Energy-efficient way to treat water contaminated with metals could be applied at mine sites

Energy-efficient approach for treating water contaminated with heavy metals could be applied at mine sites Red lagoon at the Lousal mine in Portugal. 

A new water treatment process being developed at MIT could be adapted for industrial uses such as cleaning water produced in mining or drilling operations so that the treated water can be safely disposed of or reused.  (Reference image by Gustavo Veríssimo, Wikimedia Commons) .

In a paper published in the journal Environmental Science and Technology – Water, researchers Huanhuan Tian, Mohammad Alkhadra, Kameron Conforti and Martin Bazant explain that the process could even provide a way of recovering metals that contaminate water but could be used to produce semiconductors or pharmaceuticals or other high-tech products.

The new approach focuses on removing lead or other heavy metal contaminants from water and 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 brine and a fresh stream. This mechanism allows for a 95% reduction of lead from the outgoing fresh stream.

THE PROCESS COULD EVEN PROVIDE A WAY OF RECOVERING METALS THAT CONTAMINATE WATER BUT COULD BE USED TO PRODUCE SEMICONDUCTORS OR PHARMACEUTICALS OR OTHER HIGH-TECH PRODUCTS

In principle, “this makes the process much cheaper [than currently applied methods] 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,” Bazant said in a media statement. “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.”

In the case of drinking water, sodium is typically present 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 selective removal, but it’s counterproductive since small amounts of elements such as sodium and magnesium are essential for healthy water.

The new system is the latest in a series of applications based on initial findings made 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 process, however, still has its limitations, as it has only been demonstrated at a small laboratory scale and at slow flow rates. Scaling it up to make it practical for in-home use will require further research, and larger-scale industrial uses will take even longer. 

“The main issue would be on the economic side,” Bazant said. “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.”

In the scientist’s view, the results of his research are still exciting because it shows that it is possible to tackle the issue of lead and heavy metals in water. 

“For example, 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.”

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Taxonomy

• Heavy Metal Removal
• Heavy metals