Radioactive isotopes find their way into natural waters not only through reactor accidents, but also as medical waste. Researchers have now developed a special filter membrane that efficiently removes these radionuclides from contaminated water. This could help to purify medical waste water, but also help with the disposal of the contaminated cooling water in Fukushima.
In medicine, radioactive nuclides are used for cancer treatments or as contrast media in imaging processes. These substances are usually only weakly radioactive and have a short half-life of a few hours or days. Nevertheless, they contaminate the waters if they get into the sewage system or instead have to be stored at great expense. The radioactive substances produced in nuclear power plants and in nuclear accidents, such as in 2011 in Fukushima, Japan, are significantly more hazardous to health. There, radioactive material still finds its way into the atmosphere, the groundwater and the ocean from leaks in the destroyed reactors. In addition, large amounts of contaminated cooling water are generated, which are collected in large tanks on the site. Methods for cleaning are so far hardly effective. Nevertheless, the Japanese government plans to dispose of a total of over a million liters of contaminated water in the Pacific by 2022.
Filter membrane made from whey protein and activated carbon
Scientists working with Sreenath Bolisetty from the Swiss Federal Institute of Technology in Zurich want to prevent this. Four years ago, they presented a filter membrane that has been proven to efficiently clean water from heavy metals, some radioactive elements such as uranium and precious metals such as gold or platinum. This so-called amyloid-carbon hybrid membrane also has an economic advantage: Since the filter consists mainly of whey proteins – a waste product of the dairy industry – and activated carbon, its manufacture is neither time-consuming nor cost-intensive, but it is ecologically sustainable.
To find out whether their filter membrane is also suitable for use against other radioactive nuclides, the scientists have now used the membrane to clean water samples that contained several radioactive isotopes used in medicine. This included a sample from the wastewater of a Swiss hospital, which in addition to the radionuclides contained biological pollutants such as bacterial colonies. The scientists let the contaminated waste water run through the filter membrane. Then they examined the residues in the filter with the help of so-called single-photon emission computed tomography (SPECT) and positron emission tomography (PET) for radioactive substances. After the filtration, they also measured whether radiation was still coming from the cleaned samples.
Wide range of radionuclides intercepted
The result: the filter cleaned the radioactively contaminated wastewater so efficiently that afterwards hardly any radioactive radiation was detectable. The amyloid proteins contained in the membrane had removed most of the radionuclides. With nuclides like technetium-99m, iodine-123 and gallium-68, this filter cleaning achieved efficiencies of over 99.8 percent in just one filtration step. The test of the wastewater sample from the hospital was also successful: the radioactive isotopes iodine-131 and lutetium-177 contained in the sample were almost completely removed from the water. In addition, the researchers observed no bacterial growth after the filtration.
“The filter membrane eliminates radioactive isotopes on a broad basis,” summarizes Bolisetty’s colleague Raffaele Mezzenga. In principle, all radioactive isotopes that lie between the tested extremes technetium and uranium in the periodic table bind to the amyloids in the membrane. This also includes radioactive cesium, iodine, silver and cobalt – radionuclides that occur in Fukushima wastewater. Only tritium, which occurs in large quantities there, probably does not bind to the membrane because it is too small.
Chance for Fukushima
According to the researchers, their filter could therefore also be suitable for cleaning Japanese waters. “I am already convinced that Japan could use the filter membrane immediately and thus solve a serious environmental problem,” says Bolisetty. “If our assumption is confirmed, the volume of wastewater in Fukushima could be massively reduced with the help of the filter membrane, so that no radioactive water would have to be dumped in the Pacific.” The aim of the scientists is now to convince the Japanese government of this. First negotiations with a Japanese company involved in the renovation in Fukushima are already underway.
The researchers have already come up with a solution for dealing with the pollutants that have been filtered out: “Our membrane allows the volume of waste to be massively reduced and the radiating elements to be stored compactly and dry as solids,” says Mezzenga. The filters saturated with the highly radiating elements could be stored as solids where, for example, spent fuel rods from nuclear power plants are also stored. The filtered liquids, on the other hand, could be safely discharged into the sewer system. As soon as the absorption capacity of the membrane is exhausted, it can be replaced, adds the expert.
Source: Swiss Federal Institute of Technology Zurich; Technical article: Environmental Science, doi: 10.1039 / D0EW00693A)