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

When sulfide-rich mine wastes are exposed to air and water, a series of reactions are triggered that produce acid mine drainage (AMD) and the development of newly-formed secondary mineral phases. The AMD generation is a serious environmental concern over the Puna region of Argentina, where a number of abandoned mines such as the Concordia Mine are found. Arsenic is one of the most hazardous contaminants associated with AMD due to its toxicity and high mobility in natural environments. However, it can also be scavenged by secondary minerals by either co-precipitation or adsorption. Therefore, the aim of this work is to analyse the As solid speciation in saline efflorescences precipitated in the mining site. The samples were collected from the surface of the tailings dams and the streambed of Concordia stream, and were classified according with their colour (yellow, red and blue salts). Mineralogy was characterized by XRD and SEM/EDS. μ-XRF and μ-XANES measurements of salts and reference compounds (scorodite, As(V)-jarosite, As(V) schwertmannita and Na-arsenate) were performed at the XRF beamline in the Brazilian Synchrotron 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 for each 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-ray absorption near edge structure (XANES) spectra were analysed using the Athena program. The samples mainly consist of jarosite, and a mixture of schwertmannite and hydrated Fe sulphates. The μ-XRF mapping shows a generally homogeneous As distribution in the samples but higher concentrations are more frequently observed at the surface of Fe-rich mineral grains. this is also evidenced by positive and significant correlations between As and Fe. μ-XANES spectra of all the studied samples show edge features corresponding to 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 adsorbed and/or co-precipitated onto jarosite and schwertmannite. These assignments coincide with previous XAS results and dissolution tests described in Nieva et al. [3] for the study samples. Because As is mostly associated with water-soluble phases, these efflorescences represent a hazardous source of contamination to the water reservoirs in the mine waste site. (author)