Relationship of polarity and structures of organic matter with sorption capacity for hydrophobic organic compounds

Abstract

Soil and sediment organic matter (SOM) comprises the sum total of organic materials in soil, including litter, microbial biomass, water-soluble organics, humic substances, and plant residues in varying stages of decomposition. Although the SOM content of mineral soils averages only 1-5%, SOM has a substantial impact on soil conditions (Stevenson 1994). For example, SOM is involved in the formation and stabilization of soil aggregates, promoting soil aeration and moisture retention. SOM is also the principal sorbent of hydrophobic organic compounds (HOCs), affecting their transport and bioavailability in soils and sediments (Xing and Pignatello 1998). In 1968, researchers discovered that a moderate amount of SOM had a pronounced influence on the sorption of organic compounds, unless SOM content is very low (Lambert 1968). Due to the high affinity, the interaction of SOM with HOCs may be described by the following equation: log Koc=a + b log Kow where Koc is the organic carbon-normalized sorption coefficient (i.e., Kd/foc, where foc is the fraction of organic carbon in soil), K ow is the octanol-water partition coefficient, and a and b are empirical constants. This equation has been widely used in predictive models for the movement and risk assessment of HOCs in soils and sediments, on the assumption that SOM behaves as a homogeneous partition phase, sorption occurs through partitioning, and octanol is an appropriate surrogate for SOM. In reality, SOM is very heterogeneous in composition and structure. During sedimentation and diagenesis, biopolymers are degraded and cross-linked, forming humic substances (e.g., humic acids and humin) that may be further transformed into kerogen, coal, and graphite under metamorphic conditions. Within a single soil profile, the percentage of aromatic constituents in humic acids (HAs) increases as humification progresses with depth (Table 1). Furthermore, the chemical composition of HAs sequentially extracted from a single soil is quite variable (Kang et al. 2003). The compositional and structural diversity of SOM leads to different sorptive properties for HOCs (Grathwohl 1990; Weber et al. 1992). For example, the sorption capacity of organic matter in unweathered shale and high-grade coals is more than an order of magnitude higher than that of organic matter derived from recent deposited soils, geologically immature material. Similar inferences can be drawn while comparing with the shale fraction of soils (Garbarini and Lion 1985; Grathwohl 1990). Furthermore, the measured Koc values of PCBs and fluoranthene were quite different from the Koc values calculated from equation (1) (Brannon et al. 1995). A number of investigators have reported that Koc of HOCs is predominantly influenced by the chemical characteristics of SOM (Gunasekara and Xing 2003; Khalaf et al. 2003; Kulikova and Perminova 2002; Salloum et al. 2002; Xing 1997). Nevertheless, a conclusive, distinct relationship between sorption of HOCs and the SOM characteristics has not been established. Here we summarize the literature data and our own findings on the correlation between HOC sorption and SOM characteristics with particular reference to the polarity of SOM.