Synchronous Fluorescence Spectra of Metal-Fulvic Acid Complexest Stephen E. Cabanlss Department of Chemistry, Kent State University, Kent, Ohio 44242
I Synchronous quenching spectra,
the difference between synchronous scans of ligand fluorescence before and after metal addition, have distinct and readily identifiable patterns for divalent transition metals [Cu(II), Pb(II), Ni(II),Co(II),Mn(II)], Mg(II), and Al(II1) binding to fulvic acid (FA). Quenching spectra of Fe(1II) resemble those of other transition metals at pH 5.0 but differ at pH 7.5. Principal component analyses of the spectra acquired at varying total metal concentrations demonstrate that multiple fluorescent binding sites for divalent transition metals are present in FA solutions at pH 7.5. No evidence wu found for multiple binding sites at pH 5.0. Quenching spectra of solutions of two metals show that Al(III), but not Mg(II),competes with Cu(I1) for binding sites in the FA, Al(II1) also appears to displace Mg(I1) bound to FA.
fntroduction The complexation of metal ions by naturally occurring dissolved organic matter (DOM) attracts considerable attention because of its importance in regulating metal toxicity, bioavailability, and transport in natural waters ( I ) . Fluorescence spectroscopyoffers a unique perspective on metal-DOM binding because it observes the DOM ligand directly, while most techniques measure metal concentrations (2). This study uses synchronous fluorescence spectra to examine DOM binding site heterogeneity and competition between metals for similar DOM binding sites. Binding Site Heterogeneity. Numerous studies of metal-DOM and metal-fulvic acid (FA) binding indicate that the apparent binding constant K’varies with the ratio of total metal ( M T )to total ligand (LT)concentration (3, 11, Explanations for the observed variation include the following: (A) A single type of site exists with a single intrinsic binding constant Kint,but K’ is affected by electrostatic interactions among the sites (5, 6). Increased cation binding decreases the overall negative charge of the DOM ligands, thus decreasing the effective K’ as [ML] approaches L,. (B)DOM contains several types of sites, each type having its own LT, pH dependence, and Kbv These types may exist only as computational abstracts (7), or specific structure may be imputed to them, with K,, values drawn by analogy from the literature (8). ‘A contribution of the Kent State University Water Resources Research Institute. 0013-936X/92/0926-1133$03.00/0
(C) DOM contains a very large number of binding site types described by a continuous distribution of site concentrations as a function of K (9, 10). Each of these postulates can be used to model titration data within experimental error (7). Unfortunately, extrapolation from laboratory to field conditions is very model-dependent (10). Competition for Metal-DOM Binding Sites. The presence of numerous metal ions in natural waters has lead to the concept of intermetal “competition” for organic binding sites. Competition between Ca(I1) or Mg(I1) and divalent transition metals has been invoked in model calculations to suggest that transition-metal binding by oceanic DOM ligands may be of minor concern (11). However, experiment indicates a relatively minor effect of Ca(I1) and Mg(I1) on Cu(I1)-DOM complexation (7,12). Hering and Morel (13) found that separate Ca(I1) and Cu(I1) titrations could be fitted by the same set of simple binding components, but the resulting model greatly overestimated the effect of added Ca(I1) on Cu(I1) binding by DOM. Measured competition between Cu(I1) and Cd(I1) (14) is also minor. How are these results to be explained on a mechanistic level? Possibilities include a single site with high (never fully “titrated”), different dominant sites for transition metals (low LT) and alkaline earths (higher LT),and many different, specialized sites with particular affinity for certain metals. A useful model of intermetal competition should be able to predict toxic transition metal (Cu, Cd, Pb, Cr) displacement by alkaline earth ions (Ca, Mg) ( I I ) , other transition metals (Fe, Mn) (14), or Al(II1) (15). Fluorescence Measurements of Metal-DOM Complexation. The quenching of DOM fluorescence by bound metal ions (FQ) is the basis of the quantitative metalDOM binding technique of Weber and co-workers (16-19). FQ is useful in solutions of very low DOC, but relatively insensitive to low metal concentrations which only slightly perturb the free ligand concentration (20,21). Enhancement of DOM fluorescence by metal additions has also been reported for Al(II1) (22) and Mg(I1) (23). Changes in the intensity of a single peak provide a one-dimensional indicator of complexation which cannot distinguish different causes of quenching. For example, quenching due to Cu(I1) binding is indistinguishable from quenching by H+ or Ni(I1) binding. Steady-state fluorescence is a three-dimensional measurement, with independent variables A,, and A,, and dependent variable I. Its high potential information content is often unrealized because of broad peaks and
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 6, 1992 1133
symmetry in the excitation-emission matrix (EEM) (24). Synchronous (both monochromators) scans avoid symmetrical elements and obtain sharper peaks by cutting diagonally across the EEM (25,26). Synchronous spectra of DOM and FA show more structure than either absorbance or emission spectra (27). Previous work has shown that different peaks in DOM synchronous spectra respond differently to changes in pH (28, 29). This paper examines the effect of added metal ions on the synchronous spectra of FA fractionated from surfacewater DOM and shows these quenching spectra can be used to examine directly competitive metal binding and binding site heterogeneity.
White OaK River FA
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Environ. Scl. Technol., Voi. 26, No. 6, 1992
pH 5.0
E0000
I 60000
40000
2oooo
Materials and Methods Fulvic acids were isolated by the procedure of Thurman and Malcolm (30). River water was filtered (0.45 pm) to remove particulate matter, then acidified to pH
