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Abstract  

In general, the amount of radiocesium sorbed by the five sorbents with 0.01 mol·dm–3 NaCl was in order zeolite > NiFeCN–SiO2 > montmorillonite > aerogel > silica gel. Addition of humic acid solution to the sorbents depressed the sorption of cesium by all sorbents, except for NiFeCN–SiO2 was not seen, with the greatest effect showing to the aerogel. The presence of humic acid resulted in an enhanced desorption of cesium from zeolite, NiFeCN–SiO2 and to a lesser extent from montmorillonite and silica gel. The order of cesium retention following desorption for both sorbent and sorbent/humic-acid mixtures was zeolit > NiFeCN–SiO2 > montmorillonite > silica gel. The presence of humic acid resulted in decreasing of distribution coefficient values for both sorption and desorption processes.

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Abstract  

The stability and solubility of UO2(OH)2 has been studied as a function of the humic acid concentration in 0.1M NaClO4, in the pH range from 4 to 7 under normal atmospheric conditions. The solid phase under investigation has been prepared by alkaline precipitation and characterized by TGA, ATR-FTIR, XRD, SEM and solubility measurements. According to the experimental data UO2(OH)2 is stable and remains the solubility limiting solid phase even in the presence of increased humic acid concentration in the solution. However, humic acid affects texture and particle size of the solid phase. Increasing humic acid concentration results in decreasing crystallite size of the UO2(OH)2 solid phase. Based on the solubility data, the logK sp(UO2(OH)2) has been evaluated to be −22.0±0.3 and the stability constant for the UO2(OH)HA(I) species has been estimated to be logβ 1101 = 15.3±0.5.

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Summary  

Conditional stability constants have been determined for U(IV) and U(VI) Boom Clay humic acid (BCHA) and Aldrich humic acid (AHA) complexes, under anaerobic and carbonate free conditions. The constants are needed for nuclear waste repository performance assessment purposes. The U(IV) constants were obtained by developing an approach based on the solubility product of amorphous U(OH)4. The U(VI) constants were obtained by applying the Schubert ion-exchange approach.

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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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Journal of Radioanalytical and Nuclear Chemistry
Authors: A. Paulenová, P. Rajec, J. Kandráč, G. Sasköiová, E. Tóthová, P. Bartoš, V. Švec, and R. Góra

Abstract  

Complexation equilibrium of metals by three humic acids of different origin with ultrafiltration method was investigated at pH 4 a 5 and ionic strength I = 0.1M NaClO4. Commercial (Aldrich) and two original humic acids (peat and soil, obtained by six step isolation process from the material from Trnava county, close to the NPP Jaslovské Bohunice) were used in this study. For the evaluation of the results, the model of metal ion charge neutralization upon humic acid functional group proposed by the Kim and Czerwinski was used. Complexation constants were calculated using the terms of this model (operational concentration, loading capacity). The values of log = 5.39±0.16 for yttrium, 6.15±0.16 for americium and 5.20±0.08 for lead were found. Correlation of free metal concentration and ratio of molar fraction of complexing functional groups confirms the validity of charge neutralisation model for metal and polyelectrolyte complexation study.

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Abstract  

Over the ages, the deposits of dead vegetation buried by rock and mudflows, compacted and compressed out all of the moisture; it slowly carbonized and became coal. Humic acids are natural organic acids — brown coloured biological macromolecules, formed in coal by biochemical changes (decomposition, pyrolysis) of lignocellulosic matter. From lignite coal bed, the humates were extracted in alkaline medium and isolated from the residual fraction. Humic acids were obtained by treating humantes’ solutions with HCl. Thermal analysis (TG, DTG, DTA and DSC) was used in order to establish the decomposition and thermal effects of lignite, humates, humic acids and residual matter extracted from Rovinari mines in Romania. A non-isothermal linear temperature regime was imposed to reveal all decomposition steps.

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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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Abstract  

The formation of Sr-humate precipitate was studied colourimetrically and radiometrically at different pH's. It was found that, increasing of the Sr2+ concentration or the pH value of the solution increases the precipitated complex. The competition effect of Mg2+, Ca2+, Ni2+, and Ba2+ on the complexation of Sr2+ by humic acid indicated that both Mg2+ and Ni2+ substitute Sr2+ in humate complex, while Ba2+ has a very little effect. In case of Ca2+, increasing of its concentration enhances the precipitation of Sr2+ with humic acid.

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Abstract  

Humic acids (HA) can influence the speciation of metal ions, e.g., actinide ions, and thus their migration in the environment. Therefore, knowledge of the impact of HA on the actinide migration is required to assess their transport in natural systems. However, due to the complex and heterogeneous nature of HA, there are a lot of difficulties in the thermodynamic description of their geochemical interaction behavior. A more basic understanding of the interaction processes of HA can be obtained by investigations applying HA model substances with more specific and tailored properties. This work gives a review of selected types of HA model substances (HA-alike melanoidins, synthetic HA with pronounced redox functionality, modified HA with blocked phenolic/acidic OH groups, synthetic humic substance-clay-associates), their synthesis, isotopic labeling, and characterization in comparison to isolated natural HA. Examples for their application in various geochemical studies, such as complexation, redox, sorption and migration studies with uranium as representative for actinides are presented.

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Abstract  

Complexation of Am(III) with humic acid was studied at various pHs in 0.1M NaClO4. The stability constants of the Am(III)—humate complexes were determined by a cation-exchange method. The values of log 1 and log 2 increased slightly with increases of pH from 4 to 6 and were found to be 6.9 and 11.6, respectively, at a pH of 5. Markedly larger values than these were obtained by a solvent extraction method. This discrepancy was also revealed by summarizing data from several literature sources. It is very likely that this can be ascribed to decreases in either humic acid and/or the extractant from the extraction system due to humate interactions at the aqueous-organic interface.

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