Calcium controls the ability of iron-organic aggregates to trap pollutants: example of arsenic

Article in Journal of hazardous materials

Anthony Beauvois, Martine Bouhnik-Le Coz, Charlotte Catrouillet and Mélanie Davranche (University of Rennes 1, CNRS, Géosciences Rennes) and their colleagues Jacques Jestin (CEA Saclay), Delphine Vantelon, Valérie Briois, Thomas Bizien (SOLEIL Synchrotron) publish in February 2021 in the Journal of hazardous materials an article dedicated to calcium control on the capacity of iron-organic matter aggregates to trap pollutants, including arsenic.

Organo-mineral iron-organic matter (Fe-OM) aggregates are commonly encountered in the environment. Their high affinity for pollutants, their sub-micrometric size and their low weight make them major vectors of metal pollutants. They are produced in many natural systems such as wetlands, peatlands and, more recently, through permafrost thawing. Climate change, by causing an increase in the frequency of extreme rainy events and thus soil erosion processes as well as the increase in temperatures responsible for the amplification of permafrost thawing, dramatically increased their production in the environment.

The capacity of Fe-OM aggregates to trap pollutants is controlled by their structural organization. In the case of arsenic (As), this  capacity depends on the availability of Fe nano-hydroxides surface chemical functions and their interactions with organic matter. By combining advanced technics (such as high-resolution imaging, small-angle X-ray and neutron scattering, or X-ray absorption spectroscopy), it has recently been shown that these Fe-OM aggregates exhibit a complex structural organization. Organic matter is bound to iron which inhibits the growth of Fe hydroxides. Fe is thus organized as oligomers (size less than 1 nanometer) and nano-hydroxides (size of a few nanometers). The nano-hydroxides are either isolated or aggregated and embedded in an OM aggregate forming an aggregate with a size of about 100 nanometres.

This complex structural organization depends on the geochemical composition of natural waters and, in particular, on the concentration of major cations. Among these major cations, we have demonstrated that calcium (Ca) creates cationic bridges between OM molecules and allows the formation of a micrometric organic network in which Fe oligomers and nano-hydroxides are embedded. Calcium, bound to organic matter, controls not only the oligomer/nano-hydroxides proportion but also the size of Fe nano-hydroxides and their recovery rate by organic matter, monitoring their surface reactive site availability that are responsible for trapping pollutants.

>>> For more information, see on OSUR's website