Green rust belts could help keep pollution contained

4 November 2008 by Tom Marshall

A rare kind of rust could be used to help scientists stop pollution leaking from contaminated sites into the wider environment, according to new research using powerful X-ray beams generated by specialist equipment called synchrotrons.

Diamond synchrotron

The Diamond synchrotron: six hours on a synchrotron saves six months in the lab.

Green rust is different to the brown rust that forms when metallic iron like that found in a car is exposed to water and air. Brown rust contains 'ferric' iron, which is iron in its most oxidised form, whereas green rust contains a high concentration of reduced and reactive 'ferrous' iron.

That's the key to its potential for cleaning up pollution - but also the reason researching that potential is so difficult. Green rust forms only in conditions without much oxygen. Exposed to air, it changes form and turns brown within minutes.

This reactivity is also the reason green rust could be so much help in cleaning up polluted sites - if scientists can find a way to keep it in its reactive state for long enough. The rust reduces elements like chromium, uranium, selenium and technetium, significantly reducing their solubility and mobility in the environment, and in some cases absorbing them into the rust's molecular structure.

Compounding the difficulty of studying green rust is that its particles are tiny, just a micron across and a few nanometres thick. Stacked in a column, it would take about 40,000 such particles to stretch the width of a pinhead. Normal microscopes aren't up to the job - hence the need for the multimillion-pound synchrotrons.

Dr Sam Shaw, an environmental mineralogist in Leeds University's School of Earth and Environment has been studying these unstable minerals.

He hopes green rust's unique properties will mean it can be used within a green rust filled trench or permeable reactive barrier around contaminated land sites.

Groundwater would move through the site and pick up pollutants; these would be reduced to an insoluble form and trapped in the rust's structure as the water flowed through the barrier. This would mean none of the pollutants could escape the site and harm the wider environment.

"We hope that by choosing a particular composition of green rust and the right method of emplacement, we could develop an effective permeable reactive barrier that would keep any pollutants from escaping the site," says Shaw.

"This would break the pathway of contaminant transport," he adds, explaining that it might eventually even be possible to recycle metals like chromium that had been trapped in the rust barrier.

Keeping nuclear waste contained

Another application where green rusts could be critical is in geological repositories for radioactive waste. These are proposed as a possible long-term solution to the problem of long-lived radioactive waste.

The hazardous material would be packed in stainless steel canisters and sealed by clay or concrete in rock half a kilometre underground.

As the steel canisters slowly corrode green rust could form in these repositories and provide a further barrier to the release of radioactivity.

Rust particles

Electron microscope image of green rust particles

The formation of green rust could also be promoted or emplaced in the repository and this may also help preserve the canister. Shaw's team has been working with Dr Joe Small, an environmental geochemist at the UK National Nuclear Laboratory, to investigate these possibilities.

They plan to use their data to create a computer model to represent how green rust may form and uptake contaminants over thousands of years.

But these applications would depend on forming green rust stable enough to persist in the environment and not simply convert into brown rust.

Shaw's team have begun to make big strides already; they have managed to create green rust which remains stable for hours or even days, whereas normally it decays within 20 minutes or so of exposure to oxygen.

They did this using information gathered from experiments they have performed using synchrotron technology. Synchrotrons use magnetic and electrical fields to accelerate particles round a huge ring-shaped chamber.

As these charged particles approach the speed of light, they give off radiation called 'synchrotron light'. Researchers divert this light away from the main ring and down a tangential 'beamline', where they use it like a supercharged X-ray to study the structure of matter at tiny scales.

In this case, Shaw and team used techniques called small- and wide-angle X-ray scattering to study the molecular structure of green rust as it formed in solution, using a specially-designed reaction vessel to ensure conditions very much like those under which it's thought to develop in nature.

"Green rust is so unstable that it's extremely difficult to study," says Shaw. "Using the synchrotron lets us analyse it in conditions very close to those in contaminated soil."

He adds that the facility lets them take a snapshot of the vessel's contents every two minutes, meaning researchers can track the process of formation, investigating in detail what conditions green rust needs to form and exactly how it interacts with pollutants.

Synchrotron equipment

The team's reaction cell on the beamline at Synchrotron Radiation Source in Daresbury

The facility has let the researchers work far more quickly and accurately than would otherwise have been possible; Shaw estimates that the team can do in six hours on a synchrotron what would take a researcher six months in the lab - and the data gained is more accurate and reliable, as scientists don't need to remove the rust from its controlled environment in order to test its properties.

The work was split between the Synchrotron Radiation Source facility in Daresbury, which closed in 2008 after 28 years of operation, and its replacement Diamond Light Source in Oxfordshire, which started experiments early in 2007. The team has arranged additional time at Diamond next year to continue the research.

In the longer term, Shaw hopes the research could have a commercial application. He intends to apply within a month or two for a pathfinder grant from NERC's follow-on funding programme; this will let him assess whether green rust technology could be patented and commercialised.