A simple electrostatic model was used to study the effect of pH on the binding of Cu2+ to fulvic acid in solutions containing similar amounts of dissolved organic carbon (DOC) as natural media, such as aquatic environments and soil solutions. Complexation behavior was affected by increased pH because of changes in the electrostatic interaction resulting from an increase in the negative charge on the fulvic acid molecule. Solutions of soil-extracted fulvic acid (FA), at concentrations of 25 and 35 mg L(-1), ionic strength 0.005 M, and pH 5.0, 5.5, 6.0, 6.5, 7.0, and 7.5, conditions that simulate those of natural freshwaters, were titrated with copper ion using differential pulse anodic stripping voltammetry. Assuming the formation of 1:1 complexes, the conditional binding parameters (stability constant and binding capacity) were calculated for each pH value. Use of a 1:1 electrostatic model allowed estimation of the contribution of the electrostatic effect to the ion binding reaction, at each pH value, as well as calculation of a binding constant that was not dependent on pH and which thus represented the contribution of the chemical heterogeneity. Furthermore, it was found that only a small proportion of the FA acid functional binding sites are involved in the formation of copper complexes.
The interaction of the beryllium ion [Be(II)] with
one
of the major constituent of soil organic matter, fulvic
acids (fua), was investigated. The complexation of
Be(II) by two samples of fua (amph and gran), at a
concentration of 80 mg/L, was monitored by
synchronous fluorescence spectroscopy at four pH
values (4, 5, 6, and 7). A self-modeling curve
resolution technique (evolving factor analysis) was
used in the analysis of the sets of spectra collected at
increasing Be(II):fua ratio to obtain concentration
profiles that ideally correspond to one type of binding
site. These profiles were adjusted to a 1:1 complexation model. Quite strong complexation of Be(II)
by
fua occurs at nearly neutral pH (6−7) (logarithm of
the conditional stability constant between 5.6 and 6.5),
indicating that fua probably affect markedly the
mobility of Be(II) in the
environment.
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