Chapter 15
Benzo(a)pyrene and Hexachlorobiphenyl Contaminated Soil: Phytoremediation Potential 1
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V. Epuri and Darwin L. Sorensen
Utah Water Research Laboratory, Utah State University, Logan, UT 84322-8200
Benzo(a)pyrene (B[a]P) is a carcinogenic polynuclear aromatic hydrocarbon and hexachlorobiphenyl (HCB) is a polychlorinated biphenyl (PCB) congener. Spiked, radiolabeled B[a]P and H C B mineralization, volatilization, solvent extractability, soil binding and plant accumulation were measured in soil microcosms for differences between unvegetated and vegetated treatments. Aroclor 1260 and P A H contaminated loamy sand from a New Jersey plastics plant was used. Soil was planted with Tall Fescue (Festuca arundinacea Screb.) or was left unplanted. Incubation under artificial lighting for 180 days with vegetation resulted in decreased B[a]P volatilization, increased mineralization, and increased solvent extractability but had no detectable effect on soil binding. Vegetation had no effect on H C B volatilization or soil binding but enhanced its mineralization and decreased its extracability.
Benzo(a)pyrene (B[a]P) belongs to a class of compounds known as poly cyclic aromatic hydrocarbons (PAHs). It has five fused benzene rings. Hexachlorobiphenyl (HCB) belongs to class of compounds known as polychlorinated biphenyls (PCBs). It has six chlorines attached to the biphenyl. B[a]P is a carcinogen and PCBs are toxic and are possible carcinogens (7). Since both of these compounds are toxic, there is a need to minimize their concentrations in the environment. Both B[a]P and HCB have low aqueous solubilities and are relatively non volatile (2, 3). They are persistent in the environment and not readily biodegradable. Biodégradation of HCB has been reported (4). There was a significant reduction in the concentrations of hexachlorobiphenyls in liquid media, when treated with a strain of Alcaligenes eutrophus, isolated from a PCB contaminated soil. Biodégradation of chlorobiphenyls (mono to penta) has been reported in other studies (5-75). Biodégradation of B[a]P has been reported both in liquid media and soil (16-22). In 1
Corresponding author
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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.
Downloaded by STANFORD UNIV GREEN LIBR on October 7, 2012 | http://pubs.acs.org Publication Date: April 8, 1997 | doi: 10.1021/bk-1997-0664.ch015
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EPURI & SORENSEN
Benzo(a)pyrene & Hexachlorobiphenyl
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most of the studies, specific microorganisms had been used (16-20). Mineralization of B[a]P by indigenous microorganisms has been reported in two studies (21, 22). Bioremediation is the technique of converting hazardous organic chemicals into harmless compounds using biological processes (usually microbial metabolism). Bioremediation may be enhanced by the use of vegetation, a process known as phytoremediation. The rhizosphere soil (soil adhering to the root) has higher microbial numbers, biomass and activity than the surrounding root-free soil (23) and hence enhanced degradation may be possible in the rhizosphere (24). Phytoremediation has been shown to increase the mineralization of a few pesticides (25, 26, 27). It has also enhanced the mineralization of industrial chemicals such as trichloroethylene and pentachlorophenol (28, 29). In the case of recalcitrant compounds such as PAHs having four rings or more, vegetation has enhanced the disappearance of extractable chemicals from soil (30). There has been only one study published that investigated the phytoremediation of B[a]P (30) and none for HCB. There is a need to learn more about the phytoremediation potential of carcinogenic PAHs and PCBs. We performed a laboratory scale "treatability study" of B[a]P and HCB to evaluate the potential for enhanced phytoremediation of soil contaminated with these kinds of compounds.
Materials and Methods Experimental Design. The experimental design used in this study was a randomized complete block design with factorial treatments (57). The two factors were vegetation and time. Unvegetated and vegetated treatments were used to assess the effect of vegetation and time was evaluated by sacrificing three sets of microcosms after 12, 102 and 180 days of incubation. The experimental design of the study is shown in Table 1. At day 0, when the radiolabeled compound was spiked into the soil, the compound would have been in a state of flux among phases and among various associations with the solid phase. It was anticipated that relatively rapid physical and chemical sorption of the compound would have occurred during the first few days. The situation after this "equilibration" period would be more representative of the field situation. For this reason, the first set of microcosms was sacrificed after 12 days. The decision to sacrifice the third set after 180 days was based on the economics of conducting the experiment. The second set was sacrificed after 102 days to learn about the distribution of the compound approximately midway through the experiment. Soil Sampling and Characterization. Soil was collected from CAMU 2, at the Union Carbide Corporation plant in Boundbrook, New Jersey. In 1987, a few buildings were demolished at the plant as a part of a renovation plan. During the process, a transformer valve was accidentally opened and transformer oil ran into the soil. The transformer oil contained PAHs (byproducts of petroleum refining) and PCBs (added to improve the thermal stability and electric resistivity) and thus the soil was contaminated with PAHs and PCBs.
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
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Table 1. Experimental design for phytoremediation treatability of B[a]P and HCB Factor Number Description Compound Benzo[a]pyrene; 2 Hexachlorobiphenyl Treatment 2 Vegetated; Unvegetated Time of sacrifice 3 12, 102, 180 days Replicates 3 Total Number of Microcosms 36
Soils were collectedfromtwo different locations at CAMU 2. The soil was sieved at the site through a 6.4 mm sieve, to remove large stones and building debris. The soil was then shipped to Utah State University. In the laboratory, the soil was sieved to pass a 2 mm screen and mixed thoroughly to minimize heterogeneities. It was stored at approximately 5° C until use. The results of soil characterization analyses are given in Table 2.
Table 2. Characteristics of the CAMU 2 soil samples Characteristic Value Soil Texture Loamy Sand % Sand 78 % Silt 15 % Clay 7 Field capacity (moisture content; %) 17.4 Cation exchange capacity (cmol(+)/kg) 10.7±0.4* pH 7.7 ± 0.4 Organic carbon (%) 1.46 ±0.01 Electrical conductivity (mmhos/cm) 0.30 ± 0.04 Total Kjeldahl nitrogen (%) 0.037 ± 0.003 Calcium (mg/kg) 32+11.9 Magnesium (mg/kg) 2.6 ± 0.97 Sodium (mg/kg) 5 ± 1.35 Potassium (mg/kg) 3.0 ± 0.76 Nitrate-nitrogen (mg/kg)
