Structural characterization of arsenic compounds in volcanic ashes and loessic sediments

摘要

X-ray Absorption Fine Structure spectroscopy (XAFS) was used to determine the oxidation state of Arsenic (As) and its local chemical coordination in recent and ancient volcanic ashes and Chacopampean loess sediments. XANES analysis allowed to discriminate the oxidation states of arsenic in the studied samples: As~(5+) is dominant in loess sediments while in volcanic ashes As~(-1) and As~(3+) were identified. The proposed EXAFS models fit well with the experimental data, suggesting that in loess sediments, As~(5+) could be in the form of arsenate ions adsorbed onto ferric oxyhydroxides. In recent volcanic ashes, the identified As~(-1) species is likely associated with arsenopyrite or arsenical pyrite. Besides, As~(3+) is likely related to As atoms present as impurities within the glass structure, in the form of oxy-hydroxide complexes. In ancient volcanic ashes As~(3+) is the dominant oxidation state but As~(5+) was also identified.