Sorption of heterocyclic compounds from a complex mixture of coal-tar compounds on natural clayey till

The sorption and desorption of heterocyclic organic compounds in a complex multisolute system to a natural clayey till was investigated. The compositi on of the solutes reflect a simplified composition of an aqueous phase in c ontact with coal tar. Sorption was studied for two ratios (s:1) of clayey t ill (solid) to aqueous phase (liquid). The effect of the complex mixture of solutes on sorption of the four heterocyclic compounds: benzofuran, dibenz ofuran, benzothiophene, and dibenzothiophene is evaluated by comparison wit h their sorption measured in single-solute systems. Sorption of the four co mpounds is affected by the complex mixture, with sorption decreases for all four compounds at high s:1 ratio indicating competitive sorption. The effe ct on sorption of the individual compounds is not related to solubility or hydrophobicity of the compounds. Freundlich-type isotherms are observed for all compounds in the high s:1-ratio experiments, but for the most hydropho bic compounds isotherms are close to linear. The sorption of N-compounds an d benzofuran is apparently influenced by cation exchange and dipole-dipole attraction to clay minerals. At high concentrations a dramatic increase in the sorption of the most strongly sorbing compounds is observed in the low s:1-ratio experiment. The dramatic increase in sorption appears to be a res ult of multimolecular layer sorption or condensation on surfaces in the cla yey till at high surface density of organic compounds, and the data are fit ted by BET (Brunauer, Emmet, and Teller) type 2 isotherms. The increase may or may not be induced by the presence of N-heterocyclic compounds sorbed b y cation exchange and dipole-dipole attraction. The desorption of the compo unds was studied for the low s:1 ratio where multimolecular layer formation apparently had occurred. Partially irreversible sorption, hysteric Langmui r type desorption with isotherms of very high K-1 coefficient, or behaviour reflecting dissolution of a condensed phase is observed. (C) 1999 Elsevier Science B.V. All rights reserved.