Dispersion of arsenic species from highly explosive historical volcanic eruptions in Patagonia.

Andean volcanic rocks typically have low to moderate arsenic (As) concentrations. However, elevated levels of As in groundwaters of southern South America have been reported as a consequence of weathering of volcanic glass. This study discusses the abundance, speciation and dispersion of As species in fresh volcanic ash from highly explosive (Volcanic Explosivity Index: 4-5) Patagonian eruptions, as well as the potential of As release to aqueous reservoirs. Synchrotron-based X-ray absorption and micro-focused X-ray photoelectron spectroscopies were used to evaluate As solid speciation. Batch experiments at different pH conditions were performed with the aim of understanding the controls on As release to aqueous reservoirs. Bulk chemical and mineralogical characterizations were performed by inductively coupled plasma optical emission spectroscopy, X-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy. Finally, to understand how As-bearing phases are spatially distributed after eruptions, simulations of volcanic ash emission, transport and deposition were performed. Results indicate that the concentration, speciation, and mobility of As in fresh Patagonian volcanic ash depend on the silica content of source magmas. Although the main As host in volcanic ash is Al-silicate glass, this phase is stable at neutral pH characteristic of most aqueous reservoirs. Higher contributions of As to water are associated with the more mobile As species that concentrate onto the surface of Al-silicate glass. Atmospheric dispersion simulations revealed that primary fallout of As-bearing ash has affected large areas in Patagonia, but also reached the Chaco-Pampean plain, where the presence of As-rich groundwater has been widely documented.

Long term metal release and acid generation in abandoned mine wastes containing metal-sulphides.

The sulphide-rich mine wastes accumulated in tailing dumps of La Concordia Mine (Puna of Argentina) have been exposed to the weathering action for more than 30 years. Since then, a series of redox reactions have triggered the generation of a highly acidic drainage -rich in dissolved metals-that drains into the La Concordia creek. The extent of metal and acid release in the site was analysed through field surveys and laboratory experiments. Static tests were conducted in order to predict the potential of the sulphidic wastes to produce acid, while Cu-, Zn-, Fe- and Pb-bearing phases present in the wastes were identified by XRD, SEM/EDS analysis and sequential extraction procedures. Finally, the release of these metals during sediment-water interaction was assessed in batch experiments carried out in a period of nearly two years. Field surveys indicate that the prolonged alteration of the mine wastes led to elevated electrical conductivity, pH values lower than 4 and metal concentrations that exceed the guide values for drinking water in the La Concordia stream regardless of the dominating hydrological conditions. The highly soluble Fe and Mg (hydrous)sulphates that form salt crusts on the tailings surfaces and the riverbed sediments play an important role in the control of metal mobility, as they rapidly dissolve in contact with water releasing Fe, but also Cu and Zn which are scavenged by such minerals. Another important proportion of the analysed metals is adsorbed onto Fe (hydr)oxides or form less soluble hydroxysulfates. Metals present in these phases are released to water more slowly, thus representing a potential long term source of heavy metal pollution. The obtained results are a contribution to the understanding of long term metal transformations and mobility in mine waste-impacted sites.

The role of Fe(III) modified montmorillonite on fluoride mobility: adsorption experiments and competition with phosphate.

Fluoride adsorption onto Fe(III) modified montmorillonite was investigated using batch experiments. The effect of reaction time, pH, ionic strength and phosphate, as a competitive anion, was evaluated. Kinetics indicated that adsorption obeys a pseudo-first-order rate law which involves two steps. The fast one (bulk transport/surface reaction) occurs instantaneously. The slower (diffusion in pores) takes hours to complete. The adsorption rate increases by increasing the fluoride concentration and by decreasing pH. The presence of phosphate reduces fluoride adsorption and reveals that both ions are in competition for surface sites. The reduction in fluoride adsorption when phosphate is present depends on the order of adsorbate addition. The higher fluoride adsorption occurs when both anions are added simultaneously, whereas when either fluoride or phosphate is added first, the fluoride adsorption is lower. The presence of fluoride does not have a measurable effect on phosphate adsorption. The results obtained contribute to our understanding of the mobility of fluoride in surface water which has naturally high levels of fluoride, in both the presence and absence of phosphate.

Continuous flow method for the simultaneous determination of phosphate/arsenate based on their different kinetic characteristics.

This paper proposes a new automated spectrophotometric method for the simultaneous determination of phosphate and arsenate without pre-treatment, which is faster, simpler, less expensive and hazardous than other well-known methods used with water samples. Such method is based on the different kinetic characteristics of complex formation of phosphate and arsenate with ammonium molybdate. A flow system was used in order to achieve good mixing and to provide precise time control. All the measurements were performed at the isosbestic point wavelength (885 nm). Chemical variables were optimized by factorial design (ammonium molybdate 0.015 mol L(-1), potassium antimony tartrate 1×10(-4) mol L(-1), and sulphuric acid 0.7 mol L(-1)). An appropriate linear range for both analytes (0.50-8.00µmolL(-1)), good inter-day reproducibility (4.9% [P] and 3.3% [P+As]) and a sample throughput of 6h(-1) were obtained. The detection limits are 0.4 µmol L(-1) P and 0.19 µmol (-1) [P+As] (3.3S y/x). The method was validated.

Caracterización de sedimentos provenientes del embalse San Roque y Los Molinos, Córdoba, Argentina. Propiedades superficiales y su relación con las velocidades de liberación de fosfato

El trabajo tiene como objetivo investigar las propiedades superficiales de los sedimentos de los embalses San Roque y los Molinos, Córdoba, Argentina. Para ello, se analiza el efecto del pH en los valores de carga superficial de los sedimentos, se evaluan velocidades de liberación en función del pH y temperatura y se estudia la relación entre velocidad de liberación, área superficial y carga superficial

Caracterización de sedimentos provenientes del embalse San Roque y Los Molinos, Córdoba, Argentina. Propiedades superficiales y su relación con las velocidades de liberación de fosfato

El trabajo tiene como objetivo investigar las propiedades superficiales de los sedimentos de los embalses San Roque y los Molinos, Córdoba, Argentina. Para ello, se analiza el efecto del pH en los valores de carga superficial de los sedimentos, se evaluan velocidades de liberación en función del pH y temperatura y se estudia la relación entre velocidad de liberación, área superficial y carga superficial