XAS evidence of As(V) association with iron oxyhydroxides in a contaminated soil at a former arsenical pesticide processing plant.
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.
Institut de Minéralogie et de Physique des Milieux Condenses, UMR CNRS 7590, 140 rue de Lourmel, 75015 Paris, France.
B
B
Cancès
F
F
Juillot
G
G
Morin
V
V
Laperche
L
L
Alvarez
O
O
Proux
J L
JL
Hazemann
G E
GE
Brown
G
G
Calas
D001151:Arsenic / Q000032:analysis / Q000737:chemistry*; D001152:Arsenicals / Q000737:chemistry; D005290:Ferric Compounds / Q000032:analysis / Q000737:chemistry*; D006256:Hazardous Waste; D007501:Iron / Q000737:chemistry; D010575:Pesticides / Q000737:chemistry; D012989:Soil Pollutants / Q000032:analysis* / Q000633:toxicity; D013052:Spectrometry, X-Ray Emission; D014874:Water Pollutants, Chemical / Q000633:toxicity
D001152:Arsenicals; D005290:Ferric Compounds; D006256:Hazardous Waste; D010575:Pesticides; D012989:Soil Pollutants; D014874:Water Pollutants, Chemical; C021024:ferric hydroxide; D007501:Iron; D001151:Arsenic