Engineers at MIT have developed a new approach to removing lead or other heavy-metal contaminants
from water, in a process that they say is far more energy-efficient than any other currently
used system, though there are others under development that come close.
Ultimately, it might be used to treat lead-contaminated water supplies at the home level, or to treat
contaminated water from some chemical or industrial processes.
The new system is the latest in a series of applications based on initial findings six
years ago by members of the same research team, initially developed for desalination
of seawater or brackish water, and later adapted for removing radioactive compounds from the
cooling water of nuclear power plants.
The new version is the first such method that might be applicable for treating household
water supplies, as well as industrial uses.
The findings are published in the journal Environmental Science and Technology – Water.
The biggest challenge in trying to remove lead is that it is generally present in such
tiny concentrations, vastly exceeded by other elements or compounds.
For example, sodium is typically present in drinking water at a concentration of tens
of parts per million, whereas lead can be highly toxic at just a few parts per billion.
Most existing processes, such as reverse osmosis or distillation, remove everything at once.
This not only takes much more energy than would be needed for a selective removal, but
it’s counterproductive since small amounts of elements such as sodium and magnesium are
actually essential for healthy drinking water.
The new approach is to use a process called shock electrodialysis, in which an electric
field is used to produce a shockwave inside a pipe carrying the contaminated water.
The shockwave separates the liquid into two streams, selectively pulling certain electrically
charged atoms, or ions, toward one side of the flow by tuning the properties of the shockwave
to match the target ions, while leaving a stream of relatively pure water on the other
side.
The stream containing the concentrated lead ions can then be easily separated out using
a mechanical barrier in the pipe.
In principle, this makes the process much cheaper.
The process still has its limitations, as it has only been demonstrated at small laboratory
scale and at quite slow flow rates.
Scaling up the process to make it practical for in-home use will require further research,
and larger-scale industrial uses will take even longer.
But it could be practical within a few years for some home-based systems.
For example, a home whose water supply is heavily contaminated with lead might have
a system in the cellar that slowly processes a stream of water, filling a tank with lead-free
water to be used for drinking and cooking, while leaving most of the water untreated
for uses like toilet flushing or watering the lawn.
Such uses might be appropriate as an interim measure for places like Flint, Michigan, where
the water, mostly contaminated by the distribution pipes, will take many years to remediate through
pipe replacements.
The process could also be adapted for some industrial uses such as cleaning water produced
in mining or drilling operations, so that the treated water can be safely disposed of
or reused.
And in some cases, this could also provide a way of recovering metals that contaminate
water but could actually be a valuable product if they were separated out; for example, some
such minerals could be used to process semiconductors or pharmaceuticals or other high-tech products.
Direct comparisons of the economics of such a system versus existing methods is difficult,
because in filtration systems, for example, the costs are mainly for replacing the filter
materials, which quickly clog up and become unusable, whereas in this system the costs
are mostly for the ongoing energy input, which is very small.
At this point, the shock electrodialysis system has been operated for several weeks, but it’s
too soon to estimate the real-world longevity of such a system, the researcher says.
There are places now that perform desalination of seawater using reverse osmosis, but they
have to run this expensive process twice in a row, first to get the salt out, and then
again to remove the low-level but highly toxic contaminants like lead.
This new process might be used instead of the second round of reverse osmosis, at a
far lower expenditure of energy.