Performance and kinetic studies on biosorption of Astrazon blue dye by dried biomass of Baker ’ s yeast as a low cost biosorbent

The effects of adsorbent dosage, agitation speed, and initial dye concentration on adsorption of Astrazone Blue (F2RL 200%) basic dye onto dried biomass of Baker’s yeast have been investigated in this study. The specific uptake capacity of dye decreases with the increase of sorbent dosage. The maximum dye uptake has been obtained at moderate agitation speed of 150 rpm. The amount of dye adsorbed per gram biomass increases with increasing initial dye concentration and contact time. The kinetic experimental data of the effect of initial dye concentration were analyzed using four kinetic equations including pseudo-second-order model, intraparticle diffusion model, Elovich model and the modified Freundlich model. The best fit equation was identified by four error functions; residual root mean square error, chi-square test, sum of the squares of the errors and average relative error. Modified Freundlich model gave the lowest error function values and consequently best fitting the adsorption data. A design for a batch adsorption unit using data from previous isotherm studies has been done in this study. Comparative study for the cost of Astrazone Blue dye removal with dried biomass of Baker’s yeast and commercial activated carbon (NORTI) illustrated that the expected cost of removing dye with dried biomass of Baker’s yeast is about 18.79% of that of commercial activated carbon.

waste streams (Nicibi et al., 2007).This often poses pollution problems in the form of; coloration of the water bodies, damaging the aesthetic nature of water, toxicity to the aquatic life, reduction of light penetration through water's surface and also reduction in the photosynthetic activity of aquatic organisms.They are also carcinogenic and mutagenic (Kumar et al., 2006).Moreover, they affect severely human beings by damaging the liver, kidneys, brain, central and reproductive systems (Iscen et al., 2007).Therefore, decolration of dyecontaining effluents is becoming an obligation both environmentally and for water re-use.
Dyes are classified into three categories (Mishra and Tripathy, 1993): anionic (direct, acid and reactive dyes), cationic (basic dyes) and non-ionic (disperse dyes).In particular, due to the presence of metals in their structure, basic dyes are considered one of the most toxic substances (US EPA, 1996).
Various physical, chemical and biological methods have been used for the treatment of dyecontaining wastewater.Some chemical oxidation by Fenton reagent, ozone, UV plus H 2 O 2 or NaOCl results in aromatic ring cleavage and may generate chemical sludge or by-products that are likely to be more toxic (Robinson et al., 2001).Aerobic biological treatment is known to be ineffective for dye removal but anaerobic bioremediation enables water-soluble dyes to be decolorized (Carliell et al., 1995).Adsorption technology using activated carbons, has gained favor recently because it has a high efficiency in the removal of highly stable dyes.However, this process proved to be uneconomic due to the high cost of activated carbon and also the additional cost involved in regeneration (Kumar and Porkodi, 2007).This led to directing research towards developing low cost and locally available adsorbing materials with high adsorption capacity (El-Khaiary, 2007).Microbial methods like biosorption have found utility in this context and they are applied not only to the biosorption of organic effluents such as dyes (Aksu, 2005) and phenols (Denizili et al., 2005) but also to the recovery or removal of heavy metal ions such as lead, chromium and gold from industrial effluents (Paravathi et al., 2007).
The key parameters controlling sorption of Astrazone Blue F2RL 200%, cationic (basic) dye from an aqueous solution onto dried biomass of Baker's yeast, Saccharomyces cerevisiae are expected to be biomass concentration, initial dye concentration and agitation speed thus they were the focus of study in this work.The biosorption kinetics were also investigated and the results have been analyzed by applying conventional theoretical models to fit the experimental data.Baker's yeast was selected as the biosorbent as it can be easily obtained in considerably substantial quantities at low costs.

Biosorbent preparation
Baker's yeast (product of Three Pyramids Company, Egypt, with 70% moisture by weight) was dried in a hot air oven at 60 o C overnight, then powdered using mortar and sieved to select the particle size (0.63-0.8 mm) for use as a biosorbent.

Batch biosorption experiments
A known weight of biosorbent was suspended in 50mL of known concentration of Astrazone Blue F2RL 200% dye solution of approximately pH7 in 250mL Erlenmeyer flasks and agitated at predetermined rpm in a rotary shaking incubator at 30 o C for six hours.
Samples were withdrawn at prescribed time intervals to determine the remaining dye concentration using JASCO UV/Vis/NIR spectrophotometer model V-570 at 574nm, after the separation of adsorbent by centrifugation at 4000rpm for 15 minutes using distilled water as a blank, as previously described (Farah et al., 2007).Negative controls (with no biosorbent) subjected to all experimental conditions were used as the initial dye concentrations for calculating the quantity of dye removal after biosorption experiments.
Effect of agitation speed was conducted on biosorption mixtures of 100ppm dye concentration at (100, 150, 300, 500 and 700 rpm), maintaining the biosorbent concentration constant at 0.4%.Effect of different initial dye concentration was conducted at concentrations of (100, 500 and 1000ppm), maintaining the biosorbent concentration constant at 0.4%, at agitation speed of 150 rpm.

Effect of biosorbent dosage
Fig. (1).shows the plot of equilibrium uptake capacity, q e (mg/g) and % dye removal against the biosorbent concentration (%, w/v).It was observed that the maximum percentage removal obtained (» 72%) at biosorbent concentration of 0.4% (w/v) which was the same as that obtained at 0.6% (w/v) and then decreased as the biosorbent concentration increased.Based on these results, 0.4% (w/v) of biosorbent concentration was used for further experiments.On the other hand, the adsorbed dye quantity per gram of dried biomass was maximum at biosorbent concentration of 0.2% (w/v) which was then decreased with the increase of biosorbent concentration.Similar effect was previously reported (Han et al., 2006).The primary factor explaining this performance is that at biosorbent concentration of 0.2% (w/v), adsorption sites remain unsaturated during the adsorption reaction, whereas the number of sites available for adsorption increases by increasing the adsorbent concentration to 0.6% (w/v) due to the increase of surface area.The presence of relatively higher concentration of biosorbent in the solution resulting in reduced distances between the biosorbent particles, thus making many binding sites unoccupied (Bohel et al., 2004).Also the interparticles interactions such as aggregation, overlapping and overcrowding occur at high biosorbent concentration and lead to decrease in total surface area (Iscen et al., 2007).Another reason could be due to the splitting effect of concentration gradient between dye molecules and biomass concentration causing a decrease in the amount of dye biosorbed onto unit weight of biomass (Malik, 2004).

Effect of agitation speed
Fig. (2).shows the effect of agitation speed on dye uptake, which increased from » 18mg/g at 100 rpm, with % removal of » 72% to 21 mg/g with dye removal of » 84% at 150 rpm.Then decreased at higher shaking rates (300, 500 and 700 rpm) where the dye uptake were »17, 16 and 15.5 mg/ g, respectively with dye removal of » 68%, 64% and 62%, respectively.These results indicate that the contact between biomass and dye solution is more effective at moderate agitation speed (150 rpm) this speed was thus selected for further experiments.The observed lower dye uptake at relatively lower agitation speed (100 rpm) might be due to the agglomeration of biomass particles.Increase in agitation speed up to 150 rpm might have facilitated proper contact between dye solution and biomass binding sites and thereby promoted effective transfer of sorbate ions to the sorbent sites (Asma et al., 2006).The observed decrease in dye removal beyond 150 rpm might be attributed to the increased turbulence promoted desorption of the adsorbates in the solution and hence the residual concentration of dyes were increased (Alam, 2004).

Effect of initial dye concentration
The effect of initial dye concentration on the rate of dye uptake is shown in Fig. (3).It was observed that the amount of dye adsorbed per gram biomass increased with increasing initial dye concentration and contact time.The rate of dye uptake was observed to be very rapid for the initial period of 10 min and thereafter the dye uptake process tends to proceed at a very slow rate and finally reached equilibrium within two hours.The rapid kinetics has significant practical importance as it will facilitate smaller reactor volumes ensuring efficiency and economy (Asku, 2001).The dye removal decreased from 75.44% to 47.66% with the increase in initial dye concentration (C o , 100-1000ppm).C o provides the necessary driving force to overcome the resistances against the mass transfer of dye between the aqueous and the solid phases.The increase in the C o also enhances the interaction between dye and yeast.Therefore, an increase in C o of dye enhances the adsorption uptake of Astrazone Blue per gram of adsorbent due to the increase in the driving force of the process (Mane et al., 2007).As adsorption process proceeds the driving force decreases with time.

Adsorption kinetic study
In order to investigate the adsorption processes of Astrazone Blue F2RL 200% onto dried biomass of Baker's yeast, various kinetic models were tested for the obtained data to elucidate the adsorption mechanism.

Pseudo-second-order model
This model can be represented in the following form (Ho and Mckay, 1999): Where q e and q t are the amount of dye sorbed at equilibrium and at time t (mg/g) respectively, k s is the pseudo-second-order rate constant (g/mg min) and t is the time (min).Values of q e , k s are calculated from the plot of t/q t against t as illustrated in Fig. (4).

The intraparticle diffusion model
Based on the theory proposed by Weber and Morris (1963), the interaparticle diffusion model can b expressed as follows: Where k p is the intraparticle diffusion rate constant (mg/g min 1/2 ) and c (mg/g) is a constant that gives idea about the thickness of the boundary layer, i.e. the larger the value of c the greater is the boundary layer effect (Kannan and Sundaram, 2001).According to this model, the plot of uptake, q t , versus the square root of time, t ½ , should be linear if particle diffusion is involved in the biosorption process and if these lines pass through the origin so the intraparticle diffusion is the rate controlling step (Chen et al., 2003).When the plots do not pass through the origin, this is indicative that the intraparticle diffusion is not the only rate limiting step (Poots et al., 1978).The plot of the intraparticle diffusion model at different initial concentration is shown in Fig. (5).which showed that the plots of the obtained data didn't pass through the origin indicating that the intraparticle diffusion is not the sole rate limiting step.

Elovich model
Elovich equation is also very often used to interpret the kinetics of adsorption (Cheung et al., 2000).The linear form of Elovich equation is represented as follows: Where α (mg/g min) and β(g/mg) are the Elovich constants where α and β are related to the initial sorption rate and the extent of surface coverage and activation energy for chemisorptions, respectively.Therefore, the plots of q t versus lnt as shown in Fig. (6).enable the model parameters to be determined.Kuo and Lotse (1973).The linear form of modified Freundlich equation is given as: Where q t is the amount of adsorbed dye (mg/g) at time t, k mf the apparent adsorption rate

Fig. 7: Modified Freundlich model plots
All parameters of each of the used models are summarized in Table (1).The correlation coefficients, R, for the pseudo second order kinetic model were greater than that of the intraparticle diffusion coefficients, strongly suggesting a chemisorption mechanism (Ho and Mckay, 1998), which was in agreement with the results reported in our previous study (Farah et al., 2007).

Validity of kinetic models
The adsorption kinetics of Astrazone Blue F2RL 200% onto the dried biomass of Baker's yeast was verified at different initial dye concentrations.The classical method to find out the most suitable kinetic model to represent the experimental data was the use of the correlation coefficient (R), which measures the difference between the experimental and theoretical data in linear plots only, but not the errors in kinetics curves.
Due to the inherent bias resulting from linearization, the validity of each model was determined by error function.Explanations of various error functions used in the present study are given in Table (2).
Where q t,exp. is the experimental data of the adsorption capacity (mg/g), q t,cal. is the capacity obtained by calculations from the used models (mg/ g) and N is the number of data points.
The lower the values of error function the better the fit is.Table (3) lists the values of the used error functions obtained for the studied four kinetic models.By comparing the results of the obtained values of error functions, it was found that the modified Freundlich model gave the lowest error function values and consequently best fitting the adsorption data.

Design batch adsorption from isotherm studies
Equilibrium data, commonly known as adsorption isotherms, are basic requirements for the design of adsorption systems and provide information on the capacity of the adsorbent or the amount required to remove a unit mass of pollutant under the system conditions.Assuming the batch adsorption to be a single-stage equilibrium operation, the separation process can be defined mathematically using these isotherm constants to estimate the residual concentration of dye or amount of adsorbent for desired purification (Aksu and Tezer, 2000).Based on the best fit isotherm (Farah et al., 2007), a single stage adsorber as shown in Fig. (8). is designed for different solution volumes.The solution to be treated contains V solvent (L), and the initial concentration is reduced from C 0 to C 1 (mg/L).The amount of adsorbent is M and the solute concentration increases from q 0 to q 1 (mg/g).If fresh adsorbent is used, q 0 = 0.The mass balance equates the solute removed from liquid to that picked up by the adsorbent.The mass balance equation for the sorption system can be written according to Alkan et al., (2005) as: Under equilibrium conditions C 1 → C e and q 1 → q e Since the previous study (Farah et al., 2007) confirmed that the equilibrium isotherm data for Astrazone Blue (F2RL 200 %) onto dried biomass of Baker's yeast best fitted by Langmuir isotherm equation so Langmuir isotherm is used for batch adsorber design.The Eq. ( 5) can be rearranged as Where K L (L/mg) and a L (L/g) are Langmuir constants.Equation ( 6) can be used to design the Astrazone Blue / dried biomass of Baker's yeast sorption system at different conditions of; dye percentage removal, initial dye concentrations, solution pH and volumes.Fig. 9: Shows a series of plots (90%, 80%, 70% and 60% color removal) at different initial dye concentrations (50 -250ppm) for 1 L of dye solution at 30 o C and pH=7, where K L =1.72 L/g and a L =2.29 L/mg.Mass of yeast required (g) Farah et al., (2007) studied the uptake capacity of dried biomass of Baker's yeast and commercial activated carbon (NORTI) for Astrazone Blue F2RL 200% at 30 o C and pH 7 with initial dye concentration range of 100-1000ppm.The maximum obtained values for adsorption capacity were used in this study to assess the quantity of adsorbent required to remove 1 kg of dye.The adsorbent quantities have been used as a basis for estimating the expected cost for the adsorption process.
The economic data recorded in Table (5) illustrate that, the expected cost of removing dye with dried biomass of Baker's yeast is about 18.79% of that when commercial activated carbon is used.

Conclusion
The rapid uptake, high capacity in addition to the relative low cost in removing Astrazone Blue dye using dried biomass Baker's yeast which was found to be 18.79 % of that of commercial activated carbon, proved that the dried biomass of Baker's yeast is a very attractive alternative sorbent material for conventional used sorbents.

Fig. 10 :
Fig. 10: Shows the required amount of yeast needed for the desired percentage removal (80%) of Astrazone Blue for different initial dye concentrations (25-200ppm) and solution volumes (1-10 L) at 30 o C and pH=7, where K L =1.72 L/g and a L =2.29 L/mg.