XAS Evidence of As(V) Association with Iron Oxyhydroxides in a Contaminated Soil at a Former Arsenical Pesticide Processing Plant

Abstract

The molecular-level speciation of arsenic has been determined in a soil profile in the Massif Central near Auzon, France that was impacted by As-based pesticides by combining conventional techniques (XRD, selective chemical extractions) with X-ray absorption spectroscopy (XAS). The arsenic concentration is very high at the top (>7000 mg kg(-1)) and decreases rapidly downward to a few hundreds of milligrams per kilogram. A thin layer of schultenite (PbHAsO4), a lead arsenate commonly used as an insecticide until the middle of the 20th century, was found at 10 cm depth. Despite the occurrence of this As-bearing mineral, oxalate extraction indicated that more than 65% of the arsenic was released upon dissolution of amorphous iron oxides, suggesting a major association of arsenic with these phases within the soil profile. Since oxalate extraction cannot unambiguously distinguish among the various chemical forms of arsenic, these results were confirmed by a direct in situ determination of arsenic speciation using XAS analysis. XANES data indicate that arsenic occurs mainly as As(V) along the soil profile except for the topsoil sample where a minor amount (7%) of As(III) was detected. EXAFS spectra of soil samples were fit by linear combinations of model compounds spectra and by a shell-by-shell method. These procedures clearly confirmed that As(V) is mainly (at least 80 wt %) associated with amorphous Fe(III) oxides as coprecipitates within the soil profile. If any, the proportion of schultenite, which was evidenced by XRD in a separate thin white layer, does not account for more than 10 wt % of arsenic in soil samples. This study emphasizes the importance of iron oxides in restricting arsenic dispersal within soils following dissolution of primary As-bearing solids manufactured for use as pesticides and released into the soils.

Conversion, sorption, and transport of arsenic species in geological media

“…The same is true for the two samples from the Datong Basin of Shanxi in China , which were also collected from different depths. The results are consistent with those of numerous reports citing stronger sorption of As(V) than As(III) in minerals and soils at the concentration ranges typical of natural systems (Belzile and Tessier, 1990;Goldberg, 2002;Hering, 2003, 2006;Cancès et al, 2005;Ackermann et al, 2008;Tufano and Fendorf, 2008); this is specifically true at pH < 7. At higher pH values, the extent of As(III) and As(V) adsorption is more similar (Sharma and Sohn, 2009).…”

“…Nevertheless, the reduction of sorbed As(V) results in the mobilization of the more mobile and bioavailable As(III), which is also more (twice to three times) toxic than As(V) compounds (Vaughan, 2006). Iron oxides play a pivotal role in the biogeochemical behavior of As, and provide an ubiquitous As-trapping pool in soils and sediments (Belzile and Tessier, 1990;Cancès et al, 2005;Ackermann et al, 2008). Hydrous ferric oxides (e.g., ferrihydrite) are the most reactive soil components with respect to As sorption and can take up hundreds of mg/kg As, either as As(III) or As(V) (Goldberg, 2002 neutral to alkaline pH (i.e., closer to the pK a of H 3 AsO 3 at 9.2), arsenate and arsenite both adsorb with similar efficiency onto the surfaces of hydrous ferric oxides and crystalline Fe oxides via the formation of strong inner-sphere surface complexes (Morin and Calas, 2006).…”

Mineral-Aqueous Solution Interfaces and Their Impact on the Environment

“…This is followed by studies of increasingly complex model systems, using both UHV and in situ methods, ultimately approaching the complexity of natural systems (e.g., Templeton et al, 2001;Fandeur et al ., 2009;Ona-Nguema et al, 2009;Ha et al ., 2009Ha et al ., , 2010Lee et al, 2010;Wang et al ., submitted-a) . In order to assure that the model systems chosen for study are relevant to the naturally occurring phenomena we wish to understand, parallel field studies of real environmental samples (e.g ., soils polluted with arsenic or lead) are also essential (e.g., Foster et al, 1998;Morin et al, 1999;Cancès et al ., 2005) . To illustrate this approach, we present a number of examples of experimental and theoretical studies of mineral-water interface reactions in model systems of increasing complexity as well as in real environmental systems .…”

“…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…”

Fungal Bioweathering of Mimetite and a General Geomycological Model for Lead Apatite Mineral Biotransformations

“…Conversely, secondary arsenate compounds comprise a large class of minerals that have been found in many oxidized environments, e.g., soils, mine tailings, and former industrial sites, and among them, mimetite [Pb 5 (AsO 4 ) 3 Cl] and schultenite (PbHAsO 4 ) are probably the most common (20,21). From the end of the 19th century, lead arsenates, e.g., schultenite, were employed for decades as insecticides in agriculture, increasing the concentration of arsenic in soils (20,22,23). Schultenite is a very rare mineral but is very stable in acidic solutions (20,21,24).…”