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

BIA GONZALO L.; SOTO RUEDA, ELIANA; MORS, AGUSTÍN; GARCÍA, MARÍA GABRIELA; BORGNINO, LAURA

Campinas

Encuentro; 29th Annual Users Meeting (RAU); 2019

Arsenic (As) contamination of surface and groundwater represents a serious environmental problem in many regions of Argentina. Geogenic As-contaminated waters are at equilibrium or slightly supersaturated with respect to calcite. This is the mechanism of formation of travertine rocks associated with carbonate and As-rich thermal waters in the Andean region. As rich carbonates also precipitate in soils of the Chaco-Pampean plain in the form of calcretes. As concentrations 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 the mobility of As [1].Several scientific contributions are focused on the capacity of natural calcite to retain As-oxyanions. In the last decades, it was demonstrated that As-oxyanions may adsorb onto calcite surface and/or substitute the carbonate group, in the calcite structure [2]. The aim of this study is to identify the As species in carbonates precpitated in different geologic environments associated with areas 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 and biogenic sediments were collected in the Chaco-Pampean and in the Puna region. XAS spectra were collected at the XAFS1 beamline at the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas, 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 the total As. EXAFS analysis indicate that As is mainly present in the form of As(V)-O species.Specifically, the proposed models resemble an arsenate-like structure, with the ~4 O atoms at a distance of ~1.68 Å around the As atom. The noisy EXAFS spetrum of the calcretes does not allow discriminating the second coordination shell; therefore, the obtained values can be adjusted to arsenate adsorbed onto either calcite or Fe(III)-(hydr)oxides. On the other hand, in geothermally and biogenically precipitated carbonates, the best EXAFS models suggest the presence of ~1 Fe atom 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 of arsenates adsorbed onto Fe(III)-(hydr)oxides, while the As-Ca shells are typical of arsenate substituting carbonate ions in the calcite structure.