Study on methylene blue adsorption using cashew nut shell-based activated carbon published by Ural Federal University eISSN 2411-1414 chimicatechnoacta.ru ARTICLE 2023, vol. 10(4), No. 202310401 DOI: 10.15826/chimtech.2023.10.4.01 1 of 8 Study on methylene blue adsorption using cashew nut shell-based activated carbon Hai D. Tran a , Dinh Quan Nguyen bc * a: Faculty of Environment, Ho Chi Minh University of Natural Resources and Environment, Ho Chi Minh city 700.000, Vietnam b: Laboratory of Biofuel and Biomass Research, Faculty of Chemical Engineering, Ho Chi Minh City University of Technology (HCMUT), 268 Ly Thuong Kiet, District 10, Ho Chi Minh City 700.000, Vietnam c: Vietnam National University Ho Chi Minh City, Linh Trung Ward, Thu Duc District, Ho Chi Minh City 700.000, Vietnam * Corresponding author: ndquan@hcmut.edu.vn This paper belongs to a Regular Issue. Abstract Adsorption is a widely used technique for the treatment of wastewater containing dyes, which are pollutants that can have serious impacts on the aquatic ecosystems. In this work, activated carbon (AC) was prepared from cashew nut shell (CNS) and used to adsorb methylene blue (MB) from solu- tion. The CNS AC was characterized by scanning electron microscopy, Fou- rier transform infrared spectroscopy, and nitrogen adsorption-desorption isotherms. The adsorption behavior of MB on CNS AC was investigated by varying the initial solution pH, adsorbent dosage, and initial MB concentra- tion. The results showed that the CNS AC was effective for MB removal, with an adsorption capacity of 24.8 mg/g. The adsorption nature of MB on- to the CNS AC surface was explored by analyzing the experimental data us- ing isotherm and kinetic models. The Freundlich and Dubinin- Radushkevich (D-R) isotherm models showed good agreement with the ex- perimental adsorption equilibrium results. The mean adsorption energy was found to be 22.4 kJ/mol, indicating chemical adsorption. The adsorp- tion of MB on the CNS AC followed pseudo-second-order kinetics. This study demonstrates the potential application of CNS AC for MB removal. Keywords dye removal adsorption activated carbon isotherm kinetics bio-adsorbent Received: 07.07.23 Revised: 31.07.23 Accepted: 31.07.23 Available online: 07.08.23 Key findings ● The porous and rough surface of the CNS AC is rich in functional groups containing electron donors. ● The equilibrium of MB adsorption onto the CNS AC is in good agreement with the Freundlich and D-K isotherms. ● The adsorption kinetics correspond to the pseudo-second-order. © 2023, the Authors. This article is published in open access under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction Due to the hazardous nature of common dyes, such as their poor biodegradability, toxicity, stability, and coloration, the presence of dye pollutants in water has caused major envi- ronmental problems [1, 2]. Various techniques have been developed for dye removal, including biological treatment, chemical oxidation treatment, advanced oxidation process- es, catalytic degradation, physical processes, or hybrid sys- tems [3–6]. As a prominent method among physical pro- cesses, adsorption is considered economically feasible be- cause of its highly removal-effective, operating-cost- effective, versatile, and available properties [7–9]. A large amount of CNS is being discharged in Vietnam, which poses a challenge for its management [10]. The high content of organic compounds in the CNS suggests that it should be considered as a raw material for the preparation of AC. The CNS AC exhibited a potential adsorbent for a variety of pollutants, including methylene blue (MB) [11– 14], brilliant green dye [15], and Congo red dye [16]. The CNS AC can be prepared by chemical activation using ZnCl2 [13], KOH [11, 14, 15], or H3PO4 [17]. Acid impregna- tion can be applied in the protocol of AC preparation to improve the adsorption capacity of AC via the formation of functional groups on the AC surface [18–21]. However, to the best of our knowledge, the CNS AC prepared after the http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2023.10.4.01 mailto:ndquan@hcmut.edu.vn https://www.sciencedirect.com/topics/earth-and-planetary-sciences/electron-microscope https://www.sciencedirect.com/topics/engineering/fourier-transform https://www.sciencedirect.com/topics/engineering/fourier-transform http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0002-0103-8866 https://orcid.org/0000-0001-8906-9773 https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2023.10.4.01&domain=pdf&date_stamp=2023-08-07 Chimica Techno Acta 2023, vol. 10(4), No. 202310401 ARTICLE 2 of 8 DOI: 10.15826/chimtech.2023.10.4.01 acid impregnation has not been fully investigated for its dye adsorption performance. This study aims to fill this gap by investigating the adsorption of MB on CNS AC that was prepared by HNO3 activation. The influence of exper- imental parameters on MB removal was explored. The analysis of isotherm and kinetic results revealed the na- ture of MB adsorption on the CNS AC. 2. Materials and methods 2.1. Preparation and characterization of CNS AC The CNS AC was prepared in two steps. First, the raw CNS, which had been mechanically squeezed to separate oil, was carbonized at 500 °C for 2 h to produce CNS char- coal. This charcoal was then crushed in a ball mill for 20 min. Second, the obtained charcoal was dispersed in an abundant amount of 0.5 M nitric acid (HNO3) under ultra- sonication for 15 minutes and then kept overnight. Solid particles were collected and neutralized by washing and filtrating through an 11-m filter paper (Whatmann No. 1) with distillated water. The collected solid was dried at 100 °C for 1 h. The activation process was carried out at 800 °C for 30 min in an electric furnace, which was purged with nitrogen gas to create an inert atmosphere. The functional groups and morphological structure of CNS AC were explored by Fourier transform infrared spec- troscopy (FTIR) and scanning electron microscopy (SEM) using a Prisma E SEM system and an iS5 Nicolet FTIR spectrometer, respectively. Nitrogen adsorption- desorption isotherms were performed with a Nova 2200e® system (Quantachrome Instruments, USA) to determine the pore structure. 2.2. Preparation of MB solutions A stock MB solution of 1000 ppm was prepared by dissolv- ing 1.0 g of MB (Merck) in 1 L of distilled water. The MB solutions at the desired concentrations were obtained by diluting an appropriate volume of the stock solution. A solution of 0.1 M NaOH or 0.1 M HCl was used to adjust the pH of the MB solution. 2.3. Adsorption experiments Batch adsorption experiments were conducted in a 500-ml Erlenmeyer flask, which was tightly attached with a silicone rubber stopper. The flask contained 350 ml of MB solution. A small glass tube (with an outer diameter of 4 mm) for sampling was inserted through this stopper. A magnetic stirrer was used to agitate the solution at 150 rpm. The effects of solution pH, CNS AC dosage, and initial MB concentration on adsorption performance were inves- tigated at a temperature of 300 K. To reach equilibrium adsorption, the experiment runs were carried out for 3 h. Then, samples were collected and filtered through a 0.45 m membrane to determine the equilibrium concen- tration of MB. For the kinetic study, MB adsorption was performed using the same procedures. The MB concentrations after different contact periods were measured by UV-Vis spec- troscopy at wavelength of 664 nm using an EvolutionTM 350 UV-Vis spectrophotometer. Equation 1 was used to calculate the amount of ad- sorbed MB at time t, qt (mg/g). ( )0 t t C C V q m − = (1) where, C0 and Ct are the concentrations of MB at initial and at any time t (mg/L), V is the volume of the solution (L), and m is the weight of CNS AC employed (g). The adsorption experiment was triplicated to evaluate the accuracy of the results. 3. Results and Discussion 3.1. Morphology of CNS AC The CNS AC surface has a porous and rough morphology, as shown in Figure 1. This morphology is characterized by large and interconnected open channels, which were formed by the decomposition and evaporation of volatile matter during thermal treatment [22]. This structure is proposed to facilitate the transfer of MB molecules from solution to adsorption sites [23], resulting in a fast ad- sorption rate. 3.2. Analysis of porous structure Nitrogen adsorption and desorption measurements were used to characterize the porous structure of CNS AC. The hysteresis loop in the relative pressure range of 0.3–1.0 (Figure 2) is a sign of Type IV isotherm according to the IUPAC classification, indicating the presence of mesopores in the CNS AC structure [24, 25]. This is further supported by the BJH pore size distribution (inset in Figure 2), which shows that the pore diameters are less than 4 nm, with micropores (< 2 nm) being the dominant pore type. The specific surface area was calculated to be 472 m2/g using the Brunauer-Emmett-Teller method. Figure 1 SEM image of the CNS AC. https://doi.org/10.15826/chimtech.2023.10.4.01 Chimica Techno Acta 2023, vol. 10(4), No. 202310401 ARTICLE 3 of 8 DOI: 10.15826/chimtech.2023.10.4.01 3.3. Characterization of the surface chemistry The FTIR spectrum (Figure 3) displayed characteristic peaks, which provide information on the functional groups on the surface of CNS AC. The band at 3400 cm−1 indicates the O−H stretching vibration in water molecules. The C−H sp3 stretching bond of alkane was assigned as the cause of the appearance of the peak at 2921 cm−1 [26, 27]. The sign of C−H aliphatic stretching was illustrated by a band at 2787 cm−1 [28]. The vibration of CC stretching in alkyne groups was reflected by a band at 2285 cm−1. The bands at 1632 and 731 cm−1 correspond to C=O and C−O stretching in aromatic carboxyl, phenolic ester, and conjugated ke- tonic structures [26]. C−O vibration in alcohol groups is related to a band at 1158 cm−1. The peak at 1050 cm−1 iden- tified C−C bending in the aromatic ring [29]. The presence of the groups containing  bonds (CC, C=O) and the Lew- is base groups (C=O, C−O) is proposed to promote the in- teraction of cationic MB molecules with the CNS AC sur- face [30–32]. 4. Batch adsorption studies 4.1. Effect of solution pH The pH of the solution plays an important role in the ad- sorption of dyes [32]. As shown in Figure 4, the MB re- moval and qe increased with increasing solution pH from 1 to 5. This is because (1) the functional groups on the CNS AC surface become more negatively charged as the pH val- ue increases [33], and (2) MB is deprotonated to form a positively charged species if the solution pH is higher than the pKa of MB (5.85) [34]. As a result, the electrostatic attraction between the MB molecules and the CNS AC sur- face is enhanced with increasing solution pH, leading to improved MB adsorption. However, at solution pH values above 5, the competitive adsorption of hydroxide ions (OH–) with MB molecules begins to occur, leading to a gradual decrease in MB removal [35]. Therefore, the sub- sequent adsorption experiments were performed at pH 5, where the maximum MB removal and qe were achieved. 4.2. Effect of CNS AC dosage The adsorption of MB onto CNS AC was investigated at different adsorbent dosages (0.5, 1.0, 1.5, and 2.0 g). As shown in Figure 5, MB removal increased with increasing adsorbent dosage, reaching a plateau at 1.5 g. This is be- cause the higher adsorbent dosage provides more adsorp- tive sites for MB molecules to bind to [36]. At 2.0 g of the CNS AC dosage, the MB removal was not significantly higher than at 1.5 g. The adsorption capacity decreased from 42.6 to 14.1 mg/g as the CNS AC dosage increased from 0.5 to 2.0 g. The excess of adsorptive sites at higher dosages led to an unsaturated adsorption surface, result- ing in a decrease in adsorption capacity [37]. This trend was previously reported for MB adsorption on various bi- omass-based adsorbents [38–40]. 4.3. Effect of initial MB concentration The initial dye concentration affects the dye adsorption performance, which is related to the interfacial process [41]. As shown in Figure 6, when the initial MB concentra- tion was increased from 20 to 90 mg/L, the MB removal decreased from 95.7 to 41.3%, while the adsorption capac- ity increased from 12.8 to 24.8 mg/g. Figure 2 Nitrogen adsorption-desorption isotherms (the inset: pore-size distribution). Figure 3 FTIR spectrum of the CNS AC. Figure 4 Effect of initial pH (conditions: C0 = 30 mg/L, m = 1 g, t = 3 hours). https://doi.org/10.15826/chimtech.2023.10.4.01 Chimica Techno Acta 2023, vol. 10(4), No. 202310401 ARTICLE 4 of 8 DOI: 10.15826/chimtech.2023.10.4.01 The higher the MB concentration, the greater the driving force and collision probability between MB cations and adsorptive sites [38, 41]. However, increasing the initial MB concentration causes a denser MB accumulation on the CNS AC surface, which leads to faster saturation of ad- sorptive sites [42]. Therefore, the adsorption capacity in- creased with increasing initial MB concentration, while the MB removal changed in the opposite direction. This behavior is consistent with previous reports [43, 44]. 4.4. Adsorption isotherms The nature and mechanism of the interaction between MB molecules and the CNS AC surface can be explored by an adsorption isotherm study [45]. In this study, the equilibri- um data were analyzed according to the Langmuir, Freun- dlich, and Dubinin-Radushkevich (D-R) isotherms, which were mathematically expressed as equation 2−4 [46]. = + max 1 L e e L e q K C q K C (2) = 1/n e F e q K C (3) − = 2 max DR K e q q e (4)    = +     1 1 e RT C (5) where Ce and qe are the MB concentration (mg/L) and ad- sorption capacity (mg/g) at equilibrium, respectively, qmax is the theoretical maximum adsorption capacity (mg/g), KL is the Langmuir isotherm constant (L/mg), KF is the Freundlich isotherm constant (mg/g (mg/L)n), n repre- sents the adsorption intensity, KDR is the activity coeffi- cient related to the mean sorption energy (mol2/J2),  is the Polyani potential (J2/mol2), R = 8.314 (J/mol/K) is the gas constant, and T = 300 K is the solution temperature. The constant parameters for the selected isotherm models were determined by nonlinear fitting to the exper- imental data and are presented in Table 1. Figure 7 shows the relationship between the qe and the Ce from the exper- iment (desired points) and regression (solid plots). The closeness between the isotherm model and exper- imental results was evaluated using the correlation coeffi- cient, R2. The Langmuir isotherm model assumes a mono- layer coverage of adsorbate onto a homogenous adsorbent surface [47]. The R2 value of 0.8980 (Table 1) for the Langmuir model indicates that it is unsatisfactory for de- scribing the equilibrium data obtained. This suggests that the adsorption of MB onto the CNS AC surface occurs in multilayers. This may be due to the presence of electron donors via functional groups. The Freundlich and D-R isotherm models provide bet- ter fits with the experimental data compared to the Langmuir isotherm model, as indicated by higher R2 val- ues in Table 1. Equilibrium data following Freundlich implies a multilayer adsorption of the adsorbate onto a heterogeneous adsorbent surface where there is a non- uniform distribution of the adsorption energy and affini- ties [45, 48]. The Freundlich isotherm model yielded a KF value of 14.7 mg/g (mg/L)n, which is higher than previ- ous report [49]. The high KF value reflects a high adsorp- tion capacity. The n value was determined to be 7.3 ac- cording to the Freundlich isotherm model. For n > 1, the greater the n value, the greater the adsorption intensity [45]. This resulted in a favorable adsorption for increas- ing the adsorption capacity. The KF and n values are con- sistent in explaining the mechanism of MB adsorption onto the CNS AC. Figure 5 Effect of CNS AC dosage (conditions: C0 = 30 mg/L, pH 5, t = 3 hours). Figure 6 Effect of initial concentration of MB (conditions: m = 1.5 g, pH 5, t = 3 h). Table 1 The calculated parameters for the selected isotherm models. Isotherm model R2 Calculated parameter Langmuir 0.8980 qmax = 23.28 KL = 1.32 Freundlich 0.9295 KF = 14.14 n = 7.27 Dubinin-Radushkevich 0.9316 qmax = 39.52 KDR = 10.3910 −10 https://doi.org/10.15826/chimtech.2023.10.4.01 Chimica Techno Acta 2023, vol. 10(4), No. 202310401 ARTICLE 5 of 8 DOI: 10.15826/chimtech.2023.10.4.01 The D-R isotherm considers micropores filled with ad- sorbate molecules and sorption energy at the adsorbent surface according to a Gaussian distribution [49]. As shown in the inset of Figure 2, the range of pore-size di- ameters in CNS AC is less than 4 nm, which is close to the effective diameter of the MB molecule (length of 1.45 nm, width of 0.95 nm) [50]. This allows MB molecules to transfer into the porous matrix and fill micropores. The good fit of the experimental data to the D-R isotherm (R2 = 0.932) supports this hypothesis. According to equa- tion 6, the mean adsorption energy (E) can be calculated to be 22.4 kJ/mol, which is higher than the threshold for chemical adsorption of 20 kJ/mol, implying the chemical adsorption of MB onto CNS AC [51, 52]. = 1 2 DR E K . (6) 4.5. Kinetic studies The dynamics of MB adsorption on CNS AC were investi- gated by nonlinear fitting of the experimental kinetic data with three common kinetic models: the pseudo-first-order (PFO), pseudo-second-order (PSO), and intra-particle dif- fusion (W-M) models. These models were expressed by equations 7−9 [53]. ( )−= − 11 k tt eq q e (7) = + 2 2 2 1 e t e q k t q k q t (8) − = + 0.5 t W M q k t c (9) where qt (mg/g) is the amount adsorbed of MB at contact time t, qe (mg/g) is the predicted adsorption capacity at equilibrium; k1 (1/min), k2 (g/mg/min), and kW−M (g/mg/min0.5) are rate constants of PFO, PSO, and W-M, respectively; and c (mg/g) represents the contribution of molecular diffusion through the boundary layer. The dependence of the adsorbed MB amount on the contact time is shown in Figure 8. The experimental re- sults (desired points) indicate that the MB was rapidly adsorbed onto the CNS AC in the first 60 min. This is due to the large BM concentration gradient between the bulk solution and the CNS AC surface. The obtained kinetic parameters are presented in Table 2. A comparison of the R2 values from the simulation of the three kinetic models suggests that the PSO model is the best fitting model to describe the kinetic behavior of MB adsorption on CNS AC. This indicates that the adsorp- tion rate is controlled by the chemical interaction between MB and the adsorptive sites on the CNS AC surface [42, 54]. The calculated adsorption capacity for MB (18.74 mg/g) according to the PSO model was in good agreement with the experiment result (18.65 mg/g). The adsorption mechanism of MB onto CNS AC was investigat- ed in a consensus of kinetic and isothermal analyses. 5. Limitations Experiments on competitive adsorption and applicability for real textile wastewater have not been conducted. 6. Conclusion In this work, Vietnamese CNS was activated with nitric acid to synthesize CNS AC. Figure 7 Isotherm plots for MB adsorption onto the CNS AC (con- ditions: m = 1.5 g, pH 5, t = 3 h). Figure 8 Adsorption kinetics of MB on CNS AC (C0 = 30 mg/L, m = 1.5 g, pH 5). Table 2 The calculated parameters for the selected kinetic models. Kinetic model R2 Calculated parameter PFO 0.9014 qe = 17.29 k1 = 0.11 PSO 0.9636 qe = 18.74 k2 = 9.1010 −3 W-M 0.8023 c = 6.07 kW−M = 1.19 https://doi.org/10.15826/chimtech.2023.10.4.01 Chimica Techno Acta 2023, vol. 10(4), No. 202310401 ARTICLE 6 of 8 DOI: 10.15826/chimtech.2023.10.4.01 The obtained CNS AC exhibited a porous structure with functional groups containing oxygen or -bonds, which were advantageous for the interaction with cationic mole- cules. The adsorption of MB on the CNS AC was performed at different solution pH values, adsorbent dosages, and initial MB concentrations. The experimental equilibrium data was consistent with Freundlich and D-R models, sug- gesting that the adsorption of MB onto CNS AC is a favora- ble process that occurs in multilayers on a heterogeneous surface. The mean adsorption energy was found to be 22.4 kJ/mol, confirming the chemisorption process. The kinetic results followed the pseudo-second-order kinetic model. The results provide valuable insights into the ad- sorption mechanism of MB on CNS AC, which can be used to optimize the adsorption process and improve the re- moval efficiency. ● Supplementary materials No supplementary materials are available. ● Funding This research had no external funding. ● Acknowledgments We acknowledge Ho Chi Minh City University of Technolo- gy (HCMUT), VNU-HCM for supporting this study. ● Author contributions Conceptualization: D.Q.N. Data curation: H.D.T. Investigation: H.D.T. Methodology: D.Q.N., H.D.T. Validation: D.Q.N. Writing – original draft: H.D.T. Writing – review & editing: D.Q.N. ● Conflict of interest The authors declare no conflict of interest. ● Additional information Author IDs: Hai D. Tran, Scopus ID 56226812800; Dinh Quan Nguyen, Scopus ID 15123160800; Websites: Ho Chi Minh University of Natural Resources and Envi- ronment, http://www.hcmunre.edu.vn/; Ho Chi Minh City University of Technology, http://www.hcmut.edu.vn/; Vietnam National University Ho Chi Minh City, https://vnuhcm.edu.vn/. References 1. Khan I, Saeed K, Zekker I, Zhang B, Hendi AH, Ahmad A, Ah- mad S, Zada N, Ahmad H, Shah LA. Review on methylene blue: Its properties, uses, toxicity and photodegradation. Wa- ter. 2022;14(2):242. doi:10.3390/w14020242 2. Lellis B, Fávaro-Polonio CZ, Pamphile JA, Polonio JC. Effects of textile dyes on health and the environment and bioremedi- ation potential of living organisms. 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