Measurements of trace metal species in situ in a softwater
river, a hardwater lake, and a hardwater stream were
compared to the equilibrium distribution of species calculated
using two models, WHAM 6, incorporating humic ion
binding model VI and visual MINTEQ incorporating NICA−Donnan. Diffusive gradients in thin films (DGT) and
voltammetry at a gel integrated microelectrode (GIME)
were used to estimate dynamic species that are both labile
and mobile. The Donnan membrane technique (DMT)
and hollow fiber permeation liquid membrane (HFPLM)
were used to measure free ion activities. Predictions of
dominant metal species using the two models agreed
reasonably well, even when colloidal oxide components
were considered. Concentrations derived using GIME were
generally lower than those from DGT, consistent with
calculations of the lability criteria that take into account
the smaller time window available for the flux to GIME. Model
predictions of free ion activities generally did not agree
with measurements, highlighting the need for further work
and difficulties in obtaining appropriate input data.
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 n(Pb1) = 0.92 instead of the generic value n(Pb1) = 0.60. The n(Pb1)/n(H1) ratios obtained were relatively high, indicating monodentate Pb binding to the carboxylic-type groups. The nPb2/nH2 ratios depended somewhat on the method of optimization, but the values were distinctly lower than the n(Pb1)/nH1 ratios, especially when the optimization was based on Pb bound vs log [Pb(2+)]. 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.
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