Removal of polycyclic aromatic hydrocarbons from aged-contaminated soil using cyclodextrins: Experimental study

Removal of polycyclic aromatic hydrocarbons from aged-contaminated soil using cyclodextrins: Experimental study

Environmental Pollution 140 (2006) 427e435 www.elsevier.com/locate/envpol Removal of polycyclic aromatic hydrocarbons from aged-contaminated soil usi...

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Environmental Pollution 140 (2006) 427e435 www.elsevier.com/locate/envpol

Removal of polycyclic aromatic hydrocarbons from aged-contaminated soil using cyclodextrins: Experimental study Christophe Viglianti a,b, Khalil Hanna a,*, Christine de Brauer a, Patrick Germain a a

Laboratoire d’Analyse Environnementale des Proce´de´s et des Syste`mes Industriels e INSA de Lyon, 9, rue de la Physique e 69621 Villeurbanne Cedex, France b Centre Sciences, Information et Technologies pour l’Environnement (SITE) e ENS de Mines de Saint Etienne, 158 cours Fauriel e 42023 Saint Etienne Cedex 2, France Received 8 April 2005; received in revised form 25 July 2005; accepted 6 August 2005

An innovative method using a biodegradable and non-toxic flushing agent for the depollution of industrially aged-contaminated soil. Abstract The removal of polycyclic aromatic hydrocarbons (PAHs) from soil using water as flushing agent is relatively ineffective due to their low aqueous solubility. However, addition of cyclodextrin (CD) in washing solutions has been shown to increase the removal efficiency several times. Herein are investigated the effectiveness of cyclodextrin to remove PAH occurring in industrially agedcontaminated soil. b-Cyclodextrin (BCD), hydroxypropyl-b-cyclodextrin (HPCD) and methyl-b-cyclodextrin (MCD) solutions were used for soil flushing in column test to evaluate some influent parameters that can significantly increase the removal efficiency. The process parameters chosen were CD concentration, ratio of washing solution volume to soil weight, and temperature of washing solution. These parameters were found to have a significant and almost linear effect on PAH removal from the contaminated soil, except the temperature where no significant enhancement in PAH extraction was observed for temperature range from 5 to 35  C. The PAHs extraction enhancement factor compared to water was about 200. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Cyclodextrins (CDs); Polycyclic aromatic hydrocarbons (PAHs); Aged-contaminated soil; Non-aqueous-phase liquids (NAPL); Removal

1. Introduction Soil matrices contaminated with polycyclic aromatic hydrocarbons (PAHs) abound at the sites of coke-oven

* Corresponding author. Laboratoire de Chimie Physique et Microbiologie pour l’Environnement, LCPME UMR7564, CNRS e Universite´ Henri Poincare´, Nancy I, 405, rue de Vandoeuvre, F-54600 Villers-le`sNancy, France. Tel.: C33 3 83 68 52 20; fax: C33 3 83 27 54 44. E-mail addresses: [email protected], [email protected] lcpme.cnrs-nancy.fr (K. Hanna). 0269-7491/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2005.08.002

gas plants, refineries, and many other major chemical industries. Owing to the persistence of PAHs in soil and sediments and their toxic, mutagenic, and carcinogenic effects, the remediation of PAH-contaminated sites is an important environmental issue. PAHs are present in variable amounts at most abandoned manufactured gas sites, primarily as part of coal and oil tars produced during the manufacture of town gas from coal, coke, and oil. PAH-contaminated sites are mostly found in or near cities, thus representing a considerable public health hazard.

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Contamination of soil by PAHs is frequently associated with non-aqueous phase liquids (NAPL). The presence of NAPL is a very important factor, limiting site clean-up. A number of treatment strategies, including bioremediation, exist for the in situ clean-up of contaminated sites but these are usually constrained by mass transfer limitations and PAHs low aqueous solubilities (Johnsen et al., 2005; Reid et al., 2000). Their low bioavailability is due to their high hydrophobicity, resulting in strong sorption to soil organic matter, reducing the availability of the contaminants for the soil microorganisms, thus limiting the biodegradation rate (Bardi et al., 2000; Wang et al., 1998). Attempts to increase the solubility of recalcitrant organic contaminants have been made using complexing agents which encapsulate poorly water-soluble contaminants and thus may enhance their removal from polluted site. Among the suggested enhancing-solubility compounds, cyclodextrins (CDs) have been proposed as an alternative agent (Brusseau et al., 1994; Ko et al., 1999; Tick et al., 2003). One major reason that the CD washing can clean contaminated soils is that the cyclodextrin solution can enhance the solubilization of otherwise relatively insoluble organic compounds (Hanna et al., 2004). CD present several advantages over classical enhanced-solubility agents (e.g. solvents and surfactants) such as their non-toxicity and their biodegradability (Szejtli, 1982; Verstichel et al., 2004). Previous works have focused on the cyclodextrin solubilization of individual compounds and often from lab-contaminated soil, whereas in real situations contaminated soil may contain mixtures of organic compounds issued from an aged pollution (Lee et al., 1992; Chatain et al., 2004). Furthermore, desorption from artificially contaminated soils is usually unrealistically high compared to desorption from weathered and aged coal tar-contaminated soil samples (Yeom et al., 1995; Bayard et al., 2000). Information on cyclodextrin-aided washing mechanisms regarding the removal efficiency or washing performance by CD on organic contaminated soil is very limited. In this work, two approaches were examined for predicting the main mechanism that causes the release of PAH into aqueous phase of soil. Results were used for the prediction of CD-aided washing performances for PAHs contaminated soils. Temperature can be a driving parameter to improve field performances of a washing process. Up to now, no reports have appeared concerning the temperature effect on CD washing process. In this paper, temperature effect on the contaminant mobilization from NAPL during the treatment of soils was investigated. Although CD can effectively remove organic pollutants from subsurface, more laboratory work is needed to understand the limitations of cyclodextrin-assisted remediation at high-scale. In particular, the type, the

quantity of CD and the volume of flushing solution necessary to decontaminate some kilograms of polluted soil should be detailed. In the present work, column extraction experiments were carried out to evaluate the ability of three types of cyclodextrins (b-cyclodextrin (BCD), hydroxypropylb-cyclodextrin (HPCD), methyl-b-cyclodextrin (MCD) to extract three selected PAHs from an industrially aged-contaminated soil. Enhancement extraction of compounds from soil versus each type of CD amount in flushing solution was also investigated, using CD concentration and liquid to solid ratio experiments.

2. Theory Estimate values for PAH concentrations in the aqueous phase in contact with soil containing residual organics is valuable, because predicted concentrations can aid in describing fundamental partitioning behavior of PAH, that can be used in the fate and transport modeling. Furthermore, predicted values can contribute to define the main mechanism of PAH solubilization which is useful in the assessment of a remediation technique for sites where NAPL is present in soil. An aged soil matrix was used, with a multiple-component contamination, considered to be present as an NAPL. Two main ways were considered to estimate single-PAH compound aqueous concentration obtained in the washing of this type of contamination: (i) Dissolution from the NAPL to water phase following Raoult’s law. (ii) Desorption using soil/water partition coefficients.

2.1. Dissolution model Raoult’s law can be described as: Caq ZXgS

ð1Þ

where Caq, compound molar aqueous concentration; X, compound molar fraction within the NAPL; g, compound activity coefficient within the NAPL; S, compound aqueous solubility. g can be taken equal to 1, which implies that NAPL is considered as ‘‘ideal’’. X can be expressed as: XZ

Cs MwNAPL CNAPL;s Mw

ð2Þ

where Cs, compound massic concentration in soil; CNAPL,s, NAPL massic concentration in soil; Mw, compound molecular weight; MwNAPL, NAPL molecular weight.

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As the molecular weight of the NAPL cannot be measured the common range provided in the literature for coal tar was used (200e1000 g mol1, Meta Environmental, 1990). CNAPL,s was based on Lane’s works (Lane and Loehr, 1992, 1995) who assumed that the total organic carbon (TOC, mg kg1) detected in the soil was equivalent to the amount of TOC in the tar, and that the NAPL (tar) has an average TOC of 71%. This gives: CNAPL;s Z

TOC 0:71

ð3Þ

Based on relations (1)e(3) the aqueous concentration of a single PAH solubilized from a multiple-component NAPL can be expressed as: Caq Z

0:71Cs MwNAPL S TOC!Mw

ð4Þ

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solubility of PAHs in aqueous CD solutions has been observed to increase linearly with the concentration of CD (Wang and Brusseau, 1993; Brusseau et al., 1994; McCray et al., 2001). The apparent PAH aqueous concentration Caq,app is the sum of free PAH form, and the CD-complexed form [PAH/CD]: Caq;app ZCaq CC½CD=PAH

ð8Þ

Thus, Caq;app ZCaq ð1CKCW CCD Þ

ð9Þ

where Caq, compound aqueous concentration calculated either hypothesis (i) or (ii); KCW, compound partition coefficient between CD and water or stability constant; CCD, CD aqueous concentration. Concentration of extracted PAH present in the flushing solutions (Caq,app) can be estimated by Eq. (9), based on a PAH aqueous concentration estimated by hypothesis (i) or (ii) and the partition between CD and water equilibrium constant, available in the literature.

2.2. Desorption model Lane and Loehr (1995) developed also a method to estimate PAHs aqueous concentration based on the desorption phenomenon, and the soil/water partition constant Kp found in the literature: Cs Kp ZKoc foc Z Caq

ð5Þ

where Koc, organic carbon partition coefficient; foc, organic carbon fraction present in the soil. Cs can be detailed as: Cs ZCs;0  Caq

L S

ð6Þ

where Cs,0, compound initial concentration in soil; L/S, mass of water (L) in contact with the mass of soil (S ) ratio. Then, Caq is given by: Caq Z

Cs;0 Koc foc CLS

ð7Þ

2.3. Cyclodextrin-enhancement of contaminants aqueous concentrations Previous hypothesis can predict PAH aqueous concentration in pure water with either relation (4) or (7), but these concentrations are considerably enhanced in presence of cyclodextrins. The apparent PAH aqueous concentration in presence of CD could also be estimated. Cyclodextrins seems to have a similar behavior as many cosolvent or surfactants systems. The apparent

3. Materials and methods 3.1. Chemicals b-Cyclodextrin (BCD) was obtained from RoquetteFre`res (France) and both hydroxypropyl-b-cyclodextrin (HPCD) and methyl-b-cyclodextrin (MCD) were supplied by SigmaeAldrich. All CDs were used without further purification. The solubilities of natural CDs in water are different based on the number of glucose units (6, 7, or 8, respectively, for a, b, or g-CD). BCD (native form, 1135 g mol1) is the least expensive, but unfortunately, has a limited water solubility (18.6 g L1 at 20  C, Szejtli, 1982). Thus, b-cyclodextrin is often chemically modified to enhance its water solubility. Two of its most used derivatives, HPCD (1420 g mol1) and MCD (1310 g mol1) were found to be very water-soluble (O500 g L1). 3.2. Soil characteristics The soil used in this paper was obtained from the site of a former manufacturing gas plant in the south of France. It was air-dried, crushed and passed through 2 mm sieve to remove coarse materials. The collected fraction (%2 mm) represented about 70% of the sample. Its major characteristics are given in Table 1. Coal tars are by-products of the coal carbonization process and are often present as contaminants at the sites of coke production and manufactured gas plants (U.S. E.P.A., 2004). Coal tar is a denser than water, non-aqueous phase liquid composed of a complex mixture of organic compounds, including abundant

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Table 1 Physical and chemical characteristics of the investigated soil (!2 mm fraction) Water content Organic matter content TOC pH water Clay Sand Silt

0.3% 2.7% 0.770% 8.15 1.9% 94.40% 3.70%

polycyclic aromatic hydrocarbons (PAHs) as well as more volatile components such as benzene and alkylated benzenes (Majhoub et al., 2000). The contamination of this site is about 30 years old and is mostly present as NAPL. The total amount of PAHs regarded as the US EPA 16 list was about 655 mg kg1. PAH background concentrations in soil were determined in triplicates. The reproducibility of measurement was excellent, informing of the good homogeneity of this soil. The soil was stored at 5  C. Detailed initial state of contamination is given in Table 2. 3.3. Analytical methods During experiments, aqueous samples of washing solutions (about 0.5 mL) were analyzed, (without preparation), by reverse-phase chromatography using a Waters HPLC system (Waters LC-module 1, Waters pumps 600) equipped with a UVevisible detector (UV Autochrom 162 CSI) and a reverse-phase C-18 column (Supelcosil LC-PAH, 250 mm ! 4.6 mm i.d., 5 mm). The mobile phase was a mixture of water/acetonitrile (20:80, v/v), with a flow rate of 1.0 mL min1. Wavelength used for detection was 254 nm. PAH concentrations Table 2 Initial concentration of the US EPA 16 PAHs in the studied soil (triplicates, average of the three measurements) PAH compound

mg kg1 of dried soil

Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(a)pyrene Indeno(1,2,3-c,d )pyrene Dibenzo(a,h)anthracene Benzo( g,h,i)perylene

4.4 4.3 66 48 200 71 110 71 21 23 11 9.0 8.1 3.7 1.3 3.9

Sum of US EPA 16 PAHs

655.2

were quantified with an external standard method. Water used for solutions was purified by Milli-Q system (Millipore). Acetonitrile HPLC grade (99.99%) was provided by Fluka. Three compounds, representative of 3-cycles-PAHs, have been chosen as targeted contaminants, phenanthrene (PHE), anthracene (ANT) and pyrene (PYR) for analytical reasons (short retention time and good signal at the chosen wavelength). The maximal uncertainty of the analytical methods obtained was estimated at about 5%. To verify that the complexation of PAHs with cyclodextrins do not affect the UV absorbance within the detector, three dilution experiments were conducted. A sample of washing solution containing PAHs and CD was separated into three identical fractions. Each of them was, respectively, diluted (1:2) with pure water (blank test), acetonitrile and methanol (to dissociate PAH/CD inclusion complexes), and then analyzed as mentioned before. All of the three results gave the same HPLC peak, and so, the same concentration for targeted PAHs (within a 2% range). This shows that PAHs determination is unaffected by solvents and complexing agents in our experimental conditions. 3.4. Column extraction experiments Experiments were designed to prevent soil mobilization, to measure extraction equilibrium concentrations of contaminants in conditions close to a ‘‘pump and treat’’ process. Detailed experimental set-up is described in Fig. 1. Columns were purchased from Amersham Biosciences (Germany), and were originally conceived for pharmaceutical chromatography experiments. The columns were glass-made (H: 400 mm, i.d.: 26 mm), with two end plates designed to minimize void volume, protected by nylon membranes (pore diameter 10 mm). All connections and tubing were made of Teflon. The columns were packed in incremental steps with 50 g of dry soil to establish uniform bulk density (about 7 cm height in the columns). One layer of glass beads was used to protect top and bottom nylon membrane from clogging. Tracer tests have been conducted (tracer: KBr with online conductivity sensor at the exit of column, direct flow without recirculation), and have shown that there is no preferential paths nor accumulation of tracer, and that radial diffusion is quite limited (short tailing in tracer response curve). After packing, experiments began without prior water saturation. For each experiments three columns were used, testing, respectively, a solution of BCD, HPCD and MCD, added with 0.01 M of CaCl2 to adjust the ionic strength, and 400 mg L1 of HgCl2 to prevent microbiological growth (Bayard et al., 2000). Extracting solutions are slowly pumped with a MasterFlex peristaltic pump from the buffer bottle through the columns (ascendant flow) in a closed loop. Flow rate was set at 1 mL min1.

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2 5 1 3

4

Fig. 1. Design of the experimental set-up: (1) soil (average height: 7 cm), (2) end plates with nylon membranes and glass beads layer, (3) buffer bottle, (4) magnetic stirrer, (5) peristaltic pump. All connections and tubing were made of Teflon.

The whole system, detailed in Fig. 1, was placed in a thermostatic room, and buffer bottles were protected from light by an aluminum sheet. Samples for PAHs analysis were collected from those bottles. Three temperature kinetics have been studied (5, 20, 35  C) in a thermostatic room with the same concentration of CD, except for BCD where aqueous solubility dropped too low at 5  C to be tested. The summary of various experiment realized is shown in Table 3.

4. Results and discussion 4.1. Verification of extraction mechanism Column extraction experiments were carried out with pure water as flushing solution to determine equilibrium PAHs aqueous concentration. A liquid to solid mass ratio (L/S ) of 6 was used, and tests were conducted at

22  C G 2  C. All theoretical and experimental values of equilibrium PAH concentrations are reported in Table 4. Inspection of Table 4 reveals that our experimental values shown a clear fit with Raoult’s law values, whereas soil/water partition values are at least an order of magnitude higher. Underestimated anthracene concentrations described by Raoult’s law could be caused by a non-ideal behavior of the NAPL (Majhoub et al., 2000). This could be amplified by the very low aqueous solubility of anthracene compared to a similar molecule like phenanthrene (0.062 mg L1 and 1.1 mg L1 at 25  C, respectively, for the same molecular weight, source: IUPAC-NIST Solubility Database). So in this case, desorption behavior cannot be totally neglected. Despite those limitations, the main mechanism for PAHs solubilization seems to be the dissolution of the NAPL into water phase described by Raoult’s law. This extraction behavior is consistent with previous works found in the literature (McCray and Brusseau, 1999). 4.2. Determination of steady-state at three temperatures Experiments were conducted on three columns, using respectively BCD, HPCD and MCD as flushing agents, to determine the time needed to reach a steady-state and the apparent equilibrium PAHs concentration. An L/S ratio of 3 was used. Due to the specific design of the experimental device, only the kinetic of the whole system can be observed. Evolution of PAHs aqueous concentration in the samples taken from the buffer bottles versus time shows an exponential profile before concentration plateaued (Fig. 2). This kind of profile is typical from a continuous stirred tank fed with a constant concentration solution (Caq,eq), with the same flow leaving the bottle at Caq,app. To simulate the temporal evolution of PAHs apparent aqueous concentration, a mass balance was realized on the buffer bottle, with our specific device conditions (flow from column entering bottle is equal to flow exiting bottle to the column, some volume of solution in bottle is used to fed pore volume of the column): dCaq;app q q C Caq;app  Caq;eq Z0 V V dt

ð10Þ

Table 3 Main parameters tested in various experiments Tested parameters

Kinetic exp.

CD concentration exp.

L/S ratio exp.

CD concentration (av. value, g L1) L/S ratio (kg liquid/kg soil) Temperature (  C) CD type

15 3 5, 20, 35 BCDa, HPCD, MCD

10, 30, 70, 100 2 22 HPCD, MCD

10 2, 4, 6, 8, 10 22 BCD, HPCD, MCD

a

Not tested at 5  C.

432

Table 4 Comparison of actual and predicted PAHs aqueous concentrations at equilibrium, mg L1 (Koc values: PHE: 1.39 ! 103, ANT: 1.36 ! 103, PYR: 7.29 ! 103, Lane and Loehr, 1995) Ci,aq (mg L1)

Experimental values (22  C)

Raoult’s law values (25  C)

Soil/water partition values (25  C)

Phenanthrene Anthracene Pyrene

0.13 0.04 !0.05

0.026e0.13 0.0005e0.0025 0.0009e0.0045

1.8 0.66 0.13

where q, flow rate entering or exiting the bottle; V, volume of liquid in the bottle (total liquid volume, subtracted from pore volume of column); Caq,eq, compound apparent aqueous concentration at steady-state. Relation (10) gives the following form for Caq,app time evolution: ÿ  Caq;app ZCaq;eq 1  eqt=V ð11Þ

Concentration in solution (mg.L-1)

Graphical representation of Eq. (11) (data not shown) exhibits a good fit with experimental data. This implies that PAHs concentration evolution in the buffer bottle, Caq,app, is comparable to the ones in a perfect reactor fed with a solution at the steady-state concentration (Caq,eq) with the same flow rate than we used. So, CD extraction process of PAH from soil, as well as their complexation in water can be considered as almost instantaneous. This was confirmed by batch experiments on the three types of CD (data not shown). Furthermore no mobilization peak was observed during the whole experiment (7 days). The evolution of extracted concentrations versus time was similar for all experiments, independently from temperature or CD type. Temperature is an important process parameter, so it is really noticeable that the extraction seems not very sensible to temperature variation (5, 20 and 35  C), as shown in Fig. 3. For these three experiments, time to reach a steady-state, and PAHs aqueous

15 NAP PHE ANT

12 9 6 3 0

0

5

10

15

20

25

Time (h) Fig. 2. PAHs aqueous concentrations during column experiments with HPCD (15.2 g L1), L/S ratio of 3, at 5  C. Dashed lines are simulation of a continuous stirred tank fed with a solution containing equilibrium concentration of PAHs.

Concentration in solution (mg.L-1)

C. Viglianti et al. / Environmental Pollution 140 (2006) 427e435

6

PHE ANT PYR

5 4 3 2 1 0

0

10

20

30

40

Temperature (°C) Fig. 3. Equilibrium PAHs aqueous concentrations with HPCD (15.1 g L1) for different temperatures, L/S ratio of 3.

concentration remained relatively constant. This is very interesting for a possible industrial application because others methods (organic solvents, surfactants) present a clear decrease of efficiency with decreasing temperature (Krauss and Wilcke, 2001). As enhancement of aqueous solubility of PAHs is caused by the complexation reaction, this very low dependence to the temperature is probably due to the increase of PAHs aqueous solubility with temperature (Whitehouse, 1984), counterbalanced by a destabilization of PAH/CD complexes. These complexes have a negative enthalpy of formation (about 4 kcal mol1 for anthraceneeBCD complex, Blyshak et al., 1990), and so, tend to be dissociated with the temperature increasing. This results in a relative insensibility of PAH extraction with temperature variation (within the range of temperature studied), even if more work in this field is needed to confirm this behavior. 4.3. Effect of CD concentration CD concentration effect on PAHs extraction was studied within a CD aqueous concentration range of 10e100 g L1 with an L/S ratio of 2. BCD was excluded from those experiments due to its low aqueous solubility (18.6 g L1 at 20  C). Linearity observed for PAHs release with CD concentration (Fig. 4) corroborates the theoretical approach based on Eqs. (4) and (9). This linear behavior was compared to the theoretical model (Eq. (9)) for phenanthrene and anthracene (Table 5). There is a very good fit between predicted and experimental PHE concentrations for the whole range of CD concentration, but about one fold divergence for ANT values. This could be caused again by a non-ideal NAPL, which could invalidate the use of Raoult’s law (McCray and Brusseau, 1999; Majhoub et al., 2000). It is necessary to quote here that the apparent linearity observed is valuable in our own experimental conditions (soil and CD concentrations from 10 to 100 g L1,

433

Cumulated extracted quantities (mg/kg of soil)

Concentration in solution (mg.L-1)

C. Viglianti et al. / Environmental Pollution 140 (2006) 427e435

30 PHE ANT PYR

25 20 15 10 5 0

0

20

40

60

80

100

25 PHE ANT

20 15 10 5 0

120

0

2

4

HPCD concentration (g.L )

i.e. 1e10% (w/v)). Nevertheless, the apparent linearity of PAHs removal indicates that, for practical purposes, extraction may be considered to be an ideal, equilibrium process following Raoult’s law (Eq. (4)) and relation (9) with CD presence, for most PAHs contaminants. However, non-ideal NAPL may influence the dissolution behavior of some target contaminants, even if linearity can still be observed. 4.4. Effect of successive washings Experiments on L/S ratio were carried out to test BCD, which cannot be tested in a concentration range due to its low aqueous solubility. Successive extractions were performed with the three types of cyclodextrin. Each column was circulated by a closed-loop flow of CD for 48 h then the solution was removed and replaced by a fresh one, with exactly the same characteristics, to wash the same sample of soil. As a consequence, volumes of extracting solution were cumulated so data are based on cumulated L/S ratio steps. L/S ratios flushing steps of 2 were used (50 g of soil and steps of 100 mL of solution), 5 extractions have been done for BCD and HPCD and 3 for MCD, with and an additional flushing with pure water to estimate the availability of pollutants after treatment. Fig. 5 shows the cumulated amount of PAHs extracted after Table 5 Comparison between experimental and predicted values of PAHs aqueous concentration extracted by HPCD HPCD Predicted: Experimental: Predicted: Experimental: concentration PHE PHE ANT ANT (g L1) (mg L1) (mg L1) (mg L1) (mg L1) 10 32 72.5 100

2.46 7.48 17.01 23.41

3.4 10.7 19.2 22.9

0.06 0.18 0.40 0.45

2.0 6.1 9.9 11.5

Based on Eqs. (4) and (9) with NAPL molecular weight of 1000 g mol1, KPHE Z 2400 M1, KANT Z 2950 M1 (Brusseau et al., 1994; Badr et al., 2004).

8

10

12

Fig. 5. Total extracted amount of PHE and ANT with HPCD (10 g L1) at different L/S ratios, 22  C.

each flushing step, whereas Fig. 6 exhibits the aqueous PAHs concentration for each washing step solution at equilibrium. Cumulated quantities of extracted PAHs seem to follow again a quasi-linear trend with the increase of washing solution used. This confirms previous experiments, because PAHs extracted quantities increased almost linearly with the overall quantity of CD used, whether this amount is provided by an increased concentration of CD at the same L/S ratio or increased L/S ratio. This experiment has also allowed to verify that BCD has the same extracting behavior than modified CDs even if its aqueous solubility is very limited. The specificity of this product is that PAHs aqueous concentration remained fairly constant at every flushing steps, whereas with HPCD and MCD, PAHs aqueous concentration dropped slowly. This is probably mainly caused by the very low concentrations extracted by BCD opposed to quite large ones obtained with HPCD and MCD: there maybe a depletion of the easily extractable fraction of PAHs. The final water flushing done with MCD shows that the pollutant is available after treatment. 0.31 mg L1 of phenanthrene and 0.20 mg L1 of anthracene were found in this water flush, instead of 0.13 and

Concentration in solution (mg.L-1)

Fig. 4. Equilibrium PAHs aqueous concentration with HPCD at different concentrations, L/S ratio of 2, 22  C.

6

L/S ratio

-1

5

PHE ANT

4 3 2 1 0

1

2

3

4

5

Washing step Fig. 6. Equilibrium aqueous concentrations for PHE and ANT with HPCD (10 g L1) for each washing step (L/S ratio step of 2), 22  C.

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C. Viglianti et al. / Environmental Pollution 140 (2006) 427e435 50%

ANT extraction (%)

0.04 mg L1, respectively, during a water flush done without prior treatment. This last point puts the emphasis on the necessity of a post-rinsing after a treatment with CD. Despite the fact that CDs are biodegradable and non-toxic, persistence of these compounds in the pore volume of the soil will make residual PAHs more mobile and bioavailable than before treatment, causing a probable dispersion of these contaminants.

30% 20% 10% 0%

4.5. Comparison between the removal efficiencies of three CDs

25% 20% 15% 10% 5% 0%

0

20

40

60

80

100

120

140

160

20

40

60

80

100

120

140

160

180

180

CD concentration (mmol/kg of soil) Fig. 7. Efficiency comparison between cyclodextrins for PHE extraction (percentage of recovery at 22  C).

Fig. 8. Efficiency comparison between cyclodextrins for ANT extraction (percentage of recovery at 22  C).

Whatever CD used as flushing solution, removal percentages were ranked as followed: ANT O PHE O PYR. This ranking is based on the initial state of contamination, and is different from extracted concentration ranking (PHE O ANT O PYR), based on apparent aqueous concentrations of PAHs in presence of CDs for this soil. These results indicate that CDs showed a strong efficiency for PAH solubilization and removal from soil. While surfactants may obtain comparable results, reduction of interfacial tension may cause partial mobilization of immiscible liquid (Pennel et al., 1994; Boving and Brusseau, 2000) and frequently emulsification (Okuda et al., 1996; Bai et al., 1997). Cosolvent flushing (e.g. ethanol/water) has also shown mobilization at the beginning of the treatment (Boving and Brusseau, 2000). All these phenomena were not observed for CDs during this study.

5. Conclusion Results of this work show that CD has a great potential for PAHs extraction on aged-contaminated soils. The use of CDs as a solubility-enhancement agent is effective for PAH compounds. The main mechanism of PAHs Enhancement extraction factor

PHE extraction (%)

BCD HPCD MCD

30%

0

CD (mmol/kg of soil)

Prior to comparison of the removal efficiency of three types of CD, the concentrations of CD in flushing solutions were expressed in mmol by kg of dry soil. The removal percentage of PHE and ANT using three types of CD extracting solutions are reported in Figs. 7 and 8. Data were extracted from both Sections 4.3 and 4.4, and confirmed the linearity observed for the variation of amount extracted versus CD quantity in flushing solution. Inspection of Fig. 7 reveals that cyclodextrin is an efficient agent to remove PAH from soil and that the cleaning capacity increases linearly with CD concentration. For both compounds, the removal efficiencies of CD can be ranked in the following order: MCD O HPCD [ BCD, which is consistent with the complexation equilibrium constants available in the literature. The modified CDs were more efficient than the BCD for PAH removal. When 10% of MCD solution was used as a flushing agent, 31% of PHE and 43% of ANT recovery from soil were observed. Thus, methyl-b-cyclodextrin or hydroxypropyl-b-cyclodextrin could be more suitable for remediation of PAH-polluted soils. Fig. 9 shows the enhancement extraction factor of three PAHs by HPCD and MCD compared to water used alone. PAH recovery from soil was improved by, respectively, a 150, 250 and 200 times factor for PHE, ANT and PYR, when using MCD flushing solution as compared to a pure water flushing solution. 35%

BCD HPCD MCD

40%

300 250

HPCD MCD

200 150 100 50 0

PHE

ANT

PYR

Fig. 9. Enhancement extraction factors for PAHs for modified CDs at 10%w/v.

C. Viglianti et al. / Environmental Pollution 140 (2006) 427e435

extraction from soil to the aqueous phase is the dissolution of NAPL following Raoult’s law with or without CD. Kinetic studies have shown that PAHs solubilization by CDs is a fast process. On our soil, improvement of PAHs extraction was directly proportional to CD concentration in our experimental conditions. This linearity is observed whether CD amounts are increased by higher concentration or higher volumes of solution. A very interesting point of this study is the apparent temperature independence of the extraction between 5 and 35  C, which could be industrially decisive (almost constant efficiency with changing weather conditions). Costs problematic has not been investigated in this paper but represents an important optimization parameter, as the cheapest form of cyclodextrin, BCD, is far less efficient than substituted ones (HPCD, MCD). These two derivates have close performances and the choice between them should be greatly determined by their respective costs. Based on published data, it would appear that CDs are becoming comparable in cost with surfactants. A detailed comparison on the costs of surfactants and CDs for polluted-site remediation is not found in the literature. However, since the cost of CDs has continuously decreased in recent year, investigations regarding their technical merit for subsurface remediation are justified.

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