Biological Processes for Concentrating Trace Elements from Uranium Mine Waters
Wastewater from uranium mines in the Ambrosia Lake district near Grants, New Mexico, U.S.A., contains uranium, selenium, radium and molybdenum. The Kerr-McGee Corporation has a novel treatment process for waters from two mines, sections 35 and 36, to reduce the concentrations of the trace contaminants. Particulates are settled by ponding, and the waters are passed through an ion exchange resin to remove uranium; barium chloride is added to precipitate sulfate and radium from the mine waters. The mine waters are subsequently passed through three consecutive algae ponds prior to discharge. Water, sediment and biological samples were collected over a 4-year period and analyzed to assess the role of biological agents in removal of inorganic trace contaminants from the mine waters.
Conclusions derived from chemical analyses of waters and sediments of the mine water treatment facility are:
- The concentrations of soluble uranium, selenium and molybdenum were not diminished in the mine waters by passage through the series of impoundments which constituted the mine water treatment facility. Uranium concentrations were reduced but this was due to passage of the water through an ion exchange column.
- The particulate concentrations of the mine water were reduced at least ten-fold by passage of the waters through the impoundments. Since uranium, selenium and molybdenum were associated with the suspended particulates reduction in the concentration of total suspended solids reduced the concentration of contaminants in the final effluent.
- The ponds were well oxygenated. This was probably due to the shallow depth of the ponds and growth of algae.
- The pH remained near neutral during all collections and this would indicate that the major soluble species of trace contaminants would be molybdate, uranyl carbonate species, selenite (and possibly selenate at the higher pH values).
- The temperature remained higher than 10øC in winter suggesting that plant and microbial life could remain viable and that the volume of water was great enough to lessen large temperature fluctuations.
- Phosphate and nitrate were present in high enough concentrations to support limited algal and microbial activity.
- The sediments were anoxic and enriched in uranium, molybdenum and selenium. The deposition of particulates and the formation of insoluble compounds were proposed as mechanisms for sediment enrichment.
Algal populations in the mine water treatment facility were identified and their contribution in removal of inorganic trace contaminants was assessed by field and laboratory studies. The predominant algae identified in the impoundments were the filamentous algae Spirogyra and Oscillatoria, and the benthic alga, Chara. Seasonal variations in both uranium and molybdenum levels in the filamentous algae were observed in field-collected samples. This suggested that adsorptive processes were important in the accumulation of the metals in the algae cells, since extent of adsorption depends not only on the amount of metal available per unit of surface area but also on the length of exposure of the surface to the metal. The results of 24-hour uptake experiments in the laboratory supported the field evidence. Short-term uptake of uranium and molybdenum was not observed in Chara, which accumulated both metaIs at much lower levels than the filamentous algae in the field. In the Spirogyra 24-hour test, the cell material showed a limited capacity to adsorb molybdenum, while uranium uptake increased with higher external concentrations.
The long-term laboratory studies indicated that the pond algae, in the form of particulate, decaying material, can be instrumental in removing metals from solution. However, the patterns of retention and release of uranium and molybdenum as the algae decayed in the presence of sediment indicated that maintenance of reducing conditions in the sediment or in the algal cultures was critical to the sequestering of the metals. The implication from the laboratory studies is that, while the algae are instrumental in removing metals from solution, the process was reversible unless the system contained substantial organic material. Also, while organic material accelerated the rate of removal and sometimes the extent of removal from water, retention of the metals in sediments was also reversible unless the system contained a high volume of sediment. These conclusions point to major problems in improving the existing pond system in respect to removal of uranium and molybdenum. Calculations of existing pond productivity and the current removal of uranium and molybdenum by the algae present indicated that both greater algal populations and pond areas are necessary for removal of the trace contaminants.
Large populations of microorganisms were found in the waters pumped from the uranium mines and water passing through the pond system. Of particular interest was the presence of the sulfate reducing bacteria, believed to be Desulfovibrio and/or Desulfotomaculum. Laboratory experimentation indicated that the sulfate-reducing bacteria may have a role in removal of uranium, selenium, and possibly molybdenum from solution. Sulfide production by the organisms may be important since molybdenum can be precipitated as a sulfide. The reducing conditions may be responsible for converting soluble hexavalent uranium to the insoluble quadrivalent form. Although a large population of sulfate-reducing bacteria was found in the pond sediments, there was no decrease of soluble sulfate, which remained at 700 ppm in the water flowing through the system. Reduced forms of sulfur may be oxidized by the aerobic thiobacilli in oxidizing regions, returning the sulfur to the soluble sulfate species. The system may be nutrient limited and the activity of the sulfate-reducing bacteria quite slow.
Project No. 1345639