Economical and energy-efficient approach to the treatment of water contaminated with heavy metals



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Engineers at MIT have developed a new approach to removing lead or other heavy metal contaminants from water, in a process they say is far more energy efficient than any other system currently in use, though there are others in development which are similar. Ultimately, it could be used to treat lead contaminated water supplies at the home level, or to treat water contaminated by certain 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, originally developed for the desalination of seawater or brackish water. , and later adapted to remove radioactive compounds from nuclear power cooling water. plants. The new version is the first such method that could be applicable for the treatment of domestic water supply, as well as for industrial uses.

The results are published today in the journal Environmental sciences and technologies– Water, in an article by MIT graduate students Huanhuan Tian, ​​Mohammad Alkhadra and Kameron Conforti, and chemical engineering professor Martin Bazant.

“It is notoriously difficult to remove toxic heavy metals which are persistent and present in many different water sources,” says Alkhadra. “Obviously there are competing methods out there today that perform this function, so it’s a question of which method can do it more cheaply and more reliably.”

The biggest challenge in trying to remove lead is that it is usually present in such small concentrations, far exceeded by other elements or compounds. For example, sodium is typically present in drinking water at a concentration of tens of parts per million, while lead can be highly toxic at only a few parts per billion. Most existing processes, such as reverse osmosis or distillation, remove everything at once, Alkhadra explains. This not only takes a lot more energy than would be needed for selective disposal, but it’s counterproductive as small amounts of elements like 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 shock wave inside a pipe carrying contaminated water. The shock wave separates the liquid into two streams, selectively pulling certain electrically charged atoms or ions to one side of the stream by adjusting the properties of the shock wave to match the target ions, while leaving a stream of d relatively pure water on the other side. The stream containing the concentrated lead ions can then be easily separated using a mechanical barrier in the pipe.

In principle, “it makes the process a lot cheaper,” Bazant says, “because the electrical energy you put in to make the separation really goes after the high-value target, which is the head. You don’t waste a lot of money. energy to remove sodium. ” Because lead is present at such a low concentration, “there is not a lot of current involved in removing these ions, so it can be a very cost effective way.”

The process still has its limitations, as it has only been demonstrated on a small laboratory scale and at fairly slow flow rates. Extending the process to make it practical for home use will require additional research, and larger scale industrial uses will take even longer. But it could be practical in a few years for some home systems, says Bazant.

For example, a house with a water supply that is heavily contaminated with lead might have a system in the basement that slowly treats a stream of water, filling a tank with lead-free water to use for drinking and cooking, while still leaving most of the water untreated for uses like flushing the toilet or watering the lawn. Such uses might be appropriate as an interim measure for places like Flint, Mich., Where water, mainly contaminated by distribution lines, will take many years to resolve with replacement of lines.

The process could also be adapted to certain industrial uses such as cleaning the water produced in mining or drilling operations, so that the treated water can be safely disposed of or reused. And in some cases, it could also provide a way to recover metals that contaminate water but could in fact be a valuable commodity if separated; for example, some of these 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 compared to existing methods are difficult, Bazant says, because in filtration systems, for example, the costs are mostly for the replacement of filter materials, which quickly clog and become unusable, while in this system the costs are mainly intended for the continuous energy supply, which is very low. At this point, the shock electrodialysis system has been operating for several weeks, but it is too early to estimate the actual longevity of such a system, he says.

Developing the process into a scalable business product will take some time, but “we’ve shown how it could be done, from a technical perspective,” Bazant says. “The main problem would be economic,” he adds. This includes determining the most suitable applications and developing specific configurations that would meet those uses. “We have a reasonable idea of ​​how to extend that. So it’s about having the resources,” which could be a role for a start-up rather than a university research lab, he adds. he.

“I think this is an exciting result,” he says, “because it shows that we can really tackle this important application” of cleaning lead from drinking water. For example, he says, there are now places that do reverse osmosis seawater desalination, but they have to perform this expensive process twice in a row, first to remove the salt and then again to remove low level but highly toxic contaminants like lead. This new process could be used instead of the second reverse osmosis cycle, at a much lower energy expenditure.

A new way to remove contaminants from nuclear wastewater

More information:
Huanhuan Tian et al, Continuous and selective elimination of lead from drinking water by electrodialysis shock, ACS ES&T Water (2021). DOI: 10.1021 / acsestwater.1c00234

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