New process for rinsing heavy metals from soils

When toxic heavy metals like lead and cadmium escape from factories or mines, they can pollute the nearby soil. Without easy means of removing these contaminants, fields must be cordoned off to prevent these toxins from entering the food chain, where they threaten human and animal health.

According to the Environmental Protection Agency, heavy metals have been found in thousands of locations across the country. Some have been cleaned up through a combination of federal, state and private sector efforts, but new technologies are still needed to deal with heavy metal contamination.

Today, a research team led by Stanford's materials scientist, Yi Cui, has invented a way to wash heavy metals from contaminated soils using a chemical process that looks a bit like coffee brewing.

As they describe on June 4 Nature CommunicationsThe researchers began by rinsing the contaminated soils with a mixture of water and chemicals that attract heavy metals. When this mixture enters the soil, the chemical detaches the heavy metals. The team members then collected this toxic infusion and filtered it through an electrochemical filter that captured the heavy metals out of the water. In this way, they cleaned the soil of heavy metals and recycled the water and the mixture of chemicals to filter them into more contaminated soils.

"This is a new approach to soil cleaning," said Cui, a professor of materials science, engineering and photonics. "Our next step is a pilot test to verify that what works in the lab is practical in the field and to determine the cost of this process."

Until now, his team has cleaned soil contaminated with lead and cadmium, two common and dangerous toxins, as well as copper, which is only dangerous in high concentrations. Cui thinks that this process of chemical cleaning and electrochemical filtration will work with other dangerous heavy metals like mercury and chromium, but other laboratory experiments are needed to demonstrate it.

More sacrificial plants

Cui said the project had begun two years ago when he and graduate student Jinwei Xu had been thinking about how to solve the fundamental problem: heavy metals bind to the ground and become virtually inextricable. Today, said Cui, cleaning can involve digging up contaminated soil and sequestering it somewhere. Agricultural researchers have also developed phytoremediation techniques, cultivating sacrificial plants in contaminated soil to absorb heavy metals, then harvesting them and taking them to an extraction and storage center. But phytoremediation can take several years of repeated harvests.

In search of a fast and cost effective way to extract heavy metals from contaminated fields, the researchers attempted to wash the toxic soil samples in clear water. They quickly realized that water could not break the chemical bond between heavy metals and the soil. They needed an additive to release the contaminants. They found the answer in a common chemical known as its initials: EDTA.

In retrospect, EDTA was the obvious choice because this same chemical is used to treat lead or mercury poisoned patients. The negatively charged EDTA binds so strongly to positively charged heavy metal particles that it pulls lead or mercury from the patient's tissues. The researchers felt that once dissolved in water, the negative hooks of EDTA would tear the heavy metals off the soil. The experiments have proved it. When water treated with EDTA infiltrated into contaminated soil, it would entrain heavy metals.

But the work of the team was only half done. The soil was clean, but the treated water was still toxic. They needed a way to separate EDTA from heavy metals in rinsing water and to capture these toxins once and for all.

Isolate heavy metals

Scientists knew that EDTA remained strongly negative even after capturing a positively charged metal particle. The researchers therefore constructed a sieve with electrical and chemical properties to separate negatively charged EDTA and positively charged heavy metals. The result was isolated heavy metals and a mixture of water and EDTA, ready to purify more soil.

In addition to lead and cadmium, the researchers tested the process on copper, which is only dangerous at high concentrations. Next Cui would like to conduct this experiment on other heavy metals, such as mercury, which are so toxic that they require special treatment to protect researchers. But he thinks the chemistry is so solid that he is confident in the lab's success. The big question is whether the process can be stepped up to handle tons of contaminated soil. Researchers have sought to patent the process through the Stanford Office of Technology Licensing and would like the opportunity to conduct a pilot project in a contaminated field.

"We do not really have any effective abatement technology for heavy metals," Cui said. "If this turns out to be practical on a large scale, it will be a significant advance."