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.     

Stability of nanocrystalline cellulose in aqueous KCl solutions

The electrokinetic properties and aggregation stability of nanocrystalline cellulose sols in aqueous KCl solutions have been studied at pH 5.2. Cellulose nanoparticles have been prepared via controlled degradation in a mixture of phosphotungstic and acetic acids. At electrolyte concentrations of <0.01 mol/dm³, the system is stable to aggregation. The experimentally determined threshold of fast coagulation is 0.08 mol/dm³. It has been noted that the structural component of disjoining pressure must be taken into account when considering the aggregation stability of nanocrystalline cellulose sols in terms of the DLVO theory. It has been shown that the sizes of water boundary layers decrease with an increase in the KCl concentration. The effect of medium pH on the electrokinetic potentials and sizes of aggregates has been studied.     

Effect of a Weak Electrolyte on the Critical Micellar Concentration of Sodium Dodecyl Sulfate

The critical micellar concentration of sodium dodecyl sulfate is strongly altered bytris(hydroxy-methyl)methylammonium ions. The effect of buffer solutions containing this weak electrolyte as the counterion source has been studied using various concentrations of the acid–base system as well as modifying the pH. Results show that counterion concentrations ranging from 0 to 340 × 10⁻³M induce an appreciable diminution of the critical micellar concentration from 8 to 0.7 × 10⁻³M. The analysis of data suggests that the critical micellar concentration of sodium dodecyl sulfate depends on the concentration of weak electrolytes in a way very similar to that of strong electrolytes.     

Effect of mechanochemical modification on the surfactant and structural properties of humic and himatomelanic acids

The structural properties of humic and himatomelanic acids are studied by means of ¹Н NMR spectroscopy and differential thermal analysis after mechanochemical modification of peat. The relationship between the structural modification of humic and himatomelanic acids and their surfactant properties in aqueous solutions is established. It is shown that the critical micelle concentration of transformed himatomelanic acids is halved in comparison to the initial sample, while the adsorption equilibrium constant grows by 9 times.     

Solubilization of a water-insoluble dye in aqueous NaBr solutions of alkylpyridinium bromides and its relation to micellar size and shape

In order to investigate the effect of added salt on micelle size, shape, and structure the solubilization of Orange OT in aqueous NaBr solutions of decylpyridinium bromide (DePB), dodecylpyridinium bromide (DPB), tetradecylpyridinium bromide (TPB), and hexadecylpyridinium bromide (CPB) has been examined. The solubilization powers of DePB and DPB micelles increase with increasing NaBr concentration up to 2.86 and 3.07 mol dm^–3, respectively, but above these concentrations remain unaltered. This suggests that spherical micelles of DePB and DPB can have a maximum and constant size at NaBr concentrations higher than these threshold concentrations. On the other hand, the solubilization powers of TPB and CPB micelles increase in the whole range of NaBr concentration studied. The dependencies of the solubilization powers of their micelles on the counterion concentration change at 0.10 and 0.03 mol dm^–3 NaBr, respectively, as suggests that TPB and CPB micelles undergo the sphere–rod transition at those concentrations. Orange OT is a more suitable probe for detecting the presence of the maximum- and constant-size spherical micelle than Sudan Red B.     

Determination of ammonia in the air using piezoelectric resonance sensors coated with humic acids

Natural macromolecular compounds, humic acids (HA), have been used to form gas-sensitive layers of piezoelectric resonance gravimetric sensors. HA have been extracted from humus using alkaline extraction and purified by electrodialysis through ion-exchange membranes. Species have been deposited onto the electrode surface of piezoelectric quartz resonators by air-spraying their aqueous solutions, the layer thickness being 100–200 nm. In determining ammonia with the sensor obtained on the basis of purified HA species, the detection limit is ∼10 mg/m³. Gas-sensitive layers on the basis of humic acids are more stable as compared to the conventionally used gas-sensitive layers based on pyridoxine hydrochloride or ascorbic acid.     

Oxyhumolite influence on adsorption and desorption of phosphate on blast furnace slag in the process of two-stage selective adsorption of Cu(II) and phosphate

Two-stage adsorption was used for selective removal of Cu(II) and phosphate from aqueous solutions. In the first stage, adsorption of Cu(II) and phosphate on oxyhumolite (OX) was examined. The pseudo second-order equation was found to be the best fit for the kinetic adsorption data. The adsorption capacity of OX for Cu(II) and phosphate depends on the adsorption time, the equilibrium pH influences only the adsorption of Cu(II). The high adsorption efficiency (E = 95 %, pH 3.5, 0.5 g of the solid sorbent and 50 cm³ of the solution, c = 4 mmol dm^?3) of OX for Cu(II) is caused by the presence of humic acids (HA). In the second stage, blast furnace slag (BFS) and activated blast furnace slag (BFS-A) were used to remove phosphates. The presence of OX in the first stage positively influences the adsorption efficiency of sorbents in the second stage due to the soluble humic compounds and residues of humic acids (HA) which support the precipitation of Ca-phosphates on BFS and the ions exchange reactions on BFS-A. Adsorption equilibrium of phosphate on both slags at 298 K can be well described by the Langmuir isotherm equation. Desorption of Cu(II) from OX was around 70 %. The presence of OX in the first stage also influences the desorption of phosphate bound in the second stage. Desorption efficiency of both slags for phosphate was about 60 %.     

Association of sodium dodecyl sulfate in aqueous solutions according to chemical shifts in 1H NMR spectra

Proton chemical shifts of different atomic groups in sodium dodecyl sulfate (SDS) have been studied by ¹H NMR spectroscopy as functions of surfactant concentration in aqueous solutions. Three surfactant concentration ranges of the chemical shifts have been revealed. The first range corresponds to the premicellar solutions, the second one is in the vicinity of critical micelle concentration (CMC₁), and the third range corresponds to high surfactant concentrations, at which intermicellar interactions play a significant role. The parameters of SDS association (CMC₁ and CMC₂) determined based on the concentration dependences of the chemical shifts are in satisfactory agreement with the data available from the literature. The concept of critical dimerization concentration (CDC) has been introduced for the first concentration range. The values of CDC and dimerization constant K ₂ (210 × 60 dm³/mol) have been estimated within the framework of the two-state model.     

The lon-chromatographic behavior of arsenite,arsenate, methylarsonic acid and dimethylarsinic acid on the hamilton PRP-X100 anion-exchange column

The HPLC separation of arsenite, arsenate, methylarsonic acid and dimethylarsinic acid has been studied in the past but not in a systematic manner. The dependence of the retention times of these arsenic compounds on the pH of the mobile phase, on the concentration and the chemical composition of buffer solutions (phosphate, acetate, potassium hydrogen phthalate) and on the presence of sodium sulfate or nickel sulfate in the mobile phase was investigated using a Hamilton PRP-X100 anion-exchange column. With a flame atomic absorption detector and arsenic concentrations of at least 10 mg dm^?3 all investigated mobile phases will separate the four arsenic compounds at appropriate pH values in the range 4–8. The shortest analysis time (?3 min) was achieved with a 0.006 mol dm^?3 potassium hydrogen phthalate mobile phase at pH 4, the longest (?10 min) with 0.006 mol dm^?3 sodium sulfate at pH 5.9 at a flow rate of 1.5 cm³ min^?1. With a graphite furnace atomic absorption detector at the required, much lower, flow rate of ?0.2 cm³ min^?1 acceptable separations were achievable only with the pH 6 phosphate buffer (0.03 mol dm^?3) and the nickel sulfate solution (0.005 mol dm^?3) as the mobile phase. To become detectable approximately 100 ng arsenic from each arsenic compound (100 μl injection) must be chromatographed with the phosphate buffer, and approximately 10 ng with the nickel sulfate solution.     

Influence of tartrate ions on the coprecipitation of some trace metal inner complex compounds with bis(8-quinolyl)disulphide

The coprecipitation of Fe(III) quinoline-8-thiolate (QT) with bis (8-quinolyl) disulphide has been investigated with 1 mol × dm^–3 potassium tartrate and 0.001 mol × dm^–3 aqueous tartaric acid solutions in dependence on their pH. In return QTs of Cu(II), Cd(II) and Hg(II) have been coprecipitated from 0.2, 0.1, 0.01 and 0.001 mol × dm^–3 aqueous solutions of tartaric acid after adjusting the pH to 6–7. These tartaric acid concentrations relate to their initial concentrations before the coprecipitation. The presence of tartrate ions influences the recovery of the coprecipitated Fe(III), Cd(II) and Hg(II) QTs. A complete coprecipitation of the Cu(II) inner complex compound can be achieved from the aqueous solutions indicated.     

Adsorption interactions of humic acids with biocides

The chemical composition of humic acids from brown coal (Aldrich) was determined by element analysis, ¹³C NMR spectroscopy, and potentiometric titration. The adsorption ability of humic acids with different biocides (cyproconasol, propiconasol, tebuconasol, irgarol 1051, and DCOIT) was studied. The adsorption ability of a mixture of biocides in aqueous solutions was higher than that of the individual components. The limiting concentration of humic acids at which adsorption of biocides was maximum was determined. Adsorption constants were calculated by the Freundlich equation for each biocide in aqueous solution.     

Simultaneous reactive extraction separation of amino acids from water with D2EHPA in hollow fiber contactors

Reactive extraction separation of binary amino acids from water using a microporous hollow fiber has been studied, in which the acidic extractant di(2-ethylhexyl)phosphoric acid (D2EHPA) was selected as an active carrier dissolved in kerosene. l-Phenylalanine (Phe) was extracted from an aqueous solution through the shell side of module to the organic phase through the lumen of fiber in the extraction module, in which l-Phe was then back-extracted to stripping phase in stripping module. Experiments were conducted as a function of the initial feed concentration of equimolar Phe and l-aspartic acid (l-Asp) (5 mol/m³), feed pH (3–5), the carrier concentration (0.1–0.5 mol/dm³), and stripping acidity (0.1–2 mol/dm³). The effect of process variables on the separation factor of Phe/Asp and the possible transport resistances including aqueous-layer diffusion, membrane diffusion, organic-layer, and interfacial chemical reaction were quantitatively studied and discussed. The high separation factor (β) of Phe/Asp was obtained to be 18.5 at feed pH 5 and 2 mol/dm³ of strip solution (HCl). The extraction and stripping processes appear to rely on pH dependence of the distribution coefficient of amino acids in reactive extraction system. The separation factor (β) was enhanced in hollow fiber membrane (HFM) process compared with conventional solvent process, which was a result of the counter transport of hydrogen ions.     

The effect of monoethanolamine on conductivity and efficiency of electrodialysis of acid and salt solutions

Experimental studies of the specific conductivity (SC) are carried out for aqueous solutions of organic and inorganic acids and salts including those containing different amounts of monoethanolamine (MEA), which model the absorption solutions used in purification of gas mixtures from carbon dioxide and containing heatstable salts (HSS). It is shown that the addition of MEA to binary aqueous electrolyte solutions gives rise to changes in the SC: in the MEA concentration range from 0 to ～1.5 M, the SC of the resulting ternary solutions increases but decreases again with the further increase in MEA concentration. This behavior of SC is typical also of aqueous binary amine solutions. It is shown that in the presence of MEA, the quantitative removal of dissolved acids and salts proceeds faster with the simultaneous increase in the specific energy consumption by a factor of 7–9 (up to 85.7–93.6 kJ/dm³). It is assumed that the reason for the decrease in SC and the enhancement of energy consumption at electrodialysis of mixed solutions is the probable existence of monoethanolamine both as free solvated ions and neutral molecules and as self-assembled associated structures (ion pairs and more complex particles) which involve also the ions of salts dissolved in amine-containing solutions.     

Dynamic light scattering measurements of particle size development in aqueous humic materials

Dynamic light scattering (DLS) has been used to monitor changes in aggregate sizes of aqueous humic materials as a function of solution properties. Humic and fulvic acids were dissolved at relatively low concentrations (15-30 mg L(-1)) in solutions of different temperature, cation and ethanol content, and pH. The results could be explained in terms of intramolecular contraction and intermolecular aggregation of humic polymers. The former were prevalent in soil humic acids, and less so in aquatic humic acids and fulvic acids. Increasing the temperature of humic solutions generally led to an increase in particle sizes, which was ascribed to an effect akin to surfactant clouding. The addition of cations led to either contraction or expansion, depending on the charge and concentration of the ion, and the nature of the humic material. Reducing the pH initially caused contraction, followed by growth and precipitation in more highly acidic media.     

Steady-state γ-radiolysis of aqueous methyl ethyl ketone (2-butanone) under postulated nuclear reactor accident conditions

The steady-state γ-radiolysis of aqueous solutions containing 1×10⁻³ mol dm⁻³ methyl ethyl ketone (MEK) has been studied at a dose rate of 0.12 Gy s⁻¹, 25°C and an initial pH of 10. Experiments were conducted in air-, Ar- or N₂O-purged aqueous solutions, or in Ar-purged solutions with added tert-butanol. MEK, its radiolytic products, and the change in pH resulting from MEK decomposition were analysed as a function of time (or total absorbed dose). The main initial step for the radiolytic decomposition of MEK is the H abstraction from MEK by OH, produced by γ-radiolysis of water, to form MEK radical. In the absence of O, the main decay path of the MEK radical appears to be dimerization to , -dimethyl-2,5-hexanedione. In the presence of oxygen, the MEK radical reacts primarily with O to form the MEK peroxyl radical. This radical ultimately results in a series of progressively smaller oxidation products. The formation of organic acids, and eventually CO₂, reduces the pH of the solution. This paper presents the experimental data and proposes the MEK decay kinetics and mechanism.     

Kinetics of radioeuropium sorption on Gorleben sand from aqueous solutions and ground water

Kinetics of Eu sorption on sandy sediment from Gorleben, Germany, containing humic substances, was studied by radiotracer method in batch experiments at very low europium concentration (3.4^.10^-8 mol/l), with the aim to find kinetic parameters suitable for modeling Eu migration in bed of the sediment and to elucidate the mechanism of the sorption. Experiments were evaluated using a new simplified method taking into account simultaneous sorption/desorption of Eu on the walls of sorption vessel. Five kinetic functions were tested, of which that describing diffusion in inert surface layer of sorbent proved most suitable. The effects of pH (3.0-8.8) by addition of Aldrich humic acid (10 mg/l), addition of hydrogencarbonates (5^.10^-3 mol/l) and preequilibration of the sediment with solution or of Eu with solution were examined. From the results it has been concluded that the kinetics and mechanism of the sorption strongly depend on pH. At pH 4.8 Eu is sorbed mainly as humate complex from the solution of humic acid. The addition of humic acid accelerates the sorption. Carbonate complexes of Eu are the probable forms sorbed at pH 8.8. The presence of humic substances in the slightly alkaline solutions suppresses the rate of the sorption due to slow dissociation of Eu-humate complexes.     

Effect of pH and ionic strength on the interaction of humic acid with aluminium oxide

The effect of pH and neutral electrolyte on the interaction between humic acid/humate and γ-AlOOH (boehmite) was investigated. The quantitative characterization of surface charging for both partners was performed by means of potentiometric acid–base titration. The intrinsic equilibrium constants for surface charge formation were logK _a,1 ^int=6.7±0.2 and logK _a,2 ^int = 10.6±0.2 and the point of zero charge was 8.7±0.1 for aluminium oxide. The pH-dependent solubility and the speciation of dissolved aluminium was calculated (MINTEQA2). The fitted (FITEQL) pK values for dissociation of acidic groups of humic acid were pK ₁ = 3.7±0.1 and pK ₂ = 6.6±0.1 and the total acidity was 4.56 mmol g⁻¹. The pH range for the adsorption study was limited to between pH 5 and 10, where the amount of the aluminium species in the aqueous phase is negligible (less than 10⁻⁵ mol dm⁻³) and the complicating side equilibria can be neglected. Adsorption isotherms were determined at pH ∼ 5.5, ∼8.5 and ∼9.5, where the surface of adsorbent is positive, neutral and negative, respectively, and at 0.001, 0.1, 0.25 and 0.50 mol dm⁻³ NaNO₃. The isotherms are of the Langmuir type, except that measured at pH ∼ 5.5 in the presence of 0.25 and 0.5 mol dm⁻³ salt. The interaction between humic acid/humate and aluminium oxide is mainly a ligand-exchange reaction with humic macroions with changing conformation under the influence of the charged interface. With increasing ionic strength the surface complexation takes place with more and more compressed humic macroions. The contribution of Coulombic interaction of oppositely charged partners is significant at acidic pH. We suppose heterocoagulation of humic acid and aluminium oxide particles at pH ∼ 5.5 and higher salt content to explain the unusual increase in the apparent amount of humic acid adsorbed. Received: 20 July 1999 /Accepted in revised form: 20 October 1999     

Self‐assembly of N‐carboxyethylchitosan near the isoelectric point

For the first time the possibility to obtain nanostructures by self‐assembly of chitosan polyampholytic derivative was demonstrated. The self‐assembly of N‐carboxyethylchitosan (CECh) took place only near its isoelectric point (pH 5.0–5.6). Out of the pH range 5.0–5.6, CECh aqueous solutions behaved as real solutions. Dynamic light scattering and atomic force microscopy analyses revealed that spherically shaped or rod/worm‐like nanosized assemblies were formed depending on the polymer molar mass, pH value, and polymer concentration. CECh of two different molar masses was studied in concentrations ranging from 0.01 to 0.1 mg/mL. The structures from CECh of weight‐average molar mass (M_w ) 4.5 × 10³ g/mol were spherical regardless the pH and polymer concentration. In contrast, CECh of high molar mass (HMMCECh, M_w = 6.7 × 10⁵ g/mol) formed self‐assemblies with spherical shape only at pH 5.0 and 5.6. At pH 5.2 spherical nanoparticles were obtained only at polymer concentration 0.01 mg/mL. The mean hydrodynamic diameter (D_h) of the obtained nanoparticles was in the range from 30 to 980 nm. On increasing the concentration, aggregation of the nanoparticles appeared, and at HMMCECh concentration 0.1 mg/mL, rod/worm‐like structures were obtained. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6712–6721, 2008     

Mass-transfer in hollow-fiber modules for extraction and back-extraction of copper(II) with LIX64N carriers

The extraction of Cu²⁺ ions from sulfate solutions across a hollow-fiber membrane containing LIX64N carriers dissolved in kerosene has been studied, in which Cu(II) was then back-extracted to a stripping-phase containing HCl. Experiments were conducted as a function of the initial feed concentration of Cu²⁺ (1–10 mol/m³), feed pH (2–6), the carrier concentration (0.1–0.4 mol/dm³), and stripping acidity (0.4–4 mol/dm³). A mass-transfer model was developed to predict the extent of Cu²⁺ extraction from aqueous feed in hollow-fiber contactors. The calculated time profiles of Cu²⁺ concentrations were in reasonable agreement with the experimental data (average standard deviation 9% in both extraction and back-extraction modules). The rate-controlling step(s) of such dispersion-free extraction processes were identified. It was shown that the extraction was governed by combined interfacial reaction and aqueous diffusion under the ranges studied, whereas the back-extraction was limited by combined membrane diffusion and aqueous diffusion.     

Radiation chemistry of xylenol orange in aqueous solutions at different acidities

The radiolytic decolourization and peroxide formation have been studied in aqueous solutions of xylenol orange (XO) at different acidities. The G(-XO) increases from 0.78 at pH 11, to 3.70 at pH 3. The peroxide yield also increases from 1.19 at pH 11, to 3.34 in 0.025 mol dm^-3 H₂SO₄. In alkaline solutions only the OH^. decolourizes XO whereas in acidic solutions both the H and OH^. decolourize XO though G(-XO) due to H-atoms is less. The ionization of the phenolic group in XO influences the e^-_aq reaction with it. In alkaline solution, the oxidized and reduced XO formed by OH^. and e^-_aq reactions, respectively, react together regenerating original XO. Near 0.025 mol dm^-3 H₂SO₄, there is an abstraction of H-atom from XO by HO₂ whereas at other acidities, H₂O₂ is formed by disproportionation of peroxides. Reaction schemes have been given to explain the various radiolytic yields.