sorption-desorption studies on alumina pretreated with acids: i. the anionic dye orange ii

Surface Technology, 26 (1985) 295 - 314 295

SORPTION-DES()RPTION STUDIES ON ALUMINA PRETREATED WITH ACIDS: I. THE ANIONIC DYE ORANGE II*

V. K. JAIN t , G. L. MUNDHARA and RAMESH K. MISHRA

Department of Chemistry, Ravishankar University, Raipur, Madhya Pradesh 492010 (India)

J. S. TIWARI

Government Girls College, Rajput, Madhya Pradesh (India)

(Received January 7, 1985)

Summary

Brockmann alumina was subjected to chemical pretreatment with HNO3, H2SO4 and H3PO4 of various concentrations, and samples of surface phase pH 3.5 - 8.5 (HNO3A1203(n), H2SO4-treated A1203(s)) and pH 4.0 - 8.0 (H3PO4-treated A1203(p)) were prepared. Sorption-desorption behaviour of the anionic dye Orange II (C.I.15510) with change in pH on these substrates was studied. Quantitative sorption is shown at pH ~< 4.0 (A1203(n)) , and a maximum is observed at pH 5.0 (A1203(s)) and pH 3.0 (A1203(p)). Varia- tion in the amount of sorption with time (10 min - 72 h), temperature (30 - 60 °C) and regeneration of the substrates with aqueous inorganic electrolytes is also reported. Desorption efficacy of the anions is in the order PO43- SO42-> NO3-. The acid treatment, and hence the specifically adsorbed anions (NO3-, SO42-, pO43-), appears to lower the isoelectric pH of alumina (pH 8.0). The results show the involvement of anion-exchange properties of the alumina.

1. Introduction

Chromatographic alumina normally possesses a high concentration of surface hydroxyl ions [1] and a few studies [2 - 7] have shown that chemical pretreatment of alumina promotes its ion-exchange property. Anderson and Malotky [8] have pointed out that H ÷ ion concentration and specifically adsorbed ions (e.g. SO42- and PO43-) modify the electrokinetic behaviour of the adsorbent, and these may provide new surface acidity groups, which like the original surface groups, may undergo a protolysis reaction. When adsorbed on the hydrous oxide surface, protolysable anions, such as phos-

*Presented at the Annual Convention of Chemists, Madras, 1981, Abstract No. ANAL-33.

tPresent address: Geology and Mining Department, Raipur, Madhya Pradesh, India.

0376-4583/85/$3.30 © Elsevier Sequoia/Printed in The Netherlands

296

phate and arsenate can generally perturb the structure of the electrical double layer [8]. Experimentally these anions have been found to lower the isoelectric pH or raise the zero-point of charge, by as much as four pH units. At a fixed pH, the e potential will be a function of protolysable anion adsorption, or alternatively, for a fixed degree of anion adsorption, the e potential will be a function of pH. In this sense, both species are potential determining. Possibly, they play an important role in the selective adsorption of ionic compounds on polar adsorbents.

Fuller [9] suggested that pretreatment of alumina with an acid, like HC1 or HNO3, frequently enhances its apparent anion-exchange capacity. He attributed the property to the ionization of hydroxyl groups contained in the structure of alumina as

~AI-<)H. " ~AI + + OH- (i)

~AI--OH + H +. "~AI-OH2 + (2)

It is clear that the reactions producing anion-exchange properties are favoured by low pH. The effect of this pretreatment, however, is not to convert the material to an ion exchanger, as was thought previously, but simply to convert the exchanger from the OH- form to C1- or NO3- forms. As these ions are usually much lower than the OH- ion in the anion-selectivity series of oxides and hydrous oxides, displacement of C1- or NO3- ions results in much more favourable equilibrium than the latter ion.

Jiratova and Beranek [10] studied the surface acidity and basicity of alumina to which selected ions were added. They reported that the number of acidic centres on alumina samples depends on the introduced ions in the order SO4 ~- > F- > PO43- > C1-. Singh and Clifford [11] found the following selectivity sequence on activated alumina: OH- > F- > SO42- > C1- > HCO3-.

Perusal of the literature, thus, reveals the importance of the pretreatment of alumina with acids. However, a close quantitative study of the sorption properties of alumina, pretreated with acids like nitric, sulphuric and phosphoric of various values of surface phase pH, has not been made. Hence, the present investigations were undertaken, and the present paper deals with the following aspects: (i) how far the different acid treatments of alumina affect its surface groups; (ii) the role of the specifically sorbed anions on the sorption behaviour of the anionic dye Orange II; (iii) a desorption study of the dye, adsorbed on alumina of various surface phase pH, with aqueous solutions of inorganic electrolytes.

2. Experimental details

2.1. Synthesis of alumina of various values of surface phase pH HNO3-treated alumina (A1203(n)) samples of surface phase pH 3.5 - 8.5

were prepared, as described previously [7]. Following the same general

297

method, H2SO4- and H3PO4-treated samples (A1203(s) and A1203(p)), of pH 3 . 5 - 8 . 5 and pH 4 . 0 - 8 . 0 respectively, were also prepared. In the present work, surface pH of alumina means the surface phase pH, which is measured as the bulk pH, when diluted with water to 1% equivalent aluminium oxide. The conditions for the synthesis of the substrate samples are reproduced in Tables 1 and 2. The surface areas of some of the samples were determined by the standard "cont inuous flow method" [12, 13]. Hydroxide-ion exchange capacity (Table 3) of the samples was determined by the pH titration method. The anions (NO3-, SO42-, and PO43-) present on the acid-treated samples of various values of pH were estimated [14 - 16] (Table 3).

2.2. Solute and estimation The dye Orange II (OG) C.I. 15510 was prepared in the laboratory

from diazotized sulphanilic acid and alkaline ~ naphthol [17]. The sample was subjected to repeated crystallization from ethanol:water (1:1 by volume}. The purified sample was dried at 80 °C for 24 h, and its homogeneity tested chromatographically. From the stock solution of the dye (0.2000 g 1-1) in water, 2.5 - 80 ml were withdrawn, the volume made up to 100 ml, and it was estimated colorimetrically at 485 nm, using a "Spekol" spectrocolorim- eter (Carl Zeiss, accuracy, +0.5%). Absorbance of the dye is not affected by change in pH (3.5 - 8.0) or the acid used for the pretreatment. However, the dye changes its colour at pH > 8.0, which could be restored by the addition of 2 ml of sodium acetate-acetic acid buffer of pH 4.5, before estimation. For studies below pH 3.5 the estimations were done in presence of 2 ml of 0.1 M sodium acetate solution. Adsorption experiments were carried out by equilibrating alumina (0.10 g) with dye solution of known concentration in 10 ml-volumetric flasks. The flasks were shaken thoroughly and placed in a thermostat at the desired temperature for various periods of time. The flasks were intermit tently shaken manually. After a fixed period of time, 5 ml aliquots were withdrawn from each flask, centrifuged, and the end concentrations estimated from 2.0 ml of this solution.

2.3. The variation in adsorption with time and temperature The variation in adsorption as a function of time (10 min - 72 h) and

pH was conducted at 30 + 1 °C. The process is fast on A1203(n ). Thus, in a 10 min contact period, 90% - 100% of the dye is adsorbed (pH 4.0), and 35% - 100% sorption occurs in the pH range 8.0 - 5.0, at a dye-concentration of 57.14 × 10 -s M 1-1. The equilibrium appears to be almost established in 2 h. The adsorption on A1203(s ) seems to be a relatively slow process. Thus, 60% - 75% adsorption is exhibited in the pH range 1.0 - 8.0 at the same dye concentration after 10 min. The equilibrium is almost attained in 6 h. 40% - 60% sorption is exhibited on AI203(p) at pH 1.0 - 6.0 in 10 min. It takes 24 h to reach equilibrium."

Studies of the variation with temperature in the range 3 0 - 6 0 °C (ac- curate to +0.1 °C) for a 6 h period show that with A1203(n ) of pH 4.0 - 8.0, the process is almost athermic, while for A1203(s ) (pH 1 .0 -8 .0 ) and

2 9 8

_=

~-~ .~

0

2~

~ ~.~

I I ~ ~ 1 ~ 1

~ ~ ~ I ~

i i ~ ~ i ~i

0 0 0 0 0 ~ 0 0 0 0 0 0

0 0 0 0 ~ 0 0 O 0

o~ooooo~o I ~o~

o I ~ o ~ o o o o o o o

I I ~ ~ I ~I

TABLE 2

Condi t ions for the p repara t ion of alumina samples of pI-I

299

pH o f the alumina after addition o f the acid

Strength o f the acid added (N) Volume o f the acid added (ml)

HNOa H2SO 4 HaPO4 HNO3 H2S04 H3P04

3.5 - - - - 0.01 - - - - 1.0 3.0 0.01 0.01 0.10 1.0 0.8 0.5 2.5 0.10 0.10 0.10 0.4 0.4 2.0 2.0 0.10 0.10 1.00 1.0 0.8 1.0 1.5 1.00 1.00 4.00 0.4 0.4 1.0 1.0 1.00 1.00 4.00 1.0 1.2 2.5

pH o f the alumina initially taken as 3.5 (0.1 g in 10 ml) excep t for H3PO4-treated alumina where the initial pH was taken as 4.

A12Oa(p) (pH 1.0 - 6.0), the adsorption is exothermic, with isosteric heat of adsorption values Q o f - 5 to - 1 1 kcal mo1-1 (Table 4).

2.4. Adsorption as a function of pH The results of a change in the nature of the adsorption with pH are

illustrated in Figs. 1 - 4. The process is found to depend on both the pH and the acid used for the pretreatment. Thus, with A12Oa(n ) the affinity of the dye gradually increases with decrease in the pH of the substrate. Quantitative adsorption occurs at pH < 4.0, and it drops to zero at pH 8.8. The isotherms are H-type (Fig. 1) at pH 4.8 - 6.0 and L-type at pH > 6.0. For A12Oa(s) and A12Oa(p), the adsorption increases in the pH range 1.0- 5.0 and 1.0- 3.0 respectively. The adsorption is found to decrease with increase in pH, and drops to zero at pH 8.8 and 6.5 for A12Oa(s) and Al2Oa(p) respectively. Maximum adsorption takes place at pH 5.0 (A12Oa(s)) and pH 3.0 (A12Oa(p)). For AlcOa(s) in the pH range 4.0 - 7.0 the isotherms are L-type (Fig. 2) at higher dye concentrations, with a tendency to assume an S-shape at lower concentrations. At pH < 3.5 and pH > 7.0, the isotherms are mostly S-shaped. S-type isotherms (Fig. 3) with a tendency to assume a linear nature (pH 2.5 and 2.0) and L-type isotherms (pH 3.0- 4.0) axe observed on A12Oa(p). It is also noted from Fig. 4 that at pH ~< 4.0 the adsorption is in the order A12Oa(n) > A12Oa(s) > A12Oa(p), which, however, changes to Al2Oa(s) > Al2Oa(n) > A12Oa(p) at pH I> 4.5.

2.5. Reversibility of adsorption The reversibility of the adsorption was studied in two ways: {i) keeping

the inorganic electrolytes with the dye and substrate and (ii) desorption of the loaded dye from the substrate with electrolytes.

For this purpose, adsorption studies (24 h, 30 + 1 °C) were made using the dye and electrolytes (NaNOa, Na2SO4 and NaaPO4) of the same concentration (57.14 × 10 -s M and 5.71 × 10 -s M). Substrates used in these studies

300

<

%

'T <D

N

; Ig

=

= 0 .o ..~

L

~ m

e~

e ~

odddd~do

}

% uO

+I

0 sO

II oO

oO

,.=

_=

o~ a)

301

# o

0

o

.o

o~ ~0

0

c~

0

0

0

%

× 1

~0

0

0

o

0 0

× 1

I I I I I I I I I I

I I I I I I I I I I

I l l l l l l l l

I I I I I I I I I

T ? ; J l l l ; ,

0

0

0

o z

2

N

302

SO

40

x

pH-4,S

pH = 4..8

Z0 ~ pH= 5'0

tO ~ PH:6.S pH#,7,C

0 - I I J I I I I I 10 20 30 40 50

-~ End cordon (motes / [Itre)x 10 5

Fig. i. Change in the nature of the adsorption of Orange II from aqueous solutions on HNO3-treated alumina with various values of pH (time, 24 h; temperature, 30 -+ 1 °C).

had values of pH in the ranges 2.0 - 8.0 for A1203(n ) and A1203(s ) and 1.0 - 5.0 for A1203(p). Sorption decreases significantly (Table 5) and the retarding influence is in the order PO43- > SO42- > NO3-.

Desorption studies of the dye in aqueous solutions ([OG]0 = 57.14 X 10 -s M and 5.71 )< 10 -s M), adsorbed on alumina A1203(n ) and A1203(s) of pH 2.0 - 8.0 and A1203(p) of pH 1.0 - 5.0 at 30 + 1 °C (24 h), were carried out with inorganic electrolytes (0.0001- 0.1 M, NaNO3, Na2SO4 and Na3PO4), as described previously [7]. The results (Table 6) reveal that the adsorption is quite reversible. Desorption efficacy of the anions is in the order PO43- > SO42- > NO3-, and it increases with increase in the concentration of the electrolytes. The extent of desorption depends on the pH of the substrate as well as the acid used for the pretreatment. Thus, desorption increases with increase in pH of AI203(n) (pH 2.0- 8.0). However, with A1203(s ) and A1203(p) it decreases (pH 2.0 - 5.0, pH 1.0 - 3.0), and then increases (pH 5.0 - 8.0, pH 3.0 - 5.0). Thus, the desorption is a minimum at pH

303

50

I~H ~5'0

bH'4"O p ~H=s.s

0 PH:3,$

% x

3o

o E

m 20 g o E

T t G

bH=3-0

J)H :7-0

pH,B.O

bH,~.2

0 ~ / ~ . . . - . ~ ; I I I t I J 0 10 20 ) 0 40

'rEnd conch (moles/IHre) X 10 $

Fig. 2. Change in the nature of the adsorption of Orange II from aqueous solution on H2804-treated alumina with various values of pH (time, 24 h; temperature, 30 ± 1 °C).

5.0 and pH 3.0 for A12Os(s ) and AI203(p) respectively. It is also evident (Table 6) that the amount of the dye desorbed is in the order A1203(n)> A1203(s) > AI203(p).

3. Results and discussion

3.1. Alumina and acid pretreatment Pretreatment of alumina with acid (HX) may cover the surface with the

acid anions, as shown below:

S ~AI--X + H20 (3) i

~AI'---OH + H+X - . , " ~ A 1 + IX- + H20 (4) L

>AI-OH ÷ i x- (5) [

304

Z,0

~H=3"0

D ~)H:2'0

% x 30

m

_= o E

~, 20 e .D

ul

E < 10

T

b H : 3 . 5

O ~)H =1'5

p H :~,'0

~H:I'O

I~H:5"0

0 10 20 ] 0 ~0 50

!P End concn (moles// [itre ) X 10 5

Fig. 3. Change in the nature of the adsorption of Orange II from aqueous solutions on H3PO4-treated alumina with various values of pH (time, 24 h; temperature, 30 + 1 °C).

The anions present on the surface may either be covalently bound or present as hydrated ions, depending upon the nature of the anions. The dye may be adsorbed on the substrate: (i) by ion exchange of the dye anions (single or micelles) and (ii) covalent bond formation with anionic groups of the dye (e.g. sulphonate group). It seems likely that both the mechanisms may operate, although each to a different extent, depending upon the history of the oxide surface, i.e. its surface pH, the acid used for pretreatment (and hence the anion present thereon) and the properties of the particular dye molecule. The experimental observations can now be considered.

The ion-exchange nature o f the adsorption is shown by (1) the athermic nature of the process (A1203(n)) and low values of Q

(--5 to --11 kcal mo1-1, A1203(s) and A1203(p)); (2) the significantly fast adsorption; and (3) the reversibility of the adsorption and the salt effect. In the presence of the electrolytes adsorption is retarded considerably,

and it appears that the action of the salts is one of repulsion between the dye anions and the inorganic ions at the surface of the substrates, and competi- tion between them for the ionic sites. However, the operation of an alterna-" tive covalent bond mechanism, especially at lower pH values, is shown by the relatively small effect of the electrolytes on the degree of adsorption and also by the decrease in the degree of adsorption at lower pH values, i.e. irreversible adsorption. It is also likely that after primary adsorption, the

305

o

100

80

60

20

0 HNO3-TREATED

• H2SO4-TREATE D A H 3PO4-TR EATED

&

/

/

I I I 4 6 8

pH of a[umina

Fig. 4. Change in the percentage adsorption of Orange II on (c)) HNO3-treated, (e) H2SO 4- t r ea t ed and (A) H3PO4-treated alumina with various pH of the alumina (24 h, 30 + 1 °C, [dye] 0-- 57.14 X 10-Smol 1-1).

solute may diffuse into the micropores of alumina, resulting in very slow desorption.

3.2. pH o f alumina, acid used for the treatment and adsorption 3.2.1. HNO3-treated alumina The adsorption of Orange II on A1203(n ) (Fig. 1) increases with de-

creasing pH and 100% adsorption is observed at pH < 4.0. At pH/> 4.5, the adsorption is pH dependent and the rate of NO3--dye anion exchange is directly proport ional to the concentration of the in-going ions. Thus, anion exchange appears to occur in two distinct stages: rapid exchange of the dye anions at sites on the surface, followed by a slow exchange at sites within the alumina. Since, the ionically bound nitrate content of the alumina (Table 3) appears to decrease continuously with increasing pH, the adsorption drops to zero at pH 8.8. Further, the observations can also be interpreted by the protonat ion and exchange reactions taking place at higher pH. The H ÷ ion of the ~ A 1 - - O H of the substrate at higher pH tends to dissociate from the oxygen, leaving a negative charge on the surface which will repel the dye anions, and so the adsorption gradually decreases.

306

0

0

o

<

~4

... % X

I

g

#

0 °~

I I I

I I I

0

~ x

!

r

x

m ~

~ ~ +l

0

m

C 0

0

0q.~.

t-~- ,.-4 t"-

%%

0 , ' ~

O 0 O 0

O 0 O 0

r-4

O 0 O 0

0

cO

q

O 0 O 0

O 0

O 0 O 0

0 0 0

0 0 0 0

0

0

g ~

P~

g~

307

308

~ A 1 - - O - +H+" ~ A I - - O H ° H÷ "--H + "--H: ~ A I - - O H : +

Surface at high pH Surface at low pH

(6)

3.2.2. H2SO4-treated alumina The results (Fig. 2) for adsorption of the dye on A12Oa(s) show that the

solute is essentially in the ionic form in its favourable pH range for adsorption (pH 3.0 - 6.0), pH 5.0 is the opt imum pH where adsorption is a maximum, although not 100%. At pH < 5.0, concentration of the sulphate ions on the substrate surface and also in the adsorption bath increases, and the ionic species responsible for adsorption decrease. At very low pH values of the surface (pH ~< 2.0), there may be competi t ion between the dye molecules and the undissociated acid (H2SO4), which results in a decrease in the adsorption [18]. Thus, Orange II has a low affinity in the pH range 1.0 - 2.0. At pH > 5.0, the proport ion of exchangeable sulphate decreases, resulting in a decrease in the adsorption.

3.2.3. HaPO4-treated alumina Adsorption of the dye on A1203(p) (Fig. 3) increases in the pH range

1.0 - 3.0, and then decreases at pH ~> 3.5. It drops almost to zero at pH 6.5. The dye has a maximum affinity for the substrate at pH 3.0. The process is concentration dependent. The mechanism by which aluminium oxide surface interacts with the phosphate ions is a matter of controversy. Deinemann and Helmy [19] suggested that phosphate ions chemisorb directly on the metal ions and the mechanism suggested is

~ A 1 - - O H + H2PO 4- . " ~A1- -HPO4- + H20 (7)

~ A 1 - - O H + HPO42- . " ~A1--PO42- + H20 (8)

Further, the charged-surface groups, such as --OH2 +, - -O- and un- charged =O, are not involved positively in the chemisorption reaction. Hence, phosphate ions present on the surface are mainly covalently bonded to the aluminium but a portion may also be held as loose hydrated ions. Under these circumstances, adsorption of the dye may mainly be due to covalent bond formation with the anionic group of the dye (sulphonate group). However, the possibility of ion exchange, especially at lower pH values, cannot be completely ruled out. Decrease in adsorption at pH < 3.0 can be interpreted, as in the case of A1203(s).

3.3. Isotherms The L-type isotherms indicate monolayer of the adsorbed species. From

the structural characteristics of the dye and L nature of the isotherms, a flat- orientation is likely to be most favoured. Some of the dye anions held by ion exchange appear to be associated in the form of charged micelles. H-type

309

curves indicate adsorption of ionic micelles of the dye on the charged surface. The L-type isotherms of the dye on Al203(n) at pH I> 6.5 show a change in the nature of the species being adsorbed. It is likely that in this pH range, the simple dye anion takes part in the adsorption process. However, S-type isotherms observed may be due to cooperative adsorption, and a large negative contribution of the solvent or a second solute, e.g. the undissociated acid (H2SO4 or H3PO4) , at lower pH values.

3.4. Aggregation Giles et al. [20] have shown that anionic dyes are aggregated when ad-

sorbed by ion exchange on chromatographic alumina. The aggregation of the dye anions, in the form of charged micelles, can be shown by an examination of the amount of the dye adsorbed [21 - 23]. The plateau in the adsorption isotherm is taken to represent the concentration of the dye in the solid, which is just sufficient to complete the close-packed monolayer {i.e. Am). In the present study, the turning points in the isotherms are not well defined except for the studies involving A1203(n) at certain pH values (pH 4.0 - 7.0). This may be due to the low dye concentration employed. Hence, Am values have been computed by two different methods.

(a) From the known values of the specific surface area S (m 2 g-l) of the alumina samples {Table 3) and using the relationship [24]

S-- AmNa × 10 -2° (1)

For fiat orientation, the projection area a of the dye ion is taken to be equal to 120 A 2. The value determined is shown as (Am)s.

(b) From the linear plots connecting mix and 1/(B - -x ) {Figs. 5 - 7), as suggested by Mathews [25], 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 same volume V (ml)). This has been shown as (Am)L in Table 7, in which the maximum amount of the dye adsorbed (x/m)ma~, is also recorded. The data presented in Table 7 reveal the following features. (i) In the concentration range studied (x/m)max values are generally close to the limiting values, (Am)L. The limiting values have been derived from linear plots {Figs. 5 - 7). This evaluation gives an idea of the most probable monolayer capacity Am. (ii) In the case of A1203(n) at pH ~> 4.5, the actual amount of Orange II adsorbed and the (Am)L values are generally lower than the (Am)s values. This may be due to adsorption of single dye anions. The same behaviour is shown on A1203(s ) (pH > 5.5) and A1203(p ) (pH ~> 3.5). (iii) The actual amount of the dye adsorbed on A1203(s), and hence, the (Am)L values are generally much higher than the theoretical (Am)s values, indicating aggregation of the anionic dye, during its adsorption. The aggregation of the dye anions, in the form of charged micelles, is also possible on A1203(n ) (pH ~< 4.0). The dye also shows higher adsorption than the (Am)s values on A1203(p) at pH ~< 3.0.

310

3.5. The effect o f acid treatment on the adsorption behaviour The cation exchange to anion exchange transition pH of the hydrous

aluminium oxide is pH 8.0 [26]. The acid treatment, and hence the specifically adsorbed anions (NO3- , SO42- and PO4 a-), has been found to lower the isoelectric pH, and thus, modify the electrokinetic hehaviour of the adsorbent. Further, displacement of the OH- ions from the surface by the anions may increase the anion~xchange capacity; depending on the relative position of the anions in the ion-selectivity series; i.e. O H - > P O 4 S - > 8042- > N O 3 - .

Since the NOa- ion is much the lowest in the ion-selectivity series, it has the least affinity for the alumina surface, and it can easily be exchanged by other in-going dye anions. This is shown by a regular increase in adsorption, with decrease in the pH of the substrate. HNOH treatment, thus, appears to convert alumina into a good anion exchanger. The H2SO 4 treatment appears to affect the surface of alumina in a different manner. The pre- t reatment may lower the isoelectric pH from 8.0 to a value in the vicinity of pH 5.0. This is indicated by the maximum adsorption of the dye at pH

2.0

1.8

1.6

1.4

-~ I.z

• ~ 1.o

0-4

0.2

:~l pH=E'Oz'8 /e

/ •

I

/

~ 0.4 ,

0 0-06

pH =7-O

pH:4.$

pH:&.0

I l | 0-0B 0-10 0.12 0.14 0-16

I I I I | o.~ o!~ 0',s o'.s i,o

m/x (g-ram/n~ole) xl0 3 Fig. 5. Linear plots of 1/(B - -x ) vs. m/x in the liquid-solid system Orange I ] on HNO3- treated alumina.

311

5.0. Further, SO42- ions have greater affinity for the surface and, hence, they may, by competitive effect, influence the process quite considerably at pH 5.0. At very low pH (pH < 3.0) competitive adsorption between undissociated acid molecules and the product of the reaction between the dye and acid may play a very important part in decreasing the adsorption affinity of the solute for the substrate [27]. Thus, as compared with HNOa treatment, more sites responsible for specific interactions seem to be created by H2SO4 treatment.

Phosphate, when adsorbed on the surface of the hydrous aluminium oxide, has been found to lower the isoelectric pH, by as much as four units [8]. Thus, the maximum affinity of the dye for A12Oa(p ) at pH 3.0 can be accounted. Further, since the PO4 a- ions have a high affinity for the alumina surface, it becomes difficult for the dye anions to replace them. This may be the reason for negligible affinity of the solutes at pH ~> 6.5. However, at pH ~ 3.0, competitive adsorption may play an important part in retarding adsorption of the dye. So, like H2SO4, H3PO4 may bring about the same type of changes but to a greater degree.

;I.O

10-0

9.0

7.0

%

x 6.0 _e

o 5-0

x

m 4 0

l ' 3-0

2.0

'°Io 0

I0.0 pH,5.0

9-0 pH=5.5

8'0

7.0

6-0

004 0.08

/ ': r./Y

0-12 0.16 0.20 0.24

m/x (gram/mole) x 103

pH=7"O

- - - L - - ~ 4 - - 1 - - - - - - 0.10 0 048

0.28

Fig. 6. Linear plots of 1 / ( B - - x ) vs. m / x for Orange II on H2SO4-treated alumina.

312

pH=3+0

4-0 O/e

3.G

3.2 pHI3,5 %

~ 2-8 2

x 2.~

• ! pH,&O 2.0

l 1.6

1.2 Q

0+B

0.4

0 0~5 0~0 O~S 0-20 0.25 0-30 0-35 0~0 O~S

m/x ( g r a m / m o L l ) x 1 0 3

Fig. 7. Linear plots of 1/(B - - x ) vs. m / x for Orange II on H3PO4-treated alumina.

4. Conclusion

It may be concluded that although the hydronium ion concentration plays an important role in the adsorption-desorption behaviour of the anionic dye, acid t reatment o f the alumina and hence the specifically adsorbed ions (e.g. NO3-, SO4 2- and PO4 3-) also modify the electrokinetic behaviour of the adsorbent. This, in turn, provides a different surface behaviour on a substrate like alumina.

Acknowledgments

We thank the authorities of the Directorate of Geology and Mining, Madhya Pradesh, Raipur and Dr. S. G. Tandon, Professor and Head of the Department, for the provision of research facilities.

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R e f e r e n c e s

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sorption-desorption studies on alumina pretreated with acids: i. the anionic dye orange ii

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