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.
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.
See our home page on how to order your subscription. We regret we can
only accept orders from Canada and the United States.
