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1. Geochemistry of contrasting stream types, Taylor Valley, Antarctica

   https://doi.org/10.1130/B35479.1  

   Harmon, Russell S. ; Leslie, Deborah L. ; Lyons, W. Berry ; Welch, Kathleen A. ; McKnight, Diane M. ( July 2020 , GSA Bulletin)

   Abstract The McMurdo Dry Valley region is the largest ice-free area of Antarctica. Ephemeral streams flow here during the austral summer, transporting glacial meltwater to perennially ice-covered, closed basin lakes. The chemistry of 24 Taylor Valley streams was examined over the two-decade period of monitoring from 1993 to 2014, and the geochemical behavior of two streams of contrasting physical and biological character was monitored across the seven weeks of the 2010–2011 flow season. Four species dominate stream solute budgets: HCO3–, Ca2+, Na+, and Cl–, with SO42–, Mg2+, and K+ present in significantly lesser proportions. All streams contain dissolved silica at low concentrations. Across Taylor Valley, streams are characterized by their consistent anionic geochemical fingerprint of HCO3 > Cl > SO4, but there is a split in cation composition between 14 streams with Ca > Na > Mg > K and 10 streams with Na > Ca > Mg > K. Andersen Creek is a first-order proglacial stream representative of the 13 short streams that flow <1.5 km from source to gage. Von Guerard is representative of 11 long streams 2–7 km in length characterized by extensive hyporheic zones. Both streams exhibit a strong daily cycle for solute load, temperature, dissolvedmore »oxygen, and pH, which vary in proportion to discharge. A well-expressed diurnal co-variation of pH with dissolved oxygen is observed for both streams that reflects different types of biological control. The relative consistency of Von Guerard composition over the summer flow season reflects chemostatic regulation, where water in transient storage introduced during times of high streamflow has an extended opportunity for water-sediment interaction, silicate mineral dissolution, and pore-water exchange.« less
2. Pb Mineral Precipitation in Solutions of Sulfate, Carbonate and Phosphate: Measured and Modeled Pb Solubility and Pb2+ Activity

   https://doi.org/10.3390/min11060620  

   Li, Xinxin ; Azimzadeh, Behrooz ; Martinez, Carmen Enid ; McBride, Murray B. ( June 2021 , Minerals)

   Lead (Pb) solubility is commonly limited by dissolution–precipitation reactions of secondary mineral phases in contaminated soils and water. In the research described here, Pb solubility and free Pb2+ ion activities were measured following the precipitation of Pb minerals from aqueous solutions containing sulfate or carbonate in a 1:5 mole ratio in the absence and presence of phosphate over the pH range 4.0–9.0. Using X-ray diffraction and Fourier-transform infrared spectroscopic analysis, we identified anglesite formed in sulfate-containing solutions at low pH. At higher pH, Pb carbonate and carbonate-sulfate minerals, hydrocerussite and leadhillite, were formed in preference to anglesite. Precipitates formed in the Pb-carbonate systems over the pH range of 6 to 9 were composed of cerussite and hydrocerussite, with the latter favored only at the highest pH investigated. The addition of phosphate into the Pb-sulfate and Pb-carbonate systems resulted in the precipitation of Pb3(PO4)2 and structurally related pyromorphite minerals and prevented Pb sulfate and carbonate mineral formation. Phosphate increased the efficiency of Pb removal from solution and decreased free Pb2+ ion activity, causing over 99.9% of Pb to be precipitated. Free Pb2+ ion activities measured using the ion-selective electrode revealed lower values than predicted from thermodynamic constants, indicating that the precipitatedmore »minerals may have lower KSP values than generally reported in thermodynamic databases. Conversely, dissolved Pb was frequently greater than predicted based on a speciation model using accepted thermodynamic constants for Pb ion-pair formation in solution. The tendency of the thermodynamic models to underestimate Pb solubility while overestimating free Pb2+ activity in these systems, at least in the higher pH range, indicates that soluble Pb ion-pair formation constants and KSP values need correction in the models.« less
3. The potential for aqueous fluid-rock and silicate melt-rock interactions to re-equilibrate hydrogen in peridotite nominally anhydrous minerals

   https://doi.org/10.2138/am-2020-7435  

   Lynn, Kendra J ; Warren, Jessica M ( January 2021 , The American mineralogist)

   Hydrogen is a rapidly diffusing monovalent cation in nominally anhydrous minerals (NAMs, such as olivine, orthopyroxene, and clinopyroxene), which is potentially re-equilibrated during silicate melt-rock and aqueous fluid-rock interactions in massif and abyssal peridotites. We apply a 3D numerical diffusion modeling technique to provide first-order timescales of complete hydrogen re-equilibration in olivine, clinopyroxene, and orthopyroxene over the temperature range 600-1200°C. Model crystals are 1-3 mm along the c-axis and utilize H+ diffusion coefficients appropriate for Fe-bearing systems. Two sets of models were run with different boundary compositions: 1) “low-H models” are constrained by mineral-melt equilibrium partitioning with a basaltic melt that has 0.75 wt% H2O and 2) “high-H models,” which utilize the upper end of the estimated range of mantle water solubility for each phase. Both sets of models yield re-equilibration timescales that are identical and are fast for all phases at a given temperature. These timescales have strong log-linear trends as a function of temperature (R2 from 0.97 to 0.99) that can be used to calculate expected re-equilibration time at a given temperature and grain size. At the high end of the model temperatures (1000-1200°C), H+ completely re-equilibrates in olivine, orthopyroxene, and clinopyroxene within minutes to hours, consistent withmore »previous studies. These short timescales indicate that xenolith NAM mantle water contents are likely to be overprinted prior to eruption. The models also resolve the decoupled water-trace element relationship in Southwest Indian Ridge peridotites, in which peridotite REE abundances are reproduced by partial melting models whereas the relatively high NAM H2O contents require later re-equilibration with melt. At temperatures of 600-800°C, which correspond to conditions of hydrothermal alteration of pyroxene to amphibole and talc, H+ re-equilibration typically occurs over a range of timescales spanning days to years. These durations are well within existing estimates for the duration of fluid flow in oceanic hydrothermal systems, suggesting that peridotite NAM water contents are susceptible to diffusive overprinting during higher temperature hydrothermal alteration. Thus, diffusion during aqueous fluid-rock interactions may also explain NAM H2O contents that are too high to reflect residues of melting. These relatively short timescales at low temperatures suggest that the origin of water contents measured in peridotite NAMs requires additional constraints on sample petrogenesis, including petrographic and trace element analyses. Our 3D model results also hint that H+ may diffuse appreciably during peridotite serpentinization, but diffusion coefficients at low temperature are unconstrained and additional experimental investigations are needed.« less
4. Effects of rhamnolipid biosurfactant on the dissolution and transport of silver nanoparticles in porous media

   https://doi.org/10.1039/D1EN00185J  

   Liao, Shuchi ; Liu, Chen ; Pinchbeck, Dorothea ; Cápiro, Natalie L. ; Fortner, John D. ; Abriola, Linda M. ; Pennell, Kurt D. ( September 2021 , Environmental Science: Nano)

   The effects of nanoscale silver (nAg) particles on subsurface microbial communities can be influenced by the presence of biosurfactants, which have been shown to alter nanoparticle surface properties. Batch and column studies were conducted to investigate the influence of rhamnolipid biosurfactant (1–50 mg L −1 ) on the stability and mobility of silver nanoparticles (16 ± 4 nm) in batch reactors and water-saturated columns with three solution chemistries: pH = 4 and dissolved oxygen concentration (DO) = 8.8 mg L −1 , pH = 7 and DO = 8.8 mg L −1 , pH = 7 and DO = 2.0 mg L −1 . In batch studies, the presence of rhamnolipid (2–50 mg L −1 ) reduced nAg dissolution by 83.3–99.1% under all pH and DO conditions. Improved nAg stability was observed when rhamnolipid was present in batch reactors at pH = 7 ± 0.2, where the hydrodynamic diameter remained constant (∼50 nm) relative to rhamnolipid-free controls (increased to >230 nm) in 48 hours. Column experiments conducted at pH 4.0 ± 0.2 demonstrated that co-injection of nAg with rhamnolipid (2, 5 and 50 mg L −1 ) decreased Ag + breakthrough from ∼22% of total applied mass in rhamnolipid-free columnsmore »to less than 8.1% in the presence of rhamnolipid and altered the shape of the nAg retention profile from a hyper-exponential to a uniform distribution. Column experiments performed at pH 7.0 ± 0.2 and DO levels of either ∼2.0 or ∼8.8 mg L −1 showed that co-injection of 5 mg L −1 and 50 mg L −1 rhamnolipid increased nAg mass breakthrough by 25–40% and ∼80%, respectively, enhancements in nAg stability and mobility were attributed to rhamnolipid adsorption on nAg surfaces, which effectively slowed the oxidation and thus release of Ag + , and adsorption of rhamnolipid on the porous medium, which competed for nAg attachment sites. These results indicate that the presence of rhamnolipid significantly influenced nAg dissolution and mobility under dynamic flow conditions. A mathematical model based on modified filtration theory (MFT) accurately reproduced nAg transport and retention behavior when aggregation and reaction processes were minimal and when rhamnolipid was present, providing a tool to predict the effects of biosurfactants on nAg transport in porous media.« less
5. The solubility of monazite (LaPO4, PrPO4, NdPO4, and EuPO4) endmembers in aqueous solutions from 100 to 250 °C

   https://doi.org/https://doi.org/10.1016/j.gca.2020.04.019  

   Van Hoozen, C. ; Gysi, A. P. ; Harlov, D. E. ( July 2020 , Geochimica et cosmochimica acta)

   Monazite is a light rare earth element (REE) phosphate found in REE mineral deposits, such as those formed in (per)alkaline and carbonatite magmatic-hydrothermal systems, where it occurs in association to the development of alteration zones and hydrothermal veins. Although it has been recognized that monazite may undergo replacement by coupled dissolution-precipitation processes, currently there is no model describing the compositional REE variations in monazite resulting from direct interaction with or precipitation from hydrothermal fluids. To develop such a model requires quantification of the thermodynamic properties of the aqueous REE species and the properties of the monazite endmembers and their solid solutions. The thermodynamic properties of monazite endmembers have been determined previously using calorimetric methods and low temperature solubility studies, but only a few solubility studies have been conducted at >100 °C. In this study, the solubility products (logKs0) of LaPO4, PrPO4, NdPO4, and EuPO4 monazite endmembers have been measured at temperatures between 100 and 250 °C and saturated water vapor pressure. The solubility products are reported with an uncertainty of ±0.2 (95% confidence) according to the reaction, REEPO4(s) = REE3+ + PO43−. (see table in manuscript) The REE phosphates display a retrograde solubility, with the measured Ks0 values varying several orders of magnitude overmore »the experimental temperature range. Discrepancies were observed between the experimental solubility products and the calculated values resulting from combining calorimetric data of monazite with the properties of the aqueous REE3+ and PO43− species available in the literature. The differences between the calculated and measured standard Gibbs energy of reaction (ΔrG0) for PrPO4, NdPO4, and EuPO4 increased with higher temperatures (up to 15 kJ mol−1 at 250 °C), whereas for LaPO4 these differences increased at lower temperatures (up to 8 kJ mol−1 at 100 °C). To reconcile these discrepancies, the standard enthalpy of formation (ΔfH0) of monazite was optimized by fitting the experimental solubility data and extrapolating these fits to reference conditions of 25 °C and 1 bar. The optimized thermodynamic data provide the first internally consistent dataset for the solubility of all the monazite endmembers, and can be used to model REE partitioning between monazite and hydrothermal fluids at >100 °C.« less