The Adsorption Properties of TEMPO Oxidized Cellulose against the Mixture of Methylene Blue and Rhemazol Yellow FG

Abbreviations

1  Introduction

Water, as a vital source of life, is essential for human beings, fulfilling their fundamental needs. However, with the progression of time and technology, various regions in Indonesia are grappling with the scarcity of clean water due to the rapid growth of industries, particularly the textile industry [1,2]. Rivers in the affected areas have become contaminated due to the discharge of dye waste from textile production processes [3,4].

The textile industry significantly impacts water pollution. Various processes within this industry, primarily the dyeing of fabric materials until the production of ready-to-use textiles, heavily rely on water and chemicals. These chemicals may include heavy metals, dyes, and phenolic compounds, which are often discharged directly into rivers, leading to increased pollution of natural water reservoirs [5–7].Pollution levels can be monitored through dynamics in parameters such as BOD, COD, TDS, TSS [8–10], and the presence of heavy metals like Cd, Cr, Ni, and Pb [8,11–14].

Numerous methods can be employed to reduce hazardous substances or xenobiotics in water environments, including biological treatment using microorganisms [15], membrane technology [16,17], photocatalysis [18–20], electrochemical [21,22] and adsorption [23–27]. Among these, adsorption processes have gained widespread popularity in waste treatment due to their cost-effectiveness, ease of operation, and capability to remove almost all harmful pollutants [28,29]. Several adsorbents, such as activated carbon, sponge, montmorillonite, zeolite, kaolin, and microbial-based adsorbents, can be used for color waste adsorption [5,30,31]. Another viable alternative for adsorption is bioadsorbents derived from agricultural waste [32–35]. Bioadsorbents offer several advantages, including low cost, easy availability, environmental friendliness, and high adsorption capacity [29,36,37]. Cellulose, a promising material for bioadsorbents in water purification [1,34,38,39], has limitations in color adsorption, as it cannot selectively adsorb specific color groups [28–30]. Colors can be classified into three groups based on their charge in the chemical structure: anionic, cationic, and non-ionic [40]. Therefore, surface modification of cellulose is essential to facilitate interactions with specific color groups. This modification advantageously allows the bound or adsorbed color to be reused in subsequent applications.

Various methods can enhance the ability of cellulose to absorb specific dye groups, including oxidation using periodate, potassium permanganate, and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) [40–42]. Currently, TEMPO oxidation is widely used for oxidizing cellulose surfaces due to its relatively straightforward process, resulting in the incorporation of numerous carboxylate groups onto the cellulose surfaces. Moreover, TEMPO oxidation can be conducted in water under mild conditions [41–43]. The selective nature of TEMPO oxidation allows for the transformation of hydroxyl groups at the primary C6 position of anhydroglucose units into carboxyl groups on the cellulose surface [44]. Consequently, surface modification of cellulose using TEMPO increases the negative charge content on the cellulose surface, enabling the adsorption of positively charged dye substances. During TEMPO oxidation, several chemicals are normally added as supporting materials, such as NaBr and NaOCl, which are utilized as a co-catalyst and oxidizing agent, respectively. Several studies have evaluated the effect of varying amounts of NaBr and NaOCl on the properties of oxidized cellulose, such as opacity and mechanical properties, which depend on the carboxyl content after TEMPO oxidation. Zhang et al. conducted a study on the effect of NaOCl concentration on TEMPO oxidation, and the resulting fiber was subsequently treated with periodate for further oxidation. The adsorption capacity of the treated fiber was evaluated against chrysoidine and hydrazine [45]. However, the presence of multiple oxidation processes prevents a detailed understanding of the effect of NaOCl concentration on its performance as an adsorbent for chrysoidine and hydrazine due to the subsequent periodate oxidation. Another study by Zhu et al. varied the concentration of NaOCl during TEMPO oxidation and evaluated its performance for humidity sensing [46]. The results confirmed that the NaOCl concentration linearly affected the number of carboxylic groups, and the fiber treated with a high concentration of NaOCl showed the highest performance in humidity sensing.

Based on the theories mentioned above, there was no study that has been performed about the adsorption of the dyes mixture using TEMPO oxidized cellulose. This study focuses on developing oxidized cellulose using TEMPO/NaBr in the presence of NaOCl at concentrations of 5 and 30 mmol/g of fiber. The goal is to enhance the adsorption of a mixture of methylene blue and rhemazol yellow FG dyes. To support this, various tests were conducted, including Fourier Transform Infrared (FTIR) analysis, Scanning Electron Microscopy (SEM), and X-Ray Diffraction (XRD).

2  Materials and Methods

2.1 Materials

The materials used in this study include sodium hydroxide (NaOH, Glatt Chemical-Indonesia), sodium hypochlorite, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, Sigma-Singapore), sodium bromide (HiMedia Laboratories-India), deionized water, rhemazol yellow FG (Aman Semesta Enterprise-Malaysia), methylene blue (Sigma-Singapore), hydrochloric acid (BDA Chemical-Indonesia), sodium acetate (Sigma-Singapore), phenolphthalein indicator (Arkitos Chemical-Indonesia), and cellulose (Avicel PH 101, Sigma-Singapore).

2.2 Oxidation of Cellulose Using TEMPO/NaOCl/NaBr

Approximately 0.08 g of TEMPO was dissolved in 500 mL of deionized water, and 5 g of cellulose was added to the TEMPO solution. Sodium bromide (0.05 g) and sodium hypochlorite (NaOCl) at 5 mmol/g were added to the cellulose suspension. The pH of the solution was maintained at 10–11 by adding 5 M sodium hydroxide (NaOH). The cellulose suspension was stirred for 6 h at room temperature, and then the TEMPO-oxidized cellulose was washed with deionized water. The obtained oxidized cellulose was dried at room temperature. The same procedure was repeated with NaOCl at a concentration of 30 mmol/g [47].

2.3 Acid-Base Titration

Approximately 0.1 g of TEMPO-treated cellulose was weighed and acidified using 15 mL of 0.01 M HCl for 1 h. The TEMPO-oxidized cellulose was neutralized by rinsing it with deionized water. The oxidized cellulose was mixed with 50 mL of deionized water and 30 mL of 0.25 M sodium acetate and stirred for 2 h. Then, 30 mL of the mixture was added with one drop of phenolphthalein indicator and titrated using 0.01 M NaOH [48].

2.4 Adsorption Test

Cellulose and TEMPO-oxidized cellulose were subjected to adsorption experiments using several parameters, including pH, contact time, initial concentration, recyclability, and dye separation. The samples were then separated using a centrifuge at 4500 rpm for 10 min. The obtained filtrate was tested using a UV-Vis spectrophotometer. The same procedure was repeated for methylene blue dye and binary mixtures [49,50].

2.5 Fourier Transform Infrared (FTIR) Analysis

FTIR characterization was conducted to analyze the characteristic functional groups of the cellulose samples. Approximately 1 mg of the sample was mixed with 9 mg of KBr, then placed into a press holder and pressed to form a thin pellet. The pellet was inserted into the FTIR compartment (Bruker ALPHA II Compact). FTIR analysis was performed by measuring the transmittance in the wavenumber range of 4000–500 cm−1 at a resolution of 4 cm−1.

2.6 Scanning Electron Microscopy (SEM)

The surface morphology of the cellulose and TEMPO-oxidized cellulose powder samples was observed using a Scanning Electron Microscope (SEM TM3000). The samples were coated with gold and then inserted into the specimen chamber. The samples were observed at various magnifications to understand their morphology.

2.7 X-Ray Diffractions (XRD)

XRD analysis was conducted to determine the phase structure and purity of the samples using XRD Rigaku Smartlab. The analysis was performed on cellulose and TEMPO-oxidized cellulose samples. Crystallinity testing was carried out by comparing the distances on the crystal planes and the peak intensities of the diffraction patterns with standard data. The crystallinity index was calculated using the following equation:

Crystallinity index = (crystalline intensity − amorphous intensity)/crystalline intensity × 100%

Data were obtained using an X-ray diffractometer with a Cu Kα radiation source (λ = 1.54060 Å), a scan speed of 1°/0.05 sec, and an initial and final 2θ angle range of 5°–80°.

3  Result and Discussions

The surface modification of cellulose using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)/NaOCl/NaBr was successfully carried out. TEMPO, a water-soluble compound, acts as a catalyst to oxidize primary hydroxyl groups into aldehyde and carboxylate groups [41–43,47]. Previous studies have documented that increasing the quantity of NaOCl leads to a higher concentration of carboxyl groups on the cellulose surface [47,51]. In this study, two concentrations of NaOCl (5 and 30 mmol/g) were tested for the surface modification of cellulose. The acid-base titration method was used to determine the carboxyl group content in the TEMPO-oxidized cellulose. The amount of carboxyl groups in the TEMPO-oxidized cellulose was found to be 0.5160 mmol/g for the 5 mmol/g NaOCl variation and 1.8461 mmol/g for the 30 mmol/g NaOCl variation. Both cellulose and TEMPO-oxidized cellulose samples with NaOCl concentrations of 5 and 30 mmol/g were analyzed using FTIR to identify the functional groups present. The FTIR spectra are presented in Fig. 1.

Figure 1: FTIR spectra of cellulose and oxidized cellulose samples

The FTIR spectrum depicted in Fig. 1 illustrates the functional groups present in both the cellulose and TEMPO-oxidized cellulose samples. Despite variations in peak shapes observed among the samples, the resulting spectra exhibit no significant differences. Analysis of the cellulose spectrum reveals the presence of the O-H functional group at 3332 cm−1, with the peak at 2892 cm−1 attributed to C-H stretching, and the band at 1028 cm−1 indicating C-O stretching. In the spectrum of the TEMPO-oxidized cellulose sample, a distinct peak at 1602 cm−1 is observed, corresponding to the carbonyl vibration of carboxylate in its sodium form [5,52–55].

The XRD analysis of cellulose and TEMPO-oxidized cellulose samples treated with NaOCl at concentrations of 5 and 30 mmol/g aimed to identify the phases present in the samples. The XRD results are presented in Fig. 2.

Figure 2: Diffractogram of cellulose and oxidized cellulose

The diffractogram results shown in Fig. 2 confirm that the analyzed cellulose sample belongs to cellulose I, consistent with prior research [56,57]. A shift in the diffraction peaks of the TEMPO-oxidized cellulose sample compared to cellulose is observed. Cellulose and TEMPO-oxidized cellulose at a concentration of 5 mmol/g exhibit crystallinity values of 75.94% and 81.14%, respectively. This increase in crystallinity is attributed to the reduction of amorphous regions on the cellulose surface resulting from its interaction with NaOCl. Oxidation reactions on the amorphous regions eliminate some of the amorphous areas on the cellulose surface. Aldehyde and carboxyl groups are found on both the crystalline and amorphous surfaces of TEMPO-oxidized cellulose, supported by previous studies [42,58,59]. Surface oxidation of cellulose using 30 mmol/g NaOCl results in a slight decrease in crystallinity to 80.15% compared to oxidized cellulose prepared with 5 mmol/g of cellulose. The reduction in cellulose crystallinity is attributed to the increased number of carboxyl groups on the cellulose surface, as indicated by titration results of approximately 1.8461 mmol/g. Several previous studies have reported similar findings regarding the decrease in crystallinity index [54,60–63].

The surface morphology of cellulose and TEMPO-oxidized cellulose samples treated with NaOCl at concentrations of 5 and 30 mmol/g was observed using SEM to determine the structural morphology of the cellulose surface. As shown in Fig. 3, the morphology of cellulose and TEMPO-oxidized cellulose does not differ significantly, and in general, cellulose particles appear to form aggregates. The width dimensions of cellulose and TEMPO-oxidized cellulose were measured using ImageJ 1.8.0. Cellulose exhibited an average width dimension of approximately 8 µm, whereas oxidized cellulose at concentrations of 5 and 30 mmol/g NaOCl displayed decreased average width dimensions of 6 and 4 µm, respectively.

Figure 3: Morphology of (a) cellulose, (b) oxidized cellulose 5 mmol/g NaOCl, and (c) Oxidized cellulose 30 mmol/g NaOCl

The adsorption experiment focused on TEMPO-oxidized cellulose prepared with 30 mmol/g of NaOCl due to its highest carboxylic content. The presence of carboxylic groups, confirmed through titration, enhances the adsorption capability of TEMPO-oxidized cellulose against cationic dyes, such as methylene blue. Electrostatic interactions play a significant role in the adsorption of methylene blue on the surface of TEMPO-oxidized cellulose (Fig. 4). Fig. 5 illustrates van der Waals interactions occurring during the adsorption of rhemazol yellow on the surface of TEMPO-oxidized cellulose. Various parameters were utilized in the adsorption experiments, including pH, contact time, and initial concentration.

Figure 4: Electrostatic interaction between methylene blue and TEMPO oxidized cellulose

Figure 5: Van der Waals interaction between RY and TEMPO oxidized cellulose

The surface charge of TEMPO-oxidized cellulose was influenced by the pH value of the dye. This study aimed to evaluate the effect of pH on the adsorption performance of TEMPO-oxidized cellulose. The experiments were conducted at pH 5–9 for rhemazol yellow FG and methylene blue. The adsorption capacity of rhemazol yellow FG and methylene blue was determined in separate chambers. Fig. 6 illustrates the adsorption capacity of rhemazol yellow FG and methylene blue. The anionic dye, rhemazol yellow FG, exhibited the highest adsorption capacity in acidic conditions due to the protonation of carboxylic groups. This phenomenon results in the positive charge on the surface of TEMPO-oxidized cellulose. Conversely, under alkaline conditions, the hydrogen from carboxylic groups is deprotonated, resulting in a negative charge on the surface of TEMPO-oxidized cellulose. This explains the highest adsorption capacity of methylene blue (cationic dye) under alkaline conditions.

Figure 6: The pH effect on the adsorption capacity of TEMPO oxidized cellulose against (a) methylene blue; (b) rhemazol yellow FG

Contact time during adsorption plays an important role in determining the maximum adsorption capacity at a specific contact time suitable for each dye. In this experiment, the adsorption process was evaluated using pseudo-first and second-order models. Table 1 presents the kinetic parameters of each process. According to Table 1, the adsorption process of methylene blue and rhemazol yellow FG followed the pseudo-second-order model, as concluded from the highest correlation coefficient (R2) value from each experiment.

The initial concentration is one parameter used to determine the adsorption capacity of the adsorbent. The different values of the initial concentration of the adsorbate would affect the optimum contact time for reaching equilibrium. The adsorption capacities at equilibrium have a linear correlation with the initial concentration. Two isotherm models were used in this study: Langmuir and Freundlich isotherm models.

Fig. 7 and Table 2 present the results of the effect of initial concentrations and the results of Langmuir and Freundlich isotherm models. Data in Table 2 show that the adsorption of methylene blue and rhemazol yellow FG on the matrix of TEMPO oxidized cellulose followed the Langmuir model, as concluded from the highest correlation coefficient (R2). The Langmuir model suggests that the adsorption process involves monolayer adsorption occurring on the entire surface of TEMPO-oxidized cellulose as the adsorbent [64–68]. The maximum adsorption capacities (Qm) for methylene blue and rhemazol yellow FG were approximately 601.25 and 320.44 mg/g, respectively. This result confirms that TEMPO oxidized cellulose has better adsorption capacities than adsorbents in previous studies [69–72].

Figure 7: The effect of concentration vs. adsorption capacities of TEMPO oxidized cellulose

The utilization of TEMPO-oxidized cellulose as the adsorbent could be employed for several cycles through repeated sorption and desorption processes. Fig. 8 illustrates the different behaviors of TEMPO-oxidized cellulose during the sorption and desorption of methylene blue and rhemazol yellow FG over several cycles. After 8 cycles, the sorption of methylene blue exhibited desirable adsorption capacities, approximately 95%. However, the adsorption capacities of rhemazol yellow FG decreased after several sorption–desorption cycles, reaching approximately 70%. The decrease in the adsorption capacities of rhemazol yellow FG could be caused by the decrease in the availability of binding sites [73–77].

Figure 8: Dye removal performance of TEMPO-oxidized cellulose after 8 cycles

Another experiment was conducted in this study, namely selective adsorption and intelligent separation. The adsorption of a mixed-color solution involves the simultaneous adsorption of methylene blue and rhemazol yellow FG. The adsorption test was conducted using three samples: cellulose and TEMPO-oxidized cellulose at concentrations of 5 and 30 mmol/g NaOCl. The results of the adsorption experiment for the mixed solution of Rhemazol Yellow FG and methylene blue are depicted in Fig. 9. The percentage adsorption of Rhemazol Yellow FG in the cellulose mixed-color solution was 61.77%. A decrease in the percentage adsorption was observed in the samples of oxidized cellulose at concentrations of 5 and 30 mmol/g NaOCl, with values of 58.32% and 59.52%, respectively.

Figure 9: Selective adsorption and intelligent separation of TEMPO-oxidized cellulose

The adsorption test results for methylene blue reveal that TEMPO-oxidized cellulose at 30 mmol/g NaOCl exhibits the highest adsorption at 80.17%. In contrast, the adsorption capacity of cellulose and TEMPO/NaOCl/NaBr-oxidized cellulose at 5 mmol/g was achieved at 60.27% and 75.07%, respectively. The higher content of negatively charged carboxylate groups in the TEMPO-oxidized cellulose samples facilitates interaction with positively charged colorants. This is attributed to the greater abundance of carboxylate groups compared to other samples, allowing for effective adsorption of the positively charged dyes onto the structure of the oxidized cellulose [31,55,78,79].

4  Conclusion

Oxidized cellulose was fabricated using the addition of TEMPO/NaOCl/NaBr, prepared at pH 10–11. The NaOCl concentration has a linear correlation with the carboxylic content of TEMPO-oxidized cellulose. The carboxylic content for NaOCl concentrations of 5 and 30 mmol/g was approximately 0.5160 and 1.8461 mmol/g, respectively. The successful oxidation of cellulose using TEMPO/NaOCl/NaBr was confirmed by the presence of the C = O group. TEMPO-oxidized cellulose has a higher crystallization index than cellulose. The maximum adsorption capacities of TEMPO-oxidized cellulose (30 mmol/g) against methylene blue and rhemazol yellow FG were approximately 601.25 and 320.44 mg/g, respectively. The recyclability of TEMPO-oxidized cellulose shows good performance for up to 8 cycles, with sorption capacities of methylene blue reaching up to 95%, although a decrease was observed for rhemazol yellow FG. Due to the presence of carboxylic groups on the surface of TEMPO-oxidized cellulose, the prepared adsorbent could separate the anionic and cationic groups. The degree of removal of anionic and cationic dyes depends on the amount of carboxylic content.

Declaration of Generative AI and AI-Assisted Technologies in the Writing Process: During the preparation of this work, the authors used Grammarly and OpenAI (GPT-3.5) in order to improve the language and readability. After using this tool/service, the authors reviewed and edited the content as needed and takes full responsibility for the content of the publication.

Acknowledgement: This study was supported by Institut Teknologi Sumatera through GBU 45 Research Grant.

Funding Statement: This study was supported by Institut Teknologi Sumatera through GBU 45 Research Grant with a Contract Number B/763c/IT9.C1/PT.01.03/2022.

Author Contributions: The authors confirm contribution to the paper as follows: study conception and design: I. Putu Mahendra, Kartika Dinita; data collection: Kartika Dinita; analysis and interpretation of results: I. Putu Mahendra, Kartika Dinita; draft manuscript preparation: I. Putu Mahendra, Kartika Dinita. All authors reviewed the results and approved the final version of the manuscript.

Availability of Data and Materials: The authors declare that the data supporting the findings of this study are available within the paper. Should any raw data files be needed in another format, they are available from the corresponding author upon reasonable request.

Conflicts of Interest: The authors declare that they have no conflicts of interest to report regarding the present study.

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