Arsenic speciation in geological samples using X-ray absorption spectroscopy

BIA, G; GARCIA M.G; BORGNINO L.

Campinas

Encuentro; 25th RAU annual users meeting LNLS; 2015

The Chaco Pampean plain is one of the largest regions in the world (1,000,000 Km2) affected by the presence of high concentrations of As in groundwater (Nicolli et al. 2012). The primary source of As is associated with the presence of volcanic ash spread in the loessic sediments that blanket the entire region. Bia et al. (2015) identified by XPS the presence ofAs (III)-S and As (V)-O nanocoatings, deposited on the surface of volcanic glass particles. The As contained in these coatings is easily released in contact with water, but a higher proportion of As remain within the aluminosilicate glass structure as an impurity. The aim of this work is to determine the oxidation state of arsenic, its local chemical coordination (to aradius of ~4 Å around As) and the relative proportion of the As species in volcanic Andean ashes and loess sediments. To carry out this study, volcanic ash samples of recent eruptions (Hudson, 1991 Chaitén, 2008 and Puyehue, 2011) and samples of loess were analyzed by X ray absorption fine structure spectroscopy (XAFS), using the facilities of the Brazilian Synchrotron Light Laboratory (LNLS, Brazil). XANES analysis allowed to discriminate three oxidation states of arsenic in the studied samples: As5+ is the dominant oxidation state in loess sediments while in volcanic ash samples just the oxidation states As-1 and As3+ were identified. The proposed EXAFS models fit well with the experimental data, suggesting that in loess sediments, As5+ could be in the form of arsenate ions adsorbed onto ferric oxyhydroxides, or precipitated as scorodite (FeAsO4 2H2O). In volcanic ashes, the species As-1 identified is likely associated with arsenopyrite (FeAsS) or arsenical pyrite (FeS2-2xAsx), while As3+ is likely related to As atoms present as impurities within the glass structure.