Environ. Sci. Technol. 2005, 39, 9398-9405
XAS Evidence of As(V) Association with Iron Oxyhydroxides in a Contaminated Soil at a Former Arsenical Pesticide Processing Plant B . C A N C EÅ S , † , ‡ F . J U I L L O T , * , † G . M O R I N , † V . L A P E R C H E , ‡,§ L . A L V A R E Z , ¶ O. PROUX,# J-L. HAZEMANN,⊥ G . E . B R O W N J R . , £,• A N D G . C A L A S † Institut de Mine´ralogie et de Physique des Milieux Condense´s, UMR CNRS 7590, 140 rue de Lourmel, 75015 Paris, France, Centre National de Recherche sur les Sites et Sols Pollue´s, 30 Boulevard Lahure, BP 537, 59505 DOUAI Cedex, France, Bureau de Recherches Ge´ologiques et Minie`res, EPI/ENV, 3 avenue Claude Guillemin BP 6009, 45050 Orle´ans, France, European Synchrotron Radiation Facility (ESRF), BP 120, 38000 Grenoble, France, Laboratoire de Ge´ophysique Interne et Tectonophysique, UMR CNRSsUniversite´ Joseph Fourier, 1381, rue de la Piscine, Domaine Universitaire, 38400 Saint-Martin-D’He`res, France, Laboratoire de Cristallographie, CNRS, BP 166, 38042 Grenoble Cedex 09, France, Surface and Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, and Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Road, MS 69, Menlo Park, California 94025
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 Asbearing 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 * Corresponding author phone: 33 1 44 27 50 64; fax 33 1 44 27 37 85; e-mail
[email protected]. † Institut de Mine ´ ralogie et de Physique des Milieux Condense´s, UMR CNRS. ‡ Centre National de Recherche sur les Sites et Sols Pollue ´ s. § Bureau de Recherches Ge ´ ologiques et Minie`res, EPI/ENV. ¶ European Synchrotron Radiation Facility. # Laboratoire de Ge ´ ophysique Interne et Tectonophysique, UMR CNRSsUniversite´ Joseph Fourier. ⊥ Laboratoire de Cristallographie, CNRS. £ Stanford University. • Stanford Synchrotron Radiation Laboratory. 9398
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 24, 2005
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.
Introduction From the end of the 19th century until the introduction of the insecticide dichlorodiphenyltrichloroethane (DDT) in 1947, lead arsenates, and especially schultenite (PbHAsO4), were used extensively as insecticides in orchard soils. Due to its significant solubility (1), the presence of schultenite in soils may increase the concentration of potentially bioavailable arsenic in soil solution (2-5), representing a potential environmental risk. However, once released in soil solutions, precipitation of secondary As-bearing minerals (mostly iron, calcium, and magnesium arsenates) and/or complexation with mineral surfaces (mostly iron oxides) can occur, resulting in decreases in arsenic mobility and potential bioavailability. These forms of arsenic have been identified in several laboratory studies (6-10) and field studies (11-15). However, the present study is the first to characterize these secondary As-bearing phases in a soil impacted by As-based pesticides. In this study, we combined selective chemical extractions and X-ray absorption spectroscopy to determine arsenic speciation in a soil impacted by a former arsenical pesticide processing plant located in the Massif Central at Auzon, France. We found that As(V) adsorbed on amorphous iron oxide surfaces is a major chemical species of arsenic in this contaminated soil.
Materials Historical Background. The soil chosen for this study is located in the vicinity of a former arsenical pesticide plant at Auzon (Haute-Loire, France). This industrial wasteland has been listed by the French Ministry of the Environment as one of the most polluted sites in France (16). The industrial activities of the plant started at the beginning of the 20th century and lasted until the factory was closed in 1949. The first activity at this site was the roasting of arsenic sulfides (arsenopyrite, FeAsS; realgar, AsS) from the neighboring mines in order to produce arsenolite, As2O3. Subsequently, other As-containing chemical products were manufactured at the site, including arsenical pesticides such as lead arsenate, copper arsenate, and aluminum arsenate. Some recent studies by the French Geological Survey (Bureau de Recherches Ge´ologiques et Minie`res, BRGM) and by the French National Research Centre for Polluted Sites and Soils (CNRSSP) provide strong evidence that the soils, tailings, and even the groundwaters at this site have very high arsenic levels (up to 1.2 wt % in soils and 35 mg L-1 in groundwaters) (17, 18). Geological Setting. The geological setting of the Auzon site consists of recent alluvia overlying gneissic migmatites formed during the Hercynian orogenic cycle (from 385 to 245 million years ago). These alluvia are mainly composed 10.1021/es050920n CCC: $30.25
2005 American Chemical Society Published on Web 11/15/2005
TABLE 1. Selective Extractions Procedure Applied to the Soil Samples of the Pit extractant 1 2 3
(NH4)2SO4 5 × 10-2 M pH ) 5.4 (NH4)2HPO4 5 × 10-2 M pH ) 7.0 (NH4)2C2O4/H2C2O4 0.2 M pH ) 3.0
solid/solution ratio 3/100 3/100 3/100
conditions stirring for 4 h room temperature stirring for 16 h room temperature stirring for 4 h room temperature in the dark
of clayey sand in the first few meters and of sand that may contain pebble layers at greater depth. The soils developed over this alluvial material have a very high natural background of arsenic inherited from widespread hydrothermal sulfide deposits, which characterize the French Massif Central area. Indeed, arsenic concentrations in natural soils from the French Massif Central frequently exceed 300 mg kg-1 and can even reach 1000 mg kg-1, as in the Echassie`res area (Allier, France) for instance (15, 19). The former industrial site at Auzon is cross-cut by the “Auzon” stream and is located 200 m from the confluence of the “Auzon” stream and “Allier” river. Sampling. Colored deposits were collected on the site and subsequently characterized by XRD, which showed the presence of arsenic sulfides (arsenopyrite, FeAsS; orpiment, As2S3; realgar, AsS) (data not shown). Considering the historical background of the site, these phases are likely related to source materials that were used for synthesis of As-based pesticides. This study focuses mainly on a soil profile in a partially wooded area of 5000 m2, located a few hundred meters southwest of the pesticide manufacturing plant. Previous sampling of soils from this area by the CNRSSP revealed that arsenic concentration in the topsoil horizon (until 20 cm depth) varies from place to place, ranging from 100 mg kg-1 to 12 000 mg kg-1 (17). For this study, a pit was dug down to the water table (2.70 m depth) in order to investigate the extent and nature of arsenic contamination with depth. The location of this pit corresponds to that where the concentration of arsenic in the surface horizon is among the highest in the study area (>7000 mg kg-1 As). Each soil horizon was sampled on the basis of color and texture. When no variation in color or texture was observed, a sample was collected every 20 cm. The layers in the soil profile can be described as follows: (i) below a thin litter horizon (+5 cm), the uppermost soil horizon (0-15 cm) corresponds to a clay loam layer, which is referred to as an A1 organo-mineral horizon; (ii) between 15 and 35 cm depth lies an eluviated sandy horizon referred to as an E horizon; (iii) the underlying sandy material (from 35 to 270 cm depth) likely corresponds to the alluvial bedrock. According to the U.S. soil taxonomy and considering the absence of a well-defined illuvial horizon, this soil can be ranked in the Inceptisol soil order. More precisely, since a thin discontinuous white anthropic horizon was found at 10 cm depth, the soil can be classified as a Typic Hapanthrept. The thin discontinuous white anthropic horizon found at 10 cm depth was collected separately for mineralogical and chemical analyses, and samples referred to as “soil samples” in the following text do not include this white layer.
Experimental Methods Chemical and Mineralogical Analyses. All soil samples were dried at room temperature and sieved at 2 mm. A fraction of each sieved sample, referred to as “bulk sample”, was ground to a fine powder (
