Identification of arsenic solid species in sulfide mine tailings from the Concordia Mine Argentina using X-ray absorption spectroscopy

MA GABRIELA GARCIA; LAURA BORGNINO; NIEVA EUGENIA

Campinas

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

LNLS

When mine waste rich in sulfides are exposed to the weathering agents and to the action of microorganisms, a highly acidic drainage, rich in dissolved metal(oid)s and sulfate is generated. The main process involves the oxidation of the parent sulfide and the subsequent precipitation of secondary minerals. During such process, elevated concentrations of arsenic are released to the water and then incorporated into a cycle that includes the precipitation/dissolution of As-bearing minerals as well as adsorption/desorption from Fe or Al (hydr)oxide sites.The aim of this work is to analyze the As solid speciation in sediments accumulated in the tailing dams of La Concordia mine, Argentina Puna.Samples were collected from different layers observed in the exposed walls of an oxidation profile formed in one of the tailing dams. Mineralogy was characterized by XRD and SEM/EDS. Arsenic K-edge spectra (11867 eV) were collected at beamline XAFS1 at the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas, Brazil along with some As reference material such as scorodite, Na2HAsO4.7H2O, As2S3, and NaAsO2.4H2O. The X-ray absorption near edge structure (XANES) spectra were analyzed using the Athena program [1].The main primary minerals determined by XRD and SEM/EDS are quartz, K-feldspar, zircon, and illite, as well as sulfides such as arsenical pyrite, arsenopyrite, some scarce grains of galena and polymetallic sulfides. The main secondary As minerals are As-jarosite and Fe (hydrous)sulfates, which are more abundant in the uppermost layers [2].According to the obtained results the As XANES spectra, all samples show characteristic edge features that correspond to As5+ compounds. However, samples from the bottom layers show pre-edge shoulders in the range of As+3 species. The results indicate that As5+-O compounds are dominant in all layers, but their percentages decrease with depth. In contrast, the proportion of the As3+ species, As3+-S and As3+-O, increases in depth. Considering the samples mineralogy, As(V) species likely correspond to As-jarosite, and arsenate ions sorbed onto ferric oxyhydroxides. On the other hand, As3+-S species likely correspond to polymetallic sulfides while As3+-O could be assigned to arsenite ions sorbed onto ferric oxyhydroxides.The obtained results indicate that As5+ compounds predominate all along the tailing profile, with minor contributions of As3+ species. The latter are more abundant in the bottom layers. The results suggest that after 30 years of exposure, oxidation has affected almost the entire profile. This has important environmental implications, because As is mostly associated with the more labile phases identified in the sediments.