Binding of a fluorescent hydrophobic organic probe by dissolved humic substances and organically-coated aluminum oxide surfaces

Abstract

The binding of perylene by Suwannee River humic substances in the presence and absence of colloidal-sized aluminum oxide particles was examined using a fluorescence quenching technique. Our experiments show that binding is complete within 3 min and that the fluorescence of perylene associated with dissolved and adsorbed humic substances is fully quenched as evidenced by quantum yields which approached zero for all systems. In the absence of alumina, both humic acid and fulvic acid were able to bind perylene, and the partition coefficients decreased with increasing pH and NaCl concentrations. The presence of Ca^(2+) had little effect on the binding of perylene by either of the dissolved humic substances. The adsorption of humic and fulvic acids onto alumina decreased their ability to bind perylene. For all solution conditions examined, the association of perylene with adsorbed fulvic acid was never detected. In NaCl solutions, partition coefficients for adsorbed humic acid at pH 4 were approximately half the values of those for dissolved humic acid; at pH 7 and 10, alumina-bound humic acid did not bind perylene in NaCl solutions. In contrast to the results observed for dissolved humic acid, the presence of Ca^(2+) greatly enhanced the binding of perylene by adsorbed humic acid. A major effect of solution chemistry is to alter the mechanisms by which humic substances adsorb to alumina, thereby determining how tightly the humic material is bound to the surface. The ability of weakly-adsorbed humic acid to bind perylene approaches that of the dissolved species.

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