Adsorption of Methylene Blue onto pithophora sp

By K. Vasanth Kumar, S. Sivanesan and V. Ramamurthi
June 2004

The Authors are Research Scholars at the Department of Chemical Engineering - A.C. College of Technology, Anna University, in Chennai - India.   → See also:

Abstract
Biosorption experiments were carried out in batch process for the removal of methylene from its aqueous solution using pithophora sp, a fresh water algae as an adsorbent. The sorption capacity was found to be 45.017 mg/g at 30oC for an optimum solution pH of 3.25. The dye cations were found to get adsorbed onto pithophora sp due to strong electrostatic force of attraction. Negative value of free energy change indicates the spontaneous nature of biosorption.

Key words: Biosorption, pithophora sp, methylene blue, fresh water algae

Introduction

Removal of color from dye bearing wastewaters is one of the major environmental problems because of difficulty in treating such wastewaters by conventional treatment methods, as most of the dyes are stable to light ad oxidizing agents. Presence of even minute amount of coloring substance makes it unsuitable for drinking or other recreational purposes due to its undesirable appearance. The most commonly used method for the removal of color from dye stuffs are chemical precipitation and biological oxidation [1-3]. Photo catalytic oxidation and ozone treatment are the other alternative technologies commonly employed. However these treatment methods are effective and economic only in the case where the solute concentrations are relatively high. Activated carbon is the effective alternate for the concentration of dye ions even in the case of lower solute concentration. However activated carbon adsorption is limited due to the high cost of activated carbon and the cost involved in regenerating it. The use of biomaterials for the removal of color from its aqueous solution will provide as a potential alternate to the conventional treatment techniques. The selective uptake of dye ions (solute) by microbial cells, whether dead or alive, is known as biosorption. Biosorption exploits the ability of plant biomass to sequester solute particles from aqueous solution by physiochemical mechanisms [4]. Recent investigations by several researchers proved that algae, whether dead or alive as one of the effective biosorbent for treating wastewaters [5-8]. However almost all of the works have been focused only on the removal of metal cations from wastewaters and a survey of literature indicated that no much work have been done so far on algal species for the removal of dye from wastewaters. In the present investigation, the ability of pithophora sp, a freshwater algae to adsorb dye from aqueous solution of methylene blue, a basic dye have been investigated. Methylene blue was selected as a model compound as an attempt to use pithophora sp as an adsorbent for the removal of dye from wastewaters.

Experimental

The algal biomass of pithophora sp, a fresh water algae was collected from CEG campus fountain, Anna University, India. The collected materials were washed with deionised water for a number of times to remove dirt particles and the washing process was continued till the wash water contains no color. The washed materials were then sun dried for 48 hours. The dried materials were then activated at 300oC for 50 min using muffle furnace. The activation temperature and activation time was selected based on the preliminary studies at different temperature and activation time that gives maximum sorption capacity for methylene blue removal. The activated materials were then powdered using a domestic sumeet mixie. The powdered materials were then used as adsorbents for study. The powdered materials were then sieved and the particle size in the range of 0.3 mm to 1 mm was used in the present study.

The IR Spectrum for the prepared activated algae are obtained from the sophisticated instruments laboratory, A.C.Tech, India. The spectrum was recorded in the range of 4000 cm-1 to 400 cm-1. A band at 3676 cm-1 corresponds to a hydroxyl group. Bands at 1684 and 1654 cm-1 corresponds to C=O in amides. A band at 3349.8 cm-1 corresponds to N-H groups in amides. Peak at 1458 cm-1 corresponds to C=C stretch vibrations.

The dye used in all the experiments was methylene blue. Synthetic dye solutions were prepared by dissolving weighed amount of dye in one liter of distilled water. NaOH pellets and HCl solutions used for pH studies were obtained from Qualigens fine chemicals, Mumbai, India.

Table 1. Equilibrium data for methylene blue onto pithophora sp
Co, mg/L M, g V,mL Ce, mg/L qe, mg/g Ce/qe, g/l
75 0.053 30 7.92 52.12075 0.151955
100 0.041 30 13.51 63.28537 0.213477
100 0.034 30 17.79 72.53824 0.24525
75 0.15 200 20.83 72.22667 0.288398
100 0.15 200 34.16 87.78667 0.389125

Equilibrium studies were carried out by agitating a known weight of algae to a series of beaker containing different volume of methylene blue solution of different concentrations at a desired solution at room temperature (30oC) with a constant agitation speed of 175 RPM. The different sorbate and sorbent concentrations and the volume of solution taken for equilibrium study were shown in table 1. Agitation was provided for 10 hours, which is more than sufficient time to reach equilibrium. After equilibrium the concentrations in the samples were analyzed as before.

Results and Discussions

The effect of pH is one of the important parameter affecting the biosorption process. In the present investigation, the maximum amount of color was adsorbed at a pH of 3.25 and it decreased before and after this pH. This can be explained on the basis of zero point of charge for algae. This may be due to the zero point charge for the prepared adsorbent may be found at a pH of 3. At lower pH below this point, the proton would compete effectively with the dye cations thus leaving the dye cations in the solution. At a higher pH, the algae would have a net positive charge leading to electrostatic attraction of dye cations. This confirms that there is a clear competition between protons and dye cations for the sorption sites in the algal biomass, and ion exchange was the main sorption mechanism.

The analysis and design of biosorption process requires equilibrium to better understand the adsorption process. Sorption equilibria provide fundamental physiochemical data for evaluating the applicability of sorption process as an unit operation [9]. In the present investigation the equilibrium data were analyzed using the Freundlich and Langmuir isotherm expression given by equation (1) and equation (2) respectively:

Freundlich: qe = Kf.Ce1/n   (1)

Langmuir: 2   (2)

Figure 1: Equilibrium data for methylene blue onto pithophora sp
Figure 1

The fitted equilibrium data in Freundlich and Langmuir isotherm expressions were shown in figure 1. From the figure 1, it was observed that the equilibrium data fitted very well in both Freundlich and Langmuir expression with a very higher correlation coefficient value of 0.9868 and 0.9528 respectively. The very higher correlation coefficient confirms the applicability of both the models. The calculated Freundlich and Langmuir constants Kf, n, qo and KL are given by 25.19 (mg/g)(L/g)n, 2.82, 104.1667 mg/g and 0.124 L/mg respectively. The higher adsorption capacity, qo confirms that the pithophora sp can be effectively used as adsorbents for the removal of methylene blue from its aqueous solutions. Also the higher value of qo (qo>>1) confirms that the dye cations are strongly adsorbed onto algal surface by electrostatic attraction. 7 The predicted Freundlich and Langmuir isotherm equation for methylene blue onto pithophora sp useful for design calculations were given by equation (3) and (4) respectively:

qe = 25.19Ce1/2. 82   (3)

4   (4)
The free energy change for the biosorption process was calculated using the following equation: ³

ΔG = -RT.Ln(KL)   (5)

Where, R is gas constant, T is the temperature and KL is equilibrium adsorption constant of adsorption (L/mg). The calculated ΔG values were found to be –14.431 KJ/mol. The negative value of free energy change indicates the spontaneous nature of biosorption.

Conclusions

Present investigation showed that the fresh water algae, pithophora sp can be used an adsorbent for the removal of methylene blue from its aqueous solution. The equilibrium data followed both Freundlich and Langmuir isotherm equation. The effect of pH and the higher sorption capacity suggests the electrostatic attraction of dye cations onto the algal surface. Negative values of free energy indicates the spontaneous nature of the sorption process.

References

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