Enthalpy Values Research Articles

Determining the hydrogen production potential of ConMo6Se8 chevrel phases

Abstract In this study, the Co n Mo 6 Se 8 (n = 1, 2, 3, and 4) Chevrel phases are investigated by using Density Functional Theory (DFT) to reveal their potential for photocatalytic hydrogen production. The stability conditions of these phases reveal that CoMo 6 Se 8, Co 2 Mo 6 Se 8, and Co 3 Mo 6 Se 8 satisfy the thermodynamic and mechanic stability properties, while Co 4 Mo 6 Se 8 does not satisfy any of these properties. Furthermore, the formation enthalpy of these phases shows that CoMo 6 Se 8, Co 2 Mo 6 Se 8, and Co 3 Mo 6 Se 8 can be synthesized experimentally due to having negative formation enthalpy values. Furthermore, the thermal stabilities of the machine-learning (ML) force fields are investigated by ab-initio molecular dynamics (AIMD) calculations. The electronic properties of these phases are also investigated in detail, and it is found that Co 3 Mo 6 Se 8 has a suitable band gap for photocatalytic water splitting. Concerning the investigation of the valence band and conduction band levels, it is shown that Co 3 Mo 6 Se 8 has a conduction band minimum level suitable for producing hydrogen. This study is the first attempt to reveal the hydrogen production performance of the Co n Mo 6 Se 8 (n = 1, 2, 3, and 4) Chevrel phases as far as the literature is concerned, paving the ground for future investigations in this field.

Composite Sorbents for CO2 Capture Based on Polyethyleneimine and Silica Gel: Study of Sorption Properties and Analysis of Thermal Energy Consumption

Studies of CO2 sorption by composite sorbents containing the active component (branched polyethylenimine) dispersed in the pores of mesoporous support (silica gel) have been carried out. It is shown that as silica gel pores are filled with polyethylenimine, the sorption efficiency of CO2 by the active component decreases, and the dynamic sorption capacity per 1 g of material passes through a maximum. The enthalpy values of CO2 sorption by composite sorbents have been determined. It is shown that the enthalpy values of CO2 sorption depend on the proportion of pore filling by the active component and on the characteristics of the porous structure of the support. Thermal energy consumption for regeneration of composite sorbents within the adsorption cycle was analyzed.

Study of high malachite green adsorption by organically modified clay

The removal of the toxic dye malachite green (MG) from aqueous effluents using organically modified clay (OC) was investigated in a batch system. The OC was synthesized by intercalating the cationic surfactant hexadecyltrimethylammonium bromide into Algerian montmorillonite. The study explored the effects of initial dye concentration, temperature, pH, and contact time on dye adsorption. The OC demonstrated high dye removal efficiency (99%) at an initial dye concentration of 100 mg L−1, pH 6, and 20°C. The adsorption capacity was found to be independent of pH but increased with temperature, with maximum adsorption capacities of 350, 380, and 398 mg·g⁻¹ at 20°C, 30°C, and 40°C, respectively. The adsorption process was characterized by rapid initial uptake followed by a slower rate until equilibrium was reached, with a maximum uptake of 49.45 mg·g⁻¹. The experimental data were best described by the Langmuir adsorption model, indicating monolayer adsorption on homogeneous sites. Kinetic studies revealed that the sorption rate followed a pseudo-second-order model, with a high correlation (R² = 0.999), suggesting a chemisorptive nature of the interaction. Thermodynamic analysis indicated that the process was spontaneous and endothermic, with positive enthalpy and negative Gibbs free energy values. Overall, the OC proved to be an effective and environmentally friendly adsorbent for the removal of MG dye from wastewater.

Effect of Different Temperatures on Reactions of Potassium Adsorption and Availability in Saline Soils within Nineveh Governorate, Iraq

Thermodynamic criteria were used to evaluate the adsorption and release of potassium in saline soils. The study was conducted on three sites that differ in salt content in Nineveh Governorate - Iraq, and each site is divided into two depths (surface and subsurface). The study was conducted at three temperatures (278, 298, 318 Kelvin). The results of the study showed that there was an increase in the amount of potassium adsorbed and the percentage of adsorption with an increase in the amount of potassium added in all the studied soil samples. For the ability of the soil to the adsorption, the results showed a sudden increase in the adsorption of potassium at the application of the high levels (1 and 2 mmolL-1) of potassium in all the studied soils. Also, the highest value of the ionic activity of potassium (ARk) was (14.05 × 10-3 molL-1/2), at a temperature of 298 K in the subsurface soil layer of the first site, while the lowest one was (1.61 × 10-3 molL-1/2), in the same layer of the first site at a temperature of 278 K. The values ​​of free energy of potassium, ΔGK, in the studied soils ranged from the highest value reached (-14.85×10-3 kJ) at a temperature of 278 K, to the lowest value, reached (-10.56×10-3 KJ) at a temperature of 298 K, in the subsurface soil layer of the first site. Regarding the values ​​of entropy reaction, they ranged from the highest value of -53.43 Jmol-1 in the subsurface depth of the first site at a temperature of 278 K to the lowest value of -35.46 in the same layer of the first site at a temperature of 298 K. On the other hand, the enthalpy values ​​of the studied soil samples were negative, which indicates that the reaction is of the exothermic type, where the highest negative value for enthalpy was -29.70 kJmol-1 at a temperature of 278 K in the subsurface soil layer of the first site and the lowest negative value, was -20.42 kJmol-1 in the same layer of the first site at a temperature of 298 K.

Mechanical Synthesis and Calorimetric Studies of the Enthalpies of Formation of Chosen Mg-Pd Alloys.

Despite many years of research and continuous improvements in scientific equipment, some of the thermodynamic properties of binary systems are still unknown, or are only theoretically predicted or calculated. This situation often arises from the difficulties in preparing alloys for experimental measurements. The alloys from the Mg-Pd system, especially for the Pd-rich side, are difficult to produce, and the availability of thermodynamic data is very limited. Therefore, this paper presents calorimetric studies on the standard enthalpy of formation of alloys from the Mg-Pd system, which were prepared using mechanical alloying. Three alloys (S1, S2, and S3) were synthesized, homogenized, and subjected to X-ray diffraction (XRD) analysis to investigate their phase composition. The XRD studies showed that the alloys designated as S1 and S2 were the intermetallic phases Mg6Pd and Mg0.9Pd1.1, and the S3 sample was a mixture of MgPd and MgPd3 intermetallic phases. Their heat effects, measured by drop calorimetry, were used to calculate the values of the standard enthalpies of formation of the prepared phases. The values obtained were as follows: -27.5 ± 1.1 kJ/mol at. for the Mg6Pd intermetallic phase, -72.7 ± 1.0 kJ/mol at. for the Mg0.9Pd1.1 intermetallic phase, and -46.8 ± 1.5 kJ/mol at. for the alloy which was a mixture of MgPd and MgPd3. These data were compared with values from the existing literature on the enthalpy of formation of alloys, as well as with data calculated using Miedema's model.

CALCULATION OF THERMODYNAMIC TEMPERATURES OF CHEMICAL REACTIONS OF STEPWISE IRON REDUCTION PROCESS FROM HEMATITE BY SOLID CARBON

The results of thermodynamic analysis of chemical reactions of stepwise reduction of iron from hematite by solid carbon are presented. The aims of the work are to obtain our own expressions for calculating the numerical values of Gibbs free energy depending on temperature using tabular values of standard enthalpies of formation and entropies of inorganic substances, as well as graphical dependences of Gibbs energy on temperature using formulas from literary sources and using the obtained expressions. Numerical values of boundary temperatures are obtained, above which chemical reactions of stepwise reduction of iron from hematite by solid carbon can thermodynamically proceed. It has been established that: the reduction of magnetite (Fe3O4) from hematite (Fe2O3) by solid carbon C is thermodynamically possible above the boundary temperature, the numerical value of which, according to the obtained formula, is 270°; the reduction of wustite (FeO) from magnetite (Fe3O4) by solid carbon C is thermodynamically possible above the boundary temperature, the numerical value of which, according to the obtained formula, is 658°C; the reduction of iron (Fe) from wustite (FeO) by solid carbon C is thermodynamically possible above the boundary temperature, the numerical value of which, according to the obtained formula, is 703°C, and according to formulas, taken from literary sources, is 774°C, 690°C, 680°C and 609°C, respectively, for each formula. Despite the fact that thermodynamics allows all these chemical reactions to occur above the certain above-mentioned temperature values, each of the reactions of stepwise reduction of iron from its oxides by solid carbon in a shaft reduction furnace either occurs at higher temperatures, and, which is quite obvious, with a change in the aggregate state of the reacting (initial) substances or at least one of them (given that FeO forms low-melting compounds with SiO2, this will be the reaction of reduction of iron from wustite), or does not occur at all (given that Fe2O3 and Fe3O4 do not form low-melting compounds with SiO2, and their melting temperatures are greater than 1500°C, these will be the reactions of reduction of magnetite from hematite and reduction of wustite from magnetite).

CALCULATION OF THERMODYNAMIC TEMPERATURES OF CHEMICAL REACTIONS OF STEPWISE MANGANESE REDUCTION PROCESS FROM ITS DIOXIDE BY CO GAS AND GASIFICATION OF SOLID CARBON BY DEGREES OF CHEMICAL AFFINITY OF SUBSTANCES TO OXYGEN

The results of a thermodynamic analysis of the course of chemical reactions of the stepwise reduction of manganese from its dioxide by CO gas and the gasification reaction of solid carbon (Bella-Boudoir) are presented. The goals of the work are to obtain eigenexpressions for calculating the numerical values of the Gibbs free energy depending on temperature using tabulated values of the standard enthalpies of formation and entropies of inorganic substances, as well as to construct graphical dependences of the Gibbs energy on temperature using formulas from literary sources and obtained expressions. Numerical values of the boundary temperatures are obtained above which the chemical reactions of the stepwise reduction of manganese from its dioxide by CO gas and the chemical reaction of gasification of solid carbon (Bella-Boudoir) can or cannot thermodynamically proceed. Considering the complete coincidence of the obtained data on the own (obtained) expressions using two (direct and indirect) methods, it is possible to consider with a significant degree of probability the numerical values of the boundary temperature for the reactions of stepwise reduction of manganese from its dioxide and gasification of solid carbon as reliable, which indicates the possibility of the reduction of Mn2O3 from MnO2, Mn3O4 from Mn2O3 and MnO from Mn3O4 by CO gas, the Bell-Boudoir reaction and the impossibility of the reduction of manganese from MnO by CO gas at the temperatures of the actual process in reduction furnaces. This also confirms that CO gas is not a manganese reducer at the last stage of stepwise reduction of MnO according to scheme (B), refuting any theories and assumptions about the possibility of reducing Mn from MnO by CO gas.

Using Differential Scanning Calorimetry to Measure the Energetic Properties of Residual Sludge and Catalysts from the Textile, Tannery, and Galvanic Industries

This research delved into the energetic properties of catalysts synthesized from residual sludge from the textile, galvanic, and tannery industries. The experimental process consisted of an initial heat treatment to activate their catalytic properties and a thermal analysis employing differential scanning calorimetry (DSC). This technique permitted the investigation of the materials’ thermal behavior as a function of temperature, ranging from 142 to 550 °C, effectively controlling the heating rates and pressure conditions. The data gathered were the input for constructing specific heat models through polynomial regression employing the least squares method. These models were subsequently used to estimate variations in the enthalpy and entropy for both the sludge and catalysts through integration. Third-degree polynomials primarily characterized the specific heat models that accurately represented the samples’ thermal behavior, considering variations in their physicochemical properties that influenced it. The catalysts derived from residual sludge from the textile industry exhibited the models with the most robust statistical fit. Concurrently, the catalysts from the galvanic industry displayed noteworthy similarities with the bibliographic data across various temperature points. The mathematical models determined the specific heat (Cp) as a function of temperature, which, in turn, was used to estimate the enthalpy and entropy variations in the sludge and catalysts under study. The highest enthalpy value corresponded to the sludge and catalyst obtained from the tannery industry, with a Cp of 5.60 J/g-K at 603 K and 2.45 J/g-K at 445.6 K. Finally, the third-degree polynomials showed the best mathematical models since (1) they considered the variations in the physicochemical properties that intervened in the behavior of Cp as a function of temperature; (2) they presented a better statistical fit; and (3) they showed consistency with the existing information in the literature for the textile industry and the galvanic industries.

Sustainable synthesis and environmental application of chitosan-Ocimum basilicum leaves-ZnO composite membrane for permanganate ion removal in wastewater treatment.

This study introduces a novel and environmentally friendly method for developing a cross-linked chitosan-Ocimum basilicum leaves-ZnO (ChOBLZnO) composite membrane, specifically designed for the adsorption of permanganate ions (MnO4-) from wastewater. Various characterization techniques, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) surface area analysis, were employed to confirm the membrane's synthesis quality, structural integrity, and surface properties crucial for adsorption. The BET analysis revealed a surface area of 228.64 m2/g, indicating a highly porous structure. The membrane exhibited a high adsorption capacity of 142.8mg/g and an outstanding removal efficiency of 98.5%. The adsorption kinetics were best described by the pseudo-second-order model, with a correlation coefficient (R2) of 0.998, while the Freundlich isotherm model provided the most accurate fit for the adsorption behavior, with an R2 of 0.995. Thermodynamic studies indicated that the adsorption process is spontaneous and endothermic, with negative Gibbs free energy and positive enthalpy and entropy values. Reusability tests showed that the ChOBLZnO composite membrane maintained a high removal efficiency across five cycles, with minimal performance loss. These results demonstrate the ChOBLZnO composite membrane's potential as an efficient and sustainable solution for wastewater treatment, offering both high efficiency and durability.

Comparison of extraction process, physicochemical properties, and in vitro digestion characteristics of chia seed mucilage polysaccharide.

The study explored five (acidic, alkaline, heating, ionic liquid, and urea solvent) extraction methods' effects on chia seed mucilage polysaccharide (CSM), an anionic polymeric macromolecule, regarding its physicochemical properties, structure, and digestion behavior. The results showed that extraction parameters have a considerable effect on modulating CSM properties. Significant differences emerged in the predominant chemical compositions: the carbohydrates and protein content ranged from 49.20±0.06% to 85.81±0.03%, and 3.20±0.13% to 14.57±0.30%, respectively. The structural analysis revealed that alkaline heating treatment facilitated the formation of protein-polysaccharide conjugates, resulting in reduced particle size, enhanced ζ-potential, and improved thermal stability (194.72±2.19J/g). The crystallinity of CSM varied, peaking at 42.9±0.22% without pH adjustment and heating. CSM extracted using 6M urea exhibited the lowest protein content, and crystallinity (25.50±0.09%), coupled with the highest gastrointestinal digestion rate and poorest thermal stability (with a carbohydrate degradability of 24.223±1.78% and enthalpy value of 62.82±0.32J/g). The CSM obtained under alkaline heating showed minute particles (201.1±10.35μm), the highest ζ-potential absolute value (20.95±2.28mV), and robust thermal stability (194.72±2.19J/g of enthalpy value), which is ideal for stabilizing emulsions or encapsulating thermolabile substances. Additionally, compared to monovalent cations‑sodium ions, divalent cations‑magnesium ions, is more tend to aggregate the CSM structure, resulting in larger molecular particles and a higher protein content. Elevated ionic concentration further diminished thermal stability. These findings suggest that CSM is a customizable, multi-purpose polymer that can be extracted in various ways based on the end-product requirements.

Sustainable polyurethane biocomposite foams by improved microstructure, acoustic characteristics, thermoregulation performance and reduced CO2 emission through phase change material integration

In this research, phase change material (PCM) comprised of energy storage biocomposite materials were improved with raw materials obtained from renewable resources. Modified raw material was synthesized by epoxy castor oil (ECO). After mixing the obtained modified castor oil (MCO) and polyether polyol, PCM of capric acid (CA) was supplemented. To initiate the chemical reaction, a certain amount of methylene diphenyl diisocyanate (MDI) was added to the mixture to provide cross-linking. The physical and chemical properties of the resulting biocomposite foams (BCF) were characterized. According to the results obtained, mixing MCO into polyether raw material increased the number of closed pores in BCF resulting in more CA stored in BCF cells produced. The use of MCO in BCF materials increased the CA integration ratio to 68 wt%. The melting enthalpy value of the novel BCF-PCM composites has been determined as 121.43 J/g. In a warm environment, the PCM additive creates a temperature difference exceeding 6 °C, while in a cold environment, it provides a cooling effect close to 6 °C. A more than 50 % improvement in thermal conductivity was achieved with 68 wt% PCM addition compared to pure BCF. The addition of PCM caused a reduction in tensile stress and strain values to an acceptable level. BCFs with PCM additives exhibit a sound absorption coefficient of 0.9 in narrow frequency ranges, whereas, in broad frequency ranges, this value was more than 0.7. In cold weather conditions, employing BCF-PCM material instead of traditional EPS material with a thickness of 0.1 m has led to at least a 30 % drop in CO2 emissions, no matter the type of fuel used for heating. When opting for BCF-PCM materials, the necessary heat demand was approximately 33 % less compared to using the same thickness of EPS in cold climates.

The construction of retrograded short-chain amylose beads coated with calcium alginate for increased resistant starch.

A novel strategy was developed to embed retrograded short-chain amylose (RSCA) into calcium alginate (CA) at varying concentrations (0.25%, 0.5%, 0.75%, 1%, 1.5%, and 2%) through ion crosslinking, aimed at enhancing the content of heat-resistant RS. The concentration of sodium alginate (SA) significantly influenced the thickness of the CA shell, thereby affecting the structural characteristics, physicochemical properties, and digestibility of the resulting beads. As the SA concentration increased from 0.5% to 2%, the CA shell thickness expanded from 0.612μm to 1.837μm, becoming denser and limiting the infiltration of enzymatic liquids during digestion. The initial gelatinization temperature of the beads exceeded 100°C, and the enthalpy value of the 2% SA beads was 25.40±0.17J·g-1, indicating excellent thermal stability. After heat treatment, the crystallinity reached 90.50%. The RS content of the 2% SA beads was as high as 72.66±0.15%, with an estimated glycemic index of 54.72, classifying them as low-glycemic index foods. The heat-resistant RSCA@CA developed in this study shows significant potential for application in functional foods, particularly for individuals with obesity and diabetes.

Experimental and theoretical studies on antioxidant and antibacterial properties of chitosan-gelatin functional composite films loaded with flavonoids

Chitosan-gelatin-flavonoid functional composite films were prepared with chitosan, gelatin, and three flavonoids (Naringenin, Apigenin, and Luteolin). The effect of three flavonoids on physical, antioxidant, and antibacterial properties of functional composite film was investigated from experimental and Density functional theory (DFT) simulations. The tensile strength, thermal stability, water solubility, water vapor permeability, antioxidant activity, and antibacterial activity of chitosan-gelatin-flavonoid functional composite films were improved with flavonoid (Naringenin, Apigenin, and Luteolin) incorporation. The release behavior of Apigenin from functional composite film was much lower than that of Naringenin or Luteolin. ABTS+ radical scavenging ability values of functional composite films followed: Luteolin (69.53 %) > Naringenin (41.39 %) > Apigenin (36.13 %). The antibacterial activities of functional composite films against Staphylococcus aureus followed: Luteolin (52.03 mm) > Apigenin (49.34 mm) > Naringenin (43.15 mm). The total number, location of hydroxyl groups on ring-B, and the unsaturation degree on pyrone ring of flavonoids influenced antioxidant and antibacterial activities of functional composite films. The minimum bond dissociation enthalpy values of flavonoids followed: Luteolin < Apigenin < Naringenin. The interaction energy of Apigenin-chitobiose was stronger than that of Naringenin or Luteolin. These results will shed light on flavonoid selection for functional composite films of food packaging.

Thermal performance, mechanical sensitivity and low speed impact ignition characteristics of PTFE based aluminum alloy active materials

Abstract To solve the problems of low energy release efficiency, slow reaction rate, and long ignition response time of Al/PTFE, the idea of introducing aluminum zirconium alloy powder instead of traditional Al powder is proposed in this paper. By utilizing its high calorific value, high enthalpy value, and high activity, the reaction rate and energy of PTFE based aluminum alloy active materials are promoted and enhanced, which ultimately improved and enhanced the energy release efficiency. Aluminium zirconium nickel based active materials were developed using aluminum zirconium alloy powder, and their thermal properties and mechanical sensitivity were tested. The temperature isostatic pressing preparation process based on prefabricated column was also explored. The research results indicate that the Al/Zr/Ni/PTFE formula has a higher exothermic value when Al reacts with PTFE, and it triggers a reaction in the Al/Zr/Ni alloy powder during the reaction stage; The addition of aluminum zirconium alloy powder reduces the impact and friction sensitivity of Al/PTFE materials, with Al/Zr/Ni/PTFE aluminum alloy active materials showing a slight improvement; The ignition response and energy release characteristics of Al/Zr/Ni/PTFE aluminum alloy active materials under hammer impact are significantly improved compared to Al/PTFE. Therefore, the aluminum zirconium nickel based active material system is one of the important research directions in the future.

Exploring enthalpies of CO2 and N2 adsorption on Zn- and Co-based zeolitic frameworks at varying temperatures and pressures

This study investigates the thermodynamics of CO2 and N2 adsorption on zeolitic imidazolate frameworks (ZIFs), focusing on the effects of metal ions under varying temperatures and pressures. Zn- and Co-based ZIFs were synthesized using 2-methylimidazole and characterized by XRD, FT-IR, Raman spectroscopy, and BET isotherms, confirming high microporosity. Gas adsorption experiments were performed at pressures up to 3000 kPa and temperatures between 268 and 323 K. The CO2 adsorption capacities vary from 274 to 157 cm³/g, while N2 ranged from 67 to 29 cm³/g as temperature increased. The synergistic dynamics of temperature and pressure enhanced CO2 diffusion and induced pore structure changes via a gate-opening mechanism. Isosteric enthalpy (ΔHads) values, determined using Freundlich-Langmuir/Clausius-Clapeyron and virial fits, were -22 ± 5 kJ/mol for CO2 and -15 ± 2 kJ/mol for N2, confirming that physisorption is the dominant mechanism. Gibbs free energy (ΔGads) for CO2 ranged from -12 to -18 kJ/mol, and entropy (ΔSads) from -0.01 to -0.03 kJ/mol/K, suggesting spontaneous and thermodynamically favorable CO2 adsorption, though spontaneity decreased with rising temperature. ZIFs containing Zn exhibited high CO2 adsorption capacity, while those with Co showed better CO2 selectivity over N2, highlighting their potential for efficient CO2 capture under various temperature and pressure conditions.

APPROXIMATE RELATIONSHIPS FOR DETERMINING THE ENTHALPIES OF WATER AND STEAM IN THE CALCULATIONS OF HEAT EXCHANGER DEVICES OF STEAM TURBINE INSTALLATIONS (PART I – DETERMINATION OF THE ENTHALPY OF WATER)

The work is devoted to the creation of a methodological approach for determining the thermodynamic properties of water and water vapor in the calculation of heat exchange processes for those cases when it is necessary to quickly control the operating modes of steam turbines of large power of thermal and nuclear power plants, but the use of existing tables of properties of working environments in the "manual" mode is not always suppose. In the presented part of the work the determination of the enthalpy of water is considered. Based on the analysis of the operating modes of heat exchangedevices of power units thermal and nuclear power plants of different capacities, the ranges of temperature and pressure changes of the working media during their calculations were determined. They are for water: at a pressure of 1 kPa–30 MPa, at a temperature of 1–300 °С, and for steam, respectively: 1 kPa–6 MPa, 7–450 °С. The analysis of tabular values of the working medium (water) showed that in order to present the enthalpy of water in an analytical form based on approximation equations with the required accuracy, it is advisable to consider their construction in two areas of change: in the area of vacuum (Р &lt; 0.1 MPa) and in the area of excess pressure (0.1–30 MPa), which in turn are divided into 4 ranges. When thepressure changes, this is associated with an additional effect on the temperature enthalpy value, which complicates the choice of approximate dependencies for obtaining enthalpy from the parameters of the working environment while ensuring an acceptable ratio of calculated and tabular values for solving problems. By the authors a system of regression equations was obtained, allowing to calculate the enthalpy of water in a given range of pressures and temperatures with high accuracy. To determine the area in which the measured value is located, the authors proposed the use of Antoine's formula, in which the relationship between the pressure and temperature of the medium at the boundary line of the phase transition is established. To increase its accuracy, approximation equations for temperature corrections at different pressures are proposed, which ensure that the temperature deviates from the tabulated values of thermophysical properties there are no more than ±0.04 °С.

Extracted microbial exopolysaccharides: a sustainable approach to bioremediation of nickel, cobalt and copper ions; equilibrium, kinetics and thermodynamics studies

ABSTRACT The adsorption of nickel, cobalt and copper ions onto the surface of exopolysaccharides extracted from cyanobacterial mats has been studied to provide a sustainable and efficient approach for the remediation of heavy metals. The physical characterisation of the EPS surface, including SEM, EDX and FTIR, confirmed an irregular compact structure with multiple cracks, rough macropore structure and the existence of hydroxyl, amine, amide, polysulphide and carbonyl groups. The maximum adsorption capacity (q max 47.17 , 47.02 and 46.95 mg/g for Cu(II), Co(II) and Ni(II), respectively) was achieved at pH = 6, 0.3 g/L EPS dose, 120 min duration time and 35°C. The remediation affinity of EPS decreased in the order of Ni(II) > Co(II) > Cu(II). The Langmuir model (R 2 > 0.95) better fits the adsorption behaviour than the Freundlich model (R 2 < 0.90) and the Dubinin – Radushkevich isotherm model, indicating chemical adsorption between the biosorbent and metal ions. Kinetically, the pseudo-second-order model better describes adsorption than does the pseudo-first-order model. Finally, thermodynamic studies have demonstrated that the adsorption process is spontaneously endothermic, with positive enthalpy values (ΔH°) and negative free energy (ΔG°). The positive entropy values (ΔS°) indicate an increase in disorder at the solid-liquid interface during the adsorption process.

Estimation of mixing enthalpy of the liquid Sn-Ag-Cu system at 1423 K from data on the properties of binary subsystems using geometric models of solutions

The article considers the estimation of mixing enthalpy ΔНmix of the Sn-Ag-Cu ternary system melts at a temperature of 1423 K from the calorimetric data on the mixing enthalpy of binary subsystems Ag-Cu, Ag-Sn and Cu-Sn measured earlier. The geometric models by Toop, Kohler and Muggianu, where binary data is jointly processed in each case according to a specific mathematical procedure, were applied to the ΔНmix assessment. The results of calculations by these models are presented by the compositional dependences of ΔНmix of the ternary system in the form of 3D surfaces, projections of these surfaces onto the plane of the compositional triangle, as well as isotherms plotted for individual quasi-binary sections. It was found that modeling using the Kohler’s and Muggianu’s methods gives insignificantly different results, whereas the values of the mixing enthalpy of ternary compositions in the Toop’s model are noticeably more immersed into the negative (exothermic) region. As it is known from the scientific literature, the right choice of a geometric model depends on whether the ternary system under study belongs to a “symmetric” or “asymmetric” type. The shape of the available ΔНmix isotherms of binary subsystems indicates that the Sn-Ag-Cu system is “asymmetric”. The results calculated using the Toop’s model are recognized as the most correct ones, since that model is recommended in the literature for describing “asymmetric” systems. The common limitation of all geometric models, considering only binary interparticle interactions, and the advisability of extra experimental study of three-component melts formation to reveal the possible contribution of ternary interactions to the mixing enthalpy are noted.

Mechanisms of Adsorption of Phenoxyalkanoic Herbicides on Fulvic and Humic Acids.

Our recent study demonstrated that fulvic and humic acids are the major contributors to the adsorption of phenoxyalkanoic acid herbicides in soils. At very low pH, the neutral forms of these herbicides are bound directly to fulvic and humic acids, whereas at higher pH, their anionic forms are adsorbed mainly via bridges created by Al3+ species. The number of active sorption sites associated with Al3+ species complexed with fulvic acids is pH-dependent, whereas the number of corresponding sites in humic acids is pH-independent. Based on the results of the FTIR analysis, research into adsorption thermodynamics, and molecular modeling, an attempt was made in the present study to explain the adsorption mechanisms of six phenoxyalkanoic herbicides used currently in the European Union on the surfaces of the above fractions of humic substances. The obtained values of standard enthalpy (ΔH0) for the adsorption of the anionic forms of phenoxyalkanoic herbicides on fulvic or humic acids complexed with Al3+ were in the range of physical adsorption, i.e., from -8.4 kJ/mol to -2.9 kJ/mol for the former, and from -5.3 kJ/mol to -2.4 kJ/mol for the latter. The study demonstrated that the neutral forms of phenoxyalkanoic herbicides were bound to humic substances mainly via H-bonds, π-π stacking interactions, and hydrophobic interactions. Al3+ species were complexed with fulvic and humic acids to form outer-sphere complexes. Ternary outer-sphere complexes were also created between the anionic forms of phenoxyalkanoic acid herbicides and positively charged Al3+ species complexed with fulvic acids. The mechanisms of adsorption on humic acids involved a ligand exchange between a loosely bound hydroxyl group of hydrolyzed Al3+ complexed with this adsorbent and the anionic form of the herbicide. However, in this case, adsorption took place only in the presence of sufficiently strong hydrophobic and π-π stacking interactions supported by H-bonds. These findings elucidate why phenoxyalkanoic herbicides are mobile in the soil profile and are often rapidly degraded in soils.

Preparation and application of multilayered flexible phase change material with high thermal conductivity and high enthalpy

Phase change material (PCM) uses latent heat to store heat and are widely used in thermal management of electronic components. However, endowing PCM with high thermal conductivity and high enthalpy remains a challenge. This study designs a multilayered structure, paraffin (PA)/styrene-b-(ethylene-co-butylene)-b-styrene triblock copolymer (SEBS) (PA/SEBS) and PA/SEBS/expanded graphite (EG) (PA/SEBS/EG) are prepared by melt blending and crosslinked together in sequence, named 0EG@6EG. The results show that the enthalpy value of 0EG@6EG is as high as 167.46 kJ·kg−1, and the thermal conductivity has been improved compared to traditional uniform structure. 0EG@6EG has a certain degree of self-healing ability, and the contact surface between layers could withstand a tensile strength of 0.23 MPa. The passive experiment indicates that the temperature rising rate of 0EG@6EG is 4.08 % higher than that of PA/SEBS/3EG. The active experiment shows that the highest outlet temperature difference between 0EG@6EG and PA/SEBS/3EG is 1.52 °C, and the heat released is 26.73 % higher than that of PA/SEBS/3EG. The optimal thickness ratio between PA/SEBS/0EG to PA/SEBS/6EG in 0EG@6EG is 1.2:0.8, and when the active system contacts with the layer of PA/SEBS/6EG, it is conducive to enhance heat transfer. This work provides a reference for increasing the thermal conductivity of PCM without decreasing enthalpy value.