Thermal stability of solid and aqueous solutions of humic acid

The effects of temperature on the stability of a soil humic acid were studied in the present work. Solid samples of Gohy-573 humic acid (HA) and dissolved ones in aqueous solution (pH 6.0, 0.1 mol L⁻¹ NaClO₄) were investigated in order to understand the impact of temperature on the chemical properties of the material. The methods applied to solid samples in the present investigation were thermogravimetric analysis (TGA), temperature-programmed desorption coupled with mass spectrometry (TPD-MS), and in situ diffuse reflectance infrared Fourier transformed spectroscopy (in situ DRIFTS). Humic acid samples were studied in the 25-800 °C range, with focus on thermal/chemical processes up to 250 °C. The reversibility of the changes observed was investigated by cyclic changes to specified temperature ranges (40-110 °C). All measurements were conducted under inert-gas atmosphere in order to avoid samples combustion at increased temperatures. Aqueous solutions were analyzed by UV-vis absorption spectroscopy after storage at temperatures up to 95 °C, and storage times up to 1 week. For temperatures below 100 °C experiments on solid and aqueous samples have shown results which were consistent to each other. The amount of water desorbed is temperature dependent and up to 70 °C this process was totally reversible. Above 70 °C an irreversible loss of water was also observed, which according to UV-vis spectroscopy corresponds to water produced by condensation leading to more condensed polyaromatic structures. The water released up to 110 °C was about 7 wt% of the total mass of the dried humic acid, where less than 50% corresponded to reversibly adsorbed water. At higher temperatures (>110 °C), gradual decomposition resulting in the formation of carbon dioxide (110-240 °C), and carbon monoxide (140-240 °C) takes place. Hence, thermal treatment of Gohy-573 humic acid above 70 °C results in irreversible structural changes, that could affect chemical properties (e.g., complex formation) of the material.     

Simulation of the radiation-chemical transformations of acetic acid in aqueous solutions

A general model for the radiolysis of acetic acid and its aqueous solutions is proposed. The model adequately describes experimental data on the degradation of the acid and the formation of gases (H₂, CO₂, and CH₄) in aqueous solutions at various pH values.     

Radiation-chemical transformations of coumarins in organic solvents

A spectrophotometric investigation into the radiation-chemical transformations of coumarins with different structures was performed. It was shown that the introduction of hydroxy, nitroso, and methyl groups into the coumarin molecule has a substantial effect on its radiation-chemical transformations. The solvated electron was supposed to be a reactive species responsible, for the most part, for coumarin degradation.     

Adsorption of atrazine from aqueous electrolyte solutions on humic acid and silica

The adsorption of, the still widely used, herbicide atrazine on model soil components, such as humic acid and humic acid-silica gel mixtures, was investigated in a series of batch experiments, under different experimental conditions (ionic strength, temperature, and pH). The investigation aimed at obtaining an estimate of the contribution of each of the soil components on the adsorption of atrazine from aqueous solutions. The kinetics of atrazine adsorption on humic acid showed two steps: a fast step, of a few hours duration, and a second slow step, which continued for weeks. The kinetics of adsorption data gave a satisfactory fit to the Elovich equation. Τhe adsorption of atrazine on the test substrates was found to be reversible in all cases. The atrazine uptake data on the test substrates were fitted best with the Freundlich adsorption isotherm. The ionic strength of the atrazine aqueous solutions did affect the amount of the atrazine adsorbed on the test substrates, suggesting that electrostatic forces between atrazine molecules and soil play a significant role in the adsorption process. An increase of temperature resulted in a decrease of atrazine adsorption on humic acid at low atrazine equilibrium concentrations. However, for higher levels of equilibrium concentrations (≥3 mg/L) the amount of atrazine adsorbed onto the test substrate increased as temperature increased. The calculated isosteric enthalpies of adsorption ranged between slightly exothermic at low atrazine uptake and slightly endothermic at high atrazine uptake, all values being in the range of physisorption.     

Conformational changes in humic acids in aqueous solutions

Conformational changes in humic acids in two different aqueous solutions (NaCl and NaOH) are studied by means of high resolution ultrasound spectrometry. The method is based on the measurement of parameters of ultrasonic waves propagating through the sample. The attenuation describes the decay of the amplitude of the ultrasonic wave with the distance travelled. The velocity is the speed of this wave and is related to the wavelength and the frequency of oscillation of the deformation. It is determined by the density and elasticity of the sample, which is strongly influenced by the molecular arrangement. The minimal velocity of ultrasound was observed at 1 g dm^?3 for lignitic humic acids and at 0.5 g dm^?3 for IHSS Leonardite standard. The values of compressibility as computed are almost constant up to humic acids?? content corresponding to the minimum velocity of ultrasound and then decrease with the increase in concentration. This shows that the organisation of particles in diluted and concentrated humic acids sols is different. The decrease in compressibility points to the formation of a more rigid structure, which could lead to the decrease in humic acids?? binding ability. It was confirmed that the method employed was very sensitive and could be utilised as an indicator of conformational changes in humic acids in solutions with varying concentrations.     

Peculiarities of phase transformations in aqueous isopropanol solutions

Peculiarities of phase transformations in aqueous isopropanol solutions are studied in the range from room temperature to −15°C. It is found that solution cooling results in the formation of macroaggregates with regular geometrical shapes, which are dispersed in the liquid phase and directedly move in the field of a temperature gradient. In aqueous isopropanol solutions with concentrations of 10–30 vol %, the aggregates are formed at temperatures of −10 to −15°C. The average aggregate size is several millimeters. Under the action of a temperature gradient, the aggregates move to the region of higher temperatures. At temperature gradients of 1–2 K/cm, the aggregate velocity is 1–2 mm/s. The characters of the motion and interaction of aggregates are very sensitive to the temperature distribution in a solution. After the aggregate motion ceases, crystallization of the liquid phase that initially is outside of the aggregates is observed.     

Equilibrium transformations of meso-methylated thiacarbocyanine in aqueous solutions

Equilibrium processes of dimerization and H*- and J-aggregation of 3,3′-di-(γ-sulfopropyl)-5,5′-dichloro-9-methylthiacarbocyanine (TCC) in aqueous solutions have been investigated, and the thermodynamic parameters of the processes have been calculated. At least seven equilibrium forms of TCC have been found in aqueous solution for the first time. The absorption bands of the dye have been attributed to various forms (M, D, H*, and J) with allowance for existence of stereoisomers. The aggregation numbers for J- and H*-aggregates have calculated from the dependence of the absorption spectra on the dye concentration in solutions. It has been shown that the TCC aggregates consisting of a large number of molecules are formed by the “block” mechanism via intermediate steps of the formation of dimers, J- or H*-tetramers, octamers, etc.     

Thermochemical transformations of hydrocarbonate ions in aqueous solutions

The influence of heat treatment on the reactivity index k ₁, pH value, and content of hydrocarbonate ions HCO₃⁻ in an aqueous solution was examined. The factors responsible for enhanced reactivity of thermally activated water were discussed.     

Association and colloid-chemical properties of humic acids in aqueous solutions

Colloid-chemical properties of humic acids in aqueous solutions have been studied as functions of their concentration, medium pH, and counterion nature. It has been established that, at pH < 4, Na-form humic acids exist in aqueous solutions as 30–120-nm aggregates. At pH > 4, the state of humic acids depends on the solution concentration. At low concentrations, they exist as individual molecules; as the concentration is increased, they associate to form supramolecular structures at 5 mg/dm³ and micelles at 8 g/dm³. An increase in the counterion charge has been shown to decrease the critical concentration of supramolecular structuring. Supramolecular structures have increased adsorbability on clay minerals and promote the manifestation of solubilizing properties of humic acids in solutions even at a concentration of 5 mg/dm³, which is three orders of magnitude lower than the critical micelle concentration (8 g/dm³).     

Size exclusion (radio) chromatography of aqueous humic acid solutions with cesium and strontium

The high-performance size-exclusion chromatography (HPSEC) and radiochromatography (HPSERC) was used for the identification of radiocesium and radiostrontium interaction with humic acid. It was found that the behavior of humic acid on size-exclusion chromatography is sensitive to the salt concentration and pH of the mobile phase. At lower ionic strength and in acidic region of pH, the Aldrich humic acid exhibited three main fraction within the ranges >760 kDa, 25–100 kDa and <5 kDa. Radiocesium was found in the low-molecular fractions (<1 kDa) of humic acids but radiostrontium interacts preferably with the fractions of humic acid of molecular weight within the range 2–5 kDa.     

Radiation-chemical processes in methanolic solutions of rhodium acetylacetonates

The processes of radiation decomposition of Rh/acac/₃ and Rh/acac//CO/₂ in liquid and frozen methanol-water systems have been studied. The rate constants of electron scavenging by these complexes have been measured using the pulse radiolysis and the yields of the decomposition of the complexes in liquids have been determined as the effect of -radiolysis. In frozen glassy systems the efficiency of electron scavenging by Rh/acac/₃ has been measured by both optical and ESR methods. In the last method the yields of Rh/II/ complex occurred as the transient product have also been determined.     

Photooxidation of phenol in aqueous nanodispersion of humic acid

Photooxidation of phenol sensitized by Aldrich humic acid (AHA) has been studied in an aqueous solution at neutral and basic pH. Solutions containing phenol and AHA of various concentrations were irradiated with monochromatic light at 253.7 nm or with polychromatic light within the wavelength range of 310–420 nm. The quantum yields of phenol photodegradation under these conditions were determined. At the wavelength of 253.7 nm direct degradation of phenol was much more effective than that sensitized by AHA. With polychromatic light the photooxidation was found to be strongly dependent on pH of aqueous solution and independent on AHA concentration.     

Radiation-induced transformations of threonine and its derivatives in aqueous solutions

It was found that the presence of the α,β-aminoalcohol group in the molecules of the hydroxyl-containing amino acid threonine and its derivatives is responsible for their ability to undergo free-radical degradation with C-C bond cleavage. This degradation occurs by the fragmentation of the nitrogen- and carbon-centered radicals of the initial substances.     

Adsorption of humic acid from aqueous solutions on crosslinked chitosan-epichlorohydrin beads: kinetics and isotherm studies

The adsorption of humic acid on crosslinked chitosan-epichlorohydrin (chitosan-ECH) beads was investigated. Chitosan-ECH beads were characterized by Fourier transform infrared spectroscopy (FTIR), surface area and pore size analyses, and scanning electron microscopy (SEM). Batch adsorption experiments were carried out and optimum humic acid adsorption on chitosan-ECH beads occurred at pH 6.0, agitation rate of 300 rpm and contact time of 50 min. Adsorption equilibrium isotherms were analyzed by Langmuir and Freundlich models. Freundlich model was found to show the best fit for experimental data while the maximum adsorption capacity determined from Langmuir model was 44.84 mg g(-1). The adsorption of humic acid on chitosan-ECH beads was best described with pseudo-first-order kinetic model. For desorption study, more than 60% of humic acid could be desorbed from the adsorbent using 1.0M HCl for 180 min.     

Solubility of monazite chemical components in humic acid solutions

The α-factor is a measure of the epithermal neutron flux deviation from the ideal distribution 1/E, where E is neutron energy. It defends on the position of the irradiation channel in reactor. A determination method of the α-factors in irradiation channels of Dalat reactor is presented by fitting the epithermal neutron spectrum obtained from the calculation using MCNP code. The fitting α-values were compared to those by experiment used the “Cd-ratio” method with monitors ¹⁹⁷Au–⁹⁴Zr and ¹⁹⁷Au–⁶⁴Zn. It shows that the α-values calculated from neutron spectra agree well with experimental ones. The difference between both data is about 6%.     

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.     

Diffusion in polyacrylic acid aqueous solutions

The four diffusion coefficients for the ternary system polyacrylic acid (mol. weight 5000)-polyacrylic acid (mol. weight 115000)-water have been measured at 25°C and at one average polyelectrolytes concentration. The experimental values of main and cross terms have been briefly discussed. The large cross term D ₁₂ in the system with water as solvent shows that, contrary to intuition, different molecular weight species do interact with each other.