Sorption of three heterocyclic compounds and their hydrocarbon analogue from coal-tar contaminated water in a natural clayey till

Abstract

Abstract The natural attenuation of contaminants in fractured media is controlled by biodegradation and by diffusion into the matrix. The effective rate of diffusion and the mass of contaminants thereby removed from the aqueous phase in the fracture depends on the sorption of the contaminants. The sorption of three heterocyclic compounds and their hydrocarbon analogue from an aqueous solution of a complex mixture of 25 organic coal-tar compounds including all major groups of organic coal-tar compounds in clayey till was studied in batch experiments for a large range of concentrations including concentrations approaching the effective solubility of the mixture and for two different solid:liquid (s:l) ratios. For the high s:l ratio sorption experiment the isotherms were close to linear and best fitted by Freundlich isotherms. Sorption of the four compounds was correlated with K ow , but an order of magnitude greater than predicted from a K ow - K ow relationship for aquifer materials. Sorption was primarily dominated by hydrophobic sorption. However, at high surface density carbazole sorption appeared to be influenced by dipole-dipole attraction. For the low s: 1 ratio experiment at higher concentrations a dramatic increase in sorption was observed for all four compounds. The dramatic increase is likely a result of multi-layer formation/condensation of organic compounds on mineral surfaces at high surface density of organic compounds possibly induced by the complexity of solutes. The steepness of the increase and sensitivity to total compound surface density appears to be related to compound hydrophobicity. The impact of these four compounds on groundwater quality from sources of coal-tar in fractured clayey till overburden may be significantly lower than predicted from general sorption relationships.