Direct contact membrane distillation of humic acid solutions

Direct contact membrane distillation process has been conducted for the treatment of humic acid solutions using microporous polytetrafluoroethylene and polyvinylidene fluoride membranes. The membranes were characterized in terms of their non-wettability, pore size and porosity. Water advancing and receding contact angles on the top membrane surfaces were measured. Experiments were also carried out employing pure water as feed at different mean temperatures and the water vapor permeance of each membrane was determined. Different humic acid concentrations in the feed solution, pH values and transmembrane temperature difference were tested. The direct contact membrane distillation technique is more adequate for the treatment of humic acid solutions than the applied pressure-driven separation processes, as lower membrane fouling was detected.     

Thermal decomposition and stability of fatty acid methyl esters in supercritical methanol

In recent years, non-catalytic supercritical processes for biodiesel production have been proposed as alternative environmentally friendly technologies. However, conditions of high temperature and pressure that occur while biodiesel is in supercritical fluid can cause fuel degradation, resulting in low yield. In this study, we performed the thermal decomposition of fatty acid methyl esters (FAMEs) in supercritical methanol at temperatures ranging from 325 °C to 420 °C and pressure of 23 MPa to investigate the degradation characteristics and thermal stability of biodiesel. The primary reactions we observed were isomerization, hydrogenation, and pyrolysis of FAMEs. The main pathway of degradation was deduced by analyzing the contents of degradation products. We found that if FAME has shorter chain length or is more saturated, it has higher thermal stability in supercritical methanol. All FAMEs remained stable at 325 °C or below. Based on these results, we recommend that transesterification reactions in supercritical methanol should be carried out below 325 °C (at 23 MPa) and 20 min, the temperature at which thermal decomposition of FAMEs begins to occur, to optimize high-yield biodiesel production.     

Thermal oxidation of oxynitride perovskites in different atmospheres

Oxynitride perovskites with bright and variable colour have the potential to be employed as non-toxic pigments, and to substitute colourants that contain harmful heavy metals. For this application it is extremely important to have a precise knowledge about the materials stability. The thermal stability of oxynitride perovskites in different atmospheres was measured by thermogravimetry in combination with mass spectroscopy (MS). The studied compounds, namely LaTiO₂N, SrNbO₂N and SrTaO₂N, were heated up to 1200 °C in argon–oxygen mixtures with varying oxygen contents. It was found that the thermal behaviour of the studied oxynitrides changes drastically with the oxygen concentration. When heated up in pure argon the oxynitrides transform to oxides containing transition metals of lower oxidation state and/or binary nitrides. For oxygen contents between 0.5% and 20% the samples were completely oxidised. The oxidation rate increases with oxygen content. MS analysis of the gaseous products (including N₂, NO and NO₂) reveals a complex reaction mechanism, which is strongly modified by the composition of the atmosphere.     

Thermal stability of polyurethanes

Summary The combined application of thermogravimetry reactiongas chromatography and gel-permeation chromatography permits to follow the heat degradation of polyurethane polymers in inert gas, air and water-saturated environment. The examinations give information on the rate of thermal degradation, the individual volatile degradation components, the critical points of the polymer chains and on the change of their molecular-weight distribution. Gas chromatographic examinations also permit the identification of the chain-extending components of different types of polyurethanes.     

Thermal stability of Ba1-xCaxF2 solid solutions

Ba_1-xCa_xF₂ samples with fluorite-type structure were prepared by high-energy ball milling. DSC measurements revealed exothermic decomposition at temperatures of about 450 °C. Decomposition was also observed by high temperature X-ray experiments. As a product, two phases with compositions close to pure BaF₂ and CaF₂ were obtained. The decomposition temperature observed by X-ray measurements is lower than in the DSC experiments. This might indicate that X-ray irradiation could foster the decomposition.     

Hyphenated techniques of thermal analysis for characterisation of soil humic substances

Our aim was to investigate the thermal behaviour of humic substances extracted from temperate and tropical soils by means of hyphenated techniques of thermal analysis (e. g. simultaneous thermal analysis DSC/TG coupled with mass spectrometry, MS, for the analysis of evolved gas, EGA) in order (i) to verify whether the chemical composition of isolated humic substances also reflected the differences in microbial parameters previously measured in related soil samples and (ii) to identify suitable indices of thermal stability. Our results show that the investigation of humic substances by thermal methods can provide information on soil organic matter dynamics. Differences in thermal behaviour between the two groups of soils were found. The indices of thermal stability here proposed, IR (index of thermal recalcitrance), and ID (index of thermal decomposability) clearly showed that in humic substances from tropical soils the thermally recalcitrant organic fraction dominated, whilst in temperate humic substances the opposite held. This agrees with previous results on the microbial dynamics and organic matter turnover of the respective soils and indicates that these indices of thermal stability could represent a useful tool in soil environmental quality investigations.     

Thermal stability and thermokinetics studies on silicone ceramer coatings: Part 1-inert atmosphere parameters

Two silicone ceramer coatings were synthesized and studied according to their thermal stability, using Fourier Transformation Infra-red (FTIR) spectroscopy. Differential thermal analysis was conducted on a liquid precursor to the coatings to study the phase transformations occurring in the material as the temperature was altered. The kinetics of the thermal degradation of coatings was analyzed; and the corresponding thermokinetics parameters were determined using Friedman's method, the Flynn-Wall-Ozawa method and the American standard test method E698. A thermogravimetric analysis was conducted on the liquid precursors as well as on the solidified coatings at various heating rates. The gaseous products that were obtained as a result of pyrolysis were analyzed employing an attached FTIR spectrometer.     

Thermal stability of aromatic polyesters prepared from diphenolic acid and its esters

The thermal performance of aromatic polyesters (poly(DPA-IPC), poly(MDP-IPC) and poly(EDP-IPC)) prepared from isophthaloyl chloride (IPC) with diphenolic acid (DPA) and its esters were studied with DSC and TG, and the decomposition mechanism of poly(DPA-IPC) were investigated using FTIR and integrated TG/FTIR analyses. As compared with ordinary aromatic polyesters, poly(DPA-IPC) has lower glass transition temperature (159 °C) and much lower thermal stability. It starts to decompose at about 210 °C and is characterized by two-stage thermal decomposition behavior, with active energies of decomposition of 206 kJ/mol and 389 kJ/mol, respectively. The analyses of the decomposition process and products indicate that the pendent carboxyl groups in poly(DPA-IPC) are responsible for its low thermal stability. Accordingly, a decomposition mechanism for the first stage is proposed. With this knowledge in mind, we capped the carboxyl groups in DPA with methyl and ethyl groups to prepare poly(MDP-IPC) and poly(EDP-IPC) from methyl diphenolate and ethyl dipenolate. As expected, these two polymers exhibit obviously improved thermal stability, with onset decomposition temperature of about 300 °C.     

Influence of the presence of surfactants and humic acid in waters on the indophenol-type reaction method for ammonium determination

This work has evaluated the influence of humic acid and/or surfactants in the quantification of ammonium in waters with the indophenol-type reaction method. Thymol has been employed with the colorimetric method for sample ammonium concentrations between 0.25 and 1 mg L⁻¹. In addition, SPE/diffuse reflectance method has been used for quantification of ammonium at low concentrations (between 0.025 and 0.25 mg L⁻¹). Matrix effect owing to humic substances were observed with the colorimetric method when the concentration was equal or higher than 25 mg L⁻¹. Lower concentrations of humic compounds produced matrix effects with the more sensitive SPE/diffuse reflectance method. Generalized H-point standard addition method (GHPSAM) was employed for evaluating the effect of humic acids and for eliminating the bias error produced by humic acids. Real water samples of different types were analyzed and accurate results for ammonium concentration were obtained with both procedures in presence or absence of humic acids. Cationic and anionic surfactants inhibited the derivatization reaction at percentages up to 0.001% and 0.5%, respectively, and non-ionic surfactants at percentages of 5.5%     

Thermal stability and dehydration of anapaite

The thermal behavior of anapaite, Ca₂Fe²⁺(PO₄)₂·4H₂O, has been studied by TG/DTG and DSC techniques, complemented by Fourier-transform IR spectroscopy. The anapaite sample, originating from Bellaver de Cerdena (Spain) was identified as such using X-ray diffraction and qualitative energy-dispersive analysis of X-rays. ⁵⁷Fe Mössbauer spectroscopy at various temperatures could not detect any Fe³⁺. It was found from thermal analyses and IR spectroscopy that two types of hydrogen–bonded water molecules exist in the structure. This feature is related to the distance between the hydrogen atom of a water molecule and the oxygen atom of a phosphate group, the distance between both oxygen atoms and the angle O(H₂O)–HO(PO₄). The dehydration process proceeds in two partially overlapping steps. The removal of the last two, strongly bonded water molecules is accompanied by the decomposition of the crystal structure. From TG curves, the activation energy was calculated for different intervals of dehydration reaction. For this purpose, five slow heating rates between 0.4 and 2°C/min were applied. The activation energy for the entire process was also obtained from DSC (223 kJ/mol) and found to be in reasonable agreement with the average of the various values from the TGA (233 kJ/mol). The heat of reaction for the complete dehydration was found to be 177 kJ/mol.     

Thermal stability of boron subphthalocyanines as a function of the axial and peripheral substitution

In this work, we have carried out the synthesis and thermogravimetric study of 10 different axially and peripherally substituted boron subphthalocyanines, in order to compare their thermal stabilities under non-oxidizing conditions. We demonstrate that, in general, these compounds enjoy a relatively high thermal stability, a property that is fundamental for future potential applications. The loss of the axial group is usually the first thermal degradation process to occur, and the temperature at which it takes place increases as a function of the nature of this substituent in the order: Br < OH ? OPh ∼ Cl. Peripheral substituents also have an influence on the thermal properties of subphthalocyanines, though their role is somewhat less notable.     

Thermal stability of poly(N-vinyl-2-pyrrolidone-co-methacrylic acid) copolymers in inert atmosphere

The thermal stability of poly(N-vinyl-2-pyrrolidone-co-methacrylic acid) copolymers was studied by thermogravimetry and infrared spectroscopy in inert atmosphere. The thermogravimetric curves suggested that the effective degradation of both systems occurred in the temperature range 350–500 °C with more than 60% mass loss. At this temperature, the activation energy was in the range 160–200 kJ mol⁻¹ (average values), suggesting that the degradation occurred by a random scission of the chain. The FTIR results indicated that the main volatile products of degradation are CO₂, CO and hydrocarbons (unsaturated structures) with low molecular weight. Pure PVP also showed the formation of NH₃ which was apparently suppressed in the copolymer by the formation of large amounts of CO₂ and CO. The results suggested that the thermal stability of the copolymers was essentially associated with the N-vinyl-2-pyrrolidone monomer, losing stability when the percentage of methacrylic acid in the copolymer system was increased.     

Thermal behavior of bismaleimide resin

The cure of a bismaleimide (BMI) neat resin modified with an aromatic diamine and a siloxane elastomer, has been studied by ¹³C solid state nuclear magnetic resonance. Two chemical reactions occur during the cure cycle; at a low temperature, Michael's reaction predominates, while at a high temperature the polymerization of the double bond maleimide creates the network. The degradation of this BMI material was characterized with isothermal and dynamic thermogravimetric analyses in air and in nitrogen. The BMI thermal stability is lower in nitrogen than in air. This behavior is an indication of oxygen participating in reactions at high temperatures. The activation energy (E_a) of thermal degradation was determined from isothermal data using an Arrhenius equation (In V vs. 1/T). The global E_a for the weight loss in air was found to be 91 kJ/mol. The nature and the evolution of the thermal degradation products were the combined analyzed by techniques of pyrolysis, gas chromatography and mass spectrometry. The major thermal decomposition products obtained in the temperature range of 300–700°C are identified as benzene, methyl formamide, aniline, toluene and isocyanate-derived products.     

Advanced mesoporous organosilica material containing microporous beta-cyclodextrins for the removal of humic acid from water

A new mesoporous organosilica material (beta-CD-Silica-4%) containing microporous beta-cyclodextrins (beta-CDs) has been prepared by the co-polymerization of a silylated beta-CD monomer with tetraethoxysilane in the presence of a structure-directing template, cetyltrimethylammonium bromide. Solid-state 13C and 29Si NMR studies provided evidence for the presence of covalently attached beta-CDs in the mesoporous material. Nitrogen adsorption experiments showed that beta-CD-Silica-4% material had a BET surface area of 460 m2/g and an average mesopore diameter of 2.52 nm. Small-angle powder X-ray diffraction pattern of beta-CD-Silica-4% material revealed the lack of highly ordered mesoporous structure. Adsorption experiments showed that beta-CD-Silica-4% material removed up to 99% of humic acid from an aqueous solution containing 50 ppm of humic acid at a solution-to-solid ratio of 100 ml/g.     

Prediction of humic acid content and respiration activity of biogenic waste by means of Fourier transform infrared (FTIR) spectra and partial least squares regression (PLS-R) models

Fourier transform infrared (FTIR) spectroscopy has been proven to be an appropriate analytical method for the qualitative assessment of compost stability. This study focuses on quantitative determination of two time-consuming parameters: humic acid (HA) contents and respiration activity. Reactivity/stability and humification were quantified by respiration activities (oxygen uptake) and humic acid contents. These features are also reflected by a specific infrared spectroscopic pattern. Based on this relationship partial least squares regression (PLS-R) models for the prediction of respiration activities and humic acid contents were calculated. Characteristic wavenumber regions that are assigned to the biological/chemical parameter were selected for multivariate data analysis. The coefficient of determination (R²) obtained for the humic acid prediction model from infrared spectra was 87% with a root mean square error of cross-validation (RMSECV) of 2.6% organic dry matter (ODM). The prediction model for respiration activity resulted in a R² of 94% and a RMSECV for oxygen uptake of 2.9 mg g⁻¹ dry matter (DM).     

Interaction of atrazine with Laurentian humic acid

Ultrafiltration fractionation and liquid chromatography have been applied to study the binding and hydrolysis of polar herbicide atrazine on a stoichiometrically well characterized Laurentian humic acid. The main advantage of this method over gas chromatography is the simultaneous determination of both free and bound atrazine, hydroxyatrazine and copper(II) ion with satisfactory accuracy and precision. Atrazine binding requires extensive carboxylate site protonation but the binding sites represent only a very small fraction of total carboxylate of humic acid. The results show that binding of atrazine is not competitive with binding of the hydrolysis product hydroxyatrazine, the binding capacity is reduced at higher ionic strength or by cation competition for carboxylate and the atrazine binding constant and free energy of binding can be fitted by a single value at all pH values. The differences between atrazine binding by fulvic acid and humic acid can be ascribed to the structure difference, one being a flexible linear polymer and the other a three-dimensional colloidal gel particle.     

Spectroscopic investigation on europium complexation with humic acid and its model compounds

Time resolved fluorescence spectroscopy (TRFS) of Eu(III) (an analogue of trivalent actinides) complexation with humic acid (HA) and its model compounds, namely phthalic acid (PA), mandelic acid (MA) and succinic acid (SA) has been carried out at varying concentration ratios of ligand to metal ion. The emission spectra were recorded in the range of 550–650 nm by exciting at an appropriate wavelength. The intensity of the 616 nm peak of Eu(III) was found to be sensitive to complexation. The ratio of the intensities of 616 and 592 nm peaks was used to determine the stability constants of Eu-phthalate, Eu-mandelate and Eu-succinate complexes. In the case of model compounds, the life-time was found to increase with increasing ligand to metal ratio (L/M) indicating the decrease in quenching of the fluorescence by coordinated water molecules with increasing complexation. On the other hand in the case of HA, the life-time was found to be constant at least up to L/M of 5, indicating the formation of outer sphere complex. Beyond L/M = 5 the life-time value was found to increase which can be attributed to the binding of the metal ion to the higher affinity sites in the HA macromolecule.     

Thermal stability of silver sulfathiazole-epoxy resin network

The aim of this study is to evaluate the thermal stability and thermal degradation behavior of an epoxy network based on bisphenol A modified with silver sulfathiazole and crosslinked with ethylenediamine. The sample was studied by thermogravimetric analysis coupled with differential scanning calorimetry over a range of temperature between 30 and 600 °C in N₂ atmosphere and using heating rates of 5, 10, 15 and 20 °C min⁻¹. The kinetic parameters of thermal degradation process were calculated. Fourier transforms infrared spectroscopy and mass spectroscopy coupled to thermogravimetry was used to identify the volatile products resulting from the degradation of the network. The study showed that the sample is stable up to temperatures exceeding 290 °C. The major degradation volatile products identified were: ammonia, water, carbon dioxide and compounds with aromatic structure such as bisphenol A and its degradation products.     

Dynamic conductivity measurements in humic and fulvic acid solutions

Conductivity changes of dilute aqueous humic and fulvic acids solutions were monitored after the addition of small quantities of Cu, Cd, Pb, and Zn. The solutions were stirred at a constant and reproducible rate, and measurements proceeded until stable conductivities were attained. The values were compared to KCl reference solutions treated in an identical manner, and the results showed that the humic materials significantly reduced metal ion mobilities. Pb and Cu were affected more than Cd and Zn, and especially Zn regained a significant portion of its mobility upon dispersal. Cu, in contrast, was strongly and irreversibly retarded in all cases. It was inferred that Zn underwent transient electrostatic interactions with humics at high local concentration, while associations between Cu and humics were rapid and permanent. Different humic materials showed these effects to different extents. Raising the temperature from 20 to 60 °C did not, as expected, lead to higher relative conductivities in metal-humate solutions, but showed enhanced retardation of the cations. It was noted that temperature induced aggregation (clouding) of humic solutions increased the effective sizes of the metal-humate complexes, reducing their mobilities in aqueous solution.     

Impact factors and thermodynamic characteristics of aquatic humic acid loaded onto kaolin

The adsorption of humic acid (HA) on kaolin particles was studied at various conditions of initial solution pH, ionic strength and solid-to-liquid ratio. The resulting affinity of interactions between humic acid and kaolin was attributed to the surface coordination of HA in ambient suspensions of mineral particles and the strong electrostatic force at low pH. Addition of inorganic salt can also influence the adsorption behavior by affecting the HA molecular structure, the clay particle zeta potential and so on. Equilibrium data were well fitted by the Freundlich model and implied the occurrence of multilayer adsorption in the process. In addition, the enthalpy dependent of system temperature was 79.17 kJ/mol, which proved that the mechanism of HA adsorption onto kaolin was comprehensive, including electrostatic attraction, ligand complexation and hydrogen bonding.