December 2021 ESRFnews 17

DEEP CARBON CYCLE

Jon Cartwright

Aside from the deep carbon cycle (see main text), the ESRF is helping to tackle the environmental crisis in a number of ways. For example:

This year, a group of researchers from China, the US and the ESRF showed how structural defects affect lithium-ion battery performance in a systematic study of failed cells at the ID16A nano-imaging beamline (Cell. Rep. Phys. Sci. 2 100554). Many other ESRF beamlines are used for battery research; the ESRF is soon to launch an EU battery hub based on its new access model (see News, p6) and a memorandum

of understanding with the French Alternative Energies and Atomic Energy Commission.

Last year, Yunhui Chen at University College London in the UK and colleagues used high-flux, high-energy X-rays at the ESRF s ID19 beamline to study the fast melt- solidification cycles of additive manufacturing or 3D printing in situ, in order to develop lighter, greener alloys (Appl. Mater. Today 20 100650). Additive manufacturing is the subject of many other ESRF experiments, including a PhD project with the global aluminium-alloy manufacturer Constellium within the InnovaXN doctoral programme.

Last year, scientists at the Toulouse Biotechnology Institute (TBI) in France and Carbios, a French start-up company, used the ESRF s structural-biology beamline ID30B to help solve the structure of a new enzyme that can quickly degrade plastic waste (Nature 580 216). Many scientists come to the ESRF to study photovoltaics. Last year, researchers in the Netherlands used grazing-incidence wide- angle X-ray scattering at the BM26 beamline to study the crystallisation of tin-based perovskites, a promising solar-cell material (Adv. Funct. Mater. 30 2001294).

SOLUTIONS IN THE PIPELINE

higher this concentration, the more a metal carbonate had dissolved, and the greater its solubility. Metal ions fluoresce at relatively low energies

energies that are easily absorbed by diamonds which meant that the researchers had to employ special, hollowed-out diamonds to gather data for most of the carbonates they studied. Even then, the fluorescence energy of magnesium carbonate was so low that the researchers could not gather X-ray data. For that reason, they measured solubility for manganese and zinc car- bonates, in addition to those based on calcium. With knowledge of the underlying chemistry that determines solubility, they could then extrapolate the solubility for magnesium carbonate. This was a very challenging experiment, says Rosa.

But the EBS will make it much easier, as the flux allows us to work at lower energies and detect smaller concentration changes, opening new windows into natural systems. Redfern, Rosa and colleagues found that, as carbonates

become magnesium-rich as evidence suggests they do at increasing depth they decrease in solubility by at least two orders of magnitude. The result implies that carbon- ates in subduction zones cannot easily dissolve in order to travel to the surface again, as Keleman and Manning had predicted. Instead, Redfern and colleagues estimate that most carbon-containing carbonates about two-thirds continue to sink into the Earth s interior, never to be seen again (Nat. Commun. 12 4311).

Rewriting history The implications of this are great. It means that, slowly but surely, subduction has helped to transform the Earth s cli- mate, from being hot and carbon-rich billions of years ago, to the relatively cool, hospitable place it is today. There seems to be a relation between the speed of subduction which didn t exist at all in the early Earth and the speed at which atmospheric carbon dioxide has reduced over the millennia, says Rosa. This does not mean that subduction of carbon can alle-

viate the climate crisis. The timescales are completely different, says Redfern. It takes hundreds of millions of years for a bit of carbon at the Earth s surface to be

subducted and come back again. We ve been emitting carbon from the burning of fossil fuels over just a 100- year timescale. In any case, real-world data does not lie: atmospheric carbon dioxide continues to rise, whatever is going on below ground. Yet the study does give hope for the climate in another

way. By determining in which carbonates carbon is least soluble, the researchers have identified potential hosts for carbon sequestration. In fact, according to Redfern, the entire line of research stemmed from the question of how carbonates could help suck carbon dioxide out of the atmo- sphere. We ve explored the natural route for taking carbon dioxide out of the ocean and atmosphere, he says. The chemistry of negative carbon emissions is exactly the same.

This type of geo-engineering is in its early stages (see Burying the problem , opposite), but it is becoming more and more urgent. In August, the latest report by the Intergovernmental Panel on Climate Change concluded that to achieve temperature rises less than 1.5 °C above pre-industrial levels, negative carbon emissions through geo-engineering will be a necessity, in addition to what- ever promises emerge from COP26. The idea is not so far-fetched as some may think. Redfern points out that there is a precedent for quickly sequestering a component of the atmosphere: fertiliser manufacture, which feeds on atmospheric nitrogen and has resulted in unnatural levels of the element sequestered albeit undesirably in water- ways and oceans. We humans are very good at modifying our environment, he says. 

We ve explored the natural way of taking carbon dioxide out of the atmosphere. The chemistry

of negative carbon emissions is exactly the same