Comparative Examination of Effects of Binding of Different Metals on Chromophores of Dissolved Organic Matter

This study quantified the binding of dissolved organic matter (DOM) from Suwannee River with nine metals, Ca­(II), Mg­(II), Fe­(III), Al­(III), Cu­(II), Cd­(II), Cr­(III), Eu­(III), and Th­(IV), using a differential absorbance approach. The differential spectra of DOM were closely fitted with six Gaussian bands that were present for all of the metals at varying pH values. Their maxima were located at ca. 200, 240, 276, 316, 385, and 547 nm (denoted as A0, A1, A2, A3, A4, and A5, respectively). The relative contributions and signs of the Gaussian bands were metal-specific and correlated to some degree with the covalent-bonding index of the ions and applicable complexation constants of the NICA-Donnan model. The intensity of band A4 was linearly proportional to the concentration of DOM-complexed metal, although these correlations formed two groups with different slopes, reflecting the nature of DOM–metal interactions. The results demonstrate that differential spectra yield results indicative of the nature and extent of metal and DOM interactions.