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lows multimechanistic dual-mode sorption (DMSM) or dual-reactive domain (DRDM) models (Xing and Pig-Nonlinear isotherm behavior has been reported for the sorption natello, 1997; White and Pignatello, 1999;Hu et al., of hydrophobic organic compounds (HOCs) in soil organic matter LeBoeuf and Weber, 2000a;Yuan and Xing, 2001 (SOM), but the exact mechanisms are unknown. Our objective was to provide insight into the sorption mechanism of HOCs in SOM by and references therein). studying the sorption-desorption processes of naphthalene in a min-According to these models, which consider organic materal soil, its humic fractions, and lignin. Additionally, humin and lignin ter in soil as a heterogeneous substance, SOM consists were used for studying the effects of temperature and cosolvent on of two types of amorphous domains. The domains are HOC sorption. All isotherms were nonlinear. The humin and lignin characterized as expanded and condensed, analogous to isotherms became more linear at elevated temperatures and with the rubbery and glassy synthetic polymers, respectively. addition of methanol indicating a condensed to expanded structural Sorption of HOCs to the expanded "rubbery-like" dophase transition. Isotherm nonlinearity and hysteresis increased in main generates linear isotherms due to partitioning while the following order: soil humic acid (HA) Ͻ soil Ͻ soil humin. Of the nonlinear isotherms are observed for the condensed samples, aliphatic-rich humin exhibited the largest degree of non-"glassy-like" domain due to adsorption in Langmuir-type linearity and had the highest sorption capacity for naphthalene. High nonlinearity and hysteresis in humin were most likely caused by its sorption sites. Although the dual mode sorption models condensed structure. A novel aliphatic, amorphous condensed conforhave been numerously cited in the sorption literatures, mation is proposed. This conformation can account for both high structural compositions of these domains (expanded or sorption capacities and increased nonlinearity observed for aliphaticcondensed) in SOM are not well understood (Cuypers rich samples and can explain many sorption disparities discussed in et al., 2002). the literature. This study clearly illustrates the importance of both Recent sorption (Chin et al., 1997; Perminova et al., aliphatic and aromatic moieties for HOC sorption in SOM.1999; Perminova et al., 2001) and spectroscopic studies (Xing and Chen, 1999;Johnson et al., 2001) have shown that the condensed domain is mainly attributed to aro-

The sorption of phenanthrene was examined in humic acids (HAs) from different sources: a compost, a peat soil, and a mineral soil. Sub-samples of each HA were subjected to bleaching or hydrolysis to remove predetermined chemical groups from their structures. Bleaching successfully removed a large percentage of rigid, aromatic moieties, whereas hydrolysis removed the mobile, carbohydrate components. Phenanthrene sorption by all HAs was nonlinear (N < 1). However, the phenanthrene isotherms of the bleached HAs were more linear than those of the untreated HAs, whereas the removal of the carbohydrate components by hydrolysis produced more nonlinear isotherms. The introduction of pyrene to the phenanthrene sorption system yielded more linear isotherms for all the HAs, indicative of competitive sorption. Proton spin-spin (1H T2) relaxation determined by nuclear magnetic resonance (NMR) was used to identify separate rigid (condensed) and flexible (expanded) 1H populations and to determine their distribution. These 1H domains were highly sensitive to temperature and correlated well with reported glass transition temperatures for HAs. In combination with the chemical treatments, sorption, and spectroscopic data, we were able to observe some significant relationships among chemical groups, sorption behavior, and structural characteristics.