COORDINATIVE AND HYDROPHOBIC INTERACTION OF HUMIC SUBSTANCES WITH HYDROPHILIC AL2O3 AND HYDROPHOBIC MERCURY SURFACES

The extent and kinetics of adsorption of a peat-derived humic substanc e (HS) onto a hydrophobic surface (mercury electrode) and a polar mine ral surface (gamma-Al2O3 colloids) was studied. Adsorption on the Hg-e lectrode was assessed by directly measuring the change of the double l ayer capacitance caused by the adsorption of HS on the electrode surfa ce through phase-selective a.c. polarography; the extent of adsorption of HS on the gamma-Al2O3-surface was monitored by determining the res idual HS-concentration in solution. On both surfaces, HS is adsorbed s trongly over a wide pH-range; hydrophobic interaction (i.e., expulsion from solution) prevails at the mercury surface while coordinative ads orption (ligand exchange), enhanced by hydrophobic effects, is the pre dominant mechanism at the oxide surface. Adsorption kinetics are chara cterized by an initial fast process, where even in dilute solutions (< 1 mg HS L(-1)), a high surface coverage is attained initially. True ad sorption equilibrium however, cannot be reached within hours. The slow approach to equilibrium is thought to be caused mainly by the polydis persity of HS resulting in fractionation processes, where presumably f ast-adsorbing low-molecular weight compounds are successively displace d from the surface by slow-adsorbing compounds of higher molecular wei ght. Slow molecular rearrangements of HS-molecules at the interface ca nnot be ruled out, however. Our results suggest that adsorption of hum ic substances on mineral as well as hydrophobic aquatic surfaces may l ead 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 lower-molecular we ight fractions, such as fulvic acid components, are more likely to rem ain 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 surfa ces may also be influenced by different fractions of adsorbed humic su bstances.