Removal of Methylene Blue from an Aqueous Solution using Plumbago zeylanica Leaves-Optimisation using Response Surface Methodology (RSM)

 

P. Pallavi^1*, P. King², Y. Prasanna Kumar³

¹Department of Civil Engineering, JBIT College of Engineering and Technology, Hyderabad-500085, (Telangana) India

²Department of Chemical Engineering, Andhra University College of Engineering, Visakhapatnam-530003, (Andhra Pradesh) India

³Visakha Institute of Engineering and Technology, Visakhapatnam, India

 

ABSTRACT:

Dyes are colour organic compounds which can colorize the substances usually present in the effluent water of many industries such as textiles, leather, paper, printing and cosmetics. The feasibility of removing dye from an aqueous solution using Plumbago zeylanica, a natural source was tested by biosorption process. The Plumbago zeylanica was characterized by Fourier Transform Infrared Spectroscopy (FTIR) analysis. FTIR results showed that complexation and ion exchange appears to be the principal mechanism for MB biosorption. . Batch biosorption technique was used to investigate the effects of various experimental parameters like contact time, solution pH, initial dye concentration, dosage of the biosorbent, average biosorbent size and temperature on biosorption. The biosorption data were well described by the Freundlich model, although they could be modeled by Langmuir, Temkin isotherm models. Three simplified kinetic models including pseudo-first order, pseudo-second order and Elovchi models were selected to follow the biosorption model. The pseudo-second order model agrees with experimental data well. The obtained thermodynamic parameters such as enthalpy, entropy and free energy indicated that the biosorption of methylene blue onto Plumbago zeylanica is exothermic and spontaneous process. RSM studies were conducted to optimize the biosorption process.

 

 

 

INTRODUCTION:

Out of various industries, the dyeing industry is an important sector of the chemical industry. Dyes are used in many industries such as food, paper, plastics, cosmetics, paper making and textile in order to color their products. Introducing dyes compounds into the environment also means the bad appearance of colour in the environment¹. Dyes and pigments are one of the problematic groups of pollutants.  The presence of dyes in water is undesirable since even a very small amount of these coloring agents is highly visible and may be toxic to aquatic environment ². Most of the dyes are stable against photo-degradation, bio-degradation and oxidizing agents³.

 

Dyes are toxic to some microorganisms, and may cause direct destruction or inhibition of their catalytic capabilities^4.5. Methylene blue (MB) a cationic dye causes eye burns, which may be responsible for permanent injury to the eyes of human and animals. On inhalation, it can give rise to short periods of rapid or difficult breathing, while ingestion through the mouth produces a burning sensation and may cause nausea, vomiting, profuse sweating, mental confusion, painful micturition and methemoglobinemia^{6, 7}.

 

Dye removal from wastewater has received considerable attention with several biosorbents and several classes of dye being investigated. Research is therefore needed to develop new alternative environmental friendly applications that can further exploit. In recent times, there has been increased interest in the use of agricultural waste products for dye removal by sorption from aqueous solution because of their natural availability and the high degree of dye removal achieved at laboratory scale. Plant biomass is a natural renewable resource that can be converted into useful materials and energy⁸.

 

Among these techniques biosorption has been shown to be an effective technique with its efficiency, capacity and applicability on a large scale to remove dyes as well as having the potential for regeneration, recovery and recycling of adsorbents⁹. Besides these physical characteristics, the adsorption capacity produced from different sources with different ways is strongly influenced by the chemical nature of the surface. Internal pore structure and surface characteristic play an important role in biosorption processes and depends both on the precursor used and method of preparation ¹⁰. Among the different process, biosorption is a promising removal technique that produces effluents containing very low levels of dissolved organic compounds.

 

The focus of the research is to evaluate the biosorption potential of Plumbago zeylanica for removal of methylene blue dye from an aqueous solution. Characterization of biosorbent by Fourier Transform Infrared Spectroscopy (FTIR) was done to know the different bondings that took place. Batch biosorption experiments were performed to understand the biosorption kinetics, describe the rate and mechanism of biosorption. The applicability of three kinetic models in predicting the biosorption kinetic profile was attempted.

 

MATERIALS AND METHODS:

Methylene Blue:

Methylene blue supplied by Merck, was used as a biosorbate. Methylene blue was chosen in this study because of its known strong biosorption onto solids and is often serves as a model compound for removing organic contaminants and colored bodies from aqueous solutions.  Double distilled water was employed for preparing the stock solution. A known weight of 0.1 g of  Methylene blue dye  was weighed and 1000 mg/L concentration of stock solution  was prepared and further working solutions of  concentrations (25, 50, 75 and 100 mg/L) were prepared and stored.

 

Preparation of Plumbago Zeylanica leaf powder:

The Plumbago zeylanica leaves were collected from Vignan’s Engineering College, Vadlamudi. The collected leaves were washed with deionized water for several  times, dried for 20-25 days and crushed to powder for a desired mesh size of 63-212 µm using a domestic mixture.

 

EXPERIMENTAL PROCEDURE:

Batch studies:

Biosorption experiments were carried out by agitating 25mg of Plumbago zeylanica with 50 ml of dye solution of desired concentration and pH 9 at 180rpm at constant temperature of 303K in an orbital shaker. The concentration of methylene blue in the solution before and after biosorption was determined using UV spectrophotometer at 668nm. The samples were centrifuged and liquid was analyzed for the remaining colour at 60min equilibrium contact time. The similar experiments were carried out by changing the parameters.

 

The % biosorption is given as

                                             C_{i_}^_ C_f

% Biosorption = --------------------------χ100               (1)

                                             C_i

The dye uptake onto PZ is obtained by:

 

                                                 (2)

 

Where

q_t is the amount of dye biosorbed on the PZ biosorbent surface (mg/g),

C_i is the initial concentration of solute in the solution before biosorption (mg/L),

C_f is the final concentration of solute in the solution after biosorption (mg/L),

V is the volume of the dye solution (ml) and

w is the weight of the biosorbent (g).

 

Langmuir, Freundlich, Temkin isotherms were employed to study the equilibrium biosorption. The pseudo- first order, pseudo-second order and elovich models were conducted for kinetic studies. Response surface methodology (RSM) was used to develop, improve and optimize different processes. The Central Composite Design (CCD) was used to optimize the process parameters.

 

RESULTS AND DISCUSSION:

Characterization of PZ:

The FTIR spectra of Plumbago zeylanica biosorbent before biosorption (plot a) and after biosorption of Methylene blue (plot b) are represented in Fig.1. FTIR spectra of biosorbent display number of biosorption peaks, indicating the complex nature of the Plumbago zeylanica biosorbent. The peaks were assigned to various groups and bonds in accordance with their respective wave number.

 

The FTIR spectra of the biosorbent show the broader band in the region 3565-3311 cm^-1 which indicate the presence of free or hydrogen bonded O-H groups as in pectin, cellulose and lignin on the biosorbent surface. The sharp peaks at 2920-2850 indicate the presence of aldehydes. The band at 1606 is attributed to C=O carbonyl stretching vibration. Several bands ranging from 1319 to 1053cm^-1 refer to C-O bonding of phenols indicating that Plumbago zeylanica biosorbent is rich of tannins.

 

FIG.1 (a): FTIR spectra of Plumbago zeylanica biosorbent before biosorption

 

 

FIG.1 (b): FTIR spectra of Plumbago zeylanica biosorbent after biosorption

 

Effect of contact time:

The effect of contact time with respect to change in initial dye concentrations from 25-100 mg/L is depicted in Fig.2. The percentage biosorption increases with an increase in contact time up to 60 min and reached plateau even after increase of contact time up to 135 min.  The higher biosorption rate at the initial stages of contact time was due to the availability of less number of accessible vacant sites on the biosorbent surface. The biosorption rate slows towards equilibrium due to the availability of less number of accessible vacant sites on the biosorbent surface.  The % biosorption was increased from 82.48 to 96.52 % with a contact time of 60 min at 25mg/L of initial dye concentration.

 

Effect of solution pH:

The amount of dye sorbed from an aqueous solution was found to be affected by initial solution pH (Fig.3). The effect of pH on the amount of Methylene blue adsorbed onto Plumbago zeylanica was investigated over the pH range from 2 to 10. The amount of dye sorbed was minimum at a pH 2, then increases and remains almost constant over the pH range of 9-10. A relationship was observed between the cation biosorption and the magnitude of negative charge on the surface of biosorbent, which relates to the surface functional groups ¹¹. The electrical charges on biosorbent materials are known to depend on solution pH, which is related to the ionization of the polar functional groups on the biosorbent surface. At an initial dye concentration of 25 mg/L, the maximum biosorption efficiency was observed as 96.52 % at pH 9.

 

 

Fig.2: Effect of contact time on the biosorption efficiency of MB dye onto PZ biosorbent

 

 

Fig.3: Effect of solution pH on the biosorption efficiency of MB dye onto PZ biosorbent

 

Effect of initial dye concentration:

Dye removal is highly influenced by initial dye concentration. At low concentration, the dye molecules adsorb on active sites and hence % biosorption was high. With an increase in initial dye concentrations % biosorption decreases and dye uptake increases. The number of available active sites for a given fixed dosage of solids might not be sufficient to accommodate the increased number of dye molecules in the solution at higher concentration. This led to the saturation of available sites on the biosorbent surface and decreased the percentage biosorption¹². The % biosorption with dye uptake with respect to initial dye concentration is represented in Fig.4. At an initial concentration of 25mg/L the dye uptake was increased from 12.06 to 45.38mg/g of MB at 303K and 60min.

 

 

Fig.4: Effect of initial dye concentration on the biosorption efficiency of MB dye onto PZ biosorbent

 

Effect of biosorbent dosage:

The biosorption of the dye on Plumbago Zeylanica biomass was studied by varying the sorbent dosage (0.02 to 0.2 g in 50 ml). The percentage of biosorption was increased as the biosorbent concentration increased as shown in Fig.5. This increase is due to the availability of larger surface area with more active functional
groups at higher biosorbent dosages. With an increase in biosorbent dosage proportionate increase of percentage of biosorption was observed. At 25 mg/L, the percentage biosorption obtained as 73.08 to 96.52 % for a biosorbent dosage of 0.02-0.2g.

 

Effect of average biosorbent size:

Biosorption process is related with surface area of the biosorbent directly because it determines the time required for transport within the pore to biosorption sites so particle size is very important factor affecting % biosorption. The % biosorption decreased from 96.52 to 82.64% with an increase in particle size from 63µm to 212 µm at 25mg/L initial dye concentration (Fig.6). The lesser is the size of the particle, the more will be the surface area of the particle.

 

Effect of Temperature:

Increase in temperature decreases the %removal as shown in Fig.7. With an increase in temperature, the rate of diffusion of the biosorbate molecules increases owing to the decrease in the viscosity of the solution. At high temperatures the solubility of dyes in aqueous solution increases, thus attractive forces between dye molecules and solution were stronger than attractive forces between dye molecules and surface¹³. For an initial concentration of 25mg/L, the % biosorption is 96.52% at a temperature of 303K and a contact time of 60 min.

 

 

Fig.5: Effect of biosorbent dosage on the biosorption efficiency of MB dye onto PZ biosorbent

 

 

Fig.6: Effect of average biosorbent size on the biosorption efficiency of MB dye onto PZ biosorbent

 

 

Fig.7: Effect of temperature on the biosorption efficiency of MB dye onto PZ biosorbent

 

 

 

Biosorption isotherms and its suitability:

The biosorption isotherm represents how biosorption molecules distribute between the liquid phase and solid phase when biosorption reaches equilibrium state. The analysis of the isotherm data by fitting them to different isotherm models is an important step to find the suitable model that can be used for design purpose ¹⁴.

 

Biosorption isotherm is basically important to describe how solute interacts with biosorbents, and is critical in optimizing the use of biosorbents. Biosorption isotherms study is carried out with Langmuir, Freundlich, Temkin models. The applicability of isotherm equation is compared by judging the R² values.

 

Langmuir isotherm:

Langmuir isotherm assumes monolayer biosorption onto a surface containing a finite number of biosorption sites of uniform strategies of biosorption with no transmigration of biosorbate in the plane of surface¹⁵. The linear form of Langmuir isotherm model is given by the following equation as

 

                                   (3)

 

Where C_e is the equilibrium concentration of the biosorbate (mg/L), q_e is the amount of biosorbate adsorbed per unit mass of biosorbate (mg/g), and q_max and K_L are Langmuir constants related to biosorption capacity and rate of biosorption, respectively. By plotting C_e/q_e against C_e, straight line with slope 1/q_max was obtained (Fig.8) for different temperatures. The Langmuir constants q_max and K_L were calculated from isotherm and their values are given in Table-1. The results also demonstrate the formation of monolayer coverage of dye molecule at the outer surface of the PZ biosorbent.

 

The essential characteristics of Langmuir isotherm can be expressed in terms of a dimensionless equilibrium parameter (R_L), which is defined by

 

                                                         (4)

 

Where K_L is the Langmuir constant and C_i is the initial dye concentration. The value of R_L indicates the type of the isotherm to be either unfavourable (R_L>1), linear (R_L=1), favourable (0<R_L<1) or irreversible (R_L=0). Value of R_L was found to be 0 to 1 which confirms that PZ is favourable for biosorption of MB dye under conditions’

Freundlich isotherm:

Freundlich isotherm model assumes heterogeneous surface energies, in which the energy term in Langmuir equation varies as a function of the surface coverage ¹⁶. The well-known logarithmic form of Freundlich model is given by

 

                                    (5)

Where q_e is the amount adsorbed at equilibrium (mg/g), C_e the equilibrium concentration of the biosorbate and K_f and n_f are Freundlich constants. n_f giving an indication of how favourable the biosorption process and K_f  is the biosorption capacity of the biosorbent. K_f is defined as the biosorption or distribution coefficient and represents the quantity of dye adsorbed onto PZ biosorbent for a equilibrium concentration. The slope 1/n_f ranging between 0 and 1 is a measure of biosorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to zero ¹⁷.  The plot of ln C_e vs. ln q_e gives straight lines with slope 1/n_f (Fig.9) which shows biosorption of MB follows the Freundlich isotherm. Freundlich constants are calculated from equation (5) and recorded in Table-1.

 

Temkin isotherm:

Temkin isotherm contains a factor that explicitly takes into account adsorbing species biosorbate interactions. The isotherm assumes that the heat of biosorption of all the molecules in the layer decreases linearly with coverage due to biosorbate interactions and biosorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy. Temkin isotherm is represented by the following equation

 

                                           (6)

 

The biosorption data can be analyzed according to Eq. (6). A plot of ln C_e versus q_e (Fig. 10) enables the determination of the isotherm constants A_T and B_T. A_T is the equilibrium binding constant and constant B_T is related to the heat of biosorption. The values of the parameters are given in Table-1.

 

Table-1 shows the values of parameters of three isotherms and related correlation coefficients.  From Table-1, the Freundlich model yields a somewhat better fit than the Langmuir isotherm. The values of 1/n_f from Table-1 indicate favourable biosorption process.

 

 

 

 

 

 

 

Table: 1. Isotherms constants for MB dye biosorption onto PZ biosorbent at C_i =25mg/L

w = 0.1g, d= 63-212 µm, T= 303-343 K, pH= 9 and t = 60min.

Isotherm Model	Parameters	Temperature (K)
		303	313	323	333	343
Langmuir	qmax (mg/g)	65.3594	80	101.01	70.9219	70.4225
	KL (L/g)	0.2048	0.0908	0.0439	0.0534	0.0398
	R2	0.9191	0.9637	0.9911	0.9233	0.9656
	RL=1/(1+KLCo)	0.1633	0.0398	0.0387	0.4282	0.5012
Freundlich	Kf (mg/g)/(L/g)n	12.6987	8.0719	5.1304	4.9654	3.8682
	nf (L/g)	1.8070	1.4954	1.3008	1.4803	1.4421
	R2	0.9942	0.9999	0.9936	0.9743	0.9843
Temkin	AT (L/g)	2.4058	1.0822	0.6194	0.5876	0.4494
	BT(L/mg)	188.55	164.83	152.85	188.56	22.33
	R2	0.9304	0.9633	0.9858	0.9688	0.9842

 

 

Fig.8: Langmuir isotherms at different temperatures for biosorption of MB dye onto PZ biosorbent

 

 

Fig.9: Freundlich isotherms at different temperatures for biosorption of MB dye onto PZ biosorbent

 

 

Fig.10: Temkin isotherms at different temperatures for biosorption of MB dye onto PZ biosorbent

 

Biosorption Kinetics:

Pseudo-first order kinetics:

The rate constant of biosorption is determined from the pseudo first order equation given as

 

                                      (7)

 

Where q_e and q_t are the amounts of MB adsorbed (mg/g) at equilibrium and at time t (min) respectively and k_f the rate constant of biosorption (min^-1). Values of k_f were calculated from the plots of ln (q_e-q) vs. t (Fig.11) for different initial dye concentrations of MB. The correlation coefficient values at high concentrations, where the q_e values are obtained from the linear plots. This shows that the biosorption of MB onto PZ is not a first order kinetic.

 

Pseudo second order kinetics

The pseudo second order equation is expressed as

                                 (8)

 

Where k_s (g/mg.min) is the rate constant of second order biosorption. The plot of t vs. t/q (Fig.12) shows a linear relationship, where second order kinetics is applicable. The q_e and k_s can be determined from the slope and intercept of the plot. The linear plots of t vs. t/q show a good agreement between experimental and calculated q_e values (Table-2). The values of R² for second order kinetic model are greater than 0.99 indicating the applicability of kinetic equation and second order nature of the biosorption process of MB on PZ.

 

Elovich kinetics

Elovich model gives the information about the physisorption or chemisorption mechanism. Elovich model is used to describe the second order kinetics of chemisorption and it is based on the assumption that the adsorption surface is energetically heterogeneous ¹⁸. The Elovich model is expressed as

 

                                            (9)

 

Where  is initial rate and β representing activation energy (g/mg).  A graph of ln t vs. q_e (Fig.13) is drawn where the constants are evaluated from equation (9). The constants and correlation coefficients are given in Table-2; indicated that for higher concentrations the Elovich model was not suitable to fit the data.

 

 

Fig.11: Pseudo first order kinetics at different initial dye concentrations for biosorption of MB dye onto PZ biosorbent

 

Fig.12: Pseudo second order kinetics at different initial dye concentrations for biosorption of MB dye onto PZ biosorbent

 

 

Fig.13: Elovich kinetics at different initial dye concentrations for biosorption of MB dye onto PZ biosorbent

 

Thermodynamic Studies:

Thermodynamic studies provide information about the effect of temperature on biosorption process, which is essential for the estimation of energy requirement. Thermodynamic parameters such as free energy (ΔG⁰), enthalpy change (ΔH⁰) and entropy change (ΔS⁰) can be estimated by using the equations (10, 11).

 

                                              (10)

 

 

 

                                                (11)

 

A plot was plotted between 1/T vs. ln K_D, shown in Fig.14. For different initial dye concentrations.  The values of ΔH^o, ΔS^o and ΔG^o are represented in Table-3. The negative value of ΔH^o suggests the reaction is exothermic. The negative value of ΔS^o shows an increase in the randomness at the solid solute interface during biosorption.  The negative value of ΔG^o indicates the spontaneous nature of MB biosorption onto PZ.

Table.2: Kinetic rate constants for MG dye biosorption onto PZ at C_i = 25-100 mg/L, w= 0.1g, d =63-212 µm, T=303 K, pH= 9 and t = 60 min.

Ci (mg/L)	Pseudo-first-order			Pseudo-Second-order			Elovich Model
	kf(min-1)	qe (mg/g)	R2	ks (g/mg. min)	qe (mg/g)	R2	β(g/mg)	α(mg/g.min)	R2
25	1.1238	3.1006	0.9797	0.0730	12.1951	0.999	1.9015	53.4625*106	0.9346
50	0.9836	4.9490	0.9642	0.0386	23.5849	0.999	1.0755	802.85*106	0.9489
75	0.7166	7.7477	0.9869	0.0281	34.7222	0.999	0.7207	888.59*106	0.9411
100	1.2607	15.7289	0.8931	0.0174	45.8715	0.999	0.4535	21.25*106	0.9386

 

Table.3.Thermodynamic energy parameters for the biosorption of MB dye using PZ biosorbent

Ci  (mg/L)	-ΔHo (kJ/mol)	-ΔSo (kJ/mol.K)	-ΔGo(kJ/mol)
			303K	313K	323K	333K	343K
25	37.75	0.10	7.45	6.45	5.45	4.45	3.45
50	24.90	0.06	6.72	6.12	5.52	4.92	4.32
75	28.41	0.07	7.20	6.50	5.80	5.10	4.40
100	27.33	0.07	6.12	5.42	4.72	4.02	3.32

Fig.14: Thermodynamic studies at different initial dye concentrations for biosorption of MB dye onto PZ biosorbent

 

EXPERIMENTAL DESIGN AND DATA ANALYSIS:

The aim of RSM is to find out the optimum operating conditions for a given system, or the way in which a particular response is affected by a set of variables. The quadratic response surface model over some specific region of interest was fitted to the following equation:

 

Y =b0+b1X1 +b2X2 +b3X3 +b4X4 +b11X1 2 +b22 X22 +b33X32 +b44X42 +b12X1X2 +b13X1X3 +b14X1X4 +b23X2X3 +b24X2X4 + b34X3X4        (12)

 

Where Y is the predicted response; X1, X2, X3, and X4 are independent variables; b0 is an offset term; b1, b2, b3 and b4 are linear effects; b11, b22, b33 and b44 are squared effects; and b12, b13,b14, b23, b24 and b34 are interaction terms. The ‘‘Design Expert’’ software, statistical package STATISTICA 6.0 was used for regression and graphical analysis of the data obtained. The operational conditions were independent variables which were X1 (contact time), X2 (solution pH), X3 (initial dye concentration) and X4 (temperature). The intervals of reaction parameters were selected as contact time of 50-70 min, solution pH of 5 to 13, for initial dye concentration of 15- 35 mg/L,  having a temperature of 293 to 313K. The experimental range and levels of the independent variables are indicated in Table- 4.

 

Table 4: Levels of different process variables used in CCD for removal of MB dye.

Variable	Factor	Level
		-2	-1	0	1	2
x1	Contact Time(min)	50	55	60	65	70
x2	Solution pH	5	7	9	11	13
x3	Initial Concentration (mg/L)	15	20	25	30	35
x4	Temperature (K)	293	298	303	308	313

 

 

Response Surface Methodology (RSM):

RSM was used to design the experimental runs to determine model representing the effect of all independent variables on the biosorption which in turn is used to get more accurate optimized values than traditional one factor method. The application of the RSM based on the estimates of parameters indicated an empirical relationship between the response and the input variables expressed by the following fitted second order polynomial equation,

 

Y = -56.7064+ 3.4135 X₁ +0.2353 X₂+ 1.3840 X₃+ 1.8640 X₄-0.0426 X₁²-0.2783X₂²-0.0450 X₃²-0.0207 X₄²+ 0.1220 X₁X₂+0.0249 X₁X₃+0.0036X₁X₄-0.0438X₂X₃-0.0611X₂X₄ 0.0063X₃X₄

 

The student t-distribution and the corresponding p-values, along with the parameter estimates, are listed in Table-5. The significance of each parameter was determined via p-values and the student t- test. A larger t-value and smaller p-value identifies the effect that appears to be very important ^{19, 20}. For our data, it was observed that the first order main effects of variables, namely contact time (X₁), initial dye concentration (X₃) and temperature (X₄) and their second order main effects X₁X₂, X₁X₃, X₂X₃, X₂X₄ are highly significant since their respective p-values are very small. The quantities X1, X2, X3 and X4 have positive influence while X₂X₃, X₂X₄, X₃X₄ shows a negative influence on biosorption. In order to determine whether or not the second order polynomial equation was significant to fit with experimental result, it is necessary to conduct an analysis of variance (ANOVA). The ANOVA indicates that the equation represents adequately the actual relationship between the response and the significant variables²¹. The coefficient of determination (R²) is found to be 0.95 which is very high and has advocated high correlation between the observed and the predicted value. The optimized parameters are represented in Table-6. The response surface plots for different process variables are represented in Figs.15-20.

 

Table 5: Estimated regression coefficients and corresponding t- and P- values of the model.

	Regression Coeff.	SE coeff.	t-value	p-value
constant.	-56.7064	25.18696	-2.25142	0.045776
x1	3.4135	0.58152	5.86993	0.000108
x12	-0.0426	0.00451	-9.45360	0.000001
x2	0.2353	0.98489	0.23892	0.815558
x22	-0.2783	0.02818	-9.87499	0.000001
x3	1.3840	0.40277	3.43621	0.005562
x32	-0.0450	0.00451	-9.97479	0.000001
x4	1.8640	0.42248	4.41217	0.001042
x42	-0.0207	0.00451	-4.59650	0.000769
x1x2	0.1220	0.01177	10.36235	0.000001
x1x3	0.0249	0.00471	5.28735	0.000257
x1x4	0.0036	0.00471	0.75382	0.466788
x2x3	-0.0438	0.01177	-3.71601	0.003405
x2x4	-0.0611	0.01177	-5.19179	0.000298
x3x4	-0.0063	0.00471	-1.34838	0.204644

Table 6.  Optimized process parameters

Process Parameters	Initial observed limits of process parameters	Critical values of  process parameters	Final observed limits of process parameters
Contact Time(min)	50	60.13283	70
Solution pH	5	7.76231	13
Initial dye Concentration(mg/L)	15	25.80578	35
Temperature (K)	20	34.72076	40

 

 

 

 

 

 

Fig.15. Response surface plot of initial concentration vs. temperature

 

 

Fig.16. Response surface plot of solution pH vs. initial concentration

 

Fig.17. Response surface plot of solution pH vs. temperature

 

 

Fig.18. Response surface plot of contact time vs. initial concentration

 

Fig.19. Response surface plot of contact time vs. solution pH

 

 

Fig.20. Response surface plot of contact time vs. temperature

 

 

CONCLUSIONS:

1.      The present study shows that Plumbago zeylanica is an effective biosorbent for the removal of methylene blue from an aqueous solution.

2.      The removal of methylene blue from an aqueous solution by the batch system has been investigated under different experimental conditions. Higher percentage of Methylene blue removal by Plumbago zeylanica was possible at the initial dye concentration in the solution is low.

3.      The biosorption data were fitted properly to the Freundlich model and led to determine the maximum biosorption capacity.

4.      The effect of initial dye concentration of methylene blue on the kinetics of the biosorption process has been studied.

5.      The pseudo- second order kinetic model was found to be best fit with the experimental data within the time range of biosorption, as the correlation coefficient and the accuracy of the model in predicting experimental data strongly suggest that the rate limiting step may be a chemical biosorption involving valance forces through sharing or exchange of electrons between sorbent and sorbate.

6.       Biosorption of Methylene blue on Plumbago zeylanica is favorable at lower values of temperature. The negative value of ΔG⁰ indicates spontaneous biosorption of Methylene blue on Plumbago zeylanica.

7.      The negative value of ΔH⁰ indicates that the reaction is exothermic; the negative value of ΔS⁰ represents the randomness of the reaction.

8.      The experimental optimum values of process parameters are t=60min, pH=9, C_i=25 mg/L, w=0.1g, d=63 µm at a temperature T=303K.

9.      The optimized values using RSM are t= 60.13 min, pH= 7.76, C_i=25.8 mg/L, T=307.72K.

10.    The biosorption characteristics favor PZ to be used as an effective biosorbent for the removal of methylene blue from wastewater.

 

REFERENCES:

1.       I.M.Banat, P.Nigam, D.Singh, R. Marchant, Microbial decolorization of textile-dye containing effluents: a review, Bioresource Technology. 58(1996) 217-227.

2.       G.Crini, Non-conventional low-cost adsorbents for dye removal: a review, Bioresource Technology 97(2006)1061.

3.       K.R.Ramakrishna, T.V.Viraraghavan, Dye removal using low cost adsorbents, Water Science and Technology 36 (1997) 189-196.

4.       H.M.Asfour, M.M.Nassar, O.A.Fadali and M.S. ElGuendi, Journal of Chemical Technology and Biotechnology, 35A (1985) 28-35.

5.       Z.Al-Qodah, Water Resource, 34 (2000) 2727-2733.

6.       Rajeshwarisivaraj, S.Sivalumar, P.Senthilkumar, V.Subburam, Carbon from cassava peel, an agricultural waste, as an adsorbent in the removal of dyes and metal ions from aqeous solution, Bioresource Technology 80 (2001) 233-235.

7.       W.T.Tsai, C.Y.Chang.M.C.Lin, S.F.Chien, H.F.Sun, M.F.Hsieh, and Adsorption of acid dye onto activated carbons prepared from agricultural waste bagasse by Zncl2 activation, Chemosphere 45 (2001) 51-58.

8.       D.L.Klass,Biomass for renewable Energy, Fuels and Chemicals, Academic Press, San Diego,CA,1998.

9.       G.Crini, Non-conventional low-cost adsorbents for dye removal: a review, Bioresource Technology 97(2006)1061.

10.     S.Ismadji, Y.Sudaryanto, S.B.Hartono, L.E.K.Setiawan, A.Ayucitra, Activated carbon from char obtained from vacuum prolysis of teak sawdust: pore structure development and characterization, Bioresource Technology 96 (2005) 1364.

11.     Selatina A,  Bakthi MZ,Madani A, Kertous L, Mansouri Y. Biosorption of Cd2+ from aqueous solution by a NaOH treated bacterial dead Streptomyces rimosus biomass. Hydrometallurgy 75 (2004), 11-24.

12.     P.Kalpana, V.Omkar, P.King, Removal of congo red from aqueous solutions using plant biomass: equilibrium and kinetic studies, Chemical Technology 11(2) 2016, 43-50

13.     Tan I.A.W., Ahmad A.L., Hammed B.H., Adsorption of basic dye on high-surface area activated carbon prepared from coconut husk: Equilibrium, kinetic and thermodynamic studies, Journal of Hazardous Materials 154 (2008) 337-346.

14.     M.El-Guendi, Homogeneous surface diffusion model of basic dyestuffs onto natural clay in batch adsorbents, Adsorption science Technology 8(2) (1991)217-225.

15.     W.J.Weber Jr., Physico-chemical Processes for Water Quality Control, Wiley Interscience, New York, 1972.

16.     F.Haghseresht, G.Lu, Adsorption characteristic of phenolic compounds onto coal reject derived adsorbents. Energy fuels 12 (1998) 1100-1107.

17.     Low M.J.D., Kinetics of chemisorptions of gases on solids, Chem. Rev. 60 (1960) 267-312.

18.     Perry R h and Green W D, Perrys chemical Engineers Handbook, 7th edition (Mc Graw Hill International Editions), 1998.

19.     Elibol M, Response surface methodological approach for inclusion of perfluorocarbon in actinorhodin fermentation medium, Process Biochemistry, 38 (2002) 667-673.

20.     Marban G, Valdes-Soils T and Fuertes A B, Chem Eng Sci, 61(7) (2006) 4762-4772.

21.     Kalpana.P, King.P, Equilibrium, Kinetic and Thermodynamic studies of biosorption of Methylene Blue dye using plant biomass as biosorbent: Optimization using Response Surface Methodology (RSM), International Journal of Scientific and Engineering Research, 4( 4) (2013)  1244-1252.

 

 

 

Accepted on 05.01.2018          © RJPT All right reserved

Research J. Pharm. and Tech 2018; 11(5):2006-2016.