) and ammonium phosphate. Radium was also solubilized by the heterotrophic microbial cultures in the alkaline solutions but at rates lower than those of the uranium. However, thorium was leached very efficiently. The leaching of toxic heavy metals (copper, lead) from the tailings was less intensive than that of the radioactive elements. These metals were solubilized mainly by means of microbially secreted organic acids. Copper was solubilized, although at low rates, even from the relevant sulphide minerals present in the tailings. This was due to the activity of some chemolithotrophic bacteria, mainly such related to the species Thiobacillus thioparus and Thiobacillus neapolitanus. These bacteria grow in media with neutral, slightly alkaline or slightly acidic pH values and enhance the oxidation of sulphide minerals by removing the passivation films of S~ deposited on the mineral surface as a result of different chemical, electrochemical and biological processes (9). It must be noted that portions of the radioactive elements and toxic heavy metals solubilized during the leaching were then retained by the microbial biomass in the solutions by means of different biosorption and bioaccumulation mechanisms. The acidification of the leach solutions by adding sulphuric acid (to a pH of 2.0) resulted in dramatic changes in the microflora composition of these solutions. The prevalent microorganisms under such conditions were the acidophilic chemolithotrophic bacteria related to the species Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirillum ferrooxidans. These bacteria are able to oxidize the sulphide minerals and U 4§ present in the tailings to the relevant sulphates and U 6+, respectively. Mixed enrichment cultures of acidophilic chemolithotrophic bacteria leached efficiently radioactive elements and heavy metals from the flotation tailings, especially in media supplemented with nutrients (sources of N and P) and Fe2§ ions, which are a very suitable substrate for the iron-oxidizing chemolithotrophs Thiobacillus ferrooxidans and Leptospirillum ferrooxidans. 81.9 % of the uranium was leached under such conditions within 14 days. It must be noted, however, that the natural conditions in the polymetallic deposit are not favorable for the growth and activity of the acidophilic chemolithotrophic bacteria and their role in the solubilization of pollutants from the flotation tailings is negligible. The data from this study revealed that the microbial leaching of uranium in alkaline solutions plays an essential geochemical role in the natural ecosystems and is the main mechanism connected with the radioactive pollution of these ecosystems. On the other hand, at the present time at least, the microbial leaching rates are lower than those achieved by the conventional chemical leaching of uranium by means of carbonate solutions which is largely applied under commercial-scale conditions for recovering this metal.
325 Table 5. Microflora of the drainage water used in this study Microorganisms
10 2 _ 10 3
$203 2- _ oxidizing chemolithothrophs (at neutral pH)
10 4_ 10 5
S~ oxidizing chemolithotrophs (at pH 2.0)
Fe 2+ _ oxidizing chemolithotrophs (at p n 2.0)
Anaerobic heterotrophic bacteria
10 4_ 10 5
Bacteria fermenting sugars with gas production
Aerobic heterotrophic bacteria
10 3 . 10 4
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