Evaluation of polyaromatic hydrocarbon removal from aqueous solutions using activated carbon and...

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Research ArticleReceived: 26 March 2008 Revised: 25 June 2008 Accepted: 3 July 2008 Published online in Wiley Interscience: 17 September 2008

(www.interscience.wiley.com) DOI 10.1002/jctb.2030

Evaluation of polyaromatic hydrocarbonremoval from aqueous solutions usingactivated carbon and hyper-crosslinkedpolymer (Macronet MN200)Cesar Valderrama,a,b∗ Xavier Gamisans,a Jose L Cortina,b Adriana Farranb

and F Xavier de las Herasa

Abstract

BACKGROUND: Sorption of polycyclic aromatic hydrocarbons (PAHs) on activated carbon and the Macronet polymeric sorbentMN200 was investigated to determine the effectiveness of each sorbent for removal of pollutants from aqueous solution andtheir possible use as sorbent materials for groundwater. Experiments were carried out to determine the loading capacities of afamily of PAHs (acenaphthene, anthracene, fluoranthene, fluorene, naphthalene and pyrene).

RESULTS: Activated carbon was the more effective sorbent, with maximum loadings of PAHs between 90 and 230 g kg−1, whileMN200 resin showed values of 25–160 g kg−1. Loading isotherms based on the Langmuir, Freundlich and Redlich–Petersonmodels were determined. The hydrophobic character of the pollutants appeared as an important parameter related to thesorption process. Equilibrium and kinetic parameters were used to determine the retardation factors for each PAH.

CONCLUSION: The calculated values for the simulation of barrier thickness using both sorbents, taking into account EUrequirements for PAHs in groundwater effluent, were perfectly reasonable as a first estimate. The better kinetic properties ofMN200 are evident in lower hydraulic residence times and consequently in a lower barrier thickness.c© 2008 Society of Chemical Industry

Keywords: sorption; polycyclic aromatic hydrocarbons (PAHs); Macronet polymeric sorbent (MN200); activated carbon; permeablereactive barrier (PRB)

NOTATIONq PAH loading (g PAH kg−1 sorbent)C total initial concentration of PAH in solution (mg PAH

dm−3 solution)Ceq total equilibrium concentration of PAH in solution (mg

PAH dm−3 solution)V volume of solution (dm3)ms mass of sorbent (kg)KL Langmuir sorption constant (m3 kg−1)qm maximum loading of sorbent (g kg−1)K Freundlich sorption constant (g kg−1 (g m−3)−1/n)N Freundlich exponentA, B, g constants of Redlich–Peterson isotherm (0 < g < 1)R retardation factor of target contaminantKd distribution coefficient (dm3 kg−1)ρ bulk density (kg dm−3)θ porosity of sorbentCs sorbed concentration (mg kg−1)Cw groundwater concentration (mg dm−3)Kow octanol/water partition coefficientCe time-varying concentration at effluent end of barrier

(ng PAH dm−3 solution)

C0 constant influent concentration (ng PAH dm−3 solu-tion)

L barrier length (m)v seepage velocity (m day−1)D dispersion coefficient (m2 s−1)Dm mechanical dispersion coefficient (m2 s−1)D∗ molecular diffusion coefficient (m2 s−1)αL longitudinal dispersivity (m)m empirical constantKr bulk first-order sorption rate constant (min−1)

∗ Correspondence to: Cesar Valderrama, Department of Chemical Engineering,ETSEIB, UPC, E-08028 Barcelona, Spain.E-mail: [email protected]

a Department of Mining Engineering and Natural Resources, EPSEM, UniversitatPolitecnica de Catalunya, E-08240 Manresa, Spain

b Department of Chemical Engineering, ETSEIB, Universitat Politecnica deCatalunya, E-08028 Barcelona, Spain

J Chem Technol Biotechnol 2009; 84: 236–245 www.soci.org c© 2008 Society of Chemical Industry

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PAH removal from aqueous solutions www.soci.org

INTRODUCTIONGroundwater contamination by monocyclic and polycyclic aro-matic hydrocarbons (PAHs) is typical of many pollution scenarios,as is the case at many former gas-manufacturing plant sites inEurope and the USA.1,2 Most PAHs are highly persistent in thesubsurface environment and are present in high concentrationsmany decades after contamination has occurred. In addition, theycannot be removed from the subsurface within a reasonable timeperiod by either pumping or treatment. Sorption processes areone of the alternatives for elimination of PAHs.

The sorption of hydrophobic organic contaminants fromaqueous solutions generally increases with decreasing solubilityof the compound (or increasing octanol/water partition coefficientKow) and increasing organic carbon content of the aquifer solids.Natural materials with high organic carbon content, such as coalsor bituminous shales, cause significant retardation of organiccontaminants from groundwater.3 One of the major advantagesof the sorption of PAHs lies in the fact that the persistent PAHs areremoved from the groundwater rather than simply transformedinto metabolites that may still be potentially dangerous, as occursin oxidation or reductive biotic or abiotic processes.

The sorption of PAHs is a combination of van der Waalsforces and a thermodynamic gradient determined by theirhydrophobicity, which drives them out of the aqueous solution.4

The more influential of the two forces is the van der Waals, wherebyattractive forces occur between instantaneous and induced dipolemoments of PAH molecules.5 Dipole moments depend uponpolarisability and can be estimated by the sum of the bondmoments on the molecule. An increase in the number of C&dbondC double bonds should contribute to molecular polarisability andthereby increase the van der Waals attraction strength. As a result,the heavier PAHs with more C&dbond C double bonds should sorbmore efficiently to sorbent surfaces.6

Granular activated carbon (GAC) sorption is the best availabletechnology for the control of many organic contaminants in waters.Treatment options can choose from a large variety of activatedcarbons that differ in pore structure and surface chemistry tocontrol one or more chemical compounds and mixtures.7 – 9 A highnumber of sites in North America and Europe based on sorptionrather than degradation have been installed during the last fewyears. At most of these sites, GAC has been used as the ‘reactive’sorbent material in different in situ remediation projects on PAHremoval from groundwater.10,11 GAC is particularly suitable forthe efficient sorption of high-molecular-weight organic moleculeswith lipophilic properties, as is the case of PAHs.12 – 15

However, one of the disadvantages of the sorption process is theneed to control the sorbent capacity limit and the need for disposaland regeneration of the sorbent. In the case of activated carbon,this process is achieved by thermal destruction and subsequentthermal activation of the sorbent when it is exhausted. One ofthe possibilities for overcoming this problem is to use improvedpolymer sorbents. Such materials are seen as a more attractivealternative for the removal of organic pollutants because oftheir controllable pore structures and surface characteristics. Forinstance, the commercial Amberlite XAD-4 resin was reportedas an ideal sorbent for a wide variety of aromatic compounds,especially for phenols.16 The polymer NDA-99 and its amine-modified NDA-100 (Langfang Chemical Co.) were successfullyused for the removal of both salicylic acid and 5-sulfosalicylicacid.17 In general, many hydrophobic organic pollutants can bereadily removed from wastewater by hyper-crosslinked polymeric

sorbents, which often results from a van der Waals interactionbetween the solute and sorbent phase.18

Macronet sorbents have a hyper-crosslinked structure thatmaintains both the solid macro- and microporosity at the sametime. This means having control over the pore sizes and largesurface areas. They retain their swollen state porosity and thusshow little or no change in swelling with change in permeatingliquid and are highly efficient sorbents of high-molecular-weightorganic compounds with lipophilic properties. Finally, Macronetsorbents can be easily regenerated chemically using organicalcohols and ketones.19

Permeable reactive barriers (PRBs) serve as a containment tech-nology for polluted groundwater. As contaminated groundwaterflows off-site, contaminants may be reduced by the barrier mediato concentrations below target values, which are often set as themaximum concentration levels. Design of PRBs such as funnel-and-gate systems20,21 or engineered permeable walls or extractablewalls22,23 is typically based on the use of the retardation factor Rcalculated from isotherms (Langmuir or Freundlich).

For pollution scenarios with high groundwater flow velocitiesand a common thickness of the sorption wall (1–2 m), theresidence time of groundwater will range from hours to lessthan a day. If the mean residence time is too short for equilibrium,the Kd values will not be attainable and the actual sorption ratewill be rather smaller than the expected equilibrium values (Kd),which in turn will result in early breakthrough of the contaminantthrough the sorption wall.3 Thus it is necessary to determinethe equilibrium and kinetic parameters. In previous work24 thesorption kinetic parameters were determined for removing PAHsfrom aqueous solution using Macronet resin MN200 and activatedcarbon.

The aim of this work was to determine the equilibriumparameters and to evaluate the capability of MN200 resin andactivated carbon as sorption materials for PAH removal fromthe aqueous phase. Different equilibrium models were evaluatedto accurately predict sorption properties on the expectedpollution scenarios. Based on these experimental sorption data, apreliminary estimation of material performance for PRB systems ispresented in terms of the retardation factor and barrier dimensionrequirements.

MATERIALS AND METHODSReagents and solutionsMN200 resin samples were provided by Purolite Iberia SA(Barcelona, Spain). The polymer matrix structure is a crosslinkedpolystyrene, its physical appearance is brownish-cream sphericalbeads, the particle size range is 5% maximum above 1.2 mm and1% maximum below 0.3 mm, the surface area is ∼1000 m2 g−1,the pore volume is ∼1 cm3 g−1 and the specific gravity is 1.04 gcm−3. The samples were conditioned in methanol/hydrochloricacid mixtures and then in water before being used in the sorptionexperiments.

The activated carbon sample was provided by Aguas deLevante (Barcelona, Spain). The sample used (F400) is an activatedcarbon specially tailored for the removal of organic contaminants,macromolecules, colour, taste and smell. It is a macroporousactivated carbon with a macroporous volume of 1.5 cm3 g−1 anda Brunauer–Emmett–Teller (BET) surface area of 1000 m2 g−1.The ash content is 50 mg g−1 (maximum). The product wassupplied in a coarse form (3–6 mm size fraction).

J Chem Technol Biotechnol 2009; 84: 236–245 c© 2008 Society of Chemical Industry www.interscience.wiley.com/jctb

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www.soci.org C Valderrama et al.

Six PAHs, namely acenaphthene, anthracene, fluorene, fluo-ranthene, naphthalene and pyrene, were purchased from Merck(Barcelona, Spain) and Aldrich Chemical Co. (Barcelona, Spain)Acetonitrile was obtained from Panreac (Barcelona, Spain). ThesePAHs were selected for this study because they exhibit a widerange of hydrophobicity and are of environmental concern.Additionally, most of them have been frequently used in previousresearch as model solutes in solute transport and environmentalfate studies. PAH solutions were prepared from stock solutions ofeach PAH in acetonitrile.

Sorption experimentsSorption was carried out in batch experiments at room tempera-ture (21±1 ◦C). The measured pH ranged from 5.5 to 6.5 for all PAHs.Samples (0.5 g) of sorbents were mixed mechanically in specialglass-stoppered tubes with an aqueous/organic solution (10 cm3)until equilibrium was achieved (18 h). Wet-sieved resin fractionswith a narrow particle size range of 0.5–0.7 mm for MN200 and0.6–0.8 mm for activated carbon were used. Concentrated stocksolutions of each PAH were prepared in acetonitrile. The workingsolutions with the desired concentrations were obtained by dilut-ing measured volumes of the stock PAH solutions with distilledwater, taking into account the water solubility of each PAH.

The percentage of acetonitrile and the resulting concentrationrange used for each PAH in the sorption experiments are presentedin Table 1. After phase separation in a high-speed centrifuge(Centronic BL-II, Selecta Spain, Barcelona, Spain), the extent ofsorption was calculated by the residual concentration of PAHs inthe equilibrated solution. In the case of activated carbon, samples

were filtered through a 0.45 µm filter to eliminate generated finesin order to avoid interferences in the quantification step.

The concentrations of the PAHs were determinedspectrophotometrically25 (HP 8453, Hewlett Packard, Barcelona,Spain). The absorbance values of the PAHs were determined atthe following wavelengths: acenaphthene, 281 nm; anthracene,358 nm; fluoranthene, 286 nm; fluorene, 261 nm; naphthalene,266 nm; pyrene, 272 nm. The accuracy of the results was greaterthan 95%.

The equilibrium isotherms were determined for the six PAHsin single-component experiments for both types of sorbent. Thesorption capacities of the MN200 resin and activated carbon wereevaluated as a function of PAH concentration by determining thePAH loading q (g PAH kg−1 sorbent):

q = (C − Ceq)V

ms(1)

where C is the initial concentration of the PAH in solution (mg PAHdm−3 solution), Ceq is the equilibrium concentration of the PAH insolution (mg PAH dm−3 solution), V is the volume of the solution(dm3) and ms is the mass of the sorbent (kg).

Three isotherm models (Langmuir, Freundlich andRedlich–Peterson) were used in the present study to describethe sorption data. PAH loading values were used to deter-mine the equilibrium parameters of both sorbents via theLangmuir, Freundlich and Redlich–Peterson isotherms given inEqns (2)–(4) respectively:

Table 1. Characteristics of PAHs and composition of solutions

PAHλ (nm) Structure

Water solubility at25 ◦C (mg dm−3) LogKow

Concentration (mg dm−3)(% acetonitrile)

Naphthalene 266 30.8 3.4 0–5 (0)

Acenaphthene 281 4.5 3.9 0–20 (0–10)

Fluorene 261 2.0 4.2 0–5 (0–10)

Anthracene 358 0.045 4.6 0–20 (0–40)

Pyrene 272 0.13 5.2 0–5 (0–20)

Fluoranthene 286 0.21 5.2 0–10 (0–30)

www.interscience.wiley.com/jctb c© 2008 Society of Chemical Industry J Chem Technol Biotechnol 2009; 84: 236–245

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q = KLqmCeq

1 + KLCeq(2)

where qm is the maximum loading of the sorbent (g kg−1) and KL

is the Langmuir sorption constant (m3 kg−1);

q = KC1/neq (3)

where K is the Freundlich sorption constant (g kg−1 (g m−3)−1/n)and n is the Freundlich exponent;

q = ACeq

1 + BCgeq

(4)

where A, B and g are constants of the Redlich–Petersonisotherm.26,27 The Redlich–Peterson isotherm follows the Lang-muir isotherm when g = 1 and follows the Henry’s law equationwhen A and B � 1 and g = 0. The isotherm parameters werecalculated by a trial-and-error optimisation routine.

Scanning electron microscopy (SEM) analysisA JEOL 3400 scanning electron microscope (Barcelona, Spain) withan energy dispersive system (EDS) was used to observe the surfacemorphology of the MN200 resin. Prior to analysis, samples weredried in a vacuum oven at room temperature and then coatedwith gold.

Requirements for an in situ ground-water treatment based ona sorption processThe sorption of hydrophobic organic contaminants from theaqueous phase generally increases with decreasing solubility ofthe compound (or increasing octanol/water partition coefficientKow) and increasing organic carbon content of the aquifersolids. Natural materials with high organic carbon content, suchas coals or bituminous shales, cause significant retardation oforganic contaminants from groundwater. Such materials withhigh sorption capacities may be used for passive removal ofstrongly hydrophobic contaminants in groundwater (such asPAHs). For successful use in a permeable wall, the permeability andsorption properties of the sorptive wall material (sorbent) must beoptimised.3

One of the most important requirements for in situ groundwatertreatment based on sorption processes is fast sorption kineticsto achieve high retardation factor (R) values even at highgroundwater flow velocities. This parameter could be calculatedusing equilibrium parameters obtained from the isothermsequations and the distribution factor Kd.28 Sorption isothermsare the most fundamental and informative data in a sorptionsystem. They are also very important in model prediction foranalysing and designing a sorption process.

In general, any in situ groundwater treatment system based onsorption processes must meet the following requirements:

• a relatively high permeability in comparison with that of theaquifer to prevent a steep hydraulic gradient;

• no decrease in permeability of the sorbent due to thermo-biofouling as a consequence of competitive sorption ofdissolved organic matter or growth of a biofilm that mayblock the sorbent’s pores;3

• fast sorption kinetics to achieve high R values even at highgroundwater flow velocities.

The retardation factor (R) of a target contaminant in a sorptivepermeable wall can be calculated based on the sorption coefficientor distribution constant (Kd):

R = 1 + Kdρ

θ(5)

where ρ is the bulk density and θ is the sorbent porosity. Thelatter may be interpreted as the number of pores that can bedisplaced before breakthrough of the contaminants occurs. Kd

denotes the ratio between the sorbed and aqueous concentrationof the contaminant:3

Kd = Cs

Cw(6)

where Kd is the distribution coefficient (dm3 kg−1), Cs is the sorbedconcentration (mg kg−1) and Cw is the groundwater concentration(mg dm−3).

Contaminant retardation in barrier materials is often describedby a one-dimensional advection/dispersion equation. An analyticalsolution28 of the advection/dispersion equation proposed by vanGenuchten was used to estimate the barrier design parameters:

Ce

C0= 1

2

[exp

((u − v)L

2D

)erfc

(RL − ut

2(DRt)0.5

)]

+[

exp

((u − v)L

2D

)erfc

(RL + ut

2(DRt)0.5

)](7)

where Ce is the time-varying concentration at the effluent endof the barrier end, C0 is the constant influent concentration, L isthe barrier thickness, v is the seepage velocity, D is the dispersioncoefficient and R is the retardation factor. The variable u in Eqn (7) isdefined as

u = v

(1 + 4KrD

v2

)1/2

(8)

where Kr is the first-order sorption rate constant.The use of Eqn (8) implicitly assumes that degradation of sorbed

PAHs is negligible. The retardation factor R is related to the bulkpartition coefficient Kd defined in Eqn (5) for a linear isotherm.However, sorption is nonlinear, as was shown by the equilibriumstudy for the strongly hydrophobic contaminants on both kindsof sorbent.

Several complementary approaches can be used to assessthe extent of contaminant transport in PRB systems based onsorption processes. Between them, the retardation factor R can becalculated using the distribution isotherms of either Freundlich,Langmuir or Redlich–Peterson, depending on the best fit of theexperimental data.3 According to the values obtained in this study,Langmuir isotherm parameters were used for both sorbents:

R = 1 + ρ

θ

(KLqm

1 + KLCeq

)(9)

For the steady state condition the effluent concentration (Ce) is

Ce = C0 exp

((v − u)L

2D

)(10)

Here the dispersion coefficient (D) is the sum of the mechanicaldispersion coefficient (Dm) and the molecular diffusion coefficient(D∗):

D = Dm + D∗ = αLvm + D∗ (11)

where αL is the longitudinal dispersive ability and m is an empiricalconstant that is typically assumed as m = 1 for granular media.28


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RESULTS AND DISCUSSIONKinetic evaluation of PAH extractionIn a previous study of PAH kinetic extraction by GAC and Macronetresin MN200, experiments were performed for six PAH compounds,and the data for the evolution of concentration as a function oftime were adjusted to three (pseudo-first-order, pseudo-second-order and Elovich) models in order to determine practical kineticparameters.24

Equilibrium was achieved at 100 and 300 min for GAC andMN200 respectively; the decrease in PAH concentration in theaqueous phase is steeper for MN200 compared with GAC owingto the better sorption properties of the polymeric sorbent. Forthe hyper-crosslinked polymer more than 80% was extracted after100 min, while for GAC only 50% had been extracted in this time.This slower behaviour of PAH extraction by GAC (more notableat the beginning) could be related to the ionised groups on thecarbon surface creating an electrostatic repulsion effect initially.This phenomenon is reduced as molecules of PAH are sorbed onthe surfaces of pores with low ionised groups.

The MN200 resin showed an appreciable portion of smallmicropores, which create very high internal surface areas(800–1000 m2 g−1), and an appreciable portion of macropores.The bead diameter is approximately 0.11–0.17 µm, thus creatinga more controlled and even distribution of pore sizes than inGAC, which has an extensive network of micropores (<20 Å) thatcreates a high surface area and abundant sorption sites. Thesample texture is apparent in the SEM data, possibly indicatingthe presence of large mesopores and macropores (∼0.02–0.2 µm)

providing the high surface area. The micropores provide meso-and macropores, which then allow rapid access to the internalsurfaces and promote superior kinetics in the GAC. This conceptwas determined in the kinetic study.28 The results of SEM for bothsorbent materials are shown in Fig. 1.

Analysis of the experimental data of the kinetic experiments forthe different PAHs using GAC and Macronet resin MN200 showthat a first-order reaction model provides a suitable description ofboth systems.24

PAH sorption equilibrium parametersFigures 2 and 3 show the PAH isotherms for activated carbon andMN200 resin respectively. Maximum sorption capacities variedfrom 90 to 230 g kg−1 for the activated carbon and from 25 to160 g kg−1 for the MN200 resin, depending on the nature ofthe PAH. Sorption onto synthetic polymeric sorbents is generallydriven by the attraction between the solute and the sorbent. Thesorption capacity then depends on the properties of both thesolute and the sorbent. Figure 2 shows that, in the concentrationrange evaluated, the sorption capacity of the PAHs with activatedcarbon was higher than that with MN200 resin.

The Langmuir, Freundlich and Redlich–Peterson isothermparameters for the MN200 resin and activated carbon weredetermined from the linearised forms of Eqns (2)–(4) and aresummarised in Tables 2 and 3 respectively. The determinationcoefficient was also calculated and used as the guiding parameterto establish the isotherm model that provided the best equilibriumdescription of PAH sorption on both sorbents.

MN200 GAC

Figure 1. Scanning electron micrographs of internal areas of MN200 polymer and activated carbon bead.

Langmuir A. carbon

0

50

100

150

200

250

0 20 40 60 80 100 120

Ceq (g m-3)

0 20 40 60 80 100 120

Ceq (g m-3)

q (

g k

g-1

)

0

50

100

150

200

250

q (

g k

g-1

)

Redlich-Peterson A. carbon

Figure 2. Experimental data and calculated loadings by two isotherms for six PAHs (�, anthracene; •, acenaphhene; , fluoranthene; �, fluorene; +,naphthalene; �, pyrene) on activated carbon.


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Table 2. Langmuir, Freundlich and Redlich–Peterson isothermparameters for six PAHs on Macronet resin MN200

Langmuir Freundlich Redlich–Peterson

PAH KL qm R2 K n R2 A B g R2

Naphthalene 14 160 0.99 2.1 1.0 0.96 3.9 0.02 1.0 0.99

Acenaphthene 60 38 0.99 5.4 1.9 0.96 7.0 0.2 1.0 0.98

Fluorene 45 25 0.81 6.2 3.1 0.68 2.4 0.08 1.0 0.83

Anthracene 14 35 0.87 0.4 1.1 0.77 8.0 0.7 0.9 0.70

Pyrene 77 64 0.99 5.2 1.6 0.97 5.7 0.1 0.9 0.99

Fluoranthene 39 99 0.96 2.4 1.1 0.80 3.9 0.03 1.0 0.97

Units: KL, m3 kg−1; qm, g kg−1; K , g kg−1 (m3 g−1)−1/n ; A, dm3 g−1; B,dm3 mg−1.

Table 3. Langmuir, Freundlich and Redlich–Peterson isothermparameters for six PAHs on activated carbon

Langmuir Freundlich Redlich–Peterson

PAH KL qm R2 K n R2 A B g R2

Naphthalene 192 140 0.95 30.8 2.9 0.91 121 2.6 0.7 0.94

Acenaphthene 216 111 0.77 34.6 3.8 0.93 160 1.7 0.9 0.60

Fluorene 343 145 0.95 39.2 2.7 0.88 150 2.6 0.7 0.94

Anthracene 223 232 0.95 40.2 3.0 0.69 280 5.5 0.6 0.93

Pyrene 1157 109 0.95 63.9 8.4 0.99 170 1.6 1.0 0.95

Fluoranthene 505 93 0.96 29.5 4.2 0.95 54 1.6 0.7 0.91

Units: KL, m3 kg−1; qm, g kg−1; K , g kg−1 (m3 g−1)−1/n ; A, dm3 g−1; B,dm3 mg−1.

As a general trend, the Langmuir model provides the bestdescription of PAH sorption on both activated carbon and MN200resin; however, the Redlich–Peterson model provides a gooddescription of the sorption experimental data for MN200 resin, ascan be seen in Fig. 3. The Redlich–Peterson model incorporatesfeatures of both the Langmuir and Freundlich isotherms25 and mayalso be proposed as a suitable model to describe PAH sorption onhyper-crosslinked polymer supports.

For both sorbents the Freundlich isotherm provides the poorestdescription of the sorption data for the whole range of PAHconcentrations. The Langmuir coefficient KL shows that the values

log KL vs log Kow

0.5

1.1

1.7

2.3

2.9

3.5

3 3.5 4 4.5 5 5.5 6log Kow

log

KL

A. carbonMacronet resin

Figure 4. Relation of obtained Langmuir isotherm coefficient (KL) withoctanol/water partition coefficient (Kow) for different PAHs.

for activated carbon are one order of magnitude higher than thosefor MN200 resin, and similar trends can be seen in Tables 2 and 3between the Redlich–Peterson coefficients A and B. The Langmuircoefficient KL determines the affinity of the sorbent for the soluteand corresponds to the strength of the sorption;29 thus the valuesobtained indicate a higher affinity of activated carbon towards thepollutants.

The area and structure of a sorbent’s surface also playan important role in the extraction of organic substances. Ingeneral, sorbents with more developed surfaces have pores ofsmaller average diameter. The presence of pores whose sizes arecomparable to those of the extracted molecules increases theefficiency and selectivity of sorption. An increase in the surfacearea of the sorbents also enhanced the efficiency of the recoveryof substances in the only case where the sizes of pores exceededthe molecular sizes of these substances.30 In this study the surfaceareas of the sorbents (∼1000 m2 g−1) are quite similar, whichindicates that the slight trend to increased maximum loading asthe size of the PAH increases seen in Table 2 is related to thehydrophobic character of the sorbates.

The sorption of hydrophobic compounds (PAHs) from theaqueous phase increases with decreasing solubility of thecompound (or increasing octanol/water partition coefficient Kow).The sorptive behaviour of PAHs appears to be dominatedby this hydrophobic characteristic. The sorption equilibriumconstant, from the Langmuir isotherm coefficient (KL), shows alinear dependence on the octanol/water partition coefficient Kow

(Fig. 4).

Langmuir MN200

0

20

40

60

80

100

120

0 20 40 60 80 100

Ceq (g m-3) Ceq (g m-3)

0 20 40 60 80 100

q (

g k

g-1

)

Redlich-Peterson MN200

0

20

40

60

80

100

120

q (

g k

g-1

)

Figure 3. Experimental data and calculated loadings by two isotherms for six PAHs (�, anthracene; •, acenaphhene; , fluorene; +, naphthalene; �,pyrene) on Macronet resin MN200.


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The linear dependence of log KL on log Kow reflects the con-tribution of hydrophobic interactions (π –π interactions) to thesorption ability of each sorbent. This contribution was confirmedby Penner et al.,31 who studied the relative strength of π –π in-teractions of PAHs on different sorbents. For silica-based phasessuch selectivity is provided by the plurality of alkyl chains attachedto the silica surface, whereas for polystyrene-divinylbenzene (PS-DVB) phases the shape selectivity is provided by the intrinsicstructure of the polymer matrix.31 For GAC such selectivity isprovided by the plurality of group functionalities attached to thecarbon surface.

Penner et al.31 characterised the relative strength of π –π

interactions on three different sorbents and studied the retentionof several PAHs with one to five condensed rings. Both typesof sorbent, silica-based and PS-DVB phases, retain rigid planarhydrocarbon molecules more strongly than flexible bulky ones,although non-derivatised PS-DVB sorbents have much greatershape selectivity.32,33

In the present study the PAHs evaluated had both rigid andplanar structures. The dependence of logKL on the number ofπ electrons in the solute molecules (nπ ) was evaluated for bothsorbents. Solute sorption increased gradually with the number ofπ electrons in a molecule, i.e. with the number of condensed rings.The slope of the linear dependence of logKL on nπ represents therelative strength of π –π interactions between the solutes and thesorbent,31,34 and π –π interactions are slightly stronger for GAC(slope 0.09) than for MN200 resin (0.07).

The retardation factor and distribution coefficient values calcu-lated from the equilibrium parameters obtained for both sorbentsusing Eqns (6) and (9) are presented in Table 4. The distributioncoefficients obtained were compared with those reported in theliterature for similar sorbents or similar hydrophobic contaminants.

The slope values of the log Kd vs log Kow plot for activatedcarbon (0.23) and MN200 resin (0.11) were compared with valuesreported by Noordman et al.,35 who described the behaviour ofother sorbents such as silica, hydrophobic octadecyl derivatisedsilica (ODS) and humic acid derivatised silica (HAS). These sorbentshave been used as model matrices to describe the PAH distributionin soils. The values reported were 0.42, 1.32 and 0.91 for silica, ODSand HAS respectively.35

Boving and Zhang36 studied the use of aspen wood fibres as asorbent to remove four PAHs. The average values of Kww (the aspenwood/water partition coefficient, defined in a similar way to Kow)obtained are compared with the values of Kd for activated carbonand MN200 resin in Table 4. Generally, the capacity of aspen wood

fibres is between one and two orders of magnitude lower thanthat of activated carbon or MN200 resin.

The log Kd values for acenaphthene (4.5 for GAC and 3.7 forMN200) and fluorene (4.6 for GAC and 3.8 for MN200) werecompared with those obtained by Schad and Grathwohl,3 whoevaluated three different activated carbons, namely F100, C40/4(bituminous carbons) and TE143 (coconut), as sorption materi-als for funnel-and-gate systems used for treating contaminatedgroundwater at a former gas-manufacturing plant. For acenaph-thene the three carbons showed log Kd values of 4.0, 3.9 and4.8 respectively, while the values for fluorene were 4.5, 3.9 and4.8 respectively.3 Thus the values for MN200 resin were the low-est when compared with the other sorbents, while the valuesfor activated carbon were of the same order as those reportedfor the other carbonaceous materials, particularly in the case offluorene.

Table 5 presents values of the Langmuir constant KL (naph-thalene) and distribution coefficient Kd (fluorene) reported in theliterature for different sorbents. As a general trend, the KL val-ues indicate the affinity of naphthalene towards both MN200resin and activated carbon, especially the latter; in the case offluorene the values of Kd reported for the various carbonaceoussorbents are similar to those measured for activated carbon in thisstudy.37 – 41

Preliminary estimation of barrier thickness for treated PAHgroundwater contaminationComputations were made using Eqn (10) to illustrate typicalbarrier thicknesses that would be required for MN200 resinand activated carbon sorbents. The MN200 resin and acti-vated carbon content on the barrier was assumed to be100%.

The dispersion coefficient was obtained using Eqn (11), withthe dispersivity equal to 0.1 times the barrier thickness. Themolecular diffusion coefficients for the different PAHs in a porousmedium were set assuming a tortuosity of 0.4.42 These aqueousdiffusion coefficients and first-order sorption rate constants(Kr) of PAHs are shown in Table 6. The seepage velocities ofinfluent PAH concentrations (C0) were set at 0.1 and 1 m day−1

to evaluate the velocity influence on the barrier dimensions.The evolution of the normalised concentration ratios (Ce/C0)as a function of the barrier length was calculated and ispresented in Table 6. The threshold value was selected as themaximum concentration level (MCL) defined by groundwaterregulations.

Table 4. Distribution coefficient (Kd) and retardation factor (R) calculated for activated carbon and Macronet resin MN200; comparison withdistribution coefficient (Kww) for aspen wood fibres36

Activated carbon Macronet resin MN200 Aspen wood fibres

PAH Kd R Kd R Kww

Naphthalene 2.26 × 104 5.42 × 104 2.78 × 103 4.83 × 103 2.63 × 102

Acenaphthene 1.97 × 104 4.73 × 104 2.67 × 103 4.64 × 103 NA

Fluorene 3.69 × 104 8.86 × 104 4.05 × 102 7.04 × 102 3.31 × 102

Anthracene 4.24 × 104 1.02 × 105 5.39 × 103 9.34 × 103 0.39 × 102

Pyrene 5.83 × 104 1.40 × 105 4.03 × 103 6.98 × 103 4.57 × 102

Fluoranthene 3.13 × 104 7.52 × 104 3.75 × 103 6.50 × 103 NA

NA, not available. Units: Kd and Kww, dm3 kg−1.


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Table 5. Comparison of Langmuir constant KL (naphthalene) and distribution coefficient Kd (fluorene) reported for different sorbents with valuesobtained for activated carbon and Macronet resin MN200

KL (m3 kg−1) Sorbent Reference

Naphthalene 15 Mineral sand 37

0.005 Zeolite 38

13 Spherical microporous carbon CR-1 39

0.009 Polymeric sorbent NDA-150 40

0.01 Polymeric sorbent NDA-1600 40

0.004 Amberlite XAD-4 40

14 MN200 resin This study

192 Activated carbon This study

Kd (dm3 kg−1) Sorbent Reference

Fluorene 5.0 × 104 Granular activated carbon F100 3

1.6 × 104 Granular activated carbon C40/4 3

2.0 × 104 Granular activated carbon TE143 3

7.9 × 104 C18 modified silica gel 3

0.4 × 104 Hydrophobic octadecyl derivatised silica (ODS) 35

0.002 × 104 Humic acid derivatised silica (HAS) 35

0.003 × 104 Aspen wood fibres 36

0.004 × 104 MN200 resin This study

3.7 × 104 Activated carbon This study

Table 6. Simulation of barrier thickness values (L) for activated carbon (GAC) and Macronet resin MN200 for six PAHs

PAHKr

GACKr

MN200 D∗ C0 Ce

Ce/C0(MCL)

L (v = 1)GAC

L (v = 1)MN200

Naphthalene 1.22 × 10−2 3.18 × 10−2 7.50 × 10−6 300 80 0.267 0.40 0.22

Acenaphthene 9.89 × 10−3 3.07 × 10−2 7.69 × 10−6 500 10 0.020 0.50 0.24

Fluorene 2.10 × 10−2 4.03 × 10−2 7.88 × 10−6 300 40 0.133 0.34 0.17

Anthracene 1.51 × 10−2 3.13 × 10−2 7.74 × 10−6 500 20 0.040 0.36 0.23

Pyrene 1.44 × 10−2 3.02 × 10−2 7.24 × 10−6 300 66 0.220 0.37 0.21

Fluoranthene 1.08 × 10−2 1.80 × 10−2 6.35 × 10−6 400 30 0.075 0.46 0.32

Units: Kr, min−1; D∗, cm2 s−1; C0 and Ce, ng dm−3; L, m; v, m day−1.

As an example, normalised pyrene concentrations are shown inFig. 5 as a function of barrier thickness.

Calculations for barrier design showed that the requiredthickness of a PRB using MN200 resin and activated carbon assorbent materials depends on both the source PAH concentrationsand seepage velocities. For both sorbents, barrier dimensions ofthe order of 0.1–0.2 m are enough to treat water and groundwaterat velocities of 0.1 m day−1. An increase in the seepage velocity to1 m day−1 results in an increase in the barrier dimensions to 0.3and 0.6 m for MN200 resin and activated carbon respectively.

CONCLUSIONSThe sorption equilibrium parameters for Macronet resin MN200and activated carbon to remove PAHs from aqueous solution weredetermined. Good PAH removal efficiencies were achieved withboth sorbents studied, especially activated carbon, which showedhigh sorption load capacities of 90–230 g kg−1, while the valuesfor MN200 resin were 25–160 g kg−1. The experimental datawere adjusted to sorption isotherm models (Langmuir, Freundlichand Redlich–Peterson). The Langmuir isotherm provided thebest description of PAH sorption on both activated carbon

and MN200 resin, while the Redlich–Peterson isotherm gavea good description of PAH sorption on the hyper-crosslinkedpolymer.

The sorption properties (KL and Kd) of PAHs on activated carbonand MN200 resin were directly linked to the physical propertiesand hydrophobic characteristics of the pollutants. This behaviourwas slightly more apparent for activated carbon than for MN200resin.

The calculated values for the simulation of barrier thicknessusing both sorbents, taking into account EU requirements forPAHs in groundwater effluent, were perfectly reasonable as a firstestimate. The better kinetic properties of MN200 are evident inlower hydraulic residence times and consequently in lower barrierthickness.

ACKNOWLEDGEMENTSWe wish to acknowledge the contribution of MEC projectPPQ06C02-004 (Spanish Ministry of Education and Science). Wewould also like to thank Purolite Ltd for the MN200 samplesand Aguas de Levante for the activated carbon samples. We are


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MN200

0.0E+00

2.0E-01

4.0E-01

6.0E-01

8.0E-01

1.0E+00

1.2E+00

0.00 0.05 0.10 0.15 0.20 0.25 0.30

L (m)

Ce/

C0

v=1 m day-1 v=0.1 m day-1

GAC

0.0E+00

2.0E-01

4.0E-01

6.0E-01

8.0E-01

1.0E+00

1.2E+00

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

L (m)

Ce/

C0

v=1 m day-1 v=0.1 m day-1

Figure 5. Normalised pyrene concentrations (Ce/C0) in effluent as afunction of GAC and MN200 barrier thickness for seepage velocities of0.1 and 1 m day−1.

extremely grateful to T Rovira and AM Lozano for their assistancein the sorption experiments.

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