PHASE PARTITION OF ORGANIC POLLUTANTS BETWEEN COAL TAR AND WATER UNDER VARIABLE EXPERIMENTAL CONDITIONS BORHANE MAHJOUB, EMMANUEL JAYR, RE Â MY BAYARD and RE Â MY GOURDON* M Laboratoire d'Analyse Environnementale des ProceÂdeÂs et des SysteÁmes Industriels, Institut National des Sciences AppliqueÂes de Lyon, 20 Av. Albert Einstein, 69621 Villeurbanne Cedex, France (First received 1 September 1998; accepted in revised form 1 August 1999) AbstractÐOn some abandoned manufactured-gas plant sites the presence of a particular non-aqueous phase liquid (NAPL), namely coal tar, represents a potential source of groundwater pollution. The aim of this study was to characterise the phase partition of aromatic pollutants between coal tar and water. Batch trials have been carried out in order to evaluate the state of phase partition equilibrium of pollutants between coal tar and water, under variable experimental conditions (temperature, pH, ionic strength, coal tar composition). The experimental results clearly show that coal tar cannot be considered as an ideal solution with regards to phase partition of pollutants with water. The main identi®ed mechanisms of phase partition are the quasi instantaneous solubilization of pollutants near the coal tar/water interphase, and the diusion of pollutants within the organic phase or at the interphase which controls the transfer kinetics of pollutants towards the aqueous phase. The dissolution kinetics experimentally observed were therefore correlated to a diusional model of mass transfer. The formation of a ®lm which increased resistance to mass transfer was also observed at the coal tar/water interphase. This ageing phenomenon of the coal tar/water interphase was found to be an important aspect which can have a signi®cant in¯uence on the long term fate of coal tar in soils or subsoils, as well as on soil remediation techniques. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐcoal tar, polycyclic aromatic hydrocarbon (PAH), dissolution, phase partition, interphase INTRODUCTION Non-aqueous phase liquids (NAPL) are organic liquid (or semi-liquid) phases which are not, or very poorly, miscible with water. They represent a poten- tial source of pollution of underground and surface waters in many contaminated sites (Peters and Luthy, 1993). A well-documented example of NAPL is coal tar. Soil pollution by coal tars is the result of bad practices of waste management of resi- dues from old manufactured-gas plants or other similar industries. Coal tar is a by-product of coal pyrolysis process which was used to synthesise main gas until around 1950 (Beck, 1950) or coke for the metal industry. During the operating period of these plants, part of the stored coal tar was spilt (voluntarily or accidentally) on the ground. Due to its low solubility in water and its resistance to bio- degradation, coal tar remained in the soil or migrated into the subsoil and became a potential source of groundwater pollution (Schuth et al., 1993). There are dierent types of coal tars, the compo- sition of which depends on their origin. Coal tars contain a wide variety of organic and inorganic constituents. Most of the identi®ed organics are polynuclear aromatic hydrocarbons (PAHs), hetero- cyclic PAHs, phenolic compounds (cresols, xylenols, etc), and monocyclic aromatic hydrocarbons such as benzene, toluene and xylene (EPRI, 1993). Most of these organic substances are toxic and potentially carcinogenic. Depending on the type and origin of the tar, they account for ca. 40% of the total mass of the tar. The rest is composed of pitch, free car- bon, metals and other mineral substances, along with miscellaneous heavy residual material (EPRI, 1993; EEI, 1984; Beck, 1950). When a dense non-aqueous phase liquid such as coal tar is spilt on the ground, it migrates progress- ively in depth. However, a fraction of the phase remains entrapped in the porous medium due par- ticularly to its viscosity and to capillarity eects, resulting in the residual saturation of the medium. This fraction may remain immobile, even when sub- jected to a water ¯ow (Illangasekare et al., 1995; Ill- angasekare and Ramsey, 1995). Wat. Res. Vol. 34, No. 14, pp. 3551±3560, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter 3551 www.elsevier.com/locate/watres PII: S0043-1354(00)00100-7 *Author to whom all correspondence should be addressed.