pMo

stitutemir Inmir In

sed fo

et al., 2005).Activated carbon is an expensive commodity. In the

United States its current average bulk price from the

Int. J. Miner. Process. 79 (21. Introduction

Phenol and its derivatives are present in the waste-waters of such industries as petroleum refining, leatherand textile manufacturing, olive oil manufacturing etc.in appreciable quantities (Table 1) (Cermola et al., 2004;Kent, 1992; Rengaraj et al., 2002). Phenolic compoundshave low allowable limits (0.51.0 mg/l) due to theirtoxicity to human and aquatic life and must be removed

from wastewaters in environmentally acceptable ways.Despite the existence of several physicochemical andbiological treatment techniques (solvent extraction, ion-exchange by resins, chemical oxidation by ozone,aerobic or anaerobic biodegradation, etc.), adsorptionon activated carbons is the most effective and frequentlyused technique for phenol removal (Davis and Cornx-vell, 1991; Nathanson, 1997; Monahan, 1998; Tong etal., 1998; Tchobanoglous and Stensel, 2002; DabrowskiA raw lignitic coal from Soma, Turkey was investigated to determine its potential as an adsorbent for phenol removal fromwastewaters. Kinetic batch tests demonstrated that phenol could be completely removed from solution given sufficient solidsloading and reaction time. The adsorption capacity of 10 mg/g obtained with the lignite is low compared to those achievable withactivated carbons (around 300 mg/g). However, when normalized for the surface area, the adsorption capacity was much larger forthe lignite (1.3 mg/m2) than that generally observed with activated carbons (0.050.3 mg/m2). Hydrogen-bonding of the phenolic OH with the oxygen sites on the lignite surface is the most likely mechanism for adsorption. Though water molecules also haveaffinity for the same oxygen sites, lateral benzene ring interactions make phenol adsorption energetically more favorable. Sincephenol molecules adsorbed in this fashion would project their benzene rings into solution, formation of a second layer through theaction of the dispersive interactions between the benzene rings is very likely. Residual water quality with respect to majorelements and heavy metals was within acceptable limits defined by the ASTM standards. Dissolution of organic matter from thelignite was also observed to be negligible. 2006 Published by Elsevier B.V.

Keywords: adsorption; phenol; lignite; oxidation; hydrogen-bonding; wastewaterAbstractAvailable online 24 April 2006Capacity and mechanism of

H. Polat a,, M.a Department of Chemistry, Izmir In

b Environmental Engineering Program, Izc Department of Chemical Engineering, Iz

Received 26 January 2006; received in reviCorresponding author. Tel.: +90 232 750 7531; fax: +90 232 7507509.

E-mail address: hurriyetpolat@iyte.edu.tr (H. Polat).

0301-7516/$ - see front matter 2006 Published by Elsevier B.V.doi:10.1016/j.minpro.2006.03.003henol adsorption on lignite

lva b, M. Polat c

of Technology, Urla, Izmir, Turkeystitute of Technology, Urla, Izmir, Turkeystitute of Technology, Urla, Izmir, Turkey

rm 7 March 2006; accepted 7 March 2006

006) 264273www.elsevier.com/locate/ijminpromajor producers is listed as $2.5 per kilogram with ademand of about 180 thousand tons forecasted for 2006

phenolic wastewaters are big users of energy rawmaterials (Table 1).

In this study, feasibility of using a raw lignite samplefor phenol removal from wastewater without any pre-treatment procedures was investigated. The parameterstested were the solid/liquid (S/L) ratio, initial phenolconcentration, reaction time and temperature. A mech-anism which explains the adsorption of the phenol onthe lignite surface is proposed.

2. Materials

Table 1

265H. Polat et al. / Int. J. Miner. Process. 79 (2006) 264273(Kirschner, 2006). These numbers provide compellingenough reasons for the search for cheaper adsorbents.Bentonites (Banat et al., 2000), zeolites (Roostaei andTezel, 2004; Garca et al., 1993), surfactant treatedsmectides (Shen, 2004) and montmorillenites (Jiang etal., 2002), chitin (Dursun and Kalayci, 2005), bagassefly ash (Srivastava et al., 2006) and organic beet pulp(Dursun et al., 2005) are some possible candidatesinvestigated. However, lower fractional removals andadsorption capacities obtained with these adsorbentsraise questions with respect to their efficiency andregeneration.

Though lignite has been used to prepare activatedcarbons (Streat et al., 1995; Duggan and Allen, 1997;Finqueneisel et al., 1998a,b; Aksu and Yener, 2001;

Levels of phenol reported in industrial wastewaters (Cermola et al.,2004; Kent, 1992; Rengaraj et al., 2002)

Industrial source Phenol conc., mg/l

Petroleum refineries 40185Petrochemical 2001220Textile 100150Leather 4.45.5Coke ovens (without dephenolization) 6003900Coal conversion 17007000Ferrous industry 5.69.1Rubber industry 310Pulp and paper industry 22Wood preserving industry 50953Phenolic resin production 1600Phenolic resin 12701345Fiberglass manufacturing 402564Paint manufacturing 1.1Dabrowski et al., 2005), literature is scant in any workwhere raw lignites were employed directly for phenoladsorption.1 However, as it will be discussed in theResults and Discussion sections in detail, the presenceof surface oxygen sites on carbon surface has beenreported to have a capacity for binding phenol. Ligniticcoals contain an abundance of such functional groups intheir structure. Also, their very low bulk price andabundance are the obvious advantages of the lignites aspotential adsorbents. Use of lignites in raw form alsomeans avoiding the expensive pyrolysis step necessaryfor activated carbon preparation. Furthermore, the spentlignite may not necessarily be subject to the costlyregeneration or disposal requirements. It can be burneddirectly, preferably on-site where the phenolic waste isgenerated since most of the facilities which generate

1 The work appeared in journals covered by ScienceDirect between1970 and 2006 are implied.The lignite sample was from Soma Region inWestern Turkey. The particle size was reduced tonominal passing 500 m in a laboratory jaw crusher-hammer mill-screen setup. Representative samples of250 g were stored in plastic bags under positive argonpressure. The results of the proximate and ultimateanalyses are given in Table 2 along with the surface areameasurements. Quartz, staurolite and muscovite weredetected in the lignite structure in the XRD studies.

The phenol (molecular weight of 94.1 g/mol) wasobtained from the Merc Chemical Company. At 25 C, ithas a relatively low vapor pressure of 0.41 mm Hg andsolubility in water of 8.2% by weight. Phenol solutionswere prepared daily by dissolving a known amount ofphenol in water below the solubility limit and werestirred for 30 min prior to adsorption experiments.

Double distilled water, which was passed through aBarnstead Easypure UV-Compact ultrapure water sys-tem (18.3 ), was used to prepare the solutions.Glassware was cleaned with chromic acid withsubsequent rinsing with distilled water.

3. Methods

The kinetic batch adsorption studies were carried outat a constant temperature of 25 C and at a natural pH ofabout 8 unless otherwise stated. In the tests where the

Table 2Characteristic properties of the Soma lignite sample

Physical properties Oxides, % Elements,mg/kg

Ash, % 32 CaO 33.8 Pb 420Volatile matter, % 1635 SiO2 33.4 Cu 172Fixed carbon, % 11 Al2O3 16.1 Ni 152Moisture, % 21 Fe2O3 6.0 Zn 116BET surface area, m2/g 4.79 MgO 6.1 Co 104Micropore surface area, m2/g 7.7 SO3 2.8 Cr 64Nominal particle size, m 250 Na2O 1.1 Sr 60

Calorific value, kcal/kg 2200 K2O 0.7 Cd 52

effect of S/L ratio was investigated, the solution phenolconcentration was kept constant at 100 mg/l while thelignite amount was varied between 1.25 and 40 g. Inanother set of tests, the S/L ratio was kept constant at0.05 while the initial phenol concentration was variedbetween 100 and 1000 mg/l. All adsorption tests werecarried out in 250 ml polyethylene bottles containing100 ml phenol solution on an IKA Labortech-KS125digital shaker and a Memmert Model (Germany) waterbath with temperature adjustment. Each bottle was usedto obtain a single sample to ensure a constant S/L ratiothroughout the experiments and to prevent accumulativeerror. The sampling times were 0.08, 0.33, 1.5, 3, 6, 24,

phenol completely when there is sufficient lignitesurface area in solution. According to the figure, theS/L ratios over 0.2 provide complete removal of phenol.

Fig. 2 presents the equilibrium adsorption capacities(eq, mg phenol adsorbed on each gram of lignite atextended times) as a function of initial phenolconcentration for two different temperatures. In thesetests, the S/L ratio was kept constant at 0.05 while theinitial phenol concentration was varied between 100 and1000 mg/l. It can be seen that adsorption capacityincreases significantly with increasing initial concentra-tion, but tapes off towards a seemingly limiting value ofabout 10 mg/g. Though the effect of temperature isnegligible at low initial phenol concentrations, theadsorption capacity becomes slightly lower at high

266 H. Polat et al. / Int. J. Miner. Pr48, 96 and 168 h. The supernatant solutions wereanalyzed for phenol with a UV Spectrophotometer(UVVIS 1208 from Shimadzu) at 270 nm. Todetermine the residual water quality after adsorptionfor heavy metals and major elements which may havedissolved from the lignite sample, selected supernatantsolutions were analyzed by an ICP (Varian, AES AxialLiberty Series 2). The FTIR spectra of the as-receivedand treated lignite samples were obtained in KBr using aShimazdu FTIR-8601 PC spectrometer (Kyoto, Japanwith a resolution of 1.5 cm1. The surface area wasdetermined by a Micromeritics ASAP 2010 volumetricadsorption device employing nitrogen at a temperatureof 77 K. The device uses internal algorithms whichemploy the BET and the BJH models to obtain the BETand micropore surface areas, respectively. A Soxhletevaporator-condenser was employed to determine themaximum amounts of organics dissolvable from thelignite sample. Macherey Nagel type (4040) filterpapers and membrane filters of 0.45 m from Sartorius(Minisart RC 25) were used for solidliquid separation.

An initial kinetic experiment was carried out up to200 h using a 100 mg/l phenol solution in the absence ofsolids to assess the amount of phenol loss from the

time, hours0 50 100 150

f(t)

0.0

0.2

0.4

0.6

0.8

1.0 S/L=0.4

0.0125

0.025

0. 05

0.10.2Fig. 1. Fractional phenol removals as a function of time for varying S/Lratios (f(t)= [CinCres(t)] /Cin; Cin=100 mg/l; pH=natural).reaction bottles and to test the reproducibility of theanalytical procedure. The residual phenol concentrationat the conclusion of this test was 1000.1 mg/l,indicating excellent reproducibility and no significantphenol loss.

4. Results

Kinetic adsorption studies were carried out todetermine the adsorption behavior of phenol on ligniteat various S/L ratios using an initial phenol concentra-tion (Cin) of 100 mg/l. The results are presented in Fig. 1in terms of fractional phenol removal from solution, f(t),as a function of time where f(t)= [CinCres(t)] /Cin.Here, Cres(t) is the residual phenol concentration insolution at time t. The figure shows that fractionalphenol removal increases with time and an equilibriumfractional phenol removal, feq, is approached for all S/Lratios at long reaction times. It should be noted that asthe feq value increases towards 1.0 with increasing S/Lratio. This demonstrates that it is possible to remove

Cin

200 400 600 800 1000

eq,

m

g/g

2

3

5

7

20

1

10298 K308 K

Fig. 2. Equilibrium adsorption capacities as a function of initial phenolconcentration at two temperatures. (S/L=0.05; pH=natural).

ocess. 79 (2006) 264273initial phenol concentrations for the higher temperature.

The adsorption capacity of 10 mg/g is low whencompared to the usual adsorption capacities observedwith activated carbons (100300 mg/g). This is mostprobably due to the small surface area of the lignitesample (7.7 m2/g) compared to those usually associatewith activated carbons (in the order of 1000 m2/g).

Fig. 3 gives the adsorption capacities obtained atdifferent temperatures (25, 30, 35, and 40 C) as afunction of time. The figure shows that the equilibriumadsorption capacity demonstrates a slight but steadydecrease with increasing temperature; eq decreasingfrom 1.41 to 1.22 mg/g with an increase in temperaturefrom 25 to 40 C. The solid lines in the figure weredrawn using a first-order rate equation in the form:

carbon (Filtrasorb 300, Calgon Corporation) has kF valueof 21 and n value of 1.85 (Dabrowski et al., 2005 afterDobbs and Cohen, 1980).

Fig. 3 demonstrated that the change in the adsorptioncapacity with time can be represented quite well by afirst-order rate equation. In Fig. 5, the apparent reactionconstants for the four temperatures obtained from Fig. 3

eq

2

3

57

1

10

2

3

57

10

298 KkF=0.117; n=0.70; r2=0.999

308 KkF=0.121; n=0.69; r2=0.999

1/T, x10-3 K-13.20 3.25 3.30 3.35

ln(k)

-2.0

-1.9

-1.8

-1.7

-1.6A = 9.45; Ea= -10.33 kJ/molr2 = 0.9934

267H. Polat et al. / Int. J. Miner. Process. 79 (2006) 264273ht heq1ekt 1

where k is the apparent reaction constant. It can be seenthat the agreement between the first-order equation andthe kinetic data is very satisfactory.

Among the empirical models tested (Freundlich,Langmuir and BET), the Freundlich isotherm wasobserved with the adsorption data obtained. Thisisotherm, which is known to better represent heteroge-neous adsorbent and multilayer adsorption cases (Brev,1958), can be expressed in the form:

heq kFCneq 2

In this above equation, Ceq is the equilibrium residualphenol concentration. The parameter kF is the empiricalFreundlich constant related to adsorption capacity n isthe Freundlich exponent related to the strength ofadsorption. A comparison of the experimental datawith the Freundlich isotherm is shown in Fig. 4 for 298and 308 K. The estimates of the model parameters,

time, hours50 100 150

(t), m

g/g

0.4

0.8

1.2

1.6

298 K303 K308 K313 K

Fig. 3. Effect of temperature on adsorption of phenol on the lignite.k2tThe solid lines are predicted by the equation (t)=eq(1e )

(Cin=100 mg/l; S/L=0.05; pH=natural).goodness of fit and 95% confidence intervals are alsopresented in the figure. The magnitudes of the modelparameters are rather small and remain nearly constantwith the change in temperature (kF0.119 and n0.70).Such small values are indicative of a relatively lowadsorption capacity and weak bonding (Schwarzenbachet al., 2003). As a comparison, a commercial activated

Ceq20 30 50 70 200 300 500100

1

Fig. 4. Application of the Freundlich isotherm to the adsorption data attwo temperatures. The solid lines are the best-fit lines which give theparameter estimates in the inset boxes and the broken lines show the99% confidence intervals (S/L=0.05; pH=natural).Fig. 5. The ln(k) versus 1 /T plot to calculate the activation energy, Ea,from the data given in Fig. 3.

are plotted in the form of an ln(k) vs. 1 /T plot. Accordingto the Arrhenius Equation, the slope of the line in Fig. 5should give the activation energy, Ea since

lnk lnA EaRT

3

The magnitude of Ea comes out to be 10.3 kJ/molfrom the figure. The low value of Ea also implies weakbonding between phenol and the lignite (Atkins, 1994;Yin et al., 2002).

The heat of adsorption, H, which gives the changein the enthalpy of the system upon adsorption can becalculated using the Van't Hoff Equation:

H RT1T2 lnKd1=Kd2T2T1

4

where Kd1 and Kd1 are the distribution constants at T1and T2. The distribution constant Kd is equal to the ratio

can be removed from the surface with water-washingalone, indicating that a significant fraction of the phenolis very weakly adsorbed.

The water quality with respect to heavy metals andmajor elements dissolvable from the lignite sample wasdetermined using the Standard Method ASTM D-4793(ASTM, 1995). The result was presented in Fig. 8. It canbe seen there is no significant dissolution of heavymetals. For those heavy metals which show a certainamount of dissolution, the observed values were alwaysbelow the limiting values set by the EnvironmentalRegulations for Water Quality (Classes I, II, III and IV).Also, a Soxhlet evaporator-condenser set-up was used todetermine the maximum amount of dissolvable organicsmaterial from the lignite sample. In this test, the lignitesample was repeatedly washed by evaporating, con-densing and percolating the same water through the

268 H. Polat et al. / Int. J. Miner. Prof the equilibrium concentrations of phenol adsorbed onlignite and remaining in solution at a given temperature.The heat of adsorption values as a function of initialphenol concentration in solution are given in Fig. 6. Itcan be seen that H values are negative for all initialconcentrations studied, indicating an exothermic reac-tion. Also, the absolute magnitude of H is alwayslower than 20 kJ/mol. An adsorption process isgenerally considered physical if Hb25 kJ/mol andchemical when HN40 kJ/mol.

Initial Phenol Concentration, mg/L0 200 400 600 800 1000 1200

G

, kJ/

mol

-10

-9

-8

-7

-6

H, k

J/m

ol

-50

-40

-30

-20

-10

0PhysisorptionH40 kJ/mol

Fig. 6. Heats of adsorption and Gibbs free energies for the adsorption

of phenol on the lignite as a function of initial phenol concentration.Fig. 6 presents the average Gibbs free energies for thetwo temperatures calculated using the equation:

G RT lnKd 5

The negative G values in the figure indicatespontaneous adsorption of the phenol molecules onthe lignite surface. It should be noted that the absolutemagnitudes of the Gibbs free energies are larger for lowinitial concentrations (around to 10 kJ/mol) where themonolayer coverage is more likely and show a small buta progressive decrease with increasing initial phenolconcentration.

A desorption test using only distilled water to washthe phenol from the lignite surface was also carried outto determine the fraction of weakly-bonded phenol(Fig. 7). The figure shows that about 33% of the phenol

time, hours0 50 100 150

C eq,

m

g/L

25

50

75

100

time, hours50 100 150

% d

esor

ptio

n

0

50

100

Adsorption

Desorption

Fig. 7. Adsorption and desorption of phenol from the lignite sampleafter washing with water (Cin=100 mg/l, S/L=0.05, pH=natural).

ocess. 79 (2006) 264273lignite sample until no significant changes in solution

in soGa In

Mg

ple donme

er. Prcomposition could be observed. The total amountdissolved material was found to be 1.57 mg for 1 g oflignite. Out of this amount, 0.76 mg was determined tobe comprised of Ca, Mg, Na and K. Hence, themaximum amount of organic matter dissolved fromthe lignite sample, most probably in the form of humicspecies, was about 0.81 mg for each gram of lignite.

5. Discussion

The adsorption studies carried out at different S/Lratios at a solution phenol concentration of 100 mg/l,which is usually encountered in wastewaters (see Table1), show that the lignite sample is capable of removingthe phenol completely from solution at S/L ratios of 0.2and above (Fig. 1). The studies at a constant S/L ratioand at varying initial phenol concentration, on the other

contaminants Ag Al B Ba Bi CdCo Cr Cu Fe

con

cen

trat

ion,

mg/

L

0.0

0.2

0.4

0.6

0.8

mg/

L

05

1015

Ca

Fig. 8. Dissolution of the major and trace elements from the lignite samvisible bar are below detection limits. Limiting values set by the Envir

H. Polat et al. / Int. J. Minhand, demonstrate that adsorption capacities as high as10 mg/g can be obtained with the lignite sample (Fig. 2).Further work, preferably in pilot scale, is needed todetermine the optimum conditions both with respect tothe efficiency of removal and the efficiency ofadsorption capacity.

The phenol adsorption capacity observed with thelignite is an order of magnitude lower than theadsorption capacities usually observed with activatedcarbons, which range between 50 and 300 mg/g(Coughlin et al., 1968; Franz et al., 2000; Zawadski,1988; Terzyk, 2004; Dabrowski et al., 2005). Such highvalues are due the enormous surface area of activatedcarbons. The micropore surface area is usually in theorder of 1000 m2/g for the activated carbons whereas itwas only 7.7 m2/g for the lignite sample. Therefore,when expressed in terms of per unit surface area, ligniteseems to give a surprisingly good adsorption capacity.The adsorption capacity per unit area comes out to bearound 1.3 mg/m2 for the lignite sample as opposed tothe typically observed values of 0.050.3 mg/m2 foractivated carbons.

These observations necessarily point out to twoconclusions: First, it is apparent that the fraction of thesurface area contributing to adsorption is orders ofmagnitude higher for the lignite. This is in fact not quitesurprising since approximately 80% of the microporepore volume in activated carbons belong to the poreswith diameters less than about 8 (Franz et al., 2000;Terzyk, 2003, 2004; Gauden et al., 2004). The diameterof an unhydrated phenol molecule has been to be about6.0 both experimentally (Singh et al., 1996) andcomputer simulations (Mooney et al., 1998). Since aphenol molecule must pass by another to diffuse into thepores, it is obvious that a vast amount of activated

lutionLi Mn Ni Pb Sr Tl Zn

limiting valuesmg/liter

KNa

Ag Al B Ba CdCoCrCuFe MnNi Pb ZnNa

1-5 1.01.02.0 0.150.22.01.03.03.03.02.03.0250

etermined by the ASTM analysis. The metals which do not display antal Regulations for wastewater are shown in the side bar.

269ocess. 79 (2006) 264273carbon surface would be inaccessible for phenol. On thecontrary, small angle neutron scattering studies indicatethat coals in general have negligible micropore structure(Yamada, 1996). In the case of lignites, Polat et al.(1996) observed during controlled oxidation studies thatthe majority of the surface area is due to the macro-cracks which are directly accessible from the externalsurface as have been observed.

The second conclusion arising from the observedadsorption densities is that the phenol molecules mustbe adsorbing quite generously on the lignite surface.Different parking areas have been reported for phenoldepending on its orientation on the surface; 45 2

(Mattson et al., 1969) and 52.2 2 (Puri et al., 1975) forplanar orientation and 28 2 for edge-on and 24.8 2 forend-on orientations (Singh, 1971; MacDonald andEvans, 2002). McClellan and Harnsberger (1967) givephenol parking area as 30.5 2 based on adsorption

er. Prtests. Planar and end-on orientations on the surfacenecessarily means that phenol adsorbs through thebenzene ring whereas an edge-on orientation involvesbonding through the OH functionality. Assuming anedge-on orientation for reasons which will be apparentin the following paragraphs, the adsorption density of10 mg/g (Fig. 2) would require a surface area ofapproximately 18 m2. Since the lignite sample has asurface area of 7.7 m2 per gram, it is obvious that phenolmust also be adsorbing in secondary layers.

There is a voluminous amount of work on theadsorption mechanism of aromatic compounds, specif-ically phenol, on carbon material. A comprehensivereview of this literature can be found in papers byRadovic et al. (2000), Terzyk (2004) and Dabrowski etal. (2005). It is generally accepted that aromaticcompounds adsorb on the unoxidized graphitic part ofthe surface physically through dispersive interactionsbetween the -electrons of the graphitic carbon basalplanes and those of the phenol aromatic ring (Coughlinand Ezra, 1968). However, although carbon surface alsocontain oxygen-rich sites (mainly in the form of basiccarbonylCOH, acidic carboxylCOOH and phenolichydroxylOH functionalities), the influence of thesegroups on phenol adsorption has not been resolved.Mattson et al. (1969) propose a donoracceptorcomplex formation between the negatively chargedaromatic ring and the positively charged basal plane forphenol adsorption on oxygen sites though the impor-tance of such complex formation has been seriouslyquestioned in more recent work (Franz et al., 2000;Terzyk, 2004).

Another more likely mechanism for phenol adsorp-tion on oxidized sites is hydrogen-bonding since thephenolic OH has been shown to be capable of suchbonding (Guedes et al., 2003). Such adsorption impliesthat oxidation of the carbon surface should promoteadsorption capacity. However, several works report adecrease in phenol adsorption from water with surfaceoxidation (Coughlin and Ezra, 1968; Coughlin et al.,1968; Mahajan et al., 1980; Leng, 1996; Franz et al.,2000; Terzyk, 2003, 2004; Moreno-Castilla, 2004).Since lignites are rich in oxygen surface groups (Yamanet al., 2000; Kkbayrak et al., 2000), they have a highaffinity for water (Allardice and Evans, 1971; Schafer,1972; Kaji et al., 1986; Mller and Gubbins, 1998; Leeand Reucroft, 1999). Therefore, the decrease inadsorption with oxidation is attributed to the preferentialhydration of the surface oxygen groups over thehydrogen-bonding of the OH of the phenol molecule.However, studies which demonstrate enhanced phenol

270 H. Polat et al. / Int. J. Minadsorption from aqueous solutions with surface oxida-tion also exist in the literature (Zawadski, 1988;Finqueneisel et al., 1998a,b; Tessmer et al., 1997).

Since hydrogen-bonding is an extremely fast dy-namic process with a time scale of around 0.2 ps(Asbury et al., 2004), an enormous number of bondformation and breaking must be taking place in solutionat a given instant. Though water is usually cited for itspolarity due to its high dipole moment (1.85 Debye), thephenol molecule is also strongly polarized (1.61 Debye).Such high polarity implies that the phenol moleculesmust also be forming momentary hydrogen bonds withthe surface oxygen groups. While the number ofphenolic bonds maybe smaller on the average comparedto the those formed by water, the lateral dispersiveinteractions of the benzene rings of the adsorbed phenolmolecules should also be taken into account. Dispersiveattraction between two benzene rings is twice as large asthose between two water molecules and four times aslarge as those between a water molecule and a benzenemolecule at a given separation (Polat and Polat, 2000).Removal of a phenol molecule adsorbed next to anotherwill require an additional energy for overcoming thebenzene ring interactions. Hence, such strong lateralinteractions would lead to a cumulative effect in favor ofphenol adsorption, those molecules on the surface actingas nucleation sites for further adsorption. Adsorbed inthis fashion, phenol molecules will be projecting theirbenzene rings towards the water. Such orientation maylead to the development of a secondary layer through thebenzene ring interactions.

The small values of the Freundlich parameter(n0.7) in Fig. 4 indicate physical adsorption. Thesmall activation energy in Fig. 5 (10.3 kJ/mol) and heatof adsorption values in Fig. 6 (between 6 and 20 kJ/mol) support the proposed mechanism. The negativeGibbs free energies in Fig. 6 demonstrate that adsorptionof phenol is spontaneous. The Gibbs free energy of10 kJ/mol at the low initial concentration range wheremonolayer adsorption should be dominant probablyrepresents hydrogen-bonding energy of phenol onto thesurface oxygen. The energy decreases towards 6 kJ/mol with increasing initial phenol concentration,implying that adsorption of phenol molecules as asecond layer through the benzene ring interactions wasless energetic. This is not surprising since the hydrogen-bonding energies related to the formation of the firstlayer must be higher than the dispersive interactionenergies which bind the second layer.

Finally, the FTIR spectra of phenol presents H-bonded OH stretch at 36003100 cm1 and COstretch at 13001100 cm1. It also present aromatic ring

ocess. 79 (2006) 264273absorptions of_CH at 31003000 cm1 and CC at

er. Pr1600 cm1 and 15001450 cm1 (FDM, 2002). On theother hand, the lignite sample used in this work alsogenerated these bands very broadly due to theabundance of surface oxygen groups. Therefore, theFTIR spectra of the as-received lignite sample and thelignite samples treated in water and in phenol solutionsyielded no discernible differences and are not reportedhere.

6. Conclusions

Batch studies were carried out to investigate thecapacity and mechanism of phenol adsorption on ligniteas a function of S/L ratio, initial concentration andtemperature. The specific conclusions based on thefindings of this study are as follows:

1. Complete removal of phenol from water was possibleusing the raw lignite as an adsorbent. Fractionalremoval values of 100% could be obtained for aninitial phenol concentration of 100 mg/l at S/L ratiosof 0.2 and higher.

2. Despite the low adsorption capacities per gram ofadsorbent (about a tenth of those obtained withactivated carbons), the adsorption capacity per unitadsorbent surface area was much larger (at least fourtimes of those obtained with activated carbons). Thisillustrates that phenol adsorbs generously on thelignite surface and the relatively low adsorptioncapacity is due to the smaller surface area of thelignite.

3. The primary mode of adsorption is proposed to be thehydrogen-bonding of the OH group on the phenolmolecule onto the polar oxygen sites on the lignitesurface. The relatively small difference between thepolarities of the phenol and water molecules aided bylateral interactions between the benzene rings of theadsorbed phenol molecules makes such bondingenergetically more favorable compared to thehydrogen-bonding of the water molecules to thesame oxygen sites.

4. A comparison of the adsorbed amounts and theavailable lignite surface area implies that phenolproduces a second layer on the lignite surface. Sincethe primary layer phenol molecules on the lignitewould be projecting their benzene rings into solution,the benzene ring interactions between the adsorbedphenol molecules and those in the solution are likelyto be the mechanism for the formation of thesecondary layer.

5. The results obtained in this work (negligible negative

H. Polat et al. / Int. J. Minchanges in adsorbed amounts with changes intemperature, small Freundlich parameters, activation,heat of adsorption and Gibbs free energies which areall below 20 kJ/mol) show that adsorption of phenolon lignite falls in the physisorption category andsupport the mechanism described.

6. Water quality tests carried out using the ASTMprocedure demonstrated that the effluent qualityfollowing adsorption was within environmentalstandards. The concentration levels of major ele-ments and heavy metals were well under the definedstandard levels for wastewater. The amount oforganic matter dissolved from the lignite was alsonegligible.

The above conclusions suggest that raw lignites, whichare inexpensive and abundant raw materials throughoutthe world, are viable candidates for the treatment ofphenolic wastewaters. Use of raw lignites as adsorbent inthis fashion by-passes the expensive pyrolysis proceduresfor surface activation. Moreover, the spent adsorbent maybe directly utilized on-site as an energy source where thephenolic wastewater is generated. Such possibility meansavoiding the necessary regeneration or disposal require-ments for other adsorbents.

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ASTM, 1995. Annual Book of ASTM Standards, 11.04, p. 58.Atkins, P.V., 1994. Physical Chemistry, 5th edition. Oxford University

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Capacity and mechanism of phenol adsorption on ligniteIntroductionMaterialsMethodsResultsDiscussionConclusionsReferences