A small tributary of the River Clyde in Glasgow makes the headlines every so often, when it turns a particularly alarming shade of greenish yellow. This Burn is contaminated from groundwater which has passed through waste materials that were landfilled from a local chemical works that processed chromium from the 1800s until the late 1960s. Although the plant has long since closed, the remaining waste is seeping into the environment. In its hexavalent state, Cr(VI), chromium is highly toxic, damaging DNA, causing cancer, and has the potential to pose a significant threat to humans and wildlife. While continued efforts are being made, in conjunction with local authorities, to prevent chromium from entering the Burn, and to dilute the polluted waters, this is not a problem that is unique to Glasgow. Inspired by the chemical reactions that take place in concrete, researchers from the University of Edinburgh are investigating a new technique to remove hexavalent chromium from contaminated aquatic systems around the world. Using powder X-ray diffraction (PXRD) on Diamond's I11 beamline, they've demonstrated the potential for tricalcium aluminate (C3A, Ca3Al2O6) to remove Cr(VI) from solution. Their work, recently published in RSC Advances, is a step towards a low-energy remediation technique for hexavalent chromium.
The pollution of waterways with hexavalent chromium is a growing global problem due to historical and ongoing industrial processes, including tanning, textile preservation and chrome plating. If this polluted water is left untreated, it can affect freshwater drinking water supplies and, ultimately, the marine environment.
In Glasgow, the pollution in the Burn comes from a decommissioned industrial plant that emplaced chromite ore processing residue (COPR) in landfills. The COPR contains chromium in multiple oxidation states and, unfortunately, about 25% is hexavalent chromium (Cr(VI)), the most toxic of these and also the most readily transferred to groundwater.
One common remediation strategy is to reduce soluble Cr(VI) to the less harmful and insoluble Cr(III). The COPR is, however, extremely alkaline and, at high pH (11 and above), the effectiveness of traditional environmental reductants (e.g. metallic Fe, Fe(II) and organic materials) is limited. Researchers from the University of Edinburgh are investigating a new method of remediation that would safely lock up hexavalent chromium so that it can be removed from the environment.
Lead author Dr Rebecca Rae explains:
We started looking at the mineral ettringite, which is a product of cement hydration. We realised that, in cement, ettringite forms through tricalcium aluminate reacting with a sulfate source. So we theorised that if we could mimic that with a chromate source, it might be possible to turn the chromium into cement essentially. These were our proof-of-concept experiments to see if that would work.
For these initial experiments, the team created solutions with varying concentrations of potassium chromate (K2CrO4). Then, after adding tricalcium aluminate, they took samples of the resulting liquid and solid at various intervals. Their results showed that - in all concentrations - the amount of chromium in the solution reduced, and they could identify the chromium in the solid products.
Dr Rae said:
We used a mixture of laboratory and synchrotron PXRD to fully characterise the solid products. The solid was a mixture of different phases, which were hard to separate with the lab data. But the data we collected at I11 was much higher resolution, enabling us to resolve the exact mixture of all these crystalline phases, and their evolution over time.
Their results show that this new remediation method works in the laboratory, but there's a lot more research needed before it can be tried in the field. The team is currently conducting more experiments, moving towards more realistic conditions, and may soon be ready to test it on actual water samples from the Burn in Glasgow. Dr Rae noted:
This work was at the start of the process, checking whether this works at all. These results show it's viable to take forward, and we currently have other projects based on things that have spun out from this. For example, we're looking at the stability of the solid products, to ensure that they don't leach the chromium back out. And this experiment used a simple chromium solution, so we're investigating what happens when we add in other competing ions and make the solution more like actual wastewater.
More generally, by 2025, the volume of industrial wastewater produced globally is expected to reach double its 2007 level. With many countries already experiencing water scarcity, there are many environmental, economic and public health benefits to cleaning up our water supply. Therefore, understanding remediation processes will be increasingly important, and studies such as this are essential first steps.
Senior author Dr Caroline Kirk says:
We're developing a flow cell for use on other projects, and when we've got the design right, we'll be able to do in situ experiments and watch this process happening in real-time. Now that we know the idea is feasible, we need to work on making it more and more realistic, building up to the actual environmental conditions. The advantage of this remediation method is that it creates a solid that we can remove from the burn. We would then be able to encapsulate the waste in cement that will withstand the high pH, so the chromium is unlikely to leech out. We can then bury it, as we do radioactive waste.
To find out more about the I11 beamline or discuss potential applications, please contact Principal Beamline Scientist Stephen Thompson: stephen.thompson@diamond.ac.uk.
Rae R et al. Investigating the hydration of C3A in the presence of the potentially toxic element chromium–a route to remediation? RSC advances 12.45 (2022): 29329-29337. DOI: 10.1039/D2RA04497H.
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