New carbon material can help clean up radioactive water

Researchers at Rice University and Kazan Federal University in Russia have developed a new material that can extract radioactivity from water and so could help purify the hundreds of millions of gallons of contaminated water stored after the Fukushima nuclear plant accident.

The researchers report in a paper in Carbon that their oxidatively-modified carbon (OMC) material is inexpensive and highly efficient at absorbing radioactive metal cations, including cesium and strontium. These toxic elements were released into the environment when the Fukushima plant melted down after an earthquake and tsunami in March 2011.

OMC can easily trap common radioactive elements found in water floods from oil extraction, such as uranium, thorium and radium, said Rice chemist James Tour. He led the project with Ayrat Dimiev, a former postdoctoral researcher in his lab and now a research professor at Kazan Federal University.

The material makes good use of the porous nature of two specific sources of carbon. One is an inexpensive, coke-derived powder known as C-seal F, which is used by the oil industry as an additive to drilling fluids. The other is a naturally-occurring, carbon-heavy mineral called shungite, which is found mainly in Russia.

Tour and researchers at Lomonosov Moscow State University had already demonstrated a method to remove radionuclides from water using graphene oxide as a sorbent, as reported last year in Solvent Extraction and Ion Exchange. But this new research suggests OMC is easier and far less expensive to process.

OMC is made up of carbon particles that are 10–80µm wide. Treating these particles with oxidizing chemicals increases their surface area and ‘decorates’ them with the oxygen molecules needed to adsorb the toxic metals.

While graphene oxide excelled at removing strontium, the two types of OMC were better at extracting cesium, which Tour said has been the hardest element to remove from water stored at Fukushima. The OMC was also much easier and less expensive to synthesize and use in a standard filtration system, he said.

"We know we can use graphene oxide to trap the light radioactive elements of relevance to the Fukushima clean-up, namely cesium and strontium," Tour said. "But in the second study, we learned we can move from graphene oxide, which remains more expensive and harder to make, to really cheap oxidized coke and related carbons to trap these elements."

While other materials used for remediation of radioactive waste need to be stored with the waste they capture, carbon presents a distinct advantage. "Carbon that has captured the elements can be burned in a nuclear incinerator, leaving only a very small amount of radioactive ash that's much easier to store," Tour said.

"Just passing contaminated water through OMC filters will extract the radioactive elements and permit safe discharge to the ocean," he added. "This could be a major advance for the clean-up effort at Fukushima."

The two flavors of OMC particles – one from coke-derived carbon and the other from shungite – look like balls of crumpled paper, or roses with highly irregular petals. The researchers tested them by mixing the sorbents with contaminated water and by conducting column filtration, a standard process in which fluid is pumped or pulled by gravity through a filter to remove contaminants.

For the mixing test, the researchers dispersed nonradioactive isotopes of strontium and cesium in spring water, added OMC and stirred for two hours. After filtering out the sorbent, they analyzed the particles left in the water.

OMC1 (from coke) proved best at removing both cesium and strontium from contaminated water, with its performance getting significantly better with increasing concentrations of the sorbent. A maximum 800 milligrams of OMC1 removed about 83% of cesium and 68% of strontium from 100 milliliters of water. OMC2 (from shungite) at the same concentration adsorbed 70% of cesium and 47% of strontium.

The researchers were surprised to see that plain shungite particles extracted almost as much cesium as its oxidized counterpart. "Interestingly, plain shungite was used by local people for water purification from ancient times," Dimiev said. "But we have increased its efficiency many times, as well as revealed the factors behind its effectiveness."

In column filtration tests, which involved flowing 1400 milliliters of contaminated water through an OMC filter in 100-milliliter amounts, the filter removed nearly 93% of cesium and 92% of strontium in a single pass. The researchers were able to contain and isolate contaminants trapped in the filter material.

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.