A Useful Method to Design a Multistage Sorption System

By K. Vasanth Kumar¹ and K. Porkdoi²
January 2007

  1. Department of Chemical Engineering - A.C. College of Technology, Anna University, Chennai, TN - India
  2. CIQ-UP, Department of Chemistry, Faculty of Science, University of Porto, Rua do Campo Alegre 687, 4169-007 Porto - Portugal

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Abstract
In the present report a simple procedure to design a two stage sorption system using the experimental equilibrium data of single stage sorption system was proposed. The experimental equilibrium data of basic red onto Pithophora sp, a fresh water algae was used as a case study. The present study shows that a two stage sorption system had reduced the biomass dose by 13% when compared to that of single stage sorption system.

Key words: biosorption, basic red 9, isotherm, multistage sorption, design, sorbent minimization

Notations
qnSolid phase concentration for a nth stage sorption system, mg/g
qoSolid phase concentration at time t = 0, mg/g
CnEquilibrium concentration in a nth stage sorption system, mg/g
Cn-1Initial dye concentration in a nth stage sorption system, mg/g
VVolume of dye solution to be treated, L
qeAmount of dye adsorbed at equilibrium, mg/g
CeEquilibrium dye solution concentration, mg/L
KFFreundlich isotherm constant, (mg/g)(L/g)n
1/nFreundlich exponent
ARedlich Peterson isotherm constant (L/g)
BRedlich Peterson isotherm constant (L/mg(1-1/A))
gRedlich Peterson isotherm exponent
MBiosorbent mass, g
M1Biosorbent mass required in stage 1, g
M2Biosorbent mass required in stage 1, g
KLLangmuir isotherm constant, L/mg
qmMaximum sorption capcity of sorbent, mg/g

1. Introduction

In the sorption processes, it is very important to reduce the cost of sorbent material or to minimize the usage of the high cost adsorbent in the process. Minimization of sorbent mass will be important especially for the case of sorption with high cost sorbents or when using sorbent materials which show a high potential to uptake the target compounds but are readily or easily available. In the present investigation, a model to design a two stage sorber was proposed using Langmuir isotherm in order to minimize the biosorbent mass.

The dark green colored Pithophora sp used in the present study was collected from the CEG fountain, Anna University, India. The collected species were then washed with deionised water several times to remove dirt particles. The washing process was continued till the wash water contained no color. The washed materials were then completely dried in sunlight for 10 days. The resulting half white color product were then cut into small pieces and powdered using domestic Sumeet mixie. The powdered materials were then directly used as adsorbents without any further treatment. The particle size in the range of 1-3 mm was used in the present study.

2. Experimental

The dye used in all the experiments was basic red 9; a basic (cationic) dye was obtained from CDH Chemicals, New Delhi. The details of the dye used are given in Table 1. Stock synthetic dye solutions were prepared by dissolving 1 gram of basic red 9 in 1 L of double distilled water. All working solutions were prepared from the stock solution by dilution. The NaOH pellets and HCl solution used for pH study were obtained from Qualigens Fine Chemicals, Mumbai, India.

Batch biosorption experiments were conducted by contacting known volume of dye solution of known initial dye concentration with weighed amount of biomass in a 100 mL round bottom flask running at different time intervals. Agitation was provided using a magnetic stirrer at a constant agitation speed of 250 RPM. All the experiments were carried out at a room temperature of 30°C. All the experiments were carried out at a an initial solution pH of 6. The concentration of dye ions before and after sorption was determined using UV spectrophotometer.

Biosorption equilibrium experiments were carried out by agitating 0.01 g of Pithophora sp in a series of beaker containing 30 mL of basic red 9 solution of different initial dye concentration of 60, 80, 100, 110, 120,1 30, 140 and 150 mg/L at a constant solution temperature of 30°C. The agitation was made for 12 hours, which is more than the sufficient time (predetermined by trial experiments) to reach equilibrium.

3. Results and Discussions

The three widely used Freundlich, Langmuir and Redlich Peterson isotherm was used to explain the equilibrium uptake of basic red 9 by Pithophora sp. The Freundlich (1906), Langmuir (1916) and Redlich Peterson (1959) isotherms are given by eq (1) – (3) respectively:

1   (1)

2   (2)

3   (3)

where

The parameters involved in the isotherm expressions used were obtained by non-linear method. For non-linear method, a trial and error procedure, which is applicable to computer operation, was developed to determine the isotherm parameters by minimizing the respective coefficients of determination between experimental data and isotherms using the Solver addin with Microsoft’s spreadsheet, Microsoft Excel. Figure 1 shows the experimental equilibrium data and the predicted isotherms for the sorption of basic red 9 onto Pithophora sp at 305 K. The calculated isotherm constants and their corresponding r² values are given in Table 1.
From the r² value (Table 1), it was observed that Langmuir and Redlich Peterson isotherm as the best fit isotherm to represent the equilibrium uptake of basic red 9 onto Pithophora sp. From Fig 1, it was observed that the Redlich Peterson isotherm exactly overlapped the Langmuir isotherm with the same coefficient of determination, r², value (Table 1) when the constant g equals unity. Thus Redlich Peterson is a special case of Langmuir when the constant g equals unity. The better fit of experimental equilibrium data in the Langmuir isotherm and the Redlich Peterson constant g = 1, indicates the monolayer coverage and the chemisorption of basic red 9 onto Pithophora sp. The chemisorption may be due to the polysaccharides of the algal cell walls which could provide binding groups including amino, carboxyl, phosphate and sulphate anions (Özer et al, 1999). Polysaccharides of the algal cell walls could provide binding groups including amino, carboxyl, phosphate and sulphate anions (Özer et al, 1999). The amino and carboxyl groups and the nitrogen and oxygen of the peptide bond could be available for characteristic coordination bonding with dye cations (Özer et al, 1999).

The Langmuir isotherm model was used to design a multistage sorber and for biomass optimization. The schematic diagram for a multi stage is shown in Figure 2. The solution to be treated contains V, L of dye solution of initial dye concentration Co, mg/L. The dye concentration is to be reduced from Cn-1 to Cn mg/L. M, g of biomass with solid phase concentration of qo was used to reduce the dye concentration on the biomass increases from qo mg/g to qn mg/g. The dye uptake process can be represented by a mass balance equation:

4   (4)

When fresh biomass is used at each stage, the amount of dye adsorbed on the unit mass biosorbent for a desired amount of dye removal can be obtained by rearranging eq (4) as follows:

5   (5)

If the equilibrium dye uptake follows the Langmuir isotherm, the solid phase concentration for the desired amount of dye removal can be evaluated using the equation:

6   (6)

Combining eq (5) and eq (6), the amount of biomass required for the desired removal of dye can be predicted using eq (7) as follows:

7   (7)

Eq (7) can be used to determine the amount of biomass required for any given initial dye concentration and for any desired amount of dye removal for any multistage system.

For a two stage batch sorption system, the design parameters are now explained. The design objective is to treat 50 L of basic red 9 solution of initial dye concentration 150 mg/L in the first stage. A series of equilibrium dye concentration from 140 mg/L to 10 mg/L in 10 decrements was considered in stage one of a two stage sorption system. The design plot which explains the amount of biomass needed in different two stage sorption systems are shown in Figure 3. The x-axis in Fig 3 represents the equilibrium concentration in the first stage of the two stage sorption system based on 10 mg/L of equilibrium dye concentration difference. In the sorption system number one, the design the objective is to reduce the initial dye concentration from 150 mg/L to 140 mg/L. Similarly in the sorption system 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 ,12, 13, and 14 the design objective of the first stage is to reduce the initial dye concentration from 150 mg/L to 130 mg/L, 120 mg/L, 110 mg/L, 100 mg/L, 90 mg/L, 80 mg/L, 70 mg/L, 60 mg/L, 50 mg/L, 40 mg/L, 30 mg/L, 20 mg/L and upto 10 mg/L respectively. The various process conditions at stage 1 for all the sorption systems were explained in Table 2. For all the sorption system number, the design objective of the second stage is to reduce the equilibrium dye concentration in stage 1 to 10 mg/L. The corresponding amount of biomass needed for the required amount of dye removal in stage 1 and stage 2 can be calculated eq (7). Based on the sorption system number that utilizes the minimum biomass dose to reduce the dye concentration from Cn-1 to Cn can be predicted from the plot of total biomass does required in both stages of two stage sorption system versus the equilibrium concentration in stage one as shown in Fig 3. The number enclosed in the parenthesis in the x-axis of Fig 3 represents the two stage sorption system number. From Fig 3, it can be observed that the 11th two stage sorption system with equilibrium concentration of 40 mg/L in stage one of utilized minimum biomass to achieve the desired objective of reducing 50 L of dye solution from 150 mg/L to 10 mg/L.

A similar two stage sorption systems were developed for different solution volumes to be treated for decreasing initial dye concentration from 150 mg/L to 10 mg/L. The determined amount of biomass required in each stage for the different volumes of solution to be treated to reduce the initial dye concentration from 150 mg/L to 10 mg/L were shown in Table 2. Figure 4 shows the total amount of biomass required at both the stages versus sorption system number for different volumes of basic red 9 solution. The dashed line in Fig 4 shows the minimum amount of biomass required for different volumes of dye solution to be treated. The predicted optimized biomass required for two stage sorption system to reduce the dye concentration from 150 mg/L to 10 mg/L for different dye solution volumes is given in Table 3. Table 2, also shows the biomass required to reduce the concentration from 150 mg/L to 10 mg/L in a single stage sorber. From the comparison of Table 2 and Table 3, it can be observed that for all the two stage sorption system, at optimized condition, it can be observed that a two stage sorption system had reduced the biomass dose by 13% when compared to that of single stage sorption system.

4. Conclusions

A design procedure was proposed using the Langmuir isotherm to design a two stage sorption system to minimize the amount of biomass required for the treatment of basic red 9 solution using Pithophora sp. A two stage sorption system reduced the amount of biomass required by 13 % to achieve the required amount of dye removal for any for any solution volume. The present design procedure is particularly useful if the adsorbents are costlier where the cost of adsorbent is a very important criterion in the sorption process.

Though the present study reports the sorption using a naturally available low cost biosorbent, the importance of present research will be useful when using very expensive tailor made adsorbents for the treatment of target pollutants from wastewaters.

Figures

Table 1 · Isotherm parameters for basic red 9 onto Pithophora sp at 30°C (V: 0.03 L; M: 0.01 g; pH: 6)
Freundlich Langmuir Redlich Peterson
KF,(mg/g)(L/g)n 107.4148 qm, mg/g 344.1613 A 56.32918
1/n 0.283624 KL, L/g 0.1636 B 0.163381
0.941428 0.986183 g 1
        0.985695
Table 2 · Biomass (g) required for a series of sorption system (V: 50 L; Co: 150 mg/L; C2:10 mg/L)
Sorption
system
C1
mg/L
Biomass required in stage 1, M1, g Biomass required in stage 2, M2, g
50 L 60 L 70 L 80 L 90 L 50 L 60 L 70 L 80 L 90 L
1 140 1.623 1.948 2.272 2.597 2.921 26.787 32.145 37.502 42.860 48.217
2 130 3.251 3.901 4.552 5.202 5.852 24.727 29.672 34.618 39.563 44.508
3 120 4.886 5.863 6.840 7.817 8.795 22.666 27.200 31.733 36.266 40.799
4 110 6.529 7.835 9.140 10.446 11.752 20.606 24.727 28.848 32.969 37.090
5 100 8.182 9.819 11.455 13.092 14.728 18.545 22.254 25.963 29.672 33.381
6 90 9.850 11.820 13.790 15.761 17.731 16.485 19.781 23.078 26.375 29.672
7 80 11.538 13.845 16.153 18.461 20.768 14.424 17.309 20.194 23.078 25.963
8 70 13.253 15.904 18.555 21.206 23.856 12.363 14.836 17.309 19.781 22.254
9 60 15.011 18.013 21.016 24.018 27.020 10.303 12.363 14.424 16.485 18.545
10 50 16.836 20.203 23.570 26.938 30.305 8.242 9.891 11.539 13.188 14.836
11 40 18.779 22.535 26.290 30.046 33.802 6.182 7.418 8.654 9.891 11.127
12 30 20.957 25.149 29.340 33.532 37.723 4.121 4.945 5.770 6.594 7.418
13 20 23.725 28.469 33.214 37.959 42.704 2.061 2.473 2.885 3.297 3.709
14# 10 28.848 34.618 40.387 46.157 51.926 0.000 0.000 0.000 0.000 0.000
# Biomass required in a single stage sorption system
Table 3 · Optimum biomass required for the treatment of different dye solution volume (Co: 150 mg/L; C2: 10 mg/L)
Volume of dye solution, L Optimum biomass, g
50 24.961
60 29.953
70 34.945
80 39.937
90 44.929

Fig 1 · Experimental data and isotherms for basic red 9 onto Pithophora sp
Fig 1

Fig 2 · Schematic for multistage batch sorption
Fig 2

Fig 3 · Comparison of biomass required in each stage for different equilibrium dye concentration after sorption in stage 1 for a solution volume of 50 L (Co: 150 mg/L; C2: 10 mg/L)
Fig 3

Fig 4 · Minimum biosorbent mass required for various dye volume solution in a two stage process (Co: 150 mg/L; C2: 10 mg/L)
Fig 4

References:

  1. H.M.F. Freundlich, Over the adsorption in solution, Zeitschrift für Physikalische Chemie, 57A (1906) 385-470.
  2.  I. Langmuir, The constitution and fundamental properties of solids and liquids, Journal of the American Chemical Society, 38 (1916) 2221-2295.
  3. O. Redlich, D.L. Peterson. A useful adsorption isotherm. Journal of Physical Chemistry, 63 (1959) 1024.

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