Enhanced fluoride removal from water by non-thermal plasma modified CeO2/Mg–Fe layered double hydroxides

A novel fast and efficient adsorbent based on lamellar compound namely CeO₂/Mg–Fe layered double hydroxide composite has been designed for fluoride removal from water. In order to improve fluoride removal efficiency, non-thermal plasma (NTP) was used to modify the surface state of composites. The prepared composites were characterized by powder X-ray diffraction, thermogravimetric analysis and surface area analyzer. Adsorption equilibrium and kinetics of fluoride on NTP modified composites were investigated. Experimental results indicated that the adsorption capacity was enhanced with NTP surface modification. The maximum adsorption capacity has been found to be 38.7–60.4 mg/g. The kinetic data of adsorption were found to best fit the pseudo-second-order model, while the equilibrium data were found to be well described by Langmuir model. In order to understand the mechanism of adsorption, thermodynamic parameters such as ΔG^θ, ΔS^θ and E_a were calculated. After NTP treatment, the ΔS^θ increased from − 34.7 J/mol·K to − 0.770 J/mol·K, the E_a decreased from 78.8 kJ/mol to 58.9 kJ/mol and the ΔG^θ (25 °C) decreased from − 2.62 kJ/mol to − 3.14 kJ/mol. These values indicate that the fluoride adsorption on NTP modified composites was improved.     

Crystal growing and reaction kinetic of large NaX zeolite crystals

Large crystals zeolite NaX with a uniform size of 50 μm were grown by a continuous crystallization method from seed crystals (10 μm) formed in a mother solution with the 3.5Na₂O:Al₂O₃:2.1SiO₂:1000H₂O composition. In order to grow zeolite NaX crystals to an appropriate size by the continuous method, the mother solution was fed into an autoclave a solution with various seed contents (3–20 wt.%); the autoclave has at 90 °C and the solution was added after 7, 12, 16, 19 and 24 days or at 100 °C after 7 and 9 days. The morphology of NaX zeolite crystals when viewed from the top shows an octahedron that is formed from eight equilateral triangles. These triangular faces intersect all three crystallographic axes at the same distance having an average lattice constant a = 24.9911 Å and a SiO₂/Al₂O₃ molar ratio of 2.1–2.4. The activation energy for growth of NaX zeolite crystals was found to be 43.2 kJ/mol.     

Kinetics of calcium sulfoaluminate formation from tricalcium aluminate,calcium sulfate and calcium oxide

The formation kinetics of tricalcium aluminate (C₃A) and calcium sulfate yielding calcium sulfoaluminate (C₄A₃$) and the decomposition kinetics of calcium sulfoaluminate were investigated by sintering a mixture of synthetic C₃A and gypsum. The quantitative analysis of the phase composition was performed by X-ray powder diffraction analysis using the Rietveld method. The results showed that the formation reaction 3Ca₃Al₂O₆ + CaSO₄ → Ca₄Al₆O₁₂(SO₄) + 6CaO was the primary reaction < 1350 °C with and activation energy of 231 ± 42 kJ/mol; while the decomposition reaction 2Ca₄Al₆O₁₂(SO₄) + 10CaO → 6Ca₃Al₂O₆ + 2SO₂ ↑ + O₂ ↑ primarily occurred beyond 1350 °C with an activation energy of 792 ± 64 kJ/mol. The optimal formation region for C₄A₃$ was from 1150 °C to 1350 °C and from 6 h to 1 h, which could provide useful information on the formation of C₄A₃$ containing clinkers. The Jander diffusion model was feasible for the formation and decomposition of calcium sulfoaluminate. Ca^2 + and SO₄^2 − were the diffusive species in both the formation and decomposition reactions.     

Photocatalytic reduction of Cr(VI) to Cr(III) in solution containing ZnO or ZSM-5 zeolite using oxalate as model organic compound in environment

Photocatalytic reduction of Cr(VI) to Cr(III) in aqueous solution containing ZnO or ZSM-5 zeolite under ambient condition was studied by using oxalate as model organic compound in the natural environment. ZSM-5 zeolite was characterized by X-ray diffraction (XRD), and point of zero net proton charge (PZNPC) titration. The effect of illumination time, mass content of catalyst (m/V), Cr(VI) initial concentrations, pH, ionic strength, and oxalate concentrations on the photocatalytic reduction of Cr(VI) was determined. The results indicate that the PZNPC of ZSM-5 zeolite is at pH 3.6 ± 0.1. At C[Cr(VI)_(initial)] = 2.00 × 10^?4 mol/L, pH 7.5 ± 0.1 and after illumination time of 24 h, the reduction of Cr(VI) were 1.1 × 10^?5 mol/L (no ZSM-5 zeolite, 4.0 × 10^?3 mol/L oxalate) and 1.0 × 10^?5 mol/L (0.4 g/L ZSM-5 zeolite, no oxalate), respectively; whereas the reduction of Cr(VI) achieved 1.0 × 10^?4 mol/L in the presence of 0.4 g/L ZSM-5 zeolite and 4.0 × 10^?3 mol/L oxalate. The removal of Cr(VI) from solution is dependent on pH value. The results are important for the application of zeolites in the treatment of Cr(VI) polluted solution in the natural environment.     

Hydration kinetics modeling of the effect of curing temperature and pressure on the heat evolution of oil well cement

The heat evolution of Class G and Class H oil well cements cured under different temperatures (25 °C to 60 °C) and pressures (2 MPa to 45 MPa) was examined by isothermal calorimetry. Curing pressure was found to have a similar effect on cement hydration kinetics as curing temperature. Under isothermal and isobaric conditions, the dependency of cement hydration kinetics on curing temperature and pressure can be modeled by a scale factor which is related to the activation energy and the activation volume of the cement. The estimated apparent activation energy of the different cements at 2 MPa varies from 38.7 kJ/mol to 41.4 kJ/mol for the temperature range of 25 °C to 40 °C, which decreases slightly with increasing curing temperature and pressure. The estimated apparent activation volume of the cements at 25 °C varies from − 23.1 cm³/mol to − 25.9 cm³/mol for the pressure range studied here, which also decreases slightly in magnitude with increasing curing temperature.     

Sintering behaviour of a glass obtained from MSWI ash

The sintering behaviour of a glass obtained by Municipal Solid Waste Incinerator (MSWI) bottom ash (WG) was investigated and compared with a Na₂O–MgO–CaO–SiO₂ composition (CG). The sintering activation energy, E_sin, and the energy of viscous flow, E_η, were evaluated by dilatomeric measurements at different heating rates. The formation of crystalline phases was evaluated by Differential Thermal Analysis (DTA) and X-Ray Diffraction (XRD), and observed by Scanning Electron Microscopy (SEM) and Transition Electron Microscopy (TEM). In CG, the sintering started at ≈10¹³ dPa s viscosity and E_sin (245 kJ/mol) remains constant in the measured range of shrinkage, up to 9%. In WG the densification started at ≈10¹¹ dPa s, E_sin resulted to be 395 kJ/mol up to 5% shrinkage, 420 kJ/mol at 8% and 485 kJ/mol at 10% shrinkage. The sintering rate decreased due to the beginning of the pyroxene formation and the densification stopped in the temperature range 1073–1123 K after formation of 5 ± 3% and 13 ± 3% crystal phase, at 5 and 20 K/min, respectively. Higher densification and improved mechanical properties were obtained by applying the fast heating rate, i.e. 20 K/min.     

Creep behavior of a zirconium diboride–silicon carbide composite

Flexural creep studies of ZrB₂–20 vol% SiC ultra-high temperature ceramic were conducted over the range of 1400–1820 °C in an argon shielded testing apparatus. A two decade increase in creep rate, between 1500 and 1600 °C, suggests a clear transition between two distinct creep mechanisms. Low temperature deformation (1400–1500 °C) is dominated by ZrB₂ grain or ZrB₂–SiC interphase boundary and ZrB₂ lattice diffusion having an activation energy of 364 ± 93 kJ/mol and a stress exponent of unity. At high temperatures (>1600 °C) the rate-controlling processes include ZrB₂–ZrB₂ and/or ZrB₂–SiC boundary sliding with an activation energy of 639 ± 1 kJ/mol and stress exponents of 1.7 < n < 2.2. In addition, cavitation is found in all specimens above 1600 °C where strain-rate contributions agree with a stress exponent of n = 2.2. Microstructure observations show cavitation may partially accommodate grain boundary sliding, but of most significance, we find evidence of approximately 5% contribution to the accumulated creep strain.     

Time-resolved powder neutron diffraction study of the phase transformation sequence of kaolinite to mullite

Mullite formation from kaolinite was studied by means of high-temperature in situ powder neutron diffraction by heating from room temperature up to 1370 °C. Neutron diffractometry under this non-isothermal conditions is suitable for studying high-temperature reaction kinetics and to identify short-lived species which otherwise might escape detection. Data collected from dynamic techniques (neutron diffraction, DTA, TGA and constant-heating rate sintering) were consistent with data gathered in static mode (conventional X-ray diffraction and TEM). The full process occurs in successive stages: (a) kaolinite dehydroxylation yielding metakaolinite in the ∼400–650 °C temperature range, (b) nucleation of mullite in the temperature range ∼980–992 to ∼1121 °C (primary mullite) side by side with a crystalline cubic phase (Si-Al spinel) detected in the ∼983–1030 °C temperature interval; (c) growth of mullite crystals from ∼1136 °C, (d) high (or β) cristobalite crystallization at T > ∼1200 °C and (e) secondary mullite crystallization at T > ∼1300 °C. The calculated activation energy for the kaolinite dehydration was 115 kJ/mol; for the mullite nucleation was 278 kJ/mol and for the growth of mullite process was 87 kJ/mol; finally for cristobalite nucleation the calculated apparent activation energy was 481 kJ/mol.     

Limiting factors for low-temperature performance of electrolytes in LiFePO4/Li and graphite/Li half cells

Low-temperature performance of LiBF₄ and LiPF₆-based electrolytes in LiFePO₄/Li and graphite/Li half cells was investigated. In the temperature range from 0 °C to ?40 °C, electrochemical impedance spectroscopy (EIS) results show that the charge-transfer resistance (R_ct) of graphite/Li cell decreases, the R_ct of LiFePO₄/Li cell increases, and sum resistance of LiFePO₄/Li and graphite/Li cell decreases when replacing LiPF₆ with LiBF₄. In the temperature range from 25 °C to ?40 °C, energy barrier (W) for Li-ion jump at the solid electrolyte interface (SEI) alters slightly from 16.04 kJ/mol to 13.60 kJ/mol in LiFePO₄/Li cells, but declines greatly from 46.47 kJ/mol to 19.81 kJ/mol in graphite/Li cells when using LiBF₄ instead of LiPF₆, meanwhile, activation energy (ΔG) of electrode reaction is approximately the same (～60 kJ/mol). The above results indicate that the ionic conductivity is the main limiting factor for low-temperature performance of electrolytes in LiFePO₄/Li cell, while factors related with electrolyte-interface are more crucial in graphite/Li cell than in LiFePO₄/Li cell.     

Mullitization kinetics from kaolinite incorporating nanometric Ba and Ca fluorides

The work presents the kinetic effect of nanometric BaF₂ and CaF₂ particles on kaolinite to mullite transformation. The kinetics were evaluated from dilatometric data using two different non-isothermal procedures: conversional model-fitting method and diffusional sintering analysis. From experimental data, the activation energy of mullite formation calculated from sintering (942 kJ/mol) and from conversional method (910 kJ/mol) were in good agreement with those values reported by other authors (mean value 1030 kJ/mol). After incorporation of 3 mol% of nanometric BaF₂ and CaF₂ in kaolinite and applying both analytical procedures, lower activation energies for mullite formation were obtained, assigning to the transformation the value of 635 kJ/mol for kaolinite/BaF₂ and 428 kJ/mol for kaolinite/CaF₂ composites.     

Onset coarsening/coalescence of cobalt oxides in the form of nanoplates versus equi-axed micron particles

The change of specific surface area and pore size distribution coupled with N₂ adsorption–desorption hysteresis isotherm, in particular that typical to cylindrical pores, were used to determine the onset coarsening/coalescence in the temperature range of 500–800 °C for Co(OH)₂ derived Co₃O₄ nanoplates and 700–1000 °C for CoO-derived Co₃O₄ powders (backtransformed to CoO above 900 °C) which are equi-axed in shape and microns in size. The vigorous onset coarsening/coalescence of the nanoplates and equi-axed micron particles was found to occur within minutes having apparent activation energy of 37 ± 7 kJ/mol (based on t_0.7, i.e. time for 70% surface area reduction) and 113 ± 8 kJ/mol (based on t_0.3), respectively. The surface area reduction process of the nanoplates was found to be controlled by (1 1 1)-specific coalescence besides a coarsening–repacking process more common to the equi-axed particles. The present static experimental results of coarsening–coalescence of the Co₃O₄ (below 900 °C) or CoO particles (above 900 °C) supports our previous supposition that CoO and Co₃O₄ nanocondensates could readily assemble as nanochain aggregates and further coalesce into a close packed manner below 1000 °C by the radiant heating effect in a dynamic laser ablation process.     

High compressive strength in nacre-inspired Al−7Si−5Cu/Al2O3–ZrO2 composites at room and elevated temperatures by regulating interfacial reaction

Al−7Si−5Cu/Al₂O₃−ZrO₂ composites with nacre-like structures were prepared via ice-templating and gas pressure infiltration techniques. The composites were subsequently heat-treated at 850 °C for 0, 30, 60, 90 and 120 min to regulate the interfacial reaction between Al and ZrO₂. The yield of larger (Al_1−m, Si_m)₃Zr and ZrSi₂ phases increased with longer dwell times. The compressive strength initially increased and then decreased. The highest strength was observed in composites treated for 60 min and reached 1600±40, 1261±30 and 1033±22 MPa at temperatures of 20, 150 and 300 °C, respectively. These values increased by 30−40% as compared to those of the non-treated counterparts and were 2-, 5- and 12-fold more than those of the matrix alloy, respectively, which is demonstrative of the material's excellent load-bearing capacity, particularly at elevated temperatures.     

Synthesis,crystal structure,luminescence and thermal behavior of a new energetic zinc(II) compound

An energetic material [Zn₂(btzphda)₂(H₂O)₄(dpp)₂]·2DMF·4H₂O with high decomposition enthalpy of − 748.35 J/g was prepared by the reaction of H₂btzphda, dpp and Zn(NO₃)₂·6H₂O under solvothermal conditions, where btzphda = 1,4-bis(tetrazol-5-yl)benzene-N2,N2′-diacetato, dpp = 1.3-di(4-pyridyl)propane and DMF = N,N′-dimethylformamide. The luminescence properties of H₂btzphda and [Zn₂(btzphda)₂(H₂O)₄(dpp)₂]·2DMF·4H₂O were investigated at room temperature in the solid state (Hitachi F4600 spectrofluorometer). Furthermore, the thermal decomposition behavior of the compound is characterized by differential scanning calorimetry (DSC) and thermogravimetric-differential thermogravimetric (TG-DTG) analyses. The entropy of activation (ΔH), enthalpy of activation (ΔS) and the free energy of activation (ΔG) for the decomposition temperature were ΔH = 250.64 kJ/mol, ΔS = 222.75 J·mol^− 1·K^− 1 and ΔG = 134.10 kJ/mol.     

The nature of high surface energy sites in graphene and graphite

Variations in the adsorption enthalpies of acetone to few-layer graphene and graphite nanopowders were analyzed as a function of surface coverage. The adsorption enthalpies were measured by inverse gas chromatography at low monolayer coverage levels (0.1–20%). The adsorption enthalpies increased from −13 kcal/mol at the lowest coverage to −7.5 kcal/mol. We fitted the measured adsorption enthalpies as a function of coverage using a two-state model and estimated the number of high-energy sites on both materials. The graphite powder had seven times more high-energy sites than the few-layer graphene, which explains why the adsorption enthalpies for graphite increased more slowly with increasing coverage. We also performed a theoretical study based on density functional theory calculations using a functional that accounts for dispersive interactions to elucidate the nature of the high-energy adsorption sites. The calculated adsorption enthalpies ranged from −16 to −1 kcal/mol while the adsorption enthalpy to a plain graphite surface was −9 kcal/mol. The high-energy adsorption sites were localized on surface steps and edge-cavities. The adsorption enthalpies at very low coverage therefore corresponded to adsorption on steps and edge cavities, while those measured at coverage levels of ∼4% or more reflected adsorption to the flat surface.     

Bomb calorimetry as a bulk characterization tool for carbon nanostructures

Multiwalled carbon nanotubes (MWCNTs) consisting of coaxial graphene cylinders (cylindrical MWCNTs), cones (herringbone MWCNTs) or carbon fibers were combusted in an isothermal bomb calorimeter. Their standard enthalpies of formation were determined to be 16.56 ± 2.76 kJ mol⁻¹(C – per carbon mol) for carbon fibers, 21.70 ± 1.32 kJ mol⁻¹(C) for herringbone MWCNTs and 8.60 ± 0.52 kJ mol⁻¹(C) for cylindrical ones. All materials were characterized by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, thermogravimetry, and elemental analysis. A linear correlation between the standard enthalpies of formation and D/G and G′/G Raman bands ratio (D – band is centered at 1350 cm⁻¹, G – 1585 cm⁻¹, G′ – 2700 cm⁻¹) demonstrates the applicability of bomb calorimetry for characterization of the “defectiveness” of the bulk carbon material in the sense Raman spectroscopy is widely used nowadays. Also, we show that the calorimetry may be used to estimate the oxygen content in the bulk carbon nanomaterials, as there is a linear correlation between the oxygen content (both total content and in carboxyl groups separately) and the standard enthalpies of formation for herringbone nanotubes oxidized by nitric acid.     

Effect of Cu on the ablation properties of Cf/ZrC composites fabricated by infiltrating Cf/C preforms with Zr-Cu alloys

C_f/ZrC composites were fabricated by reactive melt infiltration at 1200 °C, Low melting Zr₇Cu₁₀, ZrCu and Zr₂Cu alloys were used as infiltrators and the effect of Cu on ablation properties of the composites was investigated. The results show that the C_f/ZrC composites exhibit excellent anti-ablative properties affected apparently by the Cu contents. With the increase of Cu in infiltrators, the linear recession rates decrease from 0.0019±0.0006 to −0.0006±0.0002 mm s⁻¹, whereas the mass loss rates increase from 0.0006±0.0003 to 0.0047±0.0009 g s⁻¹. The formation of a dense ZrO₂ protective layer and the evaporation of residual Cu are in favor of their ablation resistance.     

Two-stage master sintering curve applied to two-step sintering of oxide ceramics

Tetragonal (3 mol% Y₂O₃) and two cubic zirconia (8 mol% Y₂O₃) as well as alumina green bodies were used for the construction of the Master Sintering Curve (MSC) created from sets of constant-rate-of-heating (CRH) sintering experiments. The activation energies calculated according to the MSC theory were 770 kJ/mol for Al₂O₃, 1270 kJ/mol for t-ZrO₂, 620 kJ/mol and 750 kJ/mol for c-ZrO₂. These values were verified by an alternative approach based on an analysis of the densification rate in the intermediate sintering stage. The MSCs established from the Two-Step Sintering (TSS) experiments showed at high densities a significant deflection from those constructed from the CRH experiments. This deflection was explained by lower sintering activation energy in the closed porosity stage. A new two-stage MSC model was developed to reflect the change in sintering activation energy and to describe TSS. The efficiency of TSS of four materials under investigation was correlated with their activation energies during the final sintering stage.     

Transesterification of leather tanning waste to biodiesel at supercritical condition: Kinetics and thermodynamics studies

Catalyst-free transesterification of leather tanning waste with high free fatty acid (FFA) content at supercritical condition was reported in this work. The experiments were performed in batch system at various temperatures (250–325 °C) under constant pressure of 12 MPa and methanol/fatty oil molar ratio of 40:1 for reaction time of 2–10 min. Kinetic modeling of formation of fatty acid methyl esters (FAMEs) that incorporate reversible esterification and non-reversible transesterification simultaneously was verified. The proposed semi-empirical model was fitted against kinetic experimental data over temperature range studied. The kinetic parameters (i.e. k′_TE, k′_E, and k_E′) were determined by nonlinear regression fitting. Thermodynamic activation parameters of the reactions were evaluated based on activation complex theory (ACT) and the following results are obtained: ΔG³ > 0, ΔH³ > 0, and ΔS³ < 0. The activation energy (E_a) of transesterification, forward and reverse esterification reactions was 36.01 kJ/mol, 28.38 kJ/mol, and 5.66 kJ/mol, respectively.     

Far-infrared and THz spectroscopy of 0.4PMN–0.3PSN–0.3PZN relaxor ferroelectric ceramics

Temperature dependence of the optic phonons in 0.4PbMg_1/3Nb_2/3O₃–0.3PbSc_1/2Nb_1/2O₃–0.3PbZn_1/3Nb_2/3O₃ (0.4PMN–0.3PSN–0.3PZN) ceramics were studied by means of FTIR reflection and THz transmission spectroscopy in the temperature range of −253.15 to 226.85 °C. On heating from low temperatures, the A₁ component of the strongly split TO₁ mode softens towards the Burns temperature, but the softening ceases near 126.85 °C which could be a signature of polar cluster percolation temperature. Surprisingly, the TO₂ mode also softens on heating and follows the Cochran law with extrapolated critical temperature close to the melting point.     

Onset sintering-coarsening-coalescence kinetics of calcite powders

An onset sintering-coarsening-coalescence (SCC) event based on a significant decrease of specific surface area relative to the dry pressed samples after isothermal firing in the 450–600 °C range in air was determined by N₂ adsorption–desorption hysteresis isotherm for submicron-sized calcite powders. The apparent activation energy for such a rapid SCC event was estimated as 57.5 ± 1.0 kJ/mol based on the time for 50% reduction of specific surface area without appreciable phase change of calcite. The minimum temperature to activate the SCC process, as of concern to industrial CaCO₃ applications and natural limestone formation, is 317 °C based on the extrapolation of steady specific surface area reduction rates to null.