Research Article  Open Access
Tea Stem as a Sorbent for Removal of Methylene Blue from Aqueous Phase
Abstract
The potentiality of tea stem for the adsorption of methylene blue (MB) from aqueous phase was investigated. A series of operating factors, including the initial MB concentration, contact time, pH of solution, dose of tea stem, and ionic strength of solution, were conducted to understand the effect of adsorption of MB onto tea stem. Adsorption isotherm, kinetic models, thermodynamic investigation, and regenerability of tea stem were systematically investigated in this study. The experiment results revealed that the removal efficiency decreased with MB concentration and the equilibrium time of adsorption at different initial MB concentrations was approximately at 60 min. The appropriate dose of tea stem powder was found to be 4 g/L. The pH_{zpc} of tea stem was evaluated and was observed to be 6.0 ± 0.2. The removal efficiency increased with pH ranging from 3.0 to 5.0 and remained constantly at the pH range of 5.0–11.0. The pH affected the adsorption because of the repellent power between H^{+} and dye cation. The ionic strength was found to have a negligible effect on the adsorption. The Langmuir and Temkin isotherm models were found to be the best isotherm models to elucidate the adsorption mechanism between MB and tea stem powder. The maximum adsorption capacity of 103.09 mg/g derived from the Langmuir model was much close to the experimental result. From the kinetic analysis, the pseudosecondorder model was found to be the suitable model to describe the adsorption behavior. The calculated adsorption capacities at different temperatures derived from the pseudosecondorder model ranging from 68.91 to 69.8 mg/g were well close to the experimental data. The intraparticle diffusion of MB molecules into pore structures of tea stem powder is the ratelimiting step for the adsorption process in this study. Evaluation of thermodynamic parameters including changes in enthalpy, entropy, and Gibb’s free energy indicated the adsorption mechanism between MB and tea stem powder was a spontaneous and exothermic process. The regeneration/adsorption experiments indicated that the tea stem powder efficiently remained more than 97% after five cycles using NaOH as a desorbing agent and thus be used for many times. On the basis of experimental results obtained, it is concluded that the tea stem has a considerable potential as a lowcost sorbent for removing MB from the aqueous phase.
1. Introduction
Dyes are widely applied in many fields such as textile, leather tanning industries, paper production, food technology, and medical treatment, which has propelled the development of technology and industry in recent years [1–4]. However, untreated wastewater is discharged into the environment, thereby causing river and soil pollution [5, 6]. Because of the variety, high chromaticity, refractory degradation, and poor biodegradability of dyes, this pollution has exceeded the limit in the environment and affected human health [6–9]. Solving the dyeinduced pollution problem is increasingly crucial. Current methods to dyestuff wastewater treatment include coagulation, advanced oxidation, membrane separation, biodegradation, and adsorption [10–14], all of which were limited to apply in industries because of more chemical consumption, complex operating conditions, and second pollution [15–18]. Adsorption, however, has advantages such as low cost, environmental safety, regeneration, and easy operational feasibility when compared with other methods, which attracts people to explore new approaches to solve the dyestuff wastewater problem [6, 7, 19]. Methylene blue (MB) is extensively used as a chemical indicator, biological stain, disinfectant, and medicine in water bodies such as lakes, dyeing of tissues, and treating methemoglobinemia, respectively [19, 20]. Many materials such as halloysite/MnFe_{2}O_{4} nanocomposites, kaolin, and palygorskite/carbon composite have been widely used to adsorb MB [21, 22]. In addition, the experiments were systematically conducted using kinetic, isotherm, and thermodynamic models to determine the possible adsorption mechanism.
Tea, cultivated in countries from China to Kenya, is a vital plant and wellknown nonalcoholic drink globally. It is a kind of biosorption material with a netlike structure and highly developed pore structure. After drinking, tea residue is discarded as waste. However, some studies have proven that tea waste exhibits better adsorption performance for heavy metal removal and dye removal [23–26]. Recently, researches have reported that MB can be absorbed by many materials and tea has a great potential as an adsorbent. Oolong tea is one of the important tea beverages in China. Tea stem is a byproduct from the manufacture of oolong tea. In the past, many tea stems are discarded during the manufacture of oolong tea because of low reuse. Undoubtedly, most of the wasted tea stems have been largely produced in China, leading to a severe environmental problem. To our knowledge, the use of tea stem on the removal of dyepollutants from aqueous solution is not reported. The main objective of this study was to evaluate the potential of tea stem as an alternative adsorbent for the removal of MB from aqueous solution. A series of operative factors, including contact time, initial MB concentration, dose of tea stem, pH value, and ionic strength on the removal efficiency was 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 temperatures.
2. Materials and Methods
2.1. Preparation of Tea Stem Powder
Tea stem separated from Tieguanyin species was collected from the local tea market, Anxi County, Fujian Province. Five hundred grams of tea stem were mixed with 2 L of distilled water and then were brewed at 373 K for 4 hrs, and the operation was repeated at least 6 times. The brewed solution was detected by a UVvisible spectrophotometer to confirm that the signal of brewed solution was close to the background value (absorbance signal less than 0.01). After brewing, tea stem samples were dried at 383 K for 12 hrs. The dry tea stem samples were crushed to pass through an 80mesh sieve for obtaining the desired particle sizes. The prepared tea stem powders were stored in a dry container for the adsorption experiment. Through the elemental analysis, the content of carbon, nitrogen, and hydrogen for tea stem used in this study was 48.06%, 1.35%, and 0.41%. The water content and ash content were 4.85% and 0.80%, respectively.
2.2. Analytical Instrument and Chemical Reagents
The MB concentration was analyzed using a UVvisible spectrophotometer (Shimadzu Model UV 1750) at a wavelength of 664 nm. The calibration curve was obtained by using the standard of MB reagent, and the correlation coefficient R^{2} 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 5 μm Hg at 373 K for 2 hrs.
Fourier transform infrared spectroscopy was used to identify the surface functional groups of tea stem powder. The infrared spectra were recorded on a PerkinElmer One B model FTIR spectrometer with fully computerized data storage and data handing capability. To provide adequate characterization of the wasted tea powder, the spectrum was set from a range of 400 to 4,000 cm^{−1}. A 100scan data accumulation was carried out at 4 cm^{−1} resolution.
Elementar vario EL III Heraeus CHNOS Rapid F002, equipped with a flash combustion furnace and thermal conductivity detector (TCD), was used for determination the content of carbon and nitrogen in tea stem.
2.3. Batch Studies
The concentration of MB used for the experiments of pH values, dose of tea stem, and ionic strength was controlled at 300 mg/L. The ratio of tea stem weight and MB solution volume (labelled as RM) was used, and the ranges of 2–40 g/L were investigated in this study for determining the effect of dose of tea stem on adsorption of MB. The pH value of the MB solution was adjusted to the desired value by adding a small quantity of HCl and NaOH to investigate the effect of pH on adsorption. The concentrations of MB with different pH values were measured, and the results indicated that there are no appreciable changes in the signal of UVvisible spectra of MB in the used pH range. The pH value at different concentrations of MB was measured between 4.9 and 5.1, showing a stable pH range for the adsorption experiment. All adsorption experiments were carried in conical flask with a 50 mL of MB solution and were placed on a thermocontrolled shaker at 200 rpm. After adsorption experiment, the solution was centrifuged at 5000 rpm for 10 mins, and then the supernatant solution was analyzed to determine the concentration of MB. The adsorption capacity (mg/g) and removal efficiency (%) of MB were calculated using the following equations:where C_{0} and C_{t} are the concentrations under the phase of liquid at the initial time and any time t in mg/L, respectively. Q_{t} is the amount of MB uptake at any time t in mg/g. V is the volume of the solution in liters and M is the mass of tea stem powder in gram. All adsorption experiments were performed in duplicate, and the data were further analyzed.
2.4. Investigation Point of Zero Charge (pH_{zpc}) of Tea Stem Powder
For better understanding the effect of pH in adsorption of MB onto tea stem powder, pH_{zpc} assay of tea stem powder was tested according to the report of Ali et al. [15]. A volume of 50 mL NaCl (0.01 mol/L) solution was added in flask as an electrolyte. A solution with initial pH value ranging from 3.0 to 10.0 was prepared and 4 g/L of tea stem were added in different initial pH solutions. The mixed solution was shaken at an agitation speed of 200 rpm for 12 hrs at 298 K. After shaking, tea stem powder was filtered and the final pH value was determined.
2.5. Adsorption Kinetics
Adsorption kinetic experiments were operated at different temperatures (298, 313, and 328 K) with a MB concentration of 300 mg/L and a certain mass of 0.2 g tea stem powder. Different kinetic models were applied to fit the adsorption process in this study.
The pseudofirstorder model was established by Lagergren. The linear form of the model is expressed as follows [27]:where Q_{e} and Q_{t} represent the adsorption capacity (mg/g) at equilibrium and any time t, respectively. k_{1} (min^{−1}) is the rate constant that is related to the adsorption equilibrium rate. The plot of the pseudofirstorder model is from log (Q_{e}–Q_{t}) versus t.
The pseudosecondorder model originated from Wu et al. [28]. The model is expressed as follows:where Q_{e} and Q_{t} represent the adsorption capacity (mg/g) at equilibrium and any time t, respectively. k_{2} (min^{−1}) is the rate constant related to the adsorption equilibrium rate.
The Elovich model was developed by Siddiqui et al.[29, 30]. The linearized form is expressed as follows:where Q_{t} (mg/g) represents the adsorption capacity at any time t, β is the coefficient representing rate of desorption, and α is the initial rate coefficient for the adsorption and also represents the activation energy for the chemical adsorption. α and β can be calculated from intercept and slope of the plot between Q_{t} versus ln t.
Bangham model was contrived by Bangham [31, 32]. The linearized form is expressed as follows:where C_{0} (mg/L) is the initial concentration of the solution, Q_{t} (mg/g) represents the adsorption capacity at any time t, m (g/L) and V (L) are the mass of adsorbent and volume of the solution, and k_{3} and α are constants. and log t make up the plot, where k_{3} and α can be determined from the slope and intercept.
To ascertain the specific adsorption mechanism for adsorption of MB onto tea stem powder, the intraparticlediffusion model was used to further analyze the results. According to the intraparticlediffusion model, the adsorption process is divided three stages [33]: (i) dye molecules gather in the film of liquid outside the adsorbent, (ii) dye molecules enter into the pores onto the adsorbent, and (iii) dye molecules are adsorbed by the active positions inner pores onto adsorbents and reach equilibrium. The intraparticlediffusion model is given as follows [34]:where K_{id} is the rate constant involved in the intraparticlediffusion model in mg g^{−1} min^{−0.5}, and C is the constant related to the thickness of the boundary layer. If the linearized plot passes through the origin, then the adsorption rate is only controlled by internal diffusion; if not, both the external diffusion and internal diffusion dominantly control the adsorption rate, and the higher the values of C, the greater the external diffusion effect produced [33, 34].
2.6. Isotherm Models
The Langmuir isotherm model establishes that adsorption occurs on active sites of the sorbent with equivalent energy, and a monolayer of the adsorbate exists on the solid homogeneous surface [35]. The model is expressed as follows:where C_{e} and Q_{e} are the MB concentration (mg/L) and the amount of MB adsorbed at equilibrium state (mg/g), respectively, Q_{m} is the maximum amount of the monolayer of MB occurring on the homogeneous solid surface (mg/g), and b is the Langmuir constant (L/mg). Both Q_{m} and b are calculated from the slope and intercept of the plots of C_{e}/Q_{e} versus C_{e}. The constant b was calculated from the intercept of the plot that was further determined a dimensionless separation factor, R_{L}. R_{L} confirms the characteristic of the Langmuir isotherm model, especially the type of isotherm. The R_{L} model is expressed as follows [36]:where C_{0} is the initial MB concentration, and if R_{L} > 1, then the adsorption process could be unfavorable; if 1 > R_{L} > 0, the adsorption process could be energetically favorable; if R_{L} = 1, then the adsorption process could be linear; and if R_{L} = 0, the adsorption process is irreversible.
The Freundlich isotherm model, an empirical model, is used to describe that the adsorbates are adsorbed to heterogeneous surfaces of a multilayer adsorption system [37, 38]. The model can be expressed as follows:where K_{f} and n are the rate constants related to the adsorption capacity (mg/g) and adsorption intensity and their values were obtained from the intercept and slope of the plots of Q_{e} against C_{e}. In this model, adsorption process is favorable if 1/n is closer to zero, whereas the adsorption is unfavorable if 1/n is larger than one. The adsorption is homogeneous, and no reaction occurs between the adsorbent and adsorbate when 1/n is one [39, 40].
Temkin isotherm model, taking interaction between adsorbate and adsorbent into account, assumes that (i) the heat of sorption linearly decreases with the increasing of surface coverage and (ii) bonding energies are equally distributed on the adsorbent. The linearized model is given as follows [27, 41, 42]:where R is gas constant (8.314 J/mol/K) and T is the absolute temperature, and A and b are the Temkin constants that are related to maximum bonding energies (L/mg) and the heat of sorption (J/mol), respectively. They are determined from the slope and intercept of the plot of Q_{e} against ln C_{e}.
Dubinin–Radushkevich (DR) isotherm model is applied to definite the nature of bonding happened on the heterogeneous. The linearized model is given as follows [27, 29, 43]:where Q_{e} and Q_{DR} are the amount of MB adsorbed at equilibrium state (mg/g) and the theoretical uptake of tea stem powder (mg/g). ɛ is Polanyi potential, which can be computed by the model:where R is gas constant (8.314 ∗ 10^{−3} kJ/mol/K) and T is the absolute temperature, C_{e} is the concentration of MB (mg/L) when adsorption process achieving equilibrium state, β is related to the mean free energy E (kJ/mol) of adsorption per mole of MB. E can be calculated by following model:
E value is used to evaluate physisorption or chemisorption; when E value is less than 8 kJ/mol, the adsorption is physisorption, and when E value is 8–16 kJ/mol, the adsorption is chemisorption.
2.7. Adsorption Thermodynamics
To understand the thermodynamic parameters, the Gibbs free energy, enthalpy, and entropy were considered and experiments were conducted at 298, 313, and 328 K with a 300 mg/L of MB solution. The models of thermodynamic parameters are expressed as follows [44, 45]:where k is the rate constant related to adsorption under the equilibrium state. ΔS^{0} and ΔH^{0} are the standard entropy change in J/mol K and the standard enthalpy change in kJ mol, respectively. T is the absolute temperature (K), and R is the gas constant (8.314 J/mol K). ΔS^{0} and ΔH^{0} are determined from the slope and intercept, respectively, of the plot of ln k against 1/T. The standard Gibbs free energy of adsorption (ΔG^{0}) is expressed as follows:
2.8. Regeneration and Reusability
For the regeneration process, the used tea stem powder was regenerated by mixing of 0.05 N NaOH solutions at a shaker for 60 min at 298 K. The used tea powder was separated from the suspension and washed with deionized water in ultrasonic shaker for 60 min and repeated for three times. After ultrasonic treatment, the regenerated tea stem was dried in an oven at 333 K.
3. Results and Discussion
3.1. Pore Structure and Surface Characteristic of Tea Stem
The N_{2} adsorption and desorption isotherms of tea stem powder are shown in Figure 1. A detectable hysteresis loop is observed between . This confirms that the isotherm curve corresponds to the characteristic of type IV according to the classification standard of International Union of Pure and Applied Chemistry, suggesting the major pore structure may be in the form of a cylindrical or silt shape [46]. The BET surface area of tea stem powder was measured to be 1.05 m^{2}/g. The pore size calculated using BJH method was 18.95 nm. The feature of hysteresis loop was suggested to be related to the capillary condensation associated with large pore channel. Thus, the pores on tea stem powder were macropores, mesopores, and micropores, of which the mesopores were major. XRD patterns showed that the width peaks at 2θ = 25.5 and 2θ = 34.6 were observed and assigned to the cellulose and lignin, respectively. FTIR analysis revealed that the bondedOH groups (3416 cm^{−1}), aliphatic CH group (2925 cm^{−1}), C=O stretching (1650 cm^{−1}), CH bending (1385 cm^{−1}), COC of polysaccharides (1150 cm^{−1}), and COH stretching (1062 cm^{−1}) were the main functional groups on the surface of tea stem.
3.2. Effect of Initial MB Concentration and Contact Time
Figure 2 shows the adsorption capacity and removal efficiency as a function of contact time at different MB concentrations. As shown, a rapid adsorption stage is observed in the initial contact time, which is followed by a stable adsorption stage. The adsorption capacity increases from 24.25 to 99.44 mg/g when the initial MB concentration increases from 100 to 800 mg/L. This is due to that the driving force established in the high initial concentration may have surmounted the obstruction of MB adsorption between the liquid and solid phases [47, 48]. In addition, the high MB concentration enhances the collisional frequency between MB and tea stem powder and thus increases the adsorption process [48]. The removal efficiency decreases with the initial MB concentration, which is attributed to the finite adsorption sites on the surface of tea stem powder. The equilibrium time for adsorption at different initial MB concentrations is approximately at 60 min. With time increase, the adsorption of MB stabilized, indicating that the adsorption sites saturated [49].
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3.3. Effect of the Dose of Tea Stem Powder
Figure 3 depicts the adsorption capacity and removal efficiency for different doses of tea stem powder ranging from 2 to 40 g/L. The adsorption reaches an equilibrium state when the contact time is at 60 min. The adsorption capacity decreases from 121.47 to 7.47 mg/g, and the removal efficiency increases from 80.98 to 99.50% as the contact time is progressed from 1 to 120 min. This is attributed to the amorphous and porous structures of tea stem powder and available active adsorption sites that increase with the strength of the dose of tea stem powder, and thus, decrease in driving force [29]. Based on the experimental results in this study, the appropriate dose of tea stem powder was 4 g/L because more than 90% of removal efficiency and an acceptable adsorption capacity were achieved.
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3.4. Effect of pH
The pH and pH_{zpc} are important factors for adsorption of MB onto tea stem powder. The change in pH_{zpc} provides the charge characteristic of tea stem and determines the type of charge when pH is changed in the experimental process. Figure 4 shows the effect of pH on the adsorption of MB and the graph of pH_{initial} versus pH_{final.} The removal efficiency increases from 97.57 to 99.44% as pH increases from 3.00 to 5.00. With the pH increases from 5.00 to 11.00, the removal efficiency is more than 99.00% and remains constantly. The pH_{zpc} is estimated about 6.00 ± 0.2 and the surface charges are toward to the negatively charged when pH is greater than 6.0, whereas when the pH is lower than 6.0, the tea stem surface becomes positively charged. MB is a cationic dye and possesses positively charged ions in aqueous solution. Under the 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 MB. Therefore, the adsorption efficiency of MB declines. On the other hand, higher positive charge of the MB solution and simultaneous protonation of tea stem powder at low pH cause a decrease in available adsorption sites. The electrostatic interaction between the positively charged MB and H^{+} weakens the adsorption process. Conversely, occurrence of deprotonation of the tea stem powder increases the number of active adsorption sites and reduces electrostatic force [29, 50, 51]. Therefore, the effect of pH on the adsorption of MB in this study reveals that the removal efficiency maintains constantly at the pH range of 5.0–11.0, which agrees with the pH_{zpc} of tea stem powder.
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3.5. Effect of Ionic Strength
In addition to dye, wastewater also contains large amount of inorganic salt. For a better understanding of its effect on adsorption efficiency, a series of different NaCl concentrations were investigated. Figure 5 illustrates the adsorption capacity and removal efficiency at different concentrations of NaCl. The adsorption capacity and removal efficiency decreases from 23.35 to 22.96 mg/g, and 93.40 to 91.86%, respectively, as NaCl increases from 2.00 to 8.00%. Slight decreases in adsorption capacity and removal efficiency prove that the ionic strength may be negligible on the adsorption of MB onto tea stem powder. Furthermore, the affinitive interaction between dye molecules and tea stem powder is stronger than that between Na^{+} and tea stem powder.
3.6. Study of Kinetic
3.6.1. PseudoFirstOrder and SecondOrder Models
The fitting results of the pseudo firstorder and secondorder models at different temperatures are shown in Figure 6, and the detailed kinetic parameters are listed in Table 1. The R^{2} values for the pseudofirstorder are ranged between 0.49 and 0.84 at different temperatures, while the values of R^{2} for the secondorder are more than 0.99. The predictive maximum adsorption capacities (Q_{cal}) for the pseudofirstorder and secondorder models are estimated to be 3.22, 6.45, and 1.98 mg/g and 69.93, 69.44, and 69.49 mg/g at different temperatures, respectively. The Q_{cal} values derived from pseudosecondorder are much close to the experimental values (Q_{exp}), demonstrating that the pseudosecondorder model is more appropriate for the prediction of kinetic process for MB adsorption onto tea stem powder. The idea fitting of the pseudosecondorder model means that the ratelimiting step is not the resistance of boundary layer and the adsorption of MB onto tea stem is a complicated process [52].
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3.6.2. Elovich Model
The results of the Elovich model are shown in Figure 6(c) and Table 1. The R^{2} values for the Elovich model are 0.95, 0.93, and 0.72 at 298, 313, and 328 K, respectively, showing the Elovich model could describe the adsorption of MB onto tea stem powder at 298 and 313 K rather than at 328 K. Values of α and β are 1.05 ∗ 10^{21}, 3.51 ∗ 10^{16}, 0.75, and 0.61 at 298 K and 313 K, respectively. Note that α is absolutely higher than β, which indicates that the adsorption process is the main reaction. This also explained why the initial adsorption rate of MB onto tea stem powder was high and the adsorption rate decreased with contact time. In addition, the results of the Elovich model confirmed that adsorption of MB onto tea stem powder was chemisorption, which agreed with inferences of the pseudosecondorder model.
3.6.3. Bangham Model
The results of Bangham model are shown in Figure 6(d) and Table 1. The R^{2} values for the Bangham model at 298, 328, and 313 K are 0.96, 0.96, and 0.76, respectively, which indicates that the film diffusion is the ratecontrolling step at 298 and 313 K. As the temperature increases, R^{2} value for Bangham model is smaller and proves that the film diffusion is not the ratecontrolling step.
3.6.4. IntraparticleDiffusion Model
As shown in Figure 6(e), three stages are observed from the multilinear plots. The first adsorption stage, with the highest values of K_{id}, could be controlled not only by intraparticle diffusion, but also by external diffusion to some extent because the origin was not passed. The first stage has a higher slope than the other stages. 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. Another reason may be due to the MB molecules accumulating outside the tea stem powder and the reaction among the MB cation and charges on the surfaces of tea stem powder. The second adsorption stage could be explained by that the MB molecules gradually penetrated the inner macropores and mesoporous in tea stem powder. The values of K_{id} of the third adsorption stage were small, which indicated that the adsorption reached the equilibrium state. The third adsorption stage was affected by the numbers of microspores, size of MB molecules, diffusion resistance, and the concentration of MB.
3.7. Study of Adsorption Isotherm
3.7.1. Langmuir Isotherm Model
The results of the Langmuir isotherm model are presented in Figure 7(a) and Table 2. The R^{2} value for Langmuir isotherm model was 0.99, which demonstrates the adsorption of MB onto tea stem powder is chemisorption with equivalent energy on the adsorption sites and monolayer of the MB on the tea stem powder. All values of R_{L} obtained from the Langmuir isotherm model are less than 1 and decreased with C_{e} (shown in Figure 7(b))_{.} This result attests that the adsorption of MB onto tea stem powder is favorable. The monolayer coverage of MB onto wasted tea powder is the main adsorption mechanism.
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3.7.2. Freundlich Isotherm Model
Figure 7(c) shows the Freundlich isotherm model for adsorption of MB onto tea stem powder, and relevant isotherm parameters as well as correlation coefficients are presented in Table 2. The R^{2} value for Freundlich isotherm model is 0.96, which is lower than that of the Langmuir isotherm model. The adsorption of MB onto tea stem powder is favorable because of lower 1/n value (0.29).
3.7.3. Temkin Isotherm Model
The Temkin isotherm model presented in Figure 7(d) and Table 2 also appears to have the better R^{2} in this study. The value of A, bonding energy between MB molecules and tea stem powder is 1.70 L/mg, and equally distributes. In addition, the heat of adsorption (b value) is 159.41 J/mol, which linearly decreases as MB molecules covered gradually tea stem powder. These findings suggest that the electrostatic interaction is one of the important mechanisms between wasted tea powder and MB [52].
3.7.4. DR Isotherm Model
The description of DR isotherm model is shown in Figure 7(e) and Table 2. The Q_{DR} and β for adsorption of MB onto tea stem powder are 85.53 mg/g and 2.48, respectively. The R^{2} value for DR isotherm model is 0.93, which is the lowest among the isotherm models used in this study. Therefore, the DR isotherm model probably is not suitable to elucidate the adsorption of MB onto tea stem powder.
3.8. Analysis of Thermodynamics
The relationship between ln K and 1/T and the calculated thermodynamic parameters are shown in Figure 8 and Table 3. The values of ΔG^{0} are −6.49, −6.75, and −6.62 kJ/mol at 298, 313, and 328 K, respectively, which demonstrates that the adsorption process is spontaneous. The negative ΔH^{0} (−5.18 kJ/mol) denotes that the exothermal reaction occurs between the MB and tea stem powder. The positive value of ΔS^{0} (4.60 J/mol K) signifies the increase in disorder at the interface of MB and tea stem powder. The result of negative ΔH^{0} and positive ΔS^{0} agrees with the spontaneous adsorption process. More importantly, the analysis of thermodynamic studies is similar to that of adsorption isothermal and kinetic studies.

3.9. Regeneration Studies
Regeneration and recyclability of adsorbent is important to evaluate the overall performance of adsorbent in the practical application. To explore the regenerability of tea stem, a five regeneration/adsorption cycles was carried out and the result is shown in Figure 9. Note that the total iron content was estimated approximately 2.46 ± 0.12 mg/gtea stem and its influence on regeneration can be neglected in this study. The excellent regeneration performance was found after regeneration. The removal efficiencies remain constantly (more than 97%) and no deactivation is observed, which denotes tea stem powder is a potential adsorbent for removing MB. The clear color of MB solution after five regeneration/adsorption cycles reveals that the tea stem is easily to be regenerated. The possible explanations for the excellent regeneration performance of tea stem can be attributed to the complete desorption of MB from the surface of tea stem powder when the NaOH was used as a desorbing agent. Additionally, the surface and pore structure of tea stem may be modified through the regeneration process, and the adsorption sites on tea stem powder are more active and are difficult to collapse and clog [53]. Based on the regeneration evaluation, it is deduced that the NaOH proves to be a desorbing agent and tea stem is an inexpensive, ecofriendly, and competent adsorbent for the remediation of dyes from aqueous system.
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3.10. Comparison with Other Adsorbents
The comparison of maximum adsorption capacity of tea stem for MB with those of other adsorbents in the literature is shown in Table 4. The tea stem shows the comparable adsorption capacity for MB with respect to the adsorbents. Tea stem has a higher adsorption capacity among the adsorbents in Table 4. Therefore, tea stem is a suitable adsorbent for removal of MB.
4. Conclusion
Tea stem was used as a sorbent for the adsorption of MB from aqueous phase in a batch mode. The removal efficiency decreased as the initial concentration of MB increased because of the competition among dye cations. The adsorption process achieved equilibrium when the contact time was 60 min. According to the removal efficiency and adsorption capacity, the optimum dose of tea stem powder was 4 g/L. The pH_{zpc} of tea stem powder was found to be 6.00 ± 0.20, and the pH range of 5.0–7.0 would be the optimal condition for adsorption. The ionic strength was found to have a negligible effect on the adsorption.
From the kinetics and isotherm investigation, the pseudosecondorder model performed well at 298, 313, and 328 K, denoting the adsorption process of MB onto tea stem was multistage chemisorption. Analysis of Elovich and Bangham models illustrated that film diffusion was the ratecontrolling step at 298 and 313 K. The well fitting for the pseudosecondorder model and Langmuir isotherm model revealed that the possible adsorption process was chemical, spontaneous, and monolayer adsorption. Temkin isotherm model suggested that the bonding energy between MB molecules and tea stem powder was 1.70 L/mg and equally distributed on tea stem powder. The heat of sorption was 159.41 J/mol, which linearly decreased as MB molecules covered gradually tea stem powder. The initial adsorption stage was controlled by external diffusion and intraparticle diffusion, and the second adsorption stage was controlled by the ratelimiting step. Thermodynamic studies, especially the values of ΔG^{0}, ΔS^{0}, and ΔH^{0}, concurred with the analysis results of the kinetics and adsorption equilibrium studies. Thermodynamic investigation indicated the adsorption mechanism between MB and tea stem powder was a spontaneous and exothermic process. The regeneration/adsorption experiments indicated that the tea stem powder remained more than 97% efficiently after five cycles using NaOH as a desorbing agent and thus be used for many times. Although tea stem is a potential sorbent for the removal of MB and some operating factors and their effect were carried out in a batch experiment, other reactor systems, such as a fixed bed reactor or continuous stirred tank reactor (CSTR), should be thoroughly considered in the future. In addition, some other isotherm models including Redlich–Peterson isotherm (combination of LangmuirFreundlich model), Sip model, Toth model, Koble–Corrigan model (threeparameters empirical model on the combination of the Langmuir and Freundlich) could be taken into account for a better understanding of the possible adsorption behavior. Briefly, as demonstrated in this study, tea stem powders have a huge potential as sorbents for removing dye pollution.
Data Availability
The data generated and analyzed in this manuscript are available from the correspondence author on reasonable request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Authors’ Contributions
T. C. Lee, S. Wang, and T. H. Ko conceived and designed the research and experiments. Z. Huang, Z. Mo, and G. Wang performed the experiments. Z. Wu, C. Liu, and H. Han collected data and performed the model analysis. T. C. Lee, S. Wang, and T. H. Ko wrote the paper. All authors have read and approved the final manuscript. TzanChain Lee and Shumao Wang equally contributed to this work.
Acknowledgments
This work was partially funded by the construction of modern agricultural and industrial park for Anxi County, Fujian Province, Minister of Agriculture and Rural Affairs, China (KMD18003A).
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