Elsevier

Geochimica et Cosmochimica Acta

Volume 212, 1 September 2017, Pages 119-132
Geochimica et Cosmochimica Acta

Changes in the As solid speciation during weathering of volcanic ashes: A XAS study on Patagonian ashes and Chacopampean loess

https://doi.org/10.1016/j.gca.2017.06.016Get rights and content

Abstract

X-ray absorption spectroscopy (XAS) was used to determine the oxidation state of As, local chemical coordination and the relative proportion of different As species in recent and ancient Andean volcanic ashes, as well as in Chaco Pampean loess. As K edge XANES analysis indicates that in loess sediments the dominant species is As(V) (i.e., >91%). Conversely, As(III) is dominant in all ash samples. In the Puyehue sample, only As(III) species were determined, while in both, the Chaitén and the ancient tephra samples, As(III) species accounts for 66% of the total As. The remaining 34% corresponds to As(−1) in the Chaitén sample and to As(V) in the weathered tephra. The proposed EXAFS models fit well with the experimental data, suggesting that in ancient and recent volcanic ashes, As(III) is likely related to As atoms present as impurities within the glass structure, forming hydroxide species bound to the Al-Si network. In addition, the identified As(−1) species is related to arsenian pyrite, while in the ancient volcanic ash, As(V) was likely a product of incipient weathering. In loess sediments, the identified As(V) species represents arsenate ions adsorbed onto Fe oxy(hydr)oxides, forming inner-sphere surface complexes, in a bidentate binuclear configuration.

Introduction

The presence of elevated concentrations of Arsenic (As) in both surface and groundwater sources in South America represents one of the most important environmental issues in the region. The prolonged intake of waters with elevated concentrations of As results in the well-known symptoms of chronic As poisoning, known in Argentina as Chronic Endemic Regional Hydroarsenicism (in Spanish: “Hidroarsenicismo Crónico Regional Endémico - HACRE”). The Chaco-Pampean plain of Argentina is the largest area in the world (about 1 million km2) with high As concentrations in groundwater (Bundschuh et al., 2012 and reference therein). While in Argentina the disease was first described in the early 20th century, the illness had been present since the Precolumbian Era (Arriaza et al., 2010). In the Andean region of northern Chile, arsenicosis has also been recognized as a disease that affected the earliest inhabitants (about BC 7000 and BC 2000), as evidenced by the high concentration of As found in hair, bones and skin of preserved mummies (Byrne et al., 2010, Rivadeneira et al., 2010, Kakoulli et al., 2014, Swift et al., 2015).

For decades, a great number of studies were conducted to better understand the natural sources of this hazardous pollutant, and the factors that control its mobility in the aquatic reservoirs. Previous works (Nicolli et al., 1989, Bundschuh et al., 2004, Bhattacharya et al., 2006, Sracek et al., 2009, Nicolli et al., 2010, Nicolli et al., 2012) described volcanic glass spread within the loess matrix as the primary natural source of As. While the primary and secondary sources of As in groundwater were mostly inferred on the basis of water geochemistry analysis, only a few works identified the As-bearing phases associated with loess sediments (Smedley et al., 2005, Nicolli et al., 2010, Borgnino et al., 2013, García et al., 2014). In fact, there are just a few studies reporting the chemical composition of glasses separated from loess (e.g., Nicolli et al., 1989, Nicolli et al., 2010). Recently, Bia et al. (2015) used fresh volcanic ashes collected after recent eruptions of Patagonian volcanoes as proxies for glass grains spread within loess. In this work, the authors identify the presence of As(III)-S and As(V)-O species on the glass surfaces, that were formed by chemical reactions occurring in the volcanic plume during the eruption, as well as by alteration during transport. The release of As from these coatings depends on the solubility of the As-bearing compounds and the physico-chemical conditions that predominate during transport and in the deposition location. However, the presence of As in volcanic glasses is not restricted to the above-mentioned coatings. In one of the pioneering works that characterized the solid speciation of As atoms in natural glasses, Borisova et al. (2010) reported the occurrence of oxy-hydroxide complexes, such as AsO(OH)2 and As(OH)3 in rhyolitic peraluminous glasses. The identification of the second shell of coordination (or neighboring) could not be resolved in this work; however the authors proposed that this position should be restricted to a bond via one oxygen atom to the Al-Si network of the glass, or coordinated to the major cations K/Na via electrostatic attraction in the disordered glass structure. The importance of defining the type and strength of bonds between As and the neighboring atoms lies in the control that these bondings exert on the As release during glass alteration.

Although the literature related to the As sources in the Chaco Pampean loess is abundant (Smedley and Kinniburgh, 2002, Bundschuh et al., 2004, Smedley et al., 2005) the solid speciation of As has not yet been addressed. Results of selective extraction carried out with loess samples revealed that an important fraction of As in the sediment is associated with Fe and Mn oxy(hydr)oxides (Smedley et al., 2005, Sracek et al., 2009). In view of the oxidizing conditions dominating in groundwater of the region, such association should likely represent As(V) atoms (i.e., arsenate) sorbed onto these oxides. Therefore, if the speciation and coordination of As in volcanic ash and loess are different, there is an oxidation stage that should transform the As(III) within the glass structure to the As(V) specie in the loess sediments. This stage still remains to be unraveled.

In recent decades, X-ray absorption spectroscopy (XAS) analysis have gained attention in Geoscience due to the increasing ability of beamlines to determine trace concentrations at high spatial resolutions (Alexandratos et al., 2007, Fernández-Martínez et al., 2008, Chakraborty et al., 2010, Aurelio et al., 2010). Nevertheless, XAS studies concerning the analysis of As in geological samples are still scarce (Savage et al., 2000, Zielinski et al., 2007, Borisova et al., 2010, Essilfie-Dughan et al., 2013, Root et al., 2015, Wang et al., 2016).

The purpose of this work is to contribute to the understanding of the As redox transformations that occur when volcanic ash is exposed to the Earth surface conditions for prolonged time periods. The oxidation state and the local chemical coordination of As and Fe in recently emitted ashes (the past eight years), ancient ashes (∼125,000 years) and loess sediments have been determined by synchrotron XAS at the K-edges of As and Fe. Specifically, for loess samples, the junction analysis of the As and Fe elements allowed for the discrimination of the As-bearing compounds present in the sample.

Section snippets

Volcanic ashes and loess sediments

The occurrence of discrete tephra layers, along with high contents of volcanic shards within the loess, is the result of direct contributions of volcaniclastic particles emitted during the past Andean volcanic eruptions (Zárate, 2003, Tripaldi et al., 2010). These volcanic materials were supplied from eruptive centers located along the Central and Southern volcanic zones (CVZ and SVZ, respectively). The two volcanic arcs, located in western South America, are separated by volcanically inactive

Chemical composition of volcanic ashes and loess sediments

Table 1 shows the bulk chemical composition of the analyzed samples. For the three volcanic ashes, the contents of SiO2 and Na2O + K2O range between 68.6–72.5 wt% and 7–8 wt% respectively. Therefore, according to the TAS classification (Le Maitre, 1984) Chaitén ashes are dacitic/rhyolitic in type; Puyehue ashes show a trachydacitic composition, and T ashes are rhyolitic. Loess samples are andesitic in type according to their alkalis and silica contents.

The As concentrations in both loess sediment

Discussion

Since the early work of Zoller et al. (1974) it has been well known that volcanic emissions contain important amounts of volatiles such as H2O, CO2, SO2, H2S, HCl, HF, and N2, CO, CH4 and H2 in lower proportions. In addition, volcanic gases and aerosols are enriched in many elements including alkali, alkali-earth, transition and heavy metals. Because magmatic temperatures are high, these elements are in their volatile form in gaseous volcanic emanations worldwide.

In Latin America, the presence

Conclusions

For many years, the sources and processes that control the dynamics of As in groundwaters from the large Chacopampean region have been the subject of numerous studies. Although earlier works had already considered volcanic shards spread within the loess sediments that cover the entire plain to be the primary source of As, the solid speciation of this element in the volcanic materials and its transformations during weathering remained unknown.

One important challenge in the identification of the

Acknowledgements

Authors wish to acknowledge the assistance of LNLS (Campinas-Brasil), CONICET and UNC (Argentina) for their support and the facilities used in this investigation. G. Bia acknowledges a doctoral fellowship from CONICET. L. Borgnino and M.G. Garcia are members of CICyT in Argentinás CONICET. We thank the staff of the Laboratorio Nacional de Luz Sincrotron (LNLS), particularly to Dr. Santiago Figueroa for helpful discussions and technical assistance as well as Dr. Marcelo Zárate (INCITAP) for

References (65)

  • C.I. Corkhill et al.

    Arsenopyrite oxidation – a review

    Appl. Geochem.

    (2009)
  • A.P. Deditius et al.

    A proposed new type of arsenian pyrite: composition, nanostructure and geological significance

    Geochim. Cosmochim. Acta

    (2008)
  • J. Essilfie-Dughan et al.

    Arsenic and iron speciation in uranium mine tailings using X-ray absorption spectroscopy

    Appl. Geochem.

    (2013)
  • D.M. Kröhling

    Sedimentological maps of the typical loessic units in North Pampa, Argentina

    Quatern. Int.

    (1999)
  • A.C.Q. Ladeira et al.

    Mechanism of anion retention from EXAFS and density functional calculations: arsenic (V) adsorbed on gibbsite

    Geochim. Cosmochim. Acta

    (2001)
  • D. López et al.

    Arsenic in volcanic geothermal fluids of Latin America

    Sci. Total Environ.

    (2012)
  • H.B. Nicolli et al.

    Sources and controls for the mobility of arsenic in oxidizing groundwaters from loess-type sediments in arid/semi-arid dry climates – evidence from the Chaco Pampean plain (Argentina)

    Water Res.

    (2010)
  • H.B. Nicolli et al.

    Arsenic and associated trace-elements in groundwater from the Chaco-Pampean plain, Argentina: results from 100 years of research

    Sci. Total Environ.

    (2012)
  • H.W. Nesbitt et al.

    Oxidation of arsenopyrite by air and air-saturated, distilled water and implications for mechanisms of oxidation

    Geochim. Cosmochim. Acta

    (1995)
  • G.S. Pokrovski et al.

    Experimental study of arsenic speciation in vapor phase to 500 °C: implications for As transport and fractionation in low-density crustal fluids and volcanic gases

    Geochim. Cosmochim. Acta

    (2002)
  • R.A. Root et al.

    Toxic metal(loid) speciation during weathering of iron sulfide minetailings under semi-arid climate

    App. Geochem.

    (2015)
  • K.S. Savage et al.

    Arsenic speciation in pyrite and secondary weathering phases, Mother Lode gold district. Tuolumne County, California

    Appl. Geochem.

    (2000)
  • D.M. Sherman et al.

    Surface complexation of arsenic(V) to iron(III) (hydr)oxides: structural mechanism from ab initio molecular geometries and EXAFS spectroscopy

    Geochim. Cosmochim. Acta

    (2003)
  • J. Swift et al.

    Skeletal arsenic of the pre-Columbian population of Caleta Vitor, northern Chile

    J. Archaeol. Sci.

    (2015)
  • P.L. Smedley et al.

    Arsenic associations in sediments from the loess aquifer of La Pampa, Argentina

    Appl. Geochem.

    (2005)
  • P.L. Smedley et al.

    A review of the source, distribution and behaviour of arsenic in natural waters

    Appl. Geochem.

    (2002)
  • A. Tripaldi et al.

    Petrography and geochemistry of late Quaternary dune fields of western Argentina: provenance of aeolian materials in southern South America

    Aeol. Res.

    (2010)
  • P. Wang et al.

    Ferric minerals and organic matter change arsenic speciation in copper mine tailings

    Environ. Pollut.

    (2016)
  • G.A. Waychunas et al.

    Surface chemistry of ferrihydrite: Part 1. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate

    Geochim. Cosmochim. Acta

    (1993)
  • M.A. Zárate

    Loess of southern South America

    Quater. Sci. Rev.

    (2003)
  • M.A. Zárate et al.

    The aeolian system of central Argentina

    Aeol. Res.

    (2012)
  • W.H. Baur et al.

    On the crystal chemistry of salt hydrates. VI. The crystal structures of disodium hydrogen ortho arsenate heptahydrate and of disodium hydrogen orthophosphate heptahydrate

    Acta Cryst.

    (1970)
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