Coaxing salt pollution to "bloom" from the ground for easy removal

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Salt crystals (left) grow above the surface of a brine-contaminated soil after being treated with a crystallization inhibitor(Credit: Aaron Daigh)

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Salt pollution is one of the most serious and persistent environmental problems worldwide – not the least because once it gets into the soil, it's extremely difficult to get out. Now a team of scientists at North Dakota State University (NDSU) has come up with a way to chemically coax the salt out of soil to form crystalline "blooms" that can be harvested like so many saline cauliflowers.

Salt pollution in soil can take many forms. It can come from the highly saline brine used in fracking operations, road salting to melt snow, estuary flooding, or the cumulative effect of irrigation using brackish water. But according to NDSU, once salt gets into soil it can stay there for decades or centuries. Meanwhile, the land becomes useless for cultivation and susceptible to serious erosion.

The problem is that conventional methods of handling salt-contaminated soils are often time consuming, complicated, slow, or not very useful. Some, for example, require the complete removal of the soil, which is expensive and only shifts the problem, or trying to flood the salt out, which wastes water and often merely drives the salt into the groundwater to percolate back up again.

Salt forming a crust on brine-contaminated soil(Credit: Aaron Daigh)

The biggest hurdle is that as salt water evaporates, the salt crystallizes and forms a solid crust in the soil pores, which effectively seals and protects the salt below from extraction. Looking for a way to overcome this, the NDSU team led by Aaron Daigh came across a chemical called ferric hexacyanoferrate, also known as Prussian Blue, which was being used by conservationists to protect historic sites that had been contaminated by road salt.

What Daigh's team saw was that the ferric hexacyanoferrate caused the dissolved salts to crystallize out of the soil to form blooms that were high in water content on the soil surface. These blooms could then be easily shoveled up and removed. When the chemical was tested under laboratory conditions, the team found that over a seven day period it was able to remove 27 to 57 percent of the salt depending on the type of soil – promising a quick and simple decontamination method.

According to Daigh, the technique still requires field testing and there is a concern about toxicity because ferric hexacyanoferrate releases free-cyanides as it decomposes. However, he went on to say that the breakdown is very slow, taking decades to centuries, which is enough time for microorganisms to consume any toxins that may arise.

In addition, Daigh says that ferric hexacyanoferrate is only one chemical that could produce salt blooms, so future research will consist of seeking alternatives as well as how much and how often to apply it.

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