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a Soil & Water Biochemistry Lab., Dep. of Agronomy, Univ. of Kentucky, N-122 Agricultural Sci. Bldg. North, Lexington, KY 40546-0091 USA
b Univ. of Florida Wetland Biogeochemistry Lab., Soil and Water Sci. Dep., 106 Newell Hall, P.O. Box 110510, Gainesville, FL 32611-0510 USA
edangelo{at}ca.uky.edu
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Abbreviations: A, aqueous concentration • CPs, polychlorinated phenols • DCP, dichlorophenol • EC50, effective concentration • Ka, acid dissociation constant • Kp, linear sorption coefficient • PCA pentachloroanisole • PCP, pentachlorophenol • T, total soil concentration • TCP, trichlorophenol • TeCP, tetrachlorophenol
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Chlorophenols that enter nontarget upland, wetland, and aquatic environments associate with colloidal and particulate matter and, if not photodegraded, eventually settle onto surface soils (Shiu et al., 1994). There they may be biodegraded, depending on whether degrading microorganisms are present and whether appropriate conditions exist for expression of this activity. There is still much controversy about whether the presence of microbial populations or environmental conditions limits degradation of chlorinated toxic organic chemicals (Renner, 1998).
Historically, environmental persistence of PCP and less chlorinated phenols has been attributed to the absence of degrading populations of microorganisms. However, increasing numbers of observations in diverse habitats indicate that transformation potential is widespread, but is manifested only under favorable environmental conditions. Important variables include temperature (Kohring et al., 1989), availability of electron acceptors (Haggblom et al., 1993), electron donors (Kuwatsuka and Igarashi, 1975; Chang et al., 1996), nutrients (Mileski et al., 1988; Schmidt, 1996), and toxic metals (Kuo and Genther, 1996). In wetland and aquatic systems, these properties are often present as gradients resulting in a continuum of microbial activities.
In aerobic soils, transformation of PCP by Flavobacterium and Rhodococcus spp., among others, proceeds though sequential hydroxylation and reductive removal of chlorine substituents yielding poly-hydroxybenzene compounds that are eventually mineralized to CO2 (Uotila et al., 1995; Xun et al., 1992). Several Rhodococcus spp. also methylate PCP resulting in production of volatile PCA (Middelorp et al., 1990). Several species of fungi mineralize PCP via ligninase enzymes (Mileski et al., 1988) and produce extracellular enzymes that polymerize CPs with humic substances (Bhandari et al., 1996; Ruttimann-Johnson and Lamar, 1997).
In anaerobic soils, the pathway for anaerobic transformation of PCP is sequential replacement of chlorines by hydrogen (reductive dechlorination), leading to phenol, benzoate, acetate, CO2 and CH4 (Kuwatsuka and Igarashi, 1975; Zhang and Wiegel, 1990). To date, a few anaerobic isolates having the capacity for reductive dechlorination of PCP have been discovered, and these often gain energy by coupling this process to oxidative phosphorylation (Mohn and Tiedje, 1991; Loffler et al., 1996). The resultant lesser chlorinated products from reductive dechlorination likely function as carbon and energy sources for those aerobic and anaerobic microorganisms involved (Haggblom and Young, 1990).
Predicting the persistence of microbially transformed toxic organic contaminants is currently hindered by limited knowledge of the influence of environmental variables on degradation rates (Hart, 1996). Previous experiments in numerous wetland soils, however, have demonstrated that heterotrophic microbial activities were related to soil properties (D'Angelo and Reddy, 1999), suggesting that similar relationships may also exist for transformation of toxic organic chemicals. The objectives of this study were to (i) determine whether PCP transformers are commonly found in wetland soils, and (ii) identify chemical and biological conditions that promote this activity. This investigation attempts to quantify the boundary conditions for microbial transformations of PCP in soils, including effects of concentration, sorption, electron acceptors and donors, and microbial biomass.
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Materials and methods |
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Soils were collected either under drained or flooded conditions and, hence, were initially at different water contents and redox potentials. To avoid diffusion constraints and development of microsites during transformation experiments, some soils were prepared as slurries with site water. The amount of water used to prepare slurries was arbitrary except to avoid overdilution of solids and microbial biomass. The dry bulk densities of slurries were 0.06 to 0.4 kg L-1 for organic soils and 0.1 to 0.7 kg L-1 for mineral soils. Although there was a wide range in bulk densities, these were largely attributed to differences in organic matter content. Results are presented on a soil dry weight basis unless otherwise indicated.
Pentachlorophenol Toxicity to Aerobic and Anaerobic Soil Microorganisms
Toxicity was evaluated under aerobic and methanogenic conditions in a selected organic soil (W8) and a mineral (TAL) soil using a dose–response approach. Five PCP concentrations between 0 and 3.8 mmol kg-1 were tested for effects on rates of CO2 and CH4 production and PCP transformations over an 8-wk period. Samples of W8 (5 mL) and TAL (10 mL) were transferred to serum bottles (60 mL) and sealed with teflon-lined butyl stoppers and aluminum crimps (Wheaton, Millville, NJ). Anaerobic bottles were purged with O2–free N2. Bottles were incubated in the dark at 28°C on a rotary shaker at 180 rpm. All experiments were conducted in triplicate. Separate EC50 values, defined as the effective concentration of PCP that inhibited microbial activity by 50%, were calculated for the activities of CO2 production, methanogenesis, and PCP transformation in both soils. Effective concentrations were expressed on both PCP concentration dissolved in soil solution (EC50(dissolved), µM) and PCP concentration dissolved plus sorbed to the soil (EC50(total), µmol kg-1).
Influence of Chemical Amendments on Pentachlorophenol Transformation
Before initiation of transformation studies, soil slurries were pre-incubated for 14 d to obtain either aerobic or anaerobic conditions (denitrifying, sulfate-reducing, or methanogenic). For aerobic treatments, slurry (100 mL) was incubated in glass media bottles (500 mL) fitted with teflon-lined caps (Wheaton, Millville, NJ) and opened twice a week to re-aerate the head-space. For anaerobic treatments, slurry (80 mL) was incubated in serum bottles (160 mL) fitted with teflon-lined rubber stoppers and aluminum crimps (Wheaton) and purged with O2–free N2. Different anaerobic electron acceptor treatments were imposed by amending soils with a 30-d supply of a given electron acceptor, calculated from consumption rates determined previously (D'Angelo and Reddy, 1999). Appropriate electron acceptor reducing conditions were confirmed by measuring pore water for loss of NO-3 in denitrifying treatments, loss of SO2-4 in SO2-4–reducing treatments, loss of NH+4 and production of NO-3 and SO2-4 in aerobic treatments, and headspace production of CH4 in the methanogenic treatments. Soils with high amounts of bioavailable Fe (PPP, CR, TAL) were evaluated for PCP transformation under Fe(III)-reducing conditions instead of SO2-4–reducing conditions. Iron(III) solution was prepared as described by Ghiorse (1994). All bottles were incubated horizontally with shaking. Preliminary experiments showed that this shaking protocol did not influence methanogenic activity, and previous work found it to be optimal for aerobic assays (Stark, 1996). All experiments were conducted in triplicate.
After appropriate reducing conditions were attained, slurries were spiked with PCP from a stock solution prepared in 0.05 M NaOH to give an average final PCP concentration of 0.66 mmol kg-1 dry soil. This PCP concentration was chosen because it is the median level measured at contaminated sites in the USA (ATSDR, 1998). Both aerobic and methanogenic sterile controls were included for the most biologically active soil (HLPI), and these were prepared by adding HgCl2 to a final concentration of 2% (soil dry weight basis) alone or with autoclaving at 121°C at 0.1 MPa for 1 h on three consecutive days. Sample sterility was confirmed by monitoring CO2 and methane production during the experiment.
On Days 1, 3, 6, 10, 15, 20, 25, and 30, PCP and transformation intermediates were extracted from slurries (1–3 mL) with 6 mL acetonitrile containing 0.36 M H2SO4 for 16 h on a reciprocal shaker. After centrifugation (700 x g), crude extracts were stored in amber glass vials with teflon-lined caps at 4°C before derivitization and analysis. Preliminary spike recovery experiments showed this procedure yielded >95% recovery of mixed CPs from peat and mineral soils. Similar approaches for extraction of pesticides from soil matrices have been described (Chang et al., 1996).
Additional experiments were conducted to determine whether additions of nutrients, vitamins, and electron donors could promote the methanogenic transformation of PCP in a soil that previously lacked this capacity. The protocol used was similar to that employed to examine transformations under different electron acceptor reducing conditions, except a lower PCP concentration was used (0.13 mmol PCP kg-1 equivalent to 5.8 µM in the dissolved phase) to avoid PCP toxicity. The following deoxygenated solutions (1 mL) were added to separate glass tubes (27 mL) containing PPP soil slurry (3 mL) to provide the following 16 treatments: water (control), inorganic nutrients (Owens et al., 1979), inorganic nutrients + vitamins (Owens et al., 1979), catechol, benzoate, casein, yeast extract, peptone, glucose, sucrose, maleic acid, fructose, maltose, acetic acid, ethanol, and propionate. Hydrogen (5 mL) was added to separate tubes (final concentration of 25 kPa). All carbon-based electron donor treatments were added at concentrations of 88 mmol C kg-1 soil and included amendments with inorganic nutrients + vitamins. All treatments were conducted in triplicate and, at the end of 5 wk, PCP and transformation intermediates were extracted from soils as described above.
Analytical Methods
Dissolved SO2-4 and NO-3 concentrations were determined by analyzing pore water with a Dionex 4500i ion chromatograph (Sunnyvale, CA). Headspace gases were measured using gas chromatographs equipped with detectors employing flame ionization for CH4 and thermal conductivity for O2 and CO2. Carbon in living, nonresting microbial biomass was estimated using the substrate-induced respiration technique, as described previously (D'Angelo and Reddy, 1999).
Pentachlorophenol and transformation intermediates present in extracts were prepared as acetylated derivatives and analyzed using a gas chromatograph equipped with a 63Ni electron capture detector (Nicholsen et al., 1992). The identity and concentration of PCP and transformation intermediates were determined by comparing retention times of derivitized authentic standards of the highest purity available (>98%) (Supelco, Bellefonte, PA, and Ultra Scientific, Hope, RI).
Data Analysis
Transformation rates of PCP and intermediate metabolites were described by zero-order kinetics (µmol kg-1 d-1) determined by linear regression analysis. Since experiments were initiated with high concentrations of PCP under well mixed conditions, these rates may be considered maximum velocity rates, or potentials. As experiments progressed, PCP concentrations decreased, and transformation rates became a function of PCP concentrations. Hence, first order (d-1) rates were also determined from nonlinear regression analysis. Rate constants were calculated from data generated after any lag period. The length of the lag period was defined as the period before a significant (P 
0.05) decrease in concentration was observed between successive time increments, as determined by t statistic.
In separate batch isotherm experiments, linear sorption coefficients (Kp, L kg-1) for PCP were determined for all soils examined (D'Angelo, 1998). These were then used to convert total PCP concentration (T, mmol kg-1) to aqueous concentrations (A, mmol L-1):
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is the volumetric water content (L water L-1 soil) and 
b is dry bulk density (kg L-1) of soil slurry. |
Results |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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0.38 mmol PCP kg-1 soil. In contrast, CO2 production and PCP transformation were relatively unaffected at any PCP concentrations tested under aerobic conditions. Therefore, EC50 for aerobic samples could not be calculated. However, based on the maximum concentration tested, the EC50(total) (PCP concentration on a dry soil basis) was >1.9 mmol kg-1 soil for aerobic activities compared to between 0.5 to 0.7 mmol kg-1 soil for methanogenic populations (Table 3)
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Influence of Chemical Amendments on PCP Transformation
The presence or absence of specific types of electron acceptors had significant effects on rates and mechanisms of PCP transformation in wetland soils (Fig. 2 and 3
; Tables 4–6)
. Transformation was attributed to biological activity since HgCl2 + autoclaved samples showed no changes in PCP concentration during the study (Fig. 2). Except for two soils (PPP and CR), PCP transformation occurred in each soil under at least one electron acceptor-reducing condition.
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Under denitrifying conditions, only LSM transformed PCP, as indicated by accumulation of trace amounts of the reductive dechlorination product 2,3,4,5-TeCP (Fig. 2 and 3; Table 5). Of those soils tested under Fe (III)-reducing conditions (PPP, TAL, and CR), none showed transformation of PCP (Table 5). Under SO2-4–reducing conditions, three organic soils (W2, LSM, and LAAF) showed loss of PCP. After lag periods of between 1 and 15 d, PCP was transformed at maximum rates between 23 and 40 µmol kg-1 d-1 and first-order rates between 0.065 and 0.29 d-1. The dominant mechanism of PCP transformation was sequential reductive dechlorination to tetrachlorophenol (TeCP), trichlorophenol (TCP), and dichlorophenol (DCP), which occurred primarily through the pathway PCP
2,3,4,5-TeCP3,4,5-TCP 3,5-DCP, demonstrating a preference for removal of chlorines ortho- and para- to the hydroxyl group. The agricultural LAAF soil exhibited more complex dechlorination pathways, in which 2,3,5,6-TeCP, 2,3,4,6-TeCP, and 2,3,5-TCP were also detected, indicating sequential para-, ortho-, as well as meta- dechlorination pathways. After 30 d, losses of PCP and breakdown intermediates were between 13 and 69% for these three soils (Table 5). Monochlorophenols were not detected in any of the samples.
Under methanogenic conditions, six organic soils transformed PCP, but none of the mineral soils did so (Fig. 2 and 3; Table 6). For the transforming soils, there was a lag phase of between 1 and 13 d before loss of PCP, after which maximum rates were found to be between 25 and 70 µmol kg-1 d-1 and first-order rates between 0.082 and 0.39 d-1. As observed under SO2-4–reducing conditions, the dominant mechanism for transformation was reductive dechlorination, which usually followed the sequence PCP2,3,4,5-TeCP3,4,5-TCP3,5-DCP. Again, LAAF exhibited more complex dechlorination pathways than other soils, including PCP2,3,5,6-TeCP2,3,5-TCP, 2,3,6-TCP3,5-DCP and PCP2,3,4,6-TeCP2,3,6-TCP. The sum of losses of PCP and intermediates ranged between none and 99% in 30 d (Table 6).
Rate constants for PCP transformation intermediates were determined by summing the concentrations for a given intermediate and all preceding products at each time step, and calculating the least squares fit through the points (Tables 5 and 6). For soils capable of PCP transformation under SO2-4–reducing conditions, the rate-limiting steps in overall CP loss were reductive dechlorination of TeCP, TCP, and DCP, as indicated by accumulation of these intermediates compared to the parent compound PCP. For example in LSM soil, PCP was almost completely transformed to 2,3,4,5-TeCP, which was not reduced further (Table 5). Under methanogenic conditions, most soils (except LSM) demonstrated similar dechlorination rates for CPs containing high and low numbers of chlorine substituents (Table 6).
In PPP soil maintained under methanogenic conditions and amended with nutrients, vitamins, and electron donors, PCP transformation was only observed in the water-alone control (8.1%), yeast extract (34%), peptone (37%), and acetate (5.1%) treatments (data not shown). The dominant pathway was through reductive dechlorination yielding 2,3,4,5-TeCP and 3,4,5-TCP; however, no net loss of CPs was observed.
Relationships between PCP Transformation Kinetics and Soil Properties
Regression analysis showed that none of the measured soil properties was significantly (P 0.05) correlated to the duration of the lag periods before detection of PCP transformation. Moreover, none of the measured soil properties was significantly correlated to aerobic PCP transformation rates (Table 7)
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Discussion |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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High (>10 µM) and low (<0.3 µM) PCP concentrations typically restricted transformation in soils. This inhibition was attributed to toxicity and limited bioavailability, respectively. Pentachlorophenol toxicity results from its influence on energy transduction processes carried out by the cell (Escher et al., 1996). These effects were reflected as decreased rates of CO2 production, methanogenesis, and PCP transformation.
Comparing EC50(total) values, PCP was apparently more toxic to methanogenic activities compared to aerobic activities and in mineral soils compared to organic soils. These results can at least be partially explained by differences in sorption, which was greater under aerobic conditions and in organic soils (Tables 3, 4, and 6). For example, sorption is known to provide a protective mechanism by removing PCP from the soluble (bioavailable) pool (Apajalahti and Salkinoja-Salonen, 1984). Sorption of chlorophenols is increased in soils with high concentrations of H+ (pKa of PCP = 4.74), dissolved cations, and soil organic matter (Westall et al., 1985; D'Angelo, 1998). Aerobic processes such as nitrification and sulfide oxidation tended to acidify soils, while anaerobic activities produced higher pH values (Table 2). Hence, it was expected that PCP transformations would proceed at higher concentrations in aerobic and organic soils which matched with experimental results (Table 3).
Using Eq. [1], average concentrations of PCP dissolved in soil solution ranged between 0.4 and 238 µM (Tables 4 and 6). The EC50(dissolved) for PCP transformation (i.e., concentration of dissolved PCP that decreased PCP transformation by 50%) was 10 to 14 µM for methanogenic treatments (Table 2). The EC50(dissolved) for aerobic treatments could not be calculated because inhibition was not observed at any concentration tested (up to 6 µM for the mineral soil and 23 µM for the organic soil). These results suggest that aerobic microorganisms were less affected by PCP level than methanogenic consortia. These results generally agree with others, with reported toxicity values ranging from 15 to 1900 µM for aerobes (Mileski et al., 1988; Stanlake and Finn, 1982) and 0.45 to 10 µM for anaerobes (Mohn and Kennedy, 1992; Wu et al., 1993; Uberoi and Bhattacharya, 1997).
Knowledge of toxicity threshold concentrations are key to predicting the transformation potential of PCP in soils. For example, in the electron acceptor study, PCP concentrations in the methanogenic mineral soils PPP (50 µM), CR (159 µM), and TAL (19 µM) were higher than the threshold EC50(dissolved) (µM), which explains why these soils did not transform PCP. In comparison, the TAL and PPP soils transformed PCP in the toxicity and electron donor studies when concentrations were below threshold levels.
Transformation under both aerobic and anaerobic conditions was also restricted at concentrations of PCP < 0.3 µM, at which PCP transformation rates became first order (Fig. 2d, 3a, and 3d). At low concentrations, slow desorption kinetics plays a major role in regulating transformation rates (Schlebaum et al., 1998). Moreover, in soils contaminated for long periods, PCP and intermediates may become less bioavailable as they diffuse into microbially inaccessible soil zones. Therefore rates in the present study, in which experiments were conducted in freshly contaminated soils, may overestimate actual rates. Our results and those of others (Apajalahti and Salkinoja-Salonen, 1984; Bellin et al., 1990 Mileski et al., 1988; Mohn and Kennedy, 1992), clearly show the influence of contaminant concentration on transformation of toxic organics in soils, and the role of sorption in regulating contaminant bioavailability. Sorption of PCP may be controlled by manipulating pH, types of microbial activity (e.g., aerobic and anaerobic), and organic C content in contaminated soils.
Availability of electron acceptors was also a key regulator of both rates and pathways of PCP transformation in soils. In the presence of O2, most soils produced pentachloroanisole (PCA) within 1 d after PCP treatment. Methylation is mediated by common aerobic bacteria and fungi such as Rhodococcus rhodochrous, Phanerochaete chrysosporium, and P. sordida (Middelorp et al., 1990; Lamar et al., 1990). Subsequently, PCP and PCA were lost from many soils without the appearance of chlorinated intermediates, indicating either mineralization to CO2 or chemical binding–attachment with other pesticide moieties or humic substances. Assuming degradation was the dominant mechanism, the maximum PCP loss rates of up to 77 µmol kg-1 d-1 observed in this study are typically higher than those measured previously (Briglia et al., 1994; Haggblom and Valo, 1995; Laine and Jorgensen, 1997). Higher rates in the present study perhaps reflected an exclusion of diffusion constraints by constant shaking.
Among the soil properties measured, none was significantly correlated to aerobic transformation rates, indicating that availability of C, inorganic N (ranging between 1330 and 9770 µM), and soluble P (ranging between 3 and 488 µM) were not the primary regulators in the soils tested (Table 7). Schmidt (1996), however, found a highly significant correlation between aerobic transformation rates in PCP-contaminated groundwater and soluble P between 8 and 90 µM. As observed in our study, Laine and Jorgensen (1997) also found no correlation between bacterial biomass and aerobic PCP transformation rates in pilot-scale bioremediation efforts in Finland.
Under anaerobic conditions, but not under aerobic conditions, NO-3, Fe(III), and SO2-4 inhibited PCP transformation. For most soils, lack of transformation under intermediate reducing conditions was probably not due to toxicity, since both NO-3 and SO2-4 were consumed in anaerobic treatments (Fig. 2 and 3). These results are in accordance with the paradigm that denitrifiers, Fe(III)-reducers, and SO2-4–reducers outcompeted dehalogenators for common electron-donating substrates (Chang et al., 1996; Fennel and Gossett, 1998). The co-occurrence of reductive dechlorination and SO2-4 reduction in LAAF, W2, and LSM soils, however, may reflect the ability of some dehalogenators to compete effectively with SO2-4–reducers for electron equivalents. The specific identity of electron donors required for reductive dechlorination of PCP, and comparisons of affinity constants between anaerobic microbial groups, have yet to be determined.
Under methanogenic conditions, PCP transformation in eight soils proceeded via reductive dechlorination, in which electrons derived from decomposition of organic matter replaced Cl- atoms of PCP. One can only speculate about the identity of microbial species and enzymes responsible for anaerobic PCP transformation in this study. However, enhanced transformation in PPP soil in response to the addition of the protein-based electron donors yeast extract and peptone indicated the involvement of proteolytic and amino-acid fermenting bacteria. Clostridium-like species have previously been implicated in dechlorination of CPs (Zhang and Wiegel, 1990; Madsen and Licht, 1992). These results indicate that addition of proteinaceous compounds may be a viable strategy to enrich for PCP-transforming microorganisms and to bioremediate chlorophenol-contaminated soils.
The lag time of 1 to 6 d observed prior to initiation of dehalogenation was not significantly correlated to any of the soil properties. Lag times have been observed for other microbial activities, including denitrification, SO2-4–reduction, and methanogenesis (D'Angelo and Reddy, 1999), and reductive dechlorination (Linkfield et al., 1989). Lag times may be explained by limiting environmental (electron donors and redox potential) or biological conditions (populations and induction and synthesis of enzyme systems) (Linkfield et al., 1989). In our study, however, the latter likely predominated, in view of the fact that soils were preincubated under desired reducing conditions well before PCP amendment to the soils.
Maximum PCP transformation rates under methanogenic conditions approached 70 µmol kg-1 d-1, which is higher than rates reported for many other soils (Kuwatsuka and Igarashi, 1975; Chang et al., 1996; Mikesell and Boyd, 1988). However, these reported rates are lower than that observed for a methanogenic PCP-acclimated consortia in bioreactors that transformed 10 µM PCP at 44000 µmol kg-1 d-1 (Wu et al., 1993).
Most soils that transformed PCP under SO2-4–reducing and methanogenic conditions showed preferential ortho and para dechlorination, resulting in the production of 2,3,4,5-TeCP, 3,4,5-TCP and 3,5-DCP. This pathway has been shown to be common in unacclimated microbial communities (Nicholsen et al., 1992). In the agricultural LAAF soil, however, additional meta dechorination pathways were observed, which confirmed the pathways previously observed in anaerobic soils (Kuwatsuka and Igarashi, 1975; Murthy et al., 1979). Recent discovery of toxaphene-, dieldrin-, and DDT-contaminated soils in this former agricultural field (SJRWMD, 1999) suggests the occurrence of cross-acclimation by PCP dechlorinators.
Several soil properties were highly correlated to PCP transformation rates under methanogenic conditions, including total C, N, and P, microbial C, aerobic and anaerobic C mineralization rates, and bioavailable N (Table 7). Kuwatsuka and Igarashi (1975) also observed high correlations with soil organic matter. Microbial C showed the highest correlation probably because it integrated many of the regulators of transformation (e.g., types and amounts of electron donors and enzyme systems) into one measurement. Also, since bacteria are largely composed of protein, it is plausible that dead microbial cells served as electron donors for reductive dechlorination by the degrading populations. While addition of nutrients and vitamins did not enhance PCP transformation in PPP soil under methanogenic conditions, protein-based donors did, demonstrating the primary role that electron donors and microorganisms play in regulating reductive dechlorination.
Methanogenic soils treated with 2% HgCl2 plus autoclaving did not show reductive dechlorination, indicating a requirement for biological activity. In the absence of autoclaving, however, reductive dechlorination proceeded, albeit at reduced rates, suggesting that dechlorination activity can be highly resistant to Hg (II) (Fig. 2). In contrast to methanogenic soils, PCP transformation under aerobic conditions was completely inhibited in Hg (II) treatments with and without autoclaving, indicating a higher microbial sensitivity to Hg (II). Thus co-contamination with Hg and possibly other heavy metals may be an important consideration when formulating remediation protocols.
In conclusion, this study has demonstrated the widespread geographic distribution of microorganisms capable of PCP transformation, even in systems with no known history of contamination. In addition, this study has shown the extent to which PCP transformations are regulated by selected factors, including PCP concentration, types of electron acceptors and donors, and microbial biomass. The relationships between these properties and transformation processes provided in this study may be useful in predicting environmental persistence as a function of site specific conditions, as well as provide insight about potential impediments to in situ transformation. Future studies should focus on identifying microbial species involved in transformation, and determining how chemical and biological factors influence their transformation of PCP and other toxic organics in soils.Middeldorp Briglia Salkinoja-Salonen 1990; Nicholson Woods Istok Peek 1992
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ACKNOWLEDGMENTS |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Received for publication March 11, 1999.
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