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

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)

  (2)

  (3)

where

• C_e is the equilibrium concentration (mg/L)
• q_e is the amount of dye sorbed (mg/g)
• K_F is the Freundlich isotherm constant (mg/g)(L/g)ⁿ
• n is the Freundlich exponent
• q_m is q_e for a complete monolayer (mg/g)
• K_L is sorption equilibrium constant (L/mg)
• A is the Redlich Peterson isotherm constant (L/g)
• B is the Redlich Peterson isotherm constant (L/mg^(1-1/A))
• g is the exponent, which lies between 0 and 1

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 C_o, mg/L. The dye concentration is to be reduced from C_n-1 to C_n mg/L. M, g of biomass with solid phase concentration of q_o was used to reduce the dye concentration on the biomass increases from q_o mg/g to q_n mg/g. The dye uptake process can be represented by a mass balance equation:

  (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)

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)

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)

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 C_n-1 to C_n 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 11^th 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

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

Fig 2 · Schematic for multistage batch sorption

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 (C_o: 150 mg/L; C₂: 10 mg/L)

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

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