Association of hydrophobic organic contaminants with soluble organic matter:: evaluation of the database of Kdoc values

Natural dissolved organic matter is ubiquitous in the environment and comprises a soluble compartment of any environmental system containing water. The association of contaminants with this compartment and the facilitation or hindrance of migration effected by this association can be important in the overall fate-migration process, thus quantitative evaluations of the coefficients describing association of hydrophobic organic contaminants (HOCs) with natural dissolved organic matter, from various sources, have been completed using several distinct methodologies: gas-aqueous partitioning; reverse-phase separation; dialysis; liquid-solid partitioning; fluorescence quenching and apparent solubility enhancement. Each of these methodologies and an evaluation of the quality of its results are described herein. Apparent solubility enhancement methods appear to be the most universally applicable, being subject to the least number of potential experimental artifacts. The accuracy of values obtained using the gas-aqueous partitioning and reverse-phase separation methods is suspect. Fluorescence quenching methods are limited to HOCs that fluoresce and are subject to interferences that potentially lead to positive bias. Dialysis methods are subject to anomalies that lead to a potentially negative bias of the results. Liquid-solid partitioning methods are subject to a number of potential random errors and biases. The hydrophobicity and molecular structure of the HOC, the composition and character of the natural organic matter and the background solution conditions all potentially affect the magnitude of the association coefficient. These factors are summarized and discussed. A database of coefficients and methodologies for obtaining accurate estimates of the distribution coefficient describing the association of HOCs with natural organic solids currently exists. This paper presents a first attempt at assembling a similar database and set of methodologies for accurate estimation of the coefficients quantitatively describing the association of HOCs with natural dissolved organic matter. This assembled data base is found to be lacking in terms of its predictive utility. (C) 2002 Elsevier Science Ltd. All rights reserved.