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Journal of Thermal Analysis and Calorimetry
Authors: J. Kučerík, H. Čechlovská, P. Bursáková, and M. Pekař

Abstract  

The thermodynamic stability of lignite humic acids (sodium salt) aggregates was studied by high resolution ultrasonic spectroscopy within the temperature interval from 5 to 90°C. The changes in differential ultrasonic velocity (U12) showed strong differences among humic solutions within the concentration range from 0.005 to 10 g L−1. Measurement revealed several transitions which were attributed to the weakening of humic secondary structure. Concentration around 1 g L−1 seemed to be a limit under which the change of the prevalence and importance of hydration occurred. Above this concentration the difference in U12 decreased following the temperature increase which was explained as a dominance of hydrophilic hydration. In contrast, below this concentration, the temperature dependence of U12 resulted in increasing tendency which was attributed to the prevalence of hydrophobic hydration, i.e. uncovering of apolar groups towards surrounding water. Additional experiments in which the humic sample was modified by hydrochloric acid resulted in a slight structural stabilization which lead to the conclusion that humic micelle-like subaggregates form an open-layer assemblies easily accessible for interaction with an extraneous molecule. That was partly verified by addition of propionic acid which brought about even larger reconformation of humic aggregates and exhibition of polar groups towards hydration water. The reversible changes in humate solutions induced by elevated temperatures provided the evidence about the existence of significant physical interactions among humic molecules resulting in formation of various kinds of aggregates. The nature of aggregates, mainly the stability and conformation, strongly depends on the concentration. Evidently, the changes observed in this work cannot be simply explained as expansions or conformational changes of macromolecular coils.

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Abstract  

The dried blends containing sodium and ammonium salts of lignite humic acids (humates, 0.5–10% w/w) in polyvinyl alcohol (PVA) were exposed to high dosage of γ-irradiation in the range of doses 127–806 kGy. Resulted products were then tested for their stability using thermogravimetrical analysis. As a reference the non-treated blends were used since the pure PVA exposed to γ-irradiation very quickly lost its stability and resulting consistence did not allow the stability tests. Stabilities showed a strong concentration and counterion dependency. While sodium counterion caused mostly destabilization with increasing dose, the ammonium counterion acted in an opposite way. The tests carried out in a moisturizing container revealed the changes in water absorbing capacity of irradiated samples and allowed partial explanation of humate stabilizing effect. Generally, at lower concentration of a humate the increase was observed with an increase in the γ-irradiation dose and vice versa. The results confirmed the antioxidant and stabilizing effect of humic acids added to some synthetic polymers and their applicability in materials exposed to γ-irradiation.

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. Efficient use of fertilizers by crops with humic acid addition in soils. Dr. N. R. Dhar Memorial Lecture Deepa, M. (2001): Effect of lignite humic acid on soil microorganisms . M.Sc. Thesis

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Abstract  

Humic acids represent a complicated mixture of miscellaneous molecules formed as a product of mostly microbial degradation of dead plant tissues and animal bodies. In this work, lignite humic acids were enriched by model compounds and the model-free method suggested by Šimon was used to evaluate their stability over the whole range of conversions during the first thermooxidative degradation step. The kinetic parameters obtained were used to predict the stability at 20 and 180�C, respectively, which served for the recognition of processes induced by heat and those naturally occurring at lower temperatures. Comparison of the conversion times brought a partial insight into the kinetics and consequently into the role of individual compounds in the thermooxidative degradation/stability of the secondary structure of humic acids. It has been demonstrated that aromatic compounds added to humic acids, except pyridine, increased stability of humic acids and intermediate chars. The same conclusion can be drawn for acetic and palmitic acids. Addition of glucose or ethanol decreased the overall humic stability; however, the char of the former showed the highest stability after 40% of degradation.

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Abstract  

Simultaneous DTA/TG technique has been used to study the influence of various model compounds (aromatics, organic acids, alkanes, ketone, heterocyclic and sterole) on the thermo-oxidative behavior of lignite humic acids. As a measure of stability the shift of the onset temperature of the exothermic degradation peak has been used. Further, the ratio of mass loss recorded in the high and low temperature ranges (thermogravimetric index) was used to evaluate the role of added compounds on the recombination reactions occurring during the thermooxidative degradation of humic acids. It has been demonstrated that most of added compounds play a role during those processes at relatively low concentrations (1% mass/mass) and affect the humic acid stability as well as the value of thermogravimetric index (i.e. the degree of the apparent aromaticity). It has been clearly shown, that the latter parameter reflects more the ‘qualitative’ than the ‘quantitative’ relationship between biodegradable humified parts in the extracted pool of organic matter.

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humic acids on urease . Eur J Soil Biol 44 : 166 – 171 . 10. Todd , MJ , Hausinger , RP 1989 Competitive inhibitors of Klebsiella aerogenes urease. Mechanisms of interaction with the

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Journal of Thermal Analysis and Calorimetry
Authors: Jiří Kučerík, Petra Bursáková, Alena Průšová, Lucie Grebíková, and Gabriele Ellen Schaumann

, Bursáková , P , Pekař , M 2009 The thermodynamic stability and molecular feature of lignite humic acids aggregates studied by high resolution ultrasonic spectroscopy . J Therm Anal Cal 96 : 637 – 643 10.1007/s10973

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