FREZCHEM Model Research Articles

Kainite crystallization from RO bittern: A novel approach using discontinuous evaporation

Addressing global water scarcity and achieving sustainable water management necessitates innovative solutions for handling reverse osmosis (RO) brine. This by-product, rich in high-value salts, poses environmental challenges due to its high salinity and chemical load. This study explores a novel approach for recovering valuable minerals from brine, focusing on Kainite, a mineral with substantial industrial applications and notorious for its nucleation difficulties. This research introduces a temperature-altering technique for Kainite crystallization, adjusting temperatures from 69 °C, 55 °C, 35 °C, and 25 °C, each subsequently reduced to 15 °C. Advanced characterization techniques were utilized to analyze the crystallized phases. The FREZCHEM model was used for a comparative analysis between isothermal evaporation and the experimental discontinuous evaporation results. Findings indicate that effective Kainite crystallization occurs only with discontinuous evaporation at specific temperature pairs: 55 °C to 15 °C and 69 °C to 15 °C. A novel application of Jänecke phase diagrams through the projection of crystallization path between 55 and 15 °C, and 69 and 15 °C, was instrumental in accurately predicting the crystallization sequence under these conditions. This study underscores discontinuous evaporation's potential for resource recovery from RO brine, contributing to sustainable water management.

Numerical study on the spatial–temporal distribution of solute and salt accumulation in saturated sulfate saline soil during freezing–thawing processes: Mechanism and feedback

Knowledge of freezing–thawing (F–T) cycles of sulfate saline soil is crucial regarding the spatial–temporal distribution of solutes in the soil, soil salinization, and cold region engineering in sulfate saline soil areas. F–T tests of saturated sulfate saline soils with different salt contents, water supply conditions, and the number of F–T cycles were conducted. The coupling mechanism of water and salt migration, ice–water phase change, salt crystallization–dissolution, and deformation caused by salt expansion and frost heave of saturated sulfate saline soil during F–T processes were analyzed. A water–heat–salt–mechanics coupling mathematical model for saturated sulfate saline soil under F–T cycles is proposed using the modified salt crystallization–dissolution kinetics model. The results show that the calculated profiles of temperature, water content, concentration, and soil displacement are in good agreement with the experimental data, demonstrating that the proposed coupling model can describe the coupled water–heat–salt–mechanics process of sulfate saline soil during F–T processes. Additionally, the modified salt crystallization–dissolution kinetics model employing the FREZCHEM model can theoretically reveal the processes of salt crystal growth and dissolution. Also, the water and temperature potential gradients are the main driving force for water migrating toward the freezing front. Convection and diffusion are the dominant factors for the spatial–temporal distribution of solutes of sulfate saline soil subjected to F–T cycles, causing salt accumulation and precipitation and forming subflorescence in the frozen and unfrozen layers near the freezing front and efflorescence in the soil surface–layer at a certain number of F–T cycles. Due to the blocking effect of salt accumulation and precipitation on water and salt migration, salt expansion during freezing and residual deformation during thawing gradually stabilize with the increase of F-T cycles.

Chemical characteristics and reactive transport of soil salt ions in frozen soil during the freeze and thaw period

Current researches on estimating soil salt movement during the freeze and thaw period are mainly based on total salt concentration, while ignoring the phase change of multi-component salts. In this study, the potential transport capability of soil water and salt ions in the frozen layer was investigated based on chemical characteristics and solute convection–diffusion theory. Field experiments were implemented to measure the change of total soil water, soil salt and its ion components. The soil water and salt content in liquid and solid phases were calculated by the FREZCHEM model, and then the movement of liquid soil water and soluble salt were simulated by the HYDRUS-1D model. The potential migration capability of soil water and salt components were then estimated by comparing the distribution profiles obtained by HYDRUS-1D and FREZCHEM. The results show that more than 74.5% of Cl, Na and SO4 were in liquid phase, which were less affected by precipitation-dissolution reaction and owned strong mitigation. The liquid soil water, soluble salt and salt ions in the frozen layer tended to decrease at the beginning of the freeze period and increase significantly at the end. The migration direction and quantity of various salt ions were different due to their concentration gradient and diffusion coefficient, causing the larger potential convection and dispersion quantity of Na+, Cl− and SO42- than those of Ca2+, Mg2+ and HCO3–. This study provides a new perspective for soil salt movement in frozen soil in agricultural areas with shallow groundwater table depth.

Chemical characteristics of salt migration in frozen soils during the freezing-thawing period

Soil water and salt movement in freezing-thawing periods play an important role in the agricultural ecological environment. Field experiments show a non-synchronized movement of soil water and salt during the freezing-thawing period with no definite mechanism. In this study, the non-synchronization of water and salt movement in the frozen layer (0–1 m) was analyzed from the perspective of soil salt composition combining with the convection–diffusion theory. Field experiments were carried out during two freezing-thawing periods from 2017 to 2019 in Yonglian experimental station of Hetao Irrigation District in Inner Mongolia, China. The contents of soil moisture, salt and its components (Na, K, Ca, Mg, CO3, HCO3, Cl and SO4) were measured. The correlation analysis between storage increments of total salt and its components was carried out and the FREZCHEM model was used to calculate the soluble and solid salt components. During the freezing period, the soil water storage increased by 5.42% on average, while the soil salt storage decreased by 18.69%, showing the different migration directions of soil water and salt. The migration direction of each component of salt was not exactly the same with that of the total salt. The main components causing the storage increment of soil salt were different in the three saline soils. The solute concentration gradient of soluble salt reached 3.07 mol/(L·m), which caused stronger diffusion effect to result in downward salt migration, while the convection effect drove the soil water and salt to move upwards. This is the major reason for the non-synchronization movement of soil water and salt during the freezing-thawing period. This study provides new data and perspective to understand the soil water and salt movement during the freezing-thawing period in agricultural areas with shallow groundwater table depth.

Study on the mechanism of crystallization deformation of sulfate saline soil during the unidirectional freezing process

AbstractThe freezing of sulfate saline soil involves the coupling effect of heat and mass transfer and crystallization deformation. The influence of salt content and temperature on the crystallization of ice and salt was analyzed by using a one‐side freezing experiment. Based on the effect of solute and temperature on water activity, a crystallization kinetics model of ice–water phase change in saline soil was established. The application of supersaturation in the crystallization kinetics model of solution–crystal phase change is improved with the Frezchem model. The calculated results are compared with experimental data to validate the effectiveness of the proposed model. the study shows that: (a) crystallization can be well illustrated by the proposed model based on the concepts of the formation factor of ice crystals and solution supersaturation; (b) convection and diffusion are the main means of salt redistribution, which cause the accumulation of salt above the freezing front and the formation of discontinuously distributed–layered salt crystal zones; and (c) the alternate crystallization of ice and salt increases variation i solute concentration and then increases crystallization pressure, which leads to the deformation of salt expansion.

Insights into Ceres's evolution from surface composition

AbstractInspired by the recent results of the Dawn mission, thermodynamic models of rock alteration and brine evaporation have been used to help understand the conditions under which water–rock interaction took place within the dwarf planet Ceres. This analysis constrains Ceres's early history and offers a framework within which future observations may be interpreted. A broad range of alteration conditions have been simulated using the Geochemist's Workbench and PHREEQC software, associated with the FREZCHEM model that constrains the consequences of freezing the liquid phase in equilibrium with the observed mineralogical assemblage. Comparison of the modeling results with observed surface mineralogy at Ceres indicates advanced alteration under a relatively high fugacity of hydrogen, a conclusion that is consistent with predictions for, and observations of, large ice‐rich bodies. The simulations suggest production of methane that could help regulate the redox environment and possibly form clathrate hydrates upon freezing of the early ocean. The detection of localized occurrences of natrite (sodium carbonate) at the surface of Ceres provides key constraints on the composition of fluids that are necessarily alkaline. In addition, the combined hydrothermal and freezing simulations suggest that hydrohalite may be abundant in Ceres's subsurface, similar to Earth's polar regions. The global homogeneity of Ceres's surface, made of material formed at depth, suggests a large‐scale formation mechanism, while local heterogeneities associated with impact craters and landslides suggest that some form of sodium carbonate and other salts are accessible in the shallow subsurface.

Calculation of equilibria in CO2–water–salt systems using the Frezchem model

Calculation of equilibria in CO2–water–salt systems using the Frezchem model

Modeling calcium sulfate chemistries with applications to Mars

On Mars, evidence indicates widespread calcium sulfate minerals. Gypsum (CaSO4⋅2H2O) seems to be the dominant calcium sulfate mineral in the north polar region of Mars. On the other hand, anhydrite (CaSO4) and bassanite (CaSO4⋅0.5H2O) appear to be more common in large sedimentary deposits in the lower latitudes. The tropics are generally warmer and drier, and at least locally show evidence of acidic environments in the past. FREZCHEM is a thermodynamic modeling tool used for assessment of equilibrium involving high salinity solutions and salts, designed especially for low temperatures below 298K (with one version adapted for temperatures up to 373K), and we have used it to investigate many Earth, Mars, and other planetary science problems. Gypsum and anhydrite were included in earlier versions of FREZCHEM and our model Mars applications, but bassanite (the CaSO4 hemihydrate) has not previously been included. The objectives of this work are to (1) add bassanite to the FREZCHEM model, (2) examine the environments in which thermodynamic equilibrium precipitation of calcium sulfate minerals would be favored on Mars, and (3) use FREZCHEM to model situations where metastable equilibrium might be favored and promote the formation or persistence of one of these phases over the others in violation of an idealized equilibrium state.We added a bassanite equation based on high temperatures (343–373K). A Mars simulation was based on a previously published NaCaMgClSO4 system over the temperature range of 273 to 373K. With declining temperatures, the first solid phase under equilibrium precipitation is anhydrite at 373K, then gypsum forms at 319K (46°C), and epsomite (MgSO4⋅7H2O) at 277K. This sequence could reflect, for example, the precipitation sequence in a saturated solution that is slowly cooled in a deep, warm aquifer.Because FREZCHEM is based on thermodynamic equilibrium, a crude approach to problems involving metastable equilibria is available by removing phases that may have kinetically inhibited formation. Removing anhydrite allows bassanite to precipitate at 373K, followed by gypsum at 351K (78°C), and epsomite at 277K. Removing anhydrite and gypsum allows bassanite to form from 373 to 273K. But bassanite formation from warm to cold temperatures does not seem appropriate for Mars and Earth.An explanation for spatial patterns of gypsum, anhydrite, and bassanite on Mars and Earth could be past environmental differences. Anhydrite and bassanite are favored near Mars' equator with higher temperatures, along with drier, more saline, and more acidic environments. Gypsum would be favored at the lower temperatures in the Mars polar region with wetter, lower salinity, and less acidic environments. On Earth, Ca-sulfate would likely over time largely finish re-precipitating as the more insoluble gypsum. But Mars was not in long-term moderate climates compared to Earth that strongly influenced the dominance of gypsum on Earth. So while temperature and water/acid environments for CaSO4 minerals on Mars may have been a major factor for these precipitations, the short-term moderate climates on Mars may also have influenced the prevalence of higher soluble CaSO4 species in the lower Mars latitudes.

Mirabilite solubility in equilibrium sea ice brines

The sea ice microstructure is permeated by brine channels and pockets that contain concentrated seawater-derived brine. Cooling the sea ice results in further formation of pure ice within these pockets as thermal equilibrium is attained, resulting in a smaller volume of increasingly concentrated residual brine. The coupled changes in temperature and ionic composition result in supersaturation of the brine with respect to mirabilite (Na2SO4·10H2O) at temperatures below −6.38°C, which consequently precipitates within the sea ice microstructure. Here, mirabilite solubility in natural and synthetic seawater derived brines, representative of sea ice at thermal equilibrium, has been measured in laboratory experiments between 0.2 and −20.6°C, and hence we present a detailed examination of mirabilite dynamics within the sea ice system. Below −6.38°C mirabilite displays particularly large changes in solubility as the temperature decreases, and by −20.6°C its precipitation results in 12.90% and 91.97% reductions in the total dissolved Na+ and SO42− concentrations respectively, compared to that of conservative seawater concentration. Such large non-conservative changes in brine composition could potentially impact upon the measurement of sea ice brine salinity and pH, whilst the altered osmotic conditions may create additional challenges for the sympagic organisms that inhabit the sea ice system. At temperatures above −6.38°C, mirabilite again displays large changes in solubility that likely aid in impeding its identification in field samples of sea ice. Our solubility measurements display excellent agreement with that of the FREZCHEM model, which was therefore used to supplement our measurements to colder temperatures. Measured and modelled solubility data were incorporated into a 1D model for the growth of first-year Arctic sea ice. Model results ultimately suggest that mirabilite has a near ubiquitous presence in much of the sea ice on Earth, and illustrate the spatial and temporal evolution of mirabilite within sea ice as it grows throughout an Arctic winter, reaching maximum concentrations of 2.3gkg−1.

An investigation of mineral dynamics in frozen seawater brines by direct measurement with synchrotron X‐ray powder diffraction

AbstractFrozen seawater is a composite material with a sponge‐like structure. The framework of the structure is composed of pure ice, and within the pores exists a concentrated seawater brine. When the temperature is reduced, the volume of this residual brine decreases, while its salinity increases. As a result of the paired changes to temperature and salinity, the brine eventually becomes supersaturated with respect to a mineral, resulting in the precipitation of microscopic crystals throughout the ice structure. Due to experimental constraints, the current understanding about the formation of these minerals relies on the analysis of the residual brine, rather than the mineral phase. Here synchrotron X‐ray powder diffraction was used to assess the dynamics that occur between ice, brine, and mineral phases within frozen seawater brines that were subjected to cooling and warming at subzero temperatures. The method was able to detect crystalline phases of ice, mirabilite (Na2SO4·10H2O), and hydrohalite (NaCl·2H2O). Results illustrate a highly dynamic geochemical environment where ice‐brine‐mineral interactions tend toward an equilibrium crystallization process, which supports the process of seawater freezing that is described by the Gitterman Pathway and FREZCHEM model. This study highlights the power of synchrotron techniques in observing the mineralogical dynamics of inaccessible environmental systems.

Modeling nitrogen and methane with ethane and propane gas hydrates at low temperatures (173–290 K) with applications to Titan

The FREZCHEM model was primarily designed for cold temperatures (173–298K) and high pressures (1–1000bars). Nitrogen gas (95.0%) and methane gas (5.0%) are major gases on the surface of Titan. Recently, we added nitrogen and methane gas hydrates to FREZCHEM model on Titan; and nitrogen–methane gas hydrates formed on Titan at 178K. The other common but less abundant gases on Titan are ethane (C2H6) and propane (C3H8) that can also form gas hydrates with nitrogen and methane. The specific objectives of this study were to (1) add ethane and propane to gas hydrates, including mixtures with nitrogen, methane, and carbon dioxide, and (2) explore the potential roles of gas hydrates on Titan. At 273K, the Ln(gas hydrates) were 5.095 for N2, 3.217 for CH4, 2.327 for CO2, 1.288 for C2H6, and 0.281 for C3H8. At 173K, the Ln(gas hydrates) were −4.968 for N2, −6.102 for CH4, −7.803 for CO2, −5.125 for C2H6, and −5.512 for C3H8. Apparently C2H6 and C3H8 gas hydrates change less at lower temperatures than N2, CH4, and CO2 gas hydrates. In previous papers, we added three mixed CH4–CO2, N2–CH4, and N2–CO2 binary gas hydrates. In this paper, we added ethane and propane to include new binary gas hydrate mixtures of N2–C2H6, N2–C3H8, C2H6–C3H8, CH4–C2H6, CH4–C3H8, CO2–C2H6, and CO2–C3H8. Today, there are ten binary gas hydrates in the FREZCHEM model. In the text, how to cope with more than two species is described. Simulations from 273K to 173K used a surface Titan pressure of 1.467 bars with a major gas of nitrogen (94.24%), a minor gas of methane (5.65%), and extremely minor gases of ethane (0.0038%), and propane (0.000343%). Eventually at 178K, N2·6H2O formed with 0.17694mol, CH4·6H2O formed with 0.04101mol, C2H6·6H2O formed with 6.48e−6mol, and C3H8·6H2O formed with 9.36e−7mol. Based on the atmospheric conditions of Titan, the trace gases of ethane and propane led to low gas hydrate precipitations of ethane and propane with nitrogen and methane. However, the gas concentrations could be very different in an ocean of Titan that could be 50 to 175km beneath the surface where the temperatures may increase as high as 260K. Methane gas hydrates are likely dominant beneath the surface of Titan with some minor contributions from nitrogen, ethane, and propane. Below the surface of Titan in the ocean and icy shell, gas hydrates clearly depend on the gas concentrations of methane, nitrogen, ethane, and propane as well as temperature and pressure.

Modeling nitrogen-gas, -liquid, -solid chemistries at low temperatures (173–298 K) with applications to Titan

Molecular diatomic nitrogen (henceforth, “nitrogen”) is a major gas on Venus, Earth, Mars, Titan, Triton, and Pluto; a major condensed liquid component on Titan; and major condensed ices on Triton and Pluto. Nitrogen also occurs as a component of air gas hydrates in Earth’s polar ice sheets. The Solar System’s nitrogen originally might have been produced by condensation of nitrogen ice in the outer Solar Nebula disk, or it might have first condensed as and then decomposed from nitrogen gas hydrate or ammonia. The specific objectives of this study were to add nitrogen into the FREZCHEM model (which already included ammonia) and explore some roles of nitrogen on planetary satellites, especially Titan, where nitrogen is a major atmospheric gas and a component of Titan’s hydrocarbon-rich lakes and rain. Nitrogen gases, aqueous phases, and gas hydrates were added to FREZCHEM. Nitrogen, methane, and carbon dioxide gas hydrates were parallel with respect to temperature, which allowed extensions to 173K, including for mixed N2–CH4 and N2–CO2 gas hydrates. Simulations from 273 to 173K used a surface Titan pressure of 1.467bars with a major mixing gas of nitrogen at 1.394bars (95%) and methane at 0.073bars (5%). These gas concentrations eventually led to formation of N2–CH4 gas hydrate at 178K, which suggests that these gas hydrates could be forming on the surface of Titan today (at 94K) and in sub-surfaces with higher temperatures. On Earth, air gas hydrates are common deep within polar ice sheets and are dominated by atmospheric N2/O2 gas compositions; and as a consequence, gas hydrates do not control, but are controlled by, atmospheric N2/O2 gas compositions. Given that Earth and Titan have similar atmospheric nitrogen (0.78 vs. 1.39bars) and icy environments, on neither body are gas hydrates likely the main source of atmospheric nitrogen; but rather, atmospheric gases are likely the source of minor amounts of nitrogen in gas hydrates on Earth and Titan.

Sulfite–sulfide–sulfate–carbonate equilibria with applications to Mars

Mars volcanic SO2 and H2S gas emissions are likely the dominant source of martian sulfate, and the source of sulfuric acid. Until this work, the FREZCHEM model lacked SO2 and H2S gases and associated sulfite and sulfide minerals. The specific objectives of this paper were to add these components and associated sulfite and sulfide minerals and phases into FREZCHEM, and to explore some possible roles of these chemistries on Mars. New solid phases added included the sulfites: Na2SO3·7H2O, K2SO3, (NH4)2SO3·H2O, MgSO3·6H2O, CaSO3·0.5H2O, and FeSO3·1.5H2O, and the sulfide: FeS2. The lowest eutectic of these minerals was K2SO3 (= 6.57m) at 228K. Because sulfurous acid is stronger than carbonic acid, this causes a much larger fraction of S(IV) to exist as sulfite (SO32-) at acidic to mildly alkaline pH, whereas almost none of the C is present as carbonate anion. Model calculations show that small quantities of SO2 in an early CO2-rich martian atmosphere suppressed formation of carbonates because SO2 is much more water soluble than CO2 and a stronger acid, which may be a major reason why sulfates are much more common than carbonates on Mars. Also, perhaps equally important are low temperatures that favor sulfite mineral precipitation, the oxidation of which leads to sulfate minerals. Another potentially important factor that favors sulfite/sulfide mineral formation is low pH values that cannot allow carbonate minerals, but can allow sulfide minerals such as pyrite (FeS2). The presence of pyrite, highly insoluble, would lead to sulfate minerals when oxygen becomes available in acidic environments. Major cations for both sulfites (or sulfates) and carbonates (Ca and Mg) can limit carbonates. Sulfite–sulfide volcanism on a cold, lower pH, Mars are the primary causes of high sulfate minerals (e.g., Ca and Mg sulfates), compared to volcanism on a warm, higher pH, Earth that led to more abundant carbonate minerals (e.g., Ca and Mg carbonates).

Carbon Sequestration and Release from Antarctic Lakes: Lake Vida and West Lake Bonney (McMurdo Dry Valleys)

Perennial ice covers on many Antarctic lakes have resulted in high lake inorganic carbon contents. The objective of this paper was to evaluate and compare the brine and CO2 chemistries of Lake Vida (Victoria Valley) and West Lake Bonney (Taylor Valley), two lakes of the McMurdo Dry Valleys (East Antarctica), and their potential consequences during global warming. An existing geochemical model (FREZCHEM-15) was used to convert measured molarity into molality needed for the FREZCHEM model, and this model added a new algorithm that converts measured DIC into carbonate alkalinity needed for the FREZCHEM model. While quite extensive geochemical information exists for ice-covered Taylor Valley lakes, such as West Lake Bonney, only limited information exists for the recently sampled brine of >25 m ice-thick Lake Vida. Lake Vida brine had a model-calculated pCO2 = 0.60 bars at the field pH (6.20); West Lake Bonney had a model-calculated pCO2 = 5.23 bars at the field pH (5.46). Despite the high degree of atmospheric CO2 supersaturation in West Lake Bonney, it remains significantly undersaturated with the gas hydrate, CO2·6H2O, unless these gas hydrates are deep in the sediment layer or are metastable having formed under colder temperatures or greater pressures. Because of lower temperatures, Lake Vida could start forming CO2·6H2O at lower pCO2 values than West Lake Bonney; but both lakes are significantly undersaturated with the gas hydrate, CO2·6H2O. For both lakes, simulation of global warming from current subzero temperatures (−13.4 °C in Lake Vida and −4.7 °C in West Lake Bonney) to 10 °C has shown that a major loss of solution-phase carbon as CO2 gases and carbonate minerals occurred when the temperatures rose above 0 °C and perennial ice covers would disappear. How important these Antarctic CO2 sources will be for future global warming remains to be seen. But a recent paper has shown that methane increased in atmospheric concentration due to deglaciation about 10,000 years ago. So, CO2 release from ice lakes might contribute to atmospheric gases in the future.

Modeling ammonia–ammonium aqueous chemistries in the Solar System’s icy bodies

The properties of ammonia and ammonium compounds in cold, subsurface brines are important for understanding the behavior of outer planet icy moons. The FREZCHEM model of aqueous chemistry was primarily designed for cold temperatures and high pressures, but does not contain ammonia and ammonium compounds. We added ammonia and ammonium compounds to FREZCHEM, and explored the role of these chemistries on Enceladus and Titan, mindful of their astrobiological implications. For the new FREZCHEM version, Pitzer parameters, volumetric parameters, and equilibrium constants for the Na–K–NH4–Mg–Ca–Fe(II)–Fe(III)–Al–H–Cl–ClO4–Br–SO4–NO3–OH–HCO3–CO3–CO2–O2–CH4–NH3–Si–H2O system were developed for ammonia and ammonium compounds that cover the temperature range of 173–298K and the pressure range of 1–1000bars. Ammonia solubility was extended to 173K, where NH3·2H2O and NH3·H2O precipitate, which is the lowest temperature in existing FREZCHEM versions. A subsurface “ocean” on Enceladus was simulated at 253K with gas pressures, 1–10bars, and with Na+,Cl-,HCO3-,CO2(g),CH4(g), and NH3(aq) that led to precipitation of ice, NaHCO3, and gas hydrates (CO2·6H2O and CH4·6H2O). The pH on Enceladus (Fig. 10) ranged from 5.74 to 6.76, and water activity, aw, ranged from 0.80 to 0.82, which are relatively favorable for life. A subsurface “ocean” on Titan was simulated with NH4+,Cl-,SO42-,CH4(g), and NH3(aq) over the temperature range of 173–273K that led to precipitation of (NH4)2SO4, ice, CH4·6H2O, and NH4Cl. The CH4 clathrate should float above the brine and is buoyant with respect to H2O ice, so has the potential to be a source of CH4 to replenish what has been photochemically destroyed in Titan’s atmosphere over time. The pH in the Titan simulation (Fig. 11) ranged from 11.24 to 18.03 (latter may not be accurate), and aw ranged from 0.28 to 0.72, which are relatively unfavorable for life as we know it. The Titan simulations, with total pressures of 10, 250, and 1000bars, led to similar depositions, except for ice that failed to form under 1000bars of pressure. In the past, there have been arguments for why Titan, given an early environment similar to Earth, could be a highly favorable body in our Solar System for life. But if Titan oceans are strongly alkaline with high pH whereas Enceladus’ oceans have moderate pH, as simulated, the latter would seem a better environment for life as we know it. But bear in mind, caution must be exercised in quantifying the ammonia/ammonium cases because of the complexities and limitations of these chemistries in the FREZCHEM model.

Thermophysical property anomalies of Baltic seawater

Abstract. While the thermodynamic properties of Standard Seawater are very well known, the quantitative effect of sea salt composition anomalies on various properties is difficult to estimate since comprehensive lab experiments with the various natural waters are scarce. Coastal and estuarine waters exhibit significant anomalies which also influence to an unknown amount the routine salinity calculation from conductivity measurements. Recent numerical models of multi-component aqueous electrolytes permit the simulation of physical chemical properties of seawater with variable solute composition. In this paper, the FREZCHEM model is used to derive a Gibbs function for Baltic seawater, and the LSEA_DELS model to provide estimates for the conductivity anomaly relative to Standard Seawater. From additional information such as direct density measurements or empirical salinity anomaly parameterisation, the quantitative deviations of properties between Baltic and Standard Seawater are calculated as functions of salinity and temperature. While several quantities show anomalies that are comparable with their measurement uncertainties and do not demand special improvement, others exhibit more significant deviations from Standard Seawater properties. In particular density and sound speed turn out to be significantly sensitive to the presence of anomalous solute. Suitable general correction methods are suggested to be applied to Baltic Sea samples with known Practical Salinity and, optionally, directly determined density.

Introducing the bromide/alkalinity ratio for a follow-up discussion on "Precipitation of salts in freezing seawater and ozone depletion events: a status report", by Morin et al., published in Atmos. Chem. Phys., 8, 7317–7324, 2008

Abstract. Sander et al. (2006) proposed that CaCO3 precipitation can be an important factor in triggering tropospheric ozone depletion events. Recently, Morin et al. (2008b) presented calculations with the FREZCHEM model and concluded that their results and interpretation cast doubt on the validity of this hypothesis. In this joint publication, we have re-analyzed the implications of the FREZCHEM results and show how they can be reconciled with the proposal of Sander et al. (2006). The chemical predictions of both approaches are consistent. Although an interpretation solely based on the alkalinity change in the brine does not support the conclusion of Sander et al. (2006), we show that the bromide/alkalinity ratio (which increases during the cooling of the brine) can be used as an indicator of the potential for triggering bromine explosions.

Precipitation of solid phase calcium carbonates and their effect on application of seawater <i>S<sub>A</sub></i>–<i>T</i>–<i>P</i> models

Abstract. At the present time, little is known about how broad salinity and temperature ranges are for seawater thermodynamic models that are functions of absolute salinity (SA), temperature (T) and pressure (P). Such models rely on fixed compositional ratios of the major components (e.g., Na/Cl, Mg/Cl, Ca/Cl, SO4/Cl, etc.). As seawater evaporates or freezes, solid phases [e.g., CaCO3(s) or CaSO42H2O(s)] will eventually precipitate. This will change the compositional ratios, and these salinity models will no longer be applicable. A future complicating factor is the lowering of seawater pH as the atmospheric partial pressures of CO2 increase. A geochemical model (FREZCHEM) was used to quantify the SA−T boundaries at P=0.1 MPa and the range of these boundaries for future atmospheric CO2 increases. An omega supersaturation model for CaCO3 minerals based on pseudo-homogeneous nucleation was extended from 25–40°C to 3°C. CaCO3 minerals were the boundary defining minerals (first to precipitate) between 3°C (at SA=104 g kg−) and 40°C (at SA=66 g kg−). At 2.82°C, calcite(CaCO3) transitioned to ikaite(CaCO36H2O) as the dominant boundary defining mineral for colder temperatures, which culminated in a low temperature boundary of −4.93°C. Increasing atmospheric CO2 from 385 μatm (390 MPa) (in Year 2008) to 550 μatm (557 MPa) (in Year 2100) would increase the SA and t boundaries as much as 11 g kg−1 and 0.66°C, respectively. The model-calculated calcite-ikaite transition temperature of 2.82°C is in excellent agreement with ikaite formation in natural environments that occurs at temperatures of 3°C or lower. Furthermore, these results provide a quantitative theoretical explanation (FREZCHEM model calculation) for why ikaite is the solid phase CaCO3 mineral that precipitates during seawater freezing.

Modeling aluminum–silicon chemistries and application to Australian acidic playa lakes as analogues for Mars

Recent Mars missions have stimulated considerable thinking about the surficial geochemical evolution of Mars. Among the major relevant findings are the presence in Meridiani Planum sediments of the mineral jarosite (a ferric sulfate salt) and related minerals that require formation from an acid–salt brine and oxidizing environment. Similar mineralogies have been observed in acidic saline lake sediments in Western Australia (WA), and these lakes have been proposed as analogues for acidic sedimentary environments on Mars. The prior version of the equilibrium chemical thermodynamic FREZCHEM model lacked Al and Si chemistries that are needed to appropriately model acidic aqueous geochemistries on Earth and Mars. The objectives of this work were to (1) add Al and Si chemistries to the FREZCHEM model, (2) extend these chemistries to low temperatures (<0 °C), if possible, and (3) use the reformulated model to investigate parallels in the mineral precipitation behavior of acidic Australian lakes and hypothetical Martian brines. FREZCHEM is an equilibrium chemical thermodynamic model parameterized for concentrated electrolyte solutions using the Pitzer approach for the temperature range from <−70 to 25 °C and the pressure range from 1 to 1000 bars. Aluminum chloride and sulfate mineral parameterizations were based on experimental data. Aluminum hydroxide and silicon mineral parameterizations were based on Gibbs free energy and enthalpy data. New aluminum and silicon parameterizations added 12 new aluminum/silicon minerals to this Na–K–Mg–Ca–Fe(II)–Fe(III)–Al–H–Cl–Br–SO 4–NO 3–OH–HCO 3–CO 3–CO 2–O 2–CH 4–Si–H 2O system that now contain 95 solid phases. There were similarities, differences, and uncertainties between Australian acidic, saline playa lakes and waters that likely led to the Burns formation salt accumulations on Mars. Both systems are similar in that they are dominated by (1) acidic, saline ground waters and sediments, (2) Ca and/or Mg sulfates, and (3) iron precipitates such as jarosite and hematite. Differences include: (1) the dominance of NaCl in many WA lakes, versus the dominance of Fe–Mg–Ca–SO 4 in Meridiani Planum, (2) excessively low K + concentrations in Meridiani Planum due to jarosite precipitation, (3) higher acid production in the presence of high iron concentrations in Meridiani Planum, and probably lower rates of acid neutralization and hence, higher acidities on Mars owing to colder temperatures, and (4) lateral salt patterns in WA lakes. The WA playa lakes display significant lateral variations in mineralogy and water chemistry over short distances, reflecting the interaction of acid ground waters with neutral to alkaline lake waters derived from ponded surface runoff. Meridiani Planum observations indicate that such lateral variations are much less pronounced, pointing to the dominant influence of ground water chemistry, vertical ground water movements, and aeolian processes on the Martian surface mineralogy.

Br/Cl partitioning in chloride minerals in the Burns formation on Mars

Within Gusev Crater and Meridiani Planum on Mars, the Mars exploration rovers have found Br concentrations in soils and rocks in the hundreds of ppm range. Relative to Earth compositions, these are high Br concentrations. Because of low Br concentrations on Earth, Br largely precipitates from seawater as a minor constituent in halite crystals rather than as a separate phase mineral. This is also likely to be the case for Mars. But given that the surface chemistries on Mars are significantly different than on Earth, minerals other than halite could serve as sinks for Br. The specific objectives of this paper were to (1) incorporate Br solution phase chemistries into the FREZCHEM model, (2) integrate the Siemann–Schramm Br/Cl mineral model into FREZCHEM, and (3) apply this mineral model to Br/Cl partitioning in Burns formation rocks as an indicator of past environments in the Meridiani Planum region of Mars. We showed that: (1) a molar-based model for Br substitution into halite and bischofite provided a better fit to experimental data than the standard mass-based model; (2) the concentrations of all of the soluble salts (mainly of Na, Mg, Ca, Cl, Br, and SO 4) in the Burns formation, except for Ca, were significantly related to stratigraphic depth; (3) the likely precipitation of Ca as gypsum on Mars precluded Ca precipitating as a CaCl 2 salt and thus impacts the possible minimum eutectic brine temperatures relevant to the Burns formation; (4) bischofite (MgCl 2⋅6H 2O) was a much more important sink for Br than halite; (5) Br/Cl patterns in the Burns formation, and within the three formation layers, argued in support of salt upwelling through groundwater evaporation; and (6) the high concentrations of Br in the surface layers of the Burns formation suggested that there was little water leaching and removal of soluble phases from the upper part of the stratigraphic succession.