Authors:R. Ikan, P. Ioselis, Y. Rubinsztain, Z. Aizenshtat, M. Frenkel, and K. Peters
Thermogravimetric and Rock-Eval techniques were used for the characterization of natural (humic) and synthetic (melanoidins)
substances and their hydrocarbon generation potential. A similarity between the thermal behaviour of humic substances and
of melanoidins (prepared from an excess of sugar) and the unique thermal properties of melanoidins (prepared from basic amino
acids) was observed. Rock-Eval analysis indicated that most synthetic melanoidins (also clay-complexed) generated more hydrocarbons
and related compounds than terrestrial humic substances.
Differential Scanning Calorimetry combined with Fourier transform infrared spectroscopy, was applied to the study of a number of fulvic and humic acids extracted from soils, peat, river and seawater. The thermal patterns obtained were related to the nature and origin of samples. The low-temperature endotherms were attributed to dehydration and loss of peripheral polysaccharide chains. The endotherm at 250°C observed for soil FA was ascribed to partial decarboxylation of more labile surface COOH groups, whereas the high-temperature exotherms at about 500°C were related to the degree of polycondensation of the aromatic network of the humic molecules.
Authors:S. Nagao, N. Fujitake, H. Kodama, T. Matsunaga, and H. Yamazawa
The association properties of Am with aquatic humic substances in a 0.01M NaClO4 solution at pH 6-8 were studied on the basis of molecular size distribution. Ten humic substances isolated from river water with different water quality (pH 3.9-8.0 and dissolved organic carbon (DOC) concentration of 2-40 mg/l) were used for comparing their effects on the association of Am. The molecular size distribution of Am in the presence of humic substances from an uncolored river water (DOC 2 mg/l) was different from that at the experimental systems using humic substances from brownish and high DOC (14-40 mg/l) river waters.
Authors:S. Nagao, Y. Sakamoto, T. Tanaka, and R. Rao
The association properties of Pu with aquatic humic substances in a 0.01M NaClO4 solution at pH 6–8 were studied on the basis of molecular size distribution. Seven humic substances were isolated from river
water and groundwaters using XAD extraction technique. They were used for comparing their effects on the association of Pu.
In the presence of humic acid, the dominant molecular size of Pu was 100–30 kDa. In the presence of fulvic acid, Pu exhibited
three dominant molecular sizes: 30–10 kDa, 30–5 kDa, and less than 5 kDa. The association of Pu-humus complexes might be controlled
by the molecular size distribution of humic substances and characteristics of their respective size fractions.
Authors:J. Kučerík, J. Kislinger, P. Majzlík, and M. Pekař
Application of the Arrhenius equation as the temperature function in modeling of the degradation kinetics of humic substances
brought a high positive Pearson correlation coefficient with the carbon content and a reasonable negative correlation with
the oxygen content. Ratio C/H indicating the aromaticity degree of humic samples did not show any significant correlation.
Relatively high value of correlation coefficients provided also O + N and ratios C/(O + N) and C/O, respectively. In contrast,
H, N content and natural and heat generated free radical content and their ratio gave substantially lower correlation coefficients.
The latter indicates that free radicals are probably not the main reason of the collapse of the secondary structure of humic
substances leading to their degradation.
Various approaches to the modeling of metal and radionuclide interactions with macromolecular ligands, proteins, polysaccharides
and humic substances in particular, their chemical and sorption equilibria, and the techniques used for their investigation
are concisely compared. To predict radionuclide mobility in the natural and semi-natural aqueous environment, the estimation
of the effective interaction constants, related to specific species of polyelectrolytes which are linked with the absorbancy
or absorbancy ratio in their electronic absorption spectra, should probably be preferred and developed as standard. For characterization
of the binding sites of specific molecular forms of polyelectrolyte ligands, the Scatchard plot analysis at macroconcentrations
of the metal and also, though not so effectively, the two-phase distribution at trace concentrations of radionuclide or metal
can be applied.
Humic substances (HSs) occur throughout the ecosphere in soils, waters and underground systems. The strong complexation of HSs is of importance in the migration of radionuclies in geological media. Renewed interest in stability constants of complexes of radioelements and radionuclides with humic and fulvic acids has been generated by problems associated with the nuclide migration in the environment. Use of the ion exchange method for the determination of conditional stability constants of metal-HS complexes was examined and reviewed. The complexation of HS to metal ions cannot be described in rigorous mathematical terms because of the ill-defined nature of HSs in contrast with the complexation of single ligands. Furthermore, the advantages and disadvantages of Schubert's and, Ardakani-Stevenson's, curve fitting methods were discussed. The great stabilities of HS complexes to rare earths (Yb(III), Tb(III), Eu(III), Gd(III)), americium(III), cobalt(II), uranyl(VI) and thorium(IV) were revealed.
Authors:P. Beneš, K. Štamberg, L. Široký, and J. Mizera
The sorption of trace europium, as a trivalent actinide homologue, was studied in the system Gorleben sand - aqueous solution with the aim to elucidate its mechanism. Radiotracer method (152/154Eu) and batch experiments were used. Simultaneously, the distribution of humic substances present in, or added to the system was measured. The evaluation of the sorption was complicated by the adsorption of Eu on the walls of polyethylene vials used for the experiments, which was rather high and had to be taken into consideration. It has been found that Eu sorption on Gorleben sand increases from pH 2 to pH 5-7 and then it decreases. The decrease is due to the complexation of Eu with humic substances leached from Gorleben sand at pH >7. The position of the sorption maximum depends on the composition of the solution and on the liquid-to-solid ratio. It is shifted to lower pH values in the presence of added humic acid (HA), which enhances Eu sorption at low pH values and suppresses it at pH values higher than 5. The regions of the enhancing/suppressing effects coincide with the high/low adsorption of HA on Gorleben sand, respectively. The increasing ionic strength (from 0.01 to 0.1) and europium concentration (3.4.10-8 to 9.3.10-7 mol/l) suppress the relative sorption (expressed in %) at low pH values and enhance it at pH>6-8. Addition of carbonates (5.10-3 mol/l) supports Eu sorption at pH>7.5 so that no decrease with pH is observed till pH 9. Alkaline leaching of the sand significantly changes most of the effects found. These results were qualitatively interpreted and conclusions were drawn on the mechanism of the sorption.
Authors:F. J. González-Vila, F. Martin, C. Sáiz-Jiménez, and H. H. Nimz
Thermofractography (TF) has been applied to humic and fulvic acids from four different soil types. Among the thermal products, 3,5-dihydroxybenzoic acid, catechol, 5-hydroxymethylfurfural, vanillin, phenol, furfural, guaiacol and indole were identified. These are typical fragments from lignins, microbial polyphenols, polysaccharides and proteins.
Authors:Mahmut Yildiztekin, Atilla Levent Tuna, and Cengiz Kaya
Chen , Y. , Aviad , T. ( 1990 ) Effects of humicsubstances on plant growth . In: MacCarthy , P. , Clapp , C. E. , Malcolm , R. L. , Bloom , P. R. (eds), HumicSubstances in Soil and Crop Sciences: Selected Readings . SSSA , Madison