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Chromium: the story of two sunflowers #IYPT

Van Gogh's Sunflowers masterpiece contains chromium. So do the sunflowers that cover the fields in the summer. Chromium is an extremely reactive element. The contact with the air and light makes it change its form at an extremely fast rate and this has consequences, which can be devastating.

Marine Cotte and Hiram Castillo are in the same group at the ESRF. When they started working together, back in 2011, they realised that both of them had published results on experiments on sunflowers, but not quite the same type. Cotte has had a long-standing collaboration with Letizia Monico, from the University of Perugia and CNR-ISTM (Perugia) in Italy and from the University of Antwerp in Belgium, to study the use of chrome yellows by Van Gogh, who used it extensively in his series of Sunflowers paintings.

Castillo, on the other hand, had studied where chromium and other elements such as lead or copper were localised inside plants, soils, fungi and bacteria. More importantly, he studied their toxicity in the environment. It turns out that he also studied sunflowers, like Cotte and Monico, only this time they were the real thing.

The yellow of Van Gogh's Sunflowers

Van Gogh began painting the 4th version of Sunflowers in late summer 1888 and continued into the following year. One went to decorate his friend Paul Gauguin's bedroom. The paintings show sunflowers in all stages of life, from full bloom to withering. The paintings were considered innovative for their dominant use of yellow hues, partly because newly invented pigments offered Van Gogh the possibility to exploit new vivid and bright tints. He used chrome yellow, a class of compounds usually consisting of lead chromate.

Van Gogh was no scientist, but he knew his art. His sunflowers became so iconic, that Paul Gauguin even depicted Van Gogh creating one of the paintings. What was so special about them? Van Gogh himself told his brother Theo in several missives...

Fast forward a bit more than a century and the ESRF becomes the unlikely recipient of microscopic samples of these masterpieces. Chromate being sometimes a reactive material, Cotte and Monico wondered whether the Sunflowers looked today like Van Gogh had actually painted them. They had the hypotheses that chemical changes in the paint triggered a darker tinge of the yellow.

At the ESRF's ID21, the team examined the chemical state of some paint samples. They also studied mock-up paints that they artificially aged, by exploring the effects of different lighting and humidity conditions. They discovered that the most light-sensitive chrome yellow types darken as the original chromate is reduced from its highest oxidation state (Cr(VI)) to the trivalent one (Cr(III)). The scientists were able to detect a relative amount of reduced chromium of ca. 35% on the outer surface of one of the Sunflowers paintings. The reduction process of chrome yellow triggered a change of colour of the paint surface. This implies that the Sunflowers could have looked different from what we see today.

Marine Cotte (left) and Letizia Monico during sample preparation at the ESRF.
The interest of this research is not only to find out how the Sunflowers paintings really looked, but also to learn how to preserve these artworks for the generations to come. LETIZIA MONICO, RESEARCHER AT UNIVERSITY OF PERUGIA AND CNR-ISTM (PERUGIA), Italy
Letizia Monico studied different CHROME yellow paints: from the commercially available ones of today to those from the 19th century. they were all submitted to VARIOUS accelerated aging processES.

At the ESRF the team used X-ray fluorescence (XRF) and X-ray absorption near-edge structure (XANES) spectroscopy. For the XRF, the microscopic beam size (0.9 x 0.25 µm²) made it possible to separate the study of degraded and unaffected areas, and the XANES technique proved the speciation of chrome, i.e. the reduction from chromium VI to chromium III.

Chromium: from innocuous to toxic

Today chromium is used to provide corrosion protection in paint, as well as in implants, stainless steel or in high temperature furnaces. About 35% of the Cr that arrives to soils from anthropogenic activities is Cr(VI), its most dangerous form. It became widely known twenty years ago through the film Erin Brokovich, based on a real life story, where an unemployed mother won a case against the Pacific Gas and Electric Company (PG&E), a company responsible for the serious contamination of the groundwater of the city of Hinkley, in California, with carcinogenic hexavalent chromium. PG&E had been telling Hinkley residents that they used a safer form of chromium.

For Hiram Castillo, scientist on ID21, his first contact with chromium was through the sunflowers in Mexico. "I was collaborating with professor Guadalupe de la Rosa, who lives in León, an area with a lot of sunflowers and leather tanning factories. The wastewaters from those factories contain chromium, and we wanted to check whether it made it onto the plants and in what form", he explains. They exposed the sunflower roots to chromium VI, the most toxic version of the element, and used X-ray fluorescence and X-ray Absorption Near-Edge Structure to track what reaction took place in the plant.

The results showed that the Cr(VI) is reduced to Cr(III) in all root tissues, even at the root surface and epidermis. Castillo and his colleagues understand that this is so because cell wall components and structural biopolymers in plants reduce Cr(VI) and bind Cr(III) decreasing its toxicity and mobility.

The X-ray absorption near-edge spectroscopy data showed that the static chromium is present as Cr(III) , but the scientists couldn't figure out the form of the mobile species that moves the chromium in sunflower tissues.

The root cut-out section reveals the concentration of chromium (red is the highest concentration, blue is the lowest) in the epidermis of the plant and the xylem, especially, after 72 hours of exposing the roots to a solution with Cr(VI). Chromium is bound to oxygen from the components of the cell wall and to lignin rings in the xylems and it moves up to the leaves through the xylems and accumulates in trichomes (leaf hairs).

What about Chromium-polluted soils?

So research shows that plants can transform toxic chromium into its non-toxic counterpart. What happens in chromium-polluted soils? Ana Elena Pradas, a researcher at Instituto Madrileño de Investigación y Desarrollo Rural, Agrario y Agroalimentario in Madrid (Spain), who collaborates with Castillo, has been trying to find out some answers.

She grew plants in two different soils polluted with Cr(VI) solution and an industrial sludge. At the end of six months, she carried out some X-ray absorption near-edge spectroscopy experiments on ID21 which showed that the form of the chromium in the pollution source and the characteristics of the receptor soil are crucial for the development of plants in presence of this contaminant.

"Calcareous soils with low organic matter struggle with pollution of chromium VI. In these soils we would need to add organic matter, so that the soil could increase its resilience against pollution", explains Pradas. In soils, organic matter can also reduce Cr(VI) to Cr(III).

Stabilising the chromium

As these studies have shown, the environmental impact of chromium depends on its oxidation state (Cr(VI) or Cr(III).

HIRAM CASTILLO ON THE BEAMLINE id21.

Scientists can use plants to mitigate chromium toxicity thanks to their capacity of changing its chemical form and accumulating it in their tissues (roots or stems). In similar fashion, in soils, organic matter mitigates chromium toxicity. So reforestation or adding organic matter to soil (biomass) are the potential solution.

"What you need, in order to mitigate the problem, is either a very resilient plant or a very specific organic matter composition, but the truth is that not all the polluted sites have the same characteristics, so we need to adapt to that." HIRAM CASTILLO, SCIENTIST AT THE ESRF

"The main issue is the form of chromium, and even if we know that Cr(VI) can change to Cr(III), it doesn't mean that we get rid of the problem, as the interaction with its surrounding environment might make it change form again. Ultimately, the key is stabilisation", says Castillo. The team is now focusing on getting a deep knowledge about chromium chemistry in different kinds of soils that can assist the stabilisation process and what plants are candidates to carry out this task.

CREDITS

PHOTOS: Steph Candé, Chantal Argoud, Van Gogh Museum.

TEXT: Montserrat Capellas Espuny.

REFERENCES

Monico, L., et al, Angewandte Chemie 127(47): 14129-14133.

Monico, L., et al, Analytical chemistry, 83(4): 1214-1223.

Monico, L., et al, Analytical chemistry, 85(2): 860-867.

Monico, L., et al, Journal of Analytical Atomic Spectrometry, 30(7): 1500-1510.

Guadalupe de la Rosa , et al, (2014), International Journal of Phytoremediation, 16:11, 1073-1086, DOI: 10.1080/15226514.2013.810584

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