Influence of the presence of PAHs and coal tar on naphthalene sorption in soils

Influence of the presence of PAHs and coal tar on naphthalene sorption in soils

Journal of Contaminant Hydrology 46 Ž2000. 61–80 www.elsevier.comrlocaterjconhyd Influence of the presence of PAHs and coal tar on naphthalene sorpti...

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Journal of Contaminant Hydrology 46 Ž2000. 61–80 www.elsevier.comrlocaterjconhyd

Influence of the presence of PAHs and coal tar on naphthalene sorption in soils Remy ´ Bayard ) , Ligia Barna, Borhane Mahjoub, Remy ´ Gourdon Laboratoire d’Analyse EnÕironnementale des Procedes Industriels, Institut National des ´ ´ et des Systemes ` Sciences Appliquees ´ (INSA), Building 404, 20 AÕenue A. Einstein, 69621 Villeurbanne Cedex, France Received 2 September 1999; received in revised form 9 May 2000; accepted 27 June 2000

Abstract The mobility of the most water-soluble polynuclear aromatic hydrocarbons ŽPAHs. such as naphthalene in contaminated soils from manufactured gas plant ŽMGP. sites or other similar sites is influenced not only by the naturally occurring soil organic matter ŽSOM. but also, and in many cases mostly, by the nature and concentration of coal tar xenobiotic organic matter ŽXOM. and other PAH molecules present in the medium under various physical states. The objective of the present study was to quantify the effects of these factors using batch experiments, in order to simulate naphthalene transport in soil–tar–water systems using column experiments. Naphthalene sorption was studied in the presence of Ži. solid coal tar particles, Žii. phenanthrene supplied as pure crystals, in the aqueous solution or already sorbed onto the soil, Žiii. fluoranthene as pure crystals, and Živ. an aqueous solution of organic molecules extracted from a liquid tar. All experiments were conducted under abiotic conditions using short naphthalenersorbent contact times of 24–60 h. Although these tests do not reflect true equilibrium conditions which usually take more time to establish, they were used to segregate relatively rapid sorption phenomena ŽApseudo equilibriumB . from slow sorption and other aging phenomena. For longer contact times, published data have shown that experimental biases due to progressive changes in the characteristics of the soil and the solution may drastically modify the affinity of the solutes for the soil. Slow diffusion in the microporosity and in dense organic phases may also become significant over the long term, along with some irreversible aging phenomena which have not been addressed in this work. Results showed that PAHs had no effect on naphthalene sorption when present in the aqueous solution or as pure crystals, due to their low solubility in water. Adsorbed phenanthrene was found to reduce naphthalene adsorption only when present at relatively high concentrations Žabout 120 mgrkg. in the soil. In contrast, experiments carried out with coal tar particles revealed a

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Corresponding author. E-mail address: [email protected] ŽR. Bayard..

0169-7722r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 7 7 2 2 Ž 0 0 . 0 0 1 2 5 - X

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significant effect. Naphthalene sorption appeared to be proportional to the amount of coal tar added to the sand or soil, and a much higher affinity of naphthalene for XOM Ž K oc above 2000 cm3rg. than SOM Ž K oc around 300 cm3rg. was observed. Naphthalene transport in the columns of sand or soil spiked with coal tar particles was simulated very satisfactorily with a dual double-domain model. Around 90% of naphthalene retention by coal tar was found to occur within the organic phase, suggesting a phase partition process which may be explained by the amorphous nature of the XOM and its extreme affinity for naphthalene. For SOM, however, which is present as porous microaggregates of clay and humic substances, with less affinity for naphthalene, only 1r3 of naphthalene retention was found to occur within the organic phase, underlining the significant role of surface adsorption in the short term behavior of naphthalene in soil. For longer contact times, the model simulations proposed in the present study should be coupled to slow sorption, aging and biodegradation models to describe long-term behavior of naphthalene in soil–tar–water systems. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Naphthalene; Adsorption; Partition; Soil; Modeling; Coal tar

1. Introduction The potential environmental impact of contaminated sites greatly depends on the mobility of the pollutants which is mostly determined by their distribution between the aqueous Žmobile. and solid Žimmobile. phases and mass transfer parameters. Many studies have shown that sorption is the major mechanism of retention of organic contaminants in soils. In the case of hydrophobic organic compounds ŽHOCs., sorption essentially occurs onto soil organic matter ŽSOM. ŽCalvet, 1989. generally associated with the mineral fraction. The extent of sorption depends on the organic carbon ŽOC. content and on the nature of organic matter in soils and sediments ŽGrathwohl, 1990.. Sorption usually reduces very significantly the availability of the pollutants to the soil microorganisms, thereby limiting their biodegradation and increasing their persistence in the soils ŽHatzinger and Alexander, 1995.. Numerous investigations have provided evidence on the contribution of organic matter and soil microstructure in the retention of organic contaminants in natural organic soil ŽLuthy et al., 1997.. Most authors agree on the necessity to consider the structural and compositional heterogeneity of natural organic soils to simulate instantaneous and rate-limited sorption commonly observed with HOCs. Two-site Žtwo-compartment. models have been successfully employed by numerous authors for short contact times where quasi-instantaneous and rate-limited Žtime-dependent. sorptions occur ŽWu and Gschwend, 1986; Miller and Weber, 1986; Ball and Roberts, 1991; Brusseau et al., 1991; Mahjoub and Gourdon, 1999.. However, for contact times longer than a few days and even under abiotic conditions, progressive changes in the characteristics of the soil Ždesaggregation of the microaggregates, alteration of the soil chemistry, etc., usually referred to as aging phenomena, may drastically modify the affinity of the solutes for the sorbent ŽMahjoub and Gourdon, 1999; Young and Weber, 1995.. Slow sorption processes such as diffusion in the nanoporosity may also become significant over long contact times ŽMahjoub and Gourdon, 1999; Young and Weber, 1995; Steinberg et al., 1987., along with the formation of irreversibly bound residues or the entrapment of the

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pollutants ŽPignatello and Xing, 1996. although these latter processes are favored by the microbial activity. In the case of contaminated soils from manufactured gas plant ŽMGP. sites, naturally occurring SOM is usually not predominant. The transfer of the most water-soluble polynuclear aromatic hydrocarbons ŽPAHs. is therefore influenced not only by the SOM but also, and in many cases mostly, by the nature and concentration of coal tar xenobiotic organic matter ŽXOM. present in the polluted soil. Coal tar subsurface contamination is associated with coal transformation activities ŽAziz and Melcer, 1991; Luthy and Ramaswami, 1993; Ouimet et al., 1995.. Coal tar can be considered as an anthropogenic organic material, as opposed to the naturally occurring SOM by its nature and structure. SOM consists of macromolecular mixtures of humic and fulvic acids organized in three-dimensional structures associated with clay particles in the form of porous microaggregates ŽWershaw, 1996.. Coal tar organic matter is an amorphous material constituted of a mixture of numerous aromatic organic compounds including high molecular weight PAHs that are trapped or dissolved in a dense nonaqueous phase ŽE.P.R.I., 1993.. Depending on the composition of the tar, its physical state can be solid or liquid ŽLane and Loehr, 1992.. Solid coal tar found in contaminated soils from MGP sites can be considered as AagedB coal tar. Water soluble organic compounds such as BTEX, phenol or naphthalene can be slowly released from fresh tar into the subsurface water ŽLane and Loehr, 1992; Mahjoub et al., 2000., resulting in a progressive increase of the density and viscosity of the tar which ultimately may turn into solid particles containing mostly pitch and high molecular weight PAHs. Such solid tar particles can have a significant effect on the retention of water soluble pollutants from the aqueous phase. Kopinke et al. Ž1995. have studied the sorption of hydrophobic solutes from aqueous solution onto organic materials from both coal wastewater ponds and sediments in order to determine the sorption properties of natural and anthropogenic humic-type materials. They observed in both cases a very good correlation between the sorption coefficient Ž K oc . and the octanol–water coefficient K ow . During remediation operations of MGP contaminated sites, the most heavily contaminated soils would generally be excavated and treated or disposed of in a controlled area either on site or off site. Soils from different areas of the site Žand sometimes from different sites., with variable levels of contamination, may be mixed during these operations. Some soils may contain AagedB solid tar with low contents of water-soluble contaminants, while others may contain more liquid AfreshB tar with higher concentrations of water-soluble contaminants. The presence of precipitates of heavy PAHs may even be observed in some soils. These factors may strongly affect the mobility of the relatively soluble contaminants such as naphthalene and should therefore be carefully considered to better evaluate the long-term behavior of the disposal area in a risk assessment procedure or predict the treatment efficiency, in particular when biological techniques are used. The objective of the present article is to evaluate the effects of Ži. AagedB coal tar particles, Žii. molecules of PAHs in solution in the aqueous phase, Žiii. molecules of phenanthrene sorbed onto the soil, and Živ. pure crystals of phenanthrene and fluoranthene, on the mobility of naphthalene in soil–water systems. Naphthalene has been used because it is the simplest and most water-soluble PAH. Two experimental methods have

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been used, respectively a closed stirred system Žbatch. and an open system Žcolumn. consisting of a static solid phase and a flowing aqueous phase. The batch procedure has been used to determine the sorption parameters under the different conditions tested. The column experiments have been used to verify the validity of the sorption parameters determined in batch concerning the effect of coal tar particles on the mobility of naphthalene in soil or sand. All experiments have been conducted under abiotic conditions using short naphthalenersorbent contact times in order to avoid biodegradation and aging phenomena. Slow sorption processes have therefore not been addressed in this study.

2. Materials and methods 2.1. Sorbents Fontainebleau sand ŽProlabo, France., with a particle size distribution ranging from 150 to 210 mm and no trace of carbon has been selected as a pure mineral solid phase for batch and column experiments. One natural agricultural soil sampled from La Cote ˆ Saint Andre, ´ Isere, ` France ŽCSAC., whose major characteristics are given in Table 1, has also been selected. It is a silty brown culture soil containing about 1.4% of OC Ždry weight percentage.. The soil has been gently air-dried overnight at room temperature, sieved through a 2-mm sieve to remove surface plant debris and coarse materials, sterilized by two successive irradiations with g-rays at 2.5 kGy, and stored at 48C in the dark. Autoclaving was not used because of the strong modifications of SOM induced ŽWolf et al., 1989.. Coal tar particles were extracted from a contaminated soil sampled from a 70-year old MGP site located in the Rhone–Alpes region, France. The contamiˆ nated soil was air dried overnight, coal tar aggregates and soil lumps were roughly broken with a mortar and then sieved at 2 mm. Total organic carbon ŽTOC. concentration was about 9.1% wrw in the MGP soil and 38% wrw in the tar particles. Table 2 shows the concentrations of the analyzed PAHs molecules in the tar particles. Naphthalene concentration was low but not nil Ž110 mgrkg.. However, leaching tests indicated that naphthalene was not extractable with water from the tar using three successive

Table 1 Physical–chemical characteristics of culture soil CSAC from La Cote ˆ Saint Andre´ Isere, ` France Parameter

CSAC

Sand Ž%. Silt Ž%. Clay Ž%. TOC a Ž%. CEC b Žmeqrkg. pH water

39.8 42.5 17.7 1.49 8.6 7.4

a b

TOC s Total organic carbon. CEC sCation exchange capacity.

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Table 2 PAHs concentration in coal tar particles as determined by gaseous phase chromatography with FID detection according to USEPA method 8100 Compound

Concentration Žmgrkg coal tar.

Naphthalene Acenaphtylene Acenaphtene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Chrysene BenzoŽa.anthracene BenzoŽbqk.fluoranthene BenzoŽa.pyrene IndenoŽ1.2.3-cd.pyreneqdibenzoŽa,h.antracene BenzoŽghi.perylene Total quantified PAHs

110 16 70 180 700 290 730 530 250 290 340 160 200 1300 4000

extractions of 16 h with a liquid to solid ratio of 10 according to the French standard AFNOR NF X31-210. 2.2. Analytical methods The soils were analyzed for TOC content using the Anne Ž1945. method and cation exchange capacity using the acetate ammonium method ŽRhoades, 1982.. The moisture content was determined by weight loss after 24 h at 1058C. The coal tar particles were analyzed for PAHs according to the USEPA method 8100. Naphthalene, phenanthrene and fluoranthene aqueous solutions were analyzed using a High Performance Liquid Chromatography unit ŽHPLC. equipped with a Shimadzu LP-6A isocratic pump, a 25-cm Vydac 201-TP5415 C 18 Column, a 3-cm Kromasil C 18 guard column, a Rheodyne injection valve, and a Shimadzu SPD-6A UV detector set at 254 nm. The solvent was acetonitrilerwater 80:20 ŽHPLC solvent grade, Prolabo. with a constant flow rate of 1.2 mlrmin. The two aromatic pollutants were detected by UV absorbance at l s 254 nm. 2.3. Sorbates and chemicals Naphthalene, phenanthrene and fluoranthene were purchased from Aldrich. Naphthalene aqueous stock solution was prepared by mixing an excess of fine crystals in distilled, deionized water for 48 h in the dark at room temperature, and filtering on GF-C Whatman glass fiber filter of 1.2 mm pore size ŽWhatman, Clifton, NJ, USA.. It was stored in airtight glass bottles in the dark at room temperature. Sorption experiments were conducted at 208C at initial concentrations in the range of 1–25 mgrl Žnaphthalene

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solubility limit 31.7 mgrl at 208C. by dilution of the stock solution in an aqueous solution of mercuric chloride at 400 mgrl ŽProlabo. to prevent naphthalene microbial degradation ŽWolf et al., 1989.. 2.4. Batch experiments Batch experiments were conducted in screw-cap Teflon centrifuge tubes containing the sorbent and the aqueous solution of naphthalene in a liquidrsolid ratio of three with a minimal headspace volume. The tubes were stirred on a rotary shaker at 208C in the dark. Apparent equilibrium was reached within 24 h naphthalene-sorbent contact time as previously published ŽBayard et al., 1998a.. Then, the tubes were centrifuged at 10 000 g for 15 min and the supernatant sampled for analysis of pseudo-equilibrium concentration Ž Ce .. Controls containing only the solution and no sorbent were treated in the same way to estimate losses by volatilization and adsorption on tube walls. All assays and controls were at least duplicated. Sorbed concentrations Qe were deduced by mass balance calculations and sorption isotherms obtained by plotting Qe vs. Ce . Two sets of experiments were conducted. In a first set, virgin CSAC soil was used as a sorbent and the effects on naphthalene sorption of some PAHs under different physical states were studied. Sorption experiments were conducted at 208C in a batch mode as described above by suspending the soil Ži. in an aqueous solution of naphthalene spiked with dissolved phenanthrene or pure crystals of phenanthrene or fluoranthene, or Žii. in a coal tar water extract. The tar used in that purpose was a liquid tar sampled from a buried reservoir situated on an old MGP site located in Central France ŽOuimet et al., 1995.. This tar contained around 50 000 mgrl of naphthalene, along with other light organics such as benzene, phenolic compounds, and heavy PAHs. The tar was mixed with pure de-ionized water at a LrS liquid ratio of 10 for 24 h , then the mixture was allowed to settle and the aqueous phase was filtered on Whatman GF-C filter of 1.2 mm pore size. The water extract was spiked with 400 mgrl HgCl 2 to prevent biodegradation and stored at room temperature. In a second set of experiments, different sorbents were suspended at 208C in a pure aqueous solution of naphthalene Žstill spiked with 400 mgrl HgCl 2 to prevent biodegradation. at initial concentrations Ci ranging between 1 and 25 mgrl. The sorbents used Žrespectively agricultural CSAC soil, sand, and coal tar particles. were described above. Agricultural CSAC soil was used both in its virgin, noncontaminated form and in a pre-contaminated form. Pre-contamination was done by suspending the soil for 24 h at 208C in a solution of 50r50 vrv methanolrwater containing phenanthrene at 20 or 200 mgrl, centrifuging the suspension and rinsing the soil twice with water to remove excess methanol. Controls were also conducted on virgin CSAC soil treated in the same manner but without phenanthrene addition. Coal tar particles were used by mixing them with sand or virgin CSAC soil at different concentrations. 2.5. Column experiments Preparative chromatography borosilicate glass columns ŽPharmacia. of 2.6 cm internal diameter were used with a 7-cm bed of CSAC soil Žinitially humidified at 10% wrw. or dry Fontainebleau sand, following a procedure described elsewhere ŽBayard et

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al., 1998a; Barna et al., 1998.. The sand and the soil were sterilized by two successive g irradiations at 2.5 kGy, and HgCl 2 was added at a concentration of 400 mgrl into all the solutions used in order to maintain abiotic conditions. In some experiments, the sand or the soil were manually mixed with coal tar particles prior to be packed into the columns. The packed sand or soil columns were allowed to equilibrate with a gentle close-loop circulation of 500 ml distilled sterile water Žsupplemented with HgCl 2 at 400 mgrl. for 12 h at a flow rate of 0.1 mlrmin to facilitate air displacement. The hydrodynamics of each packed column was subsequently characterized using potassium chloride ŽKCl. as a conservative tracer before adsorptionrdesorption experiments. The tubing of the peristaltic pump used were also equilibrated for 24 h at the same naphthalene concentration as the feed concentration before being connected to the columns ŽBayard et al., 1998a.. A four-way valve allowed to feed the columns with KCl solution, pure deionized water or naphthalene aqueous solution supplemented with CaCl 2 to prevent the dispersion of soil aggregates and HgCl 2 to prevent biodegradation. After the equilibrium phase, a naphthalene aqueous solution supplemented with HgCl 2 was circulated upflow through the columns Žwater-saturated conditions. at a flow rate of 72 mlrh, and naphthalene concentration was monitored on-line at the outlet with a UV detector. A sample collector allowed off-line analysis by HPLC. When the outlet concentration reached the feed concentration, the naphthalene solution was replaced in the feed by distilled water supplemented with CaCl 2 and HgCl 2 . A high flow rate was used to reduce the duration of the experiments in order to avoid possible bias due to the aging of the soil Žalteration of the organic matter, changes in microporosity, . . . . and approach the conditions of the batch experiments where a 24-h contact time was used. The typical duration of soil column experiments was about 30 h for the sorption phase and 30 h for the elution phase. The slow sorption processes were therefore not addressed in this study. 2.6. Modeling The data generated from KCl transfer column studies were analyzed by considering the column as a cascade of n Acontinuously stirred reactorB ŽCSR. to determine the hydrodynamic regime of the sand and soil column as previously described ŽBarna et al., 1998.. The transport model was resolved numerically by the finite difference method and the unknown parameters adjusted by an optimization method on experimental data obtained with the tracer. The hydrodynamic regime was shown to be a plug flow mode ŽBarna et al., 1998. with axial dispersion and no stagnant zone Ž d L s 4.5 = 10y7 m2rs.. The Peclet value P was calculated for each column. Concerning the transport of naphthalene through the soil columns, soil was considered as constituted of spherical aggregates of fine mineral particles and natural SOM, representing the major sites of adsorption of hydrophobic pollutants in water-saturated system. A double-domain model was applied, considering a nonuniform distribution of the adsorption sites constituted by SOM. Adsorption was considered to be quasi-instantaneous on the external Žsurface. domain and kinetically limited by diffusion in the internal domain ŽBarna et al., 1998. This type of model has been often applied to describe HOCs transport through soils or

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sediments ŽBarna et al., 1998; Bayard et al., 1998a; Brusseau and Rao, 1989; Piatt et al., 1996; Bouchard, 1998.. The coal tar particles were considered spherical and the sorption process was modeled also by a double-domain. Sorption on columns containing both types of sorbents Ži.e. compartments SOM and XOM. was described using a dual double-domain model. Adsorption was assumed to be rapid Žlocal equilibrium., linear and reversible for the external and internal domains: Qe s K d Ce

Ž 1.

where Ce is the solute concentration of naphthalene Žmilligram of pollutant per liter., Qe is the adsorbed concentration Žmilligram of pollutant per kilogram of soil., and K d is the distribution coefficient Žlrkg.. K d has been experimentally determined from the slopes of the sorption isotherms which have been found linear under the experimental conditions of this research. K d can be expressed as K oc with respect to the OC content: K oc s K drfoc where f oc is the OC content of the sorbent. The mass balance of the adsorbed compound at the aggregate level takes into account the rapid adsorptionrdesorption processes and the diffusion in radial pores. However, slow sorption processes Žsuch as diffusion in the nanoporosity. has not been considered in this study. A partial differential equation of second order is obtained in radial coordinates with appropriate boundary conditions. The linear driving-force approximation is commonly used to simplify and resolve this equation. An overall fluid-to-particle mass transfer coefficient, k 0 , is introduced ŽDo and Rice, 1986; Goto et al., 1990. and the second order partial differential equation becomes an ordinary differential equation having the same form as a linear kinetic equation. If the external fluid-to-particle mass transfer is rapid, the overall transfer coefficient k 0 is limited by the internal rate of transfer. For each compartment, SOM in soil aggregates and XOM in coal tar particles, one can write : For the pore Žinternal. domain with local equilibrium and mass transfer resistance : dQi ,p dt

s k d Ž FK d Ci y Q i ,p .

Ž 2.

were the subscript i refers to the CSR considered and F is the ratio of the number of internal sites to the total number of sorption sites in one compartment. The transport parameter k d is defined as ŽBarna et al., 1998.: kd s

as k 0 K d rprr q ´ p

Ž 3.

were as is the specific surface area of particles, r and rp are respectively the water’s and the particle’s densities and ´ p is the particle’s porosity. In this study k d is an adjusted parameter. For the external Žsurface. domain with local equilibrium and no resistance to mass transfer: dQi ,ex dt

s Kd

dCi dt

Ž1yF .

Ž 4.

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The total mass balance in the system is then: dCi dt

n q

t

Ž Ci y Ciy1 . s y

rp 1 y ´ r

´

Ý compartments

ž

dQi ,p dt

q

dQi ,ex dt

/

compartment

The subscript i refers to the CSR considered, n is the number of CSR determined by the hydrodynamic study, and t is the residence time of mobile water. Therefore, three parameters Ž K d , k d , and F . are needed for each compartment. Parameter K d was determined from the slopes of the sorption isotherms obtained from batch experiments. The other two model parameters were calculated by best-fit adjustment to the batch kinetic data and the column data using the Rosenbrock Ž1960. optimization method and the results obtained with the two methods were compared. The numerical values thereby calculated separately for each individual compartment SOM and XOM from two independent sets of experiments were then combined into a dual double-domain model and model predictions compared to the data obtained in a third set of experiments in the presence of the two compartments together.

3. Results and discussion 3.1. Batch studies Sorption isotherms of naphthalene at the pseudo-equilibrium attained in 24 h contact time were found to be linear in all cases under the experimental conditions of this study ŽFigs. 1 and 2.. The distribution coefficients K d of naphthalene between the sorbents and the solutions were therefore calculated as the slopes of the isotherms. Other data

Fig. 1. Naphthalene adsorption isotherm on virgin soil CSAC as compared to soil CSAC having previously adsorbed phenanthrene at 23 or 118 mgrkg. Batch test ŽT s 208C; liquid to solid ratios 3; contact time s 24 h..

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Fig. 2. Naphthalene sorption isotherms obtained in batch on Fontainebleau sand and culture soil from La Cote ˆ Saint Andre´ ŽCSAC wTOCx s1.49%. spiked with coal tar particles ŽwTOCx s 38%. at different concentrations. Batch test ŽT s 208C; liquid to solid ratios 3; contact times 24 h..

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previously published also indicated that the sorption was also well reversible under those conditions ŽBayard et al., 1998a.. Linear sorption isotherms were observed by numerous authors working under similar experimental conditions as those of this study, i.e. short contact times and low solute concentrations ŽFu et al., 1994; Kan et al., 1998.. Nonlinear sorption and hysteresis were also observed in many studies including by the authors, but usually for significantly longer contact times or higher solute concentrations ŽHuang et al., 1997; Mahjoub et al., 1999; Young and Weber, 1995; Steinberg et al., 1987; Pignatello and Xing, 1996.. Slow sorption processes related to diffusion into micro-and nanopores or into dense organic phases, or other aging phenomena such as the alteration with time of the sorbent’s characteristics or the formation of bound residues, may explain the nonlinear and the hysteresis effects. The description of these long-term effects usually requires the combination of relatively rapid kinetics of adsorption, diffusion into AsoftB organic phases or into mesopores, which are the object of this study, with the slow kinetics mentioned above, which are out of the scope of the present research. The distribution coefficients determined on soil CSAC in the presence of organic pollutants under different physical states are given in Table 3 and compared to the coefficient obtained for pure naphthalene alone onto virgin CSAC soil. Results showed that organic pollutants such as phenanthrene, or other compounds extracted from coal tar did not significantly reduce naphthalene adsorption on soil CSAC when present in solution in the aqueous phase. This can be explained by the fact that these pollutants were not significantly in competition for adsorption with naphthalene in the range of concentrations studied due to their low solubility in water. The slightly higher adsorption of naphthalene in the presence of dissolved phenanthrene or in the coal tar water extract ŽTable 3. may suggest a cooperative adsorption phenomenon but was not considered as very significant with respect to the interval of confidence on K d Ž"5%.. Even in the presence of pure crystals in excess, the concentration of phenanthrene or fluoranthene in the aqueous phase was limited by their solubilities in water Žrespectively 1.3 and 0.12 mgrl at 208C. and thereby remained much lower than naphthalene concentration, inducing no significant competition with naphthalene for adsorption.

Table 3 Effects of organic pollutants on the parameters of naphthalene adsorption onto soil CSAC a Nature of the medium of suspension

K db

c Log K oc

Pure naphthalene aqueous solution Naphthalene-solution spiked with phenanthrene at 0.6 mgrl Naphthalene in coal tar water extract Presence of fluoranthene crystals at 4000 mgrkg dry soil Presence of phenanthrene crystals at 1000 mgrkg dry soil Presence of phenanthrene crystals at 4000 mgrkg dry soil Presence of phenanthrene crystals at 8000 mgrkg dry soil

4.23 4.61 4.69 4.15 4.06 3.93 3.98

2.48 2.52 2.53 2.47 2.46 2.45 2.46

a

Batch adsorption at 208C; Liquidrsolid ratios 3; Contact times 24 h. CSAC sCulture Soil from La Cote ˆ Saint Andre, ´ Isere, ` France. b K d distribution coefficient in cubic centimeter per gram of soil with an interval of confidence of "5%. c K oc organic carbon distribution coefficient in cubic centimeter per gram of OC.

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Results also showed that naphthalene did not interact with the pure solid phases of phenanthrene or fluoranthene since its distribution coefficient with the aqueous phase was not significantly affected by the presence of the pure crystals in excess ŽTable 3.. No significant adsorption of naphthalene onto the surface of the crystals, nor phase partition into the pure solid phases, was therefore observed. Fig. 1 and Table 4 indicate, however, that phenanthrene reduced significantly naphthalene adsorption onto soil CSAC when it was adsorbed at a relatively high concentration Žclose to 120 mgrkg. onto the soil samples before they were contacted with naphthalene. At a lower concentration Ž23 mgrkg., this effect was not significant. These observations suggested that phenanthrene and naphthalene occupied the same adsorption sites on the soil since the two molecules have very similar characteristics. The rapid sorption of hydrophobic molecules such as PAHs onto organic soils was therefore shown to be controlled by a nonspecific phenomenon of adsorption onto organic surfaces of the SOM, which is known to be a rapid process ŽLuthy et al., 1997; Chiou et al., 1979; Karickhoff et al., 1979.. Indeed, sorption into the AsoftB organic phase, although also relatively rapid, would not be a competitive process ŽLuthy et al., 1997.. Adsorption onto organic surfaces is often considered as a nonlinear process ŽLuthy et al., 1997., but the linearity may be observed when working below the saturation level of the adsorption sites, which was probably the case in this work due to the low naphthalene concentrations used. In PAH-contaminated soils, the presence of sorbed PAHs may be expected to reduce naphthalene adsorption according to the results discussed above. However, previous studies showed that naphthalene sorption onto a MGP site contaminated soil was drastically higher than onto the culture soil CSAC ŽBayard et al., 1998b.. Although a priori surprising, this observation was explained by the presence in the MGP soil of XOM in the form of coal tarrclay aggregates at a relatively high concentration. To characterize better the role of XOM in naphthalene sorption, further experiments were carried out where Fontainebleau sand Žcontaining no OC. or culture soil CSAC ŽTOC content of 1.49%. were spiked with coal tar particles at different concentrations and suspended in naphthalene aqueous solutions. Sorption isotherms thereby determined at the pseudo-equilibrium attained in 24 h contact time are given in Fig. 2a and b. Fig. 2c illustrates the effect of coal tar concentration on naphthalene distribution coefficient K d Table 4 Naphthalene adsorption on virgin soil CSAC as compared to the same soil pre-contaminated with adsorbed phenanthrenea .d

Virgin CSAC soil Žcontrol CSAC with adsorbed phenanthrene at 23 mgrkg CSAC with adsorbed phenanthrene at 118 mgrkg a

K db

c Log K oc

4.47 4.13 3.22

2.53 2.47 2.36

See Table 3. See Table 3. c See Table 3. d Controls were treated in the same manner as the assays but phenanthrene was omitted in the preliminary adsorption stage. b

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in soil–tar–water or sand–tar–water systems. Batch experiments carried out on virgin Fontainebleau sand indicated no significant retention of naphthalene in the range of concentrations studied ŽFig. 2a.. The presence of coal tar particles mixed with Fontainebleau sand or CSAC soil had a very significant effect on naphthalene sorption ŽFig. 2a and b.. Moreover, distribution coefficient K d appeared to be linearly proportional to the amount of coal tar particles added to the sand or soil ŽFig. 2c.. The intercepts at the origin of the regressed lines of Fig. 2c show the distribution coefficients of naphthalene obtained with the virgin matrices Žsand or soils. when no tar was added. In the case of agricultural soil CSAC, the intercept at the origin Ž4.15 cm3rg. was very close to the K d actually measured on the virgin soil Ž K d s 4.23 cm3rg, see Table 3., indicating that naphthalene sorption occurred in an additive manner both on the SOM and the XOM. In the sand–tar–water system, the intercept at the origin was close to zero, indicating that naphthalene sorption observed in the presence of tar resulted almost exclusively from interactions with the XOM in the tar, with no significant interactions with the sand itself as already mentioned above. The Log K oc calculated in that case were therefore specific of the XOM in the tar. It was found that the Log K oc calculated for the XOM Žranging from 3.30 to 3.48. were significantly higher than those calculated for the soil–tar–water system Žranging from 2.45 to 2.76. indicating a higher affinity of naphthalene for the XOM of the tar than for the SOM of soil CSAC. The kinetics of naphthalene retention by coal tar was studied in batch conditions using sand spiked with 4000 mg of coal tar particles per kilogram of dry sand. Results are shown on Fig. 3. The data were well described by the double domain model detailed above. The first order kinetic constant k d and the distribution factor F were calculated by best-fit adjustment to the experimental data as described previously ŽBayard et al., 1998a.. Numerical values of model parameters thus calculated are given in Table 5 and

Fig. 3. Kinetics of naphthalene partition between water and coal tar particles as determined in batch in a sand–tar–water system with 4000 mg of tar per kilogram of sand ŽT s 208C, liquid to solid ratios 3.. Comparison of experimental data to model simulation using best-fit parameters given in Table 5.

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Table 5 Naphthalene sorption parameters on SOM and XOM as determined from batch ŽT s 208C; LrSs 3; 24 h contact time. and column data ŽT s 208C, flow rate of 72 mlrh. Batch data a K oc

CSAC ŽSOM. Coal tar ŽXOM.

ŽLog K oc . .d

302 Ž2.48 2026 Ž3.31.

Column data k db 0.26 0.21

F d

c

0.33 0.78

k db d

0.26 0.24

Fc d

0.33 d 0.90

a

Distribution coefficient in cubic centimeter per gram of organic matter ŽSOM or XOM.. First-order rate constant for mass transfer. c Site distribution factor, i.e. ratio of number of internal sites to the total number of adsorption sites. d After Bayard et al., 1998a,b. b

compared to the parameters calculated for CSAC in a previous work under the same experimental conditions ŽBayard et al., 1998a.. It can be seen that the major differences observed between the two types of organic matter lie in Ži. the naphthalene distribution coefficient K d as discussed above and Žii. the site distribution factor F which was much higher for XOM than SOM. In the case of XOM, most of naphthalene retention was shown to occur in the internal domain Ž F s 0.78 according to batch data., whereas for SOM most of the retention occurred on the external domain Ž F s 0.33.. This result suggested that in the experimental conditions of this study Ž24 h contact time. naphthalene sorption by coal tar particles was controlled by a mechanism of dissolution Žphase partition. whereas surface adsorption was the major mechanism for SOM. Some authors have characterized soils, sediments and aquifer solids as geosorbents comprising several domains or components with distinct sorption reactivities, whose heterogeneity may be increased by the presence of XOM ŽLuthy et al., 1997.. Based on macroscopic observations, theses authors have proposed several types of sorption mechanisms of nonpolar organics into the different domains of the geosorbents. In the present work, the XOM in the tar particles is probably composed both of an amorphous AsoftB organic phase and a more condensed AhardB one. The extremely high affinity for naphthalene Ž K oc above 2000 cm3rg, Table 5. observed for short contact times in this work are in favor of a rapid sorption phenomenon, suggesting a phase partition mechanism into the AsoftB amorphous XOM. Longer contact times would probably result into the development of slow processes such as the sorption into the AhardB domain of the tar XOM ŽLuthy et al., 1997., which have not been observed in this work. In the case of the SOM, which is present in porous microaggregates of clay and humic substances, with less affinity for naphthalene Ž K oc around 300 cm3rg, Table 5., our macroscopic observations are more in favor of a mechanism of surface adsorption onto wet organic surfaces, as discussed above. However, even in the short term, about 1r3 of naphthalene sorption was found to occur onto internal sites Ž F s 0.33., probably via relatively rapid mechanisms of diffusion and adsorption into the meso-pores or sorption into the AsoftB organic matter ŽLuthy et al., 1997.. Over longer contact times, slower processes would become significant, such as diffusion into the dense organic matter or into the microand nanoporosity of the soil aggregates, as reported by other authors ŽLuthy et al., 1997; Pignatello and Xing, 1996..

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3.2. Column studies Breakthrough curves from the column experiments were obtained by plotting the normalized effluent concentrations CrCo Žwhere C is the outlet concentration at time t and Co is the influent concentration. as a function of the normalized volume VrVo Žwhere V is the total volume passed through the column at time t and Vo the pore volume.. Several runs were conducted on Fontainebleau sand and the agricultural soil CSAC, either virgin or spiked with 4000 mg of coal tar particles per kilogram of sand or soil. Naphthalene feed concentrations ranged between 5.5 and 20.0 mgrl. Typical naphthalene breakthrough curves obtained with Fontainebleau sand and the culture soil are shown respectively in Figs. 4 and 5, where experimental data are compared to model simulations. No effect of naphthalene concentration was noted on the breakthrough curves in the range of feed concentrations studied. In sand columns, naphthalene retention was exclusively due to the presence of coal tar particles as discussed above for the batch experiments. The model parameters k d and F used for the simulations were calculated by best-fit adjustment to the column experimental data and compared to the parameters calculated from the batch data ŽTable 5.. Table 5 shows that the model parameters calculated from the column data were very close to those derived from the batch data, indicating that the contact conditions between naphthalene in aqueous solution and the sorbents Žtar or soil. were almost similar. The high flow rate used in the columns and the resulting short duration of column experiments was probably one of the reasons for the very good consistency of batch and column data. Classical column experiments use lower flow rates, sometimes in the purpose of simulating groundwater

Fig. 4. Sorption and elution of naphthalene on a column of Fontainebleau sand spiked with coal tar particles ŽXOM. at 4000 mgrkg ŽT s 208C, flow rates 72 mlrh; Co s feed concentrations11.1 mgrl; Vo s pore volumes11.4 ml.. Comparison of experimental data with model prediction considering the existence of two domains of retention Žsurface and internal. using the model parameters given in Table 5.

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Fig. 5. Column sorption and elution of naphthalene onto virgin culture soil CSAC as compared to the same soil spiked with coal tar particles ŽXOM. at 4000 mgrkg ŽT s 208C, flow rates 72 mlrh; Co s feed concentrations9 mgrl; Vo Žpore volume. s 21.6 ml.. Comparison of the experimental data with the dual double-domain predictions Žfull lines. considering the existence of two compartments of retention ŽSOM and XOM. with two domains Žsurface and internal. in each, using the model parameters given in Table 5.

circulation. Intra-particle diffusion is therefore usually quite significant in most column studies, while sorption isotherms derived from short-term batch experiments do not evaluate this mechanism. This methodological problem may result in a poor correlation between the fitting parameters derived from batch and column studies. In the present work, slow diffusional processes were not given time to occur significantly due to the high convective transport in the columns and the short contact time in the batch experiments. Similarly, aging phenomena were also very limited in both batch and column experiments due to their short duration. However, the difference observed for tar on parameter F Ž0.78 in batch vs. 0.90 in column. suggested that surface retention of naphthalene was slightly reduced in column as compared to the batch experiments. Column data confirmed that the retention of naphthalene occurred mostly through a mechanism of phase partition into the tar. The symmetry of the sorption and elution portions of the curves in Figs. 4 and 5 indicated that the retention of naphthalene by coal tar or soil particles was well reversible in the short term. Freshly sorbed naphthalene was almost totally eluted in contrast with indigenous naphthalene already present in the tar. Indigenous naphthalene has probably migrated deep into the tar particles by diffusion, making its elution back into water much slower. In addition, the aging of the tar in the contaminated soil on the MGP site may also have induced the entrapment of naphthalene. The sorption parameters determined respectively for virgin CSAC soil and the coal tar particles ŽTable 5. were combined into a dual double-domain model to simulate the

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sorptionrelution of naphthalene in the columns of soil CSAC spiked with coal tar. The model considered the existence of two distinct compartments interacting with naphthalene Žrespectively SOM and XOM., each of which divided into one internal and one external domain Žor sub-compartment.. Direct interaction between SOM and XOM were considered to be negligible as compared to interactions between each compartment and the aqueous phase. Fig. 5 shows that the model simulation was very satisfactory except for the tailing of the elution portion of the experimental curve which was not correctly described by the model. Since the tailing was not observed with the virgin soil ŽFig. 5. or with the sandrtar column ŽFig. 4., the phenomenon might reflect some nonexpected interactions between the two compartments SOM and XOM. Further studies would be needed to investigate this possible phenomenon.

4. Conclusion The sorption of naphthalene on the agricultural soil used in this study was shown not to be affected by the presence in the aqueous solution of phenanthrene or other organics extracted from coal tar. The presence of excess crystals of phenanthrene or fluoranthene did not affect either naphthalene sorption over 24 h contact time, due to the low water solubilities of these PAHs as compared to naphthalene. No significant adsorption of naphthalene onto the surface of the crystals, nor dissolution into the pure solid phases, was therefore observed. Phenanthrene was found to reduce naphthalene sorption only when it was sorbed at a relatively high concentration Žaround 120 mgrkg. onto the soil before it was contacted with naphthalene. The study also showed that the presence in the soil or sand of XOM in the form of coal tar particles modified drastically the behavior of naphthalene both in batch and column assays. In the experimental condition of the study Žshort contact time and abiotic conditions., the results suggest that the retention of naphthalene mainly occurred via a mechanism of phase partition into the coal tar ŽXOM. and surface adsorption onto the SOM. This difference may probably be attributed to the different nature and structure of the two compartments. Over short contact times, the reactive domain of the XOM is made of an amorphous organic phase with an extremely high affinity for naphthalene Ž K oc above 2000 cm3rg., whereas the SOM, exhibiting a lower affinity for naphthalene Ž K oc around 300 cm3rg. is present as porous microaggregates of clay and humic substances, more in favor of surface interactions in the short term. For SOM however, even in the short term, about 1r3 of naphthalene sorption was found to occur onto internal sites Ž F s 0.33., probably via relatively rapid mechanisms of diffusion and adsorption into the meso-pores or sorption into the AsoftB organic matter. For contact times significantly longer than those used in this study Ži.e. more than a few days., progressive changes in the characteristics of the soil and the solution may drastically modify the affinity of the solutes for the soil as reported by many authors ŽMahjoub and Gourdon, 1999; Pignatello and Xing, 1996; Young and Weber, 1995; Steinberg et al., 1987.. Many studies in the literature have also indicated that slow sorption processes such as diffusion in the micro- and nanoporosity and in dense organic phases may

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become significant over the long term, along with the formation of irreversibly bound residues or the physical entrapment of the sorbed molecules ŽPignatello and Xing, 1996; Steinberg et al., 1987.. A dual double-domain model was successfully used to describe naphthalene sorption and elution in soil columns spiked with coal tar particles in short experiments of about 60 h where slow sorption and aging processes were likely unsignificant. The model considered two reactive compartments Žrespectively XOM and SOM. which were both divided into one external domain Žwith local adsorption equilibrium and no resistance to mass transfer. and one internal domain Žwith local adsorption equilibrium and resistance to mass transfer described as a first-order kinetics.. Very consistent and reproducible numerical values were calculated for the model parameters from batch and column data. The model simulations proposed in the present study are valid for short-term and abiotic conditions. They should be coupled to slow sorption Žtaking into account the role of dense or AhardB organic matter., aging and biodegradation models to describe long-term behavior of naphthalene in soil–tar–water systems.

Acknowledgements The authors wish to thank the French industry-University Cooperative Research Network on wastes ŽReseau Cooperatif de Recherche sur les Dechets, RECORD. for ´ ´ ´ financial support.

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