The stability constants of the complex of Pu/III/ with a humic acid at pH 2.9 and 5.0 were measured using solvent-extraction technique. The organic extractant was dinonyl naphthalene sulphonic acid in sodium form /NaD/ in benzene while the humate aqueous phase had a constant ionic strength of 0.5M /NaClO4/. The total carboxylate capacity of the humic acid was determined by direct potentiometric titration to be 6.201 Meq g–1. The apparent pKa increased as the degree of ionization // increased. The 1g
1 values of the complex of Pu/III/ with humic acid have been determined to be 2.8 and 3.11 at pH 2.9 and 5.0, respectively.
Authors:A. Paulenová, P. Rajec, M. Žemberyová, G. Sasköiová, and V. Višacký
The binding of strontium by humic acid was studied in systems designed to model soil leaching conditions. Ultrafiltration experiments were conducted for a wide range of CaCl and NaCl concentrations and for the pH range 4 to 7. It was found that the recovery of humic acid slowly decreased with increasing pH. This occurred because at neutral and basic solutions the increasing degree of ionization of the carboxylate groups leads to the full repulsion state of the humic acid and to increasing of the strontium binding (strontium recovery 60% at pH 7) with this natural polyelectrolyte. The strontium complexation in the presence of calcium was studied in different series of metal concentrations at pH 5 and the ionic strength 0.1M NaCl. The results were explained on the base of the metal ion charge neutralization model of humic complexation (CNM). Loading capacity (cca 40%) and complexation constants and Sr (3.51±0.27) and Ca (3.10±0.29) with Aldrich humic acid were found.
Authors:N. Kamei-Ishikawa, K. Tagami, and S. Uchida
This study investigated selenium sorption kinetics on humic acid (HA) as a function of initial Se concentration (10–300 μg·1−1) and solid/liquid ratio (0.01–0.1). From the result, it was clear that the Se sorption kinetics on HA could be expressed
by a pseudo-second order equation. This was possible because the sorption mechanism on HA is a multiple sorption process including
specific and non specific mechanisms. Additionally, the 3-D empirical equation for the amount of sorbed Se could be determined
as a function of the initial Se concentration and solid/liquid ratio.
Humic acid sample was separated from the bottom sediments of Lake Quarun, in Egypt. It was purified and characterized by elemental
analysis, potentiometric titration, IR, UV-visible and 13C NMR spectroscopies. The product of humic acid was very low (0.009%), gray in color and has low carboxylate capacity (2.4
meq/g). The first derivative of the titration curve indicated one maximum only, which implies one kind of carboxylate groups.
The binding constant of 234Th with Lake Quarun humic acid was determined by solvent extraction. Only one parameter, β1, was required to fit the binding as a function of carboxylate concentration: the Th4+ bound to the carboxylate sites in the gray humic acid forming 1:1 complex only. The binding constant increased with the degree
of ionization and with the pKa of the humic acid.
It is suggested that Am3+ is forming different complexes with humic acids. At pH 4–5 at low concentration of Am3+ the complex is water soluble, while at higher concentration of Am3+ an insoluble complex is formed.
Authors:W. Goraczko, J. Slawiński, and K. Staninski
This study was conducted to investigate radiochemiluminescence (RCL) spectra (340–650 nm), kinetics and absorption spectra
of humic acids (HA) after their exposure of γ-radiation (absorbed doses of 1–10 kGy, 60Co) in model systems. The kinetics and spectral distribution of RCL were measured using the single photon counting method
(SPC) and cut-off filters. Absorption spectra (range 240–800 nm) of irradiated solutions indicated that post-radiative degradation/polymerization
processes take place in the HA changing their macromolecule or properties. The intensity of the delayed RCL was nonlinearly
and suggested that complex radical formation mechanisms were still involved in the post-radiative reactions.
Authors:P. Rajec, P. Gerhart, F. Macášek, I. Shaban, and P. Bartoš
The high-performance size-exclusion chromatography (HPSEC) and radiochromatography (HPSERC) was used for the identification
of radiocesium and radiostrontium interaction with humic acid. It was found that the behavior of humic acid on size-exclusion
chromatography is sensitive to the salt concentration and pH of the mobile phase. At lower ionic strength and in acidic region
of pH, the Aldrich humic acid exhibited three main fraction within the ranges >760 kDa, 25–100 kDa and <5 kDa. Radiocesium
was found in the low-molecular fractions (<1 kDa) of humic acids but radiostrontium interacts preferably with the fractions
of humic acid of molecular weight within the range 2–5 kDa.
Authors:J. Kučerík, H. Čechlovská, P. Bursáková, and M. Pekař
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.
Authors:S. Antoniou, C. Kolokassidou, K. Polychronopoulou, and I. Pashalidis
The stability of UO2CO3 has been studied as a function of the humic acid concentration in 0.1M NaClO4, in the weak acidic pH range (4.5–5) under CO2 atmosphere. 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, UO2CO3 is stable and remains the solubility limiting solid phase even in the presence of increased humic acid concentration in solution.
However, humic acid affects texture and particle size of the solid phase. Increasing humic acid concentration results in decreasing
crystallite size of the UO2CO3 solid phase. Based on the solubility data, the logKsp(UO2CO3) has been evaluated to amount −13.7±0.2 for the humic acid-free system and −13.2±0.3 for the humic acid containing system.