Aqueous Solutions Of MgSO4 Research Articles

Experimental measurements and thermodynamic modeling of dissociation pressure of CO2 hydrate in sulfate solutions

Carbon capture and storage (CCS) has been a popular strategy to mitigate climate change and has attracted significant attention from both industry and academia. CO2 can be stored in the form of CO2 hydrate in deeper locations in an ocean, which makes it a potential option for CO2 sequestration. Dissolved salts in the ocean brine can significantly affect the dissociation pressure of CO2 hydrate. Sulfate salts are one of the most common salt species in the oceanic brine. Although various experimental studies have been conducted to investigate the phase behavior of CO2 hydrate in brine, few studies focus on the effect of sulfate salts on the dissociation pressure of CO2 hydrate. In this study, we build an in-house experimental setup to investigate the effect of monovalent and divalent sulfate salts on the dissociation pressure of CO2 hydrate. The dissociation pressure of CO2 hydrate in Na2SO4, K2SO4, MgSO4, and CuSO4 aqueous solutions is measured using the isochoric pressure search method at different concentrations over the temperature range of 274.36–––282.15 K and the pressure range of 1.50–––4.03 MPa. A hybrid methodology incorporating the Soave-Redlich-Kwong Equation of State (SRK EOS), the van der Waals-Platteeuw (vdW-P) model, and the Pitzer model is applied to predict the dissociation pressure of CO2 in sulfate solutions. In addition, the dissociation enthalpies of CO2 hydrate in these sulfate solutions are calculated using the Clausius-Clapeyron equation based on the measured dissociation points. The experimental results show that the dissociation pressure of CO2 hydrate in Na2SO4, K2SO4, and MgSO4 solutions is higher than that in pure water, and the dissociation pressure of CO2 hydrate in sulfate solutions increases with an increasing salt concentration. Conversely, CuSO4 barely affects the dissociation pressure of CO2 hydrate, which is mainly attributed to the lower molar concentration of the ions compared with the other salt solutions and the ion specificity of Cu2+. The prediction results are in alignment with the experimental data measured in this study, which proves the feasibility of the thermodynamic model in predicting the dissociation pressure of CO2 hydrate in sulfate solutions. Additionally, the calculated dissociation enthalpies of CO2 hydrate show a dependence on both temperature and salt concentration. It is also revealed that Cu2+ exhibits ion specificity in affecting the dissociation enthalpy of CO2 hydrate, likely due to its distinct ability to impact the cage occupancy of CO2 hydrate compared with the other ions. These findings enhance our understanding of the impact of sulfate salts on the dissociation behavior of CO2 hydrate and offer valuable insights for CO2 sequestration.

Theoretical and practical investigation of ion-ion association in electrolyte solutions.

In this study, we present a new equation of state for electrolyte solutions, integrating the statistical associating fluid theory for variable range interactions utilizing the generic Mie form and binding Debye-Hückel theories. This equation of state underscores the pivotal role of ion-ion association in determining the properties of electrolyte solutions. We propose a unified framework that simultaneously examines the thermodynamic properties of electrolyte solutions and their electrical conductivity, given the profound impact of ion pairing on this transport property. Using this equation of state, we predict the liquid density, mean ionic activity coefficient, and osmotic coefficient for binary NaCl, Na2SO4, and MgSO4 aqueous solutions at 298.15K. Additionally, we evaluate the molar conductivity of these systems by considering the fraction of free ions derived from our equation of state in conjunction with two advanced electrical conductivity models. Our results reveal that, while ion-ion association has a minimal influence on the modification of the predicted properties of sodium chloride solutions, their impact on sodium and magnesium sulfate solutions is considerably more noticeable.

Designing Centrifugal Membrane Filters with uniform-pressure for UF/NF/RO separations

A novel Ortho-Centrifugal Membrane Filter (O-CMF) was designed and assessed to tackle common issues in conventional laboratory centrifugal membrane filters, e.g. non-uniform transmembrane pressure and concentration polarization, and to ease membrane replacement. The O-CMF, manufactured by additive 3D printing in acrylonitrile butadiene styrene, with membrane area of 0.95 cm2, was designed to process a maximum liquid sample of 9 mL. It was tested with nanofiltration of aqueous solutions of NaCl, MgSO4, and lactose (concentrations of 1–20 g/L) at transmembrane pressures of 6–24 bar. Complementary Computational Fluid Dynamics (CFD) simulations, employing a custom solver from the open-source OpenFOAM-2306 c++ toolbox, were also run to seek insight of the O-CMF main features.The permeation experiments displayed minimal impact of concentration polarization. CFD simulations showed that concentration polarization is nearly uniform and that concentrate chamber is homogenized by centrifugal convection. The CFD permeate flux predictions, based on the osmotic pressure model, closely matched experimental data, and a correlation for predicting concentration polarization was also developed.The O-CMF device represents a substantial breakthrough in the technology of centrifugal membrane filters, consistently maintaining uniform transmembrane pressure and effectively minimizing concentration polarization. The device was evaluated using nanofiltration, but its pressure range is versatile enough to cover conditions suitable for both ultrafiltration and reverse osmosis operations. This adaptability extends its application across a variety of laboratory experiments, including sample concentration, hydraulic permeability tests, and membrane screening.

Thermodynamics of the Eu(III)-Mg-SO4-H2O and Eu(III)-Na-SO4-H2O systems. Part I: solubility experiments and the full dissociation Pitzer model.

The solubility of Eu(III) was investigated under undersaturated conditions in acidic, dilute to concentrated MgSO4 and Na2SO4 solutions at T = (22 ± 2) °C. After attaining equilibrium conditions, solid phases were characterized by a multi-method approach, including X-ray diffraction (XRD), Raman and infrared (IR) spectroscopy, quantitative chemical analysis (ICP-OES) and thermogravimetric analysis (TG-DTA). A total of 45 solubility samples were investigated for the systems Eu2(SO4)3-MgSO4-H2O (19 samples) and Eu2(SO4)3-Na2SO4-H2O (26 samples). Eu2(SO4)3·8H2O(cr) was found to control the solubility of Eu(III) in all investigated MgSO4 solutions, as well as in dilute Na2SO4 systems. The transformation of Eu2(SO4)3·8H2O(cr) into the double salt Na2Eu2(SO4)4·2H2O(cr) was observed at mNa2SO4 > 0.01 mol kg-1. The latter phase is characterized by significantly lower solubility. Based on these experimental solubility measurements, thermodynamic and activity models were proposed based on the Pitzer equations considering the full dissociation of the Eu(III) species in MgSO4 and Na2SO4 aqueous solutions, i.e. deliberately excluding Eu(III)-sulfate complex formation. A combination of the geochemical calculation code PhreeSCALE and the parameter estimation code PEST was used to determine the values of solubility products and binary and ternary specific interaction parameters (β(0)ij, β(1)ij, Cϕij, θik, Ψijk).

Screening of high-efficiency crosslinking agents for poly(N,N-dimethylaminoethyl methacrylate) and performance of its nanofiltration membranes

Poly(N,N-dimethylaminoethyl methacrylate) (PDM) is a water-soluble polymer that can be crosslinked via a quaternization reaction that takes 4–20 h. Polyhalogenated hydrocarbons with different structures were screened to identify high-efficiency crosslinking agents for PDM. For this purpose, the gelation time of bulk crosslinked PDM and the separation performance of interfacially crosslinked PDM membranes were investigated. Use of bromohydrocarbons with benzylic and allylic types and high functional group density can afford the rapid quaternization reaction of PDM. PDM nanofiltration membranes were prepared via interfacially crosslinking PDM with 1,3,5-tris(bromomethyl)benzene (TBB), and the effects of membrane preparation conditions (including PDM concentration, coating time, TBB concentration, and crosslinking time) on the permeation and separation performances of as-prepared membranes were investigated. The PDM membrane prepared at a PDM concentration of 6 g/L, TBB concentration of 40 mmol/L, coating time of 30 s, and crosslinking time of 30 s exhibited a water flux of 20.3 L/(m2·h) and a salt rejection of 90%, when the membrane was treated with a 1000 mg/L MgSO4 aqueous solution at 0.8 MPa pressure. The PDM membrane was positively charged and the extent of salt rejection followed the order MgCl2 > NaCl ≈ MgSO4 > Na2SO4. The membrane also exhibited good antifouling properties and chlorine resistance.

CaF2(s) Solubility Isotherm in the CaF2–CaSO4–MSO4–H2O (M = Zn, Mn, Mg) Quaternary Systems and Their Subsystems at Various Temperatures

A ZnSO4 electrolyte solution with impurity salts such as MnSO4 and MgSO4 is often used in zinc hydrometallurgical processes over a wide temperature range. CaSO4·2H2O(s) scaling and fluorine corrosion are two encountered problems in the process. Understanding the CaF2(s) solubility behavior in MSO4 (M = Zn, Mn, Mg) aqueous solutions at various temperatures helps remove Ca2+ and F– ions from the solution. However, CaF2(s) solubility data, especially at temperatures >298 K, have not been published yet. In this work, CaF2(s) solubilities in the CaF2–CaSO4–MSO4–H2O (M = Zn, Mn, Mg) quaternary systems and its subsystems of the CaF2–MSO4–H2O (M = Zn, Mn, Mg) at 348.15 K and the CaF2–CaSO4–MSO4–H2O (M = Zn, Mg) quaternary systems and the CaF2–MgSO4–H2O ternary system at 298.15 K are systematically measured. The results show that the CaF2(s) solubility in the MSO4 (M = Zn, Mn, Mg) aqueous solution of a certain concentration at 348.15 K is considerably higher than that at 298.15 K. At 298.15 and 348.15 K, the salting-in effect of MSO4 (M = Zn, Mn, Mg) on CaF2(s) solubility is enhanced with the order ZnSO4 < MnSO4 < MgSO4. At both temperatures, CaF2(s) solubilities in CaSO4·2H2O(s)- or CaSO4(s)-saturated MSO4 (M = Zn, Mn, Mg) aqueous solutions are generally lower than those in pure MSO4 (M = Zn, Mn, Mg) aqueous solutions. Particularly, the salting-in effect of MgSO4 on CaF2 is so strong that CaF2(s) dissolves completely and is converted to CaSO4·2H2O(s) and MgF2(s) in a concentrated MgSO4 aqueous solution. Hence, this study indicates that high temperatures and high salt concentrations increase the CaF2(s) solubility, making the removal of Ca2+ and F– ions from the system challenging. Further, the existence of impurity salts MnSO4 and MgSO4 makes it more difficult.

High flux zwitterionic hybrid cellulose acetate desalination membranes: Formation of permeability-effective macropores by self-organizing zwitterionic silanes

Zwitterionic hybrid cellulose acetate membranes with improved flux and antifouling performance were produced by a modified non-solvent induced phase separation (NIPS) process in which a zwitterionic trimethoxysilane compound, (3-sulfopropylbetaine-propyl)-trimethoxysilane (SPPT), was added to the casting solutions at 1.0, 2.0, and 3.0 wt%. The successful incorporation of SPPT into cellulose acetate (CA) membranes was proved by X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and thermal gravimetric analysis (TGA). Permeate fluxes of the hybrid membranes increased significantly compared to the control cellulose acetate membrane prepared without the addition of SPPT (15 bar pressure; 2000 ppm NaCl or MgSO4 aqueous solution filtration). The hybrid membranes exhibited an increase in permeate flux up to 87 % while their salt rejections slightly decreased in NaCl solution filtration. In MgSO4 solution filtration, the flux was increased by 61 % without sacrificing salt rejection. The reason for the enhancement in the flux was the formation of macropores in the membrane bulk structures by self-organizing zwitterionic silane groups. The highest resistance to bovine serum albumin (BSA) or xanthan gum as a foulant was observed in the hybrid membranes produced with 1.0 wt% of SPPT.

固体平板と気泡間に形成される液膜の排水過程における中心膜厚に及ぼす水中の電解質の影響

The drainage and rupture of the thin liquid film formed between a bubble and a flat mica plate have been investigated. It is known that the bubble coalescence can be prevented in electrolyte aqueous solution, and its effects strongly depends on the type of electrolyte. In this study, by using two types of electrolyte aqueous solutions (0.05 M MgSO4 aqueous solution and 0.05 M CH3COONa aqueous solution) and surfactant aqueous solution (10 ppm Triton X-100 aqueous solution), evaluate the effects of impurities on the behavior of the liquid thin film. The effect of the approach velocity also is also evaluated. By using an optical interferometer and a high-speed video camera, interference fringe images are taken, and the shape of the liquid thin film formed between a bubble and a flat mica plate can be obtained. The effect of solute in transition on the thin film process in the center of the liquid film is investigated. Under conditions of 2 mm bubble diameter and the approach velocity of 50 μm/s or less, the thickness of the central film increases, and the spreading of the liquid film is suppressed when electrolytes are added into water.

Preparation of lignosulfonate-based nanofiltration membranes with improved water desalination performance.

Pulping and papermaking generate large amounts of waste in the form of lignosulfonates which have limited valorized applications so far. Herein, we report a novel lignosulfonate‐based nanofiltration membrane, prepared by using polyethylenimine (PEI) and sodium lignosulfonate (SL) via a layer‐by‐layer (LbL) self‐assembly. As a low‐cost and renewable natural polyelectrolyte, SL is selected to replace the synthetic polyelectrolyte commonly used in the conventional LbL fabrication for composite membranes. The prepared LbL (PEI/SL)7 membranes were crosslinked by glutaraldehyde (GA) to obtain (PEI/SL)7‐GA membranes with compact selective layer. We characterized (PEI/SL)7 and (PEI/SL)7‐GA membranes to study the chemical compositions, morphologies, and surface hydrophilicity. To improve the nanofiltration performances of the (PEI/SL)7‐GA membranes for water desalination, we investigated the effects of the crosslinking time, GA concentration and the NaCl supporting electrolyte on membrane structure and performance. The optimized (PEI/SL)7‐GA membrane exhibited a permeating flux up to 39.6 L/(m2·h) and a rejection of 91.7% for the MgSO4 aqueous solution 2.0 g/L concentration, showing its promising potential for water desalination. This study provides a new approach to applying the underdeveloped lignin‐based biomass as green membrane materials for water treatment.

Salt crystallization pressure as a new method to obtain micro and nanocellulose

This work evaluated the crystallization pressure of sodium and magnesium sulphates as a tool to break down and defibrillate plant cell walls, aiming at a novel, environmentally friendly and low-cost methodology to obtain micro and nanofibrilated cellulose. Elephant grass leaves both in natura and delignified were soaked in Na2SO4 and MgSO4 solutions and oven-dried at 105 ± 3 °C to promote salt crystallization, while the damage resulted from the pressure exerted by the growing crystals within the cell walls was evaluated. Na2SO4 crystallization proved to be more effective to unpack and defibrillate cell walls in comparison to MgSO4. In addition, both salts had more noticeable effects in previously delignified samples than in in natura samples. Nanofibrillated cellulose was obtained by Na2SO4 action and after shearing fibres treated with an aqueous MgSO4 solution (30% w/v), glycerol and NaOH (5% w/v).

Developing composite nanofiltration membranes with highly stable antifouling property based on hydrophilic roughness

Membrane fouling is still an inevitable main challenge to nanofiltration (NF) process, which leads to a decline in membrane performance. Inspired by fish scales, a facile method was performed to develop composite NF membranes with highly stable antifouling property based on hydrophilic roughness. γ-(2,3-glycidyloxy) trimethoxysilane (KH560) grafted onto PEI molecules during interfacial polymerization on hydrophilic microporous PTFE support. Finally, tetraethyl orthosilicate (TEOS) reacted with KH560 molecules. Results of fourier transform infrared (FTIR) spectra, FESEM and contact angle measurements indicated that micro/nano hydrophilic SiO2 spheres engendered on the membrane surface. Evaluation of membrane antifouling property showed that flux recovery ratio (FRR) of BSA, HA, SDS and Oil/water mixture for the as-prepared NF membranes were 96.0%, 95.8%, 96.8% and 95.0%, respectively. No obvious permeate flux decline and rejection fluctuation could be observed in filtration experiment of MgSO4 aqueous solution on long-term run (100 h). It indicated highly stable antifouling property, which may be due to hydrophilic microscale/nanoscale SiO2 spheres grafted onto the skin layer of the as-prepared NF membranes through chemical linkage. It is expected that these findings could provide more insight for fabrication and simultaneous modification of NF membranes.

Study of Polarization Characteristics of Corrosion Films on Magnesium in Sulfate-Containing Electrolytes

In this article, the results of studying the polarization characteristics of magnesium covered with corrosion film in aqueous solutions of MgSO4 and Na2SO4 are presented. The absence of a corrosion-free magnesium surface was shown; in this connection, it was proposed to interpret the larger values of Tafel’s coefficients obtained in the experiment from the point of view of limiting the electrochemical process by charge transfer in the film phase. Charge transfer in corrosion films obeys the regularities of particle movement in high electric fields, and it is not only cationic. According to the impedance measurements, the resistance of the oxide and hydroxide layer of the magnesium-based corrosion film in the studied solutions was calculated. The largest contribution to the restriction of charge transfer in the initial stages of corrosion is made by a dense primary film defining the polarization resistance. Correlation of transfer parameters in high electric fields with thickness and resistance of corrosion film was demonstrated.

Carbonization of Carboxylate‐Functionalized Polymers of Intrinsic Microporosity for Water Treatment

AbstractConsidering the variations in their pore characteristics and heterogeneity, the carbonization process of carboxylate‐functionalized polymers of intrinsic microporosity (PIM‐COOH) is investigated and the resulted membranes are tested for water treatment. The four distinct types of sub‐nanosized membranes with different oxygen‐to‐carbon ratios are prepared at four different temperatures: the pristine polymer membrane, crosslinked membrane via thermal decarboxylation, amorphous carbon membrane, and graphitic carbon membrane. Notably, the sub‐1 nm micropores of all the heat‐treated samples that do not exhibit a broadening pore size distribution are still retained. Nanofiltration performance is investigated using an aqueous MgSO4 solution (2000 ppm), pure water, and a series of the defect‐free, sub‐1 nm microporous membranes. While retaining the salt rejection, the water flux of the membranes increases with the pyrolysis temperature owing to their low friction property.

Insight into the roles of two typical ion clusters and their second hydration shells: Implication for the nucleation mechanism in MgSO4 aqueous solution

In this study, the ion association characteristics and the hydration dynamics of Mg2+ ions in MgSO4 aqueous solution at various concentrations were systematically investigated using density functional theory and classical molecular dynamics simulations. The results show that contact associated (CA) and solvent separated (SS) structures of Mg2+ and SO42− ions are the dominant species and play different roles in the crystallization and gelation processes of MgSO4 aqueous solution due to their distinctive association trend and the distinctive dynamics of their second hydration shells. The CA structures are more stable and the further aggregation of these clusters is sluggish in MgSO4 aqueous solution. The supersaturation or the formation of gel state can be partially attributed to the abundance of CA structures in supersaturated MgSO4 aqueous solution. In contrast, although the ion exchange in SS structures occurs frequently, the aggregation of SS structures is active and can be greatly promoted by increasing of the concentration. The dynamics of the second hydration shells around Mg2+ ions of SS structures also becomes more active as the concentration increases. The dynamic evolution characteristics of SS structures demonstrate their aggregation tendency for nucleation in the oversaturated MgSO4 aqueous solution, which helps to understand the nucleation mechanism in MgSO4 aqueous solution and the role of ion hydration dynamics in the nucleation process.

MgF2 Solubility in MgF2 + Salt + H2O Systems (Salt = MgSO4, (NH4)2SO4, NH4Cl) at 298.15 K

MgSO4 and (NH4)2SO4 are the main impurities in the hydrometallurgical systems of zinc and manganese. To understand their influences on MgF2(s) solubility, which the fluoride in these systems is expected to remove, we measured MgF2(s) solubility in NH4Cl, (NH4)2SO4 and MgSO4 aqueous solutions at 298.15 K. It was found that the MgF2(s) solubility exhibits salt-in effect in the (NH4)2SO4(aq) and NH4Cl(aq) solutions, but salt-out and salt-in effects in the MgSO4(aq) solution. Furthermore, the MgF2(s) solubility in (NH4)2SO4(aq) is higher than in NH4Cl(aq) solution. Ionic association reactions have been derived to interpret the above solubility phenomena.

Salt rejection behavior of carbon nanotube-polyamide nanocomposite reverse osmosis membranes in several salt solutions

The present paper describes the desalination performance of reverse osmosis (RO) membranes of multi-walled carbon nanotube-polyamide complex (CNT-PA) and commercial polyamide membranes in NaCl, MgCl2, MgSO4 and Na2SO4 aqueous solutions. The permeate flux, salt rejection, and salt flux were determined in a cross-flow experiment. The CNT-PA and commercial RO (PA) membranes (SWC5, Nitto Denko Co.) showed 96.0% and 99.7% salt rejection, respectively, for 0.2% NaCl aqueous solution at 0.7 MPa. The calculated salt flux was 0.38 g·m−2·h−1 (CNT-PA) and 0.07 g·m−2·h−1 (SWC5). The salt rejection increased with increasing running pressure and decreasing salt concentration. The zeta potential measurement of CNT-PA demonstrated that it is negatively charged due to the presence of CNT. Accordingly, it showed salt rejection performances against the four salt solutions (Na2SO4 > MgSO4 > NaCl > MgCl2) that differed from that of the usual PA membranes (Na2SO4 > MgSO4 > MgCl2 > NaCl). These data are explained based on the Donnan model (CNT-PA) and steric hindrance pore model (SWC5), except for the case of chlorides under low-flux or high ionic strength conditions, where the diffusion and molecular size exclusion of the salts dominate over their mas-transport. Positron annihilation lifetime spectroscopy (PALS) enabled the estimation of the pore diameters of these membranes: 0.55 nm (CNT-PA) and 0.58 nm (SWC5).

Forward Osmosis and Freeze Crystallization as Low Energy Water Recovery Processes for a Water-Sustainable Industry

A water-sustainable future chemical industry that is using water in its operations (such as the metals and mining industry) relies on the development of low energy water recovery technologies. Among them are Forward Osmosis (FO) and Freeze Crystallization (FC), two innovative processes that promise to minimize fresh water intakes in the chemical industry at a significantly lower energy input compared to conventional processes, such as Reverse Osmosis (RO), evaporation, or crystallization. FO is a membrane process that requires a concentrated draw solution (CDS) to osmotically recover water. The separation of the draw solutes from the draw solution and the regeneration of the CDS is the main source of energy consumption in the FO process. On the other hand, FC involves processes in which water is removed directly from aqueous solutions, as ice, by freezing or freeze-thaw cycling. Research advancements in the two aforementioned processes are reported. The economical effectiveness of an FO process using Trimethylamine-CO2-H2O as the CDS was evaluated through an optimization study designed and carried out in Minitab and Aspen Plus. Optimum operating conditions for the separation of the draw solutes from the product water were identified, and the most important parameters that affect its performance were determined. Further, the economic sustainability of this FO process was assessed. Regarding the FC process, the natural freezing of a 0.5 m MgSO4 aqueous solution was studied. Freezing temperatures ranged from −2 °C to −26 °C, while melting of the recovered solid, i.e., ice with entrapped solute contamination, was done at room temperature (25 °C) and at 3 °C. The optimum conditions for maximizing water recovery while reducing impurity contamination were determined.

Experimental Determination of Solubilities of Brucite [Mg(OH)2(cr)] in Na2SO4 Solutions with Borate to High Ionic Strengths: Interactions of MgB(OH) 4 + with Na2SO4

In this work, a solubility study on brucite [Mg(OH)2(cr)] in Na2SO4 solutions ranging from 0.01 to 1.8 mol·kg−1, with 0.001 mol·kg−1 borate, has been conducted at 22.5 °C. Based on the solubility data, the Pitzer interaction parameters for MgB(OH) 4 + − SO 4 2− and MgB(OH) 4 + − Na+ along with the formation constant for MgSO4(aq) are evaluated using the Pitzer model. The formation constant ( $$ \log_{10} \beta_{1}^{0} $$ = 2.38 ± 0.08) for MgSO4(aq) at 25 °C and infinite dilution obtained in this study is in excellent agreement with the literature values. The experimental data on the solubility of gypsum (CaSO4·2H2O), at 25 °C, in aqueous solutions of MgSO4 with ionic strengths up to ~ 11 mol·kg−1 were analyzed using models with and without considering the MgSO4(aq) species. The model incorporating MgSO4(aq) fits better to the experimental data than the model without MgSO4(aq), especially in the ionic strength range beyond ~ 4 mol·kg−1, demonstrating the need for incorporation of MgSO4(aq) into the model to improve the accuracy.

Water-separated ion pairs cause the slow dielectric mode of magnesium sulfate solutions.

We compare the dielectric spectra of aqueous MgSO4 and Na2SO4 solutions calculated from classical molecular dynamics simulations with experimental data, using an optimized thermodynamically consistent sulfate force field. Both the concentration-dependent shift of the static dielectric constant and the spectral shape match the experimental results very well for Na2SO4 solutions. For MgSO4 solutions, the simulations qualitatively reproduce the experimental observation of a slow mode, the origin of which we trace back to the ion-pair relaxation contribution via spectral decomposition. The radial distribution functions show that Mg2+ and SO42- ions form extensive water-separated-and thus strongly dipolar-ion pairs, the orientational relaxation of which provides a simple physical explanation for the prominent slow dielectric mode in MgSO4 solutions. Remarkably, the Mg2+-SO42- ion-pair relaxation extends all the way into the THz range, which we rationalize by the vibrational relaxation of tightly bound water-separated ion pairs. Thus, the relaxation of divalent ion pairs can give rise to widely separated orientational and vibrational spectroscopic features.

A comparison of amorphous calcium carbonate crystallization in aqueous solutions of MgCl2 and MgSO4: implications for paleo-ocean chemistry

Based on the terminology of “aragonite seas” and “calcite seas”, whether different Mg sources could affect the mineralogy of carbonate sediments at the same Mg/Ca ratio was explored, which was expected to provide a qualitative assessment of the chemistry of the paleo-ocean. In this work, amorphous calcium carbonate (ACC) was prepared by direct precipitation in anhydrous ethanol and used as a precursor to study crystallization processes in MgSO4 and MgCl2 solutions having different concentrations at 60 °C (reaction times 240 and 2880 min). Based on the morphology of the aragonite crystals, as well as mineral saturation indices and kinetic analysis of geochemical processes, it was found that these crystals formed with a spherulitic texture in 4 steps. First, ACC crystallized into columnar Mg calcite by nearly oriented attachment. Second, the Mg calcite changed from columnar shapes into smooth dumbbell forms. Third, the Mg calcite transformed into rough dumbbell or cauliflower-shaped aragonite forms by local dissolution and precipitation. Finally, the aragonite transformed further into spherulitic radial and irregular aggregate forms. The increase in Ca2+ in the MgSO4 solutions compared with the MgCl2 solutions indicates the fast dissolution and slow precipitation of ACC in the former solutions. The phase transition was more complete in the 0.005 M MgCl2 solution, whereas Mg calcite crystallized from the 0.005 M MgSO4 solution, indicating that Mg calcite could be formed more easily in an MgSO4 solution. Based on these findings, aragonite and Mg calcite relative to ACC could be used to provide a qualitative assessment of the chemistry of the paleo-ocean. Therefore, calcite seas relative to high-Mg calcite could reflect a low concentration MgSO4 paleo-ocean, while aragonite seas could be related to an MgCl2 or high concentration of MgSO4 paleo-ocean.