Rhizosphere Effects on the Degradation of Pyrene and Anthracene in

Chapter 18

Rhizosphere Effects on the Degradation of Pyrene and Anthracene in Soil 1

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Downloaded by UNIV MASSACHUSETTS AMHERST on October 6, 2012 | http://pubs.acs.org Publication Date: April 8, 1997 | doi: 10.1021/bk-1997-0664.ch018

S. C. Wetzel , M. K. Banks , and A. P. Schwab 1

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Departments ofCivilEngineering and Agronomy, Kansas State University, Manhattan, KS 66506 Polycyclic aromatic hydrocarbons (PAHs) are among the more resistant compounds found in petroleum contaminated soils and persist even after extensive bioremediation. Phytoremediation has been demonstrated to enhance the degradation of PAHs, but the mechanisms of dissipation have not been identified. The degradation of pyrene and anthracene was investigated in a laboratory study in which soil was removedfromthe rhizosphere of a long-term stand of alfalfa and compared to degradation in non-rhizosphere and sterile soil. Low molecular weight organic acids typically found in the rhizosphere were added to the soils to determine if exudation of simple organic compounds may be part of the rhizosphere effect. Dissipation in non-sterile soils was found to be much greater than in sterile soil, but there was no rhizosphere effect and the addition of organic acids did not enhance degradation. The effect of the rhizosphere on PAH degradation seems to be short-lived and requires the continued presence of roots. Polycyclic aromatic hydrocarbons (PAHs) are a group of hydrophobic organic compounds composed of varying numbers of condensed aromatic (benzene) rings and arranged in different configurations. These lipophilic chemicals are ubiquitous in the environment (usually in minute concentrations as contaminants) and are generally formed during the combustion, pyrolysis, and pyrosynthesis of organic matter. PAHs contain only hydrogen and carbon atoms with two or more benzene rings. The arrangement and number of the rings contained within the molecule results in a wide range of physical and chemical characteristics. The concern over PAHs stems from evidence of mutagenic effect in bacterial and animal cells and the carcinogenic effects in animals (7, 2). Therefore, the bioavailability of PAH contaminants is considered to be the major concern for polluted soils. The fate and transport of PAHs in soil are affected by those factors that determine the partitioning of the compounds between the solid, aqueous, and vapor Correspondingauthor

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© 1997 American Chemical Society

In Phytoremediation of Soil and Water Contaminants; Kruger, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


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WETZEL E T A L .

Degradation of Pyrene & Anthracene in Soil

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phases. Some PAHs are volatile enough that atmospheric transport can be an important environmental consideration (3), but Park (4) observed negligible volatilization from soil for PAH molecules with three or more rings. This observation appears to be the combined result of very low aqueous solubility (1 pg/L or less) and high adsorption to soil (log > 4). Leaching of PAHs also is limited by low water solubility and strong partitioning to the soil surfaces, but this can be modified by cosolvent effects and dissolution in dense nonaqueous phase liquids (5, 6). Transport of these insoluble compounds in soils also may be facilitated to a small degree by adsorption to and moving with mobile organic and inorganic colloids (7). Overall, PAHs are immobile and persistent in soil. Biodégradation is a potentially important means of removing PAHs from contaminated soil. Properly stimulated, indigenous soil microorganisms can degrade PAHs through complete mineralization (8), cometabolic degradation (9), and non specific radical oxidation (10). The availability of PAHs for biodégradation is limited by the combination of their very low aqueous solubilities and high degree of adsorption. Thus, these compounds tend to remain in the soil at relatively high concentrations after the successful bioremediation of other compounds. The soil region under the immediate influence of plant roots and in which there is proliferation of microorganisms is called the rhizosphere (77). This zone has properties that have the potential to enhance bioremediation of recalcitrant compounds because of elevated concentrations of naturaUy-CKxmrring organic materials and high microbial activities. Organic materials include low molecular weight exudates, metabolic secretions, plant mucilages, and gelatinous mucigel, and these compounds are involved in many processes including providing microbial substrates; decreased adsorption of otherwise strongly sorbed contaminants through surfactant activity; and, enhanced physical contact between roots, microorganisms, and water. The rhizosphere influence can project as much as 20 mmfromthe root (11). There is persuasive evidence that the rhizosphere community of plant roots and elevated microbial populations offer a potentially significant means by which to remediate chemically contaminated sites in situ. Laboratory and greenhouse studies have demonstrated degradation in the rhizosphere of herbicides (72,13,14), insecticides (15,16, 17), surfactants (75), and petroleum products (19, 20). Research Objectives The objectives of this laboratory research study were to: (1) Quantify the potentially beneficial effects of rhizosphere soil on the degradation of pyrene and anthracene. In this study, soilfromthe rhizosphere of alfalfa was used. (2) Assess the importance of low molecular weight organic acids typically found in root exudates on die degradation of pyrene and anthracene. (3) Apply arigorousstatistical design to the experiment to allow complete comparisons. The results of these experiments will provide important information with respect to the biodégradation of PAHs in the rhizosphere.

In Phytoremediation of Soil and Water Contaminants; Kruger, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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PHYTOREMEDIATION OF SOIL AND WATER CONTAMINANTS

Materials and Methods Soils and Treatments. The soil used in this study was an Ivan silt loam (fine-silty, mixed, mesic, Cumulic Hapludolls) obtained from the Department of Agronomy agricultural farm, Manhattan, KS. The soil was analyzed for a variety of properties and background PAH concentrations (Table I) and sieved.


Table I. Chemical and physical properties of the soil used in this study Parameter

Ρ (mg/kg) Κ (mg/kg) NH -N (mg/kg) +

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NO3-N (mg/kg)

PH CEC (cmol/kg) % Organic matter Field Capacity (%) PAH Background Anthracene (mg/kg) Pyrene (mg/kg)

Rhizosphere Soil

Non-rhizosphere soil

31 195 5.2 4.9 7.03 16 3.1 21

50 160 3.5 7.6 6.9 18 2.5 21

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Rhizosphere Effects on the Degradation of Pyrene and Anthracene in

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