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Direct Link between Fluoranthene Biodegradation and the Mobility and Sequestration of its Residues during Aging

^a Laboratoire Sols et Environnement, INPL (ENSAIA)/INRA, BP 172, 2 Avenue de la Forêt de Haye, F-54505 Vandoeuvre-les-Nancy cedex, France
^b CNRSSP, BP 537, F-59505 Douai cedex, France
^c Parc technologique ALATA, F-60550 Verneuil en Halatte, France

^* Corresponding author (sandrine.vessigaud{at}aprona.net).

Received for publication October 31, 2006.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
The aim of this study was to assess the influence of the polycyclic aromatic hydrocarbons (PAH)-degrading activity in the fate of fluoranthene in soils. Three soil samples with different degrading activities (an industrial soil, the same industrial soil after biostimulation, and an agricultural soil) were spiked with ¹⁴C-fluoranthene and incubated for 6 mo with monitoring of biodegradation and mineralization. To follow the distribution of the ¹⁴C-fluoranthene residues (i.e., ¹⁴C-fluoranthene and its degradation products) among the soil compartments, we performed successively leaching, centrifugation (to collect intra-aggregate pore water), solvent extraction, and combustion of the soil columns. In the industrial soil, no mineralization of ¹⁴C-fluoranthene was observed, and only 3% of the initial ¹⁴C-activity was non-extractable (with acetone:dichloromethane) after 165 d of incubation. The biostimulation (addition of unlabeled polycyclic aromatic hydrocarbons) increased the degrading activity in this soil (59% of ¹⁴C-fluoranthene was mineralized) and increased the residues sequestration (13% of ¹⁴C-activity was non-extractable). The microflora of the agricultural soil mineralized ¹⁴C-fluoranthene more slowly and to a lesser extent (25%) than the biostimulated soil, but a higher amount of ¹⁴C-activity was sequestered (41%). Thus, the rate and extent of ¹⁴C-fluoranthene mineralization seemed to be related to the ¹⁴C-activity sequestration by controlling the accumulation of degradation products in the soil. ¹⁴C-Fluoranthene biodegradation enhanced the concentration of ¹⁴C-polar compounds in the intra-aggregate pore water. Our results point out the close link between fluoranthene biodegradation and two key aging processes, diffusion and sequestration, in soils. Biodegradation controls the mobility and sequestration of residues by transforming fluoranthene into more polar molecules that can diffuse into the intra-aggregate pore water and then might become bound to the matrix or entrapped in the microporosity.

Abbreviations: AS, agricultural soil • HOC, hydrophobic organic contaminant • IBS, industrial biostimulated soil • IS, industrial soil • PAH, polycyclic aromatic hydrocarbon • RC, retention capacity

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Polycyclic aromatic hydrocarbons (PAHs), as main constituents of coal tar, are typical soil contaminants at former coal pyrolysis sites. In contrast to such local pollutions, agricultural soils located near traffic roads may exhibit diffuse PAH pollution due to atmospheric deposition (Wilcke, 2000). The toxic and carcinogenic potential of PAHs requires remediation of highly polluted sites. For the last two decades, bioremediation has been the subject of increasing interest (Bossert et al., 1984; Romantschuk et al., 2000; Thiele-Bruhn and Brümmer, 2005). Despite these efforts, there is a need to explain the partly unsatisfactory bioremediation results. Biological treatments often lead to residual PAHs concentrations resisting further degradation (Hatzinger and Alexander, 1995; Sabate et al., 2006). This recalcitrant fraction, which is nonavailable for microbial degradation or solvent extraction, seems to increase with increasing soil–PAH contact time, a phenomenon termed "aging" (Hatzinger and Alexander, 1995; Northcott and Jones, 2001; Semple et al., 2003). Whether this recalcitrant fraction represents an acceptable treatment endpoint requires further investigations of the mechanisms and factors involved in the aging of PAHs and in particular in the sequestration phenomenon.

Two main mechanisms have been proposed to explain the sequestration of hydrophobic organic chemicals (HOC) in soils: (i) absorption or partitioning in the organic matter and (ii) diffusion through intra-aggregate porosity. These concepts have been discussed previously (Brusseau et al., 1991; Hatzinger and Alexander, 1995; Pignatello and Xing, 1996; Richnow et al., 1997; Käcker et al., 2002; Semple et al., 2003). Absorption of HOC in the organic matter corresponds to the diffusion into the macromolecular network of organic matter. This network comprises two distinct regions (Weber et al., 1992): (i) a soft or rubbery region in which the HOC can freely diffuse and (ii) a condensed or glassy region containing rigid cavities (holes) in which the HOC can be trapped (Pignatello, 1998). HOC may also diffuse through intra-aggregate porosity (Ball and Roberts, 1991; Shor et al., 2003), where they are retarded by local sorption on hydrophobic pore walls (Pignatello and Xing, 1996; Nam and Alexander, 1998) and may become entrapped in voids resulting from constricted geometry (Werth and Reinhard, 1997). This intra-aggregate diffusion prevents HOC from being solvent extracted and protects them from microbial predation if they diffuse in pores with diameters smaller than the smallest bacteria (about 1 µm) (Semple et al., 2003).

In terms of risk assessment and water pollution, bioremediation of PAH-polluted sites raises the question of the fate of PAHs degradation products, which are sometimes more toxic than the parent compounds. Beyond its environmental importance, taking PAH degradation products into account provides a more comprehensive view of the fate and behavior of PAHs in soils. Despite the growing number of studies using labeling techniques, experiments considering the fate of degradation products are scarce (Guthrie and Pfaender, 1998; Weigand et al., 2002) and mainly focus on covalent bonds formation (Richnow et al., 1997; Käcker et al., 2002). In a bioremediation framework, where not only the parent PAH but also its degradation products are taken into account, this putative process of covalent bond formation between a metabolite and the solid phase can be considered as a third sequestration mechanism (Kästner et al., 1999). In this case, the term "sequestration" refers to the nonavailability (for solvent extraction and microbial degradation) of the parent compound and of its degradation products, collectively termed hereafter PAH residues.

The microbial activity has been found to influence the sequestration of PAHs in soils (Carmichael et al., 1997; Guthrie and Pfaender, 1998; Kästner et al., 1999; Macleod and Semple, 2003). In particular, the extent of PAH sequestration in nonsterile soils is greater than in sterile soils (Guthrie and Pfaender, 1998; Macleod and Semple, 2003). An explanation of this influence could be the formation of covalent bonds between metabolites and soil organic matter (Käcker et al., 2002). However, in a column experiment with ¹³C-anthracene, Weigand et al. (2002) observed an increase in mobility of labeled products (identified as metabolites) relative to the parent compound. The influence of microbial activity in the mobility and sequestration of PAHs residues is not fully understood (Macleod and Semple, 2003).

The objectives of the present study were to (i) investigate and compare the fate of ¹⁴C-fluoranthene residues (parent fluoranthene and degradation products) in two different soil types (agricultural and industrial soils), fluoranthene being among the most abundant PAH in both soil types (Wilcke, 2000); (ii) provide insight into the influence of the microbial activity on fluoranthene residues mobility and sequestration; and (iii) examine the possible mechanisms involved in residues sequestration, with particular attention to diffusion through intra-aggregate porosity.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Soils
The experiments were performed with three soil samples obtained from two soils: an agricultural soil (AS) and an industrial soil (IS). The AS was sampled in the surface layer of an agricultural field in northeastern France. This soil is a silty clay loam. The IS was sampled at a former coke plant site in northern France. This soil is a silt loam. The main properties of these soils are reported in Table 1. Soil samples were air dried, sieved between 1 and 5 mm, and stored in glass jars in the dark at room temperature before use. The third soil sample is a subsample of the industrial soil, which was spiked with fluorene, phenanthrene, and fluoranthene (the three major PAHs in this soil) 1 yr before the beginning of the experiment. The added concentrations of these PAHs were 180 mg kg^–1, 470 mg kg^–1, and 280 mg kg^–1, respectively. The soil sample underwent humidification/desiccation cycles until the CO₂ release due to the microflora respiration became insignificant. The sample was then air-dried and stored in the dark at room temperature until the beginning of the incubation experiment. This procedure aimed at increasing the microbial activity. This sample is hereafter referred to as industrial biostimulated soil (IBS). The main properties of the three soil samples used in this study are reported in Table 1.

Spiking Procedure and Incubation Experiment
The soils were spiked with ¹⁴C-fluoranthene. A ¹⁴C-fluoranthene methanol solution (270 µL) was added dropwise (drop size about 10 µL) to soil samples (10 g), yielding a contamination of 1.2 µg ¹⁴C-fluoranthene g^–1 corresponding to 10 kBq g^–1. The solvent was evaporated at ambient temperature for 1 h, and each soil sample was thoroughly mixed.

Soil samples were placed in small columns (Whatman Autovials; Whatman PLC, Newton, MA). These columns were 50 mm high and 20 mm internal diameter, had a 0.45-µm glass microfiber filter at the bottom, and were previously described (Guimont et al., 2005; Amellal et al., 2006). The columns were coated with an aluminium foil to prevent direct contact of the soil with the columns' polypropylene surfaces (Amellal et al., 2006). Three replicates were prepared for each soil series at each time point. At the beginning of the incubation experiment, the water content of the soils was adjusted with sterile, distilled, deionized water to 100% of the water retention capacity (RC). Soil columns replicates were placed together (groups of three columns replicates) in hermetic glass reactors (500 mL). The glass reactors were stored at 30°C ± 2°C in the dark. The incubation time varied from 0 to 165 d with seven sampling time points. A flask containing 10 mL of distilled water was placed in each glass reactor to maintain the soil water content at about 100% of the RC.

The microbial activity was monitored by CO₂ production. Over the course of incubations, CO₂ trapping solutions placed in the reactors (10 mL of NaOH 0.5 M) were periodically removed and replaced with fresh solutions. The total organic carbon mineralization (¹²C-Organic Matter, ¹²C-PAHs, and ¹⁴C-fluoranthene) corresponds to the total CO₂ trapped. The CO₂ trapped was measured by titration of the NaOH solution by HCl solution (0.2 M) after a solution of BaCl (20%) was added to stabilize the carbonate precipitates. Thymolphthalein was used as pH indicator. The ¹⁴C-fluoranthene mineralization was assessed by the ¹⁴C-activity measured in the trapping solution.

Sample Collection from Soil Solution and Soil Matrix
At intervals, we performed sequential treatments on the soil columns, including leaching, centrifugation, solvent extraction, and combustion. This procedure aimed at providing a comprehensive map of the distribution of the ¹⁴C-residues in the different soil compartments at each incubation time point. Unless stated otherwise, all radioactive solutions were analyzed for ¹⁴C-activity by mixing aliquots (1 mL) of samples with 10 mL of a scintillation cocktail (Ultima Gold; PerkinElmer, Milano, Italy) and by liquid scintillation counting on a Packard Tri-Carb 1900 CA scintillation counter (Packard Instruments, Meridien, CT). The counting time was 10 min, and quench correction was made by scintillation counter after calibration.

Leachates Collection
At each sampling interval and for each soil, three column replicates (3 x 10 g of soil) were leached to assess the amount of readily available water ¹⁴C-residues. The leaching was performed by percolating 20 mL of a CaCl₂ solution (0.01 M) through each soil column. The percolation rate was gravity driven and regulated with disposable flow control valves to about 0.33 mL min^–1. At the end of the percolation, a gas purge was performed to remove the remaining macroporal water.

Intra-aggregate Pore Water Collection
After percolation, the soil columns were centrifuged at 333 g for 5 min with a J-25 centrifuge fitted with JA-18 rotor (Beckman Instruments, Palo Alto, CA). The soil solution was collected in a sampler vial attached to the base tip of the column. This centrifugation aimed at removing the draining or gravitational water. The water content of the soil columns was reduced to 98 ± 1% of the RC (mean on values for the three soils and the seven sampling time points). After this first centrifugation, high-speed centrifugation (8316 g for 7 min) was performed. During this second centrifugation, the soil solution collected corresponded to water retained in soil by capillary forces. In this study, we considered only results regarding this water, termed intra-aggregate pore water or pore water. The volume of pore water collected (1.14 ± 0.03 mL) was similar for all soils and remained constant during the incubation. ¹⁴C-activity was measured on 0.5-mL samples.

Solvent Extraction
After the leaching and centrifugation procedures, soil samples were air dried and crushed. We performed an exhaustive solvent extraction (Accelerated Solvent Extractor 200; Dionex, Sunnyvale, CA) to assess the maximal amount of solvent extractable ¹⁴C-residues. Soil samples placed in 33-mL cells were extracted twice with a harsh method using acetone:dichloromethane (50:50, v:v) at 100°C, 140 bars for 5 min.

Soil Combustion
After exhaustive extraction, combustion was performed on soil samples to assess the amount of non-extractable ¹⁴C-residues. Aliquots of 0.45 g of soil were mixed with 0.15 g of cellulose powder and burned at 900°C with a model 307 Oxidizer (Packard Instruments). The ¹⁴CO₂ evolved was trapped with 10 mL Carbo-sorb E (PerkinElmer), and the radioactivity was counted after addition of 10 mL of a scintillating solution (Permafluor E⁺; PerkinElmer).

Nature of ¹⁴C-Residues in the Aqueous Samples
To assess the nature of ¹⁴C-residues (i.e., parent compound or degradation product) in the aqueous samples, liquid/liquid extractions were performed on the leachates (about 17.5 mL) and on the intra-aggregate pore water (about 0.64 mL). The volumes of dichloromethane used to extract the apolar ¹⁴C-residues from the leachates and the pore water were 10 and 0.3 mL, respectively. In both cases, three successive extractions were performed, each lasting 3 min. This method provided an extraction efficiency of 99.3 ± 0.1 with fluoranthene in pure water. The ¹⁴C-compounds remaining in the water at the end of the procedure are hereafter referred to as "polar residues." Those dissolved in solvent are referred to as "apolar residues."

Statistical Analysis
The error bars on the graphs represent a confidence interval of 95%. Student's t tests were performed using StatBox 6.40 (Grimmersoft, Paris, France).

	Results and Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Mineralization of ¹⁴C-Fluoranthene
The mineralization started as soon as the soils were humidified, although the soil samples were stored for a long time. All soil samples were biologically active, with CO₂ release at the beginning of the incubation between 10 µg C d^–1 g^–1 of soil (for IS and IBS) and 50 µg C d^–1 g^–1 of soil (for AS) (results not shown). The ¹⁴C-mineralization abilities were different among the three soil samples. The IS exhibited no significant ¹⁴C-fluoranthene (¹⁴CO₂) mineralization during the incubation period (Fig. 1 ). During the same period, the ¹⁴CO₂ released reached 58.9 ± 0.7% of the initial ¹⁴CO₂ associated activity in the industrial biostimulated soil (IBS) and 25.3 ± 6.3% in the AS. The mineralization kinetics and extent were also dramatically different between these soils (Fig. 1). The daily CO₂ production in AS was slower and remained steady after 30 d of incubation, whereas the daily CO₂ production in IBS reached a maximum between 20 and 40 d of incubation and then decreased sharply. The dramatic differences between IS and IBS suggest that the biostimulation procedure was effective. The microflora in IBS seem well adapted to the fluoranthene mineralization, and the rapid and high CO₂ production observed in this soil is consistent with the existence of a fluoranthene-degrading population. The low but constant CO₂ production in AS might indicate a fortuitous metabolism or a cometabolism (Soulas and Lagacherie, 2001; Haws et al., 2006). These metabolisms are often observed in soils with high microbial diversity, such as agricultural soils.

View larger version (18K): [in this window] [in a new window]   Fig. 1. Microflora activity during incubation: daily 14CO2 release (expressed as a percentage of the initial radioactivity). Errors bars represent the 95% confidence interval and are smaller than the symbol for some data points.

View larger version (17K): [in this window] [in a new window]   Fig. 2. Leaching behavior of the 14C-residues during the incubation. Errors bars represent the 95% confidence interval and are smaller than the symbol for some data points.

View larger version (17K): [in this window] [in a new window]   Fig. 4. Evolution of the quantity of 14C-residues in the intra-aggregate pore water over the course of incubation. Errors bars represent the 95% confidence interval and are smaller than the symbol for some data points.

View larger version (19K): [in this window] [in a new window]   Fig. 3. Polar fraction of the 14C-residues in the leachates. Errors bars represent the 95% confidence interval and are smaller than the symbol for some data points.

Intra-aggregate Diffusion
The amounts of ¹⁴C-residues recovered in the intra-aggregate pore water during incubation are presented in Fig. 4 . For IBS, a delay was observed between the maximum of ¹⁴C-residues in pore water relative to the maximum of ¹⁴C-residues in the leachates (Fig. 2). Except for this slight difference, the variations in the ¹⁴C-activity recovered in the pore water with time were similar to those in leachates. Similarly to the residues in the leachates, the ¹⁴C-residues in the pore water were mainly polar compound (Fig. 5 ) and were thus fluoranthene metabolites. These results confirm that ¹⁴C-fluoranthene biodegradation controls ¹⁴C-residues mobility in soils containing a microflora able to mineralize fluoranthene.

View larger version (18K): [in this window] [in a new window]   Fig. 5. Polar fraction of the 14C-residues in the intra-aggregate pore water. Errors bars represent the 95% confidence interval and are smaller than the symbol for some data points.

In IS, we observed a significantly higher percentage (p < 0.05) of polar ¹⁴C-residues in the pore water as compared with the percentage in the leachates at 20 and 40 d of incubation (Fig. 5). This result could indicate a limited or slower diffusion of the apolar fluoranthene toward the intra-aggregate porosity as compared with the diffusion of polar degradation products. A concept of sorption-retarded pore diffusion has been proposed to explain the slow desorption of PAHs from soils or sediments particles (Brusseau et al., 1991; Pignatello and Xing, 1996) on the basis of a better description of PAHs desorption rates in models including this concept. These results provide further indication that the diffusion of PAHs can be retarded by local sorption on pore walls, whereas the diffusion of less hydrophobic degradation products seems not to be affected.

This phenomenon of sorption-retarded pore diffusion was not observed in AS or in IBS. For these soils, the percentage of polar compounds was not significantly different (p < 0.05) between the leachates and the intra-aggregate pore water. The ¹⁴C-fluoranthene biodegradation might be so efficient that the fluoranthene proportion among the ¹⁴C-residues is too low to see any effect of its sorption-retarded pore diffusion.

Sequestration of the ¹⁴C-Residues
The formation of non-extractable ¹⁴C-residues during the incubation is shown in Fig. 6 . As soil-fluoranthene contact time increased, there was a significant increase (p < 0.05) in non-extractable ¹⁴C-residues for all soils. The extent of non-extractable residues formation was highly different among the three soils. After 165 d of incubation, only 2.6 ± 0.8% of the initial ¹⁴C-activity were non-extractable in IS, whereas 12.8 ± 0.8% and 40.8 ± 3.7% were non-extractable in IBS and AS, respectively (Fig. 6). The extent of non-extractable residue in IS was low as compared with other similar incubation experiments, and the extent for IBS and AS agrees well with previous studies (Kästner et al., 1999; Macleod and Semple, 2000). The extraction method used in this study was more exhaustive than usual extraction methods. The proportion of extractable residue could thus have been increased relative to other studies, and the non-extractable residue proportion consequently decreased.

View larger version (17K): [in this window] [in a new window]   Fig. 6. Formation of non-extractable 14C-residues over the course of incubation. Errors bars represent the 95% confidence interval and are smaller than the symbol for some data points.

The influence of partial degradation on the formation of non-extractable residues is further supported by the high amounts of non-extractable residues in AS. This soil, as is generally the case with rural soils, presented a high amount of bacteria (Table 1) and presumably a high bacterial activity (Andreoni et al., 2004). Moreover, its microflora were less adapted to PAH degradation than the industrial soil. These microbial characteristics have led to a high fluoranthene degradation activity but mainly consisted of incomplete degradation and therefore resulted in the accumulation of intermediates. The results on residue mobility presented previously in this article support the idea of an easier intra-aggregate diffusion of degradation products relative to the parent compound. It is therefore attractive to assume that the degradation intermediates that might have been accumulated in AS can more easily diffuse and become sequestered in the intra-aggregate porosity by entrapment or matrix binding. Conversely, in IBS, the adapted and active microflora were able to efficiently mineralize fluoranthene, and thus fewer intermediates could accumulate in this soil, resulting in fewer non-extractable residues. In IS, the microflora were presumably adapted to fluoranthene degradation but were not active enough to produce a relevant quantity of degradation intermediates that could be sequestered in the intra-aggregate porosity. The ¹⁴C-residues in this industrial soil were mainly fluoranthene as a result of the limited biodegradation activity. Moreover, the ¹⁴C-residues in this soil were mainly solvent extractable (Fig. 7 ). These results suggest that the hydrophobic and little mobile fluoranthene remained adsorbed on the soil aggregates all along the incubation. Active microflora seems necessary for the formation of fluoranthene non-extractable residues as defined in this study (non-extractable with a harsh solvent extraction).

View larger version (18K): [in this window] [in a new window]   Fig. 7. Evolution of the quantity of 14C-residues in the solvent extracts over the course of incubation. Errors bars represent the 95% confidence interval and are smaller than the symbol for some data points.

View larger version (17K): [in this window] [in a new window]   Fig. 8. Evolution of the mass balance of 14C-residues over the course of incubation. Errors bars represent the 95% confidence interval and are smaller than the symbol for some data points.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

The fluoranthene partial biodegradation resulting in the accumulation of polar intermediates has therefore two basic effects on the fate of fluoranthene residues. First, ¹⁴C-fluoranthene biodegradation increases the fraction available for leaching. Although this transfer of ¹⁴C-residues is not the main process with regard to the amounts at stake, it might have a great impact on the water pollution risk, depending on the nature of the degradation products. Second, ¹⁴C-fluoranthene biodegradation increases the residues' mobility in the intra-aggregate pore water and increases the sequestration of residues in the soil porosity. In the case of a hydrophobic PAH like fluoranthene, the two key aging processes, diffusion and sequestration, are thus controlled by a third process, the fluoranthene biodegradation activity.

	ACKNOWLEDGMENTS

 
The authors gratefully acknowledge financial support from INERIS, Charbonnage de France, Europe (Interreg III program) and the Walloon region. They also thank Christine Lors and Maxence Verhaeghe for the microbiological analysis, Virginie Serrière and Mélanie Lefebvre for the TOC analysis, and Louis Florentin for the RC measurement.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
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