EXAFS evidence of sorbed arsenic(V) and pharmacosiderite in a soil overlying the Echassières geochemical anomaly, Allier, France.
Résumé
At Echassières (Allier, France), arsenic speciation was determined in a soil developed over a micaschist where Hercynian hydrothermal mineralization, including arsenopyrite FeAsS and löllingite FeAs2, has lead to a regional As anomaly. The overlying soils which have developed from long term weathering exhibit As levels as high as 900 ppm in the richest area, where the saprolite contains up to 5200 ppm As. Analysis by powder XRD, µ-X-ray diffraction on the 20µm scale, SEM-EDS and electron micro-probe analyses revealed that As, released from arsenopyrite and/or löllingite alteration, is concentrated in a secondary iron arsenate, pharmacosiderite, (Bax,K2-2x) (Fe,Al)4 (AsO4)3 (OH)5 • 6H2O. Quantitative mineralogical analysis by Rietveld refinement indicates that the proportion of As hosted by this mineral decreases systematically from the saprolite to the topsoil (from 70 % to 30 % of the total bulk As content, respectively). EXAFS spectroscopy indicates that the main form of the As occurring with pharmacosiderite, consists of As(V) ions sorbed on iron oxides. Sorption processes, which dominate As speciation in the topsoil horizons, appear as a key mechanism able to delay As dissemination from soils to plant and surface waters, provided that pH and Eh conditions remain sufficiently acidic and oxidizing, respectively.