Natural Organic Matter and Humic Substances Interactions

[10]. In this study, DAS is applied to the exploration of the complexation behaviour of
standard Suwannee River fulvic acid.
It is also desirable to have a means to model and predict the metal complexation behaviour of
fulvic acids. While many models have been developed and utilized, the NICA-Donnan model
has emerged as a very useful means for fitting and predicting complexation behavior of fulvic
acids [11, 12]. This model uses a continuous distribution of binding site affinities to model the
direct complexation of metal cations to NOM binding sites, and models the indirect
electrostatic interactions between NOM and metal cations through distribution of potential
throughout a uniform Donnan gel phase. This study uses the NICA-Donnan model for metal
complexation to fulvic acids to fit the differential absorbance spectra generated during
complexation of SRFA with metals.
2. Methods
A Suwannee River fulvic acid (SRFA) sample was obtained from the International Humic
Substances Society (Sample # 1S101F). SRFA solutions were prepared at DOC
concentrations of 5 mg/L, and background ionic strength was established by including 0.01 M
NaClO4. The pH of NOM solutions during titrations was controlled through addition of
appropriate amounts of HClO4 or NaOH, and the volume changes associated with these
additions of acid and base were corrected for in the final data analysis. Metal concentrations
were adjusted between 0 – 1000 μg/L Cu 2+ or Pb 2+ through addition of small volumes of
dilute CuSO4 and PbClO4 solutions. Absorbance spectra were recorded in a 5 cm quartz cell
on a Perkin-Elmer Lambda 18 UV/Vis Spectrophotometer. Differential absorbance data
fitting to model calculations was done using visual MINTEQA.
3. Results and Discussion
15th IHSS Meeting- Vol. 3
Absorbance spectra for SRFA at varying metal concentrations retain the same broad and
featureless shape throughout the entire range of the titrations. However, the differential
spectra for these titrations show the emergence of clear features, as illustrated by Figure 1.
They indicate that subtle changes of the absorbance of SRFA can be detected even at copper
concentrations as low as 5 µg/L. At higher CuTOT values, the presence of several bands
becomes notable, the most prominent of which have maxima at 245 and 390 nm.
Correlations between the amount of copper bound by SRFA and changes in absorbance were
calculated using by examining the relative change of the ratio of absorbances at 350 and 390
nm (denoted as (ΔRA/RA), The selection of the wavelength of 390 nm is due to the presence of
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