1 research outputs found
Lead Binding to Soil Fulvic and Humic Acids: NICA-Donnan Modeling and XAFS Spectroscopy
Binding
of lead (Pb) to soil fulvic acid (JGFA), soil humic acids
(JGHA, JLHA), and lignite-based humic acid (PAHA) was investigated
through binding isotherms and XAFS. Pb binding to humic substances
(HS) increased with increasing pH and decreasing ionic strength. The
NICA-Donnan model described Pb binding to the HS satisfactorily. The
comparison of the model parameters showed substantial differences
in median Pb affinity constants among JGFA, PAHA, and the soil HAs.
Milne’s “generic” parameters did not provide
an adequate prediction for the soil samples. The Pb binding prediction
with generic parameters for the soil HAs was improved significantly
by using the value <i>n</i><sub>Pb1</sub> = 0.92 instead
of the generic value <i>n</i><sub>Pb1</sub> = 0.60. The <i>n</i><sub>Pb1</sub>/<i>n</i><sub>H1</sub> ratios obtained
were relatively high, indicating monodentate Pb binding to the carboxylic-type
groups. The <i>n</i><sub>Pb2</sub>/<i>n</i><sub>H2</sub> ratios depended somewhat on the method of optimization,
but the values were distinctly lower than the <i>n</i><sub>Pb1</sub>/<i>n</i><sub>H1</sub> ratios, especially when
the optimization was based on Pb bound vs log [Pb<sup>2+</sup>]. These
low values indicate bidentate binding to the phenolic-type groups
at high Pb concentration. The NICA-Donnan model does not consider
bidentate binding of Pb to a carboxylic- and a phenolic-type group.
The EXAFS results at high Pb loading testified that Pb was bound in
bidentate complexes of one carboxylic and one phenolic group (salicylate-type)
or two phenolic groups (catechol-type) in ortho position