Scholarly article on topic 'Bengal gram seed husk as an adsorbent for the removal of dyes from aqueous solutions – Equilibrium studies'

Bengal gram seed husk as an adsorbent for the removal of dyes from aqueous solutions – Equilibrium studies Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — M.C. Somasekhara Reddy, V. Nirmala

Abstract Seed husk of Bengal gram (SHBG) (Scientific Name: Cicer arietinum) was used as an adsorbent for the removal of a direct dye namely Congo red (CR), two basic dyes namely methylene blue (MB) & rhodamine-B (RB) and an acidic dye namely acid blue 25 (AB) from aqueous solutions with equilibrium isotherms. The effect of particle size, of mass of adsorbent, of agitation speed of shaker and of temperature of dye solutions was studied for understanding the interaction of dyes with adsorbent. The uptake of dyes by the adsorbent was increasing with increasing mass of the adsorbent, decreasing with increasing size of the adsorbent, and increasing with increasing agitation speed and temperature. The adsorption capacity for each dye-adsorbent system was determined by using the Langmuir isotherm. The adsorption capacity of adsorbent for MB, RB, CR and AB was 333.33, 133.34, 78.12 and 5.56mgg−1, respectively. The experimental data fit well with the Langmuir and Freundlich models. The standard enthalpy change of adsorption for MB, RB, CR and AB was 13.5, 10.97, 4.01 and 6.72kJmol−1, respectively. The average standard entropy change of adsorption for MB, RB, CR and AB is 58.2957, 36.869, 13.2138 and −3.4179kJmol−1, respectively. The adsorption of two basic dyes is much higher than that of the direct and acid dyes.

Academic research paper on topic "Bengal gram seed husk as an adsorbent for the removal of dyes from aqueous solutions – Equilibrium studies"

Arabian Journal of Chemistry (2013) xxx, xxx-xxx

King Saud University Arabian Journal of Chemistry

www.ksu.edu.sa www.sciencedirect.com

SPECIAL ISSUE: ENVIRONMENTAL CHEMISTRY

Bengal gram seed husk as an adsorbent for the removal of dyes from aqueous solutions - Equilibrium studies

M.C. Somasekhara Reddy *, V. Nirmala

Department of Basic Sciences, G.P.R. Engg. College (Autonomous), Kurnool 518 007, AP, India Received 12 July 2013; accepted 1 September 2013

KEYWORDS

Bengal gram seed husk; Adsorbent; Color removal; Isotherm

Abstract Seed husk of Bengal gram (SHBG) (Scientific Name: Cicer arietinum) was used as an adsorbent for the removal of a direct dye namely Congo red (CR), two basic dyes namely methylene blue (MB) & rhodamine-B (RB) and an acidic dye namely acid blue 25 (AB) from aqueous solutions with equilibrium isotherms. The effect of particle size, of mass of adsorbent, of agitation speed of shaker and of temperature of dye solutions was studied for understanding the interaction of dyes with adsorbent. The uptake of dyes by the adsorbent was increasing with increasing mass of the adsorbent, decreasing with increasing size of the adsorbent, and increasing with increasing agitation speed and temperature. The adsorption capacity for each dye-adsorbent system was determined by using the Langmuir isotherm. The adsorption capacity of adsorbent for MB, RB, CR and AB was 333.33, 133.34, 78.12 and 5.56 mg g-1, respectively. The experimental data fit well with the Lang-muir and Freundlich models. The standard enthalpy change of adsorption for MB, RB, CR and AB was 13.5, 10.97, 4.01 and 6.72kJmol-1, respectively. The average standard entropy change of adsorption for MB, RB, CR and AB is 58.2957, 36.869, 13.2138 and -3.4179 kJmor1, respectively. The adsorption of two basic dyes is much higher than that of the direct and acid dyes.

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1. Introduction

Synthetic coloring substances are extensively used in industries like textile, pulp & paper, tanneries, photo printing etc. The present estimate shows that more than 10,000 different dyes

* Corresponding author. Mobile: +91 94410 34599. E-mail addresses: mcsr.gprec@gmail.com, som16@rediffmail.com (M.C. Somasekhara Reddy).

Peer review under responsibility of King Saud University.

are in use across the world and a total of about 7 x 1015 tones of dyes are manufactured annually. Azo dyes occupy the major portion of them. The azo dyes are carcinogenic as well as mutagenic (Khanna et al., 1991). Therefore, the dyes must be removed from the industrial effluents before letting it into the water streams. Not only that but stringent regulations on industrial discharges also attracts the implementation of efficient treatment methods. Hence the de-colorization of the effluents prior to their disposal is most essential. A number of conventional treatment methods like biological treatment, coagulation, membrane process, ion exchange etc. are available. However, these methods suffer many disadvantages. Therefore, searchfor efficient low-cost treatment methods is being done. In this direction, already in our laboratory and

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in other laboratories, some non conventional agricultural solid wastes like coconut husk (Gupta et al., 2010), potato peel (Guechi and Hamdaoui, 2011), jackfruit peel (Jayarajan et al., 2011), mango seed kernel powder (Vasanth Kumar and Kumaran, 2005), durian peel (Hameed and Hakimi, 2008), coir pith (Motiar et al., 2011), straw (Zhang et al.,

2011), sugar extracted spent rice biomass (Rehman et al.,

2012), pistachio hull waste (Moussavi and Khosravi, 2011), grass waste (Hameed, 2009a), spent tea leaves (Hameed, 2009b), wood apple shell (Jain and Jayaram, 2010), rejected tea (Nasuha et al., 2010), yellow passion fruit peel (Pavan et al., 2008; Pavan et al., 2007), rice husk (Safaa and Bhattia, 2011), jujube seeds (Somasekhara Reddy et al., 2012), tamarind shell powder (Somasekhara Reddy, 2006), sugar beet pulp (Rasool Malekbalaa et al., 2012), sunflower seed shells (Osma et al., 2007), lemon peel (Vasanth Kumar, 2007), mandarin peels (Pavan et al., 2007), apple pomace and wheat straw (Robinson et al., 2002), coffee grounds (Hirata et al., 2002), and parthenium hysterophorus (Hem Lata et al., 2007) have been used.

The majority of articles available in the literature are relating to batch studies (Somasekhara Reddy et al., 2012). Some of the articles relating to equilibrium studies available in the literature are mentioned for better understanding (Sun and Xu, 1997; Nassar and El-Geundi, 1991; Asfour et al., 1985a,b; Noroozi et al., 2007).

The Bengal gram seed husk (BGSH) is already used for the removal of a direct dye, congo red (CR) by using batch studies (Somasekhara Reddy et al., 2013, submitted for publication) and interest led us to use the same material for the removal of different dyes like CR, MB, RB and AB through equilibrium studies to understand the removal capacity of BGSH. In batch studies, interactions between adsorbate (dye) and adsorbent (SHBG) are studied at one concentration of dye with respect to time. But in textile and dye industries, different concentrations of dyes are let out as effluents. Therefore, interactions between adsorbate (dye) and adsorbent (SHBG) at different concentrations (in a range of low to high concentrations of dye) should be studied with respect to infinite time. In this paper, SHBG is used to remove different dyes like CR, MB, RB and AB from aqueous solutions through equilibrium studies.

1.1. Theory of adsorption isotherm models

Langmuir isotherm model was applied to describe the adsorption of CR, MB, RB and AB. The model is represented by the following equation (Somasekhara Reddy et al., 2012).

Ce/qe = 1/QmaxKL + Ce/Q^ (1)

where Ce is the equilibrium concentration of CR, MB, RB and AB in solution (mg L—*), Qmax is the maximum adsorption capacity of SHBG (mg g—*) and KL is the Langmuir constant related to the adsorption energy (L mg-1). RL, a dimensionless constant, is used to determine whether adsorption is favorable or not and is calculated by (Somasekhara Reddy et al., 2012).

Rl = 1/(1 + KlCq) (2)

where C0 is the initial concentration of CR, MB, RB and AB in solution (mg L-1).

Freundlich isotherm model is also applied to describe the adsorption of CR, MB, RB and AB. Linearized in logarithmic form of Freundlich isotherm model equation is represented by (Somasekhara Reddy et al., 2012).

log qe = log Kf + (1/n) log Ce (3)

where KF is the Freundlich constant and '1/n' is the heterogeneity factor.

1.2. Thermodynamic parameters

The Clausius-Clapeyron equation (Somasekhara Reddy et al., 2012)

Kl = Ac exp(—AH°/RT) (4)

Ac = pre-exponential factor in Clausius-Clapeyron equation, AH° = standard enthalpy change, R = Gas constant and T = absolute temperature of dye solutions.

The linear form of Eq. (4) is (Somasekhara Reddy et al., 2012)

ln KL = ln Ac + (—AH°/RT) (5)

The standard enthalpy change (AH0) during the adsorption of dyes, CR, MB, RB and AB on SHBG was calculated from the slope of the plot which is made between ln KL versus 1/T using the Eq. (5).

The standard free energy change (AG0) of adsorption, AG0 (kJ/mol) was calculated from Langmuir equilibrium adsorption constant (Sun and Xu, 1997; Somasekhara Reddy et al., 2012).

AG° = —RT ln KL (6)

The standard free energy change (AG0) during the adsorption of dyes (adsorbates), CR, MB, RB and AB on SHBG was calculated by using Eq. (6).

The standard entropy change (AS0) of dyes (adsorbates), CR, MB, RB and AB on SHBG was calculated by using Eqs. (6) and (7) (Somasekhara Reddy et al., 2012).

AS° = (AH° — AG°/T) (7)

The standard entropy change (AS0) during the adsorption of dyes, CR, MB, RB and AB on SHBG was calculated from the standard enthalpy change (AH0) and the standard free energy change (AG0) by using Eq. (7).

2. Materials and methods

2.1. Preparation of adsorbent, SHBG

The preparation of adsorbent is already explained in the previous paper (Somasekhara Reddy et al., 2013, submitted for publication). However, the procedure is again given here. The seed husk of Bengal gram (SHBG) is discarded as a waste in a small-scale industry where dal of Bengal gram (which is used in the preparation of certain food items) is separated from seed of Bengal gram. This waste is used in certain areas as foodstuff to the animals in addition to use as fire wood in hotels and restaurants. The SHBG is collected from a local industry, which is in the nearby town, Nandyal and washed

thoroughly with de-ionized water for removing dirt. The dried husk material is ground and sieved to desired mesh size like >53 im <75 im. It is abbreviated as SHBG. It is used as an adsorbent for the removal of dyes like CR, MB, RB and AB.

2.2. Adsorbate

CR, MB and RB are obtained free of cost from M/S Sipka Sales Corporation, New Delhi and are used without further purification. AB is purchased from M/S Sigma-Aldrich Chemicals, Hyderabad and is used without further purification. The wave length of the maximum absorbency for MB, RB, CR and AB are 665 nm, 555 nm, 497 nm and 605 nm, respectively. The physical properties of selected dyes are shown in Table 1. The chemical structures of selected dyes are shown in Fig. 1.

Stock solution of 1000 mg L-1 was prepared by dissolving accurate quantity of the dye in double distilled water. The experimental solution was obtained by diluting the stock solution to the designed initial dye concentration.

2.3. Equilibrium studies

The equilibrium isotherms are determined by adopting the following procedure. This procedure is mentioned in Sun and Xu's research paper (Sun and Xu, 1997). 0.1 g (0.3 g in case of acid blue 25) of a solid agricultural waste material, SHBG is added to 25 ml of dye solution in a 50 ml screw type Erlenmeyer flask at room temperature (30 ± 1 0C). Each isotherm consisted of 8 dye concentrations varied from 100 to 1000 mg/l for basic and direct dyes and 100 to 500 mg/l for acidic dye. The flask containing dye solution along with SHBG are placed in a Julabo shaking water bath and agitated at 160 rpm for 3 days at constant temperature, 30 ± 1 0C. An equilibrium time of 3 days is found to be satisfactory for each dye. After this time, the samples are centrifuged at 10000 rpm for 20 min and the absorbance of supernatant solution is measured. The equilibrium concentrations of different combinations were calculated through already prepared calibration curves.

The effect of temperature (30, 45, 60, and 75 0C), size (>53 <75 im, >75 <90 pm, >90 <150 pm and >150 pm), dyes (CR, MB, RB and AB), agitation speed (160 and 100 rpm), and mass (0.1 and 0.2 g (0.3 and 0.4 g for AB dye) of SHBG on the equilibrium of different combinations of dyes is studied by keeping the remaining parameters constant and adopting the same procedure.

The amount of adsorption at equilibrium time (after 3 days) and the percentage of dye removal were calculated following Eqs. (8) and (9) (Somasekhara Reddy et al., 2012).

Chemical Structure of AB

Figure 1 Chemical structure of selected dyes.

qe = [(C0 - Ce)V\/W (8)

where C0 and Ce (mg L-1) are the liquid phase concentrations of dye at initial and at equlibrium time, respectively, V is the volume of the solution (L) and W is the mass of dry adsorbent used (g).

3. Results and discussion

The experimental results are analyzed based on Langmuir and Freundlich adsorption isotherms for understanding adsorption

Table 1 Physical properties of the selected dyes.

Dye Properties

C.I. number C.I. name kmax (nm) Class Mol. weight Purity (%)

AB 62055 Telon Blue 605 Anthraquinone 416.39 99

MB 52015 Basic blue 9 665 Thiazine 337.96 82

RB 45170 Basic violet10 555 Rhodamine B 479.0 95

CR 22120 Direct red 28 497 Polyazo 696.67 75

capacity of SHBG. The effect of adsorption of different dyes, effect of different size of particles of SHBH, effect of mass of particles of SHBH, effect of rotations per minute of shaker and effect of temperatures on the SHBH are studied and the results are given below.

The adsorption capacity of SHBH is obtained through the Langmuir adsorption isotherms which are made by plotting the graphs between Ce, versus Ce/qe. The slope and intercept of these straight line graphs are used to calculate the adsorption Langmuir adsorption isotherm constants by using the Eq. (1) (Juang et al., 1997).

The Freundlich adsorption isotherm is drawn between log Ce versus log qe. The slope and intercept of these straight line graphs are used to calculate Freundlich adsorption isotherm constants by using the Eq. (3) (Juang et al., 1997).

The Freundlich adsorption isotherm constant, KF, in general, is increased with adsorption. On the other hand, the magnitude of other Freundlich adsorption isotherm constant, n, gives an indication of the favourability and capacity of the SHBG-dyes system. It is generally stated that the values of n in the range 1-10 represent good adsorption (Namasivayam et al., 2001a,b).

3.1. Effect of dyes

The Langmuir adsorption isotherms for understanding the effect of adsorption of different dyes like MB, RB, CR and AB on the SHBG are shown in Fig. 2. The Freundlich adsorption isotherm is shown in Fig. 3. The data extracted from the Lang-muir and Freundlich adsorption isotherms are shown in Table 2.

It is observed from Table 2 that the basic dyes, MB and RB are relatively adsorbed more on SHBG than that of CR and AB. The reason is explained as follows. The agricultural waste/by-product, SHBG acquires a negative charge after mixing with aqueous dye solutions due to the presence of cellulose in the SHBG (McKay et al., 1988). The basic dyes, MB and RB are examples of dyes which ionizes to an cationic colored component D+ and anion of Cl—. The acidic dye AB and di-

400 Ce (mg/L)

Figure 2 Langmuir adsorption isotherm of CR, MB, RB and AB on SHBG. Conditions: agitation time = 72 h; C0 = 1001000 mg L—1 for CR, MB and RB (100-500 mg L—1 for AB); V = 0.025 L; temp. = 30 ± 1 oC; speed of agitation = 160 rpm; size of SHBG = >53 <75 im; mass of SHBG = 0.1 g (0.3 g in case of AB).

a 15-1

log Ce

Figure 3 Freundlich adsorption isotherm of CR, MB, RB and AB on SHBG. Conditions: as shown in Fig. 2.

rect dye, CR are examples of dyes which ionizes to an anionic colored component D— and a cation of Na + . Therefore, the positive dye ions of basic dyes (MB and RB) are fastly approached the surface of SHBG for the adsorption and it seems to be a chemical adsorption process. The approach of an acidic dye anion will suffer coulombic repulsions due to the presence of the strong anionic groups in the SHBG. In case of acidic dye, AB, the repulsions might be present between negative SHBG and negative dye ion.

The adsorption capacity of SHBG for CR is far better than that of components of sunflower stalks like skin, pith and combination of skin and pith (total) (Sun and Xu, 1997) and wood shavings(Abo-Elila and El-Dib, 1987). The comparisons are shown in Table 3. Similarly, the removal capacity of MB by SHBG (333.33 mg g—1) is more than that of components of sunflower stalks like skin (165.33 mg g—1) and total (205.41 mg g—*) (Sun and Xu, 1997). Other low cost material like fired clay is capable to remove more AB (7 mg g—*) than that of SHBG (5.56 mgg-1) (McKay et al., 1985). The comparisons are shown in Table 3.

The Freundlich adsorption isotherm constant, KF, in general, is increased with increasing adsorption. As n values are within the range of 1 < n < 10 for all dyes indicate favorable adsorption.

3.2. Effect of SHBG particle size

The effect of size of particles of SHBG on the adsorption of different dyes like CR, MB, RB and AB on the SHBG is understood. The adsorption capacity of SHBG is obtained through the Langmuir adsorption isotherms which are made by plotting graphs between Ce versus Ce/qe for different particle sizes of SHBG. The slope and intercept of these straight line graphs are used to calculate the Langmuir adsorption isotherm constants and are arranged them in Table 4. The Freundlich adsorption isotherms are drawn between log Ce versus log qe. The slope and intercept of these straight line graphs are used to calculate Freundlich adsorption isotherm constants. One typical Langmuir and Freundlich adsorption isotherm plots for the dye removal of CR are shown in Figs. 4 and 5, respectively. The extracted parameters values for all the dyes at different sizes are shown in Table 4.

From the Langmuir adsorption isotherms, as expected, the adsorption capacity of the SHBG is decreasing by increasing

Table 2 Langmuir and Freundlich adsorption constants for different dyes on SHBG.

Dye Langmuir parameters Freundlich parameters

KL(Lmg"') Qmax (mg g ') R2 KF(mgg-1)(Lmg-1)1/" n R2

CR 1.0156 78.1250 0.9956 7.7215 2.8329 0.9849

MB 5.5000 333.3333 0.9635 9.1285 1.4194 0.9824

RB 1.1333 133.3333 0.9886 4.7973 1.9124 0.9956

AB 0.0445 5.5617 0.9669 0.5724 2.9762 0.9994

Table 3 Adsorption Capacities of some adsorbents compared with that of SHBG.

Material Cationic dye (mg g ') Anionic dye (mg g ') Reference

Maize cob - 41.4 (AB) El-Geundi (1991)

Bagasse pith - 25 (AB) McKay et al. (1987)

Wood shavings - 0.7 (CR) Abo-Elila and El-Dib (1987)

Sunflower stalks

Skin 165.33 (MB) 37 (CR) Sun and Xu (1997)

Total 205.41 (MB)

Fired clay - 7 (AB) MCKay et al. (1985)

SHBG 333.33 (MB) 78.12 (CR) Present work

133.33 (RB) 5.56 (AB) Present work

Table 4 Langmuir and Freundlich adsorption constants for different sizes of SHBG for the adsorption of CR, MB, RB and AB.

Dye Size (mi) Langmuir parameters Freundlich parameters

^(Lmg-1) Qmax (mg g ') R2 KF(mgg-1)(Lmg-1)1/n n R2

CR >53 <75 1.0156 78.1250 0.9956 7.7215 2.8329 0.9449

>75 <90 0.8134 74.6269 0.9836 6.1419 2.6427 0.9145

>90 <150 0.6879 70.9220 0.9842 5.7690 2.6667 0.9188

>150 0.5915 70.4225 0.9832 4.9614 2.5458 0.9144

MB >53 <75 5.5000 333.3333 0.9635 9.1285 1.4194 0.9524

>75 <90 6.9730 270.2703 0.9733 23.5126 2.1734 0.9301

>90 <150 7.8889 277.7778 0.9565 14.5848 1.6644 0.8472

>150 8.6486 270.2703 0.9729 15.1391 1.6711 0.8743

RB >53 <75 1.1333 133.3333 0.9886 4.7973 1.9124 0.9356

>75 <90 0.6435 86.9565 0.9958 3.5950 2.0846 0.9296

>90 <150 0.8279 81.9672 0.965 5.6247 2.4131 0.9677

>150 0.8992 77.5194 0.9966 6.4328 2.6344 0.9004

AB >53 <75 0.0467 5.5617 0.9669 0.5724 2.9762 0.9394

>75 <90 0.0429 4.2088 0.9853 0.5389 3.2510 0.9709

>90 <150 0.0330 3.8835 0.9783 0.4871 3.0675 0.9749

>150 0.02323 3.8081 0.9002 0.3690 3.0600 0.7723

size of particles of the SHBG in all different types of dyes of CR, MB, RB and AB because the available surface area is decreasing with increasing the particle size. The surface area of different sizes of SHBG is shown in Table 5.

3.3. Effect of dose of SHBG

The effect of dose of SHBG on the adsorption of different dyes like CR, MB, RB and AB on the SHBG is understood by plotting the graphs between Ce versus qe (figures are not shown). The adsorption capacity of all the SHBG is obtained through the Langmuir adsorption isotherms which are made by plotting graphs between Ce versus Ce/qe. The slope and intercept

of these straight line graphs are used to calculate the Langmuir adsorption isotherm constants by using Eq. (1). The Freundlich adsorption isotherms is drawn between log Ce versus log qe and the slope and intercept of these straight line graphs are used to calculate Freundlich adsorption isotherm constants are calculated by using Eq. (3). One typical Langmuir and Freundlich adsorption isotherm plots for the dye removal of CR at different doses are shown in Figs. 6 and 7, respectively. The data extracted from the Langmuir and the Freundlich adsorption isotherms are shown in Table 6.

The adsorption capacity (which is calculated from the Langmuir adsorption isotherm) of the SHBG for all dyes, CR, MB, RB and AB is increasing with increasing mass of

â 5-1 (1)

О 4 -"S

О 32 -1 -0 -

>53<75 jim >75<90 jim >90<150 jim >150 jim

200 400 Ce (mg/L)

Figure 4 Langmuir adsorption isotherm of CR at different sizes of SHBG. Conditions: agitation time = 72 h; C0 = 1001000 mg L_1; V = 0.025 L; temp. = 30 ± 1 °C; speed of agitation = 160 rpm; size of SHBG = >53 <75, >75 <90, >90 <150 and >150 im; mass of SHBG = 0.1 g.

->53<75 |jm ->75<90 jm ■>90<150 jm >150 |m

1 2 log Ce

Figure 5 Freundlich adsorption isotherm of CR at different sizes of SHBG. Conditions: as shown in Fig. 4.

SHBG. Similar observation were observed in case of adsorption of AB on maize cob (El-Geundi, 1991) and adsorption of basic blue 69 on bagasse pith (McKay et al., 1988).

3.4. Effect of agitation speed

200 400

Ce (mg/L)

Figure 6 Langmuir adsorption isotherm of CR at different doses of SHBG. Conditions: agitation time = 72 h; C0 = 1001000 mg L—1; V = 0.025 L; temp. = 30 ± 1 oC; speed of agitation = 160 rpm; size of SHBG = >53 <75 im; mass of SHBG = 0.1 and 0.2 g.

plotting graphs between Ce versus Ce/qe. The slope and intercept of these straight line graphs are used to calculate the Langmuir adsorption isotherm constants by using Eq. (1). The Freundlich adsorption isotherms are drawn between log Ce versus log qe. The slope and intercept of these straight line graphs are used to calculate Freundlich adsorption isotherm constants are calculated by using Eq. (3). One typical Langmuir and Freundlich adsorption isotherm plots for the dye removal of CR at different agitation speed are shown in Figs. 8 and 9, respectively. The data extracted from the Lang-muir and the Freundlich adsorption isotherms are shown in Table 7.

As expected, in general, the adsorption capacity (which is calculated from the Langmuir adsorption isotherm) of the SHBG for all dyes, CR, MB, RB and AB is increasing with increasing agitation speed (Rotations per Minute, RPM) of shaker. Similar observation were observed in case of adsorption of AB on bagasse pith (McKay et al., 1988).

The Freundlich adsorption isotherm constant, KF, in general, is increased with increasing adsorption. As n values are within the range of 1 < n < 10 indicates good adsorption.

3.5. Effect of temperature

The effect of temperature on the adsorption of different dyes like CR, MB, RB and AB on the SHBG is understood by

The effect of agitation speed (RPM) on the adsorption of different dyes like CR, MB, RB and AB on SHBG is understood by plotting the graphs between Ce versus qe (Figures are not shown). The adsorption capacity of SHBG is obtained through the Langmuir adsorption isotherms which are made by

Table 5 Surface area of SHBG at different sizes.

Property Size (im) SHBG

BET surface area (m2/g) >53 <75 1.3636

>75 <90 1.0489

>90 <150 0.8950

>150 0.7410

1 2 log Ce

Figure 7 Freundlich adsorption isotherm of CR at different doses of SHBG. Conditions: as shown in Fig. 6.

Table 6 Langmuir and Freundlich adsorption isotherm constants for different doses of SHBG for the adsorption of CR, MB, RB and AB.

Dye Dose (mg) Langmuir parameters Freundlich parameters

Kl (L mg-1) Qmax R2 Kf (mg g-1) n R2

(mgg ') (L mg"1)1'"

CR 100 1.0156 78.1250 0.9956 7.7215 2.8329 0.9849

200 1.6154 109.8901 0.9581 7.4251 2.2041 0.9845

MB 100 5.5000 333.3333 0.9635 9.1285 1.4194 0.9824

200 9.9999 434.7826 0.9819 13.5988 1.3324 0.9924

RB 100 1.1333 133.3333 0.9886 4.7973 1.9124 0.9356

200 0.8364 181.8182 0.9881 4.4218 1.8132 0.9744

AB 300 0.0467 5.5617 0.9669 0.5724 2.9762 0.9894

400 0.0712 5.9809 0.9846 0.5512 2.7397 0.9862

plotting the graphs between Ce versus qe (Figures are not shown). The adsorption capacity of SHBG is obtained through the Langmuir adsorption isotherms which are made by plotting graphs between Ce versus Ce/qe. The slope and intercept of these straight line graphs are used to calculate the Langmuir adsorption isotherm constants by using Eq. (1). The Freundlich adsorption isotherms are drawn between log Ce versus

500 Ce (mg/L)

Figure 8 Langmuir adsorption isotherm of CR on SHBG at different RPM of shaker. Conditions: agitation time = 72 h; C0 = 100-1000 mg L"1; V = 0.025 L; temp. = 30 ± 1 °C; speed of agitation =100 and 160 rpm; size of SHBG = >53 <75 pm; mass of SHBG = 0.1 g.

0 12 3 4 log Ce

Figure 9 Freundlich adsorption isotherm of CR on SHBG at different RPM of shaker. Conditions: as shown in Fig. 8.

log qe. The slope and intercept of these straight line graphs are used to calculate Freundlich adsorption isotherm constants by using Eq. (3). One typical Langmuir and Freundlich adsorption isotherm plots for the dye removal of CR at different temperatures are shown in Figs. 10 and 11, respectively. The data extracted from the Langmuir and the Freundlich adsorption isotherms are shown in Table 8.

The effect of temperature is very important on adsorption of dyes on the SHBG because the dyeing of any fabric is generally carried out at higher temperature and the effluents of dye house are let out at high temperature. Therefore, for better understanding the dyeing process involved, the study of effect of temperature on the adsorption process of dyes is taken up.

The adsorption capacity (which is calculated from the Langmuir adsorption isotherm) of the SHBG for all dyes, CR, MB, RB and AB is increasing with increasing temperature. Similar observation were observed in case of adsorption of AB and basic blue 69 on bagasse pith (McKay et al., 1987), astrazone blue on wood (Asfour et al., 1985), adsorption of methylene blue and malachite green on chemically treated Psidium guava leaves (Singh and Srivastava, 1999), adsorption of methylene blue on kaolinite (Ghosh and Bhat-tacharya, 2000), adsorption of malachite green on activated carbon and activated slug (Gupta et al., 1997), adsorption of reactive red 120 and reactive red 141 on metal hydroxide sludge (Netrapradit et al., 2004), adsorption of cadmium(II) on Fe(III)/Cr(III) (Namasivayam and Ranganathan, 1995), adsorption of methyl violet on perlite (Dogan and Alkan, 2003), adsorption of copper(II) on fly ash (Panday et al., 1985), adsorption of chromium on sunflower stalks (Sun and Shi, 1998), adsorption of reactive dyes [procion turquoise HA (reactive blue 71), procion red H-E3B(reactive red 120) and remazole red RB (reactive red 198)] on synthetic clay and activated carbon (Lambert et al., 1997) and adsorption of a pesticide, propetamphos on activated carbon (Lambert et al., 1997). In contrast to this, the adsorption capacity of rice bran activated carbon for acid black 1, acid green 16 and direct red 31 (Sankar et al., 1999), adsorption capacity of sunflower stalks for copper, zinc and cadmium (Sun and Shi, 1998), the adsorption capacity of metal hydroxide sludge for reactive red 2 (Netrapradit et al., 2004), adsorption capacity of bauxite for reactive dyes [procion turquoise H-A (reactive blue 71), procion red H-E3B (reactive red 120) and remazole red RB(reactive red 198)] (Lambert et al., 1997) and adsorption

Table 7 Langmuir and Freundlich adsorption constants for different RPM on SHBG for the adsorption of CR, MB, RB and AB.

Dye RPM Langmuir parameters Freundlich parameters

KL(Lmg-') Qmax (mg g ') R2 KF(mgg-1)(Lmg-')1/n n R2

CR 100 1.0061 61.35 0.9920 7.7108 3.1319 0.8945

160 1.0156 78.13 0.9956 7.7215 2.8329 0.9449

MB 100 3.9459 270.27 0.9955 6.5073 1.3768 0.9934

160 5.5000 333.33 0.9635 9.1285 1.4194 0.9524

RB 100 0.6815 88.50 0.9900 5.8790 2.4062 0.9826

160 1.1333 133.33 0.9886 4.7973 1.9124 0.9356

AB 100 0.0228 5.31 0.9910 0.1128 1.7322 0.9861

160 0.0467 5.56 0.9669 0.5724 2.9762 0.9894

capacity of activated carbon for a pesticide, pentachlorophenol (Lambert et al., 1997) are decreasing with increasing temperature.

3.5.1. Thermodynamic parameters

The standard enthalpy change (AH0) during the adsorption of dyes, CR, MB, RB and AB on SHBG is calculated from the slope of graph which is made between ln KL and 1/T by using

300250-

d 200--Э

Ä 150 О

10050 0

50 100

Ce (mg/L)

Figure 10 Langmuir adsorption isotherm of CR on SHBG at different temperatures of dye solution. Conditions: agitation time = 72 h; C0 = 100-1000 mg L_1; V = 0.025 L;

temp. = 30 ± 1, 45 ± 1, 56 ± 1 and 75 ± 1 °C; speed of agitation = 160 rpm; size of SHBG = >53 <75 im; mass of SHBG = 0.1 g.

log Ce

Figure 11 Freundlich adsorption isotherm of CR on SHBG at different temperatures of dye solution. Conditions: as shown in Fig. 10.

the Eq. (5). (The graph made between ln KL and 1/T is not given). The standard free energy (AGO) (kJ/mol) during the adsorption of dyes, CR, MB, RB and AB on SHBG is calculated from Langmuir equilibrium adsorption constant (Sun and Xu, 1997) by using Eq. (6). The standard entropy change (AS0) during the adsorption of dyes, CR, MB, RB and AB on SHBG is calculated from the standard enthalpy change and the standard free energy change by using Eqs. (6) and (7). All thermodynamic data are shown in Table 9.

The results reveal that the standard enthalpy change (AH0) for all combinations of dyes, CR, MB, RB and AB with SHBG is positive. It indicates that all adsorption processes are endo-thermic. Similarly, the adsorption of reactive red 120 and reactive 141 on metal hydroxide sludge (Netrapradit et al., 2004), adsorption of acid orange 10 on activated carbon prepared from sugar cane bagasse (Tsai et al., 2001) and the adsorption of AB and basic blue 69 on bagasse pith (McKay et al., 1987) are also endothermic processes.

Positive standard entropy change, AS0 values for the adsorption of CR, MB and RB on SHBG at all temperatures indicate that the adsorption process is spontaneous due to increased randomness at the interface formed between solid, SHBG and the dye solutions. It is evidenced by the negative standard free energy change, AG0 values. Similar observation is made in the adsorption of malachite green by activated carbon and activated slag (Gupta et al., 1997), adsorption of reactive brilliant blue KN-R, reactive brilliant orange k-GN, reactive brilliant red K-2BP, acid anthraquinone blue and acid turquoise blue 2G on modified resin (Yu et al., 2004), adsorption of brilliant green and methylene blue by neem leaves (Sharma and Bhattacharya, 2005) and the adsorption of cadmium (II) by Fe(III)/Cr(III) hydroxide (Namasivayam and Ranganathan, 1995). But the negative entropy change for the adsorption of AB on SHBG at all temperatures explains that the adsorption of AB on SHBG is not a spontaneous process which is further evidenced by the positive free energy change.

4. Conclusions

SHBG is studied as an adsorbent for removal of different dye-stuffs like CR, MB, RB and AB in dyeing effluents with equilibrium isotherms. The results indicate that SHBG has higher maximum adsorption capacities to basic dyestuffs, namely, 333 mg of MB and 133 mg of RB per gram of SHBG, respectively. The maximum adsorption capacities of anionic dyes on

Table 8 Langmuir and Freundlich adsorption constants for different temperatures of dye solutions for the adsorption of CR, MB, RB and AB on SHBG.

Dye Temp. (°C) Langmuir parameters Freundlich parameters

KL(Lmg"') Qmax (mg g ') R2 KF(mgg-1)(Lmg-1)1/" n R2

CR 30 1.0156 78.1250 0.9956 7.7215 2.8329 0.9449

45 1.0484 80.6452 0.9917 8.1884 2.8662 0.9568

56 1.1148 81.9672 0.9920 8.4353 2.8703 0.9428

75 1.2437 84.0336 0.9919 9.5984 3.0066 0.9593

MB 30 5.5000 333.3300 0.9635 9.4820 1.4395 0.9524

45 6.5185 370.3704 0.9899 10.1765 1.3908 0.9600

56 7.3077 384.6154 0.9970 10.8968 1.3768 0.9545

75 11.1250 416.6667 0.9908 14.3285 1.3214 0.8689

RB 30 1.1333 133.3333 0.9886 4.7973 1.9124 0.9356

45 1.2754 144.9275 0.9946 5.5911 1.9732 0.9627

56 1.4677 161.2903 0.9964 6.2762 1.9592 0.9627

75 1.9855 144.9275 0.9915 8.4586 2.1580 0.9356

AB 30 0.0467 5.5617 0.9669 0.5724 2.9762 0.9394

45 0.0518 6.0976 0.9682 0.6301 2.9789 0.9627

56 0.0550 7.5358 0.9535 0.5983 2.6738 0.9561

75 0.0663 9.4697 0.9546 0.6041 2.4331 0.9681

SHBG are lower with 78 mg of CR and 5.6 mg of AB per gram of SHBG, respectively. The adsorption processes of the dye-stuffs follow Langmuir adsorption isotherms and coulombic interaction between the SHBG and the dyestuffs are the major driving force for the adsorption.

The adsorption of all dyes on SHBG increases by increasing the rate of agitation and the mass of SHBG. As expected, increase in the size SHBG decreases the uptake of all dyestuffs by SHBG.

The adsorption of all dyestuffs on SHBG is endothermic. The enthalpies of adsorption for CR, MB, RB and AB are 4, 13.5, 10.97 and 6.72kJmol~1, respectively.

The data show that SHBG has considerable potential for the removal of dyestuffs from wastewater over a wide range of concentrations.

Acknowledgments

The authors are grateful to the AICTE (All India Council for Technical Education, New Delhi) for the financial assistance through the F. No.: 8023/RID/RPS-14/Pvt (II Policy)/2011-12/Feb., 03, 2012 to carry out the present research work. The authors are also grateful to the Management, the Director and the Principal of G.P.R. Engg. College (Autonomous), Kurnool for their constant encouragement and help.

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