Rare earths from industry and hospitals end up in wastewater
Rare earth metals such as cerium and gadolinium are increasingly entering wastewater from industry and hospitals. This is shown by Eawag investigations at 63 wastewater treatment plants in Switzerland.
Without rare earth metals, almost nothing would work today. There would be no smartphones, flat screens, LED lamps, rechargeable batteries, electric motors or many other electronic devices. In the high-tech industry, for example in the automotive, electronics and energy sectors, and in medicine, it is impossible to imagine life without these valuable raw materials. It is therefore of increasing interest where the rare earths disappear after use.
On behalf of the Federal Office for the Environment (FOEN), the Eawag Water Research Institute has, for the first time, taken a closer look at rare earth elements in Swiss wastewater. For this purpose, a team of researchers from the two departments of Process Engineering and Water Resources and Drinking Water studied the sewage sludges of 63 Swiss wastewater treatment plants (WWTPs). The most important thing to know first: rare earths used in industry and hospitals not infrequently end up in wastewater.
Challenge: Distinguishing between natural and man-made sources.
First, the research team estimated the amount of rare earths that comes from natural sources. This is the only way to assess the proportion added by humans. To do this, the researchers analyzed soil samples from Switzerland, taking into account so-called PAAS values (post-archaean Australian shales), which reflect the average composition of rare earths in the earth's crust. This gave them the naturally occurring rare earth composition in Switzerland - the background pattern. In addition, the research team developed two new methods to infer the content from industrial sources from the concentrations measured in wastewater and the natural pattern.
The result: In the sewage sludge of most ARAs, the concentrations of rare earths found correspond to the natural background pattern. However, in a few WWTPs, particularly those in Yverdon, Bioggio, Hofen and Thal, the concentrations of individual rare earths were significantly elevated. The research team concludes that rare earths are not used on a large scale, but come from highly specialized applications in industry.
The highest concentrations were recorded for Cerium (also called cerium) was detected. Cerium dioxide is often used in industry as an abrasive. In Switzerland, more than 4,000 kilograms of cerium reach wastewater treatment plants every year, about half of it from industrial applications. A very large proportion of this, about 95 percent, remains in the sewage sludge. The rest ends up in the environment. The researchers therefore assume that elevated cerium concentrations will also be found in lakes, rivers and groundwater in the near future.
Rare earths such as cerium (Ce) and Gadolinium (Gd) occur naturally, but are also increasingly used in industry and hospitals. The rare earths enter wastewater treatment plants via wastewater. A large part of the cerium is separated there in the sewage sludge. Gadolinium, however, is hardly retained in the wastewater treatment plant and flows into the water with the treated wastewater.
Medical contrast agents suspected as source
A special case is gadolinium: Already 20 years ago, elevated concentrations were detected in water bodies in Europe. The source was assumed to be wastewater from hospitals. In the current Eawag study, the researchers found around 80 per cent of the total gadolinium from industrial sources in the Ramsen wastewater treatment plant near Lake Constance on the German-Swiss border. The ARA treats wastewater from the town of Singen in Germany, where a cancer center with MRI facilities is located. Gadolinium-based contrast agents are also produced in the region. The results therefore confirm the previous assumption that the gadolinium found in the sewage sludge is due to the production or use of contrast agents. Appropriate measures to reduce the input of industrial gadolinium into wastewater by industry have already been taken and will lead to a marked reduction in the gadolinium load.
Unlike the other rare earths, the concentration of lanthanum is elevated in the sewage sludge of practically all the ARAs studied. One possible cause could be biological processes that alter the retention of lanthanum in the sewage sludge. Another explanation would be the use of lanthanum-enriched fertilizers in agriculture. However, the extent to which biological processes or fertilizers are responsible for the elevated levels still needs to be investigated in more detail.
Original publication: Ralf Kaegi, Alexander Gogos, Andreas Voegelin, Stephan J. Hug, Lenny H.E. Winkel, Andreas M. Buser, Michael Berg: Quantification of individual Rare Earth Elements from industrial sources in sewage sludge. Water Research X. https://doi.org/10.1016/j.wroa.2021.100092
Fact sheet by Eawag and the Ecotox Centre: Ecotoxicity of rare earths
Bafu information: Rare engineering metals
