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 Al3+, with Suwannee River fulvic acid. Addition of Al3+ 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 similar to4.0-5.0). 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, Ca2+ and Tb3+, indicated no significant enhancement or quenching with Ca2+ and only slight quenching with Tb3+. Comparison of FT-IR spectra collected on fulvic acid alone and fulvic acid in the presence of the three class A metals (Al3+, Ca2+ and Tb3+) 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 Al3+ spectrum and the Ca2+ and Tb3+ spectra. The -OH stretch spectral region in the Al3+ spectrum has a major component shifted to higher energy (compared to fulvic acid alone or to fulvic acid in the presence of Ca2+ or Tb3+). Even more striking is the emergence of a pronounced IR band at 2407 cm(-1), which is present only in the Al3+ 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 Al3+ in fulvic acid and are major contributors to the observed spectroscopic changes associated with Al3+ 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 Al3+ and Ca2+/Tb3+ 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. (C) 2001 Elsevier Science BY All rights reserved.