XAS study on the incorporation of arsenic species in carbonates minerals of Argentina

GONZALO BIA; MARIA GABRIELA GARCIA; ELIANA SOTO RUEDA; BORGNINO, LAURA; AGUSTIN MORS

Campinas

Congreso; 29 RAU; 2019

Arsenic (As) contamination of surface and groundwater represents a serious environmentalproblem in many regions of Argentina. Geogenic As-contaminated waters are at equilibrium orslightly supersaturated with respect to calcite. This is the mechanism of formation of travertinerocks associated with carbonate and As-rich thermal waters in the Andean region. As richcarbonates also precipitate in soils of the Chaco-Pampean plain in the form of calcretes. Asconcentrations in the calcretes is ~10 ppm, while in travertines As concentrations are much higher(~3500 ppm). Regardless of their origin, these carbonates may play an important role in themobility of As [1].Several scientific contributions are focused on the capacity of natural calcite to retain Asoxyanions. In the last decades, it was demonstrated that As-oxyanions may adsorb onto calcitesurface and/or substitute the carbonate group, in the calcite structure [2]. The aim of this study is toidentify the As species in carbonates precpitated in different geologic environments associated withareas where high As concentrations have been reported in the water. X-ray absorption spectroscopy(XAS) was used to determine the oxidation state of As and their local chemical coordination.To carry out this study, three samples were analyzed: pedogenetic calcretes, geothermal andbiogenic sediments were collected in the Chaco-Pampean and in the Puna region. XAS spectra werecollected at the XAFS1 beamline at the Brazilian Synchrotron Light Laboratory (LNLS) inCampinas, Brazil.The XANES analysis allowed differentiating two oxidation states of As in the studied samples.As(V) is the dominant oxidation state (80-100%) while the As(III) represents less than 20% of thetotal As. EXAFS analysis indicate that As is mainly present in the form of As(V)-Ospecies.Specifically, the proposed models resemble an arsenate-like structure, with the ~4 O atomsat a distance of ~1.68 Å around the As atom. The noisy EXAFS spetrum of the calcretes does notallow discriminating the second coordination shell; therefore, the obtained values can be adjusted toarsenate adsorbed onto either calcite or Fe(III)-(hydr)oxides. On the other hand, in geothermallyand biogenically precipitated carbonates, the best EXAFS models suggest the presence of ~1 Featom at ~2.9 Å, ~2 Ca atoms at ~3.6 Å and ~2 Ca atoms at ~3.7 Å. The As-Fe shell is typical ofarsenates adsorbed onto Fe(III)-(hydr)oxides, while the As-Ca shells are typical of arsenatesubstituting carbonate ions in the calcite structure