Origin of As spatial heterogeneity in organic-rich by-products of riparian wetland soil oxidation using sulfur K-edge extended X-ray absorption fine structure (EXAFS)

Hélène Guénet (1) , Maya Al-Sid-Cheikh , Mélanie Davranche (1) , Delphine Vantelon (2) , Jacques Jestin (3) , Martine Bouhnik-Le Coz (1) , Patrice Petitjean (1)

1 GR - Géosciences Rennes
2 SSOLEIL - Synchrotron SOLEIL
3 LLB - UMR 12 - Laboratoire Léon Brillouin

Extensive arsenic poisoning in drinking water has been reported in Asia, Argentina, Chile and Mexico. The contamination can derive from reduction processes occurring at soil near-surface and especially in riparian wetlands. The major characteristic of these areas is that flooding induces alternating oxidizing and reducing conditions in accordance with the water saturation. It has been shown that under reduced conditions, As and Fe solubilization is increased by the presence of colloidal organic matter (OM) in soil solution. But what happens to As when the water level decreases and subsequent soil oxidation is established ? Samples consisting of reoxidation products (Fe-OM rich) of a riparian soil solution are collected on PTFE plates inserted into the upper horizons of the soil during the dry season. Geochemical analyzes showed 75% of OM, 9% of Fe and 250 mg.g-1 of As. First SEM and NanoSIMS results indicate that As is not only associated with Fe-oxides as expected in oxidized conditions, but also with OM and S-rich particles. Studies demonstrated that, in peat, As (III) is as sulphide minerals or directly bound to OM via sulfhydryl sites. However,in the oxidized precipitates, it is unlikely that As being as sulphide minerals and the sole OM-SH complexes cannot explain such concentration. Thereby, the presence of As-rich OM(S) is still unsolved. XAS (XANES and EXAFS) spectroscopy has been used to determine the local environment of sulfur since it appeared to be a key factor in the binding of As to OM. XAS was performed at the sulfur K-edge (Lucia beamline, Soleil synchrotron, Saclay) on our natural samples and several model compounds. Using XANES, sulfur could be seen in several oxidation degrees (from -II to +VI). EXAFS spectra will then provide information on the As-S, S-O, and perhaps on S-C bonding and will highlight the relation between As and OM. Thereby those results will show the ability of OM to directly or not bind As under oxidizing conditions and therefore whether wetlands are sinks or sources to As.

Domaines: Sciences de la Terre

Hélène Guénet, Maya Al-Sid-Cheikh, Mélanie Davranche, Delphine Vantelon, Jacques Jestin, et al.. Origin of As spatial heterogeneity in organic-rich by-products of riparian wetland soil oxidation using sulfur K-edge extended X-ray absorption fine structure (EXAFS). 24 ème Réunion des sciences de la Terre 2014, Oct 2014, Pau, France. pp.411, 2014, 24 ème Réunion des sciences de la terre 2014. Livre des résumés. ⟨insu-01080870⟩