RESEARCH ARTICLE

Adsorption of fluoranthene in surfactant solution on activatedcarbon: equilibrium, thermodynamic, kinetic studies

Jianfei Liu & Jiajun Chen & Lin Jiang & Xingwei Wang

Received: 1 April 2013 /Accepted: 15 August 2013 /Published online: 25 August 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Adsorption of fluoranthene (FLA) in surfactant so-lution on activated carbon (AC) was investigated. Isotherm,thermodynamic, and kinetic attributes of FLA adsorption in thepresence of the surfactant on AC were studied. Effects of ACdosage, initial concentration of TX100, initial concentration ofFLA, and addition of fulvic acid on adsorption were studied.The experimental data of both TX100 and FLA fitted theLangmuir isothermmodel and the pseudo-second-order kineticmodel well. Positive enthalpy showed that adsorption of FLAon AC was endothermic. The efficiency of selective FLAremoval generally increased with increasing initial surfactantconcentration and decreasing fulvic acid concentration. Thesurface chemistry of AC may determine the removal of poly-cyclic aromatic hydrocarbons. The adsorption process may becontrolled by the hydrophobic interaction between AC and theadsorbate. The microwave irradiation of AC may be a feasiblemethod to reduce the cost of AC through its regeneration.

Keywords Adsorption . Activated carbon . Polycyclicaromatic hydrocarbon . Surfactant . Isotherm . Kinetic

Introduction

Contamination of hydrophobic organic compounds (HOCs)in soil is a current environmental problem that has drawnmuch attention (Sehili and Lammel 2007; Wang et al. 2010).Polycyclic aromatic hydrocarbons (PAHs) are some of thetypical HOCs that originate from the combustion of carbonfuels and improper disposal of industrial materials. PAHs arefound in many industrial sites and urban soils at very highconcentrations (Baek et al. 1991; Okona-Mensah et al. 2005).PAHs are relatively stable and can survive bioremediation orchemical extraction because of their strong adsorption in soil,especially in solid organics, which means that they can remainin the environment for a very long time (Rivas 2006).Additionally, most PAHs are toxic threats to humans and theecosystem because of their carcinogenic nature (Delgado-Saborit et al. 2011; Xia et al. 2010; Xu et al. 2011).

Different physical, chemical, and biological methods andtheir combinations have been used in the remediation of PAH-contaminated sites (Gan et al. 2009). Among these methods,surfactant-enhanced soil washing shows promise in removingPAHs from polluted surfactant solutions (Mulligan et al. 2001;Paria 2008). Surfactant molecules with concentration overtheir critical micelle concentration (CMC) aggregate to formmicelles in water. In surfactant-enhanced remediation, surfac-tants can increase the solubility of HOC in water throughmicellar solubilization and improve HOC mobilization byreducing interfacial tension between water and contaminant(Guha et al. 1998). The cost of surfactant-enhanced remedia-tion technologies should be considered. Surfactant recoveryand reuse are necessary and may have an important economicrole in the remediation of PAH-contaminated sites (Cheng andSabatini 2007; Harwell et al. 1999). Ahn and colleagues foundthat activated carbon (AC) is a good material for adsorbing andrecovering surfactants because of the much higher partitioningcoefficient of HOC compared with that of nonionic surfactants

Responsible editor: Angeles Blanco

J. Liu : J. Chen (*) :X. WangState Key Laboratory forWater and Sediment Sciences ofMinistry ofEducation, School of Environment, Beijing Normal University,Beijing 100875, People’s Republic of Chinae-mail: jeffchen@bnu.edu.cn

J. LiuSchool of Civil Engineering, Henan Polytechnic University,Jiaozuo 454003, People’s Republic of China

L. JiangBeijing Municipal Research Institute of Environmental Protection,Beijing 100037, People’s Republic of China

Environ Sci Pollut Res (2014) 21:1809–1818DOI 10.1007/s11356-013-2075-1

(Ahn et al. 2007, 2008a, 2010a). Ahn and colleagues alsostudied the selective adsorption of PAH in surfactant solutionon AC (Ahn et al. 2008b, c, 2010b). Wan et al. 2011 verifiedthe capability of AC on selective removal of contaminantswhile the surfactant was recovered and reused. However,studies on adsorption kinetic models, thermodynamic param-eters, and the effect of fulvic acid on surfactant reuse have notbeen conducted.

Using fluoranthene (FLA) as the target PAH and Triton X-100 (TX100) as the nonionic surfactant, the effects of adsor-bent dosage, initial concentration of PAHs and surfactant, andaddition of fulvic acid on surfactant recovery by AC adsorp-tion were investigated. Parameters of the adsorption isotherm,kinetic models, and thermodynamic models were also deter-mined. Fourier transform infrared spectroscopy (FTIR) andscanning electron microscopy (SEM) were used to illustratethe adsorption mechanism. The regeneration of AC undermicrowave irradiation was investigated to reduce furtherwaste production and minimize the cost of the whole process.

Materials and methods

Materials

Nonionic surfactant TX100 (C8H17C6H4O(CH2CH2O)9.5H)(molecular weight, 625 g/mol; purity, >98 %) was purchasedfrom J&K Scientific, Ltd. The four-ring polycyclic aromatichydrocarbon FLA (C16H10) (molecular weight, 202.26 g/mol;purity, >99 %) was purchased from Tokyo Chemical IndustryCo., Ltd. Fulvic acid (C14H12O8) (molecular weight, 308.24 g/mol; purity, >99 %) was purchased from Mym BiologicalTechnology Company, Ltd. All chemicals were used withoutfurther purification. Commercial AC from SinopharmChemical Reagent Co., Ltd., China was used as adsorbent.AC was boiled in deionized water for 1 h, washed withdeionized water several times, and then dried at 105 °C over-night and stored in a dryer for further usage. The Brunauer–Emmett–Teller (BET) surface area, pore volume, and poresize distribution of the adsorbent were determined with nitro-gen adsorption at 77 K on ASAP 2010. Pore volume and sizedistribution were calculated by density functional theory. AnFTIR spectrum of AC was recorded at 4,000 to 500 cm−1 onNicolet NEXUS-470. The surface morphology of AC wasexamined with SEM.

Adsorption experiment

A stock solution was prepared by dissolving FLA in TX100solution. The concentrations of FLA and TX100 are 80 and5,000 mg/L, respectively. The stock solution was diluted tothe required concentrations.

The adsorption equilibrium isotherms were examinedwith a reciprocating shaker at approximately 200 rpmand maintained at 25±0.5 °C for 24 h. AC (1 g/L) wasadded into a 150-mL glass flask. Subsequently, 100 mLof solution containing both TX100 and FLA or TX100only was added. The blank recoveries (without carbon)of FLA and TX100 were investigated, and the rangewas from 94 to 98 %. All data were adjusted based on theserecoveries.

Similar to the adsorption equilibrium isotherm experi-ments, adsorption experiments at 25, 35, and 45 °C were alsoconducted to determine thermodynamic parameters. In theadsorption kinetics experiment, the aqueous samples wereobtained at 5, 10, 15, 30, 60, 120, 180, 240, 300, 360, 480,1,440, and 2,880 min. To examine the effect of AC dosage,adsorption experiments were conducted with 0.2, 0.5, 1, 2, 3,and 4 g/L of AC. To study the effect of initial concentration,adsorption experiments were conducted with 1, 2, 3, 4, and5 g/L of TX100 and a fixed FLA concentration (0.01 g/L), andwith 0.01, 0.02, 0.04, 0.06, and 0.08 g/L of FLA and a fixedTX100 concentration (5 g/L). To investigate the effect of fulvicacid, adsorption experiments were conducted with 0.02, 0.04,0.06, 0.08, and 0.1 g/L fulvic acid. In all of the adsorptionexperiments, suspension samples were placed on a reciprocat-ing shaker at approximately 200 rpm for 24 h, except thesample for the contact time test. Adsorption experiments wereconducted at 25±0.5 °C for all samples, except the sample fortemperature study.

Regeneration of the saturated AC

After adsorption, the AC was dried at 105 °C for 6 h andregenerated in a quartz reactor with 2,500 MHz microwaveirradiation at 800 W for 6 min. The experimental setup hasbeen reported previously (Liu et al. 2004). The regeneratedAC was reused for further adsorption.

Analytical methods

After the filtration with 0.2 μm PTFE filter, FLA and TX100were analyzed by using high-performance liquid chromatog-raphy (HPLC, Dionex U3000) with an ultraviolet detector andan Agilent PAH column (250×4.6 mm) packed with 5 μmparticles. And HPLC conditions were the following: Flow ratewas 1.0 mL/min, injection volume was 20 μL, UV wave-length was 230 nm, and isocratic mobile phase flow was amixture with acetonitrile/water = 85:15.

Data calculation

After adsorption by AC, the fraction of the surfactant in thesolution (fTX100) and the fraction of FLA adsorbed to AC

1810 Environ Sci Pollut Res (2014) 21:1809–1818

(FFLA) were calculated from the experimental adsorption databy using the following equations:

fTX100 ¼ CTe

CT0� 100 % ð1Þ

FFLA ¼ qF fACC F0

� 100 % ð2Þ

where CTe (in gram per liter) is the concentration of TX100 insolution after the adsorption; CT0 (in gram per liter) and CF0

(in milligram per liter) are the initial concentrations of TX100and FLA in solution before the adsorption, qF (in milligramper gram) is the concentration of FLA on AC, and fAC (ingram per liter) is the fraction of AC in solution.

The selectivity (S in Eq. 3), which was proposed by Ahnet al. (2007, 2008a), was used to evaluate the selective remov-al of FLA:

S ¼ qF

CFe� CTe

qTð3Þ

where CFe (in milligram per litter) is the concentration of FLAin solution after the adsorption and qT (in gram per gram) isthe concentration of surfactant on AC.

Results and discussion

Effect of AC dosage

Figure 1 shows the effect of AC on the adsorption of TX100(5 g/L) and FLA (80 mg/L). The fraction of FLA adsorbed on

AC increased remarkably, whereas the fraction of TX100 de-creased proportionally with increasing AC dosage. However,the increase of FLA fraction on AC slowed down at higher ACdosage. For example, FFLA increased from 39.8 to 74.5%whenAC dosage increased from 0.5 to 2 g/L; FFLA increased from80.1 to 90%whenAC dosage increased from 3 to 4 g/L, whichindicated the excess in AC concentration compared with FLAconcentration. The adsorbed TX100 leveled off at certainamounts, as reported previously (Gonzalez-Garcia et al. 2001;Narkis and Ben-David 1985).With a relatively small amount ofAC, fTX100 decreased with increasing dose of AC.

Figure 1 shows that the selectivity was higher than 1 in allcases, indicating that the adsorption of FLA in the presence ofTX100 by AC was a potentially promising method for sur-factant recovery. The fraction of TX100 decreased in propor-tion to the dose of AC. At an AC dosage of 2 g/L, 83.6 % ofTX100 remained in the solution. The adsorbed fraction ofFLA increased with decreasing FLA concentration at the sameTX100 concentration. Therefore, in practice, AC can removePAHmore efficiently because pollutant concentration in a realsetting is usually lower than the concentration used in ourexperiments.

Increasing AC dosage did not noticeably affect selectivity,which was previously reported in studies on adsorption ofPHE from TX100 by AC (Ahn et al. 2008c). When 1 g/L ofAC was used, 91 % surfactant was recovered with 53 % FLAremoval.

Adsorption isotherms

Figure 2a, b illustrates the adsorption isotherms of FLA andTX100, which are important in designing the adsorption sys-tem. Two common adsorption models, namely, the Langmuir

0 1 2 3 410

20

selectivity F

FLA

fTX100

activated carbon dosage (g/L)

sele

ctiv

ity

0.4

0.6

0.8

1.0

fTX

100 or FFL

A (%)

Fig. 1 Adsorption effect of FLA(80 mg/L) and TX100 (5 g/L)with different AC dosage

Environ Sci Pollut Res (2014) 21:1809–1818 1811

(Langmuir 1916) and Freundlich (Freundlich 1906) isothermmodels were used to analyze the relationship between FLAand TX100 that were adsorbed on AC. Linear forms of theabove two models could be represented in Eqs. 4 and 5 as thefollowing:

1

qe¼ 1

qmaxb

1

Ceþ 1

qmaxð4Þ

logqe ¼1

nlogCe þ logKF ð5Þ

where qe (in milligram per gram) is the adsorbed amount perunit mass of adsorbent at equilibrium; Ce (in milligram perliter) is the equilibrium concentration of adsorbate; qmax (inmilligram per gram) is the maximum adsorption capacity; b isthe adsorption equilibrium constant, characteristic of affinitybetween adsorbent and adsorbate; KF (mg1−1/nL1/n/g) is theFreundlich constant representing the adsorption capacity; nrefers to the adsorption capacity.

Experimental isotherm data were fitted to these equationswith linear regression analysis. Table 1 lists the coefficients ofthese two isothermmodels. Correlation coefficients of the twomodels are also calculated by fitting the experimental adsorp-tion equilibrium data.

Adsorption of FLA can be well fitted into the Langmuirisotherm at a maximum adsorption capacity of 39.98 mg/g.Only the isotherm of TX100 with FLA was considered as anexample in Fig. 2a because the adsorption isotherm of TX100did not significantly change regardless of the presence or ab-sence of FLA (p=0.0018, n=9). The regression coefficients inthis study indicate that the isotherm data of TX100 are explainedbetter using Langmuir model compared with the Freundlichmodel, which is in agreement with other studies (Erdinc et al.2010; Gonzalez-Garcia et al. 2001; Narkis and Ben-David1985; Punyapalakul and Takizawa 2006). Therefore, the ad-sorption process can comprise an adsorbate monolayer forma-tion on the outer surface of the adsorbent, which indicates thatthe presence of PAH had no significant effect on the adsorptionof surfactant on AC. This finding is consistent with results ofprevious research on the adsorption of other solutes in surfactantsolutions (Ahn et al. 2008c; Punyapalakul and Takizawa 2006).

Effect of contact time

Figure 3 shows that C/C0 was plotted against time to evaluatethe relationship between PAH concentration and contact time.

0 1 2 3 4 50.0

0.1

0.2

0.3

0.4

0.5a

Freundlich Langmuir

Equ

ilibr

ium

sor

bed

TX

100(

g/g)

Equilibrium liquid TX100 (g/L)

0.00 0.01 0.02 0.03 0.040.00

0.01

0.02

0.03

0.04

0.05b

LangmuirFreundlich

Equ

ilibr

ium

sor

bed

FLA

(g/

g)

Equilibrium liquid FLA(g/L)

Fig. 2 The fitting curves of TX100 (a) and FLA (b) with Langmuir andFreundlich model

Table 1 Langmuir and Freundlich adsorption isotherm constants

Langmuir constant Freundlich constant

qmax (mg/g) b (1/mg) R2 KF

(mg1−1/nL1/n/g)n R2

FLA 39.98 0.127 0.977 0.142 2.583 0.933

TX100 430 1.490 0.981 0.295 3.940 0.783

0 100 200 300 400 500 1500 2000 2500 30000.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

TX100FLA

C/C

0

t(min)

Fig. 3 Adsorption of TX100 (5 g/L) and FLA (80mg/L) byAC versus time

1812 Environ Sci Pollut Res (2014) 21:1809–1818

TX100 adsorption on AC was very fast within the first 30 minand then continued at a slower rate for 6 h without significantchange, which was similar to other reports (Denoyel andRouquerol 1991; Gonzalez-Garcia et al. 2001). The amount ofadsorbed TX100 on AC was the maximum amount.Additionally, the adsorption rate of FLA was lower than thatof TX100. FLA reached an equilibrium state in approximately8 h, which indicated that the adsorption of PAH was muchslower than that of the surfactant onAC. Therefore, the adsorbedTX100 can help remove FLA effectively from the solution.

Modeling of adsorption kinetics may be described by thepseudo-first-order and pseudo-second-order kinetic models.The pseudo-first-order kinetic equation has been widely usedfor the adsorption of an adsorbate from aqueous phase. Theintegral form of pseudo-first-order kinetic model (Lagergren1898) was expressed as the following,

logqe

qe− qt¼ k1t ð6Þ

where qe (in milligram per gram) and qt (in milligram pergram) were the amounts of adsorbate adsorbed on the adsor-bent at equilibrium and at time t (in minute); k1 (in per minute)was the rate constant of the first-order adsorption. Thestraight-line plots of log (qe−qt) against t were used to deter-mine rate constantk1.

The integral form of pseudo-second-order kinetic model(Vinod and Anirudhan 2003) was expressed as Eq. 7,

t

qt¼ 1

k2q2eþ t

qeð7Þ

where k2 (in gram per milligram minute) was rate constant ofthe pseudo-second-order model; k2 and qe were calculatedfrom intercept and slope of the linear plots of t/qt against t.

Table 2 lists the constants of the two kinetic models. Thedata agreed well with the pseudo-second-order kinetic modelbecause the correlation coefficient was larger than 0.99 and thecalculated qe,cal agreed with the experimental qe,exp, suggestingthat the adsorption of PAH on AC follows the second-orderkinetics. Similar results were observed in the adsorption of otherorganic compounds from aqueous solutions or in the presence

of surfactants (Cabal et al. 2009; Erdinc et al. 2010; Rodriguezet al. 2009; Valderrama et al. 2008).

Thermodynamic studies

In environmental engineering practice, both energy and entro-py should be considered to determine the spontaneous occur-rence of adsorption. Therefore, adsorption amounts of FLAand TX100 on AC were determined at 25, 35, and 45 °C. Thefollowing equations were used to determine all thermodynam-ic parameters, including changes in enthalpy (ΔH), entropy(ΔS), and Gibbs free energy (ΔG) (Nourmoradi et al. 2012):

ΔG ¼ −RTlnqeCe

ð8Þ

ΔG ¼ ΔH−TΔS ð9Þ

lnqeCe

¼ ΔS

R−ΔH

RTð10Þ

where R (in joule per mole kelvin) was the gas constant and T(in kelvin) was the absolute temperature. Thus, ΔH (in kilo-joule per mole) and ΔS (in joule per kelvin mole) could beobtained from slope and intercept of the line plotted by ln(qeCe

)versus 1/T, respectively. Table 3 lists the thermodynamicparameters.

Free energy change (ΔG) during the adsorption of FLAwasnegative, which indicated that the adsorption process wasspontaneously thermodynamic. ΔG increased with increasingtemperature, which indicated that the sorption of FLA wasfavorable at high temperature. The positive ΔH also showedthat the sorption of FLA byACwas endothermic. The positive

Table 2 The adsorption kinetic parameters adsorption of TX100 (5 g/L) and FLA (80 mg/L) by AC

qexp (mg/g) Pseudo-first-order Pseudo-second-order

k1 (1/min) qe,cal (mg/g) R2 k2 (g/mg min) qe,cal (mg/g) R2

FLA 35.976 0.0019 30.234 0.9226 0.00059 37.556 0.9970

TX100 826.63 0.0021 788.441 0.6815 16.2486 829.73 0.9991

qexp is the amount of PAH adsorbed on the adsorbent at equilibrium which obtained from experiment, qe,cal is the amount of PAH adsorbed on theadsorbent at equilibrium which used Eqs 6 and 7

Table 3 Thermodynamic parameters for the adsorption of FLA andTX100 by AC

T(K)

FLA TX100

ΔG (KJ/mol)

ΔH (KJ/mol)

ΔS (J/K mol)

ΔG (KJ/mol)

ΔH (KJ/mol)

ΔS (J/K mol)

298 −0.14 17.78 0.06 2.58 0.01 −0.29

308 −0.60 2.65

318 −1.34 2.73

Environ Sci Pollut Res (2014) 21:1809–1818 1813

ΔS indicated an increase in the degree of freedom of theadsorbed species. Similar results were observed in the adsorp-tion of other organic compounds from aqueous solutions (Chenet al. 2011; Ghaedi et al. 2012; Nourmoradi et al. 2012).However, these data showed that the free energy of TX100was non-spontaneous. Furthermore, the increase of ΔG withincreasing temperature indicated that the adsorption of TX100was more non-spontaneous at low temperature.

Effect of initial concentration on the separation of surfactantand FLA

Although surfactants can be adsorbed in soil (Da et al. 1994;Rivas 2006; Zhou and Zhu 2008; Zhu and Gu 1991), theconcentration of surfactants in soil washing solution is stillhigher than its CMC. Thus, the concentration of TX100 washigher than its CMC. Table 4 shows the effect of initialconcentration on the separation of FLA and surfactant byAC. At the same TX100 concentration (5 g/L), the adsorbedfraction of FLA and selectivity increased with decreasing FLA

concentration. However, the change of surfactant fractionswas negligible (within 1 %), which indicated that the concen-tration of FLA did not significantly affect surfactant absorp-tion on AC. At the same FLA concentration (10 mg/L), theTX100 fraction in solution and selectivity decreased, whereasthe adsorbed fraction of FLA increased with decreasingTX100 concentration.

Additionally, at the same TX100 concentration, increasedcompetition for the AC sites was observed, and the adsorptioncapability decreased with increasing FLA concentration. Thehighest selectivity (62.92) was obtained with the highestTX100 and the lowest FLA concentrations (5 g/L and10 mg/L, respectively), which indicated that surfactant con-centration should be as high as possible and the contaminantconcentration should be as low as possible to effectivelyrecover and reuse the surfactant in soil washing practice.Selectivity was over 1 in all cases, which indicated thatrelatively more contaminants were adsorbed on AC than thesurfactant, and surfactant recovery and reuse can be performedtheoretically. However, when the TX100 concentration was1 g/L, the fraction of surfactant in the solution after adsorptionby AC was 0.289, which was much lower than the value at aTX100 concentration above 2 g/L. When the TX100 concen-tration was 1 g/L and the FLA concentration was 0.01 g/L, thefraction of FLA adsorbed to AC was 0.987, which indicatedthat adsorption is a good approach for removing contaminantswith low surfactant concentrations to protect the environment.The selective adsorption of hydrophobic contaminants by ACis ideal for surfactant reuse.

Effect of humid acid on the separation of surfactant and FLA

The efficiency of surfactant recovery is also controlled by soilorganic matter (SOM) which plays an important role in the

0.00 0.02 0.04 0.06 0.08 0.106

8

10

12

14

16

selectivity F

FLA

fTX100

the humbic concentration(g/L)

sele

ctiv

ity

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

fTX

100 or FFL

A (%)

Fig. 4 Adsorption effect of FLA(80 mg/L) and TX100 (5 g/L)with different fulvic acidconcentration (pH=6.7, shakingrate is 200 rpm, shakingtemperature is 25 °C, ACdose is 1 g/L)

Table 4 Separation results of FLA and TX100 by AC adsorption

Initial TX100 (g/L) Initial FLA (g/L) fTX100 FFLA S

5 0.08 0.926 0.593 20.83

0.06 0.921 0.683 25.06

0.04 0.922 0.695 27.16

0.02 0.918 0.713 31.23

0.01 0.921 0.842 62.92

4 0.01 0.920 0.834 53.26

3 0.855 0.935 41.68

2 0.715 0.973 40.75

1 0.289 0.987 22.12

1814 Environ Sci Pollut Res (2014) 21:1809–1818

adsorption and separation of HOC in soils (García-Gilet al. 2008). These SOMs include macromolecules, suchas humic acids, fulvic acids, humin and others. In orderto understand the function of SOM, different amounts offulvic acid were added into the solution because fulvic acidwas the most mobile fraction and a major component ofdissolved organic matter in the environment, and it had notbeen extensively studied (Dai et al. 2006; Stevenson 1994;Wang et al. 2012).

Figure 4 shows the effect of fulvic acid on the separation ofFLA and surfactant on AC. The FFLAvalue showed a differenttrend from the fTX100 value. The fTX100 value was maintainedon a plateau, suggesting that fulvic acid did not significantlyaffect the adsorption of TX100 on AC even at the highestconcentration (0.1 g/L). However, the adsorbed fraction of

FLA and selectivity reportedly decreased with increasingfulvic acid concentration as reported (Ahn et al. 2008b). Forexample, the adsorbed amount of FLAwas 0.031 g/g when thefulvic acid concentration was 100 mg/L, which was 67 %compared with that of without fulvic acid. These resultsindicated that SOM had a negative effect on HOC adsorptionbecause SOM can cause an increase in FLA solubility bychanging micellar structure, directly interacting with FLA(Kilduff and Wigton 1998; Newcombe et al. 1997), orresulting from the competition between dissolved organicmatter and FLA for the limited adsorption capacity of AC.Although surfactant recovery was only slightly affected bySOM, the focus must be on the reduction of PAH adsorption.Soil washing solution with a large amount of organic matter candecrease contaminant removal significantly. Therefore, the

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ace

Wavenumbers(cm-1)

C-H C-O-CC=Ccarboxyl groups with hydrogen bonds

AC

0.0 0.5 1.0 1.5 2.00

2

4

6

8

10

b

a

Incr

emen

tal p

ore

volu

me

Pore width(Nanometers)

Fig. 5 FTIR spectra (a) and poresize distribution (b) of AC

Environ Sci Pollut Res (2014) 21:1809–1818 1815

removal of dissolved organic matter before adsorption may beimportant for surfactant reuse (Ahn et al. 2008b).

Adsorption mechanism

BETsurface area, pore volume, average particle diameter, andpore size distribution of AC were 718 m2/g, 0.85 cm3/g,0.8 mm, and 0.2 to 2.0 nm, respectively. Figure 5a illustratesthe FTIR spectrum of AC. The broad tail with a peak at 3,608to 3,320 cm−1 is due to the carboxyl groups with hydrogenbonds. The peak at 1,645 cm−1 is due to the C═C stretchingvibration in the benzene ring. The peak at 1,386 cm−1 is due tothe C–H bending vibration. A broad band at 1,116 cm−1 maybe due to the C–O–C stretching vibration.

Figure 5b shows that most micropores of ACwere from 0.2to 2.0 nm. The molecular dimension of FLA is 1.14 nm×0.95 nm×0.38 nm (Sander 1997), the size of TX100 in theform of monomer is 2.7 nm, and the average size of TX100 inthe form of micelle is 11.6 nm (Levitz et al. 1984). Microporesof AC with an entrance size of intermediate value relative tothe sizes of FLA and TX100 can only allow adsorption ofFLA, whereas TX100 micelle cannot enter the micropore.Thus, an adsorbent with pore size from 1.14 to 11.6 nmwouldbe appropriate for selective adsorption of PAHs in the surfac-tant solution. However, TX100 adsorption was much fasterand the amount of adsorbed TX100 was much higher com-pared with FLA because the concentration of TX100 wasmuch higher. Surfactant molecules adsorbed on AC can blockthe entrance of numerous micropores. Thus, the adsorptionmechanism was not the same as that of micropore filling.Therefore, it is not the pore size distribution, but the surfacechemistry of adsorbent that determines the removal of PAH.

The proposed mechanism was confirmed by SEM analysisof the surface of AC before and after adsorption. Figure 6shows that AC had a dusty texture with very high micropo-rosity before adsorption. The surface of AC after adsorptiondiffered because the micropores were not clear and the surfacewas covered by a thin film from the surface-adsorbed surfac-tant, which explains the difficult entry of FLA in the micro-pore of AC.

In soil washing practice, although surfactants can beabsorbed by soil, the concentration of the surfactant in soilwashing solution is still much higher than its CMC becausethe initial concentration of the surfactant is very high. Forexample, the optimal concentration of TX100 was 5 g/L duringsoil washing (Sheng Peng 2011). Depending on the initialsurfactant concentration, the nonionic surfactant can form ahemisphere, a hemicylinder, and a monolayer on hydrophobicsolid substrate (Arnold et al. 2011; Ferrari et al. 2004; Tiberget al. 1999). TX100 adsorption is much faster than that of FLA.Thus, TX100 adsorption on the surface of AC can lead tosurface aggregation at a concentration even lower than CMC.Therefore, aggregated or semi-aggregated micelle formation

inside the internal surface is not possible. The further adsorbedmolecules will be retained by direct hydrophobic interactionabove CMC, which indicates that the adsorption process cancomprise the formation of surfactant aggregates in the adsorbed

a

b

Fig. 6 SEM images of AC (a before adsorption; b after adsorption)

1 2 3 4 5 60

20

40

60

80

100

120

Reg

ener

atio

n ef

fici

ency

Cycles of regeneration

TX100 FLA

Fig. 7 Evolution of the regeneration efficiency of saturated AC

1816 Environ Sci Pollut Res (2014) 21:1809–1818

layer through the interaction between newly adsorbed mole-cules with those already fixed on the solid surface. Most of thePAH was adsorbed to the adsorbed surfactant aggregates ratherthan on the original carbon sites of AC (Ahn et al. 2008c), andthe dissolved PAH in TX100 micelles increased the adsorbedPAH amount on AC. Numerous studies have successfullyremoved organic compounds by surfactant-modified materials(Chen et al. 2012; Choi et al. 2008; Hong et al. 2009;Mohamed2004; Xie et al. 2012), which further confirmed that surfactantaggregation on the adsorbent surface can form micelle-likestructures that can absorb hydrophobic organic compounds.

Regeneration of AC

After adsorption experiment, AC was regenerated by usingmicrowave irradiation. After five adsorption regeneration cy-cles, the adsorption capacities of regenerated AC for TX100and FLAwere maintained at approximate 418 and 31.8 mg/g,indicating that the microwave irradiation did not significantlyaffect the surface chemistry of AC. Figure 7 shows the evo-lution of TX100 and FLA regeneration efficiency as a func-tion of the cycles. The carbon regeneration efficiencies ofTX100 and FLA could be maintained at 89.8 and 79.9 %,respectively, at the sixth cycle. The results indicated thatmicrowave irradiation of AC might be a feasible method witha reduced cost and environmental acceptability.

Conclusion

The feasibility of adsorption of hydrophobic organic compoundsin surfactant solution on AC was verified. Experimental data ofTX100 and FLA fitted the Langmuir isotherm model. Themaximum adsorption capacities of AC in the presence of sur-factant were 430 mg/g for TX100 and 39.98 mg/g for FLA. Thepseudo-second-order kinetic model agreed well with the adsorp-tion dynamic behavior of FLA and TX100 on AC, althoughTX100was adsorbedmore rapidly than FLA.A thermodynamicstudy showed that the adsorption of FLA on AC was spontane-ous and endothermic in the presence of surfactant. The additivefulvic acid had a slight effect on TX100 adsorption, but had alarge and negative effect on FLA adsorption. Furthermore, theselectivity of FLA over TX100 was much larger than 1, whichindicates that AC adsorption can be an effective material forsurfactant recovery, especially with the effective regeneration ofAC through microwave irradiation. The surface chemistry de-termines the removal of PAH in the presence of surfactant onAC adsorption.

Acknowledgments This study was financially supported by BeijingNatural Science Foundation: Mechanisms of selective recovery of sur-factant in soil washing solutions with activated carbon (8122027).

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