Indian Journal of Chemical Technology Vol. 6, March 1999, pp. 11 2- 116

Adsorption of basic dyes from aqueous solution by natural adsorbent

so Khattri * & M K Singh Departmcnt of Chemistry. Faculty of Science, Banaras Hindu Uni verSi ty, Varanasi 22 I 005 , India

Received 27 July 1998; accepted 22 February 1999

The capabilit y or the Sagaun sawdust fo r remov ing basic dyes (crystal violet. methyl ene blue, malachite grecn and rhodaminc B) from aquc(IUS so lution's has been in ves tigated. The effect of systcm variables such as concentrati on, tempcralu re. pH and parti cle si ze have been studied to understand equi librium and kinetics of adsorption process. The maxim um rcmo va l was found to be R9% at ~oncent rati on 6.0 mgL- I, temperalllre 30De and pH 7.5 .. The amoun t of the dye (crystal viol et and methylene bl ue) adsorbed, decreases from 2.773 mg g-I (86.68%) to 2.3 10 mg g I (72.19%) and 2.672 mg g- I (83. 52 'Yr) to 2.205 mg g-I (68.93%) wi tli the ri se of temperature of dye sol uti ons from 25 to 45"C indicati g the proccss to bc exothermic . The valu e of k at! of crystal violet at 25 , 3S and 45De were found to be 9. 72x I 0-2, 9.57x l 0-2 and 9.2Sx I 0-2 min - I. respcc ti vely. The adsorpti on capaci ty for dye-adsorbent system has becn de termined. The dye adsorpti on follows the Langmuir isotherm. On the has is of isotherm studies. various thermodynamic paramete rs such as t'1co, ISH nand tlS" have been calcul ated.

Deve lopmental strategies have led to environmental degradati on because the various projects are based on tec hnologies and patterns of production and consumpt ion whi ch have brought about changes in life style. ma n' s greed for quick material return s, rapid resource ex ploitat ion and product ion of waste. In addit ion, the de velopment processes have encouraged shift s in population leading to increased urbani za ti on. The quality of ground water is very important as it is depleting and being contaminated in man y part s of th e world . Colour results in aesthetic pollut ion of water in addi ti on to the toxic ity and strong in terference wi th photosynthesis. For the treatment or wastewater severa l in vesti ga ti ons have been carried out to assess the potenti al of waste mate rials l i~ e fly ash. sawdu st, bagasse ash and rice hu :-. J.. as h for the ir use as all effective alternati ve to act iva ted carbon which is quite expensive.

Po llu tion from tex tile mi ll s is a problem of fo rmidable dimensions and colour removal is the most perplexing prohlem face d by environmenta l engineers des ignin g appropria te treatment facilities for textile wa~(ewat e rs I . The abil it y of' wood to ;.tdsorb Telon Blue ~Ac i d Blue 25) has been i n vest i ga ted ~. The effect s ot contact time. partic le size and adso rpti on capacit y h:lvc bee n studi ed . Coloured dye wastes freque ntly contai n a spect rum of heavy metals and lIt her tox ic org.:tll ic pollu tants ' . Coconut husk was al so

~ For cO ITc,po ndcnce

proved to be effec ti ve for the removal of (textile dye) methy lene blue from aqueous olution-l . Deco lourisation of a d ilute solution of a basic dye stuff was carried out by using hardwood saw-dust as an adsorbent~. Keeping above in view and earlier encouraging results on removal of various dyes (methylene blue, malachite green, crystal violet and rhodamine B) using the heterogeneous mix tu re of alumin a and clal by adsorption technique, it was planned to study the adsor-ption of dyes on Sagaun sawdust whi ch is commonly ava il able and inexpensive by-product of timber indust ry.

Experimental Procedure The dyes used (methylene blue, crystal violet ,

malachi te green and rhodamine B) were supplied by Sand oz Indi a Ltd. , Bombay. The sawdust of Sagaun (Tecfona grandis) was used as adsorbent after the foll owing treatment. It was co ll ected from a local timber industry, boil ed with dilute hyd rochloric ac id and continuously washed with dist illed water to remove the surface adhered panicles and water soluble material s and then dried at a temperature of 60-80"C in an electric oven. Acid h ydroly~ed sawdust .vas finally sieved fo r particles of SO, 80 anu 100 mesh si zes.

Batch adsorption experiments ere caITicd out by shaking O.S g, of adsorbent (sawd ust) with 200 mL aqueous soluti on of dye of desired concentrati on in corning bottl es at diffe rent temperatures and pH at a

KHATTRI & SINGH: ADSORPTION OF BASIC DYES 113

constant speed in a water thermostat, the temperature of which was maintained within ± 1°C. At different intervals of time, samples were drawn out of the adsorber using a syringe. Dye concentration was measured spectrophotometrically using a Shimadzu 160A spectrophotometer in the visible range at the maximum absorbance wavelength. The effect of different parameters on the adsorption of various dyes using the adsorbent was investigated .

The SEM micrograph clearly reveals the porous structure of the Sagaun sawdust (Fig. I). There are holes and caves type of openings on the surface of the specimen which would definitely have increased the surface area availaWe for adsorption.

Results and Discussion EffecI of contact time and concentration-Fig. 2

shows that the amount of dye adsorbed in the aqueous solution increased with time and achieved equi librium in 14 'min at temperature 30 ± 1°C, and pH 7.5. Eventually a plateau is reached in almost all the curves , indicating that the adsorbent is saturated at this level. The time variation curves of adsorption are smooth and continuous, indicating the formation of monolayer coverage on the surface of adsorbent7• T he amount of adsorbate reveals the following sequence for the preference in adsorpti on, crystal vio let>methylene blue>malachite green>rhodainine B (Table I ). However, the percentage of the dye adsorbed by SagauD sawdust decreased from 89.46 to 74.03 and from 86.69 to 69 .58 for crystal violet and methylene blue, respectively and those of other dyes (malachite green and rhodamine B) also decreased on increasing the concentration of the dye from 6 to 12 mg L- 1 (Table I) .

Effect of particle size-The infl uence of contact time on three range of particle sizes of adsorbent (Sagaun sawdust) was investigated using the size ranging from 50- 100 mesh . The adsorption of dye fro m solution of concentration 12 mg L - I is found to inc rease from 68.18 % to 78 .37% with decrease in particle size of the adsorbent from 50 to 100 mesh at temperature 30 ± 1°C. The data show an increase in the rate of the d e uptake as the mean diameter (SO-100 mesh) of the sawdust decreases . This is due to i Icrease in the surface area.

dsorp1ion dynamics-The rate constant for adsorption kacl • for the removal of dye (methylene blue and crys tal violet) by Stigaun sawdust at temperatures 25. 35 and 45°C were determined using the follow ing fi rst order expression (Lagergren 1898),

Fig. l-SEM photographs of Sagaun sawdust at different magnification

4·0 r----------------,

'.,. ~ 3·0

-cr- ---o--____ -e- -- ...........

, '0 ... D Is 2·0

~::::S=~--J:. :. :.:i: ~ n--~IJF= .. =~ = = ~ ~

'" o c '" 1·0 o E

'"

o 0. o

IIWthylQnQ

~

• • • Time, mIn

Cone ·

12mgC' 8mQC1

6 mg L-1

Fig. 2-Effect of concentrati on of crystal violet and methylene blue by Sagaun sawdust (Condition : 80 mesh, temp 30 ± 1°C, p H 7.5)

k iog(q - q)= logq __ ad_1 e e 2 .303

where qe and q (both in mg g- I) are the amount of dye adsorbed at equilibrium and at any time t. respectively. A straight line plot of log (qe - q) vs. 1

114 INDIAN J CHEM. TECHNOL., MARCH 1999

Table I-Amount and percentage of dyes absorbed (PH = 7.5, temperature = 30 ± 1°C)

Concentration Cr~stal violet Meth~lene Blue Malachite Green Rhodamine 8 @f dye, mg L- I mgg-I % mgg l % mgg l % mgg l %

6.0 2.147 89.46 2.080 86.69 1.996 83.18 1.397 58.23

8.0 2.662 83.20 2.519 78.72 2.429 75 .91 1.794 56.08

12.0 3.553 74.03 3.339 69.58 3.300 68.75 2.273 47.37

Table 2-Adsorption kinetic parameters at different temperatures for crystal viotal and methylene blue

Temperature, C!1stal violet "c kad kv K

. - I min mgg- I min-In

25 9.72xI0-2 0.481 6.504

35 9.57X I 0-2 0.407 3.959

45 9.25xlO-2 0.399 2.599

indicates the applicability of the above equation (Figs 3 & 4) . The values of kad at different temperatures were calculated from the slopes of the respecti ve linear plots and are noted in Table 2. It may be concluded from the values of kad that the reaction taking place is of the first order:

Besides adsorption on the outer surface of adsorbent, there is also a possibility of transport of adsorbate ions from the solution into the pores of the adsorbent due to rapid stirring in batch reactors. This possibility was tested in terms of a graphical relationship between the amount of the dye adsorbed and the square root of time (Fig. 5). The double nature of these plots may be explained as the initi al curved portions are attributed to boundary layer diffusion effects while the final linear portions are due to intraparticle diffusion effects8. The rate constant for intraparticle diffusion kp, at different temperatures were determined from the slopes of the linear portions of the respective plots and are given in Table 2.

Effect of temperature-The amount of the ~ye (crystal violet and methylene blue) adsorbed decreases frotTI 2.773 mg g- ' (86.68%) to 2.310 mg g-' (72.19%) and from 2.672 mg g-' (83.52%) to 2.205 mg g- ' (68 .93%) with the rise of temperature of dye solutions from 25 to 45°C indicating the process to be exothermic8.

The changes in standard free energy, enthalpy and entropy of the adsorption were calculated using the equations,

Meth~lene blue

kad kv K . - I min mgg- I min- l12

9.62xlO-2 0.416 5.065

9.45xlO-2 0.396 3.212

9.15x I0-2 0.377 2.218

Ml O -fl.Co /lS o= ___ _ T

where R is gas constant and K, K' and K" are equilibrium constants at temperatures T, T, and T2 respectively. The negative values of fl.d (Table 3) indicates that the process involved is spontaneous with a high affinity of dye for the adsorbent. Further, the nega ti ve va lues of enthalpy and entropy changes (Table 3) suggest t~e exothermic nature of adsorption and random nature of adsorbing molecules. Adsorpt~1l isotherm- The study of adsorption

isotherms is helpful in determining the adsorption capacities of various adsorbents for the removal of dyes at certain temperature. Langmuir adsorption isotherm model was applied for adsorption equilibrium at all temperatures.

where Ce is the equilibrium concentration (mg L-'), qe is the amount adsorbed at equilibrium (mg g-') and QO and b are Langmuir constants rel ated to adsorption capacity and energy of a9sorption, respectively . The linear plots of Ce/qe VS. CO suggest the applicability of the Langmuir isotherms '(Fig. 6) . Values of QO and b were determined from slope and intercepts of the plots and arc presented in Table 4.

,

....

KHA TfRI & SINGH : ADSORPTION OF BASIC DYES 115

Table 3-Thermodynamic parameters for crystal violet and methylene blue

Temperature, "C -t::..G'

Crystal violet

-t::..J-f' -t::..S' Methylene blue

-t::..G' -t::..J-f' - t::..S' kl mol- I kJ mol-I l K- 1 mol- I kl mol- I kl mol-I JK- I mol-I

25

35

45

4 .638

3.520

2.524

37.84 11 1.44

34.20 99.61

4.013 34.72 103.04

2.970 30.21 88 .44

2.104

Table 4--Langmui r constants for crystal violet and methylene blue

Temperature,

°C

0-IQI 0-

01 ~

25

35

45

0·4

0·2

0·0

-0·2

-0 .4

-0·{5

-0 .8

- 1·0

-1· 2 0

Crystal violet

Q" (mg g- I)

4 .259

3.976

3.552

Time, min

b (L mg- I)

1.381

1.061

0.880

0.083

0.105

0.124

Fig. 3-Lagcrgn: n plot for adsorpti on of dye (methyJ ene blue) on Sagaun Sawdusl ((,oncli lion : cone. 8mg L- I. 80 mesh, pH 7.5)

T he equili brium paramete r RI. has been calculated fro m the re lati onshi p,

I R,=--

. i+hC,

where Co is the in it ia l concentrati on (mg L- ' ) and b is the Lan gmui r constan t (L mg- I ) . The values of RL shown in Table 4 , indi cate that adsorption of the dyes

0-I

Cli 0-

0-0

Q" (mg g- I)

04

02

00

·· 02

-04

I

4.141

3.829

3.490

- 0 6 r - 0 8

-1 0

- 1 2

0

Methylene blue

b (Lmg- I)

1.103

0 .775

0 .608

T i me, min ·

0.101

0 .138

0.170

20

Fi g. 4-Lagcrgrcn plot for adso rpti on of dye (crystal violet) on Sagaun sawdusl (Condili on : conc. 8 mg L- I, 80 mesh, pH 7.5)

(me thyle ne blues and c rysta l violet) on Sagaun sawdust is a fa vourable process because RL va lues lies between 0 and I .

£ ffect 0/ pH- Sagaun sawdust seems to be mode rate ly a good adsorbe nt fo r the removal of the dyes from aqueous so luti on at pH 7 .5, at which a lmost quantita ti ve ad sorpti on was achieved . ~ith the inc rease of pH fro m 4.2 to 7.5 , the uptake of crysta l vio let and meth ylene blue inc reases from 63.35 to 8~ . 20% and fro m 55 . 15 \0 78.72%, respecti ve ly by sagaull sawdust: and above thi s pH up to 10

116 INDIA N J C HEM . TECHNOL, MARCH 1999

40

0;- o 25 °C 0'

35°C 0- f> E 30 0 4 5°C

"' D £l L.. 0 20 '" D 0

C ::J 0 10 E «

00 0 4 5

Fig. 5-lntraparticle diffusion plot for adsorption of methylene blue on Sagaun sawdust (Condi tion : conc. 8 mg L- J , pH 7.5)

2·0r-----------------------------, Crystal vio let

- -0 - -

Methylene Temp . blue

----1·6 - - 6 -- --6----0- - -

O.OOL----'----- I _ .1 . 2 3 ee, mg L-1

4 5

Fig. 6--Langmuir plot for the adsorption of crystal violet and methylene blue on Sagaun Sawdust at different temperatures (Condition : 80 mesh, pH 7.5)

adsorption has been found to decrease gradually. Sawdust bas ically contains lignin and cellulose, and the surface of sawdust in contact with water is negatively charged. Bas ic dyes are ionized in solut ion to give coloured cat ionic dye base and this will undergo attraction approaching the anionic sawdust structure9. This can be represented as,

S(OHh+20H-+Dye++~ S(OHt2 ..... 'D e++ ..- S(OHt2 // Y

Observed decrease in adsorption beyond pH 7 .5 can be attributed to the formation of soluble hydroxy

10 complexes .

Conclusions The conclusions drawn from the present

investigations are, (i) The preliminary studies presented have shown

that Sagaun sawdust may be used as adsorbent for the removal of the basic dyes from aqueous solution.

(ii) The fitness of Langmuir model in the present system shows the formation of monolayer coverage of the adsorbate at the outer surface of the adsorbent and the process is exothermic in nature.

(iii) The mechanism involves an initial rapid rate for the dye removal due to sUlface adsorption followed by intraparticle diffusion which appears to be the rate governing step.

Acknowledgement Necessary research facilities and financial support

provided by the Head, Department of Chemistry, Banaras Hindu University, Varanasi , is gratefully acknowledged .

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