Studies on remediation of neutral red using water insoluble β-cyclodextrin (β-CD) polymers in aqueous solution

Widespread use of dyes in sectors including textiles, paper, plastics, leather, food, cosmetics, etc. causes environmental issues when dyes are discharged into water systems. Even in extremely low quantities, colors are highly visible and unwanted in the effluents from these sectors. Dumping colored waste into rivers not only diminishes the visual attractiveness of the water bodies but also hinders sunlight penetration, thereby restricting photosynthetic processes.¹ 

Dye molecules can be categorized as cationic, anionic, or non-ionic according to their ionic charge. Cationic dyes pose a greater risk compared to anionic dyes.² A short-term exposure to these dyes has been found to have negative impacts on living things. So four cationic dyes were chosen for this study.

The synthetic phenazine neutral red, also referred to as NR or 3-amino-7-dimethylamino-2-methylphenazine hydrochloride (Fig. 1), has been used for over a century in a variety of fields. This tricyclic aromatic amine incorporates two nitrogen atoms within its aromatic ring structure. Fused aromatic rings, as observed in this compound, often lead to characteristics such as high solid-to-water (S/W) ratio, low solubility in water, and resistance toward nucleophilic degradation. Nevertheless, the presence of C–N substitution in aryl compounds, exemplified by NR, facilitates water solubility, thereby improving its bioavailability.

Neutral Red (NR) is a cationic dye. It is widely used in the textile and leather industries and biological research. NR could potentially exhibit hazardous and mutagenic properties.³ Various techniques, such as microbiological methods, ion exchange, membrane filtration, ozonation, coagulation, and adsorption, are used to detect harmful ions in wastewater treatment. Among these, the adsorption method is gaining popularity due to its simplicity, cost-effectiveness, and high efficiency.^4–11 However, developing materials for adsorption that are effective, non-toxic, efficient, affordable, user-friendly, and capable of regeneration remains a significant challenge. The literature suggests that using natural, cost-effective materials that align with sustainability principles is a promising approach.^12–23 

Cyclodextrins (CDs) are cyclic oligosaccharides composed of six, seven, or eight anhydrous d-glucopyranose units connected by α (1,4) glycosidic bonds, commonly known as β-CDs, β-CDs, or γ-CDs. Cyclodextrins possess unique characteristics relevant to water treatment, as noted by researchers like Villiers. The interior surface of cyclodextrins is hydrophobic, with glycosidic oxygens and methylene proton groups forming non-covalent complexes.^24–27 In contrast, the exterior surface is hydrophilic due to hydroxyl groups. Cyclodextrins can be modified to produce various derivatives with applications in environmental research, pharmaceuticals, separation processes, life sciences, and the food industry.^28–32 

Adsorption using adsorbents like cyclodextrin-based polymers (CDPs) is an economical method, especially for the usage of wastewater, due to its likeness, convenience of use, and reasonable.^33–35 In the current study, we present a technique that unites β-Cyclodextrin (β-CD) with citric acid (CA), malic acid (MA), and tartaric acid (TA) to detach NR from aqueous solutions. Cyclodextrin possesses a porous and productive nature, accompanied by a substantial surface area. Its synthesis is straightforward, exhibiting remarkable thermal stability, easy chemical functionality, biocompatibility, and inherent non-toxicity. β-Cyclodextrin (β-CD)’s –OH groups on the outside allow for simple interactions with a variety of organic acids. While β-CD can be used for NR removal, it enhances the porosity and surface area of the compound, which improves removal effectiveness. To the best of our understanding, studies on the extraction of NR from aqueous solutions utilizing several organic acids in combination with β-Cyclodextrin (β-CD) remain unexplored.

The reagents were utilized without further purification from SD Fine (India). β-Cyclodextrin (β-CD) was 99+% pure, and citric acid (CA) was 98+% pure. All other compounds, including tartaric acid (TA), Neutral Red (NR), and malic acid (MA), were of analytical purity. 99+% pure was NR, 95+% pure was MA, and 98+% pure was TA. A precise quantity of dye fine particles was diffused in demineralized water to produce fixed compounds with a molarity of 1000 mg l⁻¹. After diluting the fixed solutions, functional solutions with an overview between 4 and 24 ppm and a pH regulated with 0.1 M HCl and 0.1 M NaOH were produced.

To attain homogeneity, dissolve 3 g of dehydrated citric acid, 6 g of β-cyclodextrin, and 1.5 g of KH₂PO₄ in an 800 ml beaker of deionized water. Mix well. After being cooked for 3.5 hours at 140 °C without stirring, the liquid was chilled. The crude product was allowed to come to room temperature before being purified by wringing it in 500 ml of liquid for five to six minutes. The product was then filtered (suctioned), dried, and heated to 50 °C. The ultimate polymer product of CA-β-CD, weighing 7.03 g and yielding 66.88%, underwent subsequent characterization and was stored under dry conditions at room temperature. Figure 2 provides an illustration of the synthesis process.

Scheme 35 was followed in the minimal adjustments that were made to the copolymerization of CD and MA. 6.75 g of CD (4.75 mmol), 3.8 g of Na₂HPO₄.12H₂O (10 mmol), and 3.82 g of MA (28 mmol) were all dissolved in a jar. The reaction mixture was intense to 100 °C for approximately sixty minutes. The final product was quickly placed on a Petri plate and desiccated in a dryer at temperatures of 160, 170, or 180 °C for durations of 60, 90, or 120 min, respectively. The initial material was then weighed and pulverized.

Expanding on the process described in the previous use of citric acid β-CD polymers, DL-tartaric acid (cross-linker) and β-CD were reacted with catalyst dihydrogen phosphate (MSP) to create TA-β-CD polymers. To obtain a homogeneous solution, 2.34 g of DL-Tartaric acid, 6 g of β-CD, and 0.75 g of KH₂PO₄ were dissolved in 135 ml of ultrapure water and stirred in an 800 ml beaker. After that, the mixture was moved and intensed, stirring not needed, to 140 °C for 3.5 h. The unprocessed product was allowed to come to room temperature before being repeatedly soaked and cleaned in deionized water, dried at 55 °C after being suction-filtered to get a uniform weight. The purified product’s weight and yield percentage were recorded. After being weighed, the product was powdered, and then it was characterized.

A range of NR concentrations in water (10–50 mg/l) were used for the adsorption experiments. A 100 ml beaker containing 20 ml of a dye 10 mg/l concentration solution was mixed with dosages of CA/β-CD, MA/β-CD, and TA/β-CD ranging from 10 to 120 mg. In an incubator, flasks were shaken for 1 h. The study investigated a number of variables, including temperature, contact times, initial dye concentration, adsorbent type, pH, and dose. Following the dye removal procedure, a calibration curve and the removal’s effectiveness were assessed using the supernatant solution, determining the maxima values for absorption at 617 nm of NR through UV–Vis spectroscopy. For every adsorbent, the standard deviation was calculated by repeating the dye deletion technique three times.

An apparent morphology study of CA/β-CD, both pre- and post-adsorption, was conducted using Scanning Electron Microscopy (SEM), as illustrated in Fig. 3. The analysis included samples at various magnifications and unlike parts. Figure 3(a) presents the exterior composition of CA/β-CD within the 10 μm array, displaying a U-shaped structure. Additionally, Figs. 3(b) and 3(c) depict the surface morphology within the 50 and 100 μm ranges, respectively, showcasing a spherical-shaped structure. The SEM images revealed the presence of porous-like structures on the material’s surface.

Surface morphology analysis of MA/β-CD, both pre and post-adsorption, was conducted using a SEM, as depicted in Fig. 4. The analysis involved samples at various magnifications and diverse regions. Figure 4(a) illustrates the exterior composition of MA/β-CD within the 20 μm array, displaying a leaf-like structure. Furthermore, Figs. 4(b) and 4(c) present the apparent morphology study of MA/β-CD within the 20 and 30 μm ranges, respectively, exhibiting a leaf-like arrangement with varying numbers of holes. The SEM images indicate the presence of porous-like structures on the material’s surface, proving beneficial for the adsorption experiments.

SEM of before and after adsorption of TA/β-CD, as shown in Fig. 5. Samples from different regions and magnifications were included in the analysis. The exterior composition of TA/β-CD in various regions of the sample within the 30 μm range is shown in Fig. 5(a), which indicates a structure resembling a porous one. The surface morphology of tartaric acid with a more porous structure within a range of 50 μm is depicted in Figs. 5(b) and 5(c). SEM pictures revealed that the material’s surface contains certain porous features, which might be helpful for the adsorption tests.

The Fourier Transform Infrared Spectroscopy (FTIR) analysis characterized the functional groups of CA/β-CD, as depicted in Fig. 6. The –OH groups found in CA/β-CD are responsible for the extensive absorption of infrared radiation in the wave number range of 3800–3200 cm⁻¹. Moreover, absorptions at 2900 cm⁻¹ show the presence of citric acid’s –CH groups, while those at 1750 cm⁻¹ correspond to the acid’s C=O groups. Moreover, the citric acid’s COOH group is at 1400 cm⁻¹. The C–O groups in beta-cyclodextrin and citric acid are responsible for the absorption at 1090–1200 cm⁻¹.

The functional groups of TA/β-CD were characterized by FTIR study, as presented in Fig. 7. It was shown that the –OH assemblies of beta-cyclodextrin and citric acid are responsible for the extensive absorption of infrared light at wave number region 3750–3150 cm⁻¹. The –CH assemblies of citric acid are supported by the absorptions of infrared radiation at wave numbers in region 2850 cm⁻¹, while the C=O assemblies are supported by those that have wave numbers at 1760 cm⁻¹. Furthermore, the citric acid COOH group is linked to the strong absorption of infrared light in wave number of 1460 cm⁻¹. However, the C–O assemblies of beta-cyclodextrin and citric acid correspond to the absorption of infrared light at a wave number of 1100–1250 cm⁻¹.

The functional groups of MA/β-CD were characterized by FTIR study, as shown in Fig. 8. Infrared radiation with a large absorption in the wave number series of 3850–3170 cm⁻¹ is attributed to the –OH found in beta-cyclodextrin and citric acid. Furthermore, absorptions at 2870 cm⁻¹ show that citric acid contains –CH, while absorptions at 1780 cm⁻¹ show that citric acid contains C=O. Moreover, the strong absorption at 1440 cm⁻¹ is consistent with citric acid’s COOH. The absorption at 1180–1230 cm⁻¹ is endorsed to the C–O present in beta-cyclodextrin and citric acid.

With the use of an adsorption system, three incredible adsorbents—CA/β-CD, MA/β-CD, and TA/β-CD—were utilized to remove NR. When using CA/β-CD, the adsorption method was more effective at removing NR than when using other polymers. Figure 9 provides evidence of the CA/β-CD’s removal capability, which stands at 92%. The NR abolition performance is better because of the presence of carboxyl groups. Better adsorption is achieved by the NR with a net efficient rate due to its higher electrostatic coupling with the CA/β-CD. Since the wide array of CA/β-CD electrostatically attracts undoubtedly charged NR molecules, the NR abolition performance of CA/β-CD adsorbents is significantly higher. The NR response was handled by CA/β-CD, TA/β-CD, and MA/β-CD, which essentially signifies the NR molecules’ adsorption from the CA/β-CD surface, reducing the deliberation of NR in the interruption. When compared to TA/β-CD and MA/β-CD, CA/β-CD displays a well abolition act instead.

A certain pH range was utilized in order to evaluate the batch’s pH impact configuration. The process outlined below was used to determine the ideal pH for NR removal. The initial pH was changed using HCl (0.1 N) and NaOH (0.1 N) solutions. Ten milligrams of adsorbent were added to each flask containing 20 ml of NR solution, and the mixture was swirled until equilibrium was reached. NR’s absorption in the UV–visible range dramatically decreases below pH 4 and above pH 6. In a similar vein, the color depth of the NR solution decreased for pH values higher than 6 and lowers than 4. Consequently, a study is investigated to examine how pH affects the process of adsorption within the pH range of 4–6. Table I illustrates the effects of varying pH on the exclusion of NR.

The data in Table I reveal that, at pH 4, all adsorbents exhibit maximum removal efficiency. Specifically, CA/β-CD demonstrates exceptionally high removal performance at pH 4, reaching 92.46%, while MA/β-CD shows comparatively lower elimination efficiency at pH 4 with a maximum of 88.15%. However, UV–visible absorbance analysis at pH 4 suggests that NR is further steady under these conditions compared to other pH levels. Consequently, all subsequent investigations in this study were conducted without altering the pH, maintaining it at 6.

The NR abolition performance is shown in Fig. 10 in relation to the remarkable dosage (10–120 mg) of adsorbents. For all drugs, the abolition act of NR showed a slightly improved trend at dosages ranging from 10 to 120 mg. For CA/β-CD, TA/β-CD, and MA/β-CD, the NR abolition act amplified somewhat when the dosage was enlarged from 10 to 100 mg. The adsorption potential and elimination performance both improved as the adsorbent dosage increased. It is clear that after treating with different amounts of adsorbents, the NR’s % removal effectiveness grew when the adsorbent dose was administered, reaching permeation at 100 mg for CA/β-CD, MA/β-CD, and TA/β-CD, respectively.

CA/β-CD, TA/β-CD, and MA/β-CD were evaluated for their removal effectiveness at varying concentrations of NR, ranging from 0 to 50 mg l⁻¹. In the experiment, 20 ml of NR solutions at various concentrations were mixed with 100 mg of adsorbent, and the mixture was left to incubate for an hour until adsorption equilibrium was reached. The reaction combination was then centrifuged, and a UV–Vis absorption spectrophotometer was used to analyze the supernatant. A study was conducted on the exclusion act of the three separate samples, TA/β-CD, MA/β-CD, and CA/β-CD. The results are revealed in Fig. 11. Adsorbents’ ability to eliminate contaminants declines as NR becomes more widely recognized. NR application of 10 mg l⁻¹, the elimination efficiency of all produced adsorbents—CA/β-CD, TA/β-CD, and MA/β-CD—turned high. Furthermore, the removal recital falls as NR awareness rises from 20 to 50 mg l⁻¹. The lack of an adsorbent surface at high NR concentrations for NR adsorption is the cause of the decline in elimination performance. At NR attention of 10 mg l⁻¹, the adsorption potential of CA/β-CD became excessive, while the adsorption proficiency of MA/β-CD became low. Ultimately, it was discovered that 10 mg l⁻¹ provides the best NR on consciousness, and this dose was used in all of the trials.

The impact of temperature on the adsorbents’ effectiveness in eliminating NR was studied within a temperature series of 30–60 °C, as illustrated in Fig. 12. The ability to extract NR reduced as the temperature upsurges from 30 to 60 °C. This decrease can be ascribed to the heightened susceptibility of electrostatic attraction among the adsorbate and adsorbents as temperatures increase. The adsorption process will be exothermic because of increasing temperature as the NR elimination performance is reduced. In parallel, growing temperatures between 30 and 40 °C caused the removal efficiency to decrease; nevertheless, at normal temperature, the elimination performance improved and changed again. The experiment showed that the best temperature for NR removal by adsorption is 30 °C.

The investigation into time-bound adsorption was conducted in-depth; the binding of NR molecules on the adsorbent occurs sequentially rather than simultaneously. In Fig. 13, the assessment of NR removal efficiency by adsorbents is presented under optimal conditions, including an adsorbent dosage of 100 mg, pH 6, and a dye concentration of 10 mg l⁻¹.

The preliminary results showed removal efficiencies of 90.39%, 83.69%, and 79.01% for CA/β-CD, TA/β-CD, and MA/β-CD, respectively, within 80 min contact duration. The accelerated NR adsorption process was attributed to the adsorbents large surface area and availability of active sites. Moreover, as no specific attractiveness existed for the adsorbent, the saturation point was observed to be reached within the 80 min duration.

The balance of dye adsorption using adsorbents was considered through the application of Langmuir and Freundlich isothermal models.³⁶ Adsorption isotherms were employed to analyze the communication among the adsorbent and adsorbate in the adsorption process. This alignment is illustrated in Figs. 14(a) and 14(b) and detailed in Table II.

The absorption kinetics in this study was initially described using the pseudo-first-order Lagergren model.³⁷ The non-linearized pseudo-second-order kinetic model³⁸ was then used to analyze the kinetic data. It seems that polymer adsorbents removed NR in a way that was more consistent with pseudo-second-order kinetics. This finding suggests that the pseudo-second-order kinetic model more closely matches the kinetic data as illustrated in Figs. 15(a) and 15(b), and Table III.

Three crucial qualities of an adsorbent are economy, recyclability, and stability. Three separate solutions containing 50 ml of HCl (0.5 M) in water, ethanol, and water–ethanol were used in this experiment to revive NR-loaded polymers. For the dye-loaded adsorbents, the three regenerating solvents—all of which include 0.5 M HCl in water—perform superiorly to the others in Fig. 16. The NR removal efficiency gradually decreased with different recycles. This leads to a progressive decrease in the elimination efficiency with each cycle. In the current design, adsorptive materials can be used for up to five sequences, with an exclusion recital of 70%–90%. Therefore, for the deduction of NR from aqueous NR solutions, adsorptive materials may be economical, effective, and efficient.

Water-insoluble β-Cyclodextrin (β-CD) polymers, specifically CA/β-CD, TA/β-CD, and MA/β-CD, were created by cross-linking β-cyclodextrin with various organic acids such as CA, MA, and TA. The resulting polymers were thoroughly characterized using advanced analytical techniques. These synthetic polymers were then tested for their ability to adsorb NR from aqueous solutions under various conditions. The study determined the optimal conditions for NR adsorption: pH = 4, adsorbent mass = 100 mg, NR concentration = 10 mg l⁻¹, contact time between 10 and 80 min, temperature = 30 °C, following the Langmuir isotherm for adsorption and pseudo-second-order kinetics for the reaction rate, along with thermodynamic analysis.

Among the polymers tested, CA/β-CD, which has a higher concentration of carboxyl groups, demonstrated the highest NR adsorption across all conditions. The carboxyl groups in CA/β-CD, which are negatively charged, enhanced electrostatic attraction with the positively charged NR, achieving an impressive removal efficiency of 92% from the aqueous solutions. These results indicate that water-insoluble cyclodextrin-based polymers could be cost-effective alternatives to expensive adsorbents for efficiently removing nitrogen from water.

The authors would like to acknowledge the support provided by Researchers Supporting Project No. RSP2024R297, King Saud University, Riyadh, Saudi Arabia. The Department of Studies in Chemistry at the University of Mysore is acknowledged by the authors for providing the laboratory facilities needed to conduct the research.

The authors have no conflicts to disclose.

Guruprasad M. Jayanayak: Conceptualization (equal); Writing – original draft (equal). S. Shashikanth: Methodology (equal); Writing – original draft (equal). Shamsuddin Ahmed: Investigation (equal); Methodology (equal). Prashantha Karunakar: Formal analysis (equal); Investigation (equal). Shareefraza J. Ukkund: Conceptualization (equal); Formal analysis (equal). Hasan Sh. Majdi: Funding acquisition (equal). Faisal M. Alfaisal: Supervision (equal). Shamshad Alam: Supervision (equal).

The data availability is available to the readers upon request from the corresponding author.