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Journal of Environmental & Analytical Toxicology
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Optimization of Biosorption Performance of Casuarina Leaf Powder for the Removal of Lead Using Central Composite Design

Srinivasa Rao J1*, Kesava Rao C2 and Prabhakar G2

1Department of Chemical Engineering, Bapatla Engineering College, Guntur, India

2Department of Chemical Engineering, S.V. University, Tirupati, India

Corresponding Author:
Srinivasa Rao J
Department of Chemical Engineering
Bapatla Engineering College
Guntur, India
E-mail: [email protected]

Received January 28, 2013; Accepted February 06, 2013; Published February 08, 2013

Citation: Srinivasa Rao J, Kesava Rao C, Prabhakar G (2013) Optimization of Biosorption Performance of Casuarina Leaf Powder for the Removal of Lead Using Central Composite Design. J Environ Anal Toxicol 3: 166. doi: 10.4172/2161-0525.1000166

Copyright: © 2013 Srinivasa Rao J, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

Rapid industrialization, urbanization and increase in population have led to increasing the pollution levels. Heavy metal pollution of water is of major concern now-a-days as water is the basic need for mankind. The present investigation is removal of lead from aqueous solutions using a new biosorbent Casuarina leaf powder. The cumulative effects of operating parameters such as initial metal ion concentration, pH of the aqueous solution, biosorbent dosage and temperature on the lead biosorption were analyzed using Response Surface Methodology (RSM). For obtaining the mutual interaction between the variables and optimizing these variables, a 24 full factorial central composite design was employed. According to ANOVA results, the proposed quadratic model for CCD fitted very well to the experimental data. The optimal set of conditions for maximum percentage biosorption of lead is found to be pH=4.988, biosorption dosage (w) =35.37 g/L, initial lead concentration (Co)=18.0555 mg/L and temperature=306.47 K and the % of biosorption calculated at these values found to be 95.73%. The Langmuir isotherm fitted well with a correlation factor of 0.9944, followed by Freundlich and Temkin. The entire biosorption process followed pseudo second order kinetics. By applying the Van’t Hoff equation the thermodynamic parameters such as enthalpy (ΔH°), entropy (ΔS°) and free energy (ΔG°) were evaluated which described the biosorption process as spontaneous, irreversible and endothermic in nature. The optimized values obtained through central composite design and one factor at a time process is in good agreement.

Keywords

Biosorption; Casuarina leaf powder; Central composite design; Isotherms; Kinetics

Introduction

Water is no alien to all the living beings upon earth. It has no barrier or bars over constituencies or continents, as it leaves only 1/4th of the land of whole ecosystem. The contaminated water may cause anemia, headache, chills, diarrhea and reduction in hemoglobin formation [1]. The influence of pollution on the global environment, through activities such as rapid industrialization, urbanization and mining operations often lead to an increase in the discharge of toxic metals, such as lead, nickel, chromium, cobalt, copper, cadmium etc., into the environment, which results in a contamination of water. These heavy metal contaminants are hazardous to the environment, because they do not naturally degrade [2]. Beyond certain limits, heavy metals are toxic to living organisms and may cause serious hazard to public health [3]. Environmental engineers and scientists are faced with the challenging task to develop appropriate low cost technologies for effluent treatment [4].

Traditional methods for removal of lead ions from solution include chemical precipitation, ion exchange, electrodialysis and membrane separations. All these methods have various disadvantages, specifically, high capital investment and operating cost, incomplete removal, low selectivity and high energy consumption. Therefore, there is a need for a cost effective treatment method that is capable of removing low concentrations of lead from solution For the last decades, biosorption or sorption of contaminants by sorbents of natural origin has gained important credibility due to the good performance and low cost of these complexing materials [5-10]. A multitude of biomass types comprising fungal biomass, bacterial biomass, algae, peat etc., have been studied for their biosorption of metals [11-14]. Agricultural wastes such as tree bark, peanut skin, hull, tobacco, tomato root tissues and plants waste have been used to remove heavy metals from water [15-17].

In view of the above, the authors tried to use a novel biosorbent Casuarina leaf powder to remove lead from aqueous solutions and report the application of Response Surface Methodology using Central Composite Design to develop a mathematical model and predict the response and check the adequacy of the model. The objectives of the present study include identifying the maximum lead uptake capacity of the Casuarina leaves powder with respect to various governing parameters. In addition, biosorption isotherms were described by using Langmuir, Freundlich and Temkin models and the kinetic experimental data were correlated by the pseudo first and second order kinetic models. Thermodynamics for biosorption of lead on Casuarina leaves powder is also studied and fitted in to Van’t Hoff equation.

Experimental Procedure

Biosorbent

Casuarina leaves were collected from the surroundings of Bapatla, Guntur. The Casuarina leaves were washed thrice with tap water and once with distilled water in order to remove adhering surface impurities. The leaves were dried in sunlight until all the moisture was evaporated. The crispy Casuarina leaves were then grinded in a ball mill to powder, separated using British Standard Sieves (BSS) and stored in dry air tight packs to prevent moisture penetration and readily used as biosorbent.

Preparation of metal solutions

Test solutions containing lead ions were prepared from Pb (NO3 )2 at different concentrations ranging from 25, 50, 100, 150, 200, 250 mg/L. The pH of each test solution was adjusted to the appropriate value by using 0.1 N HNO3 or 0.1 N NaOH solutions. All the chemicals used in preparing the stocks are of analytical grade and the water used is double distilled water prepared from Millipore ELIX-10 unit.

Batch sorption studies

The biosorption was carried out in a batch process by contacting a pre-weighed amount of the Casuarina leaves powder with a known volume of aqueous solution. Experiments were conducted in 250 ml Erlenmayer flasks containing 50 ml of 20 mg/L metal solution using single step optimization procedure. The flasks containing aqueous solution and biosorbent were agitated on an orbital shaker at 180 rpm and samples were taken at predetermined time intervals (1, 3, 5, 10, 15, 20, 25, 30, 40, 50, 60, 90, 120, 150 and 180 min) and centrifuged at 14000 rpm and the supernatant liquid was analyzed in Atomic Absorption Spectrophotometer (AAS) - Shimadzu make AA-6300 for final concentrations. Similarly the other variables were varied for a wide range: Biosorbent Size (63, 75 and 105 μm), pH of the aqueous solution (2, 3, 4, 5, 6, 7 and 8), Initial concentration of lead solution (20, 50, 80, 120 and 150 mg/L), Biosorbent dosage (5, 10, 15, 20, 25, 30, 35, 40 and 50 g/L) and Temperature (283, 293, 303, 313 and 323 K).

Process optimization

Final experimental runs for optimization were obtained through Response Surface Methodology using Central Composite Design with 24 factorial runs, 6-central points and 8-axial points. Design of Experiments (DoE) obtained based on the above optimization technique using STATISTICA software. The extent of biosorption of lead calculated at the preliminary optimum conditions is verified with the final runs for the optimum conditions. For the optimization of biosorption, the regression equation is written in terms of % biosorption of lead (Y) as function of the parameters having greater influence over the response i.e. pH (X1), Co (X2), w (X3), and T (X4). Based on experimental runs and predicted values proposed by CCD design, the following equation represents multiple regression analysis of the experimental data for the biosorption of lead:

Y = b0 + b1 X1 +b2X2 + b3X3+ b4X4 + b11X12+b22X22+ b33X32 + b12 X1X2+ b13 X1X3+ b23 X2X3+ b44 X42+ b14 X1X4 + b24 X2X4+ b34 X3X4

Results and Discussion

Effect of agitation time

Duration of equilibrium biosorption is defined as the time required for heavy metal concentration to reach a constant value during biosorption. The equilibrium agitation time is determined by plotting the % biosorption of lead against agitation time as shown in figure 1 for the interaction time intervals between 1 to 180 min. For 63 μm size of 10 g/L biosorbent dosage mixed in 50 mL of aqueous solution (Co=20 mg/L), 50.4% of lead is biosorbed in the first one minute and reached to 55.7% after 5 minutes of biosorption. The % biosorption is increased briskly up to 60 min reaching 86.5%. Beyond 60 min, the % of biosorption is constant indicating the attainment of equilibrium conditions. The maximum biosorption of 86.5% is attained for 60 min of agitation time. The rate of biosorption is fast in the initial stages because adequate surface area of the biosorbent is available for the biosorption of lead. As time increases, more amount of lead gets biosorbed onto the surface of the biosorbent due to Vanderwaal’s forces of attraction and resulted in decrease of available surface area. The biosorbate, normally, forms a thin one molecule thick layer over the surface. When this monomolecular layer covers the surface, the biosorbent capacity is exhausted. Therefore, all other experiments are conducted at this agitation time.


Environmental-Analytical-Toxicology-Effect-agitation

Figure 1: Effect of agitation time on % biosorption of lead.

Effect of biosorbent size

The variations in % biosorption of lead from the aqueous solution with biosorbent size are drawn in figure 2 with percentage biosorption of lead as a function of biosorbent size. The percentage of biosorption is decreased from 86.9% to 80.2% as the biosorbent size increases from 63 to 105 μm. This phenomenon is expected, as the size of the particle decreases, surface area of the biosorbent increases; thereby the number of active sites on the biosorbent also increases.

Environmental-Analytical-Toxicology-Biosorption-lead

Figure 2: % Biosorption of lead as a function of biosorbent size.

Effect of pH

Biosorption is controlled by pH by the influence of surface charge of the biosorbent, the degree of ionization and the species of biosorbate. In the present investigation, effect of pH on lead biosorption is obtained in the pH range of 2 to 8 of the aqueous solution (Co =20 mg/L) using 10 g/L of 63 μm size biosorbent. The effect of pH of aqueous solution on % biosorption of lead is shown in figure 3. The % biosorption of lead is increased from 64.6% to 91.4% as pH is increased from 2 to 5 and decreased beyond the pH value of 6. Percentage of biosorption is decreased to 62.1% from 84.3% for pH variation of 6 to 8. Since the pH of aqueous solution influences the solution chemistry of the heavy metals, the binding of metal ions by surface functional groups is strongly pH dependent. The increase in % removal when pH increases from 2 to 5 could be due to decrease in competition between hydrogen ions and metal species for appropriate sites on the biosorbent surface and also by the decrease in positive surface charge on the adsorbent. However, with increasing pH above 5 lead tends to hydrolyze and precipitate instead of adsorption and adsorbent was deteriorated with accumulation of metal ions, making the true adsorption studies unpredictable.

Environmental-Analytical-Toxicology-Observation-pH

Figure 3: Observation of pH along with % biosorption of lead.

Effect of initial concentration of lead

The effect of % biosorption on initial concentration of lead in the aqueous solution is shown in figure 4. The percentage biosorption of lead is increased from 73.3% to 90.1% with decrease in Co from 150 mg/L to 20 mg/L. Such behavior can be attributed to the increase in the amount of biosorbate to the unchanging number of available active sites on the biosorbent.

Environmental-Analytical-Toxicology-Variation-initial

Figure 4: Variation of initial concentration with % biosorption of lead.

Effect of biosorbent dosage

The percentage biosorption of lead is drawn against biosorbent dosage for 63 μm size biosorbent in figure 5. The biosorption of lead increased from 87.2% to 93.4% with an increase in biosorbent dosage from 5 to 35 g/L. Such behavior is obvious because with an increase in biosorbent dosage, the number of active sites available for lead biosorption would be more. The change in percentage biosorption of lead is marginal from 93.4% to 93.8% when ‘w’ is increased from 35 to 50 g/L. Hence all other experiments are conducted at 35 g/L dosage.

Environmental-Analytical-Toxicology-Dependency-biosorption

Figure 5: Dependency of % biosorption of lead on biosorbent dosage.

Effect of temperature

The effect of temperature on the equilibrium metal uptake was significant. The effect of changes in the temperature on the lead uptake is shown in figure 6. Lead uptake marginally increased from 92.2 to 94.6% with increasing temperature from 283 K to 323 K indicating that the biosorption of lead on to Casuarina leaf powder is endothermic process. Adsorption processes are normally exothermic and as the temperature increases the % adsorption decreases in accordance with Le Chatelier principle. The reverse phenomena could be activation of non living biomass under moderate temperatures and Increasing the temperature is known to increase the rate of diffusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particles, owing to decrease in the viscosity of the solution.

Environmental-Analytical-Toxicology-Effect-temperature

Figure 6: Effect of temperature on % biosorption of lead.

Isotherms

Langmuir isotherm: Irving Langmuir developed an isotherm named Langmuir isotherm. It is the most widely used simple twoparameter equation. The Langmuir relationship is hyperbolic and the equation is:

qe/qm = bCe/ (1+bCe) (1)

Equation (5.1) can be rearranged as

(Ce/qe) = 1/(bqm) + Ce/qm (2)

From the plots between (Ce/qe) and Ce, the slope {1/ (bqm)} and the intercept (1/b) are calculated. Further analysis of Langmuir equation is made on the basis of separation factor, (RL) defined as RL = 1/ (1+bCe).

0<RL<1 indicates favorable adsorption

RL>1 indicates unfavorable adsorption

RL = 1 indicates linear adsorption

RL = 0 indicates irrepressible adsorption

Langmuir isotherm is drawn for the present data and shown in figure 7. The equation obtained is Ce/qe = 0.0663142Ce + 1.0222246 with a good linearity (correlation coefficient, R2~0.9944) indicating strong binding of lead ions to the surface of Casuarina leaf powder. The maximum metal uptake of (qm) 15.0797 mg/g is observed and the separation factor obtained (RL) is 0.88617, indicating favorable biosorption.

Environmental-Analytical-Toxicology-Langmuir-isotherm

Figure 7: Langmuir isotherm for biosorption of lead.

Freundlich isotherm: Freundlich presented an empirical biosorption isotherm equation that can be applied in case of low and intermediate concentration ranges. It is easier to handle mathematically in more complex calculations.

The Freundlich isotherm is given by

qe = Kf Ce n (3)

Where, Kf (mg) represents the biosorption capacity when metal equilibrium concentration and n represents the degree of dependence of biosorption with equilibrium concentration. The above equation is represented as

ln qe = ln Kf+ n ln Ce (4)

Freundlich isotherm is drawn between lnCeand lnqe (Figure 8). The obtained equation is lnqe = 0.600397 lnCe + 0.274637; for the present data. The resulting equation has a correlation coefficient of 0.9832. The ‘n’ value (0.600397) in the above equations satisfies the condition of 0<n<1 indicating favorable biosorption. The Kf obtained was 1.882075.

Environmental-Analytical-Toxicology-Freundlich-isotherm

Figure 8: Freundlich isotherm for biosorption of lead.

Temkin isotherm: Temkin and Pyzhev isotherm equation describes the behavior of many biosorption systems on the heterogeneous surface and it is based on the following equation

qe= RT ln(ATCe)/bT (5)

The linear form of Temkin isotherm can be expressed as

qe= (RT/ bT ) ln(AT) + (RT/bT) ln(Ce) (6)

The present data are analysed according to the linear form of Temkin isotherm and the linear plot is shown in figure 9. The equation obtained for lead biosorption is: qe = 3.07614 lnCe – 0.7570 with a correlation coefficient 0.9817. The best fit model is determined based on the linear regression correlation coefficient (R2). From the figure 7-9, it is found that biosorption data are well represented by Langmuir isotherm with higher correlation coefficient of 0.9944, followed by and Freundlich and Temkin isotherms with correlation coefficients of 0.9832 and 0.9817 respectively. The validity of Langmuir model suggests the adsorption process is monolayer and adsorption of each molecule has equal activation energy. The isotherm constants are given in table 1.

Environmental-Analytical-Toxicology-Temkin-isotherm

Figure 9: Temkin isotherm for biosorption of lead.

Langmuir Freundlich Temkin
qm = 15.0797 kf= 1.882075 AT = 0.781859
b = 0.06487 n = 0.600397 bT = 818.9295
R2 =0.9944 R2 =0.9832 R2 =0.9817

Table 1: Isotherms constants.

Kinetics

The order of biosorbate – biosorbent interactions have been described using kinetic model. Traditionally, the first order model of Lagergren finds wide application. In the case of biosorption preceded by diffusion through a boundary, the kinetics in most cases follows the first order rate equation of Lagergren:

(dqt/dt)= Kad (qe – qt) (7)

Where, qe and qt are the amounts adsorbed at t, min and equilibrium time and Kad is the rate constant of the lagergren first order biosorption.

The above equation can be presented as

∫ (dqt/ (qe – qt)) = ∫ Kaddt (8)

Applying the initial condition qt = 0 at t = 0, we get

log (qe – qt) = log qe– (Kad/2.303) t (9)

Plot of log (qe–qt) versus‘t’ gives a straight line for first order kinetics, facilitating the computation of adsorption rate constant (Kad).

The pseudo second order kinetic equation given below:

(dqt/dt) = K (qe – qt)2 (10)

Where, ‘K’ is the second order rate constant.

Substituting these values in above equation, we obtain:

1/ (qe – qt) = Kt + (1/qe) (11)

Rearranging the terms, we get the linear form as:

(t/qt) = (1/ Kqe2 ) + (1/qe ) t (12)

The pseudo second order model based on above equation, considers the rate -limiting step as the formation of chemisorptive bond involving sharing or exchange of electrons between the biosorbate and biosorbent. If the pseudo second order kinetics is applicable, the plot of (t/qt) versus‘t’ gives a linear relationship that allows computation of qe and K. The first order and the second order kinetics plots are given in figures 10 and 11 respectively. The rate equations obtained and comparative lead uptake capacities are given in table 2 and table 3 respectively. The correlation coefficients indicate that the system under consideration is more appropriately described by pseudo-second order model. The regression coefficient of 0.9897 shows that that the model can be applied for the entire adsorption process. The confirmation of pseudo second order kinetics indicates that in the adsorption process, concentrations of both adsorbent and adsorbate are involved in rate determining step. In the range of studied parameters, the metal uptake is very good for Casuarina leaf powder.

Environmental-Analytical-Toxicology-First-order-kinetics

Figure 10: First order kinetics for biosorption of lead.

Environmental-Analytical-Toxicology-Second-order-kinetics

Figure 11: Second order kinetics for biosorption of lead.

Order Equation K, min-1 R2
First order log (qe-qt)=–0.019698 t–0.073348 0.04513 0.9507
Second order t/qt=0.5935 t + 1.85514 0.18987 0.9897

Table 2: Rate Equations and coefficients for biosorption of lead on Casuarina leaf powder.

Authors Biosorbent qt, mg/g
S.A. Abo-El-Enein [18] rice husk ash 158
Erdal Kenduzler [19] Amberlyst 36 88
Vijayaraghavan [20] Sargassum 20.2
Fuat Guzel [21] black carrot 5.003
Mustafa Tuzen [22] Pseudomonas aeruginosa 5.83
Present investigation Casuarina leaf powder 15.0797

Table 3: Lead uptake capacities for different biosorbents.

Thermodynamics of biosorption

Biosorption is temperature dependant. In general, the temperature dependence is associated with three thermodynamic parameters namely change in enthalpy of biosorption (ΔH), change in entropy of biosorption (ΔS) and change in Gibbs free energy (ΔG). Enthalpy is the most commonly used thermodynamic function due to its practical significance. The negative value of ΔH will indicate the exothermic/ endothermic nature of biosorption and the physical/chemical in nature of sorption. It can be easily reversed by supplying the heat equal to calculated ΔH.

The ΔH is related to ΔG and ΔS as

ΔG = ΔH – T ΔS (13)

ΔS < 1 indicates that biosorption is impossible whereas ΔS > 1 indicates that the biosorption is possible. ΔG < 1 indicates the feasibility of sorption.

The Vant Hoff’s equation is

log (qe /Ce) = ΔH/(2.303 RT) + (ΔS/2.303 R) (14)

log (qe /Ce) = – 0.40566 (1 / T) + 1.029489 (15)

Where, (qe/Ce) is called the biosorption affinity.

If the value of ΔS is less than zero, it indicates that the process is highly reversible. If ΔS is more than or equal to zero, it indicates the reversibility of process. The negative value for ΔG indicates the spontaneity of biosorption, whereas the positive value indicates non spontaneity of sorption. Experiments are conducted to understand the biosorption behavior varying the temperature from 283 to 323 K. The Van’t Hoff’s plot for the biosorption data obtained is shown in figure 12. From the plot the equation obtained is log (qe /Ce) = – 0.40566 (1 / T) + 1.029489. The values obtained from the equation are ΔG = -5964.899, ΔH = 7.7672 and ΔS = 19.71177. ΔH is positive indicating that the biosorption is endothermic. The negative value of ΔG indicates the spontaneity of biosorption. As ΔS is more than zero, it indicates the irreversibility of biosorption.

Environmental-Analytical-Toxicology-biosorption-Lead

Figure 12: Van’t Hoff’s plot for biosorption of Lead.

Optimization using Response Surface Methodology (RSM)

In order to determine an optimum condition for Pb ions removal, the parameters having greater influence over the response is to be identified. In the present study, the relationship between four independent variables and percent of Pb ions biosorption fitted well with the quadratic model. Levels of different process variables in coded and un-coded form (Table 4), Results from CCD (Table 5), Analysis of variance (ANOVA) (Table 6), Estimated regression coefficients (Table 7) and Comparison of optimum values obtained from CCD and experimentation (Table 8) for biosorption of lead using Casuarina leaf powder are presented below. For the optimization of biosorption the regression equation: % biosorption of lead (Y) is function of the pH (X1), Co (X2), w (X3), and T (X4). Based on experimental runs and predicted values proposed by CCD design.

Variable Name Range and levels
-2 -1 0 1 2
X1 Biosorbent dosage, w, g/L 25 30 35 40 45
X2 Initial concentration, Co, mg/L 10 15 20 25 30
X3 pH of aqueous solution 3 4 5 6 7
X4 Temperature, T, K 283 293 303 313 323
Table 4: Levels of different process variables in coded and un-coded form for biosorption of lead using Casuarina leaf powder.
Run no. X1(w) X2(Co) X3(pH) X4(T) % biosorption of cobalt
Experimental Predicted
1 -1.0000 -1.0000 -1.0000 -1.0000 93.18 93.21
2 -1.0000 -1.0000 -1.0000 1.0000 93.67 93.65
3 -1.0000 -1.0000 1.0000 -1.0000 93.19 93.18
4 -1.0000 -1.0000 1.0000 1.0000 93.59 93.60
5 -1.0000 1.0000 -1.0000 -1.0000 92.2 92.21
6 -1.0000 1.0000 -1.0000 1.0000 92.79 92.82
7 -1.0000 1.0000 1.0000 -1.0000 92.89 92.91
8 -1.0000 1.0000 1.0000 1.0000 93.51 93.50
9 1.0000 -1.0000 -1.0000 -1.0000 93.8 93.78
10 1.0000 -1.0000 -1.0000 1.0000 94.17 94.20
11 1.0000 -1.0000 1.0000 -1.0000 93.29 93.31
12 1.0000 -1.0000 1.0000 1.0000 93.74 93.71
13 1.0000 1.0000 -1.0000 -1.0000 91.98 92.01
14 1.0000 1.0000 -1.0000 1.0000 92.62 92.60
15 1.0000 1.0000 1.0000 -1.0000 92.28 92.27
16 1.0000 1.0000 1.0000 1.0000 92.83 92.84
17 -2.0000 0.0000 0.0000 0.0000 94.34 94.30
18 2.0000 0.0000 0.0000 0.0000 94.21 94.20
19 0.0000 -2.0000 0.0000 0.0000 94.13 94.12
20 0.0000 2.0000 0.0000 0.0000 92.28 92.25
21 0.0000 0.0000 -2.0000 0.0000 90.71 90.67
22 0.0000 0.0000 2.0000 0.0000 90.89 90.88
23 0.0000 0.0000 0.0000 -2.0000 93.77 93.73
24 0.0000 0.0000 0.0000 2.0000 94.75 94.74
25 0.0000 0.0000 0.0000 0.0000 95.6 95.60
26 0.0000 0.0000 0.0000 0.0000 95.6 95.60
27 0.0000 0.0000 0.0000 0.0000 95.6 95.60
28 0.00000 0.0000 0.0000 0.0000 95.6 95.60
29 0.00000 0.0000 0.0000 0.0000 95.6 95.60
30 0.00000 0.0000 0.0000 0.0000 95.6 95.60

Table 5: Results from CCD for cobalt biosorption by Casuarina leaf powder.

Source of variation SS df Mean square(MS) F-value P> F
Model 53.85109 14 3.8465 4649.27 0.0000
Error 0.01241 15 0.0008273    
Total 53.8635        

Table 6: Analysis of variance (ANOVA) of lead biosorption for entire quadratic model.

Terms Regression coefficient Standard error of the coefficient t-value P-value
constant -280.353 5.533036 -50.669 0.000000
X1 1.233 0.047130 26.151 0.000000
X2 -0.013 0.000220 -61.134 0.000000
X3 0.694 0.046155 15.036 0.000000
X4 -0.024 0.000220 -109.843 0.000000
X1*X1 12.272 0.232115 52.871 0.000000
X2*X2 -1.204 0.005492 -219.326 0.000000
X3*X3 2.071 0.033977 60.965 0.000000
X4*X4 -0.003 0.000055 -61.816 0.000000
X1*X2 -0.008 0.000288 -26.511 0.000000
X1*X3 -0.022 0.001438 -15.385 0.000000
X1*X4 -0.000 0.000144 -0.782 0.446215a
X2*X3 0.037 0.001438 25.468 0.000000
X2*X4 0.001 0.000144 5.998 0.000024
X3*X4 -0.000 0.000719 -0.608 0.551990a

Table 7: Estimated regression coefficients for the lead biosorption onto Casuarina leaf powder.

Variable CCD Experimental value
pH of aqueous solution 4.9884 5.0
Biosorption dosage, w, g/L 35.3702 35
Initial cobalt concentration, mg/L 18.0555 20
Temperature, K 306.4727 303
% biosorption 95.7337 93.9

Table 8: Comparison of optimum values obtained from CCD and experimentation.

Y = –280.353 + 1.233 X1 + 0.694 X2 + 12.272 X3 + 2.071 X4 – 0.013 X12 – 0.024 X22– 1.204 X32– 0.003 X42– 0.008 X1X2 + 0.022 X1X3 + 0.000 X1X4 + 0.037 X2X3 + 0.001 X2X4 + 0.000 X3X4 (16)

From the Central Composite Design, the Pareto Chart (Figure 13), Observed Vs Predicted values plot (Figure 14) and Surface contour plot (Figures 15 a-f) for biosorption of lead on to Casuarina leaf powder are presented below. The optimal set of conditions for maximum percentage biosorption of lead is pH= 4.9884 biosorption dosage (w) =35.3702 g/L, initial lead concentration (Co) =18.0555 mg/L and temperature=306.4727 K and the % of biosorption calculated at these values found to be 95.73%.

Environmental-Analytical-Toxicology-Pareto-Chart

Figure 13: Pareto Chart.

Environmental-Analytical-Toxicology-Observed-Predicted

Figure 14: Observed Vs Predicted values plot for % biosorption of lead.

Environmental-Analytical-Toxicology-Surface-contour

Figure 15a: Surface contour plot for the effects of dosage and concentration of lead on % biosorption.

Environmental-Analytical-Toxicology-effects-dosage

Figure 15b: Surface contour plot for the effects of dosage and pH on % biosorption of lead.

Environmental-Analytical-Toxicology-temperature-biosorption

Figure 15c: Surface contour plot for the effects of dosage and temperature on % biosorption of lead.

Environmental-Analytical-Toxicology-concentration-lead

Figure 15d: Surface contour plot for the effects of concentration of lead and pH on % biosorption of lead.

Environmental-Analytical-Toxicology-Surface-contour-plot

Figure 15e: Surface contour plot for the effects of concentration of lead and temperature on % biosorption of lead.

Environmental-Analytical-Toxicology-plot-effects-pH

Figure 15f: Surface contour plot for the effects of pH and temperature on % biosorption of lead.

Conclusions

The analysis of the experimental and theoretical data resulted that the equilibrium agitation time for lead biosorption is 60 minutes. The % removal of lead from an aqueous solution increases with a decrease in the particle size of the biosorbent and increases with increase in biosorbent dosage. With an increase in the initial concentration of lead in the aqueous solution, the percentage removal of lead from the aqueous solution is decreased. Percentage removal of lead from the aqueous solution increases significantly with increase in pH from 2 to 5, thereafter percentage removal decreases for further increase in pH. In the range of variables studied, percentage removal is increased from 50.44% to 93.9%. The maximum uptake capacity of 15.0797 mg/g is obtained at temperature 303 K. The kinetic studies show that the biosorption of lead is better described by pseudo second order kinetics It is also found that the data are well represented by Langmuir isotherm with higher correlation coefficient of 0.9944, followed by Freundlich and Temkin isotherms. The biosorption of lead on to Casuarina leaf powder is irreversible, spontaneous and endothermic in nature. The present study involves the use of statistical experimental design to optimize process conditions for maximal biosorption of lead from aqueous solution using CCD involving RSM. The maximum biosorption of lead (95.73%) onto Casuarina leaf powder is observed when the processing parameters are set as follows: pH = 4.988, w = 35.37 g/L, Co =18.0555 mg/L and T=306.47 K. Therefore the above said Casuarina leaf powder is effective and efficient biosorbent and is capable of removing lead.

Acknowledgements

The authors wholeheartedly thank the Bapatla Engineering College and TEQIP-World Bank for supporting the authors and sparing the equipment for Research Purpose.

References

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