Indian Journal of Chemistry Vol. 39A, June 2000, pp. 611-617

Adsorption of methylene blue on cellulose from its own solution and its mixture with methyl orange

Debashish Palit & Satya P Moulik*t

Centre for Surface Science, Department of Chemistry, Jadavpur University, Calcutta 700032, India

Received 22 March /999; revised 20 October /999

The adsorption behaviour of the cationic dye, methylene blue (MB) from its own solution and from its mixture with an anionic dye, methyl orange (MO) on cellulose has been investigated. The effects of temperature and KCl on the adsorption process have been examined. The anionic dye, MO neither individually nor in mixed condition adsorbs on the negatively charged cellulose surface under the studied conditions of temperature and ionic strength. The dye MB alone and from its mixture with MO adsorbs on the cellulose surface, the former shows lower extent of adsorption than the latter. At a tem­perature >303 K, the mixed dyes MB and MO undergo complexation. The results have been analysed in the light of the ad­sorption isotherms of Freundlich, Frumkin and Langmuir and related adsorption thermodynamics.

The adsorption of solutes on solid surfaces offers the basis for the understanding of heterogeneous cataly­sis, chromatographic separation, dyeing of textiles and clarification of effluent, etc 1

• 2

. As an adsorbent, cellulose is employed for the treatment of textile mill waste water to remove dyes or pigments3

; this most abundant polymer is used as a raw material for many products in paper and textile industries and its de­rivatives find new applications in the pharmaceutical industry in drug delivery devices4

. The dyeing of cellulose and cellulosic materials is also of great in­terest in textile and paper industries . The investiga­tion of dye adsorption on such materials is of funda­mental importance for the understanding of the dye­ing mechanism in the production of textile materials . The adsorption of dyes on charged solids is an asso­ciated field of stud/. In this direction, the adsorption of dyes on charged solids, for example, cationic dyes (including azo dyes) on alkaline chromatographic alumina, crystal violet on hydrous ferric oxides, ani­onic dye, new coccine acid red # 18 onto sludge par­ticulate, etc. can be mentioned6

-9

. Davidson has worked on the adsorption of methylene blue on oxy­cellulose prepared by the action of alkaline hypobro­mite on cotton; the dye has a high affinity for the car­boxyl group of the polymers 10

•

Although adsorption of a single component, for

tE-Mail: spmessju@vsnl.com

example, a dye on different adsorbents has been studied in plenty, such a process for mixed dyes has

been rarely examined 11•

The objective of the present research is to under­stand how ionic dyes when mixed behave in terms of adsorption onto negatively charged cellulose surface under different environmental conditions . The pair of dyes considered for the work are the cationic dye, methylene blue and the anionic dye, methyl orange. The results of study are expected to offer an under­standing of the basics of the dye adsorption in combi­nation.

Materials and Methods The cationic dye, methylene blue (3,9-bisdimethyl­

aminophenazothionium chloride) was a G.R. product of E. Merck, Germany and was used as received. The anionic dye, methyl orange (Na salt of 4, [4-(dimethylamino)phenylazo]benzenesulphonic acid) was a product of Reanal , Budapest and was used after repeated (three times) crystallization from ethanol­water mixture (I: I v/v) . The purified sample was dried at 353 K for 24 h to remove moi sture.

The cellulose used was prepared from straw. The straw was cut down to small pieces and soaked in water for seventy two hours . It was then treated with lime and alkali for twenty four hours whence it be­came free of lignin. It was thoroughly washed with di stilled water and the ~lightly yellowish material was

612 INDIAN J CHEM, SEC. A, JUNE 2000

then bleached with H202 and dried in an oven at 353 K. The dried material was then pulverised to convert it into powder.

A Shimadzu UV -vis spectrophotometer ( 160A) was employed for spectral measurements using silica cells of path lengths 1 em. Doubly distilled conduc­tivity water was used for solution preparation. All measurements were taken at constant temperature with a thermostatic arrangement with a temperature fluctuation of ± 0.1 o.

Stock solutions of the dyes (MB and MO) and electrolyte (KCI) were prepared in doubly distilled conductivity wat and stored in thoroughly cleaned, steamed coming v umetric flasks and diluted as re­quired. In the adsorption xperiment, weighed quan­tity of the cellulose (0.2 g) was taken in a series of standard-joint pyrex glass stoppered bottles (25 rnl), to each of which was added 20 rnl of the dye solution with varying initial concentration (Ci) . The bottles were covered with black marble paper to protect them from light and were shaken in a horizontal shaker for 2 h, and then placed in a thermostated wa­ter bath at the desired temperature for 20 h with oc­cassional shaking. The system was observed to reach the adsorption equilibrium after 12-14 h but was al­lowed 20 h time to ensure the completion of the proc­ess. After the equilibrium, 5 ml aliquots were with­drawn from the bottles, centrifuged and 3 rnl of each solution was used for spectrophotometric estimation.

4r---------------------------~~-.

'e 3 01

0 E

.......

e 2 ' ~

"' ~

0 2 4

106

. Ce/mol lit-1

Fig. !-Adsorption isotherm for MB-cellulose system in aqueous medium at different temperatures [curve I, 303 K ; curve 2, 313 K].

The amount of dye adsorbed was calculated from the difference in the initial concentration (C) and the equilibrium concentration (Ce) measured spectro­photometrically. The position of the maximum ab­sorbances (Amax) of the MB and MO solutions were 664 nm and 460 nm respectively. The concentrations of the experimental solutions were observed to obey the Beer-Lambert's law with molar extinction coeffi­cients, 6.25xl04 and 2:38xl04 dm3 mor 1cm·1 at 303 K for MB and MO respectively.

Results and Discussion Adsorption of M B on cellulose

The adsorption isotherms of various classes of compounds in different surface~ have been classified according to their shapes. In the literature, four types of isotherms have been reported 12

, i.e., (i) the Lang­muir type (L) with an initial concavity to the concen­tration axis, (ii) the S-type with an initial convexity to the concentration axis, (iii) the H-type resulting from extremely strong adsorption and having an intercept on the ordinate and (iv) the C-type having an initial linear portion. The adsorption isotherms of the dye MB on cellulose at different temperature are repre­sented in Fig. I. The curves are Langmuirian or L­type. The change in temperature has no effect on the shape of the isotherm. It is apparent from Fig. 1 that the adsorption of MB on cellulose shows fairly strong affinity. The interaction between cellulose surface and MB is primarily electrostatic in nature (the dye is cationic and the adsorbent is negative). The adsorp­tion process of MB on cellulose is exothermic; it de­creases with increasing temperature. This is the ex­pected and normally observed feature.

3 7

J 1

'e 2 01

0 E

....... e ' )(

<D

9

0 4 8 12 16 20 24

106. Ce/mollit-1

Fig. 2-Adsorption isotherm for MB-cellulose system in aqueous KCI medium at 303 K [curves I, 2, 3 & 4 stand for 0.0 I , 0.05, 0.1 & 0.3 M KCI].

PALIT eta/. : ADSORPTION OF METHYLENE BLUE ON CELLULOSE 613

The adsorption process is retarded by the presence of salt, KCl (Fig. 2). The increase in salt (ionic strength) decreases the attractive interaction between the cationic dye and the negative (-COOH) cellulose sites. The adsorption process is, therefore, not pre­ferred. The retardation of adsorption of dyes and polymer on solid surfaces in salt environment has been reported in the past13

·14

.

Adsorption of MB-MO mixtures on cellulose The adsorption of MO alone on cellulose has been

found to be absent (Fig. 3). The anionic dye, MO does not prefer the negative surface of cellulose. In combination with MO, the adsorption of MB has de­creased at lower temperature of 303 K. At tempera­tures 308 and 313 K, the adsorption is greater than that of pure MB . Increase of temperature has induced increasing adsorption (Fig. 4 ). Esumi et al. 15 have found that polystyrene sulphonate (PSS) forms com­plexes with ionic surfactants viz, sodium dodecyl sul­phate (SDS) and hexadecyltrimethyl ammonium chlo­ride (HT A C) in solution and consequently the ad­sorption of PSS on alumina gets enhanced. Like pure MB, the adsorption of MB from the MB-MO mixture also decreases in presence of KCI.

It may be mentioned that although most of the curves in Fig. 4 as well as in Figs I and 2 are of Langmuirian type, on the lower side of concentration

" u c 0 0 .. 0 Ill 0 <(

700 BOO

Wa v e le ngth (n m )

Fi g. 3-Spectra of pure MB and MO and their I : I mi xture befo re and after adsorption on cellulose at 31 3 K [curve I , pure MB ; curve 2, MB-MO mi xture ( 1: I) ; cut·.·-! 3, MB-MO mi xture after adsorpt ion ; curve 4, MB after adsorption ; curve 5, pure MO; cu rve 6, pure MO after adsorpt ion ].

(in some of them), the curves appear like S-shaped for deviations due to experimental errors.

Spectral features of MB-MO mixtures The decrease in MB absorption in presence of MO

in solution as well as its increased adsorption with increasing temperature are unusual features. Complex formation between the two dyes in solution is envis­aged. In Fig. 5, spectra of the individual dyes and their mixtures under different conditions are pre­sented. The decreasing absorbance of MB and that of MO particularly at higher temperature is evidenced. The two dyes undergo interaction and complex for­mation when heated for 20 h at 313 and 321 K showing changes in their spectra. The absorbance of MB is moderately reduced but that of MO suffers

2 4 6 8 10 106.Ce/ mot l it-•

Fig. 4--Adsorption isotherm for MB-MO (I: I) - cellulose system in aqueous medium at different temperatures [curve I , 31 3 K ; curve 2, 308 K ; curve 3, 303 K].

" v c 0 .D .. 0 Ill .D ...

2-00.---- - - - - - - ----- - -

soo Wa ve length ( nm)

Fig. 5-Spectra of MB and MB-MO mixture (I: I) at 3 13 K [curve I, pure [MB] = 30 ~ ; curve 2, MB -MO each of con­centration 30 J.lM ; cu rve 3, [MO] = 30 J.lM] .

614 INDIAN J CHEM, SEC. A, JUNE 2000

striking reduction and it may also disappear. The ab­sorption maximum of MB, however, remains un­changed. The formed complex (optically clear with respect to MO) is adsorbed more on cellulose com­pared to MB. The complex formation, however, is tentati vely proposed: isolation and characterization of the product is required for the elucidation of its be­haviour. It can be mentioned that at higher concentra­tion , precipitation from the oppositely charged mixed dye solution of MB and MO has been observed.

Quantifi cation of the adsorption results Attempts have been made to analyze the adsorption

results in the light of Freundl ich, Frumkin and Lang-. . I

mUir equattons at constant temperature .

Freundlich equation

log (x/m) = log k + ( 1/n) log Ce ... (l)

where k and n are constants (n> 1) and x/m and Ce are the mole of the dye adsorbed per g of the adsorbent and the equilibrium concentration of the dye in mol dm-3 respectively .

Frumkin equation

V = log(~ /55 .55) +2a8/2.303 . .. (2)

-5~------------------------~

,...... E

~ -6 o; 0

Freundlich Isotherm

-7 L__ __________ ___J_ ___ _______ ___J_4 -6 -5

log Ce Fig. 6-Plot of log (x/m) vs log Cc according to Freundlich Iso­therm fo r MB-cellulose system in KCI medium at 303 K [Lines 1-4 stand for 0.01 , 0.05, 0. 1 and 0.3M KCI respect ively].

where v = log [8-( 1-8) Ce] ' e = xlxm. where X = the mole of dye adsorbed at equilibrium and Xm = the monolayer dye adsorption density (mol g·1) and a and ~ are constants ; the other term Ce is already de­fined .

Langmuir equation

.. . (3)

where k 1

is the constant of equilibrium for the ad­sorption and desorption processes, and the other terms have been already defined .

Graphical plotting of the results in the form of log (x/m) against log Ce for equation 1, V against e for equation 2 and rn/x against 1/Ce for equation 3 can

yield the parameters of the equations 1,2 and 3. Rep­resentative plots are depicted in Figs 6-8 . The results are presented in Tables 1, 2 and 3.

The adsorption results obey the Freundlich equa-

-5-4 Frumkin Isotherm

-5 -6

-5 -8

-6-0

J)

-6-2

-6 -4

-6-6

-6-8 o.o 0 -2 0 -4 0 -6 o.s v

Fig. 7-Plot of e vs V according to Frumkin Isotherm for MB­MO ( I: I) - cellulose system in aqueous medium at di fferent tem­peratures [Lines 1-3 represent 303, 308 and 3 13 K respectively].

PALIT et al.: ADSORPTION OF METHYLENE BLUE ON CELLULOSE 615

I 0

a

E 6 E 01

........

~ 4 E ~

It>

'2

0 2 4 6 a 10

165.( 1/Ce)/lit mol-1

Fig. 8- Plot of mix vs liCe according to reciprocal Langmuir plot for MB-MO (I : I) - cellulose system in aqueous medium at differ~ ent temperatures [Lines 1-3 represent 303, 308 and 313 K respec­tively].

Table !-Results of Freundlich plot for the adsorption• of pure MB and MB :MO on cellulose under different conditions.

System Medium Temp, K n k X 10"6

MB Aqueous 303 2.56 1.32 313 5.00 0.83

O.OIM KCI 303 1.72 1.82 0.05MKCI 1.72 2.88 O. IMKCI 1.72 2.63 0.3MKCI 1.72 4.57

MB:MO(I : I) 0.005MKCI 1.75 1.32 0.05&0.1M KCI 1.75 2.51

Aqueous 303 1.29 1.41 308 2.12 0.79 313 2.04 0.71

MB:M0(1:2) 303 0.78 3.9 (2: I) 2.41 0.69

•tn the calculation of the extents of adsorption, due consideration to the dependence of the extinction coefficient on temperature particularly owing to the complex formation between MB and MO has been given.

tion reasonably well. Then and k values of equation 1 are presented in Table l . Except for two cases, the n values are nearly two; the k values vary in a reason­able range. The variation in the adjustable parameters suggests variation in the surface or the binding sites on the biopolymer (cellulose) for associating the dye MB. The temperature and salt dependencies are in­volved factors in the process.

The Frumkin plots are linear. The a and ~ values

Table 2-Results of Frumkin plot for the adsorption• of MB and MB :MO on cellulose under different conditions

System Medium Temp,K a ~xl0·6

MB Aqueous 303 1.87 9.0 3 13 3.07 3.76

O.OIMKCI 303 3.26 7.32 0.05MKCI 3.51 16.00 O.IMKCI 3.16 26.60 0.3MKCI 2.64 39.30

MB:MO(I : I) Aqueous 303 2.31 7.16 (1 :2) 2.30 10.10 (2: I) 0.86 26.00

MB:MO(I :l) 308 2.02 7.85 313 1.65 8.22

0.005MKCI 303 2.19 11.10 0.05MKCI 303 3.33 15.70 O.IOMKCI 2.29 31.20

•same conditions as in Table I

Table 3-Results of Langmuir plot for the adsorption• of pure MB and MB:MO mixtures on cellulose under different conditions

System Medium Temp, K Kxl0·5

MB Aqueous 303 2.13 313 1.20

0.05MKCI 303 0.31 O.IMKCI 303 0.23

MB:MO(I:l) Aqueous 303 0.19 l ' 308 1.78

313 2.27 0.005MKCI 303 3.00 0.05MKCI 303 1.25 O. IMKCI 303 0.39

MB:M0(1 :2) Aqueous 303 0.39 MB:M0(2:1) .. 303 1.27

•same conditions as in Table I.

are obtained from the linear least squares analysis. They, under different environmental conditions, are broadly of equal ma~itudes. The ~ increases with salt (KCI) concentration. The temperature has size­able effects on both a and ~ for the adsorption of pure MB on cellulose; fo r MB :MO(I : 1) mixture, the effects are mild.

The Langmuir plots and results (Table 3) reveal that for pure MB, the adsorption decreases with tem-

perature and hence k 1 also decreases . The k 1 values also decrease with increasing salt concentration (Fig. 9), for salt lowers the favourable electrostatic inter­action between the dye and the cellulose. This is also corroborated by the MB :MO ( 1: 1) mixture. The formed MB-MO complex at temperature greater than 303 K is favourably adsorbed on the cellulose with

616 INDIAN J CHEM, SEC. A, Jlfl\:!3 2000

2.--------------------------y~

X

' E

<D

0

langmuir Isotherm

2 -51 -1

10(/ce) /lit mol

Fig. 9-Plot of rn/x vs liCe according to reciprocal Langmuir plot for pure MB-cellulose system in aqueous KCI medium at 303 K [Lines 1-4 represent 0.3, 0.1, 0.05 and 0.0 1 M KCl respecti vely].

increasing temperature. There is no apparent prefer­ence for the adsorption of MB at constant temperature (303 K) in presence of MO at different molar ratios.

The specific surface area (S) of the cellulose sam­ple was estimated from the Xm value realised from the intercept of the reciprocal Langmuir plot (equation 3) . The value of Xm obtained was 5x10·6 mol g- 1 for all the cases.

S = Xm Ncr ... (4)

where S = the specific surface area (m2/g) ; Xm = the monolayer dye adsorption density (mol/g) ; N = the Avogadro's number (6.023xl023 molecules/mol); cr = the area occupied by a single dye molecule (m2/mole­cule).

Taking cr = 120 A2 from a literature report', a value of 3.6 m2g-1 has been obtained for S.

It is observed that the equations of Freundlich, Frumkin and Langmuir are applicable to the adsorp­tion of MB alone and in mixtures with MO on cellu­lose in aqueous and salt solutions. The derived pa­rameters are, therefore, useful to characterize the ad­sorption process. Since the adsorption results fit in the three equations, ex istence of a common basis for them is apparent. In the moderate range of concen­tration the Langmuir equation can transform into the Freundlich equation. Apparently, the Frumkin equa­tion does not show a direct correlation with either the

Langmuir or the Freundlich equation . An indirect correlation is anticipated.

Like many adsorbent/adsorbate systems, the ad­sorption of MB on cellulose is an exothermic process. The standard enthalpy change of adsorption has been calculated from the integrated van't Hoff equation, In

k' MI~" = constant -~. The enthalpy value obtained

is -45.2 kJmor' and corresponding standard free en­ergy and entropy changes calculated at 303 K from

the relations !:::..dad = - RTin k 1 and f1Siad = (t:Jfad -!:::..dad)fT are -30.9 kJmor' and -47 J mor'K' respec­tively. The negative D.ftad suggests that the ME­cellulose product is organized.

The mixed system of MB and MO has shown in­creased adsorption w.ith temperature. The standard endothermic enthalpy of adsorption for the I: I ratio has been also derived from the van't Hoff equation . The enthalpy ~alue obtained is 38.2 kJmor' and the corresponding standard free energy and entropy changes of adsorption at 303 K are -30.5 kJmor' and 226 JK1mor' respectively. The endotherrnicity of the adsorption process has been attributed to hydrophobic association of the MB-MO combine wi th the cellu­lose leading to loosening of the biopolymer configu­ration producing posit ive entropy change. Similar reports of endothermic enthalpy change are also found in recent literature 16 on the adsorption of bile salts on graphite in aqueous medium.

Conclusions The following conclusions can be drawn from the

results: I . The dye MB adsorbs on cellulose at ambient

temperature whereas MO does not. At tempera­ture above 303 K, MB and MO interact whereby the adsorption of MB in the complexed form in­creases.

2. The adsorption process under different condi­tions of temperature and MB/MO ratio is re­duced in presence. of KCI.

3. The adsorption results under different environ­mental conditions of temperature and salt obey the equations of Freundlich, Frumkin and Langmuir.

4. While pure MB is adsorbed on cellulose with negative enthalpy change, the mixture of MB and MO is adsorbed with positive enthalpy change indicating interplay of hydrophobic m­teraction.

PALIT ·et al.: ADSORPTION OF METHYLENE BLUE ON CELLULOSE 617

Acknowledgement We thank Dr. D. P. Khan of Baruipur, Calcutta

with appreciation for preparing the cellulose sample and gifting it to us. Financial support from the Jawa­harlal Nehru Memorial Fund, New Delhi to carry out the work is thankfully acknowledged.

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