Abstract: Background and Objectives: Excessive amount of methylene blue (MB) causes serious environmental problems and very difficult to decompose. Hence, the main objective of the study was to remove MB by using low cost adsorbent. Materials and Methods: The adsorbent was characterized with fourier transform infrared spectrophotometer (FT-IR) and scanning electron microscope (SEM). Various physiochemical parameters such as, contact time, initial dye concentration, adsorbent dosage and pH of dye solution were investigated in a batch-adsorption technique. Present work investigating both adsorption mechanism and adsorption kinetics of methylene blue, onto teak wood biomass from aqueous solution. Results: Equilibrium parameter has analyzed and it was found that Langmuir and Freundlich isotherm fit for data but Langmuir fit better than Freundlich isotherm. The adsorption kinetics study shows adsorption mechanism follows pseudo second order kinetics mechanism. Conclusion: It was found teak wood activated carbon can used as adsorbent for removal of methylene blue dye.
INTRODUCTION
Now-a-days, one of the major challenges for environment and human life due to their adverse effects is synthetic dyes1,2. Synthetic dyes are intensely compounds having general applications in different industries such as chemical and dye manufacturing, textile, paper, leather and plastics3,4 with structural diversity including: acidic, basic, reactive, azo, anthraquinone-based and metal complex dyes5. Those aforementioned dyes have different effects on environment, for example, they can be cause of reduction of sunlight penetration, oxygen transfer limitation and also are stable chemical and toxic for fauna and flora, when release into environment. Furthermore, they cause irreversible damages on human health such as mutagenic and carcinogenic effects6-8. Amongst these dyes, the methylene blue (MB) is widely used for dying wood, cotton and silk which can be cause of nausea, vomiting and mental confusion9,10. For this reason, removal of it from effluent containing dye is necessary. However, there are chemical and biological methods (like coagulation/flocculation, advanced oxidation processes, membrane filtration and ozonation) for treatment of dye effluents but these effluents are hardly treated by conventional biological wastewater treatments11. At the present time, neadsorption is reliable to be a simple technique and successful in water and wastewater treatment process and the success of the method largely depends on the evolution of a capable adsorbent12. Variety of absorbents has been studied to removal of MB from water and wastewater such as clay, clinoptilolite, almond gum, rice husk and fly ash etc.13-17. But, activated carbon is proved as the most efficient adsorbent because of its large surface area and highly porous structure although it has high cost limits its’ widespread use. High cost of commercially activated carbon which is a limitation to its extensive use have paved way for study of adsorption by naturally obtained and extracted activated carbon from agricultural wastes and various other sources. The purpose of this research is on the removal of cationic dye using naturally extracted and commercially obtained activated carbon. Therefore, development of activated carbon from comparatively low cost raw materials has become a growing field of interest18. Recently, agricultural waste as low cost adsorbent, availability and non-toxicity is noteworthy19. The removal efficiencies of dye in adsorption systems are often influenced by many parameters such as dosage of adsorbent, concentration of absorbate, contact time and pH. Ramirez et al.20 evaluated the production of activated carbon from two types of wood wastes, cedar and teak. Its chemical composition, morphology and textural properties were determined using different analysis technique and their application for dye removal from aqueous solutions was studied. The aim of the present study was to investigate both the adsorption mechanism and adsorption kinetics of a cationic dye, methylene blue, onto teak wood waste biomass from aqueous solution with respect to the initial dye concentration, temperature, pH and sorbent dosage.
MATERIALS AND METHODS
All the chemicals used throughout the study were supplied by E. Merck, India, double distilled water was used for preparing all of the solutions and reagents.
Adsorbate: Methylene blue (C16H18Cl N3S), as the adsorbate used in the present study, is a monovalent cationic dye. It is classified in dye classification as C.I. 42015. It has a molecular weight of 373.90 and wavelength (λmax) of 663 nm, which is a highly water soluble, supplied by BDH (India).
Adsorbent (TSAC): The teak wood (Tectona grandis) waste biomass material was collected from Thanjavur, Tamil Nadu, India during February-March, 2018. The total study duration was 10 months after collecting biomass material. The collected material was washed and air dried for 15-20 days. The dried material was cut into small pieces for further chemical modification. The ground material was mixed with equal amount of concentrated sulphuric acid and stirred for 30 min. The acid-plant material slurry was placed in a beaker and dried at 80°C in a hot air-oven. After 24 h, the thermo chemical reaction between acid and plant materials, it was proceeding by raising the oven temperature to 120°C for 90 min. After cooling, the resulting carbon washed with distilled water until a constant pH of the slurry reached. The wet carbon material was dried at 110°C and sieved into discrete particle size and stored. The adsorbent after sulphuric acid treatment was designated as TSAC.
Methylene blue dye solution: A stock solution of dye with known concentration (1000 mg L–1) was prepared. It is diluted to get different required initial concentration of the dye and used in the adsorption experiments. The initial pH was adjusted with prepared 0.1 M HCl or 0.1 M NaOH. All the adsorption experiments were carried out at room temperature.
Batch experiments: Batch experiments were conducted in bottles of 125 mL capacity. About 100 mL of the solution containing predetermined concentration of the MB dye under investigation was taken in the bottles. After the addition of known amount of adsorbent, the bottles were equilibrated for a predetermined period of time in a mechanical shaker (120 rpm). At the end of the equilibration period, the dye solution is taken and the residual dye in solution was determined. The pH of the dye solution was adjusted by using digital pH meter.
Effect of initial aqueous pH on adsorption: The pH of the dye solution is the most important factor compared to the all other factors that affect the adsorption process. The influence of pH on dye adsorption was investigated by performing experiments taking 100 mL of 20 mg L–1 of MB dye solutions and equilibrating with 0.200 g TSAC after adjusting the solution pH varying from 5.0-9.0 range for 2 h at room temperature. After equilibration time, the solutions were separated using filters and the supernatant was analyzed for residual concentration.
Effect of carbon dosage: The various concentration of adsorbent was taken in a 100 mL conical flask. Methylene blue concentration of 20 ppm was taken in that 100 ml conical flask The flask was put in a shaker at 115 rpm and 35°C. Various amounts of concentration of adsorbent were taken inside 5 different conical flasks of 0.2, 0.4, 0.6, 0.8 and 1 g, respectively. Samples were kept in each conical flask for a period of 40 min. The percentage absorbance at 664 nm was found out using a UV-spectrophotometer. The amount of adsorbent dosage was varied in the given range 0.2, 0.4, 0.6, 0.8 and 1 g.
The amount of dye adsorbed at time t, qt and at equilibrium qe was calculated from the mass balance equation:
where, qt is the amount of dye adsorbed (mg g–1) at time t, C0 is the initial dye ion concentration (mg L–1), Ct is the dye adsorbent (mg L–1) at time t, V is the volume of solution (mL) and m is the mass of the adsorbent (g). When t is equal to equilibrium contact time, Ct = Ce, qt = qe, then the amount of dye ion adsorbed at equilibrium, qe, is calculated using equation.
Adsorption isotherms: Several models of adsorption isotherm are available. The most common among them are Freundlich and Langmuir isotherm. In the present study the data obtained were tested for these two isotherms to study the relationship between the amounts of dye adsorbed with the equilibrium concentration and to find out the possibility of monolayer or multilayer adsorption.
The linear logarithmic form of Freundlich isotherm is given by:
log qe = log k+1/n log Ce
where, qe = x/m is the amount of dye adsorbed, Ce the equilibrium concentration, k and n are the constants. When log qe was plotted against logCe a straight line was obtained with slope logk and y-intercept.
The linear form of Langmuir isotherm is given by:
where, qο is the equilibrium constant, b is monolayer capacity, Ce is the equilibrium concentration and qe = x/m is the amount adsorbed per unit mass of the adsorbent.
Desorption and recycling efficiency: Desorption studies were performed with 1% H2SO4. The TSAC (100 mg/50 mL) saturated with 5 mg L–1 of MB was placed in different desorption media and was constantly stirred on a rotatory shaker at 100 rpm for 1 h. The adsorbent was separated and washed with distilled water.
Statistical analysis: The average values of duplicate runs were obtained and analysed. Error in data: ±1-2% for percentage removal ±0.005-0.01 mg g–1 for amount adsorbed.
RESULTS
Characterization of the adsorbent: The physicochemical properties of the prepared activated carbon was determined by standard procedures. The physicochemical properties are listed in Table 1.
| Table 1: | TSAC Parameters |
| TSAC: Telecommunications standards advisory committee | |
| Fig. 1: | IR spectrum of activated carbon TSAC |
FTIR analysis of activated carbon: The surface chemical characteristics of TSAC was determined by fourier transform infrared spectroscopy (FTIR) and the spectrum is given in Fig. 1. The peak at 3388 cm–1 is attributed to the stretching vibration of O–H band and aliphatic, asymmetric C–H stretching vibration of methylene group. The peaks at 2931 cm–1 in the spectra are due to stretching vibration of C-H. The band at 1750-1700 cm–1 represents the acidic carbonyl C=O stretching. The peak at 1708 cm–1 is attributed to the vibration of –COOH band. The peaks around 1000-1300 cm–1 presents C-O stretching in phenols, alcohols, acids, ethers and esters. These groups participate in MB adsorption to TSAC. The surface of carbon materials is, in general, rich in a variety of surface functional groups among which the C–O type groups are predominant and they form in ethers, acids and esters. FTIR analysis confirmed the presence of carboxylic and phenolic surface groups on the TSAC. There are hydroxyl groups (3388 cm–1, O–H stretching mode) present in the IR spectrum of the functionalized TSAC.
Scanning electron microscope analysis of activated carbon: The SEM micrograph of TSAC before adsorption is shown in Fig. 2. It is seen that TSAC has a highly porous structure.
Effect of initial aqueous pH on adsorption: In TSAC system, the increase of pH of the solution increases the adsorption of dyes from solution (Fig. 3).
Effect of carbon dosage: It was observed from the graphs that increasing the dosage increases the percentage removal of methylene blue (Fig. 4). As there was no drastic increase in the adsorption rate on increasing the dosage of adsorbent beyond 1.0 g of activated carbon, hence, from economic point of view, 1.0 g was taken as optimum dosage for removal of methylene blue. It can be attributed to the increase in adsorbent sites for more adsorption of the dye at the fixed 20 ppm.
Adsorption Isotherms: When log qe was plotted against log Ce a straight line was obtained with slope log k and y-intercept. From the Fig. 5, Freundlich isotherm is less fitted to the present adsorption study due to the less correlation coefficient value of 0.945. A plot of Ce/qe against Ce over the entire concentration range is a straight line with a slope of 1/q0 and the intercept of 1/q0b. A straight line was obtained when Ce/qe was plotted against Ce with a correlation coefficient (R2) of 0.999 indicating that the present study of data follow Langmuir isotherm (Fig. 6).
| Fig. 2: | SEM image of TSAC |
| Fig. 3: | Effect of pH on TSAC-MB system |
| Fig. 4: | Effect of carbon dosage on TSAC-MB system |
| Fig. 5: | Freundlich adsorption isotherm |
| Fig. 6: | Langmuir adsorption isotherm |
Kinetic study: The kinetics of dye sorption on the TSAC activated carbon as adsorbent was analyzed using two kinetic models. The linearized form of pseudo-first-order kinetic model and the pseudo-second-order adsorption kinetics can be expressed as shown in Fig. 7 and 8.
DISCUSSION
Many industries, such as dyestuffs, textile, paper, plastics, tannery and paint use dyes to colour their products and also consume substantial volumes of water. The presence of very small amounts of dyes in water (less than 1 ppm for some dyes) is highly visible and undesirable21. According to Chakrabarti et al.22 nearly 40,000 dyes and pigments are listed, which consist of more than 7000 different chemical structures23. Most of them are completely resistant to biodegradation processes24. Over 100,000 commercially available dyes exist and more than 7×105 t are produced worldwide annually25,26. Approximately 20% of these lost dyes enter the industrial wastewaters27. Dyes are either adsorbed through the mechanism of physisorption or chemisorption. Adsorption mechanism involves charge transfer from a charged molecule to charges surface (physisorption) or the transfer of electron from the donor molecule (adsorbate or adsorbent) to the acceptor molecule (adsorbent or adsorbate). Therefore, adsorption of dyes can be considered as a process that involves the creation electrophilic and nucleophilic centers28. Cationic dyes possess a positive charge on its molecule and are also called as basic dyes. They are usually found along with a ZnCl2 or HCl complexes29. Some of these dyes on which extensive studies have been carried out are methylene blue, malachite green, rhodamine 6G, basic yellow30-32. All these dyes are known to be toxic, capable of causing mutation, carcinogenic in nature. These dyes are capable of imparting colour to water and are soluble with it33.
| Fig. 7: | Pseudo-first-order kinetic model for removal of MB dye by TSAC |
| Fig. 8: | Pseudo-second-order kinetic model for removal of MB dye by TSAC |
Prolonged exposure to methylene blue can cause eye irritation, tachycardia, nausea, vomiting, slower the mental growth or retardation29,33. This pollutant too is known for having carcinogenic properties and cause mutation. Dyes are widely used in our environment and the possibility of experiencing its toxic impact (especially in aqueous media), any method that can reduce the concentration of dye effluence before they are discharge to the environment will contribute to the improvement of environmental quality especially when the process is ecofriendly. Therefore, the present study is aimed at investigating the potential of teak wood biomass for the adsorption of methylene blue (MB) from aqueous solutions. From the Fig. 3, the initial rate of adsorption is mainly decided by the protons release from carbon surface due to ion exchange process, which is confirmed by the measurement of pH of the solution after the adsorption process. At acidic pH, the adsorption of the dye is low up to pH 7 and then the adsorption capacity of TSAC is found to be higher at pH 9. At higher pH the protons are neutralized with OH ions, so the adsorption process increases. From the Fig 4, it was observed that the percentage adsorption of MG dye using TSAC is increased with an increase of the carbon dosage due to the increase of the activated sites available for adsorption. In TSAC, the increase in initial concentration of dye solution increased the absorption capacity. From the Fig. 5 and 6, it is very clear that adsorption of basic dye-methylene blue on TSAC follows Langmuir model.
This confirms that the removal of basic dyes from aqueous solution by activated carbon obtained from teak wood waste biomass gives monolayer coverage and the adsorption is mainly due to chemisorptions. Also, to elucidate of adsorption mechanisms was tested kinetics such as pseudo first-order and pseudo second-order and data. The adsorption kinetics well described by second order model with R2 = 0.996 that supported with Ghaedi et al.16 for kinetic and isotherm study for removal of Methylene blue34. Aboua et al.35 and Aljeboree et al.36 studies also supported the present work, that the adsorption on the studied dyes follows second-order kinetics. The isotherms studies indicate that the Langmuir isotherm was best fit with experimental data. Different kinetic models were also examined and the results indicate that the adsorption kinetics follow the pseudo-second-order rate. Generally, for cationic dye adsorption pseudo second order represents the system better29. From the desorption and recycling efficiency studies, 86% of MB was desorbed in 60 min using H2SO4 as a desorption medium (Fig. 9). Recycling efficiency of TSAC was investigated for the removal of MB. After five cycles, the adsorption efficiency of TSAC was reduced to 53 from 86% (Fig. 10). After every cycle, NaOH was used as a desorption medium to remove adsorbed MB ions from the TSAC surface. This study confirmed that the TSAC has comparatively high adsorption capacity and can be used as cost effectiveness natural absorbent.
| Fig. 9: | Desorption studies of MB dye |
| Fig. 10: | Recycling efficiency of TSAC for MB removal |
CONCLUSION
The present study indicates that activated carbon obtained from teak wood waste biomass is an effective adsorbent of methylene blue dye. The extent of adsorption decreases with increase in initial dye concentration, increases with increase in adsorbent dosage, adsorption time and pH. For adsorption isotherm, Langmuir Isotherm is more accurate because regression coefficient was R2 = 0.999 as comparison to R2 = 0.945 for Freundlich Isotherm and the rate of sorption was found to obey pseudo-second order kinetics with a good correlation coefficient. Hence, after carrying out rigorous experiments we finally came to conclusion that activated carbon prepared from Teak wood biomass can be effectively used for the removal of methylene blue effectively.
SIGNIFICANT STATEMENT
The present study was designed to investigate the adsorption capacity of teak wood activated carbon (TSAC) for methylene blue dye. The results and findings of the study conform that teak activated carbon is a good adsorbent for the removal of methylene blue dye from aqueous solution. The removal efficiency of TSAC for methylene blue dye is favoured by the present of suitable functional groups and the adsorption capacity can be optimized by taking advantage of period of contact, concentration of the adsorbate, mass of the adsorbent, pH. The adsorption of methylene dye into TSAC fitted pseudo second order kinetic model and Langmuir adsorption isotherm. Therefore, TSAC is an excellent adsorbent for the removal of methylene blue. This study will help the researchers in identifying low cost activated carbon for the removal of textile dyes in future. Thus a new theory of effluent treatment may be arrived at.
ACKNOWLEDGMENT
Authors sincerely thank St. Joseph’s College, Trichy, Tamil Nadu for FTIR spectroscopy.