Water,Air,and Soil Pollution:Focus(2006)6:299–315
DOI:10.1007/s11267-005-9024-z C Springer2006 IN-SITU PHYTOREMEDIATION OF PAHS CONTAMINATED SOILS FOLLOWING A BIOREMEDIATION TREATMENT
S′EBASTIEN DENYS?,CLAIRE ROLLIN,FRANCIS GUILLOT
and HAFID BAROUDI
INERIS,Direction des Risques Chroniques,Parc Technologique ALATA,BP2,60550France ?author for correspondence,e-mail:sebastien.denys@ineris.fr
(Received29November2004;accepted25October2005)
Abstract.Phytoremediation of pollutants in soils is an emerging technology,using different soil-plant interaction properties.For organic pollutants,such as polycyclic aromatic hydrocarbons(PAHs), phytodegradation seems to be the most promising approach.It occurs mostly through an increase of the microbial activity in the plant rhizosphere,allowing the degradation of organic substances,a source of carbon for soil microbes.Despite a large amount of available data in the literature concerning laboratory and short term PAH phytodegradation experiments,no actual?eld application of such technique was previously carried out.
In the present study,a soil from a former coking plant was used to evaluate the feasibility and the ef?ciency of PAH phytodegradation in the?eld during a three years trial and following a bioremedi-ation treatment.Before the phytoremediation treatment,the soil was homogenized and split into six independent plots with no hydrological connections.On four of these plots,different types of common plant species were sowed:mixture of herbaceous species,short cut(P1),long cut(P2),ornamental plants(P3)and trees(P4).Natural vegetation was allowed to grow on the?fth plot(P5),and the last plot was weeded(P6).Each year,representative sampling of two soil horizons(0–50and50–100cm) was carried out in each plot to characterize the evolution of PAHs concentration in soils and in soils solution obtained by lixiviation.Possible impact of the phytoremediation technique on ecosystems was evaluated using different eco-and genotoxicity tests both on the soil solid matrix and on the soil solution.
For each soil horizon,comparable decrease of soil total PAHs concentrations were obtained for three plots,reaching a maximum value of26%of the initial PAHs concentration.The decrease mostly concerned the3rings PAHs.The overall low decrease in PAHs content was linked to a drastic decrease in PAHs availability likely due to the bioremediation treatment.However,soil solutions concentration showed low values and no sign?cant toxicity was characterized.The mixture of the herbaceous species seemed to be the most promising plants to be used in such procedure.
Keywords:ecotoxicity,feasibility,?eld experiments,PAHs,phytoremediation,representative soil sampling
1.Introduction
During the last decade,concerns about persistent organic pollutant(POP)in the environment have considerably increased.Among POPs,polycyclic aromatic hy-drocarbons(PAHs)are of great interest as the accumulation of these compounds in soil might lead to signi?cant risks to Man through different exposure pathways
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(soils ingestion,contaminated plant consumption...).Indeed,PAHs were shown to be toxic and even carcinogenics to Man(Bouchez et al.,1996).For instance, benzo[a]pyrene was classi?ed by the International Agency for Research of Cancer (IARC)as probably carcinogenic(ATSDR,1995).
PAHs are by-products from the incomplete combustion or pyrolysis of organic material.Among sources of PAHs(waste incineration,residential heating...),coke production is one of the major in North of France,due to the intensive industrial coal mining during XIXth and XXth centuries.This activity led to large areas of PAHs contaminated soils.Prior to any sites rehabilitation,remediation has to be achieved.Among the existing technologies,natural attenuation is one of the sev-eral possible options.This technology relies on the processes of biodegradation, sorption,dilution or chemical reactions to reduce the mass,toxicity and mobil-ity of pollutants in soils(US-EPA,1999).Enhancing natural attenuation using the phytoremediation techniques seems to be a promising approach.Indeed the exper-imental emerging green technology that uses plants to remediate soil was already shown both for heavy metals and organic contaminants(US-EPA,2000;Alkorta and Garbisu,2001).Different strategies are behind the concept of phytoremedi-ation:uptake,volatilization,degradation of the contaminants using physiological plant mechanisms....According to(Cunningham and Berti,1993),PAHs degra-dation in the plant roots zone(de?ned as the rhizosphere,Hiltner,1904,quoted by Anderson et al.,1993)is one of the most promising phytoremediation strategy for such molecules.Ef?ciency of this method was supported by previous studies done either in batch or in column experiments(Joner et al.,2001).Comparing vegetated and fallow soil columns,Aprill and Sims(1990)observed that PAHs degradation (benzo(a)anthracene,chrysene,benzo(a)pyrene,dibenz(a,h)anthracene)was sig-ni?cantly increased in the planted soil columns.In a same soil pro?le,Olson and Fletcher(1999)showed that PAHs concentration in the horizon in which roots were developed was70%lower that in the underlying soil horizon containing no roots.Degradation of pyrene can reach74%of its initial soil content in vegetated soils whereas degradation is not signi?cant in the non-vegetated soils(Liste and Alexander,2000).
Comparable pyrene degradation rates were obtained when incubating maize exudates in soils containing14C labeled PAH,showing that plant exudates are involved in PAHs degradation(Yoshitomi and Shann,2001).
However most of the studies were conducted in laboratory conditions using la-beled or recently added molecules but were not representative of?eld conditions in which an historic pollution occurs.Recently,(Saison et al.,2004),conducted a lysimeter experiment involving a2-cm sieved soil coming from a coking plant. This study showed that,as opposed to controlled conditions experiments,phy-todegradation of PAH was not so easy to achieve over the time course of the experiment despite no toxicity of PAH to plants was observed.However,?eld ex-periment was never carried out to assess the actual feasibility of such approach in the ?eld.
IN-SITU PHYTOREMEDIATION OF PAHS CONTAMINATED SOILS301
TABLE I
Selected soil properties
pH7.6
Clay(mg kg?1)73
Loam(mg kg?1)259
Sand(mg kg?1)676
Organic Matter(g kg?1DM)502
Total N(g kg?1DM)7.7
Total P(g kg?1DM) 1.8
Total K(g kg?1DM) 3.4
The objective of this study is to test the feasibility for phytoremediation of PAHs contaminated soils in?eld conditions over a2years experiment under natural climatic conditions and following an aerobic bioremediation treatment.Soil was coming from a former coking plant located in North of France.Different plant species were grown onto six basins containing the soil.As degradation of PAHs may results in metabolites often more toxic and soluble than the parent molecule (Bouchez et al.,1995)a decrease in PAHs soil content over the time of the treatment is not suf?cient to assess the ef?ciency of phytoremediation.Measurement of soil and water toxicity are also needed to ensure that this process is appropriate and does not lead to other situations of pollution(Bispo et al.,1999).Thus,evolution of soil and leachate toxicity with time was also studied.
2.Material and Methods
2.1.S OIL CHARACTERISTICS BEFORE THE PHYTOREMEDIATION TREATMENT 1500tons of soil were collected from a former coking plant located in North of France.As suggested by the literature(Pradhan et al.,1997,1998),the soil was ?rstly remediated by aerobic incubation using two lignolytic microorganisms for 625days.This?rst treatment accounted for the reduction of the total soil PAHs content(16targeted US-EPA PAHs)from9to5g kg?1dry soil(DS).Selected soil properties are provided in Table I.Soil was homogenized using a mechanical shovel.
2.2.S ITE CONFIGURATION
Once homogenized,soil was split into four basins with no hydrological connection among them(Figure1).Each basin contained between300tons(basins2,3and4) and600tons(basin1)of soil.Underneath the contaminated soils,a schist layer drained the percolating water collected to a well,this one being located at the
302S′E BASTIEN DENYS ET AL.
Figure1.Site con?guration.
center of the site.The contaminated soil and the schist layer were separated by a geomembrane.
2.3.P LANTS SPECIES USED FOR THE PHYTOREMEDIATION TREATMENT Basins previously described were converted into plots on which phytoremediation treatment was carried out.Basins1and3were divided into two plots whereas basins2and4consisted each in1plot.Different species were sowed on each plot.
A mixture of herbaceous species was sowed on plot1:fescue,ray-grass,trifolium. Monthly,this plot was mowed between3and5cm.Plot2was vegetated using the same mixture,and mowed every three months.Plot3was manually weeded every month,plot4was let fallow.Ornamental plants(rose,gladiolus,dahlia,pink,tulip, peony,crocus and narcissus)were sowed on https://www.doczj.com/doc/d82805368.html,stly,plot6was planted with trees(oak,false acacia,alder,willow and maple tree)broadly used in France for rehabilitation of former mining facilities.
2.4.S OIL SAMPLING
An accurate evaluation of the evolution of the soil PAHs content with time was needed to estimate the actual effect of the phytoremediation treatment.This implied
IN-SITU PHYTOREMEDIATION OF PAHS CONTAMINATED SOILS303 to evaluate:
?soil PAHs content for each year of the experiment,based on an homogeneous sampling strategy among years;
?estimation of the spatial variability in soil PAHs content within plots to evaluate its impact on PAHs concentration in soil samples.
Two soil horizons were sampled:the surface horizon(0–50cm),in which the root development was considered to be maximal;the deep horizon(50–100cm)in which no roots were supposed to be present.Soil sampling was carried out once a year,in May.
During the?rst year of the trial,prior to the phytoremediation treatment,an initial soil characterization was conducted using three sampling https://www.doczj.com/doc/d82805368.html,posite, random and average soil samples were taken from each horizon and each plot in three replicates,leading to a total of72samples.During the following two years of the treatment,only average soil samples were taken from each plot(one sample per horizon)leading to12soil samples per year.
Samples were air-dried and sieved through a2mm-mesh before analytical char-acterization.
2.5.E VOLUTION IN THE SOIL PAH S CONTENT WITH TIME
2.5.1.Spatial Variability
The soil spatial variability in soil PAH content within one plot may induce a bias when comparing soil PAHs content among years.
For a given strategy,this bias was quanti?ed using the variation coef?cient(VC) de?ned by
VC=SD/M
with M:the arithmetic mean of the soil PAHs content among replicates;SD:the standard deviation associated to the arithmetic mean.
A global variation coef?cient regrouping the three sampling strategies was de-?ned as the arithmetic mean of the VC obtained for each sampling strategy.
2.5.2.Evolution Factor(EF)
Relative evolution of PAHs content in soil over the treatment was calculated through the evolution factor(EF)calculated as
EF=[PAH initial]?[PAH end]/[PAH initial]
with[PAH initial]the average soil PAHs concentration as measured in average soil sample before the phytoremediation treatment;[PAH end]the average soil PAHs
304S′E BASTIEN DENYS ET AL.
concentration as measured in average soil sample at the end of the phytoremediation treatment.
For a given plot,if in absolute value EF>VC,an evolution in PAHs content occured.An actual EF(EF actual)is then calculated as following:
EF actual=EF?VC
2.6.PAH S EXTRACTION FROM SOILS
15grams of soil were placed in a11ml metallic stainless steel cell and extracted twice with CH2Cl2at120?C at a pressure of120bars for5min using an ACE200 (Accelerated Solvent Extractor–Dionex).The CH2Cl2extracts were sampled for direct HPLC analysis(Dionex).Three replicates of each sample were analyzed.
2.7.S OIL SOLUTION EXTRACTION
Soil solution was extracted using the lixiviation test.This one give an estimation of the ability for PAHs to be mobilized in the soil solution and is currently used for the acceptation of PAHs contaminated soils in waste repositories.This test was carried out for each plot on an average soil sample according to a normalized protocol (AFNOR,1998a).Aliquots were?ltered at0.45μm using cellulose acetate?lters (SARTORIUS).
The PAHs were extracted from aliquots by liquid/liquid partitioning with dichloromethane(leachate/dichloromethane:5/1v:v).Three10min extractions periods with constant mixing were performed.At the end of the procedure,the extractant was evaporated to dryness and the residues redissolved in2ml of ace-tonitrile before HPLC detection(Dionex).
2.8.T OXICITY OF SOILS AND ALIQUOTS
Toxicity tests were carried out for both the surface and the deep horizons,on an average sample for the entire site.This sample was made by homogeneously mixing the average samples coming from the six plots.
On the solid matrix prior to the phytoremediation treatment,acute toxicity was characterized using the Eisenia fetida test(AFNOR,1994).
On the liquids(aliquots obtained from the lixiviation tests),daphnia (Ceriodaphnia dubia)and alguae(Pseudokirchneriella subcapitata)tests were car-ried out(AFNOR,1998b,2000).
A genotoxicity test was also carried out on the aliquots during the second year of the treatment.This test consisted in an in vitro micronucleus test using mice lymphoma cells(Nesslany and Marzin,1999).This test was carried out by Pasteur Institute(Lille,FRANCE).
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Plot 1
Composite 1Composite 2Composite 3Random 1Random 2Random 3Plot 2
Plot 4
Plot 6
Composite 1Composite 2Composite 3Random 1Random 2Random 3
Total PAHs content
(mg kg-1) Total PAHs content
(mg kg-1)
Figure2.Total PAHs soil content(mg kg?1)for the three composite and random samples taken from each plot.
3.Results
3.1.I NITIAL SOIL PAH S CONTENT
Prior to the phytoremediation treatment and for each plot,soil total PAHs concentra-tions for the composite and random samples were not statistically different among replicates(α=5%)(Figure2).Besides,the concentrations among plots were quite similar for each replicate.This observation is con?rmed by the comparison of the average soil PAHs content(as measured in average samples obtained for each plot).Indeed,during the?rst year of the phytoremediation treatment,average PAHs
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BASTIEN DENYS ET AL
.0
50010001500200025003000350040004500Plot 1
Plot 2Plot 3Plot 4Plot 5Plot 6
T a v e r a g e s o i l P A H s c o n t e n t (m g k g -1
)
Figure 3.Average PAHs soil content (mg kg ?1)for the average samples.
concentration ranged from 3128mg kg ?1of dry soil (DS)on plot 3to 3893mg kg ?1
DS on plot 6(Figure 3).This homogeneity in PAHs concentration was also veri?ed when considering their repartition according to their number of rings (Figure 4).For each plot,concentrations of the 4and 5-rings PAHs were the highest and the lowest for the 6-rings PAHs.3.2.S PATIAL
VARIABILITY IN SOIL
PAH S
CONTENT WITHIN EACH PLOT
Despite the homogeneity,soil sampling may induce a bias when comparing soil PAHs content among years.This bias was quanti?ed through the calculation of a variation coef?cient for soil PAHs content for each plot.The VC values varied among plots (Table II).VC ranged from 6%on plot 1to 18%on plot 5.In other words,on plot 1for instance,a 6%variation of the soil PAHs content could not be attributed to the phytoremediation treatment but to the spatial variability in the soil PAHs concentration.3.3.E FFECT
OF THE PHYTOREMEDIATION TREATMENT
ON THE SOIL PAH S CONTENT
Study of the evolution of the soil PAHs concentration among years was done using the average soil PAHs concentration.
3.3.1.Evolution of Soil PAHs Content with Time ?Evolution of the total soil PAHs content
IN-SITU PHYTOREMEDIATION OF PAHS CONTAMINATED SOILS
307
Plot 1
5001000150020002,3 rings
4 rings
5 rings
6 rings
P A H s s o i l c o n c e n t r a t i o n (m g k g -1
)
Plot 2
Plot 3
Plot 4
Plot 5
Plot 6
500100015002000
2,3rings
4 rings
5 rings
6 rings
5001000150020002, 3 rings
4 rings
5 rings
6 rings
P A H s s o i l c o n c e n t r a t i o n (m g .k g -1
)
5001000150020002,3 rings 4 rings 5 rings
6 rings
50010001500200025002,3rings
4 rings
5 rings
6 rings
5001000150020002, 3rings
4 rings
5 rings
6 rings
S o i l P A H s c o n c e n t r a t i o n (m g k g -1)
Figure 4.PAHs soil content (mg kg ?1)as a function of the number of rings.
During the phytoremediation treatment,evolution of the total soil PAHs (16tar-geted US-EPA PAHs)content was quanti?ed through the actual evolution factor (EF actual ).
For the 0–50cm horizon (Figure 5A),a decrease of PAHs soil content were obtained for plots 1,3and 6.Values for this decrease (calculated as the absolute value of EF ?VC)ranged from 2%of the initial PAHs content on plot 2to 25%of the initial PAHs content on plot 3.
For the deep horizon (50–100cm)(Figure 5B),a decrease were observed on plots 1,4and 6.Values for this decrease ranged from 2%of the initial PAHs soil
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BASTIEN DENYS ET AL .TABLE II
Variation coef?cient for each type of sampling strategy
Variation coef?cient (VC)(%)
Plot Composite Random Average Global a 1386625161110311514104141513145151919186
22
10
17
16
a
The global VC is the arithmetic mean of composite,random and average VC.
A– Surface horizon
B–Deep horizon
2000
400060001
2
34
5
6
Plot
S o i l t o t a l P A H s c o n c e n t r a t i o n (m g /k g )
2000
400060001
2
34
5
6
Plot
S o i l t o t a l P A H s c o n c e n t r a t i o n (m g /k g )
Figure 5.Evolution of soil total PAHs concentration (mg kg ?1)for each plot and each horizon.
content for plot 4to 26%of the initial PAHs content for plot 1.A increase of PAHs soil content in the deep horizon was also observed for plot 3.
?Evolution of the soil PAHs content as a function of their number of rings (Figures 6and 7)Evolution of soil PAHs content over the time was different according to the number of rings.The decrease was the highest for the 3-rings PAHs.Indeed,for each plot and each horizon it ranged from 26%(plot 5)to 55%(plot 1)of the initial 3-rings content and for the deep horizon from 28%(plot 3)to 61%(plot 1).
IN-SITU PHYTOREMEDIATION OF PAHS CONTAMINATED SOILS
309
2 rings
1
2
3
45
6
Plot
P A H s c o n t e n t (m g /k g )
3 rings
1
2
3456
Plot
4 rings
1
2
3
45
6
Plot
P A H s c o n t e n t (m g /k g
)
5-6 rings
1
2
3
45
6
Plot
Figure 6.Evolution of soil PAHs content (mg kg ?1)with time (from the ?rst year,2001to the last year 2003of the experiment)as a function of their number of rings,surface
horizon.
2 rings
Plot
P A H s c o n t e n t (m g /k g )
3 rings
1
2
3456
Plot
4 rings
Plot
P A H s c o n t e n t (m g /k g
)
5-6 rings
Plot
P A H s c o n t e n t (m g /k g )
Figure 7.Evolution of the soil PAHs content (mg kg ?1)with time (from the ?rst year,2001to the last year 2003of the experiment)as a function of their number of rings,deep horizons.
Concerning the other categories of PAHs (2,4,5and 6rings),decrease in PAHs content was lower.In the case of the 2rings and 4rings PAHs,a decrease was just observed for two plots.Maximum EF were 13%for the 2rings (deep horizon,plot 1)and 21%for the 4rings (deep horizon,plot 1).For the 5and 6rings categories,the decrease was obtained just for plot 1,reaching 12%at the surface as a maximum.An increase in PAHs content was observed for different plots and horizons,mostly for the 5and 6rings categories.For this latter,the increase concerned two
310S′E BASTIEN DENYS ET AL.
plots(plot2surface and deep horizons;plot3deep horizon).Concerning the2 rings PAH,an increase was also observed only in the deep horizons for plots3and 6.
3.4.PAH S LIXIVIATION FROM SOIL
From average soil samples,lixiviation tests were carried out to estimate the potential for PAHs to be mobilized from the soil solid matrix to the soil solution.
Evolution of total PAHs concentration with time in aliquots from lixiviation tests is given below(Table III).Results regroup the effects of both the bioremediation and the phytoremediation treatments.At the end of the phytoremediation treat-ment,the concentration was97%lower compared to the concentration obtained before the treatments.During the phytoremediation treatment,evolution in PAHs concentration in aliquots was low as the values obtained between the two years were almost similar.
When considering the results as a function of the number of rings,maximal decrease of PAHs concentration in aliquots during the treatments was obtained for the3-rings PAHs(99%),whereas the decrease was the lowest for the5and6-rings PAHs(37%).
3.5.S OIL AND LEACHATE TOXICITY
Toxicity tests concerned both the soils and the aliquots obtained from the lixiviation tests.On the soils,a solid-phase acute toxicity test was carried out;on the latter a liquid-phase chronic toxicity test was done.In2002a genotoxicity test was also done on the aliquots.
?Acute toxicity
For the soil solid phase,an acute toxicity test was carried out using the Eisenia fetida normalized test.Before the bioremediation treatment,the test showed100% of mortality for soil concentration of50%.When the soil concentration decreased down to10%,mortality was no more observed.After the bioremediation treatment, the test showed no solid toxicity for any soil samples coming from the surface horizon or the deep horizon
?Chronic toxicity
The chronic toxicity on the leachates was tested using two normalized protocols: Ceriodaphniae dubia and Pseudokirchneriella subcapitata.
For the?rst two years of the phytoremediation treatment,toxicity was not ob-served on leachate.During the last year of the treatment,no toxicity was observed on the Ceriodaphniae dubia test but a signi?cant toxicity was observed on the
IN-SITU PHYTOREMEDIATION OF PAHS CONTAMINATED SOILS
311
T A B L E I I I E v o l u t i o n w i t h t i m e o f P A H s c o n c e n t r a t i o n (μg L ?1)i n t h e a l i q u o t s o b t a i n e d f r o m t h e l i x i v i a t i o n t e s t
B e f o r e a n y t r e a t m e n t S e c o n d y e a r
T h i r d y e a r
M e a n S u r f a c e c o n c e n t r a t i o n a
h o r i z o n D e e p h o r i z o n
S u r f a c e h o r i z o n D e e p h o r i z o n
M e a n e v o l u t i o n b (%)2r i n g s 619.7n d n d 3236943r i n g s 107,400.1493568216.5246.6994r i n g s 4529.79101700905888815a n d 6r i n g s 2486.522702760164.8144.63716U S -E P A P A H s
115,036.2
3673
50282801.52616.697a
C a l c u l a t e d a s t h e a r i t h m e t i c m e a n o f t h e c o n c e n t r a t i o n s i n a l i q u o t s f r o m t h e s u r f a c e a n d t h e d e e p h o r i z o n s .b
C a l c u l a t e d a s t h e d i f f e r e n c e b e t w e e n t h e c o n c e n t r a t i o n b e f o r e a n y t r e a t m e n t a n d t h e a r i t h m e t i c m e a n o f P A H s c o n c e n t r a t i o n i n a l i q u o t s o b t a i n e d f o r t h e s u r f a c e a n d d e e p h o r i z o n s d u r i n g t h e t h i r d y e a r .n d :N o n d e t e c t e d .
312S′E BASTIEN DENYS ET AL.
Pseudokirchneriella subcapitata test.For this one,results showed that a leachate concentration from10to80%induced a signi?cant growth inhibition in72h.?Genotoxicity
No genotoxicity was observed on any leachates.
4.Discussion
An in situ trial was carried out to evaluate the phytodegradation of PAHs contami-nated soil from a former coking plant,during a two years experiment.
In terms of soil total PAHs concentration,the maximal decrease was measured on plot1(sowed with a mixture of herbaceous species).For this plot,decrease was equivalent between the surface and the deep horizons:around25%of the initial soil PAHs concentration.This value is close to the value obtained by(Vervaeke et al.,2003)using willow stands on contaminated sediment in?eld conditions who measured a decrease of23%in soil total PAHs content(13PAHs).Besides,the slow evolution in PAHs soil content over the time of the treatment con?rms the results obtained at the lysimeter scale on sieved soil samples coming from a former coking plant(Saison et al.,2004).However when comparing our data with those obtained at small scale experiments,the decrease in PAHs soil content is surprisingly low. For instance phytoremediation conducted in pots on a creosote-contaminated soil induced a higher decrease,reaching86%of the initial PAHs concentration(Joner et al.,2004).Authors related this decrease to a high soil PAHs bioavailability in spite of the old age contaminant,linked to the hydraulic conditions in the pots.
Thus PAHs bioavailability appears as a sensitive parameter which controls the ef?ciency of the phytoremediation treatment.In the present study,the long-term (625d.)bioremediation treatment which was previously carried out might have induced a depletion of the bioavailable pool of the soil PAHs.As a consequence, during the phytoremediation treatment,availability of PAHs was low and their utilization as a carbon source for microorganisms was limited,leading to a conse-quently limited PAHs degradation.
It appears that degradation was the most important for the3rings PAHs.This result con?rms previous data obtained in smaller scale experiments(Haeseler et al., 1999;Joner et al.,2004)showing that the phytoremediation treatment was the most effective for this category.The lowest evolution was obtained for the heavier PAHs (4to6rings)con?rming the recalcitrant nature of these molecules already shown (Joner et al.,2004).Results concerning degradation of the2cycles might not be here representative due to the relatively volatility of these compounds which may induce sampling and analytical bias.
In some case,an increase in soil PAHs content was observed.This effect was likely due to a nugget effect,not taken into account when using the global variation
IN-SITU PHYTOREMEDIATION OF PAHS CONTAMINATED SOILS313 coef?cient to calculate the EF actual.However,this increase can also be due to the release of some PAHs with time from the coal-tar particles contained in the soil and which could be weathered during the treatment.Concentration of PAHs in aliquots obtained after lixiviation tests showed a very low concentrations of PAHs either for total PAHs or as a function of their number of rings.The bioremedia-tion treatment induced a drastic decrease in the PAHs concentration in the liquid phase,con?rming a drastic decrease of the bioavailable pool of PAHs during the bioremediation treatment.As the results obtained for PAHs dissipation in soil,the decrease in aliquots were the most important for the3-rings PAHs whereas the 5and6rings PAHs were less concerned.Results from this lixiviation test also show that the migration of PAHs downward the soil pro?le is theoretically low, thereby having a limited effect on groundwater contamination.This can be due to the low PAHs solubility in water and their hydrophobicity as shown by other authors(Bouchez et al.,1996).Results concerning the absence of toxicity for liquid phase con?rmed results obtained by Saison et al.(2004)on leachate obtained from lysimeter experiments.Besides no genotoxicity was observed during the treatment. However these authors have shown that the soil toxicity was signi?cant at the initial time of the treatment whereas no soil toxicity was quanti?ed at the inital time of the present study despite similar PAHs content.Even if PAHs content only partly explains soil toxicity,this demonstrates that the total quantity of pollutants in soil can not be correlated to any toxicity and that the bioavailability strongly in?uence the toxicity.Here the bioremediation treatment carried out in our study likely could have induced a decrease in soil ecotoxicity because of the depletion of the PAHs bioavailable pool.
With the exception of the plot1on which herbaceous mix was sowed and harvested each month,the data obtained in terms of evolution in soil PAHs content were similar among plots,even for the weeded one.This might prove that the effect of the phytoremediation treatment is limited and that the decrease observed is possibly due to other natural attenuation phenomenon.In the case of our study, a mixture of herbaceous species seemed to be the most promising plant species to be used for phytoremediation of PAHs contaminated-soil.
5.Conclusions
The objective of this study was to test the feasibility for phytoremediation of PAHs contaminated soils in?eld conditions over a2years experiment under natural climatic conditions and following an aerobic bioremediation treatment.This was done through the characterization of the evolution of PAHs content in a soil sampled from a former coking-plant and aerobically bioremediated.The soil was split into six plots on which different plant species were sowed.One plot was weeded.
Besides the evolution in soil PAHs content during the treatment,toxicity and potential for PAHs to be mobilized from the soil solid matrix to the soil solution
314S′E BASTIEN DENYS ET AL.
were also characterized.This potential was shown to be very limited after the biore-mediation treatment.Moreover,during the phytoremediation process,the PAHs in soil solution were constant.This con?rmed the low PAHs bioavailability in the soil used for the phytoremediation treatment which consequently limited the phy-todegradation in terms of total PAHs content.The3rings PAHs were the most degraded PAHs(up to60%of degradation was obtained for this category)whereas the5and6rings seemed to be more recalcitrant.Ecotoxicity and genotoxicity tests showed no signi?cant toxicity of both the soil solid phase and the soil solution.In our case,a mixture of herbaceous species seemed to be the most promising plants to be used in such treatment.
Acknowledgments
Authors wish to acknowledge“Agence de l’Eau Artois-Picardie”and “Charbonnages de France”for funding this study.They also wish to thank Dr.
F.LE CURIEUX(Pasteur Institute,Lille,FRANCE)for the genotoxicity tests.
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