Arsenic in soluble mineral phases: μ-XRF and μ-XANES studies in saline efflorescences associated with mine wastes

LAURA BORGNINO; MARIA GABRIELA GARCIA; EUGENIA NIEVAS

Campinas

Congreso; 29 RAU; 2019

When sulfide-rich mine wastes are exposed to air and water, a series of reactions aretriggered that produce acid mine drainage (AMD) and the development of newly-formedsecondary mineral phases. The AMD generation is a serious environmental concern overthe Puna region of Argentina, where a number of abandoned mines such as the ConcordiaMine are found. Arsenic is one of the most hazardous contaminants associated with AMDdue to its toxicity and high mobility in natural environments. However, it can also bescavenged by secondary minerals by either co-precipitation or adsorption [1, 2, 3].Therefore, the aim of this work is to analyse the As solid speciation in salineefflorescences precipitated in the mining site.The samples were collected from the surface of the tailings dams and the streambed ofConcordia stream, and were classified according with their colour (yellow, red and bluesalts). Mineralogy was characterized by XRD and SEM/EDS. μ-XRF and μ-XANESmeasurements of salts and reference compounds (scorodite, As(V)-jarosite, As(V)schwertmannita and Na-arsenate) were performed at the XRF beamline in the BrazilianSynchrotron Light Laboratory (LNLS). The Kα X-ray fluorescence intensities of K, Ca,Fe, S and As were recorded so that a two-dimensional distribution could be mapped foreach sample. The data were analysed using the PyMca software. Arsenic K-edge spectra(11,867 eV) were collected from 11,740 to 12,700 eV, at room temperature. The X-rayabsorption near edge structure (XANES) spectra were analysed using the Athenaprogram.The samples mainly consist of jarosite, and a mixture of schwertmannite and hydrated Fesulphates. The μ-XRF mapping shows a generally homogeneous As distribution in thesamples but higher concentrations are more frequently observed at the surface of Fe-richmineral grains. this is also evidenced by positive and significant correlations between Asand Fe. μ-XANES spectra of all the studied samples show edge features correspondingto As(V) compounds with E0 11,873.4. The results of the linear combination fits (LCF)indicate that As(V)-O compounds likely correspond to amorphous Fe arsenates adsorbedand/or co-precipitated onto jarosite and schwertmannite. These assignments coincide withprevious XAS results and dissolution tests described in Nieva et al. [3] for the studysamples. Because As is mostly associated with water-soluble phases, these efflorescencesrepresent a hazardous source of contamination to the water reservoirs in the mine wastesite.