# Application of Wasted Oolong Tea as a Biosorbent for the Adsorption of Methylene Blue

Authors: Yunfei Hu; Yue Zhang; Yuqun Hu; Chen-Yao Chu; Jinke Lin; Shuilian Gao; Dongyi Lin; Jing Lu; Ping Xiang; Tzu-Hsing Ko
Journal: Journal of Chemistry
 (2019)

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## Abstract

Tea powder, a biosorbent prepared from wasted oolong tea, was collected as a prospective adsorbent for the adsorption of methylene blue (MB) from aqueous solution. The effect of factors on adsorption efficiency, isotherms, kinetics, and potential mechanism was carried out. Adsorption capacity of MB onto wasted tea powder increased with the MB concentration and contact time, whereas the increase in pH value and ion strength appeared to have a negative effect for the adsorption process. The adsorption efficiency increased rapidly and reached a stable state within 120 min. The optimal tea powder loading weight is suggested to be at 0.1 to 0.2 g, and the highest efficiency of 94.8% is achieved at 333 K. There were no significant changes in adsorption efficiency when the effect of temperature is considered. The Langmuir isotherm model was found to be the best isotherm models to elucidate the adsorption mechanism in this study. The maximum adsorption capacities calculated at different temperatures by the Langmuir model ranging from 312.5 to 333.3 mg·g−1 were much close to the experimental results. From the kinetic analysis, the pseudo--second-order model was found to be the best model to describe the adsorption behavior. The calculated adsorption capacities at different initial MB concentrations by the pseudo-second-order model ranging from 92.34 to 400 mg·g−1 were well close to the experimental data. The fitting results obtained from the intraparticle diffusion model suggested that the intraparticle diffusion was not the only rate-controlling step and some other mechanisms along with the intraparticle diffusion were probably involved. The intraparticle diffusion of MB molecules into pore structures of wasted tea powder is the rate-limiting step for the adsorption process in this study. The repetitive cycle experiments indicated that the wasted oolong tea powder was efficiently regenerated using NaOH and thus be used for many times.


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## Body

## 1. Introduction

Dyeing is one of the important raw materials for many industries such as dyeing, textile, printing, cosmetic, and papermaking [1, 2]. In addition to obvious color in appearance, dyeing wastewater poses a serious toxicity to the ecosystem because of its high concentration, complex organic components, and lower biodegradability. It has been estimated that about 2% of the produced dyes are directly emitted into water resources, which causes a severe environmental and health problems [3, 4]. Therefore, the removal of dyeing wastewater from aqueous solution is of great importance and crucial. Many techniques have been developed to treat dyeing wastewater including biological treatment, chemical degradation, membrane separation, and catalytic oxidation [5–8]. Among these treatments, adsorption has been considered as one of the effective and low-cost processes for dyeing removing from wastewater.Biosorbent is the material which is a byproduct of wastes from the agricultural waste material. The major advantages of biosorbent are the relative low cost, high efficiency, and no additional nutrient requirement [9]. Therefore, it has been become a popular material for environmental pollutant remediation. In the past, many agricultural waste products including grass waste, rice husk, and peel have been used for the removal of pollutants [10–13]. Thousand tones of wasted tea are produced and disposed unutilized every day in China. The main constituents of tea leaves are cellulose, hemicelluloses, lignin, tannins, and proteins. The functional groups in these compounds are mainly hydroxyl, aromatic carboxylate, amino, sulfonic, and phenolic groups, which promote the physicochemical interactions for adsorption of heavy metals and other pollutants [14]. China is the largest tea-producing country, and the production in 2010 amounted to 1475 kilotons and 35.4% of the total world production [15]. It is undoubtedly that the huge amount of wasted tea was produced in China, leading to the severe environmental problem.The main objective of this study was to evaluate the potential of wasted oolong tea powder as an alternative biosorbent for the removal of MB from aqueous. A series of operative factors, including contact time, initial MB concentration, wasted tea powder loading weight, temperature, pH value, and ionic strength on the adsorption efficiency and capacity, were experimentally investigated. The equilibrium isotherms were determined by several models to understand the mechanism of MB. Furthermore, the kinetics involved in the adsorption process was evaluated at different initial MB concentrations.


## 2. Materials and Experimental Procedure

### 2.1. Tea Sample Preparation

Tea samples used in this study were collected from a tea factory located at Anxi County, Fujian Province, and was classified to the oolong tea. The collected tea samples were washed to remove pigment in boiled water for many times and finally were flushed with deionized water. The washed tea samples were dried at 343 K for 24 h in an oven and were ground to pass through an 80-mesh sieve. The tea samples were stored in an air-tight container for further experiments. No further treatments were performed prior to the adsorption experiments.


### 2.2. Analytical Instrument

The MB concentration was analyzed using a UV-visible spectrophotometer (Shimadzu Model UV 1750) at a wavelength of 664 nm. The calibration curve was obtained by using the standard of the MB reagent, and the correlation coefficientR2 was determined more than 0.995 to ensure the accuracy.The surface area was measured with a Micromeritics ASAP 2010 instrument using adsorption of nitrogen at 77 K. Prior to adsorption measurements, the samples were degassed under a vacuum of 5μm Hg at 373 K for 2 hrs.Fourier-transform infrared spectroscopy was used to identify the surface functional groups of wasted tea powder. The infrared spectra were recorded on a Perkin-Elmer One B model FTIR spectrometer with fully computerized data storage and data-handling capability. To provide adequate characterization of the wasted tea powder, the spectrum was set from a range 400 to 4,000 cm−1. A 100-scan data accumulation was carried out at 4 cm−1 resolution.


### 2.3. Batch Adsorption Experiments

The concentration of MB and amount of wasted tea powder used for the experiments of contact time, temperature, pH values, and ion strength were set at 400 mg·L−1 and 0.1 g, respectively. The pH value was adjusted to a range of 3–11 with 0.1 N NaOH and 0.1 N HCl to investigate the effect of pH on adsorption. The concentrations of MB with different pH values were measured at a wavelength of 664 nm. Results indicated that there are no appreciable changes in spectra of MB in the used pH range. The pH value at different concentrations of MB was measured between 4.7 and 4.8, showing a stable pH range for adsorption experiment. All adsorption experiments were carried out in a conical flask with a 50 mL of MB solution and were placed on a thermocontrolled shaker with a shaking rate of 200 rpm. After the adsorption experiment, the solution was centrifuged at 5000 rpm for 10 minutes and then the supernatant solution was analyzed to determine the concentration of MB by the UV-visible spectrophotometer. The adsorption efficiency and adsorption capacity were calculated using the following equation:(1)adsorptionefficiency%=C0−CtC0×100,adsorptioncapacity=C0−CtC0×V,where C0 (mg·L−1) is the initial concentration of MB and Ct (mg·L−1) is the concentration of MB at any time t. M (g) is the wasted tea powder loading weight. Duplicate measurements were conducted for each sample, and mean values were used for the adsorption calculation.


### 2.4. Isotherm Adsorption Experiments

In the isotherm adsorption experiment, a certain 0.1 g of wasted tea powder and 50 mL of different concentrations of MB solution (100, 400, 1000, 1200, and 1500 mg·L−1) were studied at different temperatures (288, 298, 308, 318, and 333 K). After adsorption experiments, the collected samples were centrifuged and the concentration of MB in the supernatant solution was analyzed as before. Four types of isotherm models were applied to find the possible adsorption behavior between wasted tea powder and MB.


### 2.5. Kinetic Studies

Adsorption kinetic experiments were operated at 298 K with a range of MB concentration from 100 to 1500 mg·L−1, and a certain mass of 0.1 g wasted tea powder was used. Different kinetic models were applied to fit the adsorption process in this study.


## 2.1. Tea Sample Preparation

Tea samples used in this study were collected from a tea factory located at Anxi County, Fujian Province, and was classified to the oolong tea. The collected tea samples were washed to remove pigment in boiled water for many times and finally were flushed with deionized water. The washed tea samples were dried at 343 K for 24 h in an oven and were ground to pass through an 80-mesh sieve. The tea samples were stored in an air-tight container for further experiments. No further treatments were performed prior to the adsorption experiments.


## 2.2. Analytical Instrument

The MB concentration was analyzed using a UV-visible spectrophotometer (Shimadzu Model UV 1750) at a wavelength of 664 nm. The calibration curve was obtained by using the standard of the MB reagent, and the correlation coefficientR2 was determined more than 0.995 to ensure the accuracy.The surface area was measured with a Micromeritics ASAP 2010 instrument using adsorption of nitrogen at 77 K. Prior to adsorption measurements, the samples were degassed under a vacuum of 5μm Hg at 373 K for 2 hrs.Fourier-transform infrared spectroscopy was used to identify the surface functional groups of wasted tea powder. The infrared spectra were recorded on a Perkin-Elmer One B model FTIR spectrometer with fully computerized data storage and data-handling capability. To provide adequate characterization of the wasted tea powder, the spectrum was set from a range 400 to 4,000 cm−1. A 100-scan data accumulation was carried out at 4 cm−1 resolution.


## 2.3. Batch Adsorption Experiments

The concentration of MB and amount of wasted tea powder used for the experiments of contact time, temperature, pH values, and ion strength were set at 400 mg·L−1 and 0.1 g, respectively. The pH value was adjusted to a range of 3–11 with 0.1 N NaOH and 0.1 N HCl to investigate the effect of pH on adsorption. The concentrations of MB with different pH values were measured at a wavelength of 664 nm. Results indicated that there are no appreciable changes in spectra of MB in the used pH range. The pH value at different concentrations of MB was measured between 4.7 and 4.8, showing a stable pH range for adsorption experiment. All adsorption experiments were carried out in a conical flask with a 50 mL of MB solution and were placed on a thermocontrolled shaker with a shaking rate of 200 rpm. After the adsorption experiment, the solution was centrifuged at 5000 rpm for 10 minutes and then the supernatant solution was analyzed to determine the concentration of MB by the UV-visible spectrophotometer. The adsorption efficiency and adsorption capacity were calculated using the following equation:(1)adsorptionefficiency%=C0−CtC0×100,adsorptioncapacity=C0−CtC0×V,where C0 (mg·L−1) is the initial concentration of MB and Ct (mg·L−1) is the concentration of MB at any time t. M (g) is the wasted tea powder loading weight. Duplicate measurements were conducted for each sample, and mean values were used for the adsorption calculation.


## 2.4. Isotherm Adsorption Experiments

In the isotherm adsorption experiment, a certain 0.1 g of wasted tea powder and 50 mL of different concentrations of MB solution (100, 400, 1000, 1200, and 1500 mg·L−1) were studied at different temperatures (288, 298, 308, 318, and 333 K). After adsorption experiments, the collected samples were centrifuged and the concentration of MB in the supernatant solution was analyzed as before. Four types of isotherm models were applied to find the possible adsorption behavior between wasted tea powder and MB.


## 2.5. Kinetic Studies

Adsorption kinetic experiments were operated at 298 K with a range of MB concentration from 100 to 1500 mg·L−1, and a certain mass of 0.1 g wasted tea powder was used. Different kinetic models were applied to fit the adsorption process in this study.


## 3. Results and Discussions

### 3.1. Pore Structure of Wasted Tea Powder

The N2 adsorption and desorption isotherm of the wasted tea powder is shown in Figure 1. A distinct hysteresis loop can be observed for P/P0 = 0.9–10. According to the classification standard of International Union of Pure and Applied Chemistry, the isotherm curve corresponds to the characteristic of type IV, which suggests the presence of mesoporous structure with a cylindrical or silt shape [16]. The pore size calculated using the BJH method was 21.7 nm. The feature of the hysteresis loop is suggested to be related to the capillary condensation associated with large pore channel.Figure 1
N2 adsorption isotherm of wasted tea powder used in this study.


### 3.2. Analysis of Surface Structure of Wasted Tea Powder

Figure2 shows the SEM photograph of the wasted tea powder at 15000x magnification. The photograph of SEM shows that the flaky and irregular structure with a low degree of porosity is found in oolong tea. The main functional group of wasted tea powder is summarized in Table 1. An intense and broad band at 3416 cm−1 was obviously referred to the –OH group stretching vibration, which is in the form of lignin, cellulose, and hemicellulose [17]. The signals at 2925 cm−1 and 2856 cm−1 were ascribed to the aliphatic C-H group and asymmetric –CH2 vibration, respectively. The C-H alkane in the aromatic ring was detected at 1455 cm−1, which is one of the major fragrances in tea. The peaks appeared at 1238 cm−1, 1150 cm−1, and 1062 cm−1 inferred the presence of –SO3 stretching, polysaccharides, and C-O-H stretching.Figure 2
SEM photograph of wasted tea powder.Table 1
The main functional groups of wasted oolong tea powder.
Fresh tea powder
Assignment
3416
Bonded –OH groups
2925
Aliphatic C-H group
2856
Asymmetric –CH2 vibration
1651
C=O stretching, aromatic C=C, C=O
1527
Secondary amine II group
1455
C-H alkanes in aromatic rings
1384
C-H bending, -CH3 symmetric bending of CH3
1238
-SO3 stretching or bonded C-N
1150
C-O-C of polysaccharides
1062
C-O-H stretching act


### 3.3. Effect of Adsorption Time

The effect of contact time for the adsorption of MB from aqueous solution was evaluated. Figure3 shows adsorption efficiency and capacity of MB as a function of contact time at a MB concentration of 400 mg·L−1. The adsorption efficiency of MB increases with contact time and reaches a stable value at 120 min. The adsorption capacity of MB was achieved at 288.4 mg·g−1 under the experimental condition in this study, indicating that the wasted tea powder is a good candidate biosorbent for the adsorption of MB. Note that the adsorption efficiency rapidly increases when the contact reaction occurs between wasted tea powder and MB. The reasonable explanation could be attributed to the strong chemical affinity between active sites of wasted tea powder and MB.Figure 3
Effect of adsorption time on the adsorption efficiency and capacity of MB.


### 3.4. Effect of Initial Concentration of MB

To understand the effect of MB concentration on the adsorption efficiency, a series of various concentrations for MB adsorption were carried out and experimental result is shown in Figure4. As expected, the adsorption efficiency decreases with the initial concentration of MB whereas the adsorption capacity of MB increases with the initial concentration. The equilibrium adsorption capacity of MB increases from 92.9 to 400 mg·g−1 with increasing the initial concentration of MB from 100 to 1500 mg·L−1. The observation is consistent with the results obtained by Nasuha et al. and Uddin et al., in which their research showed that the initial concentration of MB is a positive influence on the adsorption capacity [18, 19]. Note that more than 90% of the adsorption capacity for MB was observed within 30 min, and thereafter, the adsorption capacity of MB remains a stable phase. This is because that the mass transfer driving force is stronger when the initial concentration of MB increases, therefore, resulting in more collisions between active sites of wasted tea powder and MB, as well as enhances the larger adsorption capacity.Figure 4
Effect of initial concentration on the adsorption efficiency and capacity of MB.
(a)
(b)


### 3.5. Effect of Wasted Tea Powder Loading Weight

Loading dose is an important factor to affect the adsorption efficiency. The effect of wasted tea powder loading weight is illustrated in Figure5. As can be seen, the adsorption efficiency rapidly increases from 70.8 to 98.7% when the loading weight was carried out from 0.05 to 0.2 g. Unlike adsorption efficiency, the adsorption capacity gradually declines with wasted tea powder loading weight. This is because of the split in the flux or the concentration gradient between solute concentration in the solution and the solute concentration in the surface of the adsorbent [20]. Based on the experimental finding in this study, the optimal wasted tea powder loading weight may be ranged from 0.1 to 0.2 g because over 90% of adsorption efficiency and an acceptable adsorption capacity (more than 100 mg MB/g tea) are achieved.Figure 5
Effect of wasted tea loading weight on the adsorption efficiency and capacity of MB.


### 3.6. Effect of Temperature

The effect of temperature on the adsorption of MB was evaluated at a range of 283–333 K using the initial concentration of 400 mg·L−1. As shown in Figure 6, it is observed that the adsorption efficiency increases with temperature. The highest efficiency of 94.8% is achieved at 333 K after 150 min. The higher temperature promotes the kinetic energy of MB and thus enhances the collision frequency between active sites of wasted tea powder and MB. On the other hand, with increasing in temperature, the diffusion rate of the adsorbate molecules across the external boundary layer and within the internal pores of the adsorbent increases because of decreasing in viscosity of the solution [21]. Some previous researchers reported that the higher temperature is unfavorable for MB adsorption. The higher temperature causes the desorption rate is more favorable than that of adsorption, which leads to a part of dye be escaped from the solid phase into the liquid phase [22, 23]. It is believed that the wasted tea powder can be used at the temperature range of 283–333 K under the experimental conditions in this study.Figure 6
Effect of temperature on the adsorption efficiency of MB.


### 3.7. Effect of pH

The pH value is one of the most important factors to govern the overall adsorption efficiency. In pilot scale dye plants, the pH of wastewater is basically depending on the types of dye. The effect of pH on the adsorption efficiency and capacity for MB in the range of 3 to 11 were investigated. As can be seen in Figure7, it is evident that the adsorption efficiency increases rapidly with increasing pH from 3 to 4.7. The adsorption efficiency increases slightly when pH arises from 4.7 to 11.1. This finding is consistent with other reports, in which their studies showed that the acidic condition is unfavorable of the adsorption reaction [24, 25]. MB belongs to cationic dye and has positive charged ions in aqueous solution. The degree of its adsorption onto the adsorbent surface is primarily affected by the surface charge on the adsorbent, which in turn is influenced by the pH of solution [26]. Under lower pH condition, the concentration of H+ ion is high, causing a competition for active sites of MB between H+ ion and cationic groups on the MB. Therefore, the adsorption efficiency of MB declines.Figure 7
Effect of pH of MB solution on the adsorption efficiency and capacity of MB.


### 3.8. Effect of Ionic Strength

In addition to dye, high concentration of inorganic salt, especially NaCl, is another compound that can be found in actual wastewater. To better understand its effect on adsorption efficiency, a series of different NaCl concentrations were studied. Figure8 shows the effect of different concentrations of NaCl on the adsorption efficiency and capacity of MB. The adsorption efficiency and capacity slightly decrease with increasing in NaCl concentration from 0 to 0.25 mg·L−1. In addition to cationic characteristic, MB is also found to exist as a dimer or as aggregates at the surface, as well as a protonated form depending on the concentration and the surface properties [27]. The effect of ionic strength on the dimerization equilibrium of MB is likely to be a function of the reduced electrostatic interaction in the presence of a high concentration of Na+ and Cl− ions. The salt screens the electrostatic interaction of opposite charges of the surface and the dye molecule; therefore, the adsorption efficiency of MB decreased with increasing the NaCl concentration.Figure 8
Effect of ionic strength on the adsorption efficiency and capacity of MB.


### 3.9. Adsorption Isotherms

Adsorption isotherm is a significant role to explore the adsorption mechanism, predict the maximum adsorption capacity of adsorbent, assess the affinity between adsorbent and adsorbate, and optimize the adsorption system design. Four adsorption isotherm models were used to fit the adsorption results, including Langmuir, Freundlich, Dubinin–Radushkevich, and Temkin models.The Langmuir model is used to describe the adsorption process that occurs on a uniform surface by monolayer adsorption, and the linear Langmuir equation can be expressed as follows:(2)CeQe=CeQmax+1QmaxKL,where Ce is the equilibrium concentration, Qe is the amount of dye adsorbed at equilibrium, Qmax is the maximum adsorption capacity of dye, and KL is the Langmuir constant. The Langmuir isotherm can be evidenced in terms of a dimensionless equilibrium parameter RL:(3)RL=11+C0KL,where C0 is the initial concentration of MB and RL indicates that the Langmuir isotherm is favorable (0<RL<1), unfavorable (RL>1), linear (RL=1), and irreversible (RL=0).The Freundlich model is applicable to a multilayer adsorption on a heterogeneous surface by adsorption sites. The linear form of the Freundlich equation is given as follows:(4)lnQe=lnKF+1nlnCe,where KF is the Freundlich constant and n is the heterogeneity factor.The Dubinin–Radushkevich model is used to describe the adsorption on both homogeneous and heterogeneous surfaces. The linear equation can be represented as follows:(5)lnQe=lnQm−Kε2,where K denotes the coefficient related to the adsorption free energy (mol2·kJ−2), Qm is the maximum adsorption capacity (mg·g−1), and ε is the Polanyi potential that can be calculated by the following equation:(6)ε=RTln1+1Ce,where R is the gas constant (8.314 J·mol−1·K−1), T is the absolute temperature (K), and Ce is the equilibrium concentration of MB.The Temkin model assumes that the heat of adsorption of the molecules in the layer decreases linearly with coverage due to adsorbent-adsorbate interactions and mainly describes the chemisorption process which dominated through electrostatic adsorption:(7)Qe=RTBTlnATCe,where BT is the Temkin constant (J·mol−1) and AT denotes the Temkin isotherm equilibrium binding constant (L·mg−1).The experimental data were fitted with four isotherm models, and the results are summarized in Table2. Aside from the Dubinin–Radushkevich model, the R2 values for other models are higher than 0.9, which implies that those models can be used to describe the adsorption behavior and the adsorption mechanism between wasted tea powder and MB is a complex reaction. The Langmuir model has the best fitting (R2 > 0.999), and the maximum capacities of MB (Qmax) for different temperatures calculated from the Langmuir model are very close to the experimental results. The monolayer coverage of MB onto wasted tea powder is the main adsorption mechanism. On the other hand, a high R2 is obtained from the Freundlich model and the heterogeneity factors are also calculated. The heterogeneity factor (n) can be used to determine the difficulty of adsorption behavior. In general, the adsorption process is easy to occur when n is larger than 2 [25]. As shown in Table 2, the heterogeneity factor (n) ranges from 4.11 to 4.90, which suggests that the adsorption process is easy to occur between wasted tea powder and MB as well as reveals the multiple coverage of MB is favorable. The Temkin model also appears to have the better fitting results in this study. This finding suggests that the electrostatic interaction is one of the important mechanisms between wasted tea powder and MB.Table 2
Adsorption isotherm fittings for the MB adsorption onto wasted tea powder.
Temperature (K)
Langmuir
Freundlich
Dubinin–Radushkevich
Temkin
Q
exp (mg·g−1)
Q
max (mg·g−1)
K
L (L·mg−1)
R
2
n
K
F (L·mg−1)
R
2
B (mol2·kJ−1)
Q
max (mg·g−1)
R
2
B
T (J·mol−1)
A
T
R
2
288
311.6
312.5
0.061
0.999
4.63
78.67
0.967
3.811
261.5
0.815
57.81
2.74
0.991
298
322.4
333.3
0.042
0.999
4.11
68.41
0.976
6.633
265.4
0.791
52.35
1.33
0.988
308
325.2
333.3
0.055
0.999
4.37
75.91
0.974
3.496
268.7
0.795
56.81
2.06
0.988
318
319.6
322.6
0.060
0.999
4.63
80.54
0.963
2.881
266.2
0.819
62.81
2.88
0.994
333
309.7
312.5
0.075
0.999
4.90
84.89
0.945
2.349
263.1
0.840
70.78
4.12
0.988


### 3.10. Adsorption Kinetics

Adsorption kinetics study provides useful information regarding the adsorption efficiency and engineering design for scale-up operations. The adsorption kinetic can be elucidated using various types of mathematic models, of which one most widely used is the Lagergren rate equation [28, 29]. In this study, the adsorption kinetic was analyzed using the pseudo-first-order model, the pseudo-second-order model, and intraparticle diffusion model. The description of these models can be represented by the following equation.Pseudo-first-order model:(8)lnQe−Qt=lnQe−K1t.Pseudo-second-order model:(9)tQt=1K2Qe2+1Qe,where Qe (mg·g−1) and Qt (mg·g−1) are the amounts of MB adsorbed at equilibrium at any time t (min) and K1 and K2 are the pseudo-first-order rate and pseudo-second-order rate constants.The intraparticle diffusion model is applied to assess the rate-controlling step in the porous structure [30]. It can be expressed as follows:(10)Qt=Kidt+C,where Qt (mg g−1) is the amounts of MB adsorbed at equilibrium at any time t (min), Kid is the intraparticle diffusion rate constant, and C denotes the greater effect of the boundary layer on molecule diffusion.The fitting results from pseudo-first-order and second-order models are listed in Table3. The R2 values for pseudo-second-order model were larger than 0.998, which are better than those obtained for the pseudo-first-order model. The Qmax values derived from the pseudo-second-order model are much close to the experimental value, Qexp, indicating that the pseudo-second-order model is more appropriate for the prediction of the kinetic process for MB adsorption onto wasted tea powder. The perfect fitting of the pseudo-second-order model means that the rate-limiting step is not the resistance of the boundary layer [31]. The adsorption process is controlled by the chemisorption involving valence forces through the sharing or exchange of electrons between the adsorbent and the adsorbate [32, 33]. However, the pseudo-second-order model contains the external liquid film diffusion, intraparticle diffusion, and adsorption on the surface of the adsorbent; this model provides a more comprehensive and accurate description of the adsorption mechanism between wasted tea powder and MB [34–36].Table 3
Adsorption kinetic fitting results for the MB adsorption onto wasted tea powder.
Initial MB concentration
Q
exp (mg·g−1)
Pseudo-first-order
Pseudo-second-order
Q
cal (mg·g−1)
K1 (min−1)
R
2
Q
cal (mg·g−1)
K2 (min−1)
R
2
100
92.94
29.78
0.039
0.974
92.34
0.0044
0.999
400
288.36
92.55
0.032
0.981
294.10
0.0012
0.999
1000
367.64
144.82
0.033
0.813
370.37
0.0006
0.999
1200
374. 11
76.63
0.024
0.868
370.37
0.0014
0.998
1500
404.76
72.67
0.027
0.703
400.00
0.0018
0.999Figure9 shows the plot of intraparticle diffusion model for the adsorption of MB onto wasted tea powder. Note that all the plots are nonlinear over whole time ranges and do not pass through the origin. This illustrates that the intraparticle diffusion is not the only rate-controlling step and some other mechanisms along with the intraparticle diffusion are probably involved [37, 38]. After further analysis, it is observed that the adsorption process tends to be followed by the two linear stages. The first linear stage is due to the transportation of MB molecules from the bulk solution to the external surface of the wasted tea powder by diffusion through the boundary layer, and the second one seems to refer to the diffusion into meso/micropores of the wasted tea powder structure. As can be seen from Figure 9(b), the first linear stage has higher slope than the second linear stage. This implies that the resistance of mass transfer between the external boundary layer film of liquid and adsorbent is relatively smaller, resulting in the rapid adsorption rate for the first linear stage. The intraparticle diffusion of MB molecules into pore structures is the rate-limiting step for the adsorption process onto wasted tea powder.Figure 9
The plot of intraparticle diffusion model for the adsorption of MB at various initial concentrations. (a) The original plot and (b) the fitting results.
(a)
(b)


### 3.11. Regeneration and Recyclability Investigation

For practical applications, regeneration and recyclability of the adsorbent is important to evaluate the overall performance of the adsorbent. For the regeneration process, the used tea powder was regenerated by mixing of 0.5 N NaOH solutions and deionized H2O at the shaker for 1 hr at 298 K, respectively. The used tea powder was separated from the suspension and washed by deionized water in the ultrasonic shaker for three times and dried in an oven at 333 K. The adsorption efficiency of used tea powder is shown in Figure 10. The adsorption efficiency decreased with regeneration cycles for both the cases. It is evident that the wasted tea powder regenerated by NaOH possesses 61% adsorption efficiency after third cycles. The adsorption efficiency dropped rapidly with regeneration cycles for the case of H2O, suggesting that the H2O is not a suitable reagent for the regeneration process. The decrease in adsorption efficiency is attributed to the some solubilized parts of the wasted tea powder, changed superficial structures of wasted tea powder and subsequently leads to loss and blockage of adsorption sites [25, 39]. Based on the regeneration evaluation, it is deduced that the NaOH proves to be a desorbing agent and wasted oolong powder is an inexpensive, ecofriendly, and competent adsorbent for the remediation of dyes from the aqueous system.Figure 10
Regeneration and recyclability tests for the adsorption of MB.


## 3.1. Pore Structure of Wasted Tea Powder

The N2 adsorption and desorption isotherm of the wasted tea powder is shown in Figure 1. A distinct hysteresis loop can be observed for P/P0 = 0.9–10. According to the classification standard of International Union of Pure and Applied Chemistry, the isotherm curve corresponds to the characteristic of type IV, which suggests the presence of mesoporous structure with a cylindrical or silt shape [16]. The pore size calculated using the BJH method was 21.7 nm. The feature of the hysteresis loop is suggested to be related to the capillary condensation associated with large pore channel.Figure 1
N2 adsorption isotherm of wasted tea powder used in this study.


## 3.2. Analysis of Surface Structure of Wasted Tea Powder

Figure2 shows the SEM photograph of the wasted tea powder at 15000x magnification. The photograph of SEM shows that the flaky and irregular structure with a low degree of porosity is found in oolong tea. The main functional group of wasted tea powder is summarized in Table 1. An intense and broad band at 3416 cm−1 was obviously referred to the –OH group stretching vibration, which is in the form of lignin, cellulose, and hemicellulose [17]. The signals at 2925 cm−1 and 2856 cm−1 were ascribed to the aliphatic C-H group and asymmetric –CH2 vibration, respectively. The C-H alkane in the aromatic ring was detected at 1455 cm−1, which is one of the major fragrances in tea. The peaks appeared at 1238 cm−1, 1150 cm−1, and 1062 cm−1 inferred the presence of –SO3 stretching, polysaccharides, and C-O-H stretching.Figure 2
SEM photograph of wasted tea powder.Table 1
The main functional groups of wasted oolong tea powder.
Fresh tea powder
Assignment
3416
Bonded –OH groups
2925
Aliphatic C-H group
2856
Asymmetric –CH2 vibration
1651
C=O stretching, aromatic C=C, C=O
1527
Secondary amine II group
1455
C-H alkanes in aromatic rings
1384
C-H bending, -CH3 symmetric bending of CH3
1238
-SO3 stretching or bonded C-N
1150
C-O-C of polysaccharides
1062
C-O-H stretching act


## 3.3. Effect of Adsorption Time

The effect of contact time for the adsorption of MB from aqueous solution was evaluated. Figure3 shows adsorption efficiency and capacity of MB as a function of contact time at a MB concentration of 400 mg·L−1. The adsorption efficiency of MB increases with contact time and reaches a stable value at 120 min. The adsorption capacity of MB was achieved at 288.4 mg·g−1 under the experimental condition in this study, indicating that the wasted tea powder is a good candidate biosorbent for the adsorption of MB. Note that the adsorption efficiency rapidly increases when the contact reaction occurs between wasted tea powder and MB. The reasonable explanation could be attributed to the strong chemical affinity between active sites of wasted tea powder and MB.Figure 3
Effect of adsorption time on the adsorption efficiency and capacity of MB.


## 3.4. Effect of Initial Concentration of MB

To understand the effect of MB concentration on the adsorption efficiency, a series of various concentrations for MB adsorption were carried out and experimental result is shown in Figure4. As expected, the adsorption efficiency decreases with the initial concentration of MB whereas the adsorption capacity of MB increases with the initial concentration. The equilibrium adsorption capacity of MB increases from 92.9 to 400 mg·g−1 with increasing the initial concentration of MB from 100 to 1500 mg·L−1. The observation is consistent with the results obtained by Nasuha et al. and Uddin et al., in which their research showed that the initial concentration of MB is a positive influence on the adsorption capacity [18, 19]. Note that more than 90% of the adsorption capacity for MB was observed within 30 min, and thereafter, the adsorption capacity of MB remains a stable phase. This is because that the mass transfer driving force is stronger when the initial concentration of MB increases, therefore, resulting in more collisions between active sites of wasted tea powder and MB, as well as enhances the larger adsorption capacity.Figure 4
Effect of initial concentration on the adsorption efficiency and capacity of MB.
(a)
(b)


## 3.5. Effect of Wasted Tea Powder Loading Weight

Loading dose is an important factor to affect the adsorption efficiency. The effect of wasted tea powder loading weight is illustrated in Figure5. As can be seen, the adsorption efficiency rapidly increases from 70.8 to 98.7% when the loading weight was carried out from 0.05 to 0.2 g. Unlike adsorption efficiency, the adsorption capacity gradually declines with wasted tea powder loading weight. This is because of the split in the flux or the concentration gradient between solute concentration in the solution and the solute concentration in the surface of the adsorbent [20]. Based on the experimental finding in this study, the optimal wasted tea powder loading weight may be ranged from 0.1 to 0.2 g because over 90% of adsorption efficiency and an acceptable adsorption capacity (more than 100 mg MB/g tea) are achieved.Figure 5
Effect of wasted tea loading weight on the adsorption efficiency and capacity of MB.


## 3.6. Effect of Temperature

The effect of temperature on the adsorption of MB was evaluated at a range of 283–333 K using the initial concentration of 400 mg·L−1. As shown in Figure 6, it is observed that the adsorption efficiency increases with temperature. The highest efficiency of 94.8% is achieved at 333 K after 150 min. The higher temperature promotes the kinetic energy of MB and thus enhances the collision frequency between active sites of wasted tea powder and MB. On the other hand, with increasing in temperature, the diffusion rate of the adsorbate molecules across the external boundary layer and within the internal pores of the adsorbent increases because of decreasing in viscosity of the solution [21]. Some previous researchers reported that the higher temperature is unfavorable for MB adsorption. The higher temperature causes the desorption rate is more favorable than that of adsorption, which leads to a part of dye be escaped from the solid phase into the liquid phase [22, 23]. It is believed that the wasted tea powder can be used at the temperature range of 283–333 K under the experimental conditions in this study.Figure 6
Effect of temperature on the adsorption efficiency of MB.


## 3.7. Effect of pH

The pH value is one of the most important factors to govern the overall adsorption efficiency. In pilot scale dye plants, the pH of wastewater is basically depending on the types of dye. The effect of pH on the adsorption efficiency and capacity for MB in the range of 3 to 11 were investigated. As can be seen in Figure7, it is evident that the adsorption efficiency increases rapidly with increasing pH from 3 to 4.7. The adsorption efficiency increases slightly when pH arises from 4.7 to 11.1. This finding is consistent with other reports, in which their studies showed that the acidic condition is unfavorable of the adsorption reaction [24, 25]. MB belongs to cationic dye and has positive charged ions in aqueous solution. The degree of its adsorption onto the adsorbent surface is primarily affected by the surface charge on the adsorbent, which in turn is influenced by the pH of solution [26]. Under lower pH condition, the concentration of H+ ion is high, causing a competition for active sites of MB between H+ ion and cationic groups on the MB. Therefore, the adsorption efficiency of MB declines.Figure 7
Effect of pH of MB solution on the adsorption efficiency and capacity of MB.


## 3.8. Effect of Ionic Strength

In addition to dye, high concentration of inorganic salt, especially NaCl, is another compound that can be found in actual wastewater. To better understand its effect on adsorption efficiency, a series of different NaCl concentrations were studied. Figure8 shows the effect of different concentrations of NaCl on the adsorption efficiency and capacity of MB. The adsorption efficiency and capacity slightly decrease with increasing in NaCl concentration from 0 to 0.25 mg·L−1. In addition to cationic characteristic, MB is also found to exist as a dimer or as aggregates at the surface, as well as a protonated form depending on the concentration and the surface properties [27]. The effect of ionic strength on the dimerization equilibrium of MB is likely to be a function of the reduced electrostatic interaction in the presence of a high concentration of Na+ and Cl− ions. The salt screens the electrostatic interaction of opposite charges of the surface and the dye molecule; therefore, the adsorption efficiency of MB decreased with increasing the NaCl concentration.Figure 8
Effect of ionic strength on the adsorption efficiency and capacity of MB.


## 3.9. Adsorption Isotherms

Adsorption isotherm is a significant role to explore the adsorption mechanism, predict the maximum adsorption capacity of adsorbent, assess the affinity between adsorbent and adsorbate, and optimize the adsorption system design. Four adsorption isotherm models were used to fit the adsorption results, including Langmuir, Freundlich, Dubinin–Radushkevich, and Temkin models.The Langmuir model is used to describe the adsorption process that occurs on a uniform surface by monolayer adsorption, and the linear Langmuir equation can be expressed as follows:(2)CeQe=CeQmax+1QmaxKL,where Ce is the equilibrium concentration, Qe is the amount of dye adsorbed at equilibrium, Qmax is the maximum adsorption capacity of dye, and KL is the Langmuir constant. The Langmuir isotherm can be evidenced in terms of a dimensionless equilibrium parameter RL:(3)RL=11+C0KL,where C0 is the initial concentration of MB and RL indicates that the Langmuir isotherm is favorable (0<RL<1), unfavorable (RL>1), linear (RL=1), and irreversible (RL=0).The Freundlich model is applicable to a multilayer adsorption on a heterogeneous surface by adsorption sites. The linear form of the Freundlich equation is given as follows:(4)lnQe=lnKF+1nlnCe,where KF is the Freundlich constant and n is the heterogeneity factor.The Dubinin–Radushkevich model is used to describe the adsorption on both homogeneous and heterogeneous surfaces. The linear equation can be represented as follows:(5)lnQe=lnQm−Kε2,where K denotes the coefficient related to the adsorption free energy (mol2·kJ−2), Qm is the maximum adsorption capacity (mg·g−1), and ε is the Polanyi potential that can be calculated by the following equation:(6)ε=RTln1+1Ce,where R is the gas constant (8.314 J·mol−1·K−1), T is the absolute temperature (K), and Ce is the equilibrium concentration of MB.The Temkin model assumes that the heat of adsorption of the molecules in the layer decreases linearly with coverage due to adsorbent-adsorbate interactions and mainly describes the chemisorption process which dominated through electrostatic adsorption:(7)Qe=RTBTlnATCe,where BT is the Temkin constant (J·mol−1) and AT denotes the Temkin isotherm equilibrium binding constant (L·mg−1).The experimental data were fitted with four isotherm models, and the results are summarized in Table2. Aside from the Dubinin–Radushkevich model, the R2 values for other models are higher than 0.9, which implies that those models can be used to describe the adsorption behavior and the adsorption mechanism between wasted tea powder and MB is a complex reaction. The Langmuir model has the best fitting (R2 > 0.999), and the maximum capacities of MB (Qmax) for different temperatures calculated from the Langmuir model are very close to the experimental results. The monolayer coverage of MB onto wasted tea powder is the main adsorption mechanism. On the other hand, a high R2 is obtained from the Freundlich model and the heterogeneity factors are also calculated. The heterogeneity factor (n) can be used to determine the difficulty of adsorption behavior. In general, the adsorption process is easy to occur when n is larger than 2 [25]. As shown in Table 2, the heterogeneity factor (n) ranges from 4.11 to 4.90, which suggests that the adsorption process is easy to occur between wasted tea powder and MB as well as reveals the multiple coverage of MB is favorable. The Temkin model also appears to have the better fitting results in this study. This finding suggests that the electrostatic interaction is one of the important mechanisms between wasted tea powder and MB.Table 2
Adsorption isotherm fittings for the MB adsorption onto wasted tea powder.
Temperature (K)
Langmuir
Freundlich
Dubinin–Radushkevich
Temkin
Q
exp (mg·g−1)
Q
max (mg·g−1)
K
L (L·mg−1)
R
2
n
K
F (L·mg−1)
R
2
B (mol2·kJ−1)
Q
max (mg·g−1)
R
2
B
T (J·mol−1)
A
T
R
2
288
311.6
312.5
0.061
0.999
4.63
78.67
0.967
3.811
261.5
0.815
57.81
2.74
0.991
298
322.4
333.3
0.042
0.999
4.11
68.41
0.976
6.633
265.4
0.791
52.35
1.33
0.988
308
325.2
333.3
0.055
0.999
4.37
75.91
0.974
3.496
268.7
0.795
56.81
2.06
0.988
318
319.6
322.6
0.060
0.999
4.63
80.54
0.963
2.881
266.2
0.819
62.81
2.88
0.994
333
309.7
312.5
0.075
0.999
4.90
84.89
0.945
2.349
263.1
0.840
70.78
4.12
0.988


## 3.10. Adsorption Kinetics

Adsorption kinetics study provides useful information regarding the adsorption efficiency and engineering design for scale-up operations. The adsorption kinetic can be elucidated using various types of mathematic models, of which one most widely used is the Lagergren rate equation [28, 29]. In this study, the adsorption kinetic was analyzed using the pseudo-first-order model, the pseudo-second-order model, and intraparticle diffusion model. The description of these models can be represented by the following equation.Pseudo-first-order model:(8)lnQe−Qt=lnQe−K1t.Pseudo-second-order model:(9)tQt=1K2Qe2+1Qe,where Qe (mg·g−1) and Qt (mg·g−1) are the amounts of MB adsorbed at equilibrium at any time t (min) and K1 and K2 are the pseudo-first-order rate and pseudo-second-order rate constants.The intraparticle diffusion model is applied to assess the rate-controlling step in the porous structure [30]. It can be expressed as follows:(10)Qt=Kidt+C,where Qt (mg g−1) is the amounts of MB adsorbed at equilibrium at any time t (min), Kid is the intraparticle diffusion rate constant, and C denotes the greater effect of the boundary layer on molecule diffusion.The fitting results from pseudo-first-order and second-order models are listed in Table3. The R2 values for pseudo-second-order model were larger than 0.998, which are better than those obtained for the pseudo-first-order model. The Qmax values derived from the pseudo-second-order model are much close to the experimental value, Qexp, indicating that the pseudo-second-order model is more appropriate for the prediction of the kinetic process for MB adsorption onto wasted tea powder. The perfect fitting of the pseudo-second-order model means that the rate-limiting step is not the resistance of the boundary layer [31]. The adsorption process is controlled by the chemisorption involving valence forces through the sharing or exchange of electrons between the adsorbent and the adsorbate [32, 33]. However, the pseudo-second-order model contains the external liquid film diffusion, intraparticle diffusion, and adsorption on the surface of the adsorbent; this model provides a more comprehensive and accurate description of the adsorption mechanism between wasted tea powder and MB [34–36].Table 3
Adsorption kinetic fitting results for the MB adsorption onto wasted tea powder.
Initial MB concentration
Q
exp (mg·g−1)
Pseudo-first-order
Pseudo-second-order
Q
cal (mg·g−1)
K1 (min−1)
R
2
Q
cal (mg·g−1)
K2 (min−1)
R
2
100
92.94
29.78
0.039
0.974
92.34
0.0044
0.999
400
288.36
92.55
0.032
0.981
294.10
0.0012
0.999
1000
367.64
144.82
0.033
0.813
370.37
0.0006
0.999
1200
374. 11
76.63
0.024
0.868
370.37
0.0014
0.998
1500
404.76
72.67
0.027
0.703
400.00
0.0018
0.999Figure9 shows the plot of intraparticle diffusion model for the adsorption of MB onto wasted tea powder. Note that all the plots are nonlinear over whole time ranges and do not pass through the origin. This illustrates that the intraparticle diffusion is not the only rate-controlling step and some other mechanisms along with the intraparticle diffusion are probably involved [37, 38]. After further analysis, it is observed that the adsorption process tends to be followed by the two linear stages. The first linear stage is due to the transportation of MB molecules from the bulk solution to the external surface of the wasted tea powder by diffusion through the boundary layer, and the second one seems to refer to the diffusion into meso/micropores of the wasted tea powder structure. As can be seen from Figure 9(b), the first linear stage has higher slope than the second linear stage. This implies that the resistance of mass transfer between the external boundary layer film of liquid and adsorbent is relatively smaller, resulting in the rapid adsorption rate for the first linear stage. The intraparticle diffusion of MB molecules into pore structures is the rate-limiting step for the adsorption process onto wasted tea powder.Figure 9
The plot of intraparticle diffusion model for the adsorption of MB at various initial concentrations. (a) The original plot and (b) the fitting results.
(a)
(b)


## 3.11. Regeneration and Recyclability Investigation

For practical applications, regeneration and recyclability of the adsorbent is important to evaluate the overall performance of the adsorbent. For the regeneration process, the used tea powder was regenerated by mixing of 0.5 N NaOH solutions and deionized H2O at the shaker for 1 hr at 298 K, respectively. The used tea powder was separated from the suspension and washed by deionized water in the ultrasonic shaker for three times and dried in an oven at 333 K. The adsorption efficiency of used tea powder is shown in Figure 10. The adsorption efficiency decreased with regeneration cycles for both the cases. It is evident that the wasted tea powder regenerated by NaOH possesses 61% adsorption efficiency after third cycles. The adsorption efficiency dropped rapidly with regeneration cycles for the case of H2O, suggesting that the H2O is not a suitable reagent for the regeneration process. The decrease in adsorption efficiency is attributed to the some solubilized parts of the wasted tea powder, changed superficial structures of wasted tea powder and subsequently leads to loss and blockage of adsorption sites [25, 39]. Based on the regeneration evaluation, it is deduced that the NaOH proves to be a desorbing agent and wasted oolong powder is an inexpensive, ecofriendly, and competent adsorbent for the remediation of dyes from the aqueous system.Figure 10
Regeneration and recyclability tests for the adsorption of MB.


## 4. Conclusions

In this study, a wasted oolong tea powder was successfully used as a biosorbent for the adsorption of MB from aqueous solution. A series of operative parameters, including contact time, MB concentration, wasted tea powder loading weight, operation temperature, pH value of solution, and ion strength of solution, were investigated. Experimental results revealed that the adsorption capacity of MB increased with the contact time, MB concentration, and pH value of solution. The adsorption data were well fitted by the Langmuir and Temkin isotherm. The maximum adsorption capacity could be predicted to be 333.3 mg·g−1 when the temperature was controlled at 298 K and 308 K based on the Langmuir model simulation. Kinetic studies revealed that the adsorption process follows the pseudo-second-order model. The intraparticle diffusion model provided a reasonable reason to interpret the rate-limiting step for the adsorption process in this study. The regeneration and recyclability experiment suggested that the NaOH is a suitable desorbing agent for the regeneration and the wasted oolong tea powder is a readily available and ecofriendly biosorbent for the removal of dyes from the aqueous system.



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*Source: 4980965-2019-01-16.xml*