Influence of chemical characteristics of humic substances on the partition coefficient of a chlorinated dioxin

Abstract

The partition coefficients (K_oc) of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) with respect to a variety of humic substances (HSs) were evaluated by a method involving solid-phase microextraction and gas chromatography–electron capture detection. The log K_oc values for each of the HS samples were in the range of 6.4–7.7. The log K_oc values for HAs from tropical peat, brown forest and ando soils were in the range of 7.3–7.6, similar to the calculated value for the octanol–water partition coefficient (log K_oc = 7.56). In contrast, the log K_oc values for FAs and peat HAs were 0.5–1 unit lower than the calculated value. The parameters for the polarity of HSs, as calculated from (N + O)/C, O/C atomic ratios and the carboxyl group content, were numerically similar related to the log K_oc for HpCDD. These results show that the K_oc values for HpCDD are significantly influenced by the polarity of HSs.

Introduction

It has been reported that the partitioning of hydrophobic organic pollutants (HOPs) into humic substances (HSs), the latter of which are widely distributed in soil and aquatic environments, influences their ecotoxicity (Lee et al., 1993, Haitzer et al., 1998) and environmental fate (Murphy and Zachara, 1995, Lee and Kuo, 1999, Terashima et al., 2003, Osako and Kim, 2004). The chemical property of interest for the partitioning of HOPs into HSs is frequently referred as the soil–water partition coefficient normalized to organic carbon (K_oc). In general, K_oc values can be calculated from the linear relationship, like in a Stern–Volmer plot, for lower concentrations of HOPs and organic carbon in HSs (Gauthier et al., 1987, Terashima et al., 2003). However, recent studies have reported nonlinear relationships when a wide range of concentrations of HOPs is used at arbitrary concentrations of HSs (Weber et al., 1992, Xing et al., 1996, Huang and Weber, 1997, Xing and Pignatello, 1998). This can be attributed to an adsorption-like process as well as partitioning, because HSs are heterogeneous macromolecules and have a variety of binding sites for HOPs (Xing, 1998).

Assuming soil organic carbon to be homogeneous, it should be possible to calculate the K_oc values from the relationships between K_oc and the hydrophobic indices of HOPs such as water solubility and octanol–water partition coefficients (K_ow) (Karickhoff, 1981, Kim and Lee, 2002). However, the characteristics of soil organic matter are actually different, and this can lead to variations in K_oc. For example, in the case of polycyclic aromatic hydrocarbons (PAHs), variations of the levels in K_oc of as much as one order of magnitude have been reported, depending on the type of soil organic matter being considered (Gauthier et al., 1987, Grathwohl, 1990). In addition, for the cases of PCBs and fluoranthene, the measured K_oc values were quite different from the calculated values from K_oc–K_ow empirical relationships (Brannon et al., 1995). Thus, a number of investigators have focused on relationships between the K_oc of HOPs and the chemical characteristics of HSs (Paolis and Kukkonen, 1997, Xing, 1997, Kulikova and Perminova, 2002, Salloum et al., 2002, Gunasekara and Xing, 2003, Khalaf et al., 2003). For example, the K_oc value for pyrene increases with increasing molecular weight, absorptivity at 280 nm and the aromatic carbon content of the HS (Gauthier et al., 1987, Chin et al., 1997, Tanaka et al., 1997).

Since the water solubility of polychlorinated dibenzo-p-dioxins (PCDDs) is extremely low (0.4–36.1 ng l⁻¹ for hexa-, hepta- and octachlorodibenzo-p-dioxin), it is difficult to analyze PCDD species (unbound and bound forms) in aqueous solution in the presence of HSs. Thus, in the case of PCDDs, only a limited number of measured K_oc values have been reported. In a previous study, we developed a simple method for determining the K_oc for 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD), which involved a solid-phase microextraction (SPME) method and gas chromatography–electron capture detection (GC–ECD) (Yabuta et al., 2004). In this study, the K_oc values could be calculated using the linear correlation of the concentration ratio of HpCDD in the absence of HSs to unbound HpCDD with the concentration of organic carbon in the HSs. The objective of the present study was to evaluate the K_oc values of HpCDD for a variety of HSs and to elucidate the structural factors of HS that may affect the K_oc values. In the present study, HpCDD was selected as an example of the predominant contributor to the toxicity equivalent (TEQ) of PCDDs in aquatic and soil environments (Schuhmacher et al., 1997).