Understanding arsenic-ulexite interactions in evaporite environments: Evidence from XRPD, micro-XRF, micro-FT-IR, and XPS studies.

World-class borate deposits often form from As-rich waters, this study addresses the understudied association of arsenic (As) species with evaporite borates, focusing on the Puna region's borate deposits (Central Andes of Argentina). The research aims to characterize the association between borate minerals and high As concentrations in brines and thermal waters. To achieve this, five borate samples were collected from the Olaroz salt flat nucleus and thermal springs, alongside associated water samples. Comprehensive analytical techniques, including ICP-MS, ICP-OES, synchrotron-based micro-XRF, XRPD, Rietveld analysis, micro-FT-IR, and XPS, were employed to determine bulk and surface chemical compositions, mineral identification, and solid speciation of As and boron. The study reveals that under oxidizing conditions and in absence of organic matter, aqueous arsenic species interact with ulexite through a stepwise process involving charge neutralization, cationic bridge formation, and surface complex formation with polyborate and As(V) oxyanions. However, in environments associated with microbial mats or organic-rich sediments, the dissolved As(V) is reduced to As(III), which forms complexes with functional groups of organic matter. The coexistence of As(III) and As(V) in specific layers suggests potential remediation strategies targeting organic matter for the removal of the more toxic As(III) in similar geological settings.

Synchrotron XAS study on the As transformations during the weathering of sulfide-rich mine wastes.

This paper describes the weathering processes that occurred across two tailing dumps in the Concordia mine (Puna of Argentina) with the purpose of evaluating the formation of secondary As-bearing minerals due to arsenopyrite alteration. After 30 years of exposure, the gradual oxidation of the wastes produced a number of secondary mineral phases containing As in different chemical arrangements. Synchrotron-based X-ray absorption spectroscopy was used to determine both, As and Fe solid speciation and to identify the formed As-bearing minerals. The results reveal that in the first stages of oxidation, As released from arsenopyrite is adsorbed/substituted in the jarosite structure partially inhibiting its dissolution. When pH values in the system slightly increase As-jarosite transforms into schwertmannite, where the released As could be re-adsorbed or co-precipitated. When the available adsorption sites become oversaturated with As, the precipitation of amorphous ferric arsenates may occur. The latter, likely constitute the more labile As fractions in the sediments and are therefore the main phases contributing As to the nearby environments. These amorphous and labile phases are more abundant in the uppermost layers of the profiles, where oxidation has taken place for a more prolonged time-lapse. The described transformations are enhanced by the acidic pH, the absence of minerals attenuating the acidity and the high sulfate and As concentrations in pore water.

Mechanisms of fluoride release in sediments of Argentina's central region.

We analyze the sources and mechanisms that control the release of fluoride from sediments collected from two types of aquatic reservoirs in the central region of Argentina: 1) mountainous rivers draining crystalline rocks from the Sierras Pampeanas ranges, and 2) shallow aquifers in loessic sediments. The assessment was performed on the basis of experimental work and the study of chemical and mineralogical characteristics of sediments in contact with F-rich waters of the studied region. The chemical and mineralogical compositions of sediments were analyzed by ICP/OES, DRX, and SEM-EDS. Batch experiments were conducted to evaluate the kinetics of fluoride release under variable pH and ionic composition of the solution. The enhanced release of fluoride at more acidic pH, the inhibition of release in Ca-rich solutions and the positive significant linear trends between Ca(2+) and fluoride concentrations suggest that the dissolution of a Ca/F-bearing phase (like fluorapatite) strongly controls the dynamics of fluoride in the early stages of water-sediment interaction, particularly under acidic conditions. Calculations revealed that the dissolution of an amount of FAp equivalent to that estimated in the studied sediments may widely account for the values measured in the leaching experiments at pH6, whatever the dominant cation in the solution. Under such conditions, dissolution of FAp (present as coatings onto glass surfaces or as detritic grains) is likely the major primary source of fluorine in the studied sediments. Contribution from biotite may be also considered as a source in fluvial sediments. When adequate surfaces are present the released anions may partially be scavenged from the solution by adsorption at acidic pH. Increasing alkalinity in the aquatic reservoirs may then release the adsorbed fluoride through desorption or through competition with other anionic species. Comparing both mechanisms, dissolution predominates at lower pH while desorption is the main contribution under alkaline conditions.