Aquatic Geochemistry最新文献

Given their environmental abundances, it has been long hypothesized that geochemical interactions between reactive forms of manganese and nitrogen may play important roles in the cycling of these elements. Indeed, recent studies have begun shedding light on the possible role of soluble, ligand-bound Mn(III) in promoting abiotic transformations under environmentally relevant conditions. Here, using the kinetic data of Karolewski et al. (Geochim Cosmochim Acta 293:365–378, 2021), we provide the chemical mechanism for the abiotic oxidation of nitrite (NO₂⁻) by Mn(III)-pyrophosphate, Mn^IIIPP, to form nitrate (NO₃⁻). Nitrous acid (HNO₂), not NO₂⁻, is the reductant in the reaction, based on thermodynamic and kinetic considerations. As soluble Mn(III) complexes react in a one-electron transfer reaction, two one-electron transfer steps must occur. In step one, HNO₂ is first oxidized to nitrogen dioxide, ·NO₂, a free radical via a hydrogen atom transfer (HAT) reaction. We show that this inner sphere reaction process is the rate-limiting step in the reaction sequence. In step two, ·NO₂ reacts with a second Mn^IIIPP complex to form the nitronium ion (NO₂⁺), which is isoelectronic with CO₂. Unlike the poor electron-accepting capability of CO₂, NO₂⁺ is an excellent electron acceptor for both OH⁻ and H₂O, so NO₂⁺ reacts quickly with water to form the end-product NO₃⁻ (step 3 in the reaction sequence). Thus, water provides the O atom in this nitrification reaction in accordance with the O-isotope data. This work provides mechanistic perspective on a potentially important interaction between Mn and nitrogen species, thereby offering a framework in which to interpret kinetic and isotopic data and to further investigate the relevance of this reaction under environmental conditions.

Studies done on small tropical west-flowing river catchments located in the Western Ghats in southwestern India have suggested very intense chemical weathering rates and associated CO₂ consumption. Very less studies are reported from these catchments notwithstanding their importance as potential sinks of atmospheric CO₂ at the global scale. A total of 156 samples were collected from a small river catchment in the southwestern India, the Payaswini–Chandragiri river Basin, during pre-monsoon, monsoon and post-monsoon seasons in 2016 and 2017, respectively. This river system comprises two small rivers originating at an elevation of 1350 m in the Western Ghats in peninsular India. The catchment area is dominated by biotite sillimanite gneiss. Sodium is the dominant cation, contributing ~ 50% of the total cations, whereas HCO₃⁻ contributes ~ 75% of total anions. The average anion concentration in the samples varied in the range HCO₃⁻ > Cl⁻ > SO₄²⁻ > NO₃⁻ > F⁻, whereas major cation concentration varied in the range Na⁺  > Ca²⁺  > Mg²⁺  > K⁺. The average silicate weathering rate (SWR) was 42 t km⁻² y⁻¹ in the year 2016 and 36 t km⁻² y⁻¹ in 2017. The average annual carbon dioxide consumption rate (CCR) due to silicate rock weathering was 9.6 × 10⁵ mol km⁻²y⁻¹ and 8.3 × 10⁵ mol km⁻² y⁻¹ for 2016 and 2017, respectively. The CCR in the study area is higher than other large tropical river catchments like Amazon, Congo-Zaire, Orinoco, Parana and Indus because of its unique topography, hot and humid climate and intense rainfall.

Over the world, the available lithium (Li) resources are reserved mainly in closed-basin brines, with high Li concentration (> 150 mg/L) and low Mg/Li ratio (< 10) being critical for Li extraction using precipitation-based methods. In order to investigate the enrichment of Li over Mg during the formation of Li brine deposits, batch water–rock interacting experiments between igneous rocks and aqueous solutions were carried out under low (25, 50 and 75 °C) and high (200, 300 and 400 °C) temperature conditions. Our results show that for the experiments using water and accomplished under 25 °C, the Mg and Li concentrations vary from 0.470 and 0.782 mg/L in the solution interacted with Li-rich granite, to 5.626 and < 0.002 mg/L in that interacted with basalt, with Mg/Li ratio being slightly higher than those of the igneous rocks. By contrast, while a NaCl or Na₂SO₄ solution was used, the Mg and Li concentrations can be improved by up to tens of times, and the Mg/Li ratio also increased slightly. Lastly and above all, with increase in the water–rock interacting temperature from 25 to 400 °C, the Mg and Li concentrations in all solutions vary conversely and the Mg/Li ratio decreases by orders of magnitude, leading to the formation of Li-rich brines with very low Mg/Li ratios at temperatures above 200 °C. By comparing the results from our experiment to those from Li-rich springs, rivers and closed-basin brines, we conclude that water evaporation over time is fundamental for the concentration of Li in brines, meanwhile high-temperature hydrothermal processes are key to the formation of Li brine deposits with low Mg/Li ratios.

Equilibrium-kinetic modeling allows investigating metal behavior in the water–rock-organic matter system with time to evaluate anthropogenic effects on the environment. In the article, the interactions of stagnant mine drainage water of the flooded mine “Arsenic” with ore and gangue minerals were simulated using different organic matter incorporation approaches. If the model is closed to humic substances (no additional organic matter input), most fulvic acids are bound in the Fe fulvate complex. While under the removal of Fe fulvate from the model, the Cu fulvate becomes prevalent, the contribution of the fulvate complexes with Zn, Mg, and Ca also increases. This scenario simulates the organo-mineral complexes behavior well and allows identifying the sequence of metal binding to organic ligands as follows Fe > Cu > Zn > Mg > Ca. The second scenario imitates the constant input of organic matter to the model (open system regarding humic substances). The dissolved metal concentrations in the model solution are extremely high in comparison to the mine drainage water. This scenario demonstrates that excessive input of organic matter leads to the accumulation of the metals in a dissolved form and blocks the secondary mineral formation despite the faster dissolution of the primary minerals under a more acidic pH than in the first scenario. However, despite the differences between the model solution and the mine drainage water, this scenario is useful to address specific issues associated with changes in natural and anthropogenic conditions. Both scenarios show the importance of organic matter incorporation to the equilibrium-kinetic models.

The aim of this study was to develop an analytical method to determine free concentrations of Europium (Eu(III)) in natural waters. Europium(III) in solution was detected using cathodic stripping voltammetry after complexation with N-nitroso-N-phenylhydroxylamine (cupferron). Optimization of analytical parameters allowed us to detect nanomolar levels of Eu(III) in solution. Free Eu(III) in solution was measured using the Donnan membrane technique in which a natural solution (the “donor”, containing various ligands) is separated from a ligand-free solution (the “acceptor”) by a cation-exchange membrane. This membrane allows only non-colloidal cationic species to pass through it, and after an adequate time equilibrium is reached between both compartments. Total Eu(III) concentration can then be quantified in the acceptor solution and related to free Eu(III) in the natural sample. Due to its high valency, free Eu(III) tends to adsorb strongly to the cation-exchange membrane. In order to determine the physicochemical conditions minimizing this adsorption, we analyzed solutions of different Eu(III) and Ca(II) (as background ion) concentrations. Results showed that 100 mM of Ca(II) were necessary to make adsorption of Eu(III) onto the membrane negligible. The optimized setup was then used to quantify Eu(III) complexation in a Eu(III)-dissolved organic matter solution.

Mountains are among the most sensitive ecosystems to climate change, and one of the most evident signs of climate-related effect is the continuous net loss of ice from the cryosphere. Several studies showed that meltwater from glaciated and perennially frozen areas can profoundly affect alpine aquatic ecosystems. Here, we present the impressive temporal increase in solutes in Lake Leit, a mountain lake in the Central Alps that is impacted by an active rock glacier. During the last 30 years, concentrations of sulfate and base cations increased by factors of 4 and 3, respectively. Atmospheric deposition, the only catchment external source, could be excluded as possible cause. The inlets have sulfate and base cations concentrations that were up to double the concentrations of the lake outlet confirming the presence of catchment internal sources. The highest concentrations were measured in the springs at the bottom of the rock glacier. Ground surface temperatures of the rock glacier indicated a high probability of permafrost occurrence, while the annual mean air temperature after the mid-1980s showed continuously positive deviations from the long-term average values (period 1961–1990) with increasing values after 2010. δ³⁴S of sulfate in the inlets and the outflow indicated that sulfate in Lake Leit mainly derived from dissolution of gypsum/anhydrite even if its presence is not confirmed by the Geologic Atlas of Switzerland. Because of these results, we postulate the presence of subsurface traces of sulfate-bearing evaporites, probably associated with Triassic metasediments. These deposits are very common in the closer surroundings. We further hypothesize that the thawing of permafrost affects the flow path of water enabling its contact with fresh highly weatherable minerals increasing the overall weathering rate and shifting the relative ionic composition in the discharge toward the ions that originate from the most soluble minerals. This study shows that increased permafrost thawing in the future can influence water quality in high-alpine settings.

Arsenic (As), iron (Fe), and manganese (Mn) contents were measured in sediment nodules and associated pore waters obtained from sediment cores collected from a salt marsh on Pólvora Island (southern Brazil). Sediment cores were obtained when brackish water dominated the estuary, at two different environments: an unvegetated mudflat colonized by crabs (Neohelice granulata), and a low intertidal stand vegetated by Spartina alterniflora. We determined the percentage of nodules in each depth interval of the cores, along with redox potential, and As, Fe, and Mn contents of the nodules. The mineralogy of the nodules was investigated, and results showed they are mainly composed by quartz, phyllosilicates, and amorphous Fe–Mn oxides/oxyhydroxides. Pore water results showed that bioturbation by local crabs supports oxygen penetration to depths of ca. 25 cm below the salt marsh surface, with lower Fe contents in pore water associated with the brackish period. However, S. alterniflora growth appears to have a greater impact on sediment geochemistry of Fe, Mn, and possibly As due to sulfate reduction and the associated decrease in pore water pH. Higher Fe concentrations were observed in the pore waters during the period of brackish water dominance, which also corresponded to the S. alterniflora growth season. The study demonstrates that differences in geochemical conditions (e.g., Fe content) that can develop in salt marsh sediments owing to different types of bioirrigation processes (i.e., bioirrigation driven by crabs versus that related to the growth of S. alterniflora) play important roles in the biogeochemical cycling of As.