Hybrid Experimental/theoretical Models for Acidic Sulfate Waters

Introduction: The discovery of ferric hydroxides/oxyhydroxides, jarosites (iron-bearing sulfates thought to have formed via aqueous activity) and other sulfate-bearing minerals in the Meridiani region provides strong evidence that liquid water once flowed on the Martian surface [1-3]. It is suggested that sulfate deposits enriched in iron and mixed iron oxyhydroxide were precipitated form meltwaters [4], thought to have been acidic. Terrestrial acidic and sulfate-rich sites such as Rio Tinto and El Jaroso in Spain, Iron Mountain in California, and some lakes in Western Australia are exceptional sites for the study of acidic waters, its associated precipitates and efflorescent salts, and sulfate-forming processes in general in which the presence of microorganisms may be required. The characterization of the hydrogeochemistry of these sites is crucial in order to describe all of the different processes and settings of iron and sulfate deposition on Earth. Further, as these localities host mineralogical signals detected on Meridiani and other sites on Mars, the comparison may help to understand how these minerals formed. It is crucial then to characterize the range of environmental stability for each of these mineral, i.e., to classify the minerals as a function of their precipitation order, or increasing solubility in a given aqueous system. The reverse application of those precipitation models to the current Martian iron sulfate mineral phases will help obtaining a complete picture of the acidic waters that once flooded the planet’s surface via describing phase relations in those waters. In this work we show the investigations on synthetic acidic aqueous systems that mimic the conditions of the natural waters of the Rio Tinto Mars analog site. The physico-chemistry of two synthetic systems of special relevance: iron(II) sulfatesulfuric acid-water, and iron(III)-sulfuric acid-water, is reported. Additionally, we investigated the mass transport properties of a third system, sulfuric acid-water. These systems are non-ideal multicomponent liquid mixtures whose structure is not known, mainly because there exists a large uncertainty in the available data for the activity coefficients in multicomponent systems. Techniques based on spectroscopy do allow the direct determination of the mole fractions of all components in a mixture. We have developed a procedure for the measurement of the physico-chemical properties of these systems using Raman spectroscopy. Raman spectroscopy: The wavelength of a small portion of the light scattered from a sample is shifted from the wavelength of the incident light. Furthermore, this shift in wavelength depends only on the chemical bonding responsible of the scattering. Molecular vibrations of a huge number of complexes are determined by specific wavelengths in the Raman spectra. So, in principle, a single Raman spectrum of a sample – which does not have to prepared or damagedcan reveal the molecular structure of it. Chemical composition and abundance of species may also be retrieved from a Raman spectrum upon certain processing. Results: Accurate quantitative analyses of concentrations of species present in the system iron(II) sulfate-sulfuric acid-water have been achieved, along with the characterization of the equilibriums in this system in terms of the activity coefficients product. This is essential in order to establish physico-chemical models of acid sulfate waters as this particular system turns out to have a highly non-ideal behavior. Additionally, Raman spectroscopy has provided a means to corroborate previous findings on the formation of iron(II) hexahydrate complexes in the mentioned system. The activity coefficients product (Fig. 1) has been calculated as a function of iron(II) sulfate concentration with iron concentrations exceeding Rio Tinto’s. The logarithmic relationship between the activity coefficients product and the concentration of iron (II) sulfate found for this system gives an idea of the high non-ideality if this system, and allows to extrapolate values beyond the limits of salt concentrations used in this work.