Sorption—desorption studies of anionic dyes on alumina pretreated with acids

Sorption—desorption studies of anionic dyes on alumina pretreated with acids

Colloids and Surfaces, 29 (1988) 373-389 Elsevier Science Publishers B.V., Amsterdam 373 - Printed in The Netherlands Sorption-Desorption Studies ...

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Colloids and Surfaces, 29 (1988) 373-389 Elsevier Science Publishers B.V., Amsterdam

373 -

Printed

in The Netherlands

Sorption-Desorption Studies of Anionic Dyes on Alumina Pretreated with Acids V.K. JAIN*, G.L. MUNDHARA

and J.S. TIWARI

Department of Chemistry, Rauishankar University, Raipur, Madhya Pradesh 492010 (India) (Received 3 December

1986; accepted in final form 31 August 1987)

ABSTRACT Sorption-desorption behaviour of the anionic dyes naphthol blue-black (NB) and lissamine green ‘BN’ (LG) on chromatographic alumina, pretreated with mineral acids, is described. Alumina samples of surface-phase pH 1.0-8.0 were prepared and studied for their sorption behaviour. The sorption was found to vary with surface pH of the substrate and acid used for pretreatment. Quantitative sorption was shown at pH 6 4.0 (NB) and pH < 3.5 (LG) on A1203 treated with HNO, [A&O,(n) 1, and maximum sorption occurred at pH 5.0 (NB) and pH 5.5 (LG) on Al,O, treated with H,SO, [ Al,O,(s) ] and pH 2.5 (NB) and pH 3.0 (LG) on A&O, treated with H,PO, [ A&O3 (p)] . Variation in sorption with time (10 min-72 h), temperature (30-60°C) and regeneration of the substrates with aqueous electrolytes is also reported. Desorption efficacy of anions was in the order: PO!- > SOi- > NO,. The acid-treatment, and hence the specifically adsorbed anions (NO;, SO:-, PO:- ) , appears to modify the sorption properties of alumina significantly. It appears that the controlling force for adsorption is predominantly ion exchange. A few synthetic mixtures of the dyes were separated by column chromatography, using inorganic electrolytes as eluents.

1. INTRODUCTION

Separation techniques based on the phenomenon of adsorption have been extensively employed in many fields of research and technology. However, chemical pretreatment of the adsorbent layer, which would enhance or decrease the capacity for differential separation, appears to be a worthwhile area of investigation. Chromatographic alumina possesses a high concentration of surface hydroxyl ions [ 11, and on chemical pretreatment, its ion-exchange properties are enhanced [ 2-71. Anderson and Malotky [ 81 pointed out that H+ ion concentration and specifically adsorbed anions (e.g., SO:-, PO:- ) modify the electrokinetic behaviour of the adsorbent, and may provide new surface groups, which like the original surface groups, may undergo a protolysis reaction. The adsorbed protolysable anions can perturb the structure of the *Present

address: Geology and Mining Department,

0166-6622/88/$03.50

Raipur, Madhya Pradesh,

0 1988 Elsevier Science Publishers

B.V.

India.

374

electrokinetic double layer. It was observed [ 81 that the anions lower the isoelectric point or raise the zero point of charge, by as much as four pH units. At a fixed pH, the zeta potential is a function of protolysable anion adsorption, or alternatively, for a fixed degree of anion adsorption, the zeta potential is a function of pH. Therefore, both species, which are potential determinants, may have a role in the selective adsorption of ionic compounds, like dyes, on polar adsorbents. The studies conducted by Fuller [ 91, Jiratova and Beranek [ lo], Singh and Clifford [ 111 emphasised the importance of pretreatment of polar adsorbents with acids. In our earlier communication [ 121, the sorption-desorption behaviour of the anionic dye Orange II (OG) on alumina pretreated with acids (HNO,, H,SO, and H,PO,) was described. To elaborate upon the work, two other anionic dyes, naphthol blue-black (NB) (Michrome No. 1113) and lissamine green ‘BN’ (LG) (C.I. 44090)) were studied. Distribution coefficients for the three dyes and their column chromatographic separation, based on these studies, were also investigated. 2. EXPERIMENTAL

DETAILS

HNO,/H,SO, and H,PO1-treated alumina samples of surface-phase pH 3.5-8.5 and pH 4.0-8.0, respectively, were prepared by washing the original Brockmann alumina (100 g, 100-200 mesh) with acids of different strengths (Table 1). Surface area, OH- ion-exchange capacity and anion content (NO; /SO:- /PO:- ) of the different samples were determined (Table 2). The dyes NB (E. Merck) and LG (Chroma) were used after repeated crystallisation from ethanol-water mixture (1:l v/v) till the absorbance was constant, and chromatography showed the presence of only one component. The purified samples were dried at 80°C for 24 h to remove moisture. The II,,, values for NB and LG were determined to be 620 and 629 nm, respectively. Stock solutions of each dye were prepared in water (0.57 mmol 1-l ) by dissolving 0.3525 and 0.3297 g 1-l of NB and LG, respectively. From the stock solution, 2.5-80.0 ml aliquots were withdrawn, made up to 100 ml, and estimated calorimetrically after suitable dilution. A ‘Spekol’ spectrocolorimeter (Carl Zeiss, accuracy ? 0.5% ) was employed for estimation of the dyes. Absorbance of the dyes is not affected by change in pH (1.0-8.5)) the acid used for the pretreatment or elevated temperature ( 30-60 oC ) . Adsorption experiments were conducted by equilibrating alumina (0.1 g) with aqueous dye solutions (10 ml) of known concentration. Desorption was studied with aqueous inorganic electrolytes. The procedure for desorption was to equilibrate (24 h, 30 + 1 “C) the adsorbate (10 ml, 0.57 mmol 1-l) with the substrate (0.1 g) , reduce the volume of the solution to 5 ml, followed by addition of the desorbing agent (5 ml). After 30 min, a 5-ml aliquot was withdrawn

8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8

8.8

8.8 8.8 8.8 8.8 8.8 8.8 8.8 6.0 6.0 5.0 5.0 4.0 8.8 8.8 8.8 8.8 8.8 8.8 8.8 5.0 5.0 -

0.010 0.0125 0.015 0.015 0.175 0.175 0.020 0.025 0.040 0.050 0.075 0.10

0.025 0.030 0.05 0.075 0.075 0.10 0.10 0.050 0.050 0.10 0.50 2.00 0.10 0.10 0.20 0.30 0.35 0.40 0.50 0.20 0.80 -

-

H,PO,

50 20 50 20 50 50 50 50 50 50 50

50

50 50 50 50 100 50 100 50 100 50 100 300

H,SO,

3.5 4.0 4.5 4.8 5.0 6.0 7.0 8.0

pH of the alumina

90 93 96 96 91 94 92

93

AMb(n)

AI&(P) 70 73 74 67 73 -

~l,O:~(s)

105 105 107 103 107 113 109

Surface area ( S k 5 m* g - ’ )

0.182 0.161 0.095 0.029 0.028 0.019 0.013 0.006

403(n)

0.253 0.251 0.206 0.193 0.142 0.103 0.094

Al&(s)

0.821 0.513 0.333 0.204 0.121 -

40, (P)

Anion content of the sample (mg-ion g- ’ )

8.6 8.4 8.0 7.5 7.2 6.8 6.0 5.5 5.2 5.0 4.5 4.0

HNOB

8.6 8.4 8.2 7.8 7.3 6.7 6.2 5.7 5.2 4.7 4.2 3.7

H,SO,

8.2 7.4 6.9 6.4 5.9 5.4 4.9 4.4 3.9 -

H,PO,

pH of the wash liquid

0.72 0.58 0.50 0.44 0.42 0.40 0.37 0.30

403 ( n 1

-

1.04 0.98 0.92 0.75

1.73 1.39 1.17

&On(s)

0.93 0.28 0.16 0.07 0.04 -

AMA (P)

Ion-exchange capacity (meq gg ’ )

2 5 8 10 12 15 18 5 18 -

-

H,PO;

HNO,

H&Q

HNO,

AI&(P)

AI& ( n )

AI&k(s)

Time of contact during shaking (min)

Strength of acid added (200 ml) for pretreatment (N)

Initial pH of the alumina (100 g)

Surface area, anion content and hydroxide ion-exchange capacity of alumina samples of various pH

TABLE 2

“Hours.

8.5 8.2 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.8 4.5 4.0 3.5

Surface-phase pH of the prepared alumina

Synthesis of alumina samples of various surface-phase pH

TABLE 1

376

and estimated. The solution ment after 30 and 60 min.

left in the flask was again given the same treat-

3.RESULTS

3.1. Variation in adsorption with time The variation in adsorption with time (10 min-72 h) and pH was examined at 30 5 lo C. The process was fast on A1203 (n). Thus, at pH 4.0 and with lomin contact, quantitative adsorption occurred. However, the rate of adsorption appeared to decrease with increase in pH (4.0-8.0). The equilibrium was established in 2 h. The adsorption on A&O3 (s) and Al,O,( p) seemed to be a relatively slow process. The equilibrium period was attained after 6 h [A12G3(s)l and24h [4&(~)1. 3.2. Variation of adsorption with temperature Studies on the variation of adsorption with temperature ( 30-60 + 0.1 ‘C, 6 h) showed that with A&O3 (n) of pH 4.0-8.0, the process was almost athermic for NB and exothermic for LG. The isosteric heats of adsorption (Q) were calculated using the Claussius-Clapeyron equation, Q ( isosteric)

= -p

RT,T, T 1 -T

C, 2

%

2

For the same amount of dye adsorbed, concentrations ( C1, C,) , coresponding to different temperatures ( T1, T,),were determined from the isotherms (not shown) and then computed to calculate Q. The Q values of LG were - 3 to - 9 kcal mol-‘. Exothermic nature of adsorption was also shown by the dyes on Al,O,( s) (pH 2.0-8.0, 6 h) and Al,O, (p) (pH 2.0-6.0, 6 h). The Q values werelow (-3to -24kcalmol-lforNBand-4to -1Okcalmoll’forLG). 3.3 Adsorption as a function of pH Changes in the nature of adsorption with pH ( 30 -t 1 oC, 24 h) are illustrated in Figs l-6. It was observed that with A1203 (n),the affinity of the dyes for the substrate increases gradually with decrease in pH. Quantitative adsorption occurred at pH < 4.0 (NB) and pH 6 3.5 (LG) , and it dropped to zero at pH 8.8 for NB, and was very low in the case of LG. The adsorption on A1203 (s) increased in the pH range 1.0-5.0 (NB) and 2.0-5.5 (LG) and then decreased in the pH range 5.5-8.8 (NB) and 6.0-8.8 (LG) . The maximum adsorption took place at pH 5.0 ( NB) and 5.5 (LG). For A1203 (p), the adsorption increased in the pH range 1.0-2.5 (NB) and 2.0-3.0 (LG) and then decreased with increase in pH and dropped to zero at pH 6.5 (NB ) and 7.0 (LG) . Max-

I

I

I

IO

I

I

20 -End

concn

I 30

( moles/

I

I LO

I 50

litre)X lo5

Fig. 1. Change in the nature of adsorption of naphthol blue-black from aqueous solutions on HNO,treated alumina of varying pH. (Time, 24 h; temperature, 30 + 1 “C.)

imum adsorption was thus observed at pH 2.5 (NB) and 3.0 (LG). It was noted that at pH < 4.0 andpH > 8.0 for NB andpH 54.5 (LG), the adsorption was in the order: A1203 (n ) > A1203 ( s ) > A&O, ( p 1, which, however, changes to A1203 (s I> A&O, ( n ) > A1203 ( p ) at pH 4.5-7.5 ( NB) and pH > 5.0 for LG. The isotherms observed were of H, L and S type (Figs l-6 ) .

-End

concn (moles/litre

)X105

Fig. 2. Change in the nature of adsorption of lissamine green ‘BN’ from aqueous solutions on HNO,,-treated alumina of varying pH. (Time, 24 h; temperature, 30 ?I 1 “C.)

3.4. Adsorption as a function of pH in presence of electrolytes Adsorption of the dyes in the presence of electrolytes ( NaNO,, Na2S04 and Na,PO,) ( 30 k lo, 24 h) was investigated using identical dye and electrolyte concentrations, i.e., 0.57 mmol 1-l. The data are shown in Table 3. Sorption was observed to decrease significantly, and the order of retarding influence was: PO;- > SO:- > NO,. The three types of alumina did not show any affinity for the solutes at pH > 5.0 ( NB ) and pH 3 5.0 ( LG ) [ A1203( n ) ] , pH Z 6.0 [Al,O,(s)] andpH >4.0 (NB) andpH 24.0 (LG) [A1203(p)],withNaaP04 present in the adsorption bath. In addition, it was observed that in the presence of the electrolytes, the optimum pH for adsorption remained the same.

379 60

pti=5,0 pH=L.0 50

t

P

LO

3 E

30

I

,.

pH=7.0

20

IO

l

End

concn

(moles/

litrel

x lo5

Fig. 3. Change in the nature of adsorption of naphthol blue-black from aqueous solutions on H,SO,treated alumina of varying pH. (Time, 24 h; temperature, 30 fr 1°C.)

3.5. Reversibility of adsorption The dyes adsorbed on the substrates [ A&O3 (n ) and A&O3( s) of pH 2.0-8.0 andAl,O,(p) ofpH 1.0-5.0 (NB) and2.0-5.0 (LG)] at30-tl”C (24h) were selected for desorption studies. Aqueous solutions of NaN03, Na2S04 and Na,PO, of varying concentrations (0.1 to 1 x 10m4M) were tested as desor-

PH =6.5

----+End

concn (moles/litre)X

IO5

Fig. 4. Change in the nature of adsorption of l&amine green ‘BN’ from aqueous solutions on H,SO,-treated alumina of varying pH. (Time, 24 h; temperature, 30 f 1 “C.)

bents. The observations (Table 4) reveal that the adsorption is quite reversible. Desorption efficacy of the anions was in the order: PO:- > SOi- > NO,, and it increased with increasing concentration of the electrolytes. The extent of desorption was found to depend on pH of the substrate as well as the acid used for the pretreatment. Thus, desorption increased with increase in pH [ A&O3( n ) ] ( pH 2.0-8.0 1. However, with A1203( s ) and A&O3 ( p ) it decreased [ pH 2.0-5.0 for A&O,(s) and pH 1.0-2.5 ( NB ) and pH 2.0-3.0 ( LG) in the case of A1203(p) 1. Desorption was found to increase at pH 5.0-8.0 for A&O3(s) and pH > 2.5 (NB) and pH >3.0 (LG) in the case of A&,0,(p). Thus, desorption was minimum at pH 5.0 (NB and LG) and pH 2.5 (NB) and pH 3.0

381

,

I -End

concn

(moles/litre

I X 10'

Fig. 5. Change in the nature of adsorption of naphthol blue-black from aqueous solutions on H,PO,treated alumina of varying pH. (Time, 24 h; temperature, 30 ? lo C. ) ( LG ) for A1203 ( s ) and A&O3 ( p) , respectively. It was observed that degree of desorption was in the order: A1204 ( n) > A1203 ( s) > A1203 ( p ) . 4. DISCUSSION

4.1. pH and sorption

The apparent ion-exchange nature of alumina is due to amphoteric of the hydroxyl groups, as shown below:

reactions

382

0 0

16 _

End

2L wncn

32

Imoles/lltrel

LO

X IO5

Fig. 6. Change in the nature of adsorption of lissamine green ‘BN’ from aqueous solutions H,,PO,-treated alumina of varying pH. (Time, 24 h; temperature, 30 f 1 “C.)

on

TABLE 3 Percentage decrease in the adsorption of anionic dyes (naphthol blue-black and lissamine green ‘BN’) from aqueous solutions in the presence of electrolytes on acid-treated alumina samples of varying pH” pH of Al,%(n) alumina NaNO,, ~~ NB 2.0 3.0 4.0

5.0 6.0 7.0 8.0

LG

Al&(s)

AM&(P)

Na,SO,

Na,PO,

NaNO, -~

Na$O,

Na,PO, ~~~

NaN03

Na2S0,

Na,PO,

NB

NB

NB

NB

LG

NB

NB

NB

NB

30.5 54.9 27.3

48.3 71.4 1.7 26.7 10.0 48.1 54.7 74.0 2.6 27.3 9.7 55.6 50.1 87.9 68.6 71.4 44.2 75.0 73.1 91.6 93.4 NA

LG

LG

3.7 3.0 9.4 31.6 23.0 59.2 1.1 15.5 7.1 33.3 43.8 49.0 95.7 0.9 25.4 10.7 57.3 54.4 NA NA 0.7 21.0 10.7 60.7 67.5 NA NA 0.6 11.1 13.9 56.8 61.4 NA NA NA 3.1 6.8 58.1 58.1 NA NA 0.9

“Temperature, 30f 1°C; time, 24 h; [dye],=057 adsorption.

LG

7.3 7.7 15.4 16.4 11.2 12.3

NA 10.6 14.0 32.1 NA 8.1 25.9 38.7 NA 14.5 39.4 54.5 NA mmol l-l,

LG

LG

NA 32.5 72.0 NA NA NA -

[electrolyte],=0.57

LG

NA NA -

NA -

mmol 1-l; NA, no

LG

NA -

383 TABLE 4 Percentage desorption of anionic dyes (naphthol blue-black and lissamine green ‘BN’) adsorbed on different acid-treated alumina samples by electrolytes (0.1 M)” pH of AlzGr Cd alumina NaNOzI -----NB

LG

ALO, ( 8)

Al&:,(p)

Na,SO,

Na:*PO,b NaNO,

Na,SO,

Na,PO,

NaNO,,

Na,SO,

Na,PG,

NB LG

NB LG NB

NB

NB LG

NB

LG

NB

LG

NB LG

85.8 74.3 80.1 86.9 QD -

36.7 71.6 QD -

QD QD QD QD QD -

QD QD QD -

LG

2.0 2.5

6.6 5.2 QD QD QD QD 30.4 32.5 QD _ _ _ _ _ _ _ _ _

4.0 5.0 6.0 7.0 8.0

12.5 25.0 38.2 52.1 54.0

10.0 28.1 31.1 39.1 42.9

QD QD QD QD QD

QD QD QD QD QD

QD QD QD QD QD

“Temperature: 30+ 1°C; [dye],=0.57 “0.01 M. >Al

-OH

-

+%

LG

QD 10.3 21.9 95.2 QD 6.2 5.8 93.0 QD 7.9 5.3 94.1 QD 8.7 12.8 96.2 QD 17.4 18.2 QD

86.3 QD 87.6 _ _ _ 24.3 76.0 QD 80.7 37.2 67.8 QD 75.7 75.8 61.9 QD 70.8 72.9 QD 86.7 86.7 QD 88.9 -

QD QD QD QD QD -

mmol I-‘; QD, quantitative desorption.

>Al+

+

OH-

(onion

exchange)

>AIO-

+

H+

(cation

exchange)

Pretreatment of the substrate with an acid ( H+X- ) may cover the surface with the acid anions, and these anions may bind to the surface in different ways, depending upon the nature of anions. >A1

-

X

+

H,O

The network of the oxide may react or coordinate with H+ ions of the acid ( HN03, H&SO,, H,PO,) incorporating the diffusible anions (NO,, SOi- or PO:- ) . Thus, anion-exchange behaviour is imparted to the surface when the concentration of the acid increases, the surface acquires increasing positive charge, and so more diffusible anions enter. This is shown by an increase in the amount of the anions on the pretreated samples (Table 2). Thus, sorption of anionic species is likely to be enhanced with decrease in pH of the substrate surface. The ion-exchange nature of adsorption is also indicated by the fast adsorption, low values of isosteric heat of adsorption, retardation of sorption in the presence of electrolytes and the desorption characteristics. However, a mechanism involving the formation of covalent bonds, especially at lower pH values, cannot be ruled out. This is indicated by the small effect of the electrolytes on the degree of sorption at lower surface pH values. In

384

addition, specifically adsorbed iour quite significantly.

ions (X- ) may modify the adsorption

behav-

4.2. Structural characteristics and adsorption When the structural characteristics of the dyes are considered, a few important indications can be made regarding their adsorption behaviour. The general rules defined by Roosens [ 131 and Cummings et al. [ 141 may be of help in interpreting the observations. OG is a monosulphonate mono-azo dye, while NB belongs to the disulphonate-diazo group of dyes. LG has also a disulphonate group but lacks the azo group. The coverage factor [ 151 of the dyes, as calculated from the relationship, C.F. = 1.2 x 10m7 (ionic wt) 2.g3,was 2.8, 13.0 and 14.3 for OG, LG and NB, respectively. So, NB anions have greater affinity as compared to the other two dyes, which is reflected in terms of their Kd values. Distribution coefficients (I&) for adsorption were calculated using the relation: K =Amount d

of the solute on solid Weight of solid

Amount of the solute in solution I

Volume of solution

The Kd was determined by batch experiment. A few Kd values utilised for chromatographic separations are shown in Table 5. In the pH range 4.0-7.0, NB showed a greater affinity for the substrates in comparison to LG and OG. The lesser affinity of LG as compared to NB and OG (pH 6 7.0) may be due to the absence of an azo group in its molecule [ 131, and also its relatively weaker acidic character, while its greater affinity in the alkaline pH range may be ascribed to its weak amphoteric character. 4.3. Aggregation of dyes Anionic dyes are known to aggregate in the form of charged micelles, when adsorbed on alumina [ 161. The concentration of the dyes needed to obtain a close-packed monolayer (Am) can be computed in two ways: (i) using the formula [ 171: S=AmXiVXaX

10v2’

where S is the surface area of the solid, N is Avogardro’s number, a is the projection area of the dye and the value determined is denoted by (Am) s; (ii) from the linear plots of l/ (B -x) versus m/x, as suggested by Mathews [ 181, where B is the number of moles of solute present in a volume V (ml) which have been equilibrated with m (g) of the adsorbent and x moles are adsorbed (leaving behind B-x moles in the same volume V) . The latter is denoted by (Am) L. This evaluation gives us an idea of the most probable monolayer ca-

TABLE 5 Separation of anionic dyes (orange II, naphthol blue-black and lissamine green ‘BN’) on acidtreated ahmina” Sepn Constituents pH of No of mixture alumina and its height (cm) Nitric acid-treated alumina 1 OG pH 5.5 NB 12 and (20x1 cm LG column) 2 OG pH 6.0, and 6 NB 3 OG pH 7.0 and 7 LG 4 LG pH 5.0 and 7 NB Sulphuric acid-treated alumina 5 OG pH 5.0 and 7 NB 6 OG pH 6.0 and 8 LG 7 LG pH 4.0, and 9 NB

Kd values Adsorption x 102 (ml g-‘) 1.24 2.18 11.36 1.02

Eluent used Dye eluted (Concn. (M) and Desorption volume (ml)) x 10-2 (g ml-‘)

Percentage recovery

17.52b 13.29’ 0.56b 4.65 0.28b 2.73 0.50

OG

NaNO, (0.1,50)

99.7

LG

Na,PO, (0.001,25)

99.5

NB OG

Na,PO, (0.01,30) NaNO, (0.1,50)

99.4 99.7

7.93 0.26

NB OG

Na2S0, (0.1,30) NaNO, (0.1,50)

99.5

QD”

0.71 2.57

0.26 0.41

LG LG

Na,PO, (0.1,30) NaN03 (0.1,50)

99.6

21.19

0.26

NB

Na2S0, (0.1,30)

99.5

6.70

0.68

OG

Na,PO, (0.001,50)

99.6

QA’ 4.55

0.27 4.41

NB OG

Na,PO, (0.01,25) Na,PO, (0.001,45)

99.6 99.6

7.76 1.97

0.79 2.29

LG LG

Na,PO, (0.01,25) Na,SO, (0.01,50)

99.7

33.64

0.48

NB

Na,PO, (0.01,35)

99.6

99.8

99.6

99.4

“Column, quick fit glass column (10 X 1 cm) ; mixture for separation, 5.0 ml aqueous solution of each dye ( 0.23 mm01 1-l ) ; time of contact before elution 1 h [ A1203( n ) ] and 2 h [ Al,O, (s ) ] . bValuesare for the first eluent used in the separation. ‘Values are for the second eluent used in the separation. dQuantitative desorption. *Quantitative adsorption.

pacity(Am).

(x/m),, is the maximum amount of the dye adsorbed. The linear plots and the (x/m) mBIL values were obtained through equilibrium adsorption studies conducted in the concentration range 2.28-0.57 mmol 1-l. One of the representative figures of the linear plots is shown in Fig. 7.

Q

pH.C.8 0

1.0

pH.5.0

.

pH=6.0

.

1.6

pH:

I.6

1.2

0.0

04

0.2

0 0

0 P m/x

Fig. 7. Linear plots of l/(B-z) HNO,,-treated alumina.)

versus

(gram/ m/x

mole

1 x IO3

in liquid-solid

system. (L&amine

green ‘BN’ on

In Table 6, the monolayer capacity (Am) L, and (Am) s are reported along with (x/m ) maxvalues. Upon analysis of the results it is seen that: (a) In the concentration range studied, (x/m) maxvalues are generally close to the limiting (Am), values. (b ) In case of A1203(n) (OG and NB, pH z 4.5; LG, pH > 6.0)) the actual amount of the dyes adsorbed and the (Am) L values were lower than the (Am ) s values, indicating adsorption of single dye anions. The same behaviour was observed for A1203(s) (OG and NB, pH > 5.5) and A1203(p) (OG and NB, pH a3.5). (c) With A1203(s) the (x/m) maxand (Am) L values were much higher than the calculated (Am) s values, indicating aggregation of the anionic dyes, during their adsorption. The aggregation of dye anions in the form of charged micelles

-

-I

NB

LG

43.48 40.00 34.48 _ 23.26

66.67 62.50 51.28 _ 44.44

70.07 70.07 68.56 70.82

LG

NB

.-

-

-

.

,,,

--

.

_

LG

61.80 61.80 113.03 95.34 123.46 107.53 96.86 105.79 104.87 125.00 60.47 100.56 116.28 _ 65.26 87.67 67.57 111.11 62.46 45.20 86.05 51.28 105.26

.

NB

LG

77.60 87.39 80.61 85.13

68.44 77.08 71.10 71.10 75.09

110.54 77.69 121.65 102.04 78.35 69.08 111.12 125.00 78.35 _

NB

NB

(AmJL LG

NB

(AmJs LG

12.66 19.61 55.75 49.17

44.09 277.18 52.82 57.14 66.52 50.31 96.15 58.82 52.74 46.52 41.15 48.54 47.62 52.74 46.52 23.17 26.32 27.18 52.74 46.56 _ _ _ _ _

LG

12.06 18.13

154.60 81.53 38.35 24.13 _

NB

( x/mJ max

(AmJs

(x/~Jrn,x

(Am),

H,PO,

HP%

“All values are in moles per gram times 106. b(AmJL values computed from linear plots connecting l/(B -x) and m/x. ‘(AmJs values computed from surface area values of the samples.

57.61 52.42 46.98 36.81

40.43 37.78 31.12 _ 20.00

LG

4.8 5.0 5.5 6.0 6.5 7.0

NB

(AmI sc

82.00 85.46 84.03 88.50 67.81 59.81 60.90 72.21 64.10 78.13 67.81 59.81

LG

(Am)nb

2.5 3.0 3.5 4.0 4.5

NB

pH of HNO, alumina wmkn,x

Computation data of Am (monolayer capacity) of anionic dyes (naphthol blue black and &amine green ‘BN’) on acid-treated alumina samples of various pH”

TABLE 6

388

is also possible for the dyes on A1203( n) (OG and NB, pH ~4.0, LG, pH < 5.0). The dyes also show higher adsorption than that indicated by the (Am) s values on A1203(p) (OG and NB, pH d 3.0; LG, pH < 3.5). (d) In the pH range 4.0-5.0, the adsorption on A1203( n) may be micellar in nature and (Am) L values were in the order: NB < LG < OG, which is the inverse order of basicity. (e) However, in the pH range 4.0-5.0 [ A1,03 (s) ] and 3.0-4.0 [A&O,(p) 1, the adsorption appears to be monodispersed and (Am) L values follow the order LG < NB < OG, which is the inverse order for cross-sectional area. The values for cross-sectional area determined for flat-orientation of the dye ions were 120,220.5 and 250 A” for OG, NB and LG, respectively. 5. CONCLUSION

The results of this investigation provide ample evidence that the ion-exchange capacity of alumina can be suitably promoted by its treatment with acids, e.g., HN03 and H,SO,. The acid treatment, and hence the adsorption of specific anions (NO,, SO:-, PO:- ) , lowers the isoelectric point, and thus, modifies the electrokinetic behaviour of the sorbent. In addition, displacement of the OH- ions from the surface by the anions may increase anion-exchange capacity; depending on the relative position of the anions in the ion-selectivity series, i.e., OH- > PO!- > SO:- > NO;. Thus, NO; ion can be easily exchanged by adsorbing dye anions. The H,SO, treatment appears to affect the substrate surface in a different manner. The pretreatment may lower the isoelectric point of alumina from 8.0 to pH N 5.0, as indicated by maximum adsorption at pH 5.0. As SOi- ions have greater affinity for the surface, they may influence the process quite considerably at pH > 5.0 (probably through a competitive effect) : At pH < 3.0, the competition between undissociated acid molecules and the product of the reaction between the dye and acid plays a prominent part, and this decreases adsorption of the solutes [ 191. The maximum affinity of the solutes for A1203(p ) at about pH 3.0 may be due to lowering of the isoelectric point (by as much as 5 pH units). The higher affinity of PO:- ions for surface may lower affinity of the solutes. Thus, as compared to HNO, treatment, more sites involved in specific interactions seem to be created by H,SO, and H3P04 treatment of alumina with the latter being more effective. These findings also have practical importance, as a greater proportion of the solutes is desorbed in the presence of other inorganic ions of higher basicity. Thus, synthetic mixtures of these dyes (binary and ternary) in aqueous solutions were separated by column chromatography on alumina of varying pH. The experimental conditions are summarised in Table 5. The difference in the values of Kd was the basis for such separations. K,, (distribution coefficient for desorption) values were calculated according to the relation:

389

K

= D

Amount

of solute in solution

Volume of the solution

Amount of solute in the solid Weight of solid I

ACKNOWLEDGEMENTS

We are thankful to Dr S.G. Tandon, Professor and Head of the Department and Dr S.S. Dave, Director, Geology and Mining, M.P., for the encouragement and provision of facilities.

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