Arsenic speciation in geological samples using X-ray absorption spectroscopy

BIA, G.; GARCÍA M. G; BORGNINO, L.,

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) affectedby the presence of high concentrations of As in groundwater (Nicolli et al. 2012). Theprimary source of As is associated with the presence of volcanic ash spread in the loessicsediments 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 higherproportion of As remain within the aluminosilicate glass structure as an impurity. The aim ofthis 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 Andeanashes 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 Xrayabsorption fine structure spectroscopy (XAFS), using the facilities of the BrazilianSynchrotron Light Laboratory (LNLS, Brazil). XANES analysis allowed to discriminatethree oxidation states of arsenic in the studied samples: As5+ is the dominant oxidation statein 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, suggestingthat in loess sediments, As5+ could be in the form of arsenate ions adsorbed onto ferricoxyhydroxides, or precipitated as scorodite (FeAsO4?2H2O). In volcanic ashes, the speciesAs-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 glassstructure.