# Removal of Methylene Blue from Aqueous Solution Using Black Tea Wastes: Used as Efficient Adsorbent

Authors: Azmat Ullah; Muhammad Zahoor; Waqar Ud Din; Mian Muhammad; Farhat Ali Khan; Aamir Sohail; Riaz Ullah; Essam A. Ali; H. C. Ananda Murthy
Journal: Adsorption Science & Technology
 (2022)

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## Abstract

The biosorbent black tea wastes (BTW) after preliminary treatments was used in this study for the removal of methylene blue (M.B) from aqueous solution. The removal of M.B from aqueous solution was studied as a function of time, initial concentration of M.B temperature, pH, and BTW dosage. The optimum time for equilibration was achieved in 3 min. The optimum dosage of adsorbent was found to be 0.4 g. Various kinetic models were applied to the sorption kinetic data in which the obtained data was best explained by the pseudo-second-order model (R2=0.99) with a rate constant K2 of 0.0714–0.0763 g.mg-1 min-1. Additionally, the calculated amount of adsorption was approximately equal to the experimentally determined value. The isotherm data was best fitted to the Langmuir model rather than the Freundlich model. The intraparticle diffusion model exhibited the process to be diffusion dependent. The various organic functional groups on the surface of BTW played a significant role in the sorption of the selected dye. Consequently, BTW has the prospective to act as a potential sorbent for the removal of other contaminants from aquatic media as well.


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## Body

## 1. Introduction

Dyes are organic compounds that are widely utilized in the textile, paper, food, printing, plastic, drinks, leather, and pharmacology industries [1–3]. In general, dyes can be categorized as anionic (acidic dyes) and cationic (basic dyes). Among them, azo dyes (anionic) are that class of dye which contain nitrogen-nitrogen double bonds and are considered to be the largest group of organic dyes but at the same time are quite toxic. Some of them have been reported to have carcinogenic potentials, whereas same effect has been observed for the metabolites formed from them after degradation [4]. Thus, there is extensive need to reclaim them from industrial effluents before entering into water bodies. Despite of the harmful effects, dyes are still significant and important group of chemicals used for coloring material objects. There are over 1×106 commercially available dyes, with an annual output of about 7×105 metric tons, according to the reports [5]. As mentioned before, because of their complex aromatic structure and nonbiodegradable nature, dyes in waste water pose a significant threat to living organisms, making their detection and removal a difficult job [6].Dyes are excessively entered into water reservoirs a number of industrial sources. These sources include textile, paper, beauty products, food, and polymers. The continuous entry of dyes into aquatic media poses severe health problems in human beings and also destroys the ecosystem. Most of these dyes are extremely poisonous, mutagenic, and carcinogenic in nature [7–10].Methylene blue (M.B) is an aromatic and widely used synthetic cationic dye with a high adsorption capacity that is commonly used in the dyeing of silk, wool, and cotton in the industrial process. M.B is also used to treat diseases such as duck hepatitis B, psoriasis, and West Nile virus. Despite of medicinal application, some health complication is associated with its use, for example, permanent eye damage, vomiting, gastritis, breathing difficulties, nausea, mental confusion, painful micturition, tissue necrosis, cyanosis, and methemoglobinemia-like syndromes. Methylene blue is also highly carcinogenic in living organisms. Methylene blue is not regarded as extremely toxic but its ingestion through the mouth produces a burning impression [11]. Therefore, it is exceptionally important to remove it from industrial waste water before discharging it into the environment. Different physiochemical methods have been used for the removal of various contaminants, namely, solvent extraction, membrane filtration [12–15], coagulation, and chemical oxidation [16] from wastewaters. Unfortunately, these techniques are not cost-effective and also have a number of disadvantages, including a high reagent and energy consumption, as well as the production of poisonous clay or other byproducts that must be disposed of after completion of the processes. Furthermore, certain colors used in textile industry are extremely difficult to eliminate using traditional waste disposal methods since they are light and oxidizing agent resistant and unchanged by aerobic digestion [17]. As a result, new procedures for removing dyes are being extensively employed in the industries. Although activated carbon is the most effective adsorbent but due to high cost of preparation, its use becomes impractical in most parts of the world. Therefore, scientists these days are trying to use waste biomass as greener adsorbent.A lot of studies have been reported on various low-cost adsorbent for dye removal from water [8–19]. Out of which waste biomasses and commercial byproducts have been intensively investigated for their potential to eliminate dye from aqueous media, as they are readily available and cheap green sources, e.g., corncob, modified leaves of plants, and rice husk [18].Due to the massive consumption of tea around the world, numerous researchers have employed tea waste as an excellent adsorbent. After water, tea is the world’s second most consumed nonalcoholic liquid. In 2013, global tea output was 5.1 million tons, with China accounting for 36% of total production, followed by India (21.2%), Kenya (7.8%), Sri Lanka (7.0%), Turkey (4.8%), Vietnam (4.6%), and Iran (4.6%). The remaining 15.3% comes from other countries. Turkey is the world’s fifth largest tea grower, with an annual output of 212,400 tons and the highest per capita tea consumption. Turkey produces around 150,000 tons of tea annually (dry basis) and 30,000 tons of tea waste per year [20]. The tea wastes are discarded as such which finally finds its way to river and canals.The aim of present study was to use the tea wastes in a useful way as a biosorbent for methylene blue. Different isothermal and kinetic models were employed to estimate the adsorption parameters of selected dye on tea waste-based biosorbent.


## 2. Materials and Methods

### 2.1. Materials

Black rea wastes (BTW) was collected from local market. The BTW sample was treated with hot double distilled water to remove dust and other materials. The treatment was applied to remove certain pigments remained in BTW. After treatment, the BTW was dried in an electric oven at 75-80°C for 12-14 h. The dried BTW sample was then grinded into a fine powder form using a grinder. The dried and grinded BTW was then stored in a glass reagent bottle till further studies. Methylene blue (M.B) was provided by the Institute of Chemical Sciences University of Malakand. The chemicals NaOH and HCl were purchased from Merck (Germany). The schematic diagram of the whole process is shown in Figure1.Figure 1
Schematic representation of decolorization of M.B by BTW.


### 2.2. Batch Adsorption Experiments

In batch sorption experiments, 30 mL of M.B solution in a 100 mL glass reagent bottle were contacted with specified amount of adsorbent. Different concentration solutions of M.B were obtained from 800 mg.L-1 stock solution using dilution method. For adsorption kinetics about 0.40 g of BTW was added to 30 mL M.B solution during kinetic study and stirred for different interval of time. During isotherm studies, different initial concentrations of M.B, i.e., from 100 mg.L-1 to 400 mg.L-1 in 30 mL volume were contacted with 0.4 g of BTW powder. The adsorption isotherm data was then interpreted for different isotherm models. For optimum dose of BTW, different masses of BTW were added in the range of 0.10-0.50 g to a series of flasks containing 30 mL solution each. Out of the tested doses, 0.40 g amount was selected as optimum dose of BTW for the subsequent experiments. The pH on M.B adsorption was studied in range 1-12 where pH of solutions were adjusted through solution of NaOH and HCl. The M.B concentration after adsorption in each experiment was determined at 665 nm using a double beam UV/visible spectrophotometer (Shimadzu Japan).


### 2.3. Calibration Curve of M.B

As mentioned above, the concentrations of the remaining dye in solution were determined at 665 nm. To avoid tedious calculations, a calibration curve as shown in Figure2 was constructed.Figure 2
Calibration curve for methylene blue.


### 2.4. Data Analysis

The experimental data was fed (equations (1) and (2)) to calculate different adsorption parameters:
(1)Q=Xo−Xtm×V,(2)%R=Xo−XtXo×100.In equation (1), Q is the amount of M.B adsorbed at different time interval, Xo and Xt are the initial concentration and concentration at time t of M.B dye, respectively, whereas m is the mass of BTW in grams and V is the volume in liter (L) of M.B dye solution. %R represents the percent removal of M.B dye.


## 2.1. Materials

Black rea wastes (BTW) was collected from local market. The BTW sample was treated with hot double distilled water to remove dust and other materials. The treatment was applied to remove certain pigments remained in BTW. After treatment, the BTW was dried in an electric oven at 75-80°C for 12-14 h. The dried BTW sample was then grinded into a fine powder form using a grinder. The dried and grinded BTW was then stored in a glass reagent bottle till further studies. Methylene blue (M.B) was provided by the Institute of Chemical Sciences University of Malakand. The chemicals NaOH and HCl were purchased from Merck (Germany). The schematic diagram of the whole process is shown in Figure1.Figure 1
Schematic representation of decolorization of M.B by BTW.


## 2.2. Batch Adsorption Experiments

In batch sorption experiments, 30 mL of M.B solution in a 100 mL glass reagent bottle were contacted with specified amount of adsorbent. Different concentration solutions of M.B were obtained from 800 mg.L-1 stock solution using dilution method. For adsorption kinetics about 0.40 g of BTW was added to 30 mL M.B solution during kinetic study and stirred for different interval of time. During isotherm studies, different initial concentrations of M.B, i.e., from 100 mg.L-1 to 400 mg.L-1 in 30 mL volume were contacted with 0.4 g of BTW powder. The adsorption isotherm data was then interpreted for different isotherm models. For optimum dose of BTW, different masses of BTW were added in the range of 0.10-0.50 g to a series of flasks containing 30 mL solution each. Out of the tested doses, 0.40 g amount was selected as optimum dose of BTW for the subsequent experiments. The pH on M.B adsorption was studied in range 1-12 where pH of solutions were adjusted through solution of NaOH and HCl. The M.B concentration after adsorption in each experiment was determined at 665 nm using a double beam UV/visible spectrophotometer (Shimadzu Japan).


## 2.3. Calibration Curve of M.B

As mentioned above, the concentrations of the remaining dye in solution were determined at 665 nm. To avoid tedious calculations, a calibration curve as shown in Figure2 was constructed.Figure 2
Calibration curve for methylene blue.


## 2.4. Data Analysis

The experimental data was fed (equations (1) and (2)) to calculate different adsorption parameters:
(1)Q=Xo−Xtm×V,(2)%R=Xo−XtXo×100.In equation (1), Q is the amount of M.B adsorbed at different time interval, Xo and Xt are the initial concentration and concentration at time t of M.B dye, respectively, whereas m is the mass of BTW in grams and V is the volume in liter (L) of M.B dye solution. %R represents the percent removal of M.B dye.


## 3. Results and Discussion

### 3.1. Effect of pH on Percent % Removal of M.B

The effect of pH on the adsorption of M.B on BTW is graphically shown in Figure3 as studied in the pH range of 1-12. The maximum adsorption occurs at the pH 8 and then the adsorption rate decreased. The high degree of adsorption at pH 8 may be due some positive interaction of certain functional groups such as amino, hydroxyl, and ethereal present on the surface of BTW with that of the M.B [21]. These oppositely charged ions attract each other strongly at pH 8.Figure 3
Effect of pH on methylene blue removal.


### 3.2. Effect of BTW Dosage

The effect of BTW on the adsorption of M.B is shown in Figure4. With the increase in BTW amount, the uptake of M.B molecules also increased and then becomes steady after 0.40 g. Thus, 0.4 g of BTW was selected as maximum sorbent dosage that was then used in subsequent experiments. The rise in the percent removal of dye with increasing dose is due to the fact that the larger amount of mass provided a larger number of active sites for the biosorption, which causes the rise in the percent of removal as already reported in the literatures by Alencar et al. [22] and Tural et al. [23].Figure 4
Effect of dosage on M.B removal.


### 3.3. Adsorption Isotherm Studies

The effect of dye concentration on adsorption studied in range 20 mg.L-1–140 mg.L-1 is shown in Figure 5. It is clearer from the Figure 4 that as concentration of M.B increases from 20 mg.L-1 to 100 mg.L-1, the rate of adsorption increases. The plot becomes almost linear at 100 mg.L-1, further increase in concentration of M.B has no effect on the removal of M.B from aquatic media. At lower initial concentrations of M.B, there were fairly insufficient dye molecules for the large number of available active sites, hence resulting in rapid uptake by sorbent. With the increase in the initial concentrations of M.B, a gradual decrease in the percentage removal of dye was observed which was due to the saturation of the available active sites [24].Figure 5
Adsorption isotherm plot of M.B on BTW.


### 3.4. Langmuir Isotherm Model

Linear equation [25] for the Langmuir isotherm model is given:
(3)CeXe=1KLXm+CeXm,where Xeis the equilibrium concentration, Xm is the maximum sorption capacity (mg.L-1), and KL is the Langmuir isotherm constant. The value of KL is related to free energy of sorption process, which corresponds to the affinity of between the surface of sorbate molecules and sorbent. It also defines the mode of adsorption [26, 27].The experimental values of constants in equation are given in Table1. After plotting Ce/Xe versus Ce, linear plot was obtained as shown in Figure 6. It indicates that the experimental data fitted well to the Langmuir isotherm model, verifying monolayer M.B adsorption on a homogenous surface of BTW [26]. The value of Xm was calculated from the slope of plot while KL (affinity constant or energy constant) was calculated from the intercept and its values are given in Table 1.Table 1
Adsorption isotherm parameters of M.B on BTW.
Isotherm model
Parameter
Values
R
2
Langmuir
K
L
0.0740
0.997
X
M
3.367
Freundlich
K
f
2.32
0.927
1
n
0.367Figure 6
Langmuir isotherm plot for adsorption of M.B on BTW.


### 3.5. Freundlich Isotherm Model

The Freundlich isotherm model [28] is given.
(4)logqe=logKf+1nlogCe,where 1/n and Kf are Freundlich isotherm constants and can be obtained by plotting log Xe versus Q as shown in Figure 7. The constant Kf indicates the quantity of the M.B adsorbed on the surface of the BTW, while the constant 1/n shows the heterogeneity of the system or adsorption intensity. The value of 1/n<1 shows normal adsorption [29]. The R2 value (0.927) was far away from unity as compared to same parameter value of the Langmuir model. This reveals that the Freundlich model could not be obeyed by the experimental data.Figure 7
Freundlich isotherm plot for adsorption of M.B on BTW.


### 3.6. Adsorption Kinetics

Adsorption kinetics is one of the most features of such processes that dictate the solute uptake level/the adsorption ability of biosorbent that help in deciding the possible applicability of the adsorbent to be used in industry [30]. The effect of contact time on adsorption is shown in Figure 8. It is clear from the figure that the rate of adsorption is higher in the first three minutes which then slows down. The faster rate is due to availability of active sites on the surface of BTW at start of experiments. As the sites are saturated with M.B molecules, the rate of adsorption slows down. It is due to the increasing competition between the residual dye molecules for the remaining active sites on the surface of biosorbent. Similar results have been reported earlier by Tang et al. [31], Kumar and Jena [32], Pang et al. [33], and Heidarinejad et al. [34].Figure 8
Adsorption kinetic plot of M.B on the surface of BTW at different temperatures.


### 3.7. Pseudo-First-Order Model

To calculate the kinetic constants of M.B adsorption on BTW, the Lagergren pseudo-first-order rate (equation (5)) was applied to the adsorption kinetic data.
(5)LogXe−Xt=logXe−K12.303t,where Xe is the amount of M.B adsorbed at equilibrium time and Xt is the amount of M.B adsorbed at time t. K1 is the pseudo-first-order rate constant. The constants in the equation were obtained by plotting time (t) vs. lnXe−Xt (Figure 9). The values of constants are given in Table 2. The results obtained for this model were not in accordance with the experimental data. The lower value of R2 is less than unity at temperature 303 and 313 K showing that this model does not fit well the kinetic data.Figure 9
Pseudo-first-order kinetic plot of M.B onto BTW.Table 2
Constant of pseudo-first-order kinetics.
Temperature
293 K
303 K
313 K
q
max
0.175
0.167
0.235
K
1
3.49
3.705
3.565
R
2
0.994
0.960
0.9278


### 3.8. Pseudo-Second-Order Model

This model can be given as follows [30, 35]:
(6)tXt=1K2Xe+tXtThis is the linear form of the pseudo-second-order kinetic model whereXe is the amount of M.B adsorbed at equilibrium, Xt is the amount of M.B adsorbed at time t, and K2 is the pseudo-second-order rate constant. The values of constants (Table 3) were obtained by plotting t/Xt vs. time (Figure 10). The value of R2 is almost equal unity at all temperatures showing that the kinetic data could be bitterly accommodated by this model and adsorption process is chemical in nature where the sharing or exchange of electrons between sorbate and biosorbent has been taken place. Similar results have been reported by Li et al. [36], X. Li and Y. Li [37], Weng et al. [38], Ravi and Pandey [39], Pathania et al. [40], and Qian et al. [41].Table 3
Pseudo-second-order kinetic constants.
Temperature
q
t
×
10
−
4
mg
.
g
−
1
K
2
R
2
293 K
33.33
0.0762
0.9985
303 K
……
0.0714
0.9980
313 K
……
……
0.9902Figure 10
Pseudo-second-order kinetic plot of M.B onto BTW.


### 3.9. Intraparticle Diffusion Model

Intraparticle diffusion mechanism was used to investigate the diffusion mechanism of adsorption process. For intraparticle diffusion, equation (7) [42] is used:
(7)qt=Kdifft1/2+C.In equation (7), Kdiff (mgg-1 min-1) is intraparticle diffusion rate constant, qt is the amount of M.B adsorbed on BTW at time (t), t1/2 is the square root of time, and C is the thickness of boundary layer. A linear plot (Figure 11) was obtained by plotting qt vs. t1/2. The thickness of boundary layer is obtained from the intercept of the linear plot, while Kdiff (mgg-1 min-1) was obtained from the slope of linear plot. The values of constants obtained from linear plot are tabulated in Table 4. It was observed from the linear plot of intraparticle diffusion plot that the process of M.B adsorption on BTW is a diffusion-controlled process [43].Figure 11
Intraparticle diffusion plot.Table 4
Constant of intraparticle diffusion.
Temperature
293 K
303 K
313 K
K
diff
12.191
14.802
13.601
C
19.004
19.467
25.434
R
2
0.9237
0.9902
0.9794


## 3.1. Effect of pH on Percent % Removal of M.B

The effect of pH on the adsorption of M.B on BTW is graphically shown in Figure3 as studied in the pH range of 1-12. The maximum adsorption occurs at the pH 8 and then the adsorption rate decreased. The high degree of adsorption at pH 8 may be due some positive interaction of certain functional groups such as amino, hydroxyl, and ethereal present on the surface of BTW with that of the M.B [21]. These oppositely charged ions attract each other strongly at pH 8.Figure 3
Effect of pH on methylene blue removal.


## 3.2. Effect of BTW Dosage

The effect of BTW on the adsorption of M.B is shown in Figure4. With the increase in BTW amount, the uptake of M.B molecules also increased and then becomes steady after 0.40 g. Thus, 0.4 g of BTW was selected as maximum sorbent dosage that was then used in subsequent experiments. The rise in the percent removal of dye with increasing dose is due to the fact that the larger amount of mass provided a larger number of active sites for the biosorption, which causes the rise in the percent of removal as already reported in the literatures by Alencar et al. [22] and Tural et al. [23].Figure 4
Effect of dosage on M.B removal.


## 3.3. Adsorption Isotherm Studies

The effect of dye concentration on adsorption studied in range 20 mg.L-1–140 mg.L-1 is shown in Figure 5. It is clearer from the Figure 4 that as concentration of M.B increases from 20 mg.L-1 to 100 mg.L-1, the rate of adsorption increases. The plot becomes almost linear at 100 mg.L-1, further increase in concentration of M.B has no effect on the removal of M.B from aquatic media. At lower initial concentrations of M.B, there were fairly insufficient dye molecules for the large number of available active sites, hence resulting in rapid uptake by sorbent. With the increase in the initial concentrations of M.B, a gradual decrease in the percentage removal of dye was observed which was due to the saturation of the available active sites [24].Figure 5
Adsorption isotherm plot of M.B on BTW.


## 3.4. Langmuir Isotherm Model

Linear equation [25] for the Langmuir isotherm model is given:
(3)CeXe=1KLXm+CeXm,where Xeis the equilibrium concentration, Xm is the maximum sorption capacity (mg.L-1), and KL is the Langmuir isotherm constant. The value of KL is related to free energy of sorption process, which corresponds to the affinity of between the surface of sorbate molecules and sorbent. It also defines the mode of adsorption [26, 27].The experimental values of constants in equation are given in Table1. After plotting Ce/Xe versus Ce, linear plot was obtained as shown in Figure 6. It indicates that the experimental data fitted well to the Langmuir isotherm model, verifying monolayer M.B adsorption on a homogenous surface of BTW [26]. The value of Xm was calculated from the slope of plot while KL (affinity constant or energy constant) was calculated from the intercept and its values are given in Table 1.Table 1
Adsorption isotherm parameters of M.B on BTW.
Isotherm model
Parameter
Values
R
2
Langmuir
K
L
0.0740
0.997
X
M
3.367
Freundlich
K
f
2.32
0.927
1
n
0.367Figure 6
Langmuir isotherm plot for adsorption of M.B on BTW.


## 3.5. Freundlich Isotherm Model

The Freundlich isotherm model [28] is given.
(4)logqe=logKf+1nlogCe,where 1/n and Kf are Freundlich isotherm constants and can be obtained by plotting log Xe versus Q as shown in Figure 7. The constant Kf indicates the quantity of the M.B adsorbed on the surface of the BTW, while the constant 1/n shows the heterogeneity of the system or adsorption intensity. The value of 1/n<1 shows normal adsorption [29]. The R2 value (0.927) was far away from unity as compared to same parameter value of the Langmuir model. This reveals that the Freundlich model could not be obeyed by the experimental data.Figure 7
Freundlich isotherm plot for adsorption of M.B on BTW.


## 3.6. Adsorption Kinetics

Adsorption kinetics is one of the most features of such processes that dictate the solute uptake level/the adsorption ability of biosorbent that help in deciding the possible applicability of the adsorbent to be used in industry [30]. The effect of contact time on adsorption is shown in Figure 8. It is clear from the figure that the rate of adsorption is higher in the first three minutes which then slows down. The faster rate is due to availability of active sites on the surface of BTW at start of experiments. As the sites are saturated with M.B molecules, the rate of adsorption slows down. It is due to the increasing competition between the residual dye molecules for the remaining active sites on the surface of biosorbent. Similar results have been reported earlier by Tang et al. [31], Kumar and Jena [32], Pang et al. [33], and Heidarinejad et al. [34].Figure 8
Adsorption kinetic plot of M.B on the surface of BTW at different temperatures.


## 3.7. Pseudo-First-Order Model

To calculate the kinetic constants of M.B adsorption on BTW, the Lagergren pseudo-first-order rate (equation (5)) was applied to the adsorption kinetic data.
(5)LogXe−Xt=logXe−K12.303t,where Xe is the amount of M.B adsorbed at equilibrium time and Xt is the amount of M.B adsorbed at time t. K1 is the pseudo-first-order rate constant. The constants in the equation were obtained by plotting time (t) vs. lnXe−Xt (Figure 9). The values of constants are given in Table 2. The results obtained for this model were not in accordance with the experimental data. The lower value of R2 is less than unity at temperature 303 and 313 K showing that this model does not fit well the kinetic data.Figure 9
Pseudo-first-order kinetic plot of M.B onto BTW.Table 2
Constant of pseudo-first-order kinetics.
Temperature
293 K
303 K
313 K
q
max
0.175
0.167
0.235
K
1
3.49
3.705
3.565
R
2
0.994
0.960
0.9278


## 3.8. Pseudo-Second-Order Model

This model can be given as follows [30, 35]:
(6)tXt=1K2Xe+tXtThis is the linear form of the pseudo-second-order kinetic model whereXe is the amount of M.B adsorbed at equilibrium, Xt is the amount of M.B adsorbed at time t, and K2 is the pseudo-second-order rate constant. The values of constants (Table 3) were obtained by plotting t/Xt vs. time (Figure 10). The value of R2 is almost equal unity at all temperatures showing that the kinetic data could be bitterly accommodated by this model and adsorption process is chemical in nature where the sharing or exchange of electrons between sorbate and biosorbent has been taken place. Similar results have been reported by Li et al. [36], X. Li and Y. Li [37], Weng et al. [38], Ravi and Pandey [39], Pathania et al. [40], and Qian et al. [41].Table 3
Pseudo-second-order kinetic constants.
Temperature
q
t
×
10
−
4
mg
.
g
−
1
K
2
R
2
293 K
33.33
0.0762
0.9985
303 K
……
0.0714
0.9980
313 K
……
……
0.9902Figure 10
Pseudo-second-order kinetic plot of M.B onto BTW.


## 3.9. Intraparticle Diffusion Model

Intraparticle diffusion mechanism was used to investigate the diffusion mechanism of adsorption process. For intraparticle diffusion, equation (7) [42] is used:
(7)qt=Kdifft1/2+C.In equation (7), Kdiff (mgg-1 min-1) is intraparticle diffusion rate constant, qt is the amount of M.B adsorbed on BTW at time (t), t1/2 is the square root of time, and C is the thickness of boundary layer. A linear plot (Figure 11) was obtained by plotting qt vs. t1/2. The thickness of boundary layer is obtained from the intercept of the linear plot, while Kdiff (mgg-1 min-1) was obtained from the slope of linear plot. The values of constants obtained from linear plot are tabulated in Table 4. It was observed from the linear plot of intraparticle diffusion plot that the process of M.B adsorption on BTW is a diffusion-controlled process [43].Figure 11
Intraparticle diffusion plot.Table 4
Constant of intraparticle diffusion.
Temperature
293 K
303 K
313 K
K
diff
12.191
14.802
13.601
C
19.004
19.467
25.434
R
2
0.9237
0.9902
0.9794


## 4. Conclusions

In this study, black tea wastes were used as biosorbent for the removal of M.B from aqueous media. The methylene blue was efficiently recovered from solution through the biosorbent. The sorption of methylene blue was studied as a function of time, initial concentration of methylene blue, temperature, pH of methylene blue, and sorbent dosage. The kinetic studies were recorded for three different temperatures, i.e., 293 K, 303 K, and 313 K. The equilibrium time for the removal of methylene blue on black tea wastes was achieved within 3 min. The fast rate of kinetics shows that methylene blue dye adhered to the surface of adsorbent very quickly. The adsorption isotherm data was well explained by the Langmuir adsorption isotherm while the adsorption kinetic data by pseudo-second-order kinetic model rather than pseudo-first-order kinetic model. Maximum removal of methylene blue occurs at pH 8. The optimum dosage of black tea wastes was selected 0.4 g in all experiments. The adsorbent was effective in the removal of selected dye, and its use could be extended to other dyes as well.



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