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Hydrological Sciences -Journal- des Sciences Hydrologiques,40,4, August 1995

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The effect of an anionic surfactant on the mobilization and biodégradation of PAHs in a creosote-contaminated soil L. DESCHÊNES, P. LAFRANCE & J. P. VILLENEUVE INRS-Eau, Université du Québec, 2800 rue Einstein, CP 7500, Sainte-Foy, Québec, Canada, G1V 4C7

R. SAMSON Department of Chemical Engineering, BIOPRO Research Center, Ecole Polytechnique, University of Montreal, CP 6079, suce. Centre-ville, Montréal, Québec, Canada, H3C 3A7 Abstract The mobilization and biodégradation of 13 PAHs sorbed in a creosote-contaminated soil were assessed in the presence of sodium dodecyl sulphate (SDS). In a mobilization experiment, the soil was mixed with SDS solutions (0.005 to 1 % w/v) and the PAH concentrations in the aqueous phase were determined. In a biodégradation experiment, soil residual PAH concentrations were monitored during 45 weeks in the presence of 10, 100 and 500 mg kg"1 SDS. The SDS was effective in mobilizing three- and four-ring PAHs. Increasing SDS concentration resulted in the mobilization of very low water-soluble five- and six-ring PAHs. The SDS (100 and~ 500 mg kg 4 ) significantly decreased the biodégradation of fluorene, phenanthrene and all of the four-ring PAHs. PAHs with more than four rings were not biodegraded. The surfactant [a14C]SDS was readily biodegraded. Even if SDS may be efficient in mobilizing PAHs in soil pore water or groundwater, it will not improve PAH biodégradation. Such a result must be considered when using anionic surfactants in the perspective of biological treatment of PAHs.

Effet d'un agent tensio-actif anionique sur la mobilisation et la biodégradation de HAP présents dans un sol contaminé par la créosote Résumé La mobilisation et la biodégradation de 13 HAP adsorbés sur un sol contaminé par la créosote ont été étudiées en présence de dodecyl sulphate de sodium (DSS). Pour la mobilisation, le sol a été mélangé avec des solutions de DSS (0.005 à 1 % p/v) et la concentration des HAP a été mesurée en phase aqueuse. Pour la biodégradation, les concentrations résiduelles de HAP dans le sol ont été mesurées durant 45 semaines en présence de 10, 100 et 500 mg kg"1 DSS. Le DSS a été efficace pour mobiliser des HAP de trois et de quatre anneaux. Une augmentation de la concentration en DSS se traduit par la mobilisation de HAP de cinq et de six anneaux qui sont normalement très peu solubles dans l'eau. Le DSS (100 et 500 mg kg"1) diminue de façon significative la biodégradation du fluorène, du phénanthrène et de tous les HAP de quatre anneaux. Les HAP de plus de quatre anneaux n'ont pas été biodégradés. Le tensio-actif [a 14 e]DSS a été aisément dégradé. Le DSS pourrait être efficace pour mobiliser les HAP dans l'eau du sol:

Open for discussion until I February 1996

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L, Deschênes et al. cependant, il ne faciliterait pas la biodégradation de ces HAP. Ce résultat doit être considéré lorsqu'un tensio-actif anionique est utilisé en vue du traitement biologique des HAP.

INTRODUCTION In recent years, several restoration technologies have been proposed to remediate contaminated soils that affect groundwater quality. Among these, biological remediation techniques constitute an attractive and cost-effective alternative (Heitzer & Sayler, 1993). However, the efficiency of the biological treatment of soils containing hydrophobic organic contaminants is highly dependent on contaminant bioavailability. In fact, low water-soluble contaminants such as Polycyclic Aromatic Hydrocarbons (PAHs) tend to sorb strongly to soil and this property makes them difficult to biodegrade. The use of surface active agents (surfactants) to increase the mobilization and possibly the biodégradation of PAHs is thought to be a solution to the problem. Surfactant soil washing may constitute an in situ remedial action procedure to mobilize the sorbed PAHs and to facilitate their removal by pumping groundwater. If not carefully applied, the use of surfactants in soil remediation has many disadvantages such as potential toxicity for soil biomass, adsorption and precipitation of the surfactant and the possible spreading of desorbed contaminants into the groundwater (West & Harwell, 1992). The results of recent works (Abdul et ah, 1990, 1992) suggest that surfactant washing is promising for the remediation of soil containing hydrophobic contaminants. One of the most common anionic surfactants, sodium dodecyl sulphate (SDS) has been shown to be effective in removing a PAH (anthracene) from a contaminated sand (Dipak et al., 1994). SDS has also been use in a pilot scale surfactant washing technology (Clarke et al., 1992). In the perspective of biological treatment, some recent studies have focused on the effects of surfactants on the biodégradation of contaminants like PAHs (Bury & Miller, 1993; Laha & Luthy, 1991, 1992). A study conducted by Tiehm (1994) showed that SDS increased the water solubility of a crystalline PAH (phenanthrene) but also inhibited its biodégradation by a mixed culture adapted to the PAH. It has been suggested that surfactants at low concentrations may be useful for in situ bioremediation of sites (Aronstein et al., 1991, Aronstein & Alexander, 1993) but there has been insufficient study of this in soils. Unfortunately, in most studies, radio-labelled or other freshly added compounds were used and few were done using aged contaminated soils. In fact, the PAHs freshly added in reconstituted systems may not present the same bioavailability as in aged soil in which the PAHs are expected to be strongly sorbed. The weathered residues of PAHs originating from aged contamination, as opposed to freshly added PAHs, have been shown to be more resistant to leaching and to microbial degradation (Weissenfels et al., 1992). The efficiency of SDS in the removal of weathered PAHs residues must

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be determined before it can be considered for in situ or above-ground extraction and bioremediation of contaminated soils. The purpose of this study was to verify the effect of SDS on the mobilization and biodégradation of 13 PAHs sorbed on an aged creosote-contaminated soil. In this study, the term mobilization refers to the increase in aqueous phase concentration (apparent solubility) of PAHs as a result of the removal (e.g. incorporation of PAHs within surfactant micelles) of soil sorbed or deposited compounds.

MATERIALS AND METHODS Soil description and characterization The contaminated soil containing weathered PAH residues came from a woodpreserving industrial site in Delson, Québec (Canada). The soil was chosen for its representativeness of aged contamination by creosote but was not characterized for its hydrological properties since no remediation procedure was undertaken at the site. A bulk sample (20 kg) was collected (5-30 cm depth) at the exit of a creosote autoclaving unit where the dripping of treated wood had occurred over approximately 20 years. The site permitted the obtention of presumably highly contaminated soil as required for the monitoring of residue biodégradation. The top of the soil surface (0-5 cm) was discarded since its biological properties and microfloral composition may have differed from that of the unsaturated zone as a result of light exposure and greater variations in soil transient properties (temperature and water content). The soil was homogenized, sieved at 2 mm and kept at 4°C in the dark until use. The PAH concentrations were greater than the Quebec criteria C (Table 1) which represents a highly contaminated soil (MENVIQ, 1988) unsuitable for residential, commercial or industrial use. As determined by the hydrometer method (Gee & Bauder, 1982), the sandy loam soil contained 13% clay, 16% silt and 71% sand. The water content at field capacity was 18.5% as measured by a pressure membrane extractor (Soilmoisture Equipment Corp. Model 1000) operating at 30 kPa. The soil pH was 7.5. Soluble inorganic constituents were extracted (10 g soil with 25 ml of KC1 2 g H) and assayed by HPLC (Spectra-Physics Model SP8100) using a conductivity detector (Waters Model 430). The sodium and potassium content of the soil were 40 mg kg"1 and 1540 mg kg"1, respectively. The contents in sulphate and chloride were 110 mg kg"1 and 2090 mg kg"1, respectively. The orthophosphate ( < 12 mg kg"1), nitrate ( < 19 mg kg"1) and ammonium ( < 6 mg kg"1) were non-detectable. The C/N ratio was found to be 90 as determined using a CHN analyser (Control Equipment Corp. Model 240XA). In face of the low nitrogen and phosphorous levels, a mineral salt medium or MSM (Greer et al., 1990) was added in microcosms used in biodégradation experiments to provide required nutrients to the soil biomass.

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For the soil characterization (Table 1), PAHs from dried soil were Sohxlet extracted for 32 h using methylene chloride. The extract was then concentrated by the Kuderna-Danish method to avoid loss of the volatile compounds. A fraction of the concentrate was cleaned on a silica gel column (250 mm x 10 mm). The silica gel (100-200 mesh) was previously activated at 105 °C for 16 h. The mineral oil and greases fraction was first eluted by 35 ml hexane, PAHs were thereupon eluted with 75 ml hexane containing 20% methylene chloride and PCP was then eluted with 35 ml methanol. PAHs and PCP were analysed using GC/MS (see PAH analysis below). Hexane containing mineral oil and greases was evaporated and a known volume of reagent grade CFC-113 (trichloro-l,l,2-trifluoro-l,2,2-ethane) was added: mineral oil and greases were then determined by infrared spectrophotometry.

Table 1 Chemical characterization of the creosote-contaminated soil PAH naphthalene acenaphthylene acenaphthene fluorene phenanthrene anthracene fluoranthene pyrene benzo(a)anthracene chrysene benzo(b)fluoranthene benzo(k)fluoranthene benzo(a)pyrene indeno(l ,2,3-cd)pyrene dibenzo(a,h)anthracene benzo(g,h,i)perylene total PAHs pentachlorophenol mineral oil & greases

Concentration (mg kg'1)

Quebec criterion (mg kg 4 )

114 17 247 181 700 126 705 485 128 127 80 48 46 24 3 13

50 100 100 100 50 100 100 100 10 10 10 10 10 10 10 10

3042

200

103

5

4843

5000

Chemicals [a14C]SDS (35.5 mCi mmol"1, purity >98%) was purchased from Sigma (St Louis, MO, USA). All solvents were pesticide grade (purity > 99.7%) and common chemicals were reagent grade. The SDS purity was >99% (Boehringer, Mannheim, Germany). The standard 4-fluorobiphenyl (Riedel-DeHaen, Germany), p-terphenyl (Aldrich Chem. Co., Milwaukee, WI, USA) and 1,2,3-trichlorobenzene (Aldrich, Milwaukee, WI, USA) were >99% in purity.

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Mobilization experiment The mobilization of PAHs contained in the creosote-contaminated soil was assessed in the presence of 0, 0.005, 0.01, 0.05, 0.075, 0.1, 0.25, 0.75, 0.5 and 1% (w/v) of SDS. Each set of experiments was performed in triplicate. This range of concentration was chosen since it includes the critical micellar concentration (CMC) of SDS which is approximately 0.23% (Clarke et al., 1992). Indeed, many studies (Dipak et al., 1994) have reported a substantial increase of the solubilization of hydrophobic organic contaminant(s) for SDS concentrations above the CMC. The mobilization was performed following an adapted procedure of the USEPA Method 1312 which is used to test the leaching of contaminants from soil (Chiang et al., 1989). For each experiment a soil sample (100 g) was mixed with 220 ml of SDS solution in an "end over end" shaker for a period of 16 h. The leachate was then extracted using a filtration system (Millipore). PAHs in the liquid phase were extracted using methylene chloride (liquid-liquid extraction) and then quantified by GC/MS (see PAH analysis below). Biodégradation experiment Addition rate of SDS To determine the addition rate necessary to maintain sufficient SDS concentrations in the soil microcosms, SDS biodegradation experiments were performed in 125 ml glass serum bottles equipped with a C0 2 trap (5 ml glass tube filled with 1 ml KOH 1 mol 1"1). Twenty grams of moist soil were added to the microcosms. Other studies (Loehr, 1992) reported that the range of water content for which biodégradation of organic contaminants in soil is optimum is 30-50% to 80-90% of soil field capacity. In this study, the water content was then adjusted to 80% of soil field capacity with mineral salt medium (MSM) solutions containing the [a14C]SDS (100 000 dpm per microcosm) and the appropriate unlabelled SDS amount to give final concentrations of 10, 100 and 500 mg kg"1 of SDS. The exact activity of each aqueous stock solution was determined using a scintillation counter before their addition to microcosms. Microcosms were immediately sealed with a crimp seal Teflon valve (type Mininert, Supelco Inc., Bellefonte, PA, USA) and were incubated at 20°C in the dark. For each set of experiments, abiotic controls, in which 0.02% sodium azide was added, were set up to assess any abiotic loss of the [a14C]SDS. Periodically, KOH containing the trapped 14 C0 2 was removed for analysis and replaced by fresh alkali. KOH samples (2 ml) mixed with 18 ml liquid scintillation cocktail (ACS scintillation cocktail, Amersham) were counted for trapped 14 C0 2 with a liquid scintillation counter (Packard Tri-Card Model 4530, Packard Instrument Co. Inc.) using the appropriate quenching curve. Biodégradation of PAHs in the contaminated soil To study the effect of SDS on PAH biodégradation, 1 litre glass jars with Teflon covers were

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used. A total of 12 microcosms was prepared to study four different treatments in triplicate. The four treatments were: (a) soil without SDS, (b) soil with 10 mg kg"1 SDS, (c) soil with 100 mg kg 1 SDS and (d) soil with 500 mg kg-1 SDS. For each soil microcosm (350 g), the water content was adjusted to 80% of soil field capacity with sterile MSM solutions in which the appropriate amount of the SDS had been previously dissolved. Abiotic controls were made by addition of 0.02% sodium azide. Each week, the treated soil was mixed to enhance oxygen availability, and at two weeks intervals the previously selected amounts of surfactants were added in an appropriate MSM volume to maintain the water content at 80% of soil field capacity. Typically, the soil oxygen consumption was 750 mg 0 2 kg"1 in the first seven days of incubation, as determined independently on samples (200 g) freshly amended with MSM solution and using an electrolytic respirometer (Bioscience Management ER-100 Respirometer, Bethlemem, PA, USA). It was verified that the weekly renewal of the microcosm atmosphere (75 % v/v) supplied sufficient oxygen to satisfy the soil oxygen consumption, and thus no anaerobic conditions were assumed to have occurred in the microcosms. Incubation was at 20°C in the dark for a period of 45 weeks. Soil was periodically sampled for chemical analysis. At the end (45 weeks) of the experiments, total heterotrophic bacteria was found to have increased typically from 5.6 x 105 to 1.2 x 108 CFU g"1. Although these results indicated appropriate conditions for the growth of carbon-utilizing microorganisms, no attempt was made to characterize further the soil microflora. Statistical analysis PAH biodégradation in treatments with and without SDS were evaluated at the 95 % confidence level by an analysis of covariance (ANCOVA) using computer software packages (Statistical Analysis System Institute, 1989). The statistical analyses were performed on the logarithmically transformed individual data obtained during 11 weeks for the three-ring PAHs and during 45 weeks for the four-ring PAHs. Analytical methods Liquid-liquid PAH extraction Nine grams of NaCl and 45 ml of the extracted aqueous phases were added to 60 ml glass serum bottles. A recovery standard (4-fluorobiphenyl) was added and results were corrected for the recovery efficiency. Each bottle was then sealed with a crimp seal Teflon valve and agitated at 400 rpm for 10 min. With a syringe, 3 ml of methylene chloride was added and the bottles were agitated for 20 min (400 rpm). Samples were centrifuged at 2000 rpm for 10 min. Using a syringe, 1 ml of the solvent was taken for PAH analysis. Soil PAH extraction Residual soil PAHs were extracted as follows: 8 g samples were dried by mixing with 12 g anhydrous sodium sulphate and recovery standards (4-fluorobiphenyl and p-terphenyl) were added to the soil.

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All

The soil was ground with a mortar to obtain a fine dried powder and was put in 50 ml glass tubes with Teflon-lined screw caps. Twenty ml of methylene chloride were added and the tubes were shaken for 4 h on a Wrist Action shaker at full speed. The tubes were then left under the hood for 20 min in order to allow sedimentation of the soil and to obtain a clear solvent phase for analysis. The water content of a portion of each sample (5 g) was determined in order to compile results on a dry-weight basis. PAH analysis An injection standard (1,2,3-trichlorobenzene) was added to solvent samples and PAH concentrations were analysed using GC/MS (Hewlett Packard 5890 Gas Chromatograph connected to a HP 5970 Mass Spectrometer Detector) following the USEPA method 8270. Separation was obtained by injection of 1 /xl of the extract on a 30 m x 0.25 mm DB-5 capillary column with a 0.25 /xm stationary phase (J & W Scientific Inc., Folsom, CA, USA) with helium as the carrier gas. The following temperature programme was used: 55°C hold 3 min, raise to 280°C at a rate of 4°C miff1. The injector and detector temperatures were 275°C and 280°C, respectively.

RESULTS AND DISCUSSION Mobilization experiment Results showed that SDS was quite effective in mobilizing sorbed PAHs in contaminated soil (Fig. 1). In fact, at SDS concentration of 0.5% (w/v) and greater, a substantial increase of the mobilization of fluorene, phenanthrene, anthracene, fluoranthene and pyrene occurred. For example, at a concentration of 1% of SDS, the aqueous concentrations of pyrene and fluoranthene were 21.6 mg l"1 and 24 mg l"1 compared to only 0.04 mg H and 0.06 mg l"1, 35 - : 30 -

: 25 - -

1

1

1

_|

1

.:-

—•— pyrene —•— fluoranthene

f

:

-

-i

is -

/ ;

è cp

20 '

|

10 5 -

o^ 0.2

0.4

0.6

0.8

SDS concentration (%)

Fig. I Mobilization of PAHs sorbed in the contaminated soil by different concentrations of SDS.

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respectively, in pure water. Pyrene andfluoranthene,having four rings, were more mobilized than the three three-ring PAHs studied. Contrary to pyrene and fluoranthene, the two other four-ring PAHs, benzo(a)anthracene and chrysene were not detected in the aqueous phase when no SDS was added (Table 2). This may be explained by the low aqueous solubility of these PAHs, 0.014 mg l"1 and 0.002 mg l"1, respectively (Dzombak & Luthy, 1984) compared to those of pyrene (0.135 mg Ï"1) and fluoranthene (0.26 mg l"1). In fact, the detection limit of the analysis method used being around 0.015 mg l"1, these PAHs may not have been detected. Increasing the concentration of SDS resulted in the mobilization of high molecular weight PAHs like benzo(a)pyrene that are only slightly soluble in pure water. Detectable concentrations of benzo(a)anthracene, chrysene and of the five- and six-ring PAHs were obtained in the aqueous phase when using a SDS concentrations of 0.1%. The increase of PAH mobilization at an SDS concentration of 0.25% (Table 2) corresponds with the CMC (0.23%) of the surfactant. For SDS concentrations greater than 0.1%, important aqueous concentrations of these high molecular weight PAHs were obtained and the Quebec criterion C for groundwater quality was exceeded (MENVIQ, 1988). These results showed that in field application of SDS, the monitoring of these mobilized PAHs is required and further aboveground treatment is necessary to meet water quality criteria. Table 2 High moiecular weight PAH concentrations (jxg l"1) in aqueous phase after soil washing with SDS PAH

SDS concentration ( % )

0to0.075a 0.10 0.25 0.50 0.75 ________ _ _ _ _ _ chrysene ND 29 91 328 1300 benzo(b)fluoranthene ND 28 89 244 838 benzo(k)fluoranthene ND 21 65 178 634 benzo(a)pyrene ND 17 60 153 581 indeno(l,2,3-cd)pyreneND 6 47 96 272 dibenzo(a,h)anthracene ND ND ND 43 ND benzo(g,h,i)petylene ND 8 46 83 216 a 0, 0.005, 0.01, 0.05, 0.075% SDS;b ND: not detectable (detection limit; 15 /ig l"1).

1.0 _ 4020 2490 1946 1801 851 343 708

Biodégradation experiment Determination of the surfactant addition rate In Fig. 2, results showed that [a14C]SDS at the three studied concentrations was readily biodegraded in the contaminated soil. After 10 days of incubation, approximately 60% of the radio-labelled SDS was completely mineralized following the recovery as [14C]C02 of the carbon-14 unit bound to the sulphate group of the molecule. If the use of [a14C]SDS allowed the verification of the complete mineralization of the surfactant molecule, one may also note that such

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typical microcosms studies permit the recovery of only a fraction of the added radioactivity (in this study 60%) as [14C]C02. This limited recovery was mainly attributable to the fraction of the radio-labelled compound being incorporated into cellular components or converted to other compounds. In this study (Fig. 2), the same asymptote observed at 20 days for the different SDS concentrations suggests that for such long term incubation, the yield coefficient — describing the conversion efficiency of substrate to microorganisms mass — was constant in all microcosms, i.e. independent of the total initial SDS concentration. Such a constant yield coefficient can possibly be explained by the absence, for such a relatively low substrate (SDS) concentration range, of an inhibitory effect due to an excess of substrate. However, a mass balance approach was not applied to verify the relative conversion of [a14C]SDS to soil biomass or to other unrecovered compounds. On the other hand, increasing SDS concentration appeared to reduce its own rate of mineralization at the early stage of biodégradation (i.e. days 0-5). For example, for the 0-2 day period (four data points), the mean mineralization rate for [a14C]SDS (initial quantity: 22 /xg kg"1) was 5.2, 3.8 and 3.2 /ng kg"1 day"1 in the presence of 10, 100 and 500 mg kg"1 SDS, respectively. Given these results, a two weeks SDS addition rate was chosen to insure that SDS remained active during all the biodégradation experiment for PAHs (45 weeks).

o

s

to

is

20

Time (d) 14

Fig. 2 Biodégradation of [a C]SDS in the contaminated soil.

Effect of SDS on the biodégradation of sorbed PAHs After 11 weeks of incubation, the three-ring PAHs were almost completely degraded. For this period, the SDS (100 and 500 mg kg"1) significantly decreased the fluorene and phenanthrene biodégradation (Fig. 3). Indeed, after two weeks of treatment, the mean residual concentration of fluorene was 51 mg kg"1 for the soil without

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SDS, compared to 117, 87 and 67 mg kg"1 for the soil amended with 500, 100 and 10 mg kg"1 of SDS, respectively. Similarly, in soil with the same SDS concentrations as previously mentioned, 276, 239 and 169 mg kg"1 of phenanthrene was found, compared to 145 mg kg"1 in the soil with no SDS. In contrast, the SDS did not significantly affect anthracene biodégradation. The biodégradation of four-ring PAHs (Fig. 4) was slower than that of three-ring PAHs. This agreed with results obtained by Park et al. (1990) showing that the biodégradation rates of PAHs is as follows: two-ring > threering > four-ring PAHs. After 45 weeks of incubation without SDS, nearly 95% of the fluoranthene and pyrene was biodegraded compared to 90% for benzo[a]anthracene. Chrysene, which showed the lowest aqueous solubility (0.002 mg l"1 compared to 0.26, 0.135, and 0.014 mg l"1 for fluoranthene, pyrene and benzo(a)anthracene, respectively) exhibited the lowest biodégradation efficiency (80%).

fluorene

Eg U Q D Q

abiotic no SDS 500 mg»g SDS 100 mg/kg SDS 10 mg/kg SDS

time (w)

Fig. 3 Residual concentrations of three-ring PAHs in the contaminated soil at day 0 and after two weeks of treatment in the presence and absence of SDS. Error bars represent one standard deviation about the mean of three replicates.

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The addition of 10 mg kg"1 of SDS did not significantly affect the biodégradation of the four PAHs studied. However, the addition of 100 and 500 mg kg"1 of SDS was shown to slow down the PAH biodégradation. In fact, soils with no SDS and those treated with the lowest SDS concentration (10 mg kg"1) exhibited the most biodégradation. Statistical analysis result (P-value) obtained from all the data monitored during 45 weeks are shown in Table 3. The decrease in PAHs biodégradation may be explained by the high biodegradability of SDS (Fig. 2) and its possible preferential use by the indigenous microflora as a substrate. No significant abiotic loss of the four-ring PAHs was observed (Fig. 4). High molecular weight PAHs like benzo[a]pyrene were not biodegraded (Fig. 5) nor were benzo(b)- and benzo(k)fluoranthene, indeno(l,2,3-cd)pyrene, dibenzo(ah)anthracene and benzo(g,h,i)-perylene (not shown).

Table 3 P-values for comparison between the treatment with no SDS and the treatment with SDS (LS means)* PAH

fluoranthene pyrene benzo(a)anthracene chrysene

SDS concentrationi (mg kg~') 10

100

500

0.4671 0.8582 0.2213 0.6894

0.0001 0.0001 0.0001 0.0001

0.0001 0.0001 0.0001 0.0001

* Comparisons were made using all concentration data monitored during 45 weeks (complete biodégradation curve, not shown).

CONCLUSION This study has shown that SDS is quite effective in increasing the concentration of PAHs in the aqueous phase, suggesting that this surfactant might be used for above-ground soil washing or for in situ treatment to recover these compounds. Such an increase in the mobilization of PAHs may however contribute to the spreading of these contaminants to previously uncontaminated sections of soil. In fact, the results showed that increasing SDS concentration resulted in the mobilization of toxic high molecular weight PAHs such as benzo(a)pyrene which is only slightly soluble in pure water. In such a case, the preservation of groundwater quality would require the monitoring and the effective control of the transport of mobilized PAHs. On the other hand, this study showed that the addition of SDS would not be effective in improving the biotreatment of creosote-contaminated soil. Moreover, the addition of SDS at high concentrations reduced the PAH biodegradation. This was attributable to its use as a preferential substrate by the indigenous microflora. The SDS was also found to be readily mineralized by the indigenous microflora. In the case of in situ bioremediation, these results

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fluoranthene

abiotic uoSDS 500 mg/kg SDS 100 mg/kg SDS 10 mg/kg SDS

time (wj

4S

Fig. 4 Residual concentrations of four-ring PAHs in the contaminated soil at day 0 and after 45 weeks of treatment in the presence and absence of SDS. Error bars represent one standard deviation about the mean of three replicates.

could possibly provide an advantage since the persistence of the surfactant in the aquifer is an important consideration. The high biodegradability of SDS suggested that this surfactant will not persist for long periods of time in groundwater after the treatment. However, as with above-ground treatment, the SDS would have to be injected at a determined rate and concentration to maintain its efficiency for the duration of the treatment.

The effect of an anionic surfactant on PAHs ~

ISO

m e g 100

483

abiotic no SDS Benzo(a)pyrene 500 mg/kg SDS 100 mg/kg SDS 10 mg/kg SDS

3

-o 0

time (w)

45

Fig. 5 Residual concentrations of benzo(a)pyrene in the contaminated soil at day 0 and after 45 weeks of treatment in the presence and absence of SDS. Error bars represent one standard deviation about the mean of three replicates.

These observations need to be verified with different soil types containing weathered PAH residues. Extrapolation of these results to other anionic surfactants used for remediating contaminated soils should be made cautiously. It is expected that other anionic surfactants may possibly produce an analogous effect on the removal of PAHs, provided that they exhibit surface active and biodegradability characteristics similar to that of SDS. In order to develop effective remediation techniques, research is thus needed on the behaviour and fate of both PAHs and surfactants injected in soil pore water or groundwater. Acknowledgements The authors gratefully thank C. Beaulieu, S. Deschamps, D. Ouellette and M. Leduc from the Institut de Recherche en Biotechnologie for technical support. The valuable statistical work of B. Clément, Ecole Polytechnique, was greatly appreciated. The financial support of L. Deschênes by the Fonds pour la formation de Chercheurs et l'Aide à la Recherche, Government of Québec, and from the Natural Sciences and Engineering Research Council of Canada is acknowledged.

REFERENCES Abdul, A. S., Gibson, T. L. & Rai, D. N. (1990) Selection of surfactants for the removal of petroleum products from shallow sandy aquifers. Ground Wat. 28, 920-926. Abdul, A. S., Gibson, T. L., Ang, C. C , Smith, J. C. & Sobczynski, R. E. (1992) In situ surfactant washing of poly chlorinated bipheny Is and oils from a contaminated site. Ground Wat. 30, 219-231. Aronstein, B. N. & Alexander, M. (1993) Effect of a non-ionic surfactant added to the soil surface on the biodégradation of aromatic hydrocarbons within the soil. Appl. Microbiol. Biotechnol. 39, 386-390. Aronstein, B. N., Calvillo, Y. M. & Alexander, M. (1991) Effect of surfactants at low concentrations on the desorption and biodégradation of sorbed aromatic compounds in soil. Environ. Sci. Technol. 25, 1728-1731. Bury, S. J. & Miller, C. A. (1993) Effect of micellar solubilization on biodégradation rates of hydrocarbons. Environ. Sci. Technol. 27, 104-110. Chiang, T. C , Valkenburg, C. A. & Miller, D. A. (1989) Performance testing of method 1312 QA support for RCRA testing: project report. USEPA report no. EPA/600/4-89/022.

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