Role of loosely bound humic substances and humin in the bioavailability of phenanthrene aged in soil
Introduction
It is now widely accepted that aging or sequestration phenomenon of some hydrophobic contaminants exists in soil, sediment, and aquifer (Alexander, 1995, Luthy et al., 1997). Also, it is well known that behavior of aged compounds is different from that of freshly added chemicals in the environment. It is typical that aged compounds show resistance to solvent extraction (Kelsey et al., 1997) and desorption (Kan et al., 1994). Such resistance results in reduced availability to bacteria (Nam et al., 1998) and higher organisms as well including plants (Bowmer, 1991), earthworms (Kelsey and Alexander, 1997), and guinea pigs (Umbreit et al., 1986).
Despite the current consensus on the aging of hydrophobic contaminants in the natural environment, it is still not clear which mechanisms are involved in the aging phenomenon. Among the possible mechanisms are the association of organic compounds with natural organic matter (Carroll et al., 1994) and the penetration of contaminants into small pores in soil (Wu and Gschwend, 1986). An additional model has been proposed originating from observations of different competitive effects in the sorption of organic contaminants (Xing et al., 1996). According to this model, natural organic matter has two different sorptive domains that interact with organic contaminants; partitioning domain and hydrophobic hole domain. The hydrophobic hole domain exhibits competitive sorption behavior and may be responsible for the desorption- and extraction-resistant fractions of aged contaminants. This concept is consistent with the findings that small pores with hydrophobic surfaces are responsible for resistant desorption (Werth and Reinhard, 1997) and declined bioavailability of contaminants to bacteria (Nam and Alexander, 1998).
Weber and Huang (1996) proposed that the hydrophobic hole domain is located between loose, amorphous humic materials and mineral surfaces and is composed of highly compact humic materials, which is typical to humin. Earlier studies have demonstrated that humin has macromolecular aliphatic chains as major constituents (Almendros and Gonzalez-Vila, 1987, Almendros and Sanz, 1991) and has significant amounts of small pores on its surface (Malekani et al., 1997). In these regards, it seems reasonable to hypothesize that the hydrophobic hole domain, which is proposed to be responsible for persistence of organic compounds, may exist in humin fraction of soil organic matter. The present study was thus conducted to determine a potential role of humin and alkaline-extractable humic substances in the sequestration and bioavailability of phenanthrene in soil.
Section snippets
Soil samples
Lima loam was collected from the Aurora Experimental Farm of Cornell University (Ithaca, NY). The soil is composed of 32.0% sand, 45.5% silt, and 22.5% clay (pH 6.89) and has 2.99% of organic carbon. The soil was passed through a 2-mm sieve, air-dried, and sterilized by gamma irradiation (2.5 Mrad) from a 60Co source (Ward Laboratory, Cornell University, Ithaca, NY). The samples were stored in 50-ml screw cap tubes at room temperature until use.
Aging of phenanthrene
Unlabeled and [9-14C] phenanthrene (Sigma Chemical
Distribution of phenanthrene in humic substances after aging
An experiment was conducted to determine the distribution of phenanthrene in humic substances after aging. After 1, 7, and 100 days of aging of phenanthrene in Lima loam, the soil was treated with alkali to separate humic and fulvic acids from humin-mineral fraction. Phenanthrene was extracted from each component of humic substances, and the amounts were determined chromatographically. As shown in Table 1, more than 90% of initial amount was found in humin-mineral fraction regardless of aging
Discussion
The data show that most phenanthrene was found mainly in humin-mineral complex rather than in humic and fulvic acids whether the polycyclic aromatic hydrocarbon was freshly added or aged. Operationally, humin is defined as the fraction of mineral-bound humic materials that can not be extracted with either alkali or acid. Since humin often comprises more than 50% of organic carbon in soil (Rice and MacCarthy, 1990) it may play an important role in the association of organic contaminants in soil.
Acknowledgements
The authors wish to thank Dr. Martin Alexander for financial support and guidance for this work and Dr. Joseph J. Pignatello for invaluable discussion during the preparation of this manuscript. We also thank the financial support by the Research Division of Seoul National University/Hanyang University for Social Infrastructure and Construction Technology.
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