Removal of congo red from wastewater by adsorption onto waste red mud

Removal of congo red from wastewater by adsorption onto waste red mud

Copyright 0 1997 Elsevier Science Ltd Printed in Great Britain All rights reserved PII: SOO45-6535(96)00385-Z 0045.6535/97 $17.00+0.00

REMOVAL OF CONGO RED FROM WASTEWATER BY ADSORPTION

ONTO WASTE RED MUD

C NAMASIVAYAM* and D J S E ARASI Environmental Chemistry Division Department of Environmental Sciences

Bharathiar University Coimbatore - 641 046 Tamil Nadu - INDIA

(Received in USA 23 June 1996; accepted 23 August 1996) ABSTRACT

Waste red mud, an industrial byproduct, generated during the processing of bauxite ore, is recycled

for the adsorption of congo red from aqueous solution

Adsorption kinetics were studied using the parameters such

as dye concentration, adsorbent dose, agitation time and pH

Adsorption followed first rate expression The equilibrium

adsorption data obeyed both Langmuir and Freundlich

isotherms The adsorption capacity of the red mud for the

dye was 4.05 mg/g Adsorption was found to be nearly

quantitative at pH 2.0 Effect of pH and desorption studies

suggest that the mechanism of adsorption is mostly ion

exchange.0 1997 Elsevier Science Ltd All rights reserved

Key words : Waste red mud, congo red, adsorption isotherms

pH effect

* Author to whom correspondence should be addressed

E-mail: sandflas250.bharathi.ernet.in

INTRODUCTION

effluent which is easily detected and readily traced back to

its source Most dyes are stable to biological degradation

Coloured waters are often objectionable on aesthetic grounds

for drinking and other agricultural purposes Colour affects

the nature of the water by inhibiting sunlight penetration

carcinogenic and mutagenic (1) Hence there is a need to

remove dyes from wastewaters before it mixing with receiving

waters

The treatment of dyes in industrial wastewaters

poses several problems since dyes are generally stable to

photodegradation and oxidation (2); hence, they cannot be

Adsorption of dyes provides an attractive alternative for

popular adsorbent and has been used with great success

However, high costs in the procurement of activated carbon

adsorbents used for the removal of dyes and heavy metals

bagasse and paddy straw (7) and industrial solid waste such

as Fe(III)/Cr(III) hydroxide(8) and fly ash and coal(g)

Namasivayam and coworkers employed red mud for the treatment

of dairy wastewater (10) and for the removal of chlorophenol

(11) from aqueous solutions Zouboulis et al (12) have

employed red mud for the removal of nickel

The objective of this study was to evaluate the feasibility of using red mud for the treatment of wastewater

containing congo red, a textile dye as a typical case (13)

EXPERIMENTAL

Materials

Waste red mud obtained from M/S Mettur Aluminium Factory (Mettur, Tamil Nadu, India) was used as

the adsorbent It was thoroughly washed with distilled

water to remove very fine particulate ( < 53 /A) and dried

at 60°C for 5 h The powder was then used for adsorption

studies Congo red was obtained from CIBA - GEIGY (Bombay,

India)

Methods

Batch adsorption experiments were carried out

by agitating 50 mL of dye solution at natural pH (7.3) with

250 mg of adsorbent in glass bottles at 140 rpm at room

temperature (30 ? 2'C) using a shaker machine The dye

solution was separated from the adsorbent by centrifugation

photometrically by monitoring the absorbance changes at the wavelength of maximum absorption (495.8 nm) using a Hitachi spectrophotometer (Model U-3210, Tokyo)

For studies on the effect of pH upon dye removal, the initial pH of the dye solution was varied from

2 to 11 In the pH range 7.3 to 11.0 there was no change in absorbance with pH But in acidic conditions the absorbance

of the dye changed Hence the per cent removal of the dye was determined from absorbance values of dye solutions after adjusting the final pH to 7.3 The data for Langmuir and Freundlich adsorption isotherms were taken from batch adsorption experiments evaluating effect of agitation time, and initial dye concentration and adsorbent dose on per cent removal, respectively

Batch desorption studies were carried out as follows: After adsorption experiments with 50 mL of dye solution of 10 mg/L and 250 mg of adsorbent for an agitation time greater than the equilibrium time i.e 100 min, the supernatant dye solution was discarded The dye loaded adsorbent was washed gently to remove any unadsorbed dye Several such samples were prepared Then the spent adsorbent samples were agitated with 50 mL of water adjusted to different pH values for 100 min The desorbed dye was separated from the adsorbent by centrifugation and estimated

RJZ!3ULTSANDDISCUSSION

Effect of contact time and concentration

Figure 1 shows effect of agitation time and initial concentration on adsorption of congo red by red mud

The removal of dye was rapid in the initial stages of

contact time and gradually decreased with lapse of time

until equilibrium

The equilibrium time was 90 min for all the dye concentrations used The equilibrium uptake of dye decreased

from 36.5 $ to 25.7 % as dye concentrations increased from

10 to 40 mg/L The removal curves were single, smooth and

continuous indicating monolayer coverage of dye on outer

surface of adsorbent

20 40 60 80 loo 120 140 160 180 200

Agdatlon lime (min)

Fig 1 Effect of agitation time and dye concentration on

removal: Dye concentration:(O)- 10 mg/L,(D) - 20

mg/L,(A)- 30 mg/L,(e)40 mg/L, Adsorbent dose - 250

Adsorption dynamics

The rate constant for adsorption of congo red

by red mud was studied using Lagergren rate equation (4),

log10 (qe

kadt

- 9) = log10 qe _

2.303

(1)

where qe and q are the amounts of dye adsorbed (mg/g) at equilibrium and at time t (sin), respectively, and kad is the rate constant of adsorption Linear plots of loglo (qe - 9) vs t were obtained for different dye concentrations, which indicate that the adsorption process follows the first order rate expression (Fig 2) Adsorption

rate constants (kad) are presented in Table 1 Adsorption

rate constants for congo red (50 mg/L) on biogas waste

slurry (4) waste banana pith (5), orange peel (6) and waste

Fe(III)/Cr(III) hydroxide (8) were reported to be 0.028,

0.106, 0.0406 and 0.34 l/min, respectively

Table - 1

Rate constants for adsorption

Concentration Adsorption

of congo red rate constant

Langmuir isotherm

Langmuir isotherm is valid for monolayer adsorption on a surface containing a finite number of

identical sites The model assumes uniform

adsorption on the surface and no transmigration

in the plane of the surface The Langmuir

represented by the equation (6)

energies of

of adsorbate isotherm is

(2)

where, C e is the concentration of dye (mg/L) at equilibrium The constant Q, signifies the adsorption capacity and b is related to the energy of adsorption Linear plot of C,/q, vs

'e shows that the adsorption follows Langmuir isotherm (Fig 3) The values of Q, and b were calculated from the slope and intercept of the linear plot and are presented in Table 2 The applicability of Langmuir isotherm suggests the monolayer coverage of congo red on the surface of red mud The Q, and b values for the adsorption of congo red by some non-conventional adsorbents are shown in Table 2

Ce hg/L)

Fig 3 Langmuir plot for dye adsorption

Table - 2 Langmuir constants

Waste Fe(III)/Cr(III) hydroxide 44.00 0.0505 9

work

The essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant, called equilibrium parameter, RL, which is defined by,

1 + bco

where b is the Langmuir constant and Co is the initial dye concentration (mg/L) The RL values were found to be between

0 and 1, indicating favourable adsorption for congo red on red mud for all the concentrations studied(6)

Freundlich isotherm

Freundlich equation was also applied for the

X 1 log10 - = log10 kf + -

log10 ce (4)

where x is the amount of dye adsorbed (mg), m is the weight

of the adsorbent used (g), Ce is the equilibrium dye concentration in solution (mg/L) and kf and n are constants incorporating all factors affecting the adsorption process, such as adsorption capacity and intensity Linear plot of

log10 x/m vs log10 'e shows that the adsorption also follows Freundlich isotherm (Fig 4) In general, higher the

kf value, greater will be the adsorption capacity The values of k, and n are shown in Table 3 along with those of

L

other non-conventional adsorbents reported

05

1

Corr coetft =0.9904 o-4

in literature

03-

o- -0 \ - -3 2 -

kl;c,Ce

Table - 3

Freundlich constants

work

Effect of pH

When initial pH of the dye solution was increased from 2 to 11 the per cent removal decreased from

98 to 9 (Fig 5) The final pH after adsorption was higher

than the initial pH in the initial pH range 2 to 7 This is due to the contribution of alkalinity of red mud (13); hence,

final pH is important to explain the per cent removal The

per cent removal vs final pH is also shown in Fig 5c The

decrease in adsorption with increase in pH may be explained

on the basis of aqua complex formation and subsequent acid-

base dissociation at solid/solution interface (13)

0 I I I I I ,& ,

Final pH

Initial pH

Fig 5 A Effect of initial pH on per cent removal of dye

B Effect of initial pH on Final pH

C Per cent removal vs Final pH

In acid medium, positive charge develops on the surface of

oxides of adsorbent and may be written as

H+

"\

where M stands for Al or Si present in the red mud Since

the solution is acidified by hydrochloric acid, the outer

surface of positively charged interface will be associated

with Cl- ions The chloride ions are exchanged with dye anions

M - OHa+ / Cl- + Dye- > "\

M - OH2+ / Dye- + Cl-

0

(6)

With an increase in pH, positive charge on the

oxide/solution interface decreases At pHs above the pH-zpc

of the adsorbent i.e 8 - 10, the adsorbent surface becomes

negatively charged and will be associated with positively

charged ions of the solution in the following manner:

Thus there are no exchangeable anions on outer surface of the adsorbent at higher pHs

consequently the adsorption decreases Similar trend

observed in the adsorption of congo red on wollastonite

the and was (14) (7)

Desorption studies

Desorption studies help elucidating the mechanism of adsorption and recovery of dyes and adsorbent This may make the treatment process economical The per cent desorption increases with increase in the pH of the aqueous medium (Fig 6) This is just opposite to the pH effect The observation in the desorption studies again confirms the mechanism of adsorption stated in the pH effect

01 3 I 4 I 5 I I I I I I

7 8 9 10 11

PH

Fig 6 Effect of pH on per cent desorption

CONCLUSIONS

1 Red mud, a waste byproduct, from a bauxite processing

industry, can be effectively used as an adsorbent for the removal of congo red from wastewaters

2 The adsorption followed both Langmuir and Freundlich

isotherms The Langmuir adsorption capacity was 4.05 m9/9

3 Almost quantitative dye removal occurred at the initial

pH of 2.0

4 The kinetic data may be useful for environmental

technologist in designing treatment plants for colour removal from wastewaters enriched with congo red

5 As the adsorbent i.5 discarded as waste in bauxite processing industry, the treatment method is expected

to be economical

ACKNOWLEDGEMENT

Authors are thankful to Bharathiar University authorities for providing Central Instrumentation Lab facilities

REFERENCES

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Fuller's earth and fired clay as adsorbents for dye stuffs Equilibrium and Rate constants Water, Air,

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