EXAFS evidence of sorbed arsenic(V) and pharmacosiderite in a soil overlying the Echassières geochemical anomaly, Allier, France

Abstract

Abstract. -At Echassières (Allier, France), arsenic speciation was determined in a soil developed over a micaschist where Hercynian hydrothermal mineralization, including arsenopyrite FeAsS and löllingite FeAs 2 , 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, (Ba x ,K 2-2x ) (Fe,Al) 4 (AsO 4 ) 3 (OH) 5 • 6H 2 O. 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. Résumé. -La spéciation de l'arsenic a été déterminée dans un sol développé sur un micaschiste affecté par des minéra-lisations à arsénopyrite FeAsS et löllingite FeAs 2 d'âge hercynien qui ont conduit à la mise en place d'une anomalie géochimique régionale en As, à Échassières, Allier, France. Les sols qui se sont développés au cours d'une longue pé-riode d'altération présentent des concentrations atteignant 900 ppm dans les horizons de surface des zones les plus riches, où la saprolite contient jusqu'à 5200 ppm d'As. Les analyses par diffraction des rayons X sur poudre, par micro-diffraction à l'échelle de 20µm ainsi que les analyses par MEB-EDS et microsonde électronique révèlent que l'As, libéré par l'altération de l'arsénopyrite et/ou de la löllingite, est piégé dans un arséniate de fer secondaire, la pharmacosidérite, (Ba x ,K 2-2x ) (Fe,Al) 4 (AsO 4 ) 3 (OH) 5 • 6H 2 O. L'analyse minéralogique quantitative par affinement Rietveld indique que la fraction de l'As total piégé dans cette phase diminue de 70 % dans la saprolite à moins de 30 % dans les horizons de surface. La spectroscopie EXAFS indique que les ions As(V) adsorbés à la surface des oxydes de fer représentent la forme majoritaire de l'arsenic accompagnant la pharmacosidérite. Les processus d'adsorption, qui dominent la spéciation de l'arsenic dans les horizons de surface apparaissent comme un mécanisme important capable de retarder la dissémination de l'arsenic du sol vers les eaux et les plantes, à condition que les conditions de pH et de Eh du sol restent suffisamment acides et oxydantes.

Highlighting the wide variability in arsenic speciation in wetlands: A new insight into the control of the behavior of arsenic

“…The best fit was chosen by minimizing the reduced χ² which depends on the number of independent parameters, the number of fitted parameters and the uncertainty of the data points. For As, the addition of multiple scattering tends to improve the quality of the fit (Morin et al, 2002;Voegelin et al, 2007). In the 5-3 µm and 3-0.2 µm fractions, the triangular As-O-O (MS1, degeneracy = 12), collinear As-O-As-O (MS2 degeneracy = 4) and non-collinear As-O-As-O (MS3, degeneracy = 12) paths were added for the fit of the As (V) O 4 -tetraedron.…”

Mineral-Aqueous Solution Interfaces and Their Impact on the Environment

“…Although not as well publicised, there are also many examples of As-contaminated soils and sediments in other areas that impact humans . Some of the well-studied areas include the Mother Lode Gold District of the Sierra Nevada foothills, California (Foster et al ., 1998), a natural As geochemical anomaly in central France (Morin et al ., 2002), a former industrial plant in central France (Cancès et al, 2005), an acid mine drainage (AMD) system in southern France , and the naturally polluted aquifer sediments in the Ganges Delta, Bangladesh introduced above . All of these As pollution problems are impacted by sorption/desorption…”

“…Arsenic geochemical anomalies also provide examples of long-term regional contamination from which geochemists can learn about the transformations of As-bearing minerals and the mobility of this toxic contaminant over long periods of geological time . Guillaume Morin conducted one of the first studies of an As geochemical anomaly that has resulted in long-term diffuse contamination of soils and provides important information on the trapping of As in stable forms over geological time periods (Morin et al ., 2002) . The area Guillaume investigated is located at Echassières (Allier, France) in the Massif Central, where soils have developed on schists enriched in As over tens of km 2 .…”

“…The primary As-bearing minerals in the underlying schists are arsenopyrite, löllingite ( As, Cu, and Zn concentrations as a function of soil depth. (c) XRD patterns of soils from different depths (primary pharmacosiderite = Ph; muscovite = Mu; biotite = Bt; after Morin et al, 2002).…”

“…These results suggest that a major proportion of the arsenic is sorbed on, or co-precipitated with, poorly crystalline HFO or sorbed onto crystalline Fe-(oxyhydr)oxide Figure 15.13 (a) Fourier-filtered, second-neighbour contributions to the As K-edge EXAFS spectra of the Echassières soil, showing evidence for pharmacosiderite in the saprolite and As adsorbed on HFO (As-Fh) in the upper soil horizons. (b) crystal structure of pharmacosiderite and As adsorption complexes on FeOOH (after Morin et al, 2002).…”