Controversy over new method to cut atmospheric CO₂ levels.

Researchers at the University of Massachusetts, US, have stumbled on what they believe could be the most effective route yet to reducing atmospheric carbon dioxide levels. They were surprised to find that by mixing liquefied CO₂ with pulverised limestone in water, an emulsion was formed (D Golomb et al, Environ. Sci. Technol., 2004, 38, 4445). This emulsion, they say, could be suitable for release into the ocean where it would then become sequestered. This represents ’the only way’ forward for reducing anthropogenic CO₂ from the atmosphere, team leader Dan Golomb told Chemistry World.

However, the method may not find favour with environmental groups. ’Ideas like this miss the point with deadly accuracy,’ said Rob Gueterbock, Greenpeace climate campaigner. ’The only way we know we can tackle climate change is to reduce emissions at source through energy efficiency and renewable energy. Dumping our carbon waste into the ocean would be a dangerous experiment we can’t afford to undertake.’

Golomb and colleagues are aware of the opposition they face and initially set out to develop a benign way of sequestering CO₂. Alternative methods for sequestering have been pro-posed, including releasing liquid CO₂ directly into the sea at depths of 1000m, which acidifies the surrounding sea water and is harmful to sea life, and ’iron fertilisation’ designed to increase the growth of algal blooms and increase CO₂ uptake (See Chemistry World, May 2004, p15).

The team’s initial idea was to mix water with CO₂ under pressure and to add a slurry of pulverised limestone to neutralise the carbonic acid formed. However, they were surprised to find that, under certain conditions, a stable macro-emulsion was formed. This macro-emulsion, which Golomb calls a ’globulsion’, consists of tiny CO₂ droplets finely coated in limestone, dispersed in the water. The macro-emulsion which is denser than sea water could then be released into the sea at a depth of little over 500m - a significant reduction in time and cost of transportation. In addition, the globulsion is formed with half the weight of limestone that the team originally thought necessary to fully neutralise the CO₂. The group has also achieved similar results when using olivine (magnesium iron silicate) and fly-ash, a by-product of thermal power stations.

The globules are stable in the laboratory over several hours with no obvious signs of degradation, and once released would ’rain out’ and sink to the ocean floor, says Golomb. He feels that eventual dispersion of the globulsion should not cause many problems as the release is well below the photic zone of the ocean where the majority of sea life is. ’There are not a lot of critters down there,’ he said. ’There are critters down there but I’m not even sure these globules would do any harm’.

But Gueterbock is swift to counter these claims. ’The deep oceans, even below 500m, are by no means abiotic zones,’ he said. ’They are just some of the poorest studied ecosystems on the planet. Species compositions and overall biodiversity will vary from location to location,’ he adds, ’but it is simply inaccurate to state that there are "not a lot of critters" at these depths.

’Finally, "only" needing half a tonne of limestone for every tonne of CO₂ would mean chopping up billions of tonnes of limestone every year. It might work in the lab, but rather like the southern ocean iron fertilisation proposals, it is totally impractical.’

Golomb’s group is now working with collaborators at the Monterey Bay Aquarium Research Institute, California, US, on a practical delivery system for the macro-emulsion, working to carry out a pilot-scale project. Golomb is optimistic the method will win the backing of environmental groups and plans to present preliminary results at the Seventh international conference on greenhouse gas control technologies in Vancouver, Canada, in September.

Vikki Allen