Influence of humic acid on bioavailability of heavy metals in sediments

Abstract

Char and soot (black carbon, BC) are highly surface-active materials that can play an important role in contaminant fate and bioavailability in soils and sediments. This report summarizes our research on the influence of adsorbate structure and BC properties on adsorption of organic compounds, and the possible attenuation of BC surface activity by humic substances in the environment. The BC was a maple wood char formed at 400 ~ in air and under other conditions. The solutes were polar and apolar aromatic compounds. The following points will be discussed. (1) By comparing adsorption to nonporous graphite, it was shown that molecular sieving effects (steric restriction) in the char micropore system occur in the order of increasing substitution on the benzene ring and in the order of increasing fused ring size. (2) By accounting for hydrophobic and steric effects it was shown that aromatic rings substituted with strongly pi-electron-accepting functional groups, such as nitro, undergo pi-pi electron donor-acceptor (EDA) interactions with the pi-donor polyaromatic (graphene) surface of the char. The free energy of adsorption due to pi-pi EDA interactions correlated with the free energy of molecular complexation in chloroform with model compounds representing the graphene surface (napthalene, phenanthrene, pyrene). (3) A series of chars were prepared of different polarity (O content) but similar surface area and pore size distribution by varying temperature and atmosphere. Singleand bi-solute experiments showed that polar interactions with surface O are not a significant driving force for adsorption of polar compounds. Rather, surface O attracts water molecule clusters that inhibit adsorption of both polar and nonpolar compounds by competition. (4) Aging of char particles in a soil-water suspension strongly reduced char SA and sorption of added benzene. Studies were conducted with dissolved humic (HA) acid, HA-char co-precipitates, A13+-HA-char co-flocculates, and soybean oil representing humic lipid components. Suppression of adsorption increased with adsorbate size, consistent with a competitive effect on the external surface and the adsorbates ability to penetrate micropore networks. Suppression was much greater for the co-flocculated char. SA by the N2 BET method at 77 K was greatly reduced by due to pore plugging by inflexible humic molecules at pore throats.