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An unfortunate consequence of many industrial and
manufacturing practices, from textile factories to metalworking operations, is
the release of heavy metals in waterways. Those metals can remain for decades,
even centuries, in low but still dangerous concentrations.
Ridding water of trace metals “is really hard to do,”
says Joseph Calo, professor emeritus of
engineering who maintains an active laboratory at Brown University.
He notes the cost, inefficiency, and time needed for such efforts. “It’s like
trying to put the genie back in the bottle.”
That may be changing. Calo and other engineers at Brown
describe a novel method that collates trace heavy metals in water by increasing
their concentration so that a proven metal-removal technique can take over. In
a series of experiments, the engineers report the method, called the cyclic
electrowinning/precipitation (CEP) system, removes up to 99% of copper,
cadmium, and nickel, returning the contaminated water to federally accepted
standards of cleanliness. The automated CEP system is scalable as well, Calo
says, so it has viable commercial potential, especially in the environmental
remediation and metal recovery fields. The system’s mechanics and results are
described in Chemical
Engineering Journal.
A proven technique for removing heavy metals from water
is through the reduction of heavy metal ions from an electrolyte. While the
technique has various names, such as electrowinning, electrolytic
removal/recovery or electroextraction, it all works the same way, by using an
electrical current to transform positively charged metal ions (cations) into a
stable, solid state where they can be easily separated from the water and
removed. The main drawback to this technique is that there must be a high-enough
concentration of metal cations in the water for it to be effective; if the cation
concentration is too low—roughly less than 100 ppm—the current efficiency
becomes too low and the current acts on more than the heavy metal ions.
Another way to remove metals is through simple chemistry.
The technique involves using hydroxides and sulfides to precipitate the metal
ions from the water, so they form solids. The solids, however, constitute a
toxic sludge, and there is no good way to deal with it. Landfills generally
won’t take it, and letting it sit in settling ponds is toxic and
environmentally unsound. “Nobody wants it, because it’s a huge liability,” Calo
says.
The dilemma, then, is how to remove the metals
efficiently without creating an unhealthy byproduct. Calo and his co-authors,
postdoctoral researcher Pengpeng Grimshaw and George Hradil, who earned his
doctorate at Brown and is now an adjunct professor, combined the two techniques
to form a closed-loop system. “We said, ‘Let’s use the attractive features of
both methods by combining them in a cyclic process,'” Calo says.
It took a few years to build and develop the system. In
the paper, the authors describe how it works. The CEP system involves two main
units, one to concentrate the cations and another to turn them into stable,
solid-state metals and remove them. In the first stage, the metal-laden water
is fed into a tank in which an acid (sulfuric acid) or base (sodium hydroxide)
is added to change the water’s pH, effectively separating the water molecules
from the metal precipitate, which settles at the bottom. The “clear” water is
siphoned off, and more contaminated water is brought in. The pH swing is
applied again, first redissolving the precipitate and then reprecipitating all
the metal, increasing the metal concentration each time. This process is
repeated until the concentration of the metal cations in the solution has
reached a point at which electrowinning can be efficiently employed.
When that point is reached, the solution is sent to a
second device, called a spouted particulate electrode (SPE). This is where the
electrowinning takes place, and the metal cations are chemically changed to
stable metal solids so they can be easily removed. The engineers used an SPE
developed by Hradil, a senior research engineer at Technic Inc., located in Cranston, R.I.
The cleaner water is returned to the precipitation tank, where metal ions can
be precipitated once again. Further cleaned, the supernatant water is sent to
another reservoir, where additional processes may be employed to further lower
the metal ion concentration levels. These processes can be repeated in an
automated, cyclic fashion as many times as necessary to achieve the desired
performance, such as to federal drinking water standards.
In experiments, the engineers tested the CEP system with
cadmium, copper, and nickel, individually and with water containing all three
metals. The results showed cadmium, copper, and nickel were lowered to 1.50,
0.23, and 0.37 ppm, respectively—near or below maximum contaminant levels
established by the Environmental Protection Agency. The sludge is continuously
formed and redissolved within the system so that none is left as an
environmental contaminant.
“This approach produces very large volume reductions from
the original contaminated water by electrochemical reduction of the ions to
zero-valent metal on the surfaces of the cathodic particles,” the authors
write. “For an initial 10 ppm ion concentration of the metals considered, the
volume reduction is on the order of 106.”
Calo says the approach can be
used for other heavy metals, such as lead, mercury, and tin. The researchers
are currently testing the system with samples contaminated with heavy metals
and other substances, such as sediment, to confirm its operation.
