Coordinative and hydrophobic interaction of humic substances with hydrophilic Al 2O 3 and hydrophobic mercury surfaces

Abstract

The extent and kinetics of adsorption of a peat-derived humic substance (HS) onto a hydrophobic surface (mercury electrode) and a polar mineral surface (γ-Al 2O 3 colloids) was studied. Adsorption on the Hg-electrode was assessed by directly measuring the change of the double layer capacitance caused by the adsorption of HS on the electrode surface through phase-selective a.c. polarography; the extent of adsorption of HS on the γ-Al 3O 3-surface was monitored by determining the residual HS-concentration in solution. On both surfaces, HS is adsorbed strongly over a wide pH-range; hydrophobic interaction (i.e., expulsion from solution) prevails at the mercury surface while coordinative adsorption (ligand exchange), enhanced by hydrophobic effects, is the predominant mechanism at the oxide surface. Adsorption kinetics are characterized by an initial fast process, where even in dilute solutions (<1 mg HS L −1), a high surface coverage is attained initially. True adsorption equilibrium, however, cannot be reached within hours. The slow approach to equilibrium is thought to be caused mainly by the polydispersity of HS resulting in fractionation processes, where presumably fast-adsorbing low-molecular weight compounds are successively displaced from the surface by slow-adsorbing compounds of higher molecular weight. Slow molecular rearrangements of HS-molecules at the interface cannot be ruled out, however. Our results suggest that adsorption of humic substances on mineral as well as hydrophobic aquatic surfaces may lead to a progressive and selective immobilization of certain fractions of humic substances. It is probable that the higher-molecular weight fractions accumulate at aquatic interfaces, whereas lowermolecular weight fractions, such as fulvic acid components, are more likely to remain in solution. This may have significant effects on the qualitative composition and reactivity of dissolved vs. particulate organic carbon and on the residence time of different fractions of humic substances in natural systems. In turn, the chemical reactivity of particle surfaces may also be influenced by different fractions of adsorbed humic substances.