Martina Klučáková

Composition, particle size, charge, and colloidal stability of pH-fractionated humic acids

PurposeThe purpose of this study was to divide humic acids into several fractions by means of their dissolving in buffers with different pH values and to characterize obtained humic fractions with respect to their composition, structure, particle size, and charge. Relationships between determined characteristics of fractionated humic acids and pH values of used buffer solutions and the method of fractionation was investigated.Materials and methodsHumic acids were fractionated by means of two different methods: the subsequent dissolution in buffers adjusted to different pH and the sequential dissolution in buffers with increasing pH values. Composition, structure, and properties of the obtained humic fractions were studied using elemental analysis, FT-IR spectroscopy, UV/VIS spectroscopy, and light scattering methods.Results and discussionAs expected, fractions obtained by subsequent dissolution were more heterogeneous than those prepared by sequential extraction. Fractions obtained at lower pH values contained higher amounts of aromatic and carboxylic groups, while those extracted at higher pH values were rich in aliphatic and/or peptide groups. Fractions extracted close to neutral pH region had some specific properties. Functional groups dissociated in a range of pH values depending on the chemical structure of the molecules. Weaker carboxylic groups could dissociate in less acidic solutions, and more aromatic fractions could be dissolved. Conformational changes and deaggregation process differed with the fractionation procedure and concentration of studied solutions. A bimodal distribution of particle sizes and higher values of polydispersity were obtained for some for less concentrated solutions of humic fractions.ConclusionsObtained humic fractions behave as anionic heterogeneous ligands with many carboxylic and phenolic groups of different strength, and present polyfunctional and polyelectrostatic effects due to different functional sites and net charged groups. Two main processes can affect their properties and behavior in aqueous solutions: dissociation of acidic functional groups and breaking of humic aggregates into smaller molecular associations or humic molecules. An important parameter affecting the spatial arrangement of obtained humic fractions is their concentration.

On the role of humic acids’ carboxyl groups in the binding of charged organic compounds

Interactions of humic acids (HAs) with two cationic dyes (methylene blue and rhodamine 6G) were studied using a unique combination of diffusion and partitioning studies in HAs, containing hydrogels and batch sorption experiments. In order to investigate the involvement of carboxyl groups of HAs in these interactions, all experiments were performed for both, the original lignite HAs and HAs with selectively methylated carboxyls. The results of the diffusion experiments confirm that the interactions between the solute and humic substances have a strong impact on the rate of diffusion process. Surprisingly, the effect is almost equally approved for original and methylated HAs. On the other hand, the results of batch sorption experiments show strong improvement of the sorption capacity (methylated HAs), which is explained by changed morphology of alkylated HAs. The comparison of the results of diffusion and adsorption experiments shows that the diffusion experiments simulate the transport of solutes in natural humics containing environment more reasonably.

Characterization of pH-fractionated humic acids with respect to their dissociation behaviour

Humic acids were divided into several fractions using buffer solutions as extraction agents with different pH values. Two methods of fractionation were used. The first one was subsequent dissolution of bulk humic acids in buffers adjusted to different pH. The second one was sequential dissolution in buffers with increasing pH values. Experimental data were compared with hypothesis of partial solubility of humic acids in aqueous solutions. Behaviour of humic fractions obtained by sequential dissolution, original bulk sample and residual fractions obtained by subsequent dissolution at pH 10 and 12 agrees with the hypothesis. Results demonstrated that regardless the common mechanism, solubility and dissociation degree of various humic fractions may be very different and can be estimated using parameters of the model based on the proposed mechanism. Presented results suggest that dissolving of solid humic acids in water environment is more complex than conventional solubility behaviour of sparingly soluble solids.

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.

The Role of Concentration and Solvent Character in the Molecular Organization of Humic Acids

The molecular organization of humic acids in different aqueous solutions was studied over a wide concentration range (0.01–10 g·dm−3). Solutions of humic acids were prepared in three different media: NaOH, NaCl, and NaOH neutralized by HCl after dissolution of the humic sample. Potentiometry, conductometry, densitometry, and high resolution ultrasound spectrometry were used in order to investigate conformational changes in the humic systems. The molecular organization of humic acids in the studied systems could be divided into three concentration ranges. The rearrangements were observed at concentrations of ~0.02 g·dm−3 and ~1 g·dm−3. The first “switch-over point” was connected with changes in the hydration shells of humic particles resulting in changes in their elasticity. The compressibility of water in the hydration shells is less than the compressibility of bulk water. The transfer of hydration water into bulk water increased the total compressibility of the solution, reducing the ultrasonic velocity. The aggregation of humic particles and the formation of rigid structures in systems with concentrations higher than 1 g·dm−3 was detected.

Comparison of thermal and chemical stability of Cu–humic complexes

PurposeThe purpose of this study was to investigate relationships between chemical and thermal stabilities of Cu–humic complexes. The study of the chemical stability was based on pedological methods used for the determination of the bond strength of metal ions in soils by chemical leaching agents. The samples with various contents of the Cu(II) ions and their bond strength were put to the thermal analysis in order to correlate their thermo-oxidative behavior with their stability determined by leaching.Materials and methodsThe humic acid was extracted from the South-Moravian lignite by standard alkaline extraction. The humic sample was used in two different forms: as the solid powder and as the hydrogel prepared by the acidic precipitation of humate. Six various concentrations of copper(II) solutions were used for the complexation of the humic powder and the hydrogel, in order to study the influence of their initial concentration on both the determined stabilities of the prepared complexes. Their chemical stability was assessed in terms of the Cu(II) ions release from the humic acid structure into two different extraction agents (MgCl2 and HCl solutions). Their thermo-oxidative behavior was investigated employing the thermogravimetry.Results and discussionThe complexation capacity of the humic hydrogel was higher in comparison with the humic powder. The amounts extractable from the Cu–humic complexes by the used leaching agents are higher for the humic powder, which shows on the lower chemical stability. The thermal degradation of the prepared complexes proceeds in several steps and this character remains also after the removal of the mobile and the ion-exchangeable fractions by the MgCl2. The elimination of these fractions as well as the extraction of the strongly bound Cu(II) ions shift the thermal degradation to higher temperatures. The incombustible residue increases with the Cu(II) content in the complexes except for the samples extracted by the HCl.ConclusionsThe form of humic sample used for the preparation of the Cu–humic complexes influences both the chemical stability and the thermal one. The main reason is probably a better accessibility of the functional groups in the humic gel, which enables forming stronger binding copper(II) ions. The results showed that the thermal and chemical stabilities are closely related, which corresponds with the shift of the thermal degradation to higher temperatures after removing the less stable fractions from the humic complexes.

Comparative Study of Binding Behaviour of Cu(II) with Humic Acid and Simple Organic Compounds by Ultrasound Spectrometry

Several simple organic (hydroxy)acids as models of principal functional moieties of humic acids and the copper ion as a model ion were used to investigate the metal-humic interactions. The quantum chemical calculations have been performed showing differences in the interaction strength and stability of the metal-acid complexes formed by different aromatic mono- and polycarboxylic acids and their hydroxyl-containing analogues. In this study, new results from the high resolution ultrasound spectroscopy will be presented. The ultrasound propagation velocity, which is sensitive to the intermolecular interactions, has been measured during titration of the models by solution of the copper(II) salt. The dependence of ultrasound velocity on added amount of the copper(II) ions enabled to follow the metal interactions with the individual functional groups and to elucidate the effect of structure of complexing molecule on its binding with the metal. Changes in the slope of the dependence were used to find the saturation of binding and to follow changes of hydration of interacting species. The differences observed for the individual models show that there are active centers not only with various strength and stability of the formed complexes but also with their various rigidity and ability of conformational changes.

Complexation of Copper(II) with Humic Acids Studied by Ultrasound Spectrometry

Copper biogeochemistry is controlled by bonding to natural organic matter. The soluble forms of bonded copper can be more biologically active due to the higher migration in environmental systems and instability of some copper-humic complexes. In this work, the interactions of copper(II) ions with humic acids are studied by means of high-resolution ultrasound spectrometry. It was found that the stoichiometry of the formed complexes is strongly influenced by the organization of humic acid in solution and by the final pH value in equilibrium. Although the ratio between the added copper and humic acids in all used concentrations was constant and the initial pH value was neutral, we observed significant differences between the individual systems. The highest binding ability was determined for a humic content of 0.5 g·dm−3. More diluted and more concentrated systems were able to bind lower amounts of copper. The implemented method is very sensitive and can be utilized not only for monitoring copper binding but also as an indicator of conformational changes of humic acid in solutions with varying concentration.

Study of interactions between hyaluronan and cationic surfactants by means of calorimetry, turbidimetry, potentiometry and conductometry

The thermodynamics of the micelle formation of the cationic surfactants tetradecyltrimethylammonium bromide (TTAB) and cetyltrimethylammonium bromide (CTAB) with and without the addition of hyaluronan of two molecular weights was studied in aqueous solution by titration calorimetry. Macroscopic phase separation, which was detected by calorimetry and also by conductometry, occurs when charges on the surfactant and hyaluronan are balanced. In contrast, turbidimetry and potentiometry showed hyaluronan-surfactant interactions at very low surfactant concentrations. The observed differences between systems prepared with CTAB and TTAB indicate that besides the electrostatic interactions, which probably predominate, hydrophobic effects also play a significant role in hyaluronan interactions with cationic surfactants.

Fluorescence Spectroscopy Study of Hyaluronan–Phospholipid Interactions

Abstract Capability of phospholipids with positive charge to form complexes with hyaluronan in aqueous solutions, in a similar way as traditional cationic surfactants, was investigated by fluorescence probes. DPPC and lecithin aggregate in aqueous solution to form micelle-like structures capable to solubilize hydrophobic molecules. Changes in aggregation behavior after adding hyaluronan were observed only in the case of lecithin. Further, nonionic biocompatible surfactant was used as additional dispersion environment in phospholipid–hyaluronan system with phospholipid molecules acting as a physical linker bonding micelles and biopolymer.