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Journal of Colloid and Interface Science 261 (2003) 32–39www.elsevier.com/locate/jcis

Adsorptive removal of methylene blue from colored effluentson fuller’s earth

G. Atun,a,∗ G. Hisarli,a W.S. Sheldrick,b and M. Muhlerc

a Department of Physical Chemistry, Faculty of Engineering, University of Istanbul, 34850 Avcilar-Istanbul, Turkeyb Department of Analytical Chemistry, Faculty of Chemistry, University of Ruhr, D-44780 Bochum, Germanyc Department of Technical Chemistry, Faculty of Chemistry, University of Ruhr, D-44780 Bochum, Germany

Received 3 July 2002; accepted 9 January 2003

Abstract

The adsorption behavior of methylene blue (MB) on four fuller’s earth (FE) samples of varying compositions was investigatedspectrophotometric technique to obtain information on the color removal. The distribution coefficient (KD) increased with an increase in thinitial concentration (C0) of the dye, attained a maximum value, and decreased again at higher initial concentrations. Dye solutionscolorless for aC0 value corresponding to maximumKD. A progressively increased flocculation behavior in the clay suspensions was oband the maximum value ofKD corresponds to optimum flocculation of the clay. TheKD values were found to decrease exponentially aftethe solution again became colored while the amount adsorbed increased with an increase in the initial concentration of MB. Only adata obtained for this region could be defined by adsorption isotherm equations. The shifts of theC0 values corresponding toKmax

D towardhigher concentrations were correlated with the composition of FE samples by using XRF, XRD patterns, and SEM images. The intemperature on MB adsorption was also studied and thermodynamic parameters were calculated. 2003 Elsevier Science (USA). All rights reserved.

Keywords:Fuller’s earth; Dye adsorption; Methylene blue; Adsorption isotherm; Distribution coefficient

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1. Introduction

Fuller’s earth (FE) used as an adsorbent has been knto exist in the Mihaliccik region of Turkey. It contains vaous amounts of dioctahedral smectites (Ca-montmorillonnatural zeolites (analcime), and loughlinite, which is a kof sepiolite [1]. All of these minerals exhibit a strong affiity for heteroaromatic cationic dyes. The visible absotion spectrum of cationic basic dyes such as methyblue (MB) [2–11], thionine (TH) [12–15], crystal viole(CV) [9–11], acridine orange (AO) [7,16,17], and rhodam6G (R6G) [18] has been used recently in the determtion of surface properties of several types of clay minesuch as smectites [2–8,12,15–17], zeolites [13,14], andolite [9,10,18]. Smectites are specified as 2:1 layered cand swell in water. Surface charge of smectites arisesproton adsorption–desorption reactions on surface hydrgroups while layer charge results from substitutions in

* Corresponding author.E-mail address:gultena@istanbul.edu.tr (G. Atun).

0021-9797/03/$ – see front matter 2003 Elsevier Science (USA). All rights rdoi:10.1016/S0021-9797(03)00059-6

-

octahedral alumina layer, which is separated from aqous solution by a tetrahedral silicate layer [19]. The strture of zeolite channels contains large cages with windshowing different openings [13]. Sepiolite is a fibrous cmineral with fine channels running parallel to its fibershas an open structure exhibiting microfibrous morpholwith a large micropore volume due to the existence oftercrystalline cavities [9,21]. Monomers, protonated catiodimers, and higher aggregates of dye occur on the extsurface or in the basal plane, cages, cavities, or chanof these minerals depending on the size of the dye mcule, dye loading, solution pH, the type of exchangeacations, surface acidity, and the swelling properties ofclay. Since each form of dye absorbs visible light at a difent wavelength, a visible spectroscopic technique hassuccessfully used to study clay–dye interactions in claypensions. The MB–clay interaction has been claimed toan extremely sensitive fingerprint for the detection of sface properties of clays such as surface charge, layer chsurface area, and exchange capacity [3,6,8,9]. There is,ever, some disagreement about the correct value of su

eserved.

G. Atun et al. / Journal of Colloid and Interface Science 261 (2003) 32–39 33

gend thebleithheen-in-

ionstionsn theaveag-

entaterFEata

s inrptio

al-plese-mECa-i-

-rayam-rawFE.chough

ith

lib-atereremer

ofed

su-ea-th ofeakd-

tionon

erkersn as as,red,

e

e.

les,instres.es.re-d aad-vesra-on-t ofntra-to

y bethed atith

area calculations because it was recently shown that theerally accepted adsorption geometry is questionable anMB molecules adsorb a tilted arrangement [6,8]. VisiMB spectra in dilute solution show a band associated wmonomeric MB at 665 nm with a shoulder at 605 nm. Tobserved red shift of the MB monomer band in clay suspsions arises from the interaction of dye molecules in theternal domains of tactoids. MB in aqueous clay suspensshows metachromatic behavior even at lower concentrawith respect to the aqueous dye solution. The decrease imonomer band and the appearance of bands at lower wlengths are attributed to formation of dimers and highergregates on the external surface [2,4–8].

The aim of this study is to produce a satisfactory efflufor discharge into receiving waters or for reuse as a wsupply from highly concentrated MB solutions by usingsamples with different compositions. The adsorption dhave also been correlated to the ratio of clay mineralFE samples and have been used to determine the adsocapacity for MB of the adsorbents.

2. Experimental

Four FE samples from different deposits in the Mihliccik region were used for batch experiments. FE samwere kindly provided by Professor M. Yeniyol from the Gology Department of Istanbul University and Y. Bilge froBensan A.S. X-ray diffraction (XRD) paterns of the four Fsamples revealed that they were composed mainly ofmontmorillonite, loughlinite, and analcime [1]. The chemcal composition of the FE samples was analyzed by Xfluorescence spectrometry (XRF). Analyses of the FE sples are given in Table 1. The surface morphology of thematerial and the product obtained after MB treatment ofwere examined by scanning electron microscopy (SEM)

Adsorption of MB was performed by using a batmethod. The clay samples were crushed and sieved thra 200-µm sieve. A liquid–solid ratioV/m of 100 cm3 g−1

was chosen. Thus 0.05 g of clay was put into contact w5 cm3 of dye solution in a polypropylene beaker of 25 cm3

for 4 h (i.e., a time period long enough to reach equirium) at a given temperature in a thermostatic shaker/wbath and left overnight. Later, the clay suspensions wcentrifuged at 4000 rpm for 30 min and the equilibriuMB concentration was determined by using a Perkin ElmUV–vis. spectrophotometer model 554 with silica cellslength 1 cm. The initial MB concentrations were chang

-

-

n

in the range of(5 × 10−5)–(2 × 10−2) M. Except forthe largest concentrations generally no dilution of thepernatant was required prior to spectrophotometric msurements. Absorbances were determined a waveleng665 nm, which corresponds to the maximum absorption pof MB monomers in water, or at the maximum of the reshifted monomer peak.

Kinetic experiments were made to compare adsorprates of MB on FE and on commercial activated carb(CAC). Beakers containing 5 cm3 of MB solution of 3.25×10−3 mol dm−3 were immersed in a thermostatic shakcontrolled at 298 K. Adsorbents were added to the beaand the timing was started. The suspensions were shakefunction of time from 1 min to 4 h. At various time intervalsamples were taken and MB concentration was measuafter centrifugation as already described.

The distribution coefficient for adsorptionKD (cm3 g−1)was derived using the formula

(1)KD = C0 − C

C0

V

m,

whereC0 andC are the initial and final concentration of thdye solution (mol dm−3), respectively,m (g) is the weight ofadsorbent, andV (cm3) is the volume of the aqueous phas

3. Results and discussion

3.1. MB adsorption isotherms

As an example of the adsorption isotherms of FE sampFig. 1 depicts the amounts of MB adsorbed by FE-2 agathe equilibrium concentrations for various temperatuThree regions can be distinguished on the isotherm curv

In the first region (I), an S-shaped isotherm which corsponds to a slow increase, followed by a fast growth, anregion growing to a plateau is observed at very low dye loing, except for the lowest temperature. After that, the curshow a behavior in which at lower equilibrium concenttions the adsorption is higher than at higher equilibrium ccentrations in the second region. However, the amounadsorbed dye increases depending on the initial concetion for all cases. The adsorption of MB on FE leadschanges in the absorption spectra of the dye. As maseen from visible spectra for the first region in Fig. 2maximum wavelength of the monomer peak is observe675 nm. This represents a red shift by about 10 nm w

Table 1Chemical analyses (wt%) of the FE samples used in this study

Clay SiO2 Al2O3 MgO CaO Na2O Fe2O3 TiO2 MnO K2O P2O5

FE-1 51.09 10.09 10.57 4.38 3.83 4.19 0.42 0.06 2.45 0.01FE-2 51.70 9.51 11.06 4.10 4.85 3.95 0.38 0.06 2.00 0.01FE-3 52.42 7.73 12.55 1.90 6.60 3.14 0.29 0.05 1.64 0.01FE-4 50.23 6.29 13.61 1.84 6.81 2.25 0.22 0.04 1.07 0.00

34 G. Atun et al. / Journal of Colloid and Interface Science 261 (2003) 32–39

atdraw

n at

olu-ace

ionsthe

n-pH

es inved

theandringori-llars anolor

on-at thedyeces

ole-ughak ispro-

ionionacter-olorkeds in

E isricorb-id–ointofav-the

me-houl-largetelymedndsfaced in-bedisfaceFEature.

y byd thes.inermvery

het arer toedfreethend

Fig. 1. Equilibrium profiles for MB solution in contact with FE-2 samplesvarious temperatures. The curves in the first and second regions wereby hand through the data.

Fig. 2. Visible spectra of MB in aqueous solution and after equilibratioa point corresponding to the first, second, and third regions.

respect to the maximum observed for dilute aqueous stions. At this stage dye penetrates into the interlayer spwhere the bathochromic shift could be due to p-interactbetween the aromatic entity and the oxygen plane ofclay minerals and/or micropolarity/microacidity of the iterlayer surface. In our unpublished study, the averagevalue at the zero point of charge (i.e., pHzpc) of FE sam-ples was determined as 8.7. Since solution pH changthe range of 6 to 8 during sorption experiments it is belie

n

that the positively charged surface hydroxyl groups onedges of clay repel the similarly charged MB cationshence they enter the interlayer of minerals without coveof outer exchange sites [12]. The dye molecules mustent themselves to find a suitable position in the interlamespaces of the adsorbent. Since this is a difficult procesS-shaped isotherm is observed in this region and the cchanges from light blue to dark with increasing dye ccentration. However, an absence of S-shaped behaviorlowest temperature may be due to the competition ofmolecules with water molecules in the interlamellar spabecause it is well known that the adsorption of water mcules on a solid is higher at lower temperatures. Althothere are positively charged surface sites on FE, no peobserved at the longer wavelength corresponding to thetonated form of adsorbed dye.

In the second region (II), as the initial MB concentratincreases the equilibrium concentration of MB in solutdecreases and finally reaches zero. This does not charize an adsorption behavior. In this region the change of cfrom dark blue to colorless can be followed by the naeye. The clay flocculates due to aggregation of the cationthe interparticle space of the flocs. At an excess of MB, Fgradually peptized, forming small tactoids with monomeMB in the interlayer space and at the same time adsing dimeric and polymeric MB cationic species at the solliquid interfaces. Colorless solution (CS) appears at the pof optimum flocculation of FE, indicating the highest ratesedimentation due to size of the flocs forming. This behior suggests that all structural negative charges of FE insuspension are neutralized by cationic MB species. Tidependent spectra for the second region show that the sder at 610 nm starts to decrease, and at 575 nm a newpeak appears. At equilibrium, the 610-nm peak compledisappears while the 575-nm peak increases. It is assuthat the 575-nm band in the MB spectra in Fig. 2 correspoto trimers or higher aggregates of dye on the external surand 610-nm band corresponds to dimers at the outer anternal surfaces of the clay [7,15]. The amount of adsordye (qcs) at the optimum flocculation point (i.e., at CS)considered to correspond to coverage of the clay surwith MB molecules. The layer charge distribution on thelayers is heterogeneous because of its heterogeneous nNeutralization of the higher negative layer charge densitcationic MB species promotes greater agglomeration anvalue ofqcs increases as the layer charge of FE increase

In the third stage of adsorption (III), the MB solutionsequilibrium are once again colored and the H type isothis observed, indicating that the adsorbent surface has ahigh affinity for MB molecules. The wavelengths of tabsorption maximum and shoulder of the supernatan665 and 605 nm, respectively. These values are similathose of MB in water rather than to those of MB adsorbon dispersed clay particles, indicating the presence ofMB molecules in the water phase. The interaction ofmontmorillonite, sepiolite, and some zeolite with MB a

G. Atun et al. / Journal of Colloid and Interface Science 261 (2003) 32–39 35

with

axi-dat-rbedtheeel-adbease

ldns.

ad-n

the

tte.fE

intotiveuirhelp

us

in

nd

other basic dyes has been studied by several authorssimilar results [7,13,15,18].

The curves show clear plateaus corresponding to mmum adsorption capacity (qm) of FE. Although the S-shapecurves in the first region move toward the right, indicing a lower coverage for higher temperature, the adsoamount of MB increases with increasing temperature inthird region. This may be attributed to the difficulty of thmonomers attaining a suitable orientation in the interlamlar space at higher temperature. On the other hand, thesorption mechanism in the third region is considered toion exchange and in general the exchange capacity increas the temperature increases.

Only adsorption data for the third region in Fig. 1 coube fitted to Freundlich and Langmuir isotherm equatioThe linear form of the Freundlich isotherm is defined by

(2)logq = logk + n logC,

whereq is the amount of dye adsorbed per unit mass ofsorbent (in mol g−1) andC is the equilibrium concentratioof dye (mol dm−3).

As is seen in Fig. 3, logq vs logC curves give straighlines according to the Freundlich isotherm equation. TFreundlich isotherm parametersk and n are shown inTable 2. The Freundlich exponentn gives information abousurface heterogenity and surface affinity for the soluFor favorable adsorption, 0< n < 1. Since the degree ofavorability increases asn approaches zero all four Fsamples show a very high affinity for MB species.

Since the Freundlich isotherm equation does not takeaccount the maximum adsorption capacity, as an alternadata for the H type isotherm were fitted to the Langmequation. The adsorption data were analyzed with the

-

s

,

Fig. 3. Freundlich isotherms for third region of MB adsorption at variotemperatures.

of the linear form of the Langmuir isotherm,

(3)C/q = 1/Kqm + (1/qm)C,

whereqm is the maximum adsorption capacity of dye (mol g−1), and if the equilibrium concentrationC is referredto standard concentrationC0 = 1 mol dm−3, the adsorptionequilibrium constantK is found to be nondimensional.

As seen in Fig. 4 from the linear relation betweenC/q

andC, the adsorption equilibrium constantK and maximumadsorption capacityqm of adsorbents were calculated a

Table 2Isotherm parameters of MB adsorption on FE samples and parameters forKmax

D

Clay sample T Freundlich parameters Langmuir parameters Parameters forKmaxD

n k × 104 r qm × 104 K × 10−4 r C0 × 103 qcs× 104

(K) (mol g−1) (M) (mol g−1)

FE-1 290 0.116 2.94 0.982 1.50 3.77 0.999 0.85 0.84298 0.093 5.20 0.992 3.15 6.17 0.999 1.50 1.49318 0.083 6.95 0.951 4.25 3.13 0.999 2.00 1.90333 0.080 9.15 0.948 4.48 14.00 0.999 2.35 2.30

FE-2 290 0.079 3.09 0.961 1.78 16.27 0.999 0.90 0.98298 0.068 5.89 0.962 3.47 20.48 0.999 2.00 2.00318 0.050 8.13 0.958 5.65 21.53 0.999 3.60 3.50333 0.061 9.42 0.994 6.11 36.58 0.999 3.80 3.79

FE-3 290 0.059 3.55 0.950 2.95 22.19 0.999 2.50 2.50298 0.057 6.62 0.911 4.15 36.58 0.999 3.25 3.22318 0.051 7.84 0.986 5.79 29.65 0.999 3.63 3.62333 0.060 9.33 0.995 6.00 56.81 0.999 3.80 3.79

FE-4 290 0.058 5.15 0.967 3.96 38.08 0.991 3.00 2.99298 0.061 8.51 0.977 6.02 45.59 0.999 3.35 3.30318 0.069 9.12 0.977 6.23 56.24 0.999 3.70 3.69333 0.065 10.47 0.999 6.34 79.02 0.995 3.80 3.79

36 G. Atun et al. / Journal of Colloid and Interface Science 261 (2003) 32–39

us

andere-

anden

in-of

em-

val-

rves

ard2).a

t ofin-gne1].ngth-1er-lin-

B

snhowthatand,theo-

itieslin-

E,aceCS,ndt toasalniteuntn,

oneear

n theareRD

veredulesty to

as

Fig. 4. Langmuir isotherms for third region of MB adsorption at variotemperatures.

these are also shown in Table 2. BothK andqm increasedfrom FE-1 to FE-4.

Using the appropriate constants for the FreundlichLangmuir equations, the theoretical isotherm curves wpredicted using known values ofC. Figure 1 shows a comparison of the experimental points with the FreundlichLangmuir isotherms. Both isotherms gave good agreemwith the experimental data for the four temperatures.

3.2. Distribution coefficients dependence on initialconcentration of MB

Our investigation has focused particularly on determing the initial concentration of MB for the appearancecolorless solution depending on clay composition and tperature. This can be easily determined from theC0 valuecorresponding to the maximum of the logKD vs C0 curves.Before the appearance of colorless solution (BCS) theues ofKD increased exponentially with increasingC0 whileafter coloring appeared (ACS) they decreased. The cufor BCS and ACS intersect at a point corresponding toC0for the occurrence of CS. These points for CS shifted towhigherC0 values from FE-1 to FE-4 (see Fig. 5 and TableAs can be seen in Table 1, the amounts of MgO and N2Oincrease from FE-1 to FE-4 whereas the amounts of Al2O3,CaO, and Fe2O3 decrease. This suggests that the amounloughlinite, which is a Na-sepiolite, in the FE samplescreases toward FE-4. Sepiolite is a natural hydrated masium silicate mineral with a needle-like morphology [20,2These microscopic fibers have dimensions ranging in lefrom 0.2 to 2 µm. A comparison of SEM images for FEand FE-4 in Fig. 6 shows fibrous loughlinite particles intspersed with a larger fraction in FE-4. Since both lough

t

-

Fig. 5. The change of logKD values against initial concentration of Msolution for FE samples.

ite and montmorillonite have ad spacing of 12.9 Å, peakcorresponding to 2θ of 6.8◦ overlap in the XRD patterns iFig. 7 for the FE samples. These XRD patterns also sthat this peak increases from FE-1 to FE-4, indicatingthe fraction of these minerals increases. On the other hthe XRF data in Table 1 support the fact that especiallyfraction of loughlinite increases rather than that of montmrillonite. The increase in the maximum adsorption capactoward FE-4 may be correlated with the enhanced loughite fraction (Table 2).

To better understand the interaction of dye with FSEM images and XRD patterns of a raw clay surfwere compared with the MB adsorbed surface at BCS,and ACS. At the BCS, the intensity of peaks of MB aloughlinite in the XRD patterns decreases with respecthe raw materials. On the other hand, at the CS, the bspacing is hardly ever observed for MB adsorbed loughliand montmorillonite. Since at this point the adsorbed amoof MB is equal to the point producing optimum flocculatiosilicate layers arranged very irregularly with respect toanother [3]. At ACS, the peaks of MB adsorbed clay appagain. This suggests that MB aggregates adsorbed osurface are diffused interparticles and clay silicate layersreoriented. The SEM images corresponding to these Xpatterns indicate that the surfaces at BCS and CS are coby MB aggregates while at ACS some of the dye molecare diffused inside and the surface shows some similarithe raw material.

3.3. Effect of temperature on MB adsorption

The values listed in Table 2 show that the values ofC0corresponding toKmax

D shift toward higher concentrations

G. Atun et al. / Journal of Colloid and Interface Science 261 (2003) 32–39 37

Fig. 6. A comparison of SEM images of FE-1 and FE-4 samples.

alleres o98,ot

awten-era-ated

entature.s

re

ated

temperature increases from 290 to 333 K. However, smchanges were observed for higher temperatures. A seriexperiments were conducted with water without MB at 2318, and 333 K to study the transformation of clay in hwater.

A comparison of SEM images and XRD patterns of rmaterial and hot-water-treated FE showed that peak insities of XRD patterns increased with increasing tempture and SEM images showed that clay layers are separ

f

,

forming small tactoids of larger spacing. So, the adsorbpossesses a higher surface area and a more porous nAs a result of these phenomena, the adsorption capacitieqcsat CS, the maximum adsorption capacitiesqm, and the equi-librium constantsK increased with increasing temperatu(Table 2).

The change of free energy of adsorption can be calculfrom equilibrium adsorption constants:

(4)�G0 = −RT lnK.

38 G. Atun et al. / Journal of Colloid and Interface Science 261 (2003) 32–39

Fig. 7. A comparison of XRD patterns of FE samples.

pe

s ofesef

m is-re-

Thermodynamic parameters, standard enthalpy�H 0 andentropy�S0 of adsorption, were obtained from the sloand intercept of the lnK vs 1/T plot according to therelation

(5)lnK = �S0

R− �H 0

RT.

The results presented in Table 3 show that the value�G0 are negative, indicating favorable adsorption. Thvalues (around−30 kJ mol−1 at 298 K) are in the middle ophysical adsorption and chemisorption [22]. Since�H 0 isclose to zero and�S0 is positive, the adsorption mechanisis an entropy-controlled process.

Table 3Thermodynamic parameters for MB adsorption on FE

Clay sample �H0 × 102 �G0 �S0

(kJ mol−1) (kJ mol−1 ) ( kJ mol−1 K−1)

FE-1 1.58 −27.34 0.92FE-2 1.29 −30.30 1.01FE-3 1.28 −31.74 1.06FE-4 1.27 −32.28 1.08

3.4. Comparison of adsorption rate of MB on FE andactivated carbon

Since the adsorption rate of MB is very high, the dtribution coefficients initially increase exponentially. The

G. Atun et al. / Journal of Colloid and Interface Science 261 (2003) 32–39 39

in

testhatof

imere,hes

m-E-1tiontheis

ccu-The

id-ntra

in-eat-seonFEFE

ore,t forforbout

l,

.

198

aday

y,

(1)

a,

115

177

7)

5.87

.

25

id

Fig. 8. Comparison of adsorption rates of two FE samples with CAC3.25× 10−3 M MB solution (i.e., near CS).

fore, logKD was plotted against time. The adsorption raof FE-3 and FE-4 samples near CS were compared withof the CAC. As shown in Fig. 8 the adsorption capacitiesboth FE samples are higher than that of CAC and the trequired to attain equilibrium is much shorter. FurthermoFE is a locally available, low-cost adsorbent in Turkey. Taverage price of FE samples is $0.04/kg whereas CAC cost$20/kg.

4. Conclusion

MB is found to adsorb strongly on the surface of FE saples and the adsorbed amount of MB increases from Fto FE-4. Three regions were observed on the adsorpisotherm curves. The shape of the isotherm belongs toS type for low dye loading, whereas an H-type isothermobserved in high concentrations. On the other hand, a flolation behavior is observed at moderate concentrations.interaction of MB with clay in this region cannot be consered as dye adsorption because the equilibrium conce

-

tion of MB in solution decreases as dye-interacted claycreases. The adsorption process of MB on FE, for the trment of MB effluents, allowed effluent suitable for reuto be produced. The results show that MB adsorptionFE is very fast compared to that on CAC and also thatsamples have higher adsorption capacity. Furthermoreis approximately 500 times cheaper than CAC. TherefFE can be used as a highly effective low-cost adsorbenthe removal of cationic dyes. The optimum conditionthe production of colorless effluent was determined as a318 K.

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