Aquatic Geochemistry最新文献

In this study, the release of elements and in particular U from five Austrian orthogneiss and granite samples into a CO₂-bearing solution was investigated to describe the initial phase (24?h) of leaching focusing on the impact of ferrous (hydro)oxide formation. Experiments were conducted at ambient temperature by flushing CO₂:N₂ gas through the reactive solution (pH_initial?~?4.3) at a liquid:solid ratio of 10:1 with and without a reducing agent. The chemical evolution of the leaching solution was dominated by incongruent dissolution of silicates showing a parabolic kinetic behavior due to protective surface formation most likely caused by precipitation of amorphous Fe^III/Al hydroxides. However, the relative distribution of Ca, Mg and Sr in the leaching solution excellently traced the individual bulk rock composition. The mobilization of U was highly prevented under oxidizing conditions by sorption onto ferrous (hydro)oxides, which were precipitating through ongoing silicate leaching. Therefore, the leaching behavior of individual U-bearing minerals was less relevant for U release. At reducing conditions, the above elements were accumulated in the solution, although an oversaturation regarding U^IVO₂ was calculated. This indicates its inhibited formation within the experimental run time. The composition of experimental leaching solutions did not reflect analyzed groundwater compositions from investigated local rock-type aquifers indicating that reaction rate constants of siliceous rocks significantly differ between values found in nature and in the laboratory. Change in active mineral surface areas with ongoing weathering, accumulation of secondary precipitates, leached layer formation and given reaction time are key factors for distinct elemental release.

Manganese oxides, typically similar to δ-MnO₂, form in the aquatic environment at near neutral pH via bacterially promoted oxidation of Mn(II) species by O₂, as the reaction of [Mn(H₂O)₆]²⁺ with O₂ alone is not thermodynamically favorable below pH of ~?9. As manganese oxide species are reduced by the triphenylmethane compound leucoberbelein blue (LBB) to form the colored oxidized form of LBB (λ_max?=?623?nm), their concentration in the aquatic environment can be determined in aqueous environmental samples (e.g., across the oxic–anoxic interface of the Chesapeake Bay, the hemipelagic St. Lawrence Estuary and the Broadkill River estuary surrounded by salt marsh wetlands), and their reaction progress can be followed in kinetic studies. The LBB reaction with oxidized Mn solids can occur via a hydrogen atom transfer (HAT) reaction, which is a one-electron transfer process, but is unfavorable with oxidized Fe solids. HAT thermodynamics are also favorable for nitrite with LBB and MnO₂ with ammonia (NH₃). Reactions are unfavorable for NH₄⁺ and sulfide with oxidized Fe and Mn solids, and NH₃ with oxidized Fe solids. In laboratory studies and aquatic environments, the reduction of manganese oxides leads to the formation of Mn(III)-ligand complexes [Mn(III)L] at significant concentrations even when two-electron reductants react with MnO₂. Key reductants are hydrogen sulfide, Fe(II) and organic ligands, including the siderophore desferioxamine-B. We present laboratory data on the reaction of colloidal MnO₂ solutions (λ_max?~?370?nm) with these reductants. In marine waters, colloidal forms of Mn oxides (<?0.2?μm) have not been detected as Mn oxides are quantitatively trapped on 0.2-μm filters. Thus, the reactivity of Mn oxides with reductants depends on surface reactions and possible surface defects. In the case of MnO₂, Mn(IV) is an inert cation in octahedral coordination; thus, an inner-sphere process is likely for electrons to go into the empty e ^*_g conduction band of its orbitals. Using frontier molecular orbital theory and band theory, we discuss aspects of these surface reactions and possible surface defects that may promote MnO₂ reduction using laboratory and field data for the reaction of MnO₂ with hydrogen sulfide and other reductants.

The salt assemblages precipitated during evaporation of concentrated brine collected from Gasikule Salt Lake (GSL) were studied to better understand the formation of potassium deposits in the Qaidam Basin. The study included isothermal evaporation at 25?°C in the laboratory and solar evaporation in the ponds at GSL field. Brines increased in density and became moderately acidic (pH?≈?5.30) while major ion geochemistry and precipitate mineralogy all showed broad agreement between both systems. Four salt assemblages were identified in the isothermal evaporation experiment: halite?→?halite?+?hexahydrite?→?halite?+?bischofite?+?carnallite?→?bischofite. Alternately, three salt assemblages were recognized in the solar evaporation: halite?→?halite?+?epsomite?+?carnallite?→?halite?+?carnallite?+?bischofite. The key difference in salt assemblages between the two systems is attributed to differences in relative humidity and temperature conditions. Although the GSL has deep spring inflow recharge, the high abundance of MgSO₄ salts demonstrates that the salt assemblages are similar to normal seawater evaporation. Thus, different proportions of deep spring inflow and river water could form both MgSO₄-deficient potassium evaporite and normal seawater potassium evaporites. Therefore, nonmarine water may form diverse potassium evaporite deposits in continental basins when the geological structure as well as hydrogeological and climatic conditions is appropriate.

Organic and inorganic precipitates are both characteristic in the active hypogenic karst area of Buda Thermal Karst in Hungary. As an active system, it is a good natural laboratory to study ongoing precipitation processes. Because of anthropogenic influence and the complexity of spring environments, it is challenging to reveal all the governing factors in the process of precipitation. In situ experiments, i.e. artificially controlled natural systems simplify the complexity by adding, excluding or stabilizing influencing parameters during the experiment. CO₂ degassing drives changes in the physicochemical parameters of spring waters from the discharge along their flow path. The rate and spatial extension of these changes depend on local hydrogeological, geological, climatic, topographical etc. factors, affecting precipitation processes. In this study, two one-day-long in situ experiments were executed to examine the physicochemical parameter changes of thermal water in a tunnel. The integration of the results with reactive transport models revealed the physicochemical processes of ingassing and degassing and predicted CaCO₃ precipitation along the flow path. Small-scale roughness of the channel surface seemed to further influence pH and concentration of HCO₃^?. After 6?weeks of thermal water flowing, organic precipitate (biofilm) formed close to the discharge and then, with a sharp change, inorganic precipitate (calcite) dominates a bit further from the discharge. In situ experiments and connected numerical simulations revealed the role of CO₂ degassing and calcite precipitation in the changes of physicochemical parameters, but organic precipitates also have to be considered near the discharge.

Oxygen profiles were measured in the sediments of the Gulf of Aqaba (Red Sea), an oligotrophic marine system affected by episodic seasonal flash floods and intense aeolian dry deposition. Sediment cores were retrieved from shallow (15–45?m), intermediate (250–561?m) and deep (700?m) water sites of south–north and east–west transects. Dissolved oxygen concentrations were measured simultaneously by using microelectrodes and microoptodes immediately after sampling and after transportation. Oxygen penetration depths were found to increase from 2 to 5?mm at the shallow water sites with sandy permeable sediments to 10–21?mm at the deeper sites with cohesive muddy sediments. This increase corresponds to decrease in oxygen diffusive fluxes at the sediment–water interface and oxygen consumption rates with depth. Oxygen consumption rates exhibit local maxima at the oxic–anoxic sediment boundary, which may be attributed to oxygen reduction coupled to oxidation of dissolved Fe(II) and Mn(II) at deep and intermediate water sites and of hydrogen sulfide at shallow water sites. Microelectrodes and microoptodes measurements of cohesive sediments from deep and intermediate water sites yielded similar results. By comparison, the microoptodes displayed more robust measurements than microelectrodes in sandy near-shore sediments. This was attributed to their flexible fiber structure that is less likely to break or to abruptly displace sand particles. After transportation of sediment cores from Eilat to Beer Sheva followed by ≤?24-h storage, no changes in oxygen fluxes and consumption rates were detected.

Fluorescence of the hydrophobic acid fraction (HPOA) of Suwannee River natural organic matter and Tb³⁺ excitation spectra were measured in tandem using the instantaneous and time-resolved emission modes. The intensity of HPOA fluorescence decreased in the presence of Tb³⁺, while the intensity of the emission from Tb³⁺ cations bound by HPOA increased by up to several orders of magnitude due to energy transfer (ET) from HPOA to Tb³⁺ ions. To determine intrinsic ET and fluorescence quenching (FQ) coefficients, NICA–Donnan modeling was carried. It showed that phenolic groups in HPOA dominated both the ET and FQ processes and that the binding of Tb³⁺ by HPOA could be described using the non-ideality parameter n_Tb, median binding constant log (tilde{K}_{text{Tb}}) for the phenolic sites and intrinsic ET and FQ coefficients (denoted as η_TbΦ and α_TbΦ), and were 0.48, 8.5, 1385 and 0.12, respectively. The high value of the energy transfer coefficient of Tb³⁺ ions bound by the phenolic groups in HPOA is indicative of both the match between the electronic levels of the donor and acceptor, and the short distance between them. The deviation of the data of Nica–Donnan modeling of the ET and FQ dependence of versus [Tb]_total for a 1.0?M ionic strength highlights the need to quantify the distribution of donor–acceptor distances in HPOA molecules in more detail.

The examination of past and new chemical–isotopic data (²H/¹H–¹⁸O/¹⁶O, ¹¹B/¹⁰B and ⁸⁷Sr/⁸⁶Sr ratios) shows the meteoric origin of the Sawa Lake (Muthanna Governorate, Iraq) and its connection with the local aquifers, which feed the lake via the groundwater emerging from its floor through fault systems. The chemical and isotopic evaporation models are traced by geochemical computer codes by using a different composition of some potential inflows to the lake (e.g., the Euphrates River and Dammam aquifer). The main product of the chemical evaporation models is gypsum, as confirmed by the mineralogical examination of the sediment and the surrounding outcrops. A strong ¹⁸O–²H enrichment is a consequence of the evaporation effect in arid regions; δ¹⁸O–Cl models and δ¹¹B?=?+?23.4‰ exclude the contribution of any seawater-derived fluids. This latter value along with ⁸⁷Sr/⁸⁶Sr?=?0.707989 suggests a mixed origin from the Eocene–Miocene aquifers. The isotope and chemical evaporation paths from the meteorically recharged sources match the lake composition. However, compositional switches from NaCl toward MgCl₂ occurred in the last decade and are related to post-drought periods, showing that the interaction of the recharging waters with the local soils (Na–Mg exchange and/or the leaching of the top layer salts) have a role in the chemical composition. This demonstrates that the lake is significantly influenced by climatic variations.

For many years, the Dead Sea suffers from an annual inflow deficiency of about one billion cubic meters, flood and baseflow. The water level changes are related to the majority of surface water inflows diverted for irrigation purposes, in addition to intensive loss of water by the high rate of evaporation and industrial water use. This causes the Dead Sea water level to decline about 35?m within the last 50?years for a long-term average of about 0.79?m per year. The changes in the hydrochemical composition were simulated experimentally to determine the changes that take place as a function of brine water evaporation level and its density. The Total Dissolved Solids (TDS) and the density of the Dead Sea water varies as a function of its water evaporation level changes. It was found that the density variation is not following a linear function with respect to water volume changes. But it follows the total amount of precipitate that occurred at different water levels. The electrical conductivity (EC) changes with respect to time and the prevailing temperature. There was no formula to calculate the high salinity of brine water above the normal ocean water. Consequently, the EC measurements were adopted to represent the Dead Sea water salinity. But in this research a converging factor (0.80971) has been found to convert the TDS values into salinity values. On contrary, the pH values revealed an inverse relationship with respect to the evaporation levels.

The rate of non-oxidative galena dissolution in seawater compositions over the pH range of 2–4.5 was determined from batch reactor experiments. The derivative at zero time of a polynomial fit of the Pb concentration versus time data for the first 30?min was used to determine the rate. A plot of R_Gn (rate of galena dissolution) versus pH for data from six experiments is linear (R²?=?0.96), with a slope of 0.5. The rate equation describing the rate of galena dissolution as a function of hydrogen ion activity is

Varying the concentration of dissolved oxygen produced no significant effect on the measured rates. The activation energy, based on four experiments carried out over the temperature range of 7–30?°C, is 61.1?kJ/mol.