SPECTROSCOPIC AND CHEMICAL-BINDING PROPERTIES OF HUMIC ACIDS IN WATER

The fluorescence properties of humic acids (HA) from different origins have been examined in detail, The rather diffuse absorption and emiss ion spectra can be dissected into components nominally associated with families of chromophores on the basis of their distinctive behavior i n titration and probe binding experiments. Principal contributions to the emission spectra that appear with maxima at 460, 490, and 530 nm c orrespond to absorption features that appear at approximately 360, 400 , and 470 nm, respectively, Humic acid fractions of higher molecular w eight (particularly for solutions at higher pH) give rise to fluoresce nce that is less intense and shifted to longer wavelengths. Phase-shif t fluorometry measurements are consistent with the role in emission of multiple chromophores, with a minimal fit of the data to three expone ntials with corresponding lifetimes ranging 0.6-8 ns. On interaction ( complexation) with methyl viologen (paraquat, MV(2+)), an electron tra nsfer agent and effective static quencher of HA fluorescence, some dis crimination among families of chromophores is observed. A solvatochrom ic probe, styryl-7, provides evidence for both electrostatic binding o f dye aggregates to humic acid, and the sequestering of dye monomers i n biopolymer microdomains at different ratios of HA and dye, The photo physical and binding properties of a ''synthetic'' humic acid prepared by oxidative polymerization of L-DOPA compared favorably with those o f HA's of natural origin.