Pertanika 10(3), 341 - 347 (1987)

Mixed Adsorbent for Colour Removal from Aqueous Solution

SUN1L KUMAR KHARE1, VISHWANATH SINGH Land RAM MOHAN SRI V ASIA VA '1 Department of Mechanical Engineering

Institute of Technology,Banaras Hindu University,Varanasi — 221 005, India

Key words: Mixed adsorbent; fly ash-wollastonite; victoria blue; diffusion; mass-transfer;Langmuir isotherm; pH desorption.

ABSTRAK

Campuran homogen fly-ash' dan wolastonit (1:1) telah digunakan sebagai penyerap bercam-pur untuk mengkaji pemisahan victoria biru (CI Basic Blue-26t 44045) daripada larutan akuesnya.Parameter-parameter yang dikaji adalah kepekatan pewarna awal, suhu dan pH larutannya. Kinetikjerapan menunjukkan proses tersebut adalah proses pembauran terkawal. Data keseimbangan padasuhu berbeza-beza memenuhi isoterima Langmuir. Penjerapan dan penyahjerapan maksimumvictoria biru daripada antara muka fly-ash' dan wolastonit masing-masing berlaku pada pH 8.5 danpH3.0

ABSTRACT

A homogenous mixture of fly ash and wollastonite (1:1) has been used as mixed adsorbent tostudy the removal of victoria blue (CI Basic blue — 26, 44045) from its aqueous solution. The para-meters studied were initial dye concentration, temperature and pH of the solution. The kinetics ofadsorption indicate the process to be diffusion controlled. The equilibrium data at different tempe-ratures fit well into the Langmuir isotherm. Maximum adsorption and desorption of victoria bluefrom the interface of fly ash-wollastonite was noted at pH 8.5 and pH 3.0 respectively.

INTRODUCTIONThe colour of the effluents from the textile andcarpet industry causes widespread industrialwaste disposal problems. Dyes used in such in-dustries are particularly difficult to remove byconventional waste treatment methods becauseof their stability towards light and oxidizingagents and resistance towards aerobic digestion(Weber and Morris, 1962; Poots et al, 1976).The adsorption technique using activatedcarbon (ICI, 1971; McKay, 1984) as adsorbent isgenerally employed for colour removal fromwater and wastewaters. However, high costs anddifficult procurement of activated carbon

restrict its use in developing countries like India.Recently a number of unconventional adsor-bents have been used for colour removal fromwastewater by several investigators (Alexanderand McKay, 1977; Poots et al, 1976, 1978;Singh et al., 1984; Khare et al, 1987). Thereappears to be limited information on the use ofmixed adsorbents for wastewater treatment. Insome cases, the adsorption capacity of mixedadsorbents for the removal of undesirable sub-stances from water was found to be more than itsconstituents (Panday et al., 1986). This paperdescribes the results of a feasibility study using ahomogenous mixture (1:1) of fly ash and wolla-

Key to author's name: Khare, S.K., Singh, V.N. and Srivastava, R.M.

Department of Applied Chemistry, Institute of Technology, Banares Hindu University.

SUNIL KUMAR KHARE, VISHWANATH SINGH AND RAM MOHAN SRIVASTAVA

stonite as an adsorbent for the removal ofvictoria blue from its aqueous solutions at dif-ferent concentrations, temperatures and pH.

MATERIALS AND METHOD

The AdsorbentFly ash was obtained from Obera ThermalPower Plant, Mirzapur (U.P.) and wollastonitewas supplied by M/s Wolkem Pvt. Ltd., Udaipur(Rajasthan), India. These materials were sievedto bSu size and used as such without any treat-ment. An equal amount of sieved fly ash andwollastonite was mixed to obtain a homogenousmixture (1:1) and employed as mixed adsorbentin the present investigations. The surface area ofthe mixed adsorbent was determined using a"Three Point" N 2gas adsorption method, modelQpl Quantasorb Surface Area Analyser (QuantaChrome Corp., USA) and the average particlesize was measured by HIAC-320, Model 8002917(Royco Inst. Div., USA). The porosity anddensity of the adsorbent were determined usingmercury porosimeter and specific gravity bottlerespectively. The different constituents of theadsorbent were analysed by Indian StandardMethods of Chemical Analysis (1960) and theresults are presented in Table 1 along with othercharacteristics.

The Adsorbate and its SolutionVictoria blue (Basic blue — 26) was supplied byBDH and used without further purification. Allsolutions of dye were prepared with deionised-distilled water and the desirable pH of thesolution was maintained using 0.5 M HC1 orNaOH solution.

Batch ExperimentsBatch adsorption experiments were carried outby shaking different glass bottles containing 1.0gmixed adsorbent (fly ash-wollastonite) and 50mlaqueous solution of victoria blue of desired con-centration at various temperatures and pH. Thespeed of shaking was maintained at 125 r.p.m.for each run. After predetermined time inter-vals, the suspensions were centrifuged and theresidual dye concentrations were determinedusing a spectronic — 20, Spectrpphotometer at575nm. The desorption experiments were per

formed by shaking l.Og used fly ash — wolla-stonite containing fixed amount of victoria bluewith 50ml deionised — distilled water at diffe-rent pH at 30°C.

RESULTS AND DISCUSSIONThe chemical analysis of fly ash — wollastonitemixture (Table 1) shows that the oxides of sili-con, calcium and aluminium are the main con-stituents while other oxides are present in traceamounts. It is thus expected that the dye willbe mostly adsorbed either by any one or by acombined influence of the major oxides present.

TABLE 1Chemical analysis of adsorbent and other

characteristics

Constituents

SiO2

CaOA ' 2 ° 3F e

2 ° ,MgO

Ignition loss

Specific surface area ( m 2 g ~ ] )

Mean particle diameter (uxa)

Porosity

Density (g cm ~5)

% by weight

52.28

25.35

13.07

0.76

0.47

8.07

3.05

0.50

0.18

3.055

Effect of ConcentrationWith an increase in victoria belue concentrationfrom 1.0X10 6to 1.0X10 5M, the amount ofdye adsorbed was noted to increase from 0.0246to 0.1938mg g ' (Figure 1) while percentageremoval decreased from 98.50 to 77.59%, indi-cating that dye removal using the adsorptiontechnique is highly concentration dependent.The equilibrium time was noted to be 60 min. inall cases and is found to be independent of con-centration of the solution. The plots in Figure 1are single smooth and continuous leading tosaturation, suggesting the possibility of theformation of monolayer coverage of victoria blueon the fly-ash wollastonite surface (Panday, etai, 1986).

342 PERTANIKA VOL. 10 NO. 3, 1987

MIXED ADSORBENT FOR COLOUR REMOVAL FROM AQUEOUS SOLUTION

40r

To 30

20

80 " 120 160

Tim* (mini

200 2*0

Fig. 1: Time variation of removal of victoria blue byfly ash-wollastonite at differentconcentrations: (0) 1.0x10 ~6M5.0X10 6Mand 1.0x10 ~5M.Conditions: Temperature 30 ±0.1 °C;pH: 8.5

Adsorption DynamicsThe rate constant of adsorption, K (min ~l) ofvictoria blue on fly ash-wollastonite was deter-mined using the following rate expression(Lagergren, 1898):

l o g ( q , - q) = I o gq 2.303t •-•(1)

where, q and q are the amount of dye adsorbed(mg g ') at equilibrium and at time, t respec-tively. A straight line plot of log (q^ - q) Vs t(Figure 2) suggests the applicability of the aboveequation. The value of rate constant was cal-culated from the slope and found to be 6.60 X10 'min 'at30°C.

In a rapidly stirred batch reactor, the adsorbatemolecules generally diffuse into the interior ofthe porous adsorbents. During such processes itis probably reasonable to assume that the rate ofuptake of adsojpbate is not limited by mass-transfer from the bulk liquid to the externalsurface of the particle. If this is the case, onemight postulate that me rate limiting step isintraparticle diffusion and the amount adsorbedshould vary almost proportionally with square-root of time rather than time itself (Weber andMorris, 1963). The plots of amount of victoriablue adsorbed Vs t05(Figure 3a) are linear for awide range of contact period. The slight devia-tions from linearity at the initial stages suggestthat the pore diffusion is not predominant at theinitial stages of the contact. The plots also do notpass through the origin suggesting that themechanism of victoria blue adsorption proceedsfrom boundary layer mass-transfer across theinterface to the intraparticle diffusion within thepores of adsorbent (Poots et al., 1978).

-10

Fig. 2: Plots for (0) rate constant of adsorption;mass transfer coefficient of victoria blue onfly ashwollastonite.Conditions: Concentration: 1.0x10 5M;Temperature: 30±0.1°C; pH: 8.5

Fig. 3: Plots of amount adsorbed Vs. V forvictoria blue adsorption on fly ash-wollastonite at different particle size

53}im (0) 63 - 53jim and (0) 75 - 63)fjim.Conditions: Concentration: 1.0x10 ~5M;Temperature: 30°C±0.1°C; pH: 8.5

The rate constants of intraparticle trans-port, k (Table 2) were calculated from thelinear portion of the plots of q Vs t05(Poots etaL, 1976) for different particle size range at30°C and solution concentration of 1.0 X 10 ~5M.A self consistancy check is then possible, inwhich a plot of rate constant of intraparticle dif-fusion Vs square of the inverse of particle

PERTANIKA VOL. 10 NO. 3, 1987 543

SUNIL KUMAR KHARE, VISHWANATH SINGH AND RAM MOHAN SRIVASTAVA

Rate constants

Particle size

(um)

53

63-53

75-63

TABLE 2of intraparticle diffusion

(mgg minv 1

0.021

0.023

0.025

wm =

diameter (Figure 3b) yielded straight line. Thevalue of pore diffusion coefficient, D was cal-culated using the relationship given below (Helf-ferich, 1962).

D = 0.030

where, D is the pore diffusion coefficient (cm2

sec *), r the radius of adsorbent particle (cmand 11 is the time for half adsorption (min)). Thevalue of D was found to be 2.50 X 10 10cm 2sec "'suggesting that the rate of reaction is mainlygoverned by intraparticle diffusion during thelatter stages of contact (Michelsen et aL, 1975)although at the initial stages, a rapid surfaceadsorption takes place because at this stage thedye molecules have only to overcome the poten-tial barrier across the boundary of the solid-solu-tion interface (Poots et aL, 1978).

The value of surface mass-transfer coeffi-cient for the transport of victoria blue from bulkto the solid phase at 30°C was calculated usingthe following diffusion model (McKay et aL,1981):

1l n ( C 1 + mk)+ k )

mk1+mk

1+mk

where, C is the initial dye concentration(mg 1 - 1 ) ; C (mg 1 ]), the dye concentration insolution'after time t ; k ( l g " 1 ) i s the Langmuirconstant obtained by multiplying Q° and b; |3i

(cm sec"1) is the mass transfer coefficient; m(g 1 ~1) and S s (cm ~') are the mass and thespecific surface of adsorbent per unit volume ofparticle free slurry respectively. The values of mand S were calculated from the equations:

and

S - m

. . . (4)

.-.(5)

where, W is the weight of adsorbent (g); V, thevolume of solution, (1); d , the particle diameter(cm); p , the density of adsorbent (g 1 ') andG is the porosity of adsorbent.

P The plot of l n ( C / C o - 1/(1+mk) againsttime gives a straight line (Figure 2) and the valueof mass-transfer coefficient was calculated fromthe slope and intercept of the plot and found tobe 2.73X10 5cmsec ]at 30°C. This value indi-cates that the velocity of mass transfer of victoriablue from solution onto fly ash-wollastonite israpid enough (McKay et aL, 1981) to suggest theuse of this adsorbent for the treatment of waste-waters enriched in victoria blue.

A dsorption IsothermThe equilibrium results at different concentra-tions and temperatures were analysed in the lightof the following rearranged Langmuir equation:

C

q e . . . (6 )

where, C is the equilibrium concentration of vic-toria blue in solution (mg 1 ~!) and Q° and b areconstants signifying the adsorptive capacity andenergy of adsorption respectively. The values ofQ° and b were calculated from slopes and inter-cepts of the straight line plots of C/q Vs C(Figure 4) and are given in Table 3. The validity

10

C (MflfS

Fig. 4: Langmuir isotherm at different temperatures30°C, (0) 40°C and (0) 50°C

544 PERTANIKA VOL. 10 NO. 3, 1987

MIXED ADSORBENT FOR COLOUR REMOVAL FROM AQUEOUS SOLUTION

TABLE 3Langmuir constants at different temperatures

Temperature

(•c)30

40

50

Graphical

(mg g g - 1 )

0.801

0.909

1.002

values

b(Irng"1)

0.074

0.084

0.098

Regression

0.805

0.908

1.018

values

b( I m g 1 )

0.075

0.086

0.094

of the Langmuir isotherm for the present systemwas further confirmed by the regression analysisof the equilibrium data at different tempera-tures. The regression equations relating C/qp

andC are found as:and

peratures using the following relationships (Jainetal, 1979):

C/qe= 1.24186 C + 16.55820 at 3,. CC/q]= 1.10125C + 12.80525at^°C

* = 0.98619 C +

..(7)

.. (8)10.38098 at 50°C ...(9)

The values of Q° and b were calculated(Table 3) by comparing equation (6), (7), (8)and (9) and found to be almost equal to thegraphical values. This confirms the applicabilityof Langmuir isotherm at different temperaturesin the present system.

Effect of TemperatureThe effect of temperature on the removal ofvictoria blue may be described in terms of varia-tions of adsorption capacity, Q° of adsorbentwith temperature of the systems. The removalcapacity of mixed adsorbent increases from0.801 to 1.002 mgg" 1 (Table 3) by increasingthe temperature of victoria blue solution from 30to 50°C at pH 8.5. The increase in uptake of dyewith temperature may be explained by the en-hanced rate of intraparticle diffusion of dye,within the pores of adsorbent as the diffusion isan endothermic process (Bye et ai, 1982). Theisosteric heat of adsorption, calculated with thehelp of Gibb's Helmholtz equation, is always afunction of the surface coverage and its valuewas found to be 4.305kcal mol -1for the surfacecoverage of 0.248mg g " ]of adsorbent.

The thertnodynamic parameters for theadsorption of victoria blue on fly ash-wolla-stonite, have been calculated at different tem-

AG° - - R T l n KT .T

.(10)

...(11)and

A G ° = AH° - TAS°

where, K , K1 and K" are the equilibrium cons-c c c *

tants, obtained from the limiting slopes of theadsorption isotherm at zero concentration at dif-ferent temperatures (Kipling, 1965). Otherterms have their usual significance.

The values of AG° at 30, 40 and 50°C werefound to be -0.747, -0.966 and -1.782kcalm o l 1 respectively, indicating that the inter-action of dye with adsorbent is spontaneous andfeasible with a high preference of victoria blue forfly ash-wollastonite (Panday et ai, 1985). Thepositive value of AH° ( + 5.877kcal mol "*) at30°C) further indicates the endothermic natureof the process and possibility of chemical bond-ing between the adsorbate and the adsorbent(Panday et ai, 1985). The positive entropychange ( + 21.86 e.u. at 30°C) indicates somestructural changes at the solid-solution interface.

Effect ofpHWith an increase in solution pH from 3.0 to 8.5, theextent of removal increased from 0.0032mg g -1

(12.99%) to 0.0246mg g"1 (98.66%) at1.0 X10 "6 M dye concentration and 30°C(Figure 5a). Similar results were reported earlierin the case of cationic adsorption (Pandav. et ai,1985;Kharee*a/., 1987).

The oxides of silicon, calcium and alumi-nium of the mixed adsorbent develop electrical

PERTANIKA VOL. 10 NO. 3, 1987 345

SUNIL KUMAR KHARE, VISHWANATH SINGH AND RAM MOHAN SRIVASTAVA

Fig. 5: Effect of pH on (0) adsorption anddesorption of victoria blue from fly ash-wollastonite.Conditions: Concentration: 7.0x70 6 M,Surface concentration 0.589mg g },Temperature 30±0.1°C

charge on their surface in contact with wateraccording to the pH of the solution which is asfollows:

H 'M f —* M

X O H

(Basic dissociation)

OH

M

(Acidic dissociation)

where, M stands for Si, Ca and Al. Except silica,all other oxides will possess positive charge for apH range of interest (Panday, et ai!, 1986)because the zero point charge (pH ) o\ SiO t,CaO and Al^O^are 2.2, 11.0 andZ 8.3 respec-tively. Thus, most of the dye cations at pH 8.5,will be adsorbed by negatively charged silicaat surface site of adsorbent. However, a little con-tribution of alumina for victoria blue adsorptionat lower pH cannot be ruled out.

The uptake of dye cations with negativelycharged oxide surface may also be explained interms of ion-exchange mechanism in the follow-ing manner:

' HMO Jor + Dye.+....C1

)Na +Dye +

...(14)

As the solution pH decreases, the negativecharge density on the surface decreases which

results in an unfavourable condition for thecationic adsorption, consequently decreasing theextent of removal of victoria blue at lower pH ofthe solution.

Desorption StudiesMaximum desorption of 0.3653mg (61.93%)was observed (Figure 5b) from the surface flyash-wollastonite (l.Og) containing 0.5898mgvictoria blue per gram of adsorbent at pH 3.0and 30°C. As the pH of the aqueous suspensionwas further increased, the desorption of dye wasdecreased. The difference in the extent ofdesorption at pH 3.0 and 10.5 (Figure 5b) clearlysuggests strong bonding between the adsorbatespecies and the active surface sites of adsorbent.The weakly bonded dye cations were detachedfrom the surface at neutral pH while they weremostly desorbed in acidic medium due to the exchange with H + ions.

CONCLUSIONA homogenous mixture of fly ash and wolla-stonite (1:1) has been found quite effective invictoria blue removal from aqueous solution andtherefore may be considered as potential adsor-bent for colour removal from water and waste-waters. The efficiency of the removal process isenhanced by decreasing the concentration andincreasing temperature and pH of the solution.The results of desorption studies show that a con-siderable amount of dye may be recovered fromthe surface of adsorbent particularly at low pH.The data thus obtained are useful for colour re-moval from water and wastewater using batch orstirred tank flow reactor, where head-lossthrough bed column reactor would be prohi-bitive.

ACKNOWLEDGEMENTSThe authors are grateful to the Head, Depart-ment of Applied Chemistry, Institute of Techno-logy, Banaras Hindu University, for providinglaboratory facilities, and to Dr. K.K. Panday,Research Associate, School of BiochemicalEngineering, Institute of Technology, BanarasHindu University, Varanasi, India, for expertadvice and invaluable discussion throughout thestudy.

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(Received 11 June, 1987)

PERTANIKAVOL. 10 NO. 3, 1987 347