Removal of chrome dye from aqueous solutions by mixed adsorbents: Fly ash and coal

Wat. Res. Vol. 24, No. 1, pp. 45-50, 1990 Printed in Great Britain.All rightsreserved

0043-1354/90$3.00+0.00 Copyright © 1990PergamonPress plc

REMOVAL OF CHROME DYE FROM AQUEOUS SOLUTIONS BY MIXED ADSORBENTS: FLY ASH A N D COAL G. S. GUPTA,G. PRASAD and V. N. SINGH Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi--221005, India

(First received June 1987; accepted in revised form July 1989) Abstract--The removal of Omega Chrome Red ME (a popular chrome dye) from its aqueous solutions by adsorption on a homogeneous mixture of fly ash and coal in different proportions has been carried out. It has been noted that low adsorbate concentration, small particle size of adsorbent, low temperature and acidic medium favour the removal of the dye. A 100% removal of the said dye was achieved at 10 mg 1-i 30oc, 2.0 pH and 53 #m particle size, using a 1: 1 ratio of fly ash and coal. The kinetics and mass transfer studies were made using the models suggested by Lagergren and McKay et aL, respectively. The equilibrium data fit well in the Langmuir model of adsorption, showing the formation of monolayer coverage of dye molecules at the outer surface of the adsorbent. Effect of temperature was explained on the basis of solubility and chemical potential of the adsorbate. An attempt has been made to explain the results thus obtained on the basis of various physiochemical properties of the solid--solution interface involved in the process of removal.

Key words---chrome dye, fly ash, coal, adsorption kinetics, pore diffusion, mass transfer and Langmuir's model

INTRODUCTION

mixed in the desired proportions. The Indian Standard Methods (1960) were applied for investigation of the adsorbent (Table 1). All chemicals used were of analytical reagent grade and supplied by BDH. The dye used was kindly donated by M/s. Sandoz India Ltd, Bombay. In batch adsorption experiments 1.0 g of mixed adsorbent was shaken with 50 ml aqueous solution of dye of varying concentration and pH in different glass bottles at various temperatures in a shaking water bath at a constant speed of 125 rpm. At the end of predetermined time intervals the adsorbent was removed by centrifugation and supematant was analysed for chrome dye spectrophotometrically (Bausch and Lomb, Spectronic-20) at maximum adsorbance wavelength. An appropriate pH and 0.05 ionic strength of dye solution were maintained by 0.05 M HC1/0.1 M NaOH and NaCIO4, respectively.

Woollen and carpet industries mostly use chrome dyes for dyeing purposes. In the course of the dyeing and washing of carpets, it is but natural that the dye should come into contact with water and soil and consequently adversely affect the health of living beings and fertility of the soil (Walsh et al., 1980; Ajmal and Khan, 1985). Chromium present in the dye effluents sometimes also causes myelotoxicological, carcinogenic, mutagenic, teratological and other severe effects on animals (Martin and Holdich, 1986; Anne et al., 1986). Its ingestion may cause pain, vomiting, nausea, haemorrhage and vigorous diarrhoea (Jain et al., 1977). In water reuse technology, various techniques have been employed in the past for the removal of dye from water and wastewater (Perineau et al., 1982; Singh et al., 1984; McKay et al., 1985, 1986; Gupta and Bhattacharya, 1985). Adsorption is one of these techniques which is comparatively more useful for such removal. In the present communication the authors have given some useful data obtained from the study of removal of chrome dye from water by adsorption.

RESULTS AND DISCUSSION

Effect o f contact time and initial dye concentration on adsorption The results shown in Fig. 1 indicate that the remaining concentration of dye in solution decreases with time up to 100 min and thereafter it becomes constant for each concentration. This shows that equilibrium is attained at 100 min, which is irrespective of the concentration of dye solution. However, with increase in the concentration, the amount of dye present in the solution also increases at various intervals of time.

MATERIALS AND METHODS

The coal and fly ash were obtained from Singrauli Coalfields and Obera Thermal Power Plant Mirzapur, respectively, both located in India. Both adsorbents were passed separately through 53, 75 and 125/am sieves and used as such without any pretreatment, just after being well

Effect o f particle size o f the adsorbent on adsorption With the increase of adsorbent particle size from 53 to 125/~m, the remaining dye solution concentration 45

G. S. GUPTA et al.

46

Table 2. Effect of various mixtures of fly ash and coal Mixture ratios Percentage (fly ash + coal) removal

Table 1. Characterization* and physical properties+ of the fly ash, coal and fly ash+coal (1:1) Adsorbents

Constituents SiO, A1203 CaO Fe, 03 MgO TiO, MnO PzO5 Sulphite Alkalioxide Ignition loss Mean particle diameter (cm) Surface area (m2g ~) Porosity Density (gem 3)

Fly ash

Coal

56.04 25.90 2.22 1.26 0.94 -

59.15 25.77 1.45 9.04 0.96 1.58 O.11 0.47 0.42 1.03

Fly ash + coal ( I : 1)

57.60 25.83 1.84 5.15 0.96 0.79 0.05 0.24 0.21 0.51 13.64 6.82 48 x 10 4 50 × 10 ~ 49 × 10 4 5.77 0.38 3.42

13.43 0.34 3.51

9.60 0.36 3.46

1:i 1:2 1:3 2:1 3:t

Conditions: initial dye concentration, 10mgl ~: adsorbent particle size, 53 ,um; temperature, 30C: pH 4.2

Adsorption dynamic~ T h e rate c o n s t a n t for a d s o r p t i o n o f the c h r o m e dye o n the m i x e d a d s o r b e n t (1:1) was d e t e r m i n e d using L a g e r g r e n ' s e q u a t i o n (1898): kad log(qe - q ) = logqe -- 2 . ~ '

*All values are in percentage by weight. fOnly for 53/~m size mixed adsorbent.

increases f r o m 0.7300 to 3.4161 m g I ~, while percentage r e m o v a l decreases f r o m 92.70 to 65.84 at 1 0 m g l t c o n c e n t r a t i o n , 30°C, 4 . 2 p H a n d 5 3 p m a d s o r b e n t particle size using a 1 : 1 ratio o f fly ash a n d coal (Fig. 1). This m a y be explained o n the basis o f the surface area available for the a d s o r p t i o n o f dye w h i c h is g r e a t e r in smaller particle sizes ( P o o t s et al., 1976a, b).

Effect o f various ratios of fly ash and coal mixture on adsorption T h e results p r e s e n t e d in Table 2, s h o w that the p e r c e n t a g e r e m o v a l o f dye increases as the ratio o f coal in the fly a s h - c o a l m i x t u r e increases. This is d u e to the higher p e r c e n t a g e o f coal a n d c o n s e q u e n t l y g r e a t e r surface area available for the a d s o r p t i o n o f dye species in a m i x t u r e c o n t a i n i n g a higher p e r c e n t a g e o f coal.

20t 16

E

0

40

80

o

o

o

lD

ID

ID

I ~ I • L120 160 2OO

O-

I g.~ 240 280

Time (rain) Fig. 1. Effect of contact time, dye solution concentration and adsorbent particle size on the removal of Omega Chrome Red ME by fly ash-coal (1:1). (O) 2 0 m g l -~, ( O ) 10mgl -t, (l-l) 5 m g l - L (Conditions: 5 3 # m partical size, 0.05M ionic strength, 4.2pH, 30°C.) ((~) 125#m, (ID) 75ttm, ( 0 ) 53#m. (Conditions: 10mgt -I concentration, 0.05 M ionic strength, 4.2 pH, 30°C.)

(1)

dig

- 15

8

t

where, q~ a n d q ( b o t h in m g I ~) are the a m o u n t s o f dye a d s o r b e d at e q u i l i b r i u m a n d at time t (min), respectively, a n d kad (min -~) is the rate c o n s t a n t for a d s o r p t i o n o f dye. T h e values o f kad at different t e m p e r a t u r e s were calculated f r o m the slopes o f the respective linear plots o f log(qe - q ) vs t (Fig. 2) a n d n o t e d in Table 3. It m a y be c o n c l u d e d f r o m the values o f kad that the reaction taking place is o f the first order. In a b a t c h r e a c t o r with rapid stirring, there is also a possibility that the t r a n s p o r t o f a d s o r b a t e ions f r o m s o l u t i o n into the p o r e s o f the a d s o r b e n t is the rate c o n t r o l l i n g step ( W e b e r a n d M o r r i s , 1963; P o o t s et al., 1978). This possibility was tested in t e r m s o f a graphical r e l a t i o n s h i p b e t w e e n the a m o u n t o f dye a d s o r b e d a n d the s q u a r e r o o t o f time (Fig. 3). T h e

0

:i

92.70 9489 96.10 92.34 91.83

;tO

40

60 80 100 120 Time (min) Fig. 2. Lagergren plot [(O) 30;C; (O) 40°C; (Q) 50°C] and McKay et aL plot [((~) 30'~C] for the removal of Omega Chrome Red ME by fly ash-coal (1:1). (Conditions: 10mg 1-] concentration, 53 ~m particle size, 0.05 M ionic strength, 4.2 pH.) Table 3. Adsorption kinetic parameters at different temperatures Temperature k,,~ kp /~ ('C) (min ~) (min~2) (cm2s ~1 30 40 50

2.92× I0 2 2.90× 10 : 2.84× 10 :

2.63× 10 2 2.41 × 10 2 2.04x 10 -2

2.82×10 ~o 1.75x 10 "~ 1.15 × 10 io

Conditions: initial dye concentration, I0 mg I ~; adsorbent particle size, 53 ,urn; ratio oftty and coal, 1: 1; temperature, 30"C; pH 4.2

Removal of chrome dye by adsorbents O5

25-

j°J I

47

0.4

/

E

;

. / J

1

?y 5

,qE I 2

I 4

I 6 T ~

I 8 (rain)

I 10

I 12

0

1/2

I

I

2

[

4

I

6

Ce (mg

8

I

10

I

12

t -1)

Fig. 3. Weber and Morris plot for the rate constants of pore diffusion of Omega Chrome Red ME during its removal by fly ash-coal (1:1). (Q) 30°C, (qS) 40°C, (©) 50°C. (Conditions: 10 mg 1-t concentration, 53/am particle size, 0.05 M ionic strength, 4.2 pH.)

Fig. 4. Langmuir plot for the adsorption of Omega Chrome Red ME on fly ash-coal (I:1). (Q) 30°C. (~5) 40°C, (O) 50°C. (Conditions: 10mgl -t concentration, 53#m particle size, 0.05 M ionic strength, 4.2 pH.)

double nature of these plots may be explained as: the initial curved portions are attributed to boundary layer diffusion effects (Crank, 1965) while the final linear portions are due to intraparticle diffusion effects (McKay et al., 1980). The rate constant for intraparticle diffusion, kp, at different temperatures was determined from the slopes of the linear portions of the respective plots and are given in Table 3. The pore diffusion coefficient,/3, at different temperatures was determined by using the following equation (Bhattacharya and Venkobachar, 1984):

(time -l ) as the rate constant for adsorption of dye (time- l ) and is equal to 1.53 x 10- 2 min- l indicating that the rate of adsorption of dye may be diffusion controlled (#l" S~ ~ k,o) similar to our earlier findings (Panday et al., 1985).

tl/2 =

0.03 ro~ /3

(2)

where, t~/2(min) is the time for the adsorption of half amount of dye, r0 (cm) is the radius of adsorbent. The values o f / 3 (Table 3) were found in the order of 10-1°cm: s -l indicating that the process is governed by diffusion but pore diffusion is not the only rate limiting step (Michelson et al., 1975).

Mass transfer analysis The mass transfer analysis for the adsorption of Omega Chrome Red ME on mixed adsorbant was carried out using the McKay et al. (1981) equation: In (Cc-~0 1 +1= l n)m k

1 +mkmk l+m------~kk'fll'Ss'tmk (3)

where m (g ! -l) is the mass of mixed adsorbent per unit volume of particle free slurry, k (Q °b, 1 g-l) is the Langmuir constant and fll (cm s- l ) and S~ (cm- l ) are the mass transfer coefficient and the outer surface of mixed adsorbent (1:1) per unit volume of particle free slurry, respectively. A straight line plot of ln[(Ct/Co)-{1/(1 +ink)}] vs t (Fig. 2) shows the applicability of the above equation for the present system. The value of 81 was calculated from the plot and found to be 1 . 8 6 x 1 0 - S c m s -l at 10mgl -~ concentration, 30°C, 4.3 pH and 53 #m adsorbent particle size using the 1:1 ratio of fly ash and coal. The product of fit and Ss has the same unit

Adsorption isotherm The analysis of equilibrium data for the adsorption of the chrome dye on mixed adsorbent (1 : 1) has been done in the light of the rearranged Langmuir isotherm model: Ce

qe

1

Ce

QOb I- QO

(4)

where, C~ (mg 1- ~) is the equilibrium concentration of dye, and Q0 and b are Langmuir constants related to the capacity and energy of adsorption, respectively. The linear plot of Ce/qe VS Co at different temperatures (Fig. 4) suggests the applicability of the above model for the present system, showing formation of monolayer coverage of the adsorbate at the outer surface of the mixed adsorbent. This was also experimentally confirmed by extending the time of adsorption to 48 h, wherein the amount of adsorption remained the same as at 100 min and no multilayer curve was obtained. The values of Q0 and b at different temperatures were determined from the plots and are given in Table 4 along with their regression values. The values obtained in both cases are very much comparable. The essential characteristic of the Langmuir isotherm may be expressed in terms of a dimensionless equilibrium parameter RL (Hall et al., 1966) using the following equation: 1

RL = 1 + bC----~o"

(5)

The values of RL for the studied system at different temperatures were found to be in between zero and one (Table 4) showing favourable adsorption of the dye on the mixed adsorbent (I:1).

G.S. GUPTAet al.

48

Table 4. Langmuir constants at different temperatures Graphical values

Regression values

Temperature CC)

QC~ (mgg i)

b (Img 1)

Q0 (mgg 1)

b (Img J)

Equilibrium parameter R[

30 40 50

0.7553 0.6938 0.6455

2.9422 0.8479 0.3756

0.7645 0.7025 0.610l

2.6963 0.7831 0.4275

0.0329 0.1055 0.2103

Conditions: initial dye concentration, 10 mg I ~; adsorbent particle size, 53 .am; ratio of fly ash and coal, 1: I; temperature, 30'~C; pH4.2

Effect of temperature on adsorption The rise of temperature affects the solubility and the chemical potential of the absorbate, the latter being a controlling factor for the adsorption. It has been reported earlier (Singh, 1974) that if the solubility of the adsorbate increases with increase in temperature the chemical potential decreases and both the effects i.e. solubility and normal temperature effects, work in the same direction, causing a decrease in the adsorption. On the other hand if temperature has the reverse effect on the solubility then both the said effects will act in the opposite direction and the adsorption may increase or decrease depending upon the predominant factor. In the present study, as the chrome dye has the positive temperature coefficient,

the adsorption of the dye should decrease with rise in temperature. This is born out by the results.

Effect of pH Figure 6 shows the effect of pH on the removal of chrome dye. It is observed that the concentration of dye left in solution increases from zero (100% removal) to 8.62 mgl ~ (13.80% removal) with the rise of pH from 2.0 to 12.0, at 10mgl -~ dye concentration, 30°C and 53/~m adsorbent particle size using the 1 : 1 ratio of fly ash and coal. The results obtained at various pH values, have been explained on the basis of aqua-complex formation and its subsequent acid-base dissociation at the solid-solution interface (Ahmed, 1966): O

O

\ /

M---O + H---4)H

,

O

o

o

o

M--OH~ + OH-

(6)

O

where, M stands for AI, Ca, Si, etc. Since the solution is acidified by hydrochloric acid the outer surface of positively charged interface will be associated with chloride ions. The chloride ions are exchanged with dye anions as given below: O

E

o

/

o.-

2

\

0

1 40

I 80

I 120

I 160

I 200

I 240

[ 280

/

M---OH~-/C1 + Dye

O

Time (rnin)

Fig. 5. Effect of temperature on the removal of Omega Chrome Red ME by fly ash-coal (1 : 1). (O) 30°C, ( ~5) 40°C, (O) 50°C. (Conditions: 10mgl -~ concentration, 53pm particle size, 0.05 M ionic strength, 4.2 pH.)

O --,

/

M~H2/Dye

+ CI-

(7)

O 10-

E

when pH of the dye solution rises the adsorbent surface becomes negatively charged, which repels the dye anions resulting in low adsorption at higher pH. In addition to this, the functional oxidized groups present on the surface of coal play a major role with the change in pH of the system in removing the dye contents from water (Frumkin, 1930): the mechanism involved at the interface in this case is given below:

6

~2

C~O + H 2 0 ~ - C 2+ + 2OH0

~

4

6

8

10

12

11

pH

Fig. 6. Effect of pH on the adsorption of Omega Chrome Red ME on fly ash-coal (1:1). (Conditions: 10mgl -I concentration, 53/~m particle size, 0.05 M ionic strength, 30°C.)

CxO 2 + H20~-Cx

O2+ + 2 O H - .

(8) (9)

In the neutral to acidic pH range: C 2+ + Dye ~-Dye.C~+

(10)

C~O 2+ + Dye- ~-Dye. CxO 2+

(1 l)

Removal of chrome dye by adsorbents

49

Table 5. Comparative study of the adsorptive properties and cost of activated carbon, coal, flu ash + coal (1 : 1) and fly ash for Omega Chrome Red ME.

kad

kp

(mg g-~ Equilibrium Price per Temperature Percentage (rain- ~) min- ]/:) Q0 b period ton Adsorbents (°C) removal x l0 -2 x l0 -: (mgg -I) (I mg-1) (rain) (Rs.) Activated carbon 30 98.41 5.99 3.59 2.0944 6.9440 40 84.13 5.55 3.02 1.7218 0.2115 60 40,000.00 50 71.43 5.23 2.15 0.9799 0.0753 Coal 30 96.35 3.34 7.18 0.8119 3.0777 40 83.94 2.40 5.92 0.6781 1.2627 90 3000.00 50 71.17 2.26 6.62 0.6231 0.4585 Fly ash + coal (1:1) 30 92.70 2.92 2.62 0.7553 2.9422 40 82.48 2.90 2.41 0.6938 0.8479 100 1550.00 50 70.80 2.84 2.20 0.6101 0.3756 Fly ash 30 91.37 2.00 3.03 0.7286 2.1960 40 79.86 1.91 2.60 0.6400 0.8224 120 100.00 50 69.06 1.78 2.32 0.5199 0.5495 Conditions: initial dye concentrations, 10 mgl i; adsorbent particle size, 53/~m; ratio of fly ash and coal, 1:1; pH 4.2 When the p H of the system turns from the neutral to alkaline range, due to the c o m m o n ion effect ( O H - and D y e - ) the percentage of dye anions in the solution decreases, therefore, adsorption of the chrome dye decreases at higher p H (Panday et al., 1984; Huang and Wu, 1975).

Comparative study o f adsorption properties o f activated carbon, fly ash, coal and fly ash 4- coal (I : 1)for Omega Chrome Red M E It may be seen from Table 5 that the dye removal is in the order: activated carbon > coal > fly ash + coal > fly ash and the equilibrium period is minimum for activated carbon. The values of kad, kp, Q0 and b at different temperatures are also given in this table.

Cost analysis o f the adsorbents with respect to activated carbon As is clear from Table 5 fly ash is much cheaper than activated carbon while its mixture with coal (1:1) comes next. The adsorption capacities of the fly ash and fly ash + coal (1 : 1) are nearly three times less than that of activated carbon. Although, activated carbon may be used four to five times after regeneration, fly ash as well as its mixture with coal would still prove to be cheaper without regeneration, because fly ash-coal is found in abundance in India and also used fly ash-coal, which is much stronger, can be utilized in preparing refractory bricks. CONCLUSION The following conclusions may be drawn from the present investigations. (1) An homogeneous mixture of fly ash and coal in different proportions shows a good adsorption capacity towards Omega Chrome Red ME. However, with the increase in the percentage o f coal in the fly ash-coal mixture the percentage adsorption of the dye increases due to greater surface area available for the adsorption of the dye in a mixture containing a higher percentage of coal.

(2) The mechanism involves an initial rapid rate for the dye removal due to surface adsorption followed by intraparticle diffusion which appears to be the rate governing step. (3) The fitness of Langmuir's model in the present system shows the formation of m o n o layer coverage of the adsorbate at the outer surface of the adsorbent. (4) The products fll'Ss and kao have the same dimension and magnitude. This further lends support to a diffusion controlled process. (5) The positive temperature coefficient suggests the exothermic nature of the process involved herein. (6) The high extent of adsorption in acidic medium is due to the aqua-complex formation and its subsequent acid-base dissociation at the solid-solution interface. (7) Comparative studies of the adsorption capacities and costs of the activated carbon, fly ash, coal and fly ash--coal mixture ( l : l ) show that the mixture of fly ash and coal ( l ' l ) may substitute the activated carbon.

Acknowledgements--The award of a scholarship to G.S.G. by University Grants Commission, New Delhi and donation of the dye by Sandoz India Ltd, Bombay, are gratefully acknowledged. REFERENCES

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G.S. GUPTA et al.

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