Verlag des Forschungszentrums Jülich
JUEL-4046
Humic acid is very heterogeneous in terms of physical and chemical
properties. To reduce the chemical heterogeneity of the extracted humic acid
(from the Ap horizon of the Orthic Luvisol, Merzenhausen, Germany), a
fractionation scheme using the ultrafiltration technique was used to obtain
eight size fractions of the humic acid. The extracted humic acid and its
fractions were characterized by potentiometric acid-base titration, eiemental
analysis and different spectroscopic methods (NMR, UV-VIS and FT -IR
spectroscopy). Clear chemical differences between the humic acid size
fractions were observed. Smaller size fractions of the soil humic acid
contained more chargeable functional groups and larger percentage of
aromatic carbon than the larger size fractions. Conversely, the percentage of
aliphatic carbon increased with increasing apparent molecular weight.
Moreover, the solid-state 19F-NMR was used to study the sorptive uptake of
hexafluorobenzene by the humic acid and its fractions. It was found that
humic acid molecules have different chemical environments into which
organic pollutant such as hexafluorobenzene can sorb. Small humic acid
molecules have at least three sorption sites ("rigid", "soft" and other new
domains) that are more clearly defined and homogeneous than the sorption
domains found in larger humic acid molecules.
The effect of the pH and the electrolyte concentration on the adsorption of the
humic acid onto alumina surfaces as weil as on the colloidal stability of these
systems were studied. To better understand the binding mechanisms these
results were also compared to those of polyacrylic acid. The adsorption of
humic acid or polyacrylic acid to alumina varied with pH and electrolyte
concentration, suggesting that the conformation of the humic acid or
polyacrylic acid in solution significantly determines their structures on the
mineral surface. At low pH « point of zero charge (PZC) of alumina),
increasing amounts of humic acid or polyacrylic acid are adsorbed on the
alumina surface with increasing concentrations of solutes, resulting in a
charge reversal from positive to negative net total particle charge whilst at
high pH (> PZC), the electrophoretic mobility was shifted to more negative
values. The colloidal stability of the alumina dispersions containing increasing
amounts of the added humic acid and polyacrylic acid, respectively, was
monitored using the dynamic light scattering technique. The maximum
aggregate size was observed around the zero electrophoretic mobility,
indicating the importance of the charge neutralization mechanism.
By comparing the adsorption of the humic acid fractions on alumina surfaces,
it was found that the adsorbed amount increases with increasing humic acid
molecular size. Furthermore, an increase in the rise of the adsorption
isotherm in the plateau regions by increasing the humic acid fraction size was
also observed which indicates a higher contribution of the hydrophobic
interactions due to the increase in the aliphatic carbon and the decrease in
the chargeable groups.
The sorption results of 2,4-dichlorophenol on the immobilized humic acid at
different pH and salt concentrations revealed that the value of the sorption
coefficient decreases as the ionic strength decreases or the pH value
increases which gives direct evidence for the importance of the conformation
of the immobilized HA. These sorption isotherms can be described as a linear
isotherm, which indicates that the sorption of 2,4-dichlorophenol is
predominantly a partitioning process between the aqueous phase and the
immobilized humic acid.
Khalaf, Moustafa
Effect of the fractionation and immobilization on the sorption properties of humic acid
III, 114 S., 2003
Humic substances modify the surface of inorganic soil constituents changing
the nature and amount of adsorption sites for contaminants and also
influencing the particle-particle interactions and thus the mobility and transport
behavior of the soil particles. The aim of this PhD work was to investigate the
effects of the interactions between two important soil components, aluminum
oxide and humic acid, on the sorption behavior of 2,4-dichlorophenol under
laboratory conditions selected to model the soil systems.
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