Green rust – a good guy!

When you encounter rust of the brown variety it is, by all accounts, bad news. If your pride and joy car, favourite bike or most useful tools fall prey to rusting you lament their ageing and take steps to remove the rust and restore your metal object to its former glory. But what about green rust? What exactly is green rust and how does it form?

In the mineral world, green rust is very much the new kid on the block. It only made it onto the official mineral list in 2004 when it was given the mineral name fougerite. This is because you don’t find it very often and, when you do, you have just a few minutes to study it before it transforms into common brown rust.

Aggregate of green rust particles each approx 1 micron in size, copyright Sam Shaw, University of Leeds

However, a few forward thinking environmental science groups around the world are turning their attention to green rust. Here in the UK, we are pioneering studies that we hope will harness green rust’s potential to immobilise toxic and radioactive species in contaminated land (e.g. radioactive waste repositories), including uranium, chromium, selenium and zinc.

We know that the unique chemistry of the green rust particles mean it can transform (reduce) the contaminants into insoluble and less mobile forms preventing the contamination from spreading and dramatically reducing their bioavailability.

The problem with green rust is that it only forms in soils and sediments when the level of oxygen is very low, and in air it will transform to normal brown rust within a few minutes. This has prevented researchers from finding out exactly how and under what conditions it forms. To overcome these problems we have recreated the conditions conducive to green rust formation within a contaminated land site on the beamline at Diamond. Using a complex chemical reaction cell, and with the unique power and intensity of the X-ray beam, we have been able to gain a unique insight in to the atomic scale formation and crystallisation of green rust under conditions close to which it forms in contaminated land environments. This work will provide valuable information to predict how and where green rust will form and what effect it will have on the behaviour and migration of pollutants within soil and groundwater.

Time-resolved scattering experiments conducted at the Synchrotron Radiation Source and Diamond Light Source have allowed us to directly observe the formation and transformation pathways of green rust under condition close to those in which they may form in contaminated soils. In particular, we made the first in situ observation of the transformation of brown rust (ferric oxyhydroxide) nanoparticles to various green rust phases at the atomic scale. The studies have shown how the nanoparticles form and what chemical constituents and environmental/geochemical conditions are required to form and stabilise different types of green rust.

Combining the information from of the mineral formation experiments and synchrotron (SRS) studies into the speciation/molecular chemistry of various contaminants (e.g. uranium, zinc, selenium) we have been able to determine when, how and under what conditions they are immobilised by GR. Finally, recent studies at Diamond we have been able to show the incorporation of caesium into the GR structure during its formation.

We are just starting to build up a picture of the potential of green rust as a remediation technology for contaminated land such as radioactive waste repositories. This is significant not only for the work that is needed to clean up the land that has been polluted in the past, but also to responsibly plan for dealing with contamination events in the future.

Diamond is enabling us carry out our experiments incredibly fast and being able to bring the contaminated landsite onto the beamline means that we are, for the first time, seeing exactly how the green rust is effecting the toxic and radioactive species in the soil. On the beamline, we can do in one day what it would take years for us to achieve back in the laboratory. This may bring the reality of a green rust remediation technology within our grasp within years.

Work is ongoing to fully understand, on the nanoscale, how green rust is managing to immobilise toxic and radioactive species in the soil, linking the contaminant chemistry to what is actually happening to the mineral at the atomic and nano scale. Also, to predict how and when it will form in a site and what influence this will have on contaminant movement and bioavailability.

We are also making progress on altering the state of the green rust itself, to make it a more stable compound to enable us to harness the potential of green rust for a longer period of time.

Dr Sam Shaw, University of Leeds