Adsorption of some acid dyes from aqueous solutions . ont6 neutral alumina
Mihir Desai, Arti Dogra, Sambhav Vora &P Bahadur*
Department of Chemistry, South Gujarat University, Surat 395007, India and
RNRam
Department of Chemistry, M S University, Baroda 390 002, India Received IS April 1997; revised 30 July 1997.
The kinetics and isotherm studies on the adsorption of five acid dyes, viz., Acid Orange 6. Acid Orange 7,Ethyl Orange, Acid Red 88 and Acid Blue I 13 on neutral alumina have'been carried out spectrophotometrically. In each case the absorption process follows first order kinetics. For all the five dyes the rate of adsorption shows an increase with increase in dye concentration (0.5 - 6.0 x 10-5M),decrease inpH (4 - 8.5), decrease in temperature (30 - 50°C) and amount of adsorbent (0. I - 0.6g). The adsorption shows the order: Acid Orange 6> Acid Orange 7 > Ethyl Orange>
Acid Red 88 > Acid Blue I 13.The dye adsorption follows the Langmuir and Freundlich isotherms; the various isotherm constants and thermodynamic parameters for the adsorption of dyes have been calculated and discussed.
Textile industrial effluents contain a large number of metal complex dyes, which are non-oxidisable by biological and chemical treatment methods because of their structural complexities. As most of these dyes are toxic in nature'v, their high concentration may cause many waterborne diseases and increase the biological oxygen demand (BOD) of the receiving water'. The treatment of texti Ie effluents prior to their discharge into the receiving water is thus a problem of practical interest.
Adsorption of dyes onto solid/water interface has been found to be an efficient and economically cheap process'l" for removing such pollutants using various adsorbents'<'". The investigations, however, have been limited to the adsorption isotherm without much emphasis on the time rate study of the absorption processll-13. The main objective of the present work is to investigate the kinetics of the removal of five anionic dyes, Ethyl Orange, Acid Orange- 7, Acid Orange-6, Acid Red-88 and Acid Blue-113 using neutral alumina as an adsorbent.
Materials and Methods
Chromatographic grade active neutral alumina (Glaxo, Bombay, India) of 100-140 mesh size was treated with boiling distilled water to remove surface impurities, if any. It was then dried at 85°C for about
16h and then stored in a dessicator at room tempera- ture (~h). The specific surface area of the neutral alumina was found to be 129 m2/g. Ethyl Orange (Sigma), Acid Orange-6 (Aldrich), Acid Orange-7 (Sigma), Acid Red-88 (Colourtex) and Acid Blue-
113 (Colourtex) were precipitated from N,N-di- methylformamide solution using acetone.
ETHYL ORAN GE
,,~.~
~,O~"i
OH~,O$
ACID RED <II
'O"b,.,~~.~ ~~
~ /-- ~ h
ACIO BLUE 113 503 No
(II
DESAI et al.:ADSORPTION OF ACID DYES ON NEUTRAL ALUMINA 939
An accurately weighed quantity 'of the dye(l) was dissolved in doubly distilled water to prepare stock solution (l.OxIO-3 M).Experimental solutions of the desired concentration were obtained by successive dilutions.
Kinetic studies
Neutral alumina (O.Sg) was kept in contact with an appropriate concentration (0.S-6.0x I0-5M)of the dye solution (300 ml) at constant temperature. The ad- sorbent-adsorbate system was shaken continuously using an electric device. At suitable time intervals, shaking was arrested so that the necessary withdraw- als of the sample soultion (3.0 ml) could be made.
The concentration of the solution was measured by a spectrophotometer (Spectronic-20, Bausch and Lomb, USA) at the respective Amax values. The amount of the adsorbed dye on neutral alumina sur- face was determined by the difference between the initial and remaining concentrations of dye solution.
Several sets of experiments were carred out by vary- ing the amount of the adsorbent, the temperature and the concentration of the dye solution as well as the pH of the solution.
Isotherm studies
Inorder to study the adsorption isotherm, neutral alumina (0.1 g) was kept in contact with 50 ml of dye solution of different concentrations (3.OxI 0-5M to 6.Sxl 0-5M)for 4 h to allow attainment of equilibrium at constant temperature ranging from 25 to 50°C.
Results and Discussion
Figure 1 shows a representative plot for the ad- sorption of different dyes with time. Under different conditions, viz., concentration of absorbent, tempera- ture andpH, the general trend remain the same, i.e., adsorption is fast during the initial stages and an
"g: 6
....o
-
>< 5
]
~ 4
"
C J
1:
h
"B
(;
l :L-~~~~L--Lo 10 15 20 25__
~-W30 35
TIME.min
Fig.1 - Time variation of adsorption of dyes onto neutral alumina with concentraion of dye solutioni(3 x 10-5M)
appreciable fraction of the total uptake (nearly 50%
of the toal amount of the dye) is completed within a short interval of time (- 10 min). The process, how- ever, slows down later and proceeds gradually to saturation. The uptake ofthe dye with time increases with concentration of dye solution (Table I). The time variation curves of absorption are smooth and continuous, indicating the formation of monolayer converage on the surface of the neutral alumina. It also indicates that a single and uniform process is operative throughout and is free of any appreciable induction period for the initiation of adsorption. The interaction leading to the final surface is thus simple and unaccompanied by any complicating secondary process. The linear behaviour of log [dye] versus log [initial rate] shows that the process is of first order as shown in Fig. 2. The amount adsorbed reveals the following sequence for the preference in absorption:
Acid Orange 6 > Acid Orange 7 > Ethyl Orange>
Acid Red 88 > Acid Blue 113.
Table I-Time rate study of adsorption onto neutral alumina with variation of concentration of dye solution at 30±0.1 °C
Dye (Cone. of (Amount Initial
the dye adsorbed rate x 103
solution) ingg-] g/g" min-]
x 105M adsorbent at equilibrium)
x 103
Ethyl Orange 1.0
2.0 3.0 4.0 2.0 3.0 4.0 5.0 3.0 4.0 5.0 6.0 0.5 1.0 2.0 3.0 0.5 1.0 2.0 3.0
1.47 3.02 4.54 5.96 3.22 4.70 6.30 7.87 5.13 6.78 8.25 9.45 0.47 1.37 2.69 3.84 1.00 1.77 3.06 3.82
0.070 0.145 0.180 0.238 0.186 0.233 0.274 0.343 0.933 1.333 1.466 1.517 0.045 0128 0.430 0.710 0.091 0.120 0.180 0.233 Acid Orange-?
Acid Orange-S
Acid Red-88
Acid Blue-I 13
The acid dyes used were all from the class of azo dyes with similar structures. A possible explanation for the difference in adsorption of the dyes can be given on the basis of their molecular weight and structural complexity. It appears that ~yes with sim- pler molecular structures adsorb more and take more time for saturation of adsorption. In other words, the equilibrium adsorption and adsorption kinetics are dependent upon the molecular dimensions of the dyes studied.
The uptake of the dye was found to decrease with an increase in temperature (Table 2). It alsoreveals that dye adsorption on oxide surface is favoured at lower temperature. The energy of activation calcu- lated from Arrhenius plot is very low (Fig. 3) which
-2.4 ...---,
¥
•••-l1,
~o
_4.4 t.<::..---""_-1-._""""-_i-- ...••••._-'---'
-~.J~ -4·95 - 4·55 - 4-15
~og [Oy,)
Fig.2 - Plot of log (initial rate) versus log [dye]
Dye
Table 2-The influence of temperature on adsorption of acid dyes onto neutral alumina
Temp. (Amount adsorbed in Initial rate x 103(gig) Energy of activation
(0C)
ss"
adsorbent at min-' (Ea,kJmor')equilibrium) x 103
Ethyl Orange" 30
3S
40 4S 30
3S
40 45 30 35 40 45 30
3S
40 45 30 35 40 45
• Initial concentration of dye solution =3.0 x I
o'
MtInitial concentration of dye solution =4.0 x 10-5M : Initial concentration of dye solution =1.0x IO-sM
Acid Orange-7·
Acid Orange-e"
Acid Red-SSl
Acid Blue-133:
4.54 O.ISO
4.16 0.IS8
3.65 0.196
3.37 0.203
4.70 0.233
4.48 0.241
4.17 0.253
3.88 0.261
5.93 0.131
5.14 0.185
4.13 . 0.192
2.71 0.210
1.37 0.401
1.27 0.623
1.\3 0.869
1.04 0.923
1.77 0.805
1.67 0.862
1.47 0.943
1.30 1.027
6.44
5.23
3.33
7.32
5.88
DESAI et al.: ADSORPTION OF ACID DYES ON NEUTRAL ALUMINA 941
shows that adsorption of dyes onto neutral alumina is Table 3- Time variation of adsorption withpHof the dye
fast. solution at 30±0.1 °C
Table 3 indicates that the extent of adsorption Dye pHof (Amount Initial decreases as the pH of the solution increases (from solution adsorbed rate x 103
in gg" (glg)min-I
acidic to basic) keeping the initial concentration of adsorbent at
the solution constant, while the corresponding value equilibrium)
of equilibrium constant increases. It happens so be- x 103
cause neutral alumina normally contains varying Ethyl Orange• 4.10 5.07 0.240
amounts of water molecules especially those which 5.30 4.96 0.260 either exist as surface hydroxyl groups or adsorbed
water'", 7.10 4.86 0.275
AI (OH)3 ~ Al (OHio!-+OH- 8.30 4.75 0.306
AIO.OH2 ~ AIO.O- +2H+ Acid Orange- 7• 4.10 5.14 0.246
In water, these OH- ions are released from the 5.30 5.02 0.268
surface and the cationic centres remaining on the
surface would be the source of attraction for anionic 7.10 4.93 0.294 dyeslS. Our unpublished studies on the adsorption of 8.30 4.80 0.323 anionic dyes onto silica did not show any significant Acid Orange-s" 4.30 3.17 0.149
adsorption (results not given) which support these
5.85 2.84 0.189
views. The improved adsorption at lowerpH may be explained on the assumption that as the surface comes
7.30 1.91 0.222
in contact with water it is surrounded by hydroxyl
groups. On increasing the concentration of the hydro- 8.45 1.56 0.250 gen ion in the dye solution, the surface OH- ions Acid Red-88: 4.20 1.90 0.625
would get neutralized by protonation which facili- 5.60 1.65 0.840 tates the diffusion of dye molecules in the vicinity of
the adsorbent. On the other hand, a diminished ab- 7.80 1.37 0.921 sorption at higher pH may be due to the abundance 8.70 1.24 1.023 of OH- ions and consequently ionic repulsion be- Acid Blue-I 13: 4.20 2.56 0.116
tween the negatively charged surface and the anionic 5.60 2.21 0.120
dye molecules'".
A study of the kinetics of adsorption was made 7.80 1.70 0.133
using various amount of the adsorbent, while keeping 8.07 1.28 0.180
the dye concentration constant at constant tempera- • Initial concentraion of dye solution =3.0 x 10-5M
tInitial concentration of dye solution =4.0 x 10-5M : Initial concentration of dye solution =1.0 x 10-5M
-L.)
...•
/IC·).a
:::
:'
~ -J.)-u'--_.J..._-'-- ---'"---'-~
),)0 ).2S ).20 ).IS ).10 lOS
1/1 X 10~ Ii'
~
Fig.3 - Plot of log (initial rate) versus IIT
ture. The nature of adsorption was abserved to be same as described above (Table 4) .
Adsorption isotherms
The plots of dye uptake against equilibrium con- centration (Ce) (Fig.4) indicate that adsorption in- creases initially with concentration but then reaches saturation. The equilibrium data for the adsorption of all the five dyes on neutral alumina fit the Freundlich adsorption isotherm according to the equation :
Xlm
=
KF C1eln ... (I)Table 4-The influence of various amounts of neutral alumina on adsorption of acid dyes at 30±0.1 °C
Dye Amount of
adsorbent (g)
Ethyl Orange 0.3
0.4 0.5 0.6 0.3 0.4 0.5 0.6 0.1 0.2 0.3 0.4 0.3 0.4 0.5 0.6 0.3 0.4 0.5 0.6 Acid Orange 7·
Acid Orange 6T
Acid Red 88t
Acid Blue 113t
(Amount adsorbed in gg"
adsorbent at equili- brium) x 103
8.30 5.87 4.54 3.69 8.38 6.02 4.70 3.79 13.04 11.52 9.74 7.52 2.59 1.83 1.37 0.86 3.25 2.29 1.77 1.39
• Initial concentration of dye solution =3.0 x 10-5M tInitial concentration of dye solution =4.0 x 1
o'
MtInitial concentraion of dye solution =1.0x
ro'
MInitial rate x 103
(g/gjmin"
0.206 0.213 0.232 0.253 9.188 0.219 0.241 0.264 0.783 0.846 0.933 1.015 0.124 0.249 0.481 0.799 0.903 1.102 1.383 1.545
where, Xlm is the amount adsorbed in gg", KFis a rough measure of the adsorption capacity (i.e. the intensity of adsorption) and C« is the equilibrium concentration of the dye solution. The linear plot of logXlm versus log Ce (Fig.5) indicates the applica- bility of the Freundlich adsorption isotherm and for the present system exhibits a monolayer coverage of the adsorbate on the outer surface of the adsorbent.
The values of KF and lIn calculated from the intercept and the slope of the log-log plot (Table 5) for all the temperatures were used for computing ~J.1 (i.e., the standard affinity of the dye) using the equa- tion ~J.1
=
-RTInKF. It is seen from the results that the values of ~J.1decrease with an increase in temperature (Table 5) indicating greater absorption affinity at lower temperatures 17.The monolayer formation for the present system has further been confirmed by the linear plots ofCe/qe versus Ce (Fig. 6) according to the langmuir iso- therm :
Ce/qe
=
IIQob +Ce/Qohere, QO and b are the Langmuir constants related to the capacity and energy of adsorption, respec- tively. The values of QO and b calculated from the slopes and intercepts of the above plot are given in Table 5. The QO values were found to remain almost constant over the concentration range studied.
IO~--'---~
e
--go , .
....
go~'"
; 4
2 3
Cc X ,05.M
4 5
Fig.4 -- Variation of adsorption of dyes on neutral alumina with equilibrium concentration at 30°C
'"
...Jo
-3 .7L..- __.L...-__ """- __ -'- __ -'- __ --"
-50 -L5 -LO -35 -)0 Log Ce
Fig.5 - Log-log plots of variation of adsorption with con- centration of dyes at 30°C
DESAI et aJ.: ADSORPTION OF ACID DYES ON NEUTRAL ALUMINA 943
Table 5-Values ofthennodynamic parameters and Freundlich &Langmuir isotherm constants
Temp. 6f,! -6G -M! -ss Freundlich constant Langmuir constant
(0C) (klmof") (klrnol") (klrnol") (Jmol"
deg")
KF IIn QO b
(I g-I) (mgg-I) (1 mg")
EthyI Orange
30 3.11 6.00 57.13 0.268 0.569 1.44 0.334
35 2.90 5.75 57.01 0.322 0.596 1.41 0.295
40 2.26 5.39 23.31 57.25 0.419 0.631 1.36 0.251
45 1.72 5.12 57.20 0.521 0.661 1.31 0.231
50 1.66 5.04 56.50 0.538 0.677 1.27 0.207
Acid Orange-7
30 1.08 2.83 38.60 0.649 0.567 3.74 0.334
35 0.86 2.91 38.30 0.714 0.583 3.56 0.301
40 0.62 lOI 14.97 38.60 0.788 0.601 3.40 0.277
45 0.45 3.09 38.90 0.843 0.616 3.28 0.251
50 0.24 3.19 39.10 0.914 0.632 3.14 0.222
Acid Orange-6
25 3.67 2.15 48.67 0.219 0.411 2.53 0.468
30 3.63 1.90 48.70 0.226 0.454 1.81 0.297
35 2.12 \.65 16.66 48.70 0.423 0.583 1.39 0.383
40 2.01 1.38 48.78 0.440 0.647 1.28 0.303
Acid Red-88
30 3.67 2.33 28.61 0.428 0.647 0.92 0.560
35 3.59 \.96 11.00 29.35 0.407 0.658 0.61 0.600
40 3.37 \.69 29.74 0.365 0.662 0.43 0.650
45 3.21 1.37 30.28 0.371 0.675 0.17 0.682
Acid Blue-I 13
30 2.52 \.91 35.28 0.272 0.598 0.58 0.750
35 2.37 1.52 35.97 0.252 0.604 0.42 0.860
40 2.15 1.17 12.60 36.06 0.228 0.610 0.20 0.720
45 2.06 \.13 36.52 0.218 0.613 0.14 0.840
Thermodynamic parameters
The change in standard free energy may be calcu- lated using the following relationship
dG=-Rl1nK ... (2)
where K is the equilibrium constant and can be determ ined as
A+dye A dye (surface complex)
where A is the adsorbent.
Thus
K=concentration of dye present on surface remaining concentration in solution
The change in enthalpy was determined from the slope ofl inear plot of logKversus liT (Fig.7) and the entropy by,
~=AH-AG
T
The values for dG, ~ and Ware given in Table 5.
The negative values of dG are indicative of a spontaneous process with a high affinity of the dye to the surface ofthe adsorbent. Exothennicity may be explained on the basis of negative values of the enthalpy change (W) in the adsorption of anionic dyes on the neutral alumina surfaces. A negative entropy change (~ found in the present study may
"
:r
NO
;c., 3
!!••
u 2
Fig.6 - Plot of Cdqe versus Ce for the adsorption of dyes onto neutral alumina at 30°C
-1.2r---.
],)0 l·25 no rrs 310 I/T X10l.K·1
Fig.7 - Plot of log K(equilibrium constant) versus liT for dyes [e Acid Orange 6; 6 Ethyl Orange; • Acid Orange 7;®Acid Red 88; 0Acid Blue 113]
be understood in terms of restriction of the movement of molecules to two dimensions on the surface, as against three dimensions in the bulk. Inother words, a decrease in entropy change is indicative of the decrease in the randomness of the system 17.
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