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    Uranium removal vital to public health


    By Chris Hansen

    Lakes in the Canadian Shield region often have high levels of uranium, posing problems for communities using them as a water source. ES&E Photo

    The radioactive chemical element, Uranium (U) is not at all rare. There is more uranium in the earth’s crust than cadmium, bismuth, silver, mercury and iodine. High concentrations of uranium are found in an ore deposit that covers much of central Canada. A uraniumrich band of earth runs across the top of Saskatchewan, through central Manitoba, along the top of the Great Lakes, eventually heading in a south eastern direction cutting through eastern Ontario. Naturally occuring uranium can be found around eastern areas of Canada all the way south to the New England area of the United States.

    Some important uranium ores include pitchblende, uraninite, carnotite, autunite, and torbenite, which are found throughout the country. Uranium in groundwater may occur in all areas near this natural ore deposit.

    These ores are sources of nuclear fuel and can produce tremendous amounts of energy in comparison to fossil fuels. One pound of uranium yields as much energy as three million pounds of coal.

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    When ingested, uranium attacks the bone structure and the kidneys. This destruction takes place over a long period of time due to the exceptionally low radioactivity per unit volume of the two major isotopes, U-235 and U-238. Evidence has shown that the cancer-causing alpha particle emission closely resembles that of radium and that the health effects published for radium can be used as a surrogate for uranium. The current detection level for uranium is 0.5 pCi/l (picocuries per litre), and the Canadian Federal Drinking Water Guidelines indicate a maximum contaminant level (MCL) of 20 mg/l (30 pCi/l).

    Treatment methods

    There are several treatment methods for reducing the level of uranium in water, including distillation and electrodialysis reduction. Perhaps the most effective method is reverse osmosis. The RO membrane works as a molecular filter that rejects positively and negatively charged ions based on molecular weight. Uranium and uranium complexes are very heavy, which allows the RO process to work in the 95 – 99 percent rejection range. Reverse osmosis does have limitations around process efficiency and may incur high maintenance and operational costs due to the effects of mineral deposition on the membrane. There may also be difficulty dealing with high rejection waste volumes from a waste management standpoint.

    Lime softening has proven to be effective, but is also very pH-dependent. The addition of lime in the dosage range of 100 – 200 mg/l should raise the pH to 10 – 11. This has proven to be effective for removing 80 percent of the uranium present. To obtain higher levels of removal, it is necessary to add magnesium carbonate (MgCO3) with the lime. When dosage levels of MgCO3 are in the 100 mg/l range, tests show that greater than 95 percent of the uranium can be removed. Lime softening will produce a sludge cake containing uranium and disposal of the residual solids must be considered.

    Another treatment method is conventional coagulation/filtration, using aluminum sulfate (alum) or a ferrrous sulfate. In the dosage range of 10 – 30 mg/l and a pH of less than or equal to 10, it is possible to remove greater than 90 percent of the uranium present. This process is also pH-dependent. Tests show that lowering the pH to the 4 – 8 range will result in a drop in performance to below the 50 percent removal range. Like lime softening, the management of residual solids from coagulation and filtration must be taken into consideration.

    Ion exchange is an effective means of reducing the level of uranium in water. Both cation and anion resins have been evaluated for uranium removal. Cation resin is most effective when used in the hydrogen form. It is thought that when the uranium carbonate complex passes through the acid bed, it is reduced to a uranium cation. This is very effective for obtaining removal rates in the 90 – 95 percent range, but the effluent will have a pH of less than or equal to 3.5. Cation resin in the sodium form can produce results in the 70 percent removal range when the pH is less than 7.

    Anion resin has proved to be very effective due to the fact that in surface and groundwater supplies, the uranium will usually exist as an anion complex. Tests show that anion resin in the chloride form is capable of reducing uranium levels by 90 – 99 percent. Ion exchange often proves to have the highest process efficiency resulting in low aqueous waste volumes.

    In summary, there are several effective means of reducing the uranium content in ground and surface water supplies. The best treatment method would have to be determined for individual applications and requirements.

    Chris Hansen is V.P. Engineered Systems/Community Water Systems at Kinetico Incorporated. For more information, contact Community Water Canada at 519-927-9500.