A new method for removing lead from drinking water
Engineers have designed a relatively low-cost, energy-efficient approach
to treating water contaminated with heavy metals.

Date:
September 23, 2021
Source:
Massachusetts Institute of Technology
Summary:
Engineers designed a relatively low-cost, energy-efficient approach
to treating water contaminated with heavy metals such as lead.



FULL STORY ========================================================================== 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, in a paper by MIT graduate students Huanhuan Tian, Mohammad Alkhadra, and Kameron Conforti, and professor of chemical
engineering Martin Bazant.

"It's notoriously difficult to remove toxic heavy metal that's
persistent and present in a lot of different water sources," Alkhadra
says. "Obviously there are competing methods today that do this
function, so it's a matter of which method can do it at lower cost and
more reliably." 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, Alkhadra explains. 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 uses a process called shock electrodialysis, in which
an electric field is used to produce a shockwave inside an electrically
charged porous material carrying the contaminated water. The shock
wave propagates from one side to the other as the voltage increases,
leaving behind a zone where the metal ions are depleted, and separating
the feed stream into a brine and a fresh stream. The process results in
a 95 percent reduction of lead from the outgoing fresh stream.



==========================================================================
In principle, "this makes the process much cheaper," Bazant says, "because
the electrical energy that you're putting in to do the separation is
really going after the high-value target, which is the lead. You're
not wasting a lot of energy removing the sodium." Because the lead is
present at such low concentration, "there's not a lot of current involved
in removing those ions, so this can be a very cost-effective way."
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,
Bazant says.

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, the researchers say.

Direct comparisons of the economics of such a system versus existing
methods is difficult, Bazant says, 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,
he says.

Developing the process into a scalable commercial product will take
some time, but "we have shown how this could be done, from a technical standpoint," Bazant says. "The main issue would be on the economic side,"
he adds. That includes figuring out the most appropriate applications
and developing specific configurations that would meet those uses. "We
do have a reasonable idea of how to scale this up. So it's a question
of having the resources," which might be a role for a startup company
rather than an academic research lab, he adds.

"I think this is an exciting result," he says, "because it shows that
we really can address this important application" of cleaning the lead
from drinking water. For example, he 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.

The research received support from a MathWorks Engineering Fellowship
and a fellowship awarded by MIT's Abdul Latif Jameel Water and Food
Systems Lab, funded by Xylem, Inc.

========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by David
L. Chandler. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Huanhuan Tian, Mohammad A. Alkhadra, Kameron M. Conforti, Martin Z.

Bazant. Continuous and Selective Removal of Lead from Drinking
Water by Shock Electrodialysis. ACS ES&T Water, 2021; DOI: 10.1021/
acsestwater.1c00234 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210923115612.htm

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