Batch Column Studies 4101 Effect of Bed Height

The observed curves of RR158 adsorption on CAC's at three different bed heights are displayed in Fig. 54, plotted as the percentage dye removal versus time and Fig. 55 shows the breakthrough curves. The uptake of RR158 increased from 73.60% to 92.49% with increase in bed height from 6 to 10 cm. This rise in dye uptake was due to the increase in mass of adsorbent in the column resulting in an increase in the surface area of adsorbent, which provided more binding sites for adsorption [1].

Results indicated that volume varies with bed depth. This displacement of the front of adsorption with the increase in depth can be explained by mass transfer phenomena that take place in this process. When bed depth is reduced, axial dispersion phenomena predominates in the mass transfer and reduces the diffusion of species. The solute thus does not have enough time to diffuse into the whole of the adsorbent mass thus leading to a reduction in dye uptake.

The breakthrough curves (Fig. 55) had high R2 values of 0.972 for bed height of 6 and 8 cm and 0.990 for bed height of 10 cm, and so followed the characteristics "S" shape profile produced in an ideal adsorption system. Less sharp breakthrough curves were obtained at higher mass of adsorbent. The variation in breakthrough

2.000

Fig. 53 The linearized intra particle diffusion adsorption isotherms of RR158 by CAC's. Experimental conditions: room temperature; agitation rate: 200 rpm; biomass dosage: 20 g/L; pH: 2

Table 12 Summary of results of intra particle diffusion model

Dye concentration (mg/L)

R2 value of graph

a

kid (h-1)

Adsorbent dosage: 7.5 g/L

20

0.040

-0.528

37.325

40

0.942

0.508

1.052

60

0.751

0.088

12.853

80

0.970

0.032

15.596

100

0.829

0.130

9.226

120

0.939

0.108

15.704

Adsorbent dosage: 10 g/L

20

0.799

0.198

8.620

40

0.488

0.598

1.641

60

0.586

0.449

1.563

80

0.947

0.093

12.794

100

0.397

0.053

23.659

120

0.740

0.099

16.634

Adsorbent dosage: 12.5 g/L

20

0.510

0.370

4.020

40

0.357

1.194

0.096

60

0.703

0.357

4.710

80

0.801

0.743

0.472

100

0.031

0.046

17.458

120

0.107

0.739

5.248

Adsorbent dosage: 15 g/L

20

0.694

0.474

3.266

40

0.795

0.657

1.426

60

0.704

0.173

15.776

80

0.010

0.013

16.943

100

0.384

0.578

1.510

120

0.120

0.076

14.191

Adsorbent dosage: 17.5 g/L

20

0.807

0.920

0.832

40

0.633

0.751

1.084

60

0.295

0.037

24.434

80

0.082

-0.035

26.607

100

0.016

0.010

19.861

120

0.645

0.266

5.623

Adsorbent dosage: 20 g/L

20

0.827

0.496

5.383

40

0.906

0.781

1.253

60

0.808

0.323

8.892

80

0.827

0.681

1.358

100

0.354

0.134

0.057

120

0.220

0.038

19.999

Fig. 54 Graph of dye removal versus time for batch column experiment. Experimental conditions: pH: 2, volume of dye: 250 mL, initial dye concentration of dye: 120 mg/L
Fig. 55 Breakthrough curves of adsorption of RR158 dye on CAC's at various bed heights. Experimental conditions: pH: 2, input volume of dye: 250 mL, initial dye concentration: 120 mg/L

shape with CAC's mass is mainly due to the relatively large adsorption zone that is CAC's near the bottom of the adsorption column, which comes into contact with dye solution after CAC's near the top of column, is exhausted. Therefore, the mass transfer zone (MTZ) has its own characteristics that are dependent upon the nature

Fig. 56 Dye removal at varying initial dye concentration. Experimental conditions: pH: 2, volume of dye input: 150 mL, bed height: 6 cm

of adsorbent-adsorbate interactions in addition to the experimental conditions such as mass of adsorbent and initial dye concentration.

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