statistical analysis of congo red dye removal using ...boehm titration [4] for boehm titration, 1.0...

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 19 (2017) pp. 8788-8804

© Research India Publications. http://www.ripublication.com

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Statistical Analysis of Congo Red Dye Removal Using Sawdust Activated

Carbon

M.Srinivas Kini*, K.Balakrishna Prabhu, Ankit Gundecha and Devika .U

Department of Chemical Engineering, Manipal Institute of Technology, Manipal, Karnataka-576104, India.

*Orcid ID: 0000-0001-6408-0538, (*Corresponding author)

Abstract

In the present study, an adsorbent was prepared from Ben

Teak sawdust by chemically treating and carbonizing with

phosphoric acid. Using Design Experts software, 2 level

factorial studies and Response surface methodology studies

using Box-Benkhen design model were carried out and a

relationship between Qe and pH, temperature, concentration of

dye, amount of adsorbent, agitation speed were derived for

Congo red- Saw Dust Carbon(SDC) . Maximum Congo-red

removal of 96.764 % and adsorption capacity of 149.507 mg/g

was found at pH 2, 30 °C, 125 rpm, 150 mg/L and 1gram/L of

reaction mixture for an optimum contact time of 2.5 hours.

Adsorption isotherm obtained fitted well into both Freundlich

and Langmuir equation but Freundlich isotherm fits better

with 1/n ranging from 0.4944 to 0.5894 and R2 ranging from

0.92333 to 0.952. The qmax from Langmuir isotherm was

established to be 209.7 mg/g.

Keyword: Sawdust Carbon, Congo red, Box-Benkhen,

Response surface methodology, Langmuir

The freshwater is consumed in mainly three segments like

agricultural, industrial and domestic segments and this has led

to the production of large amounts of wastewater comprising

innumerous pollutants. One of the main type of the

contaminants is dyes. Dye pollution has become a significant

problem. The release of dye-bearing wastewater into water

bodies from different industries poses problems as these dyes

are toxic in nature causing damage to the natural surroundings

and disrupting the life of aquatic beings. Dyes are toxic and

concentration as low as 0.005mg/L is visible which captures

the attention of both public and authorities.

Currently, there are more than 100,000 marketable dyes and

the annual production of these dyes is assessed to be 7×105 -

1×106 tonnes per year [1]. It is established that 10-15% of the

used dyes enter the environs through waste [2]. The main end

users of dyes are textile, dyeing, paper and pulp, tannery and

paint industry. Hence the wastewater of these industries in

addition to those from plant producing dyes tend to contain

dyes in adequate amount.

Various methods that have been adopted for dye removal over

the years are microbial degradation, precipitation, membrane

filtration, electrochemical destruction, ozonation,

photochemical process, Ion exchange and reverse osmosis.

Adsorption is a familiar separation technique and preferred

method for dye elimination from wastewater [3]. Adsorption

has been found to be successful technique over other

treatment methods due to its less first cost, greater flexibility,

ease of design and operation, insensitivity to toxic dyes.

Though adsorption on commercially activated carbon (AC) is

preferred in most of the industries for effluent treatment but

issues like losses during regeneration and high cost limit their

use. In this context, an earnest effort is made to present an

inexpensive adsorbent for dye removal.

The study involves preparation of Ben Teak sawdust carbon

(SDC) as an adsorbent for elimination of Congo red(CR) dye.

Batch studies incuded parameters such as pH, adsorbent

concentration, initial CR concentration, temperature and

agitation rpm. The influence of these parameters on

adsorption capacity was examined using 2 Level factorial and

Response Surface Methodology (RSM) studies employing

Design Expert software. The adsorption data were fitted using

suitable isotherm.

MATERIALS & METHODS

Preparation of the adsorbate

Congo red dye solution of 1000mg/L was prepared by

dissolving 1 g of CR powder in 1L standard flask using

distilled water.

Figure 1: Chemical structure of Congo-Red



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Preparation of the adsorbent

Saw dust of Ben Teak wood was dried at 105°C in a drier for

5 hours to minimize moisture content. The dried saw dust was

mixed thoroughly with 88% ortho-phosphoric acid. The

mixture was then carbonized in a muffle furnace at 300°C for

3 hours. The carbonized sample was first washed with water

followed by 2% of sodium bicarbonate (NaHCO3) to

neutralize the pH. The washed sample was dried at 110°C in a

drier for 24 hours. The dried sample was grinded and sieved

through a mesh size of 150 µm. From the BET studies the

surface area of SDC was 211.34 m2/g and the pore volume

was 0.1888 cc/g.

LEVEL FACTORIAL AND RSM STUDIES

Factorial and RSM studies were carried out by analysing the

process of adsorption for 100mL of reaction mixture

according to the change in pH, temperature, amount of

adsorbent, concentration of dye and agitation speed. The

range of these parameters is as follows:

a) pH – 2 to 9

b) Temperature – 30°C to 50°C

c) Amount of adsorbent – 0.1 to 2 g

d) Concentration of dye – 25 mg/L to 150 mg/L

e) Agitation speed – 125 rpm to 200 rpm

A table of experiments was created using the design expert

software. The factorial and RSM studies were carried out by

varying 5 parameters. Hence, a table of 32 (25) experiments

was created. Box-Benkhen design was employed for the RSM

studies. The experiments were carried out according to the

table for a preliminary contact time of 6 hours in 250mL

conical flask containing 100mL of reaction mixture. The

filtrate from each experiment was analysed using the UV

spectrophotometer and corresponding Qe, percentage removal

values were calculated using the following equations.

Where,

Co – Initial conc. in mg/L V – Volume of CR solution in L

Ct – Final conc. in mg/L M – Amount of adsorbent in g

And,

Boehm Titration [4]

For Boehm titration, 1.0 g of the SDC was mixed with 15mL

solution of Sodium bicarbonate (0.1M), Sodium carbonate

(0.05M) and Sodium hydroxide (0.1M) for determining acidic

groups and 0.1M Hydrochloric acid for determining basic

groups/sites respectively at 32 oC for more than 48 hours with

intermittent swirling. Consequently, the aqueous mixture were

back titrated with Hydrochloric acid(0.1M) for acidic and

Sodium hydroxide (0.1M) for basic groups. The quantity and

nature of acidic groups were calculated by considering that

Sodium hydroxide neutralises carboxylic, lactonic and

phenolic groups, sodium carbonate neutralises carboxylic and

lactonic groups and that sodium bicarbonate neutralises only

carboxylic groups. Neutralisation points were known using

pH indicator of phenolphthalein solution.

Fourier transform infra-red (FTIR) Studies

The FTIR analysis was carried out in a FTIR

spectrophotometer 8400S manufactured by Shimadzu having

a wavenumber range of 4000-400 cm-1. In this study, the

adsorbent is first mixed with KBr (about 10 times the weight

of the sample) in a mortar at ambient temperature. The

mixture is then placed in a hydrostatic pressure (pressure of 5

tonnes is applied) to make a thin transparent pellet. The pellet

is then transferred to the FTIR spectrophotometer for analysis

Adsorption Isotherms

Langmuir and Freundlich, were used to fit the experimental

data points for Congo-Red adsorption on absorbent at three

temperatures, 30, 40 and 50°C. 100mL of dye with

concentrations of 25, 50, 75, 100 and 150mg/L were used at

each temperature and 0.1gram of adsorbent was added at pH

3.3. The agitation speed of 125rpm was maintained for all

experiments.

RESULT AND DISCUSSION

Calibration Curve of Congo-red

The calibration curve provided with the relationship between

the measured absorbance and the CR concentration of the dye

at λmax = 499nm.



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Figure 2: Calibration Curve of Congo-red

Table 1: Boehm Titration Results

Groups Before adsorption of congo-red After adsorption of congo-

red

Carboxylic groups (mmol/g) 1.5 0.7

Phenolic groups (mmol/g) 5.5 3.7

Lactonic groups (mmol/g) 0 0

Basic groups (mmol/g) 6.5 6.3

From the above table we can infer that, predominantly

carboxylic and phenolic groups existing on the exterior of the

adsorbent are responsible for the adsorption of congo-red.

FTIR Studies

Figure 3: FTIR before adsorption of CR on SDC



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Figure 4: FTIR after adsorption of CR on SDC

From Figure 3, peaks with wavenumber 1704.96 cm-1 and

1589.23 cm-1 are predominant and from Figure 4., peaks

having wavenumber 3433.06 cm-1, 1704.96 cm-1 and 1596.95

cm-1 are predominant. On comparing the two figures, it can be

inferred that Alcohol (O-H, stretch, H-bonded), Carbonyl

(C=O, stretch) and Nitro (N-O, stretch) functional groups are

accountable for the adsorption of congo-red on the exterior of

the adsorbent. The results obtained from FTIR analysis are in

concordance with Boehm titration results.

LEVEL FACTORIAL STUDIES

The 2 Level factorial studies were carried out using the

Design Expert software. The following table illustrates the

results obtained after carrying out each experiment for 6

hours.

Table 2: Factorial Studies for adsorption of CR on SDC

Std Run Block pH Amt of adsorbent

g

Conc of dye

mg/L

Temp

Deg C

Agitation speed

rpm

Qe

mg/g

%

removal

6 1 Block 1 9.00 0.10 150.00 30.00 100.00 127.603 85.068

7 2 Block 1 2.00 2.00 150.00 30.00 100.00 7.326 97.685

32 3 Block 1 9.00 2.00 150.00 50.00 200.00 6.797 90.634

29 4 Block 1 2.00 0.10 150.00 50.00 200.00 149.42 99.614

15 5 Block 1 2.00 2.00 150.00 50.00 100.00 7.479 99.724

25 6 Block 1 2.00 0.10 25.00 50.00 200.00 24.917 99.669

31 7 Block 1 2.00 2.00 150.00 50.00 200.00 7.355 98.071

9 8 Block 1 2.00 0.10 25.00 50.00 100.00 24.9174 99.669

3 9 Block 1 2.00 2.00 25.00 30.00 100.00 1.183 94.710

23 10 Block 1 2.00 2.00 150.00 30.00 200.00 7.471 99.614

26 11 Block 1 9.00 0.10 25.00 50.00 200.00 19.876 79.504

20 12 Block 1 9.00 2.00 25.00 30.00 200.00 0.9979 79.830

10 13 Block 1 9.00 0.10 25.00 50.00 100.00 20.124 80.496

1 14 Block 1 2.00 0.10 25.00 30.00 100.00 24.835 99.340

16 15 Block 1 9.00 2.00 150.00 50.00 100.00 7.3182 97.570



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2 16 Block 1 9.00 0.10 25.00 30.00 100.00 20.619 82.479

19 17 Block 1 2.00 2.00 25.00 30.00 200.00 1.2128 97.020

21 18 Block 1 2.00 0.10 150.00 30.00 200.00 149.669 99.770

8 19 Block 1 9.00 2.00 150.00 30.00 100.00 7.268 96.914

18 20 Block 1 9.00 0.10 25.00 30.00 200.00 14.2562 57.024

22 21 Block 1 9.00 0.10 150.00 30.00 200.00 95.207 63.470

11 22 Block 1 2.00 2.00 25.00 50.00 100.00 1.237 99.008

12 23 Block 1 9.00 2.00 25.00 50.00 100.00 0.985 78.844

14 24 Block 1 9.00 0.10 150.00 50.00 100.00 126.446 84.297

13 25 Block 1 2.00 0.10 150.00 50.00 100.00 150 100.00

24 26 Block 1 9.00 2.00 150.00 30.00 200.00 6.946 92.617

28 27 Block 1 9.00 2.00 25.00 50.00 200.00 0.692 55.370

30 28 Block 1 9.00 0.10 150.00 50.00 200.00 88.26 58.843

4 29 Block 1 9.00 2.00 25.00 30.00 100.00 0.754 60.330

27 30 Block 1 2.00 2.00 25.00 50.00 200.00 1.188 95.040

5 31 Block 1 2.00 0.10 150.00 30.00 100.00 149.83 99.889

17 32 Block 1 2.00 0.10 25.00 30.00 200.00 24.256 97.024

The results obtained were analysed using ANOVA (Analysis of Variance) and the following results were obtained for adsorption

capacity:

Table 3: ANOVA Analysis Table - I

Source Sum. of

Squares

df Square of Mean F - Value p-value

Prob. > F

For Model 91638.46 19 4823.08 104.96 < 0.0001 significant

A-pH 743.21 1 743.21 16.17 0.0017

B-Conc of Ad 38493.73 1 38493.73 837.67 < 0.0001

C-Conc of dye 26011.66 1 26011.66 566.05 < 0.0001

D-Temperature 0.14 1 0.14 3.023E-003 0.9571

E-Agitation

speed

197.03 1 197.03 4.29 0.0606

AB 700.04 1 700.04 15.23 0.0021

AC 588.78 1 588.78 12.81 0.0038

AD 0.41 1 0.41 8.880E-003 0.9265

AE 184.16 1 184.16 4.01 0.0684

BC 20651.41 1 20651.41 449.40 < 0.0001

BD 0.21 1 0.21 4.562E-003 0.9473

BE 188.29 1 188.29 4.10 0.0658

DE 0.011 1 0.011 2.444E-004 0.9878

ABC 590.33 1 590.33 12.85 0.0038

ABD 0.36 1 0.36 7.880E-003 0.9307

ABE 175.68 1 175.68 3.82 0.0742

ADE 1.555E-003 1 1.555E-003 3.385E-005 0.9955

BDE 0.077 1 0.077 1.673E-003 0.9681

ABDE 9.574E-003 1 9.574E-003 2.083E-004 0.9887

Residual 551.44 12 45.95

Cor Total 92189.90 31



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The F-value of 104.96 obtained from the model implies the it

is significant. There is a 0.01% chance that a "Model F-Value"

this large could happen due to noise.

St. Dev. 6.78 R2 0.9940

Mean 39.89 Adj R2 0.9845

CV. % 16.99 Pred R2 0.9575

PRESS 3921.35 Adequate Precision 27.864

The "Pred R-Squared" of 0.9575 agrees with the "Adj R-

Squared" of 0.9845."Adequate Precision" gives the ratio of

signal to noise. The desirable ratio should be greater than 4.

Adequate Precision of 27.864 point toward an adequate

signal. This model can be used to navigate the design space.

Final Equation with Actual Factors:

a) Percentage Removal = …(3)

+103.72242

-2.96926 * pH

-3.84462 * Amt of adsorbent

-0.010409 * Conc of dye

+0.061542 * Amt of adsorbent * Conc of dye

From the above equation we can infer that, percentage removal has a linear relationship with pH, amount of adsorbent and

concentration of dye. There is also a significant interaction between amount of adsorbent and concentration of dye.

b) Qe = …(4)

-12.75372

+6.64088 * pH

+6.04961 * Conc of Ad

+1.13263 * Conc of dye

+0.015267 * Temperature

+0.069641 * Agitation speed

-3.36255 * pH * Conc of Ad

-0.041310 * pH * Conc of dye

-8.33008E-00 * pH * Temperature

-0.038379 * pH * Agitation speed

-0.54153 * Conc of Ad * Conc of dye

-3.76566E-004 * Conc of Ad * Temperature

-0.032813 * Conc of Ad * Agitation speed

+4.35940E-005 * Temperature * Agitation speed

+0.020668 * pH * Conc of Ad * Conc of dye

+5.45320E-003 * pH * Conc of Ad * Temperature

+0.019346 * pH * Conc of Ad * Agitation speed

+9.25388E-006 * pH * Temperature * Agitation speed

-6.12732E-005 * Conc of Ad * Temperature * Agitation speed

-1.38722E-005 * pH * Conc of Ad * Temperature * Agitation speed

From the above equation we can infer that, adsorption

capacity has a linear, 2 factor interaction and 3 factor

interaction with the parameters taken into consideration.

Response Surface Methodology (RSM) Studies

Adsorption experiments were conducted as per design

developed with response surface Box-Benkhen design

methodology. The experiments were performed in 250mL

conical flasks and 100mL reaction mixture for 6 hours.



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Table 4: RSM Studies for adsorption of CR on SDC

Std Run Block pH Amt of

adsorbent

in g

Conc of

dye in

mg/L

Temp in °C Agitation

speed in

rpm

Qe in mg/g % removal

38 1 Block 1 5.50 2.00 87.50 30.00 162.50 4.2596 97.362

34 2 Block 1 9.00 1.05 87.50 40.00 125.00 7.981 95.780

15 3 Block 1 2.00 1.05 150.00 40.00 162.50 14.27 99.890

11 4 Block 1 5.50 0.10 87.50 40.00 200.00 68.8846 78.725

30 5 Block 1 5.50 1.05 150.00 40.00 125.00 13.941 97.589

32 6 Block 1 5.50 1.05 150.00 40.00 200.00 13.9047 97.330

33 7 Block 1 2.00 1.05 87.50 40.00 125.00 8.282 99.384

2 8 Block 1 9.00 0.10 87.50 40.00 162.50 48.039 54.901

42 9 Block 1 5.50 1.05 87.50 40.00 162.50 7.9158 94.989

44 10 Block 1 5.50 1.05 87.50 40.00 162.50 7.9158 94.989

23 11 Block 1 5.50 0.10 150.00 40.00 162.50 138.077 92.051

40 12 Block 1 5.50 2.00 87.50 50.00 162.50 4.209 96.219

26 13 Block 1 9.00 1.05 87.50 30.00 162.50 7.923 95.076

12 14 Block 1 5.50 2.00 87.50 40.00 200.00 4.132 94.460

36 15 Block 1 9.00 1.05 87.50 40.00 200.00 7.725 92.703

24 16 Block 1 5.50 2.00 150.00 40.00 162.50 7.161 95.487

43 17 Block 1 5.50 1.05 87.50 40.00 162.50 7.9158 94.989

6 18 Block 1 5.50 1.05 150.00 30.00 162.50 140.073 98.050

5 19 Block 1 5.50 1.05 25.00 30.00 162.50 1.9194 80.615

39 20 Block 1 5.50 0.10 87.50 50.00 162.50 81.577 93.230

4 21 Block 1 9.00 2.00 87.50 40.00 162.50 4.0519 92.616

16 22 Block 1 9.00 1.05 150.00 40.00 162.50 13.794 96.564

25 23 Block 1 2.00 1.05 87.50 30.00 162.50 8.2307 98.769

20 24 Block 1 5.50 1.05 87.50 50.00 200.00 7.688 92.264

28 25 Block 1 9.00 1.05 87.50 50.00 162.50 7.857 94.280

7 26 Block 1 5.50 1.05 25.00 50.00 162.50 2 84.000

21 27 Block 1 5.50 0.10 25.00 40.00 162.50 21.6923 86.769

17 28 Block 1 5.50 1.05 87.50 30.00 125.00 7.8864 94.637

9 29 Block 1 5.50 0.10 87.50 40.00 125.00 68.269 78.022

1 30 Block 1 2.00 0.10 87.50 40.00 162.50 87.1923 99.648

10 31 Block 1 5.50 2.00 87.50 40.00 125.00 4.2057 96.131

3 32 Block 1 2.00 2.00 87.50 40.00 162.50 4.321 98.769

37 33 Block 1 5.50 0.10 87.50 30.00 162.50 62.039 70.901

14 34 Block 1 9.00 1.05 25.00 40.00 162.50 1.523 64.000

22 35 Block 1 5.50 2.00 25.00 40.00 162.50 0.5154 41.232

41 36 Block 1 5.50 1.05 87.50 40.00 162.50 7.9158 94.989

35 37 Block 1 2.00 1.05 87.50 40.00 200.00 8.274 99.296

18 38 Block 1 5.50 1.05 87.50 50.00 125.00 7.989 95.868

29 39 Block 1 5.50 1.05 25.00 40.00 125.00 2.058 86.460

46 40 Block 1 5.50 1.05 87.50 40.00 162.50 7.9158 94.989



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8 41 Block 1 5.50 1.05 150.00 50.00 162.50 14.102 98.718

45 42 Block 1 5.50 1.05 87.50 40.00 162.50 7.9158 94.989

19 43 Block 1 5.50 1.05 87.50 30.00 200.00 7.322 87.868

27 44 Block 1 2.00 1.05 87.50 50.00 162.50 8.304 99.648

31 45 Block 1 5.50 1.05 25.00 40.00 200.00 2.043 85.840

13 46 Block 1 2.00 1.05 25.00 40.00 162.50 2.2857 96.000

The following fit summary was obtained for adsorption capacity :

Table 5: Fit Summary for adsorption capacity of SDC

Source. Sum. of Squares df Square of Mean F- value p-value

Prob. > F

Mean vs Total 20602.07 1 20602.07

Linear vs Mean 25686.77 5 5137.35 8.22 < 0.0001 Suggested

2FI vs Linear 7457.02 10 745.70 1.28 0.2872

Quadratic vs 2FI 8791.02 5 1758.20 5.03 0.0025 Suggested

Cubic vs Quadratic 6997.09 15 466.47 2.68 0.0599 Aliased

Residual 1739.60 10 173.96

Total 71273.58 46 1549.43

The following model summary analysis was obtained for adsorption capacity :

Table 6: Model Summary Analysis for adsorption capacity of SDC

Source. St. dev R2 Adjusted R2 Predicted R2 “PRESS”

Linear 24.99 0.5069 0.4453 0.3296 33971.16 Suggested

2 FI 24.17 0.6541 0.4811 0.0849 46370.15

Quadratic 18.69 0.8276 0.6896 0.3103 34946.78 Suggested

Cubic 13.19 0.9657 0.8455 -1.1972 1.113E+005 Aliased

*PRESS – Predicted Residual Sum of Sqaures

The following fit summary was obtained for percentage removal:

Table 7: Fit summary for percentage removal of CR

Source. Sum. of Squares df Square of Mean F- value p-value

Prob. > F

Mean vs Total 3.775E+005 1 3.775E+005

Linear vs Mean 2401.10 5 480.22 4.67 0.0019 Suggested

2FI vs Linear 1323.94 10 132.39 1.42 0.2177

Quadratic vs 2FI 1032.91 5 206.58 2.94 0.0320 Suggested

Cubic vs Quadratic 1649.41 15 109.96 10.20 0.0004 Aliased

Residual 107.76 10 10.78

Total 3.84E+005 46 8347.99



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The following model analysis was obtained for pecentage removal:

Table 8: Model analysis for percentage removal of CR

Source. St. dev R2 Adjusted R2 Predicted R2 “PRESS”

Linear 10.14 0.3685 0.2896 0.1385 5612.54 “Suggested”

2 FI 9.64 0.5718 0.3576 -0.1464 7468.85

Quadratic 8.38 0.7303 0.5145 -0.0788 7028.67 “Suggested”

Cubic 3.28 0.9835 0.9256 -0.0586 6896.72 “Aliased”

From Table 6, it can be concluded that quadratic model gives

a better fit for adsorption capacity as the R2 value is high and

adjusted-R2, predicted-R2 are in reasonable agreement.

Similarly from Table 8, Linear model fits better for

percentage removal.

Final Equation with actual factors:

a) Percentage removal = (5)

+80.17899

-1.88362 * pH

+3.81764 * Amt of adsorbent


+0.19342 * Temperature

-0.025644 * Agitation speed

b) Qe = (6)

-66.46694

-0.82266 * pH

-53.87947 * Amt of adsorbent


-2.02430 * Temperature

+0.73953 * Agitation speed

+2.92362 * pH * Amt of adsorbent

+3.27657E-004 * pH * Conc of dye

-9.95000E-004 * pH * Temperature

-4.72381E-004 * pH * Agitation speed

-0.46206 * Amt of adsorbent * Conc of dye

-0.51549 * Amt of adsorbent * Temperature

-4.83719E-003 * Amt of adsorbent * Agitation speed

-0.050421 * Conc of dye * Temperature

-2.27200E-006 * Conc of dye * Agitation speed

+1.75600E-004 * Temperature * Agitation speed

-0.26491 * pH2

+30.43680 * Amt of adsorbent2

+2.35444E-003 * Conc of dye2

+0.078653 * Temperature2

-2.27614E-003 * Agitation speed2



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Analysis of RSM Studies

Figure 5: Effect of amount of SDC and conc. of CR on adsorption capacity of SDC. (pH 2, 30°C, 125 rpm)

Figure 6: Effect of pH and amount of SDC on adsorption capacity of SDC. (150 mg/L of CR, 30°C, 125 rpm)

Figure 7: Effect of temp. and amount of adsorbent on adsorption capacity of SDC. (150 mg/L of CR, pH 2, 125 rpm)



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Figure 8: Effect of temp. and agitation rpm on adsorption capacity of SDC. (150 mg/L of CR, 1 g/L of SDC, pH 2)

Figure 9: Effect of pH and temp. on adsorption capacity of SDC. (150 mg/L CR, 1 g/L of SDC, 125 rpm)

Figure 10: Effect of SDC dosage on percentage CR removal . (150 mg/L of CR, pH 2, 30°C, 125 rpm)



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Figure 11: Effect of pH on percentage CR removal (150 mg/L of CR, 1g/L of SDC, 30°C, 125 rpm)

Figure 12: Effect of temperature on percentage removal of CR. (150 mg/L of CR, 1 g/L of SDC, pH 2, 125 rpm)

Figure 13: Effect of agitation rpm on percentage CR removal (150 mg/L of CR, 1 g/L of SDC, pH 2, 30°C)



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Effect of pH – The isoelectric point of congo-red is 3[5]. At

pH below the isoelectric point, CR exists mostly in the

molecular form and above the isoelectric point it exists in its

dissociated form. By and large the adsorption of an anionic

dye drops with increase in pH, due to two reasons (i) the

negative charge on the surface of the adsorbent (ii) excess

OH- ions in the solution that compete for the adsorption sites.

The adsorption of congo-red was studied in the pH range of 2-

9 where Qe decreased from 149.462 mg/g to 119.15 mg/g and

percentage removal decreased from 96.79% to 76% with

increase in pH. This behaviour may be due to greater positive

charge on the exterior of the SDC.

Effect of initial concentration of dye – The equilibrium

adsorption capacity (Qe) increased from 24.8333 mg/g to

149.462 mg/g with increase in initial CR concentration from

25 to 150 mg/L due an increase in the mass gradient between

the CR solution and the SDC and thus acting as a driving

force for the transfer of CR molecules from the bulk to the

SDC surface. However the percentage removal decreased

from 97.2931% to 96.7878% with increase in the initial CR

concentration from 25 to 150 mg/L, since the surface area

available and the active sites saturate leading to a decrease in

percentage decolourization.

Effect of temperature – Both percentage removal and

adsorption capacity increased from 85.641% to 96.41% and

128.4615 mg/g to 144.6154 mg/g respectively with rise in

temperature from 30°C to 50°C. By increase in temperature,

the solubility of the dye decreases which results in greater

adsorption. An increase in adsorption may also be due to rise

in the movement of the large CR ion with temperature, as

many molecules may gain sufficient energy to endure

interaction with active sites on the surface of SDC.

Effect of SDC dosage – Percentage removal of congo-red

increased from 96.7878% to 99.23% with increase in SDC

dosage from 0.1 to 2 g whereas Qe decreased from 149.462

mg/g to 7.35578 mg/g with increase in SDC dosage. Increase

in percentage removal can be ascribed to increased SDC

surface area and presence of additional adsorption sites.

Whereas decrease in adsorption capacity may be due to

overlapping of sorption sites as a result of overcrowding of

SDC particles..

Isotherm Modelling

The optimum conditions in terms of amount of adsorbent (Ben

Teak Sawdust), concentration of dye, pH, agitation speed and

temperature were maintained throughout to model the

optimum isotherm model. The non-linear forms of Langmuir

and Freundlich isotherm equations were used and their

applicability was determined by comparing the correlation

coefficients (R2) and the model with the highest R2 value is

considered as the best fit.

Langmuir isotherm theory is based on the assumptions that the

surface of the adsorbent is uniform with no interaction of the

adsorbed molecules. The entire adsorption happens through

the identical mechanism and at the maximum adsorption, only

a monolayer is formed. The Langmuir equation can be written

in the following form [6]:

Where,

Qe = equilibrium adsorbent capacity (mg/g)

Ce = equilibrium conc. of dye (mg/L)

b = constant associated to the affinity of binding sites (L/mg)

Qo = Maximum adsorption capacity of adsorbent (mg/g)

The Freundlich isotherm is derived by assuming a

heterogeneous surface with a non-uniform distribution of heat

of adsorption over the surface and is used for dilute solution[7].

The equation is:

Qe = KF Ce (1/n) (8)

Where,

Ce= equilibrium conc. in the dye solution (mg/L)

Qe = adsorption capacity at equilibrium (mg/g)

KF = amount adsorbed at unit concentration

n = constant.

The following are the graphs obtained through isotherm

studies performed at 30°C, 40°C and 50°C. The graphs are

obtained by plotting the adsorbent capacity, Qe (mg/g) against

the final dye concentration, Ce (mg/L).



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Figure 14. Langmuir and Freundlich Isotherms at 30°C for adsorption of CR on SDC. (1 g/L of SDC, pH 3.3, 125 rpm)




8802


Table 9. Langmuir and Freundlich isotherm constants at 30°C for CR adsorption on

30°C

Langmuir Freundlich

Co (mg/L) Qe (mg/gm) RL Qo (mg/gm) b (L/mg) R2 KF (mg/gm) (L/g)n 1/n R2

25 22.76923 0.2276 166.673 0.135735 0.92546 28.21234 0.4944 0.92333

50 47.07692 0.1284

75 71.76923 0.08944

100 89.23077 0.06861

150 128.4615 0.04681

SDC. (1 g/L of SDC, pH 3.3, 125 rpm)

Table 10:. Langmuir and Freundlich isotherm constants at 40°C for CR adsorption on SDC. (1 g/L of SDC, pH 3.3, 125 rpm)

40°C

Langmuir Freundlich

Co (mg/L) Qe (mg/gm) RL Qo (mg/gm) b (L/mg) R2 KF (mg/gm) (L/gm)n 1/n R2

25 23.84615 0.1558 184.095 0.21685 0.92546 36.626 0.5444 0.93174

50 48.38462 0.0845

75 73.07692 0.0579

100 93.07692 0.04413

150 139.2308 0.02986



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Table 11: Langmuir and Freundlich isotherm constants at 50°C for CR adsorption on SDC. (1 g/L of SDC, pH 3.3, 125 rpm)

50°C

Langmuir Freundlich

Co (mg/L) Qe (mg/gm) RL Qo (mg/gm) b (L/mg) R2 KF (mg/gm) (L/gm)n 1/n R2

25 24.38462 0.106830369 209.7036 0.33442 0.94671 51.588 0.5894 0.952

50 49.23077 0.056429369

75 73.76923 0.038340761

100 96.46154 0.029033866

150 144.6154 0.019545067

The results show slight deviation from the fitted non-linear

equation of Langmuir model as indicated by the high value of

R2 ranging from 0.925 to 0.947. The highest R2 value was

obtained at 50°C. From the literature, when the R2 value is

greater than 0.89, the adsorption data follows the Langmuir

model. Moreover, the value of Qo of activated Ben Teak wood

was calculated to be 209.7mg/g at 50°C and an initial

concentration of 150mg/L. The fitting of data to Langmuir

model indicates both the homogeneous nature of Ben Teak

sawdust surface and the formation of monolayer of Congo-

Red dye molecule at its exterior surface.

The dimensionless separation factor (RL) indicates a

characteristic of Langmuir isotherm. The RL values obtained

were in the range of 0 to 1 indicating good adsorption under

all temperatures and initial CR concentrations.

The non-linear model of Freundlich isotherm was used to find

the intercept values of KF and the slope (1/n) together with R2.

The highest value of R2 obtained using Freundlich model

(0.952) was slightly higher than that obtained using Langmuir

model. The values of (1/n) obtained were in the range 0 and 1

thus indicating the favourable nature of the process and

heterogeneous nature of the adsorbent surface. Therefore,

Freundlich model is also well fitted using the data.

Hence, it can be conclude that Ben Teak is indeed a good

adsorbent of Congo-Red dye using both the Langmuir and

Freundlich model. The applicability of both Langmuir and

Freundlich model shows that both monolayer adsorption and

heterogeneous energy distribution of active sites on the

surface of the SDC exist under the experimental conditions

employed.

CONCLUSION

Chemically activated and carbonised ben-teak saw dust can be

considered a suitable adsorbent for removal of textile dyes

from aqueous solution. The percentage

removal/decolourisation increased with (i) increase in SDC

mass (ii) decrease in concentration of dye (iii) decrease in pH

(iv) increase in temperature and (v) decrease in agitation

speed. Whereas, Adsorption capacity increased with (i)

decrease in SDC mass (ii) increase in concentration of

dye(iii)decrease in pH and (iv) increase in temperature.

According to RSM studies, adsorption capacity fitted better in

quadratic equation whereas percentage removal fitted better in

linear equation. The adsorption isotherm was successfully

defined by Freundlich isotherm.

ACKNOWLEDGEMENTS

Authors gratefully acknowledge assistance received in the

form of experimental facilities for conducting this research

work from Manipal University, Manipal, India.

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statistical analysis of congo red dye removal using ...boehm titration [4] for boehm titration, 1.0...

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