Fe speciation in mine tailings using X-ray absorption spectroscopy

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

Campinas

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

LNLS

In the last three decades, the minewastes of La Concordiamine (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 sulfidesand the subsequent formation of secondary minerals such as sulfates, oxides, oxy(hydr)oxidesulfates, etc. The corresponding redox reactions involve the generation ofacidity and the release to the water of a number of potentially toxic elements suchas heavy metals and As [1]. Feplays an important role in such systems as it may precipitate in the form ofamorphous Fe (hydr)oxides or Fe sulfates, which act as sinks for these toxicelements. The aim ofthis work is therefore, to determine the oxidation state of iron, its localchemical coordination (to a radius of ~5 Å around Fe) and the relativeproportion of the Fe species in mine wastes from La Concordia Mine.To carry out this study, sediment sampleswere collected from the exposed walls of an oxidation profile formed in one ofthe mine tailing dams. XAFS spectra (including the X-ray absorptionnear-edge structure region or XANES, and the extended X-ray absorption finestructure 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 XAFS1at the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas, Brazil. Fivescans were collected for each sample, and then merged. Data analysis wasperformed with the Athena and Artemis packages based on the IFEFFIT program [2]. First derivate of XANES spectra allowedto discriminate four different Fe-bearing phases in the studied samples: Fe associatedwith phylosilicates, oxides, sulfides and sulfate mineral. The proposed EXAFSmodels fit well with the experimental data, suggesting that the firstcoordinated shell around a central Fe atom is associated with Fe (III)octahedrally coordinated with oxygen in the uppermost layers. The proportion ofthese species decreases with depth in the profile. On the contrary, Feassociated with sulfides compounds form a structure of a central Fe associatedwith 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 secondcoordinated shell is associated with K, Fe and Al in the uppermost, intermediateand bottom layers respectively. According to the obtained results, thespecies Fe(III)-O is likely associated with sulfate, oxides and phyllosilicatesminerals present in all layers of the profile, while Fe(II)-S is likely relatedto Fe atoms in pyrite or arsenopyrite.