Fluorescence and FT-IR spectroscopic studies of Suwannee River fulvic acid complexation with aluminum, terbium and calcium.

In this study fluorescence emission and IR spectroscopy have been used to investigate the interaction of the class A (oxygen seeking "hard acid") metal Al(3+), with Suwannee River fulvic acid. Addition of Al(3+) ion results in a significant enhancement in fulvic acid fluorescence emission (at lambda(em)=424 nm) and significant red shift of the excitation wavelength (from lambda(ex)=324 nm to lambda(ex)=344 nm) at low pH values (pH approximately 4.0-5.0). At pH 4.0 (0.1 M ionic strength), where the predominant aluminum ion species is the "free" (aquo) ion, the fulvic acid fluorescence reaches 142% of the value in the absence of added metal ion. Analysis of the pH 4.0 and pH 5.0 fluorescence enhancement data with the nonlinear (single site) model of Ryan and Weber indicated binding constants in the range of 4.67.10(4)-2.87.10(6) M(-1) and concentrations of ligand sites in the range of 18.6.10(-6)-24.0.10(-6) M, both consistent with previous studies performed on both aquatic and soil fulvic acids. Companion fluorescence experiments performed on two other class A metal ions, Ca(2+) and Tb(3+), indicated no significant enhancement or quenching with Ca(2+) and only slight quenching with Tb(3+). Comparison of FT-IR spectra collected on fulvic acid alone and fulvic acid in the presence of the three class A metals (Al(3+), Ca(2+) and Tb(3+)) provides strong evidence for the involvement of carboxyl carbonyl functions in the binding of all three metal ions, which is not unexpected. The spectra also reveal, however, a very pronounced difference in the 4000-2000 cm(-1) IR spectral region between the Al(3+) spectrum and the Ca(2+) and Tb(3+) spectra. The -OH stretch spectral region in the Al(3+) spectrum has a major component shifted to higher energy (compared to fulvic acid alone or to fulvic acid in the presence of Ca(2+) or Tb(3+)). Even more striking is the emergence of a pronounced IR band at 2407 cm(-1), which is present only in the Al(3+) spectrum. The results of fluorescence and IR experiments with the model compounds salicylic acid and phthalic acid further confirm that both salicylic acid-like sites and phthalic acid-like sites are likely complexation sites for Al(3+) in fulvic acid and are major contributors to the observed spectroscopic changes associated with Al(3+) ion complexation. From a comparison of both the fluorescence and IR spectral results for all three class A metals, differing most strongly in the value of their ionic index, it seems clear that major sources of the deviation in spectral properties between Al(3+) and Ca(2+)/Tb(3+) is the unusually high value of its charge density and relatively low propensity for involvement in covalent bonding interactions (very high ionic index and relatively low covalent index in the Nieboer and Richardson classification of environmental metals), as well as affinity for certain functional groups.