Multi-Wavelength Molecular Fluorescence Spectrometry for Quantitative Characterization of Copper(II) and Aluminum(III) Complexation by Dissolved Organic Matter
Conditional stability constants and binding capacities are important parameters with which to estimate the biological availability of metal ions in aqueous solution in the presence of dissolved natural organic matter (fulvic acid, organic matter in natural waters or in aqueous extracts of forest litter). Determination of these parameters depends on analytical methods that can distinguish between free and organically bound metal ions. This speciation is difficult, mainly because natural organic matter typically is a complex mixture. In this paper, multi-wavelength molecular fluorescence spectrometry is evaluated prototypically as a method for the determination of stability constants and binding capacities for Cu(II) and Al(III) complexation by dissolved organic matter in a juniper leaf litter extract. Equilibrium ion exchange quantitation and electron spin resonance spectroscopy served as quantitative and qualitative reference methods, respectively. Three types of binding site for Cu and Al could be differentiated qualitatively by the reaction patterns of various wavelength regions of the total luminescence spectrum of the leaf litter extract in response to increasing metal ion addition. For both Cu (pH 6) and Al (pH 5), binding parameters for the two types of binding site forming the most stable complexes were deduced self-consistently from reactions evaluated at selected excitation/emission wavelength pairs.
© 1996 American Chemical Society. Received for review July 18, 1995. Revised manuscript received November 22, 1995. Accepted December 17, 1995. This research was supported by U.S. NSF Grant CES-8896204 as well as funds of the Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland. The authors thank Dr. F.-R. Chang for suggesting the method of reducing the number of variables described in the Experimental Section and Dr. N. Senesi for critically reviewing the interpretation of the ESR spectra. Gratitude is expressed to U. Beutler for providing some control measurements.