Changes in arsenic speciation through a contaminated soil profile: a XAS based study.

An impacted soil located near an industrial waste site in the Massif Central near Auzon, France, where arsenical pesticides were manufactured, has been studied in order to determine the speciation (chemical forms) of arsenic as a function of soil depth. Bulk As concentrations range from 8780 mg kg(-1) in the topsoil horizon to 150 mg kg(-1) at 60 cm depth. As ores (orpiment As2S3, realgar AsS, arsenopyrite FeAsS) and former Pb- and Al-arsenate pesticides have been identified by XRD at the site and are suspected to be the sources of As contamination for this soil. As speciation was found to vary with depth, based on XRD, SEM-EDS, EPMA measurements and selective chemical extractions. Based on oxalate extraction, As is mainly associated with amorphous Fe oxides through the soil profile, except in the topsoil horizons where As is hosted by another phase. SEM-EDS and EPMA analyses led to the identification of arseniosiderite (Ca2Fe3+3(AsVO4)3O2.3H2O), a secondary mineral that forms upon oxidation of primary As-bearing minerals like arsenopyrite, in these topsoil horizons. These mineralogical and chemical results were confirmed by synchrotron-based X-ray absorption spectroscopy. XANES spectra of soil samples indicate that As occurs exclusively as As(V), and EXAFS results yield direct evidence of changes in As speciation with depth. Linear combination fits of EXAFS spectra of soil samples with those of various model compounds indicate that As occurs mainly As-bearing Fe(III)-(hydr)oxides (65%) and arseniosiderite (35%) in the topsoil horizon (0-5 cm depth). Similar analyses also revealed that there is very little arseniosiderite below 15 cm depth and that As(V) is associated primarily with amorphous Fe oxides below this depth. This vertical change of As speciation likely reflects a series of chemical reactions downward in the soil profile. Arseniosiderite, formed most likely by oxidation of arsenopyrite, is progressively dissolved and replaced by less soluble As-bearing poorly ordered Fe oxides, which are the main hosts for As in well aerated soils.

Behavior and fate of industrial zinc oxide nanoparticles in a carbonate-rich river water.

The present study precisely describes the solubility patterns of commercial uncoated and organic coated ZnO NPs (nc-NPs and c-NPs, respectively) in a natural carbonate-rich water and the physicochemical processes involved. NPs transformation rates were determined with the Donnan Membrane approach (DMT, to obtain Zn(2+) concentration) and ultrafiltration (i.e. Zn(2+) and Zn bound to small organic ligands) and modeled with VMinteQ. XPS measurements evidenced the presence on native nc-NPs of a Zn(OH)2 layer which accounts for almost 22% of total Zn. This Zn(OH)2 phase is more soluble than ZnO, and could control the early dissolution steps of the nc-NPs in our system. Indeed, nc-NPs display a fast (<1 h) dissolution step reaching 19 µM Zn in solution (<1% of the total initial zinc concentration). Comparatively, c-NPs progressively release zinc during the first 48 h, to finally reach a maximum of 197 µM (10% of total Zn), which is 10 times the maximum value measured for nc-NPs. Over the long term, dissolved Zn concentrations decrease in both systems, corresponding to the neoformation of carbonate phases observed by TEM imaging. The kinetic modeling allows highlighting two different ranges of time, corresponding to (i) first 10h with a fast precipitation (kp(')=-182.10(-4)) related to a highly oversaturated solution with respect to carbonate zinc mineral and (ii) a second slower precipitation step (kp(')=-8.10(-4)), related to the embedding of NPs in the precipitated carbonate matrix. The steady state is reached after 3 months of interaction.

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.