Environ. Sci. Technol. 1997, 31, 3244-3250
Fate and Toxicity of Chlorophenols, Polychlorinated Dibenzo-p-dioxins, and Dibenzofurans during Composting of Contaminated Sawmill Soil M . M I N N A L A I N E , * ,†,‡ JUKKA AHTIAINEN,† NADJA WÅGMAN,§ LARS G. O ¨ BERG,§ AND KIRSTEN S. JØRGENSEN† Finnish Environment Institute, Laboratory, Hakuninmaantie 4-6, FIN-00430 Helsinki, Finland, Department of Applied Chemistry and Microbiology, University of Helsinki, P.O. Box 56, FIN-00014 Helsinki University, Finland, and Institute of Environmental Chemistry, Umea˚ University, S-90187 Umea˚, Sweden
We studied the fate of chlorophenols during the composting of sawmill soil and impregnated wood to see whether chlorophenols, in addition to mineralization, would form any harmful metabolites. The toxicity assessed by luminescent bacteria tests decreased during the composting, and it followed the chlorophenol concentrations in the compost piles. The threshold value for chlorophenol toxicity appeared to be 200 mg of total chlorophenols/kg dry weight. Polymerization of chlorophenols was studied by determining the molecular weight distribution of organic halogen compounds during the bioremediation. Organic chlorine compounds appeared in high molecular weight sizes, indicating that binding to soil organic matter had taken place during the long exposure time to the wood preservative Ky-5 in the contaminated soil. No major polymerization occurred during the composting, but the polymerized fraction was not either degraded or remobilized. Large amounts of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/ Fs) originating from the wood preservative were found in the compost piles, but their concentrations did not significantly change during the bioremediation process. The congener composition of PCDD/Fs resembled the one in original wood preservative. Since PCDD/Fs were released from the wood chips of the former Ky-5 dipping basin and not degraded during the bioremediation, it is not recommended to treat PCDD/F-contaminated wood chips in biopiles.
Introduction Many xenobiotics are known to incorporate into humus (1, 2) through abiotic or biotic reactions. Organic contaminants incorporated into soil are considered to be resistant against microbial remobilization (3) or biodegradation (4), and their bioavailability is considered to be small (5). Nevertheless, very little is known about the heterogeneous high molecular weight chlorinated fraction that is formed during (co)* Corresponding author phone: (358) 9 40300884; fax: (358) 9 40300880; e-mail:
[email protected]. † Finnish Environment Institute. ‡ University of Helsinki. § Umea ˚ University.
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polymerization, and it is not known if the polymerization is safe and an irreversible pathway for removal of contaminants. Recent findings suggest that at least some remobilization may take place. Certain chemical compounds such as oil, dissolved organic matter in compost, or surface active biologically produced compounds (e.g., rhamnolipids) may act as surfactants and release large lipophilic contaminants such as polychlorinated biphenyls (PCBs) or polynuclear aromatic hydrocarbons (PAHs) from soil and thus increase their bioavailability (6-9). Simultaneous oil contamination may enhance the mobility of PCDD/Fs in soil-water systems (10). Furthermore, synthetic chlorolignin model compounds have been shown to be bioavailable to rats (11). Thus, the immobilization of chlorophenols into soil as a bound residue, by dimerization or polymerization, is not a favorable remediation event. Chlorophenol-contaminated sawmill soil in Finland is often also contaminated with PCDD/Fs as well as with polychlorinated phenoxyphenols (PCPPs) since the technical wood preservative called Ky-5 that was produced and used in Finland as well as in other Scandinavian countries contained these dimeric compounds as impurities (12-17). Dioxins and other dimeric compounds may also be biologically formed in contaminated soil. White rot fungi may form PCDD/Fs from chlorophenols via peroxidase enzymes (18) or from chlorinated anisyl compounds that they produce de novo (19). Food is the major source of PCDD/Fs for humans (20). Chickens and cows have been found to accumulate PCDD/Fs from contaminated soil even at as low a concentration as 45 ppt as international 2,3,7,8-TCDD toxic equivalents, I-TEQ (5.6 mg kg-1 as total PCDD/F concentration) (21). Although non-chlorinated dibenzo-p-dioxin and dibenzofuran has been shown to be biodegradable to some extent (22-25), only few papers report biological breakdown or dehalogenation of PCDD/Fs (26-31). Bioremediation of PCDD/Fs is also hindered by their low availability to microorganisms. This is due to the low concentration of PCDD/Fs and their high affinity to soil. In soil bioremediation, there is often a need for cheap and reliable methods for assessing the efficiency of the bioremediation process. Toxicity tests allow the analysis of overall toxicity even when every chemical compound present is not known. The inhibition of light production by the luminescent bacterium Vibrio fisheri indicates disturbance of the energy metabolism of this heterotrophic bacterium since the luminescence pathway is a direct branch of the electron transport chain. Hence the change in bacterial luminescence, in the presence of soil extracts, can be used as an indicator of soil toxicity. This method is thus sensitive to compounds that act as uncouplers of the electron transport chain and hereby cause acute toxicity. Chlorophenols and many of their derivatives belong to this group of compounds (32), whereas PCDD/Fs have a different toxic mechanism and cause mainly chronic toxicity (33). Due to the polarity of the chlorophenols, they may leach into the groundwater and thus cause a threat to the humans and to the environment. PCDD/Fs are less mobile since they adsorb to the organic matter in soil. The main PCDD/Fs exposure from contaminated soil to human is skin contact or via inhalation of dust particles. Recent studies indicate that chlorophenols rather than PCDD/Fs are the main cause for detrimental health effects to human in contaminated sawmill areas (34). Removing the chlorophenols from the contaminated sawmill soil reduces the risk for substantial spreading of the pollution. In our earlier studies, we have shown an effective chlorophenol degradation and mineralization in compost piles of chlorophenol-contaminated
S0013-936X(97)00233-2 CCC: $14.00
1997 American Chemical Society
a DCP dichlorophenol; TCP, trichlorophenol; TeCP, tetrachlorophenol; PCP, pentachlorophenol. b The wood chips were contaminated with chlorophenols. c Time 0b: after 9 weeks, 1 m3 of heavily contaminated soil was further added to piles 1-3, and 1 m3 of fresh bark was added to piles 1 and 3. d The other five chlorophenol congeners analyzed were under detection limit in all the samples. e Chlorophenols were analyzed as described by Laine and Jørgensen (35, 36).
