Fe speciation in mine tailings using X-ray absorption spectroscopy

NIEVA N.E.; BORGNINO L; GARCÍA M.G; BIA, G.

Campinas

Encuentro; 26th annual Users meeting LNLS/CNPEM; 2016

LNLS

In the last three decades, the mine wastes of La Concordia mine (NW Argentina) remained exposed to the weathering agents without a proper treatment. The main process occurring in these systems is the oxidation of the primary sulfides and the subsequent formation of secondary minerals such as sulfates, oxides, oxy(hydr)oxide sulfates, etc. The corresponding redox reactions involve the generation of acidity and the release to the water of a number of potentially toxic elements such as heavy metals and As [1]. Fe plays an important role in such systems as it may precipitate in the form of amorphous Fe (hydr)oxides or Fe sulfates, which act as sinks for these toxic elements. The aim of this work is therefore, to determine the oxidation state of iron, its local chemical coordination (to a radius of ~5 Å around Fe) and the relative proportion of the Fe species in mine wastes from La Concordia Mine.To carry out this study, sediment samples were collected from the exposed walls of an oxidation profile formed in one of the mine tailing dams. XAFS spectra (including the X-ray absorption near-edge structure region or XANES, and the extended X-ray absorption fine structure region or EXAFS) of the sediments and Fe reference material such as (FeS2, FeOOH, FeAsO4) were collected at iron K-edge (7112 eV) at beamline XAFS1 at the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas, Brazil. Five scans were collected for each sample, and then merged. Data analysis was performed with the Athena and Artemis packages based on the IFEFFIT program [2]. First derivate of XANES spectra allowed to discriminate four different Fe-bearing phases in the studied samples: Fe associated with phylosilicates, oxides, sulfides and sulfate mineral. The proposed EXAFS models fit well with the experimental data, suggesting that the first coordinated shell around a central Fe atom is associated with Fe (III) octahedrally coordinated with oxygen in the uppermost layers. The proportion of these species decreases with depth in the profile. On the contrary, Fe associated with sulfides compounds form a structure of a central Fe associated with six neighbours of S atoms; this species is dominant in the bottom layers, while it was not determined in the uppermost layers of the tailings. The second coordinated shell is associated with K, Fe and Al in the uppermost, intermediate and bottom layers respectively. According to the obtained results, the species Fe(III)-O is likely associated with sulfate, oxides and phyllosilicates minerals present in all layers of the profile, while Fe(II)-S is likely related to Fe atoms in pyrite or arsenopyrite.