Efficient removal of basic yellow 28 dye from water using facilely synthesized ZnO and Mg₃B₂O₆ nanostructures

Abstract

Basic yellow 28 dye, used extensively in the textile and leather industries, poses significant environmental and health risks, including allergic reactions, skin irritation, and respiratory problems. This study reports the Pechini sol-gel synthesis of novel ZnO/Mg₃B₂O₆ nanostructures for the decontamination of basic yellow 28 dye from aqueous solutions. The nanostructures were synthesized by calcining at 650 and 850 °C for 5 h, producing ZM650 and ZM850, respectively. The average crystallite sizes were 39.28 nm for ZM650 and 51.03 nm for ZM850. BET surface areas were 70.71 m²/g for ZM650 and 48.13 m²/g for ZM850. FE-SEM and HR-TEM analyses revealed distinct morphological structures, with ZM650 exhibiting a dense aggregation of rod-like particles and ZM850 showing larger clusters. The maximum adsorption capacities were 381.68 mg/g for ZM650 and 303.03 mg/g for ZM850. The optimum adsorption was observed at a pH of 10, a contact time of 70 min, and a temperature of 298 K. Regeneration using a 6 M HCl solution demonstrated efficient reusability over five cycles. The adsorption process followed pseudo-second-order kinetics and the Langmuir isotherm, indicating monolayer adsorption. Also, the adsorption process was found to be physical and exothermic.

Introduction

The primary source of organic dye contamination in water is industrial activities, especially those linked to the textile, leather, paper, and plastic manufacturing^1–3. During the dyeing process, a significant portion of dyes, often ranging from 10 to 15%, fail to bind to the fabric or material, resulting in their release into wastewater. Additional sources include household waste from the use of dyed products, agricultural runoff containing dye residues from treated seeds, and improper disposal of dye-containing products^4,5. These dyes, due to their synthetic origin and complex chemical structure, are typically resistant to biodegradation, leading to their persistence in aquatic environments. Organic dyes pose several environmental and human health risks. Environmentally, they reduce light penetration in water bodies, disrupt photosynthesis in aquatic plants, and adversely affect the entire aquatic ecosystem. Some dyes are toxic to aquatic life, leading to mortality and reduced biodiversity^6,7. In humans, exposure to certain dyes has been linked to various health issues, including skin irritation, respiratory problems, and even carcinogenic effects^8,9. The presence of dyes in drinking water sources can lead to chronic health conditions, making it crucial to address their contamination. The worldwide water crisis is exacerbated by the growing contamination of water resources with synthetic dyes, particularly from the textile, leather, and other industrial sectors. According to the World Health Organization, approximately 25% of human health diseases are triggered by environmental pollution, with synthetic dye pollutants playing a significant role¹⁰. This issue is also highlighted in the United Nations Sustainable Development Goal 6 (SDG6), which aims to ensure clean water and sanitation for all by 2030 ¹¹. Contaminants like synthetic azo dyes are known to be carcinogenic, mutagenic, and resistant to biodegradation, posing serious risks to aquatic ecosystems and human health¹². Basic yellow 28 dye, also known by its IUPAC name 2-[4-(Dimethylamino)phenylazo]benzoic acid ethyl ester, is a synthetic azo dye widely used in the textile and leather industries. Its complex aromatic structure makes it highly resistant to biodegradation, contributing to environmental persistence and contamination of water bodies. The molecular structure of basic yellow 28 is shown in Fig. 1.

Fig. 1.

Molecular structure of basic yellow 28 dye.

It is known to cause allergic reactions and skin irritation upon contact. Prolonged exposure can lead to respiratory problems, including asthma and bronchitis. More concerning is its potential carcinogenicity; studies have indicated that certain breakdown products of basic yellow 28 dye can induce mutations and cancer in living cells^13,14. This makes the management and removal of this dye from water sources a public health priority. Several methods have been developed to remove organic dyes from contaminated water, such as adsorption, electrochemical, biological, and photocatalytic degradation. Several methods have been developed to remove organic dyes from contaminated water, such as adsorption^15,16, electrochemical^17,18, biological¹⁹, and photocatalytic degradation^20,21. Adsorption involves the adhesion of dye molecules to the surface of solid materials. Adsorption is favored over other dye removal methods for several reasons. It is relatively simple and cost-effective, does not require complex equipment, and can be applied to a wide range of dyes. Moreover, the substances employed as adsorbents can commonly be regenerated and reused, establishing the process sustainable²². Electrochemical methods use electrical currents to degrade dye molecules into less harmful substances. These methods are effective but can be costly and require significant energy input^23,24. Photocatalytic degradation utilizes light-activated catalysts, such as titanium dioxide, to break down dye molecules. This method is effective but requires specific conditions, such as UV light, which can limit its applicability²⁵. Biological methods utilize microorganisms to break down dyes. Although these methods are eco-friendly, they can be slow and often struggle to effectively degrade complex synthetic dyes²⁶. Nano-metal oxides have emerged as highly effective adsorbents for organic dyes because of their special attributes. Their high surface area promotes numerous reactive sites for dye decontamination, enhancing removal efficiency. Their stability and reusability make them ideal candidates for large-scale water treatment applications^27,28. The development of novel adsorbents has become critical in addressing the increasing challenges posed by environmental pollutants, particularly in water treatment applications. Traditional adsorbents often suffer from limitations such as low adsorption capacity, poor selectivity, and difficulty in regeneration. In contrast, novel adsorbents, including nanocomposites and functionalized materials, offer superior adsorption properties due to their high surface area, enhanced reactivity, and tailored functionality. These materials have been successfully applied in the removal of a wide range of pollutants, including heavy metals, dyes, and organic contaminants. Their ability to be regenerated and reused makes them ideal for sustainable environmental remediation solutions. Recent studies have demonstrated the effectiveness of novel magnetic biochar composites, metal-organic frameworks (MOFs), and functionalized nanoparticles in removing toxic substances from water, highlighting their potential in large-scale applications^29–31. The current study introduces a novel ZnO/Mg₃B₂O₆ nanostructure adsorbent, synthesized through a cost-effective Pechini sol-gel method, with the goal of addressing these limitations and improving the efficiency of dye removal from aqueous environments. The Pechini sol-gel procedure serves as a multifunctional and cost-effective strategy for synthesizing nano-metal oxides with controlled morphology and size. This method involves the use of chelating agents, such as citric acid, and polymeric precursors, like ethylene glycol, to form a gel. Upon thermal treatment, this gel decomposes to produce highly pure and homogenous nano-metal oxides³². ZnO has been widely recognized for its superior photocatalytic activity, large surface area, and high adsorption capacity, making it one of the most effective materials for removing synthetic dyes from water^33–35. Furthermore, Mg-based materials, particularly Mg₃B₂O₆, exhibit excellent stability and provide additional adsorption sites, which enhance the overall dye removal efficiency³⁶. The combination of ZnO with Mg₃B₂O₆ forms a highly effective composite that leverages the advantages of both components, including enhanced adsorption capacity. This synergy makes ZnO/Mg₃B₂O₆ composites promising candidates for water treatment applications, particularly for removing persistent organic dyes such as basic yellow 28. Hence, the current research introduces the synthesis of novel ZnO/Mg₃B₂O₆ nanostructures using the Pechini sol-gel method, with citric acid as a chelating agent and ethylene glycol as a crosslinker. The research further explores the application of these novel ZnO/Mg₃B₂O₆ nanostructures in treating water pollution caused by basic yellow 28 dye, a common pollutant with known toxicity. By leveraging the high adsorption capacity and reusability of the synthesized nanostructures, this study aims to provide an efficient and sustainable solution for the removal of basic yellow 28 dye from aqueous environments. The Pechini sol-gel method was chosen for the synthesis of ZnO/Mg₃B₂O₆ nanostructures due to its ability to control particle size, produce homogenous materials, and operate at relatively moderate temperatures, resulting in highly pure nanostructures with enhanced surface area and reactivity. These characteristics are crucial for improving adsorption efficiency. Adsorption was selected for dye removal because of its simplicity, cost-effectiveness, and the ability to regenerate the adsorbent, making it a highly sustainable option for large-scale water treatment.

Experimental

Chemicals

In this study, the following chemicals were used: boric acid (H₃BO₃, 99% purity), zinc nitrate hexahydrate (Zn(NO₃)·6 H₂O, 98% purity), potassium chloride (KCl, 99% purity), magnesium nitrate hexahydrate (Mg(NO₃)₂·6 H₂O, 98% purity), hydrochloric acid (HCl, 37% concentration), citric acid (C₆H₈O₇, 99% purity), sodium hydroxide (NaOH, 98% purity), ethylene glycol (C₂H₆O₂, 99% purity), and basic yellow 28 dye (C₂₁H₂₇N₃O₅S, dye content ≥ 90%). All chemicals were procured from Sigma-Aldrich.

Synthesis of ZnO/Mg3B2O6 nanostructures

First, 6.36 g of Mg(NO₃)₂·6H₂O was solubilized in 50 mL of distilled water. Then, 1.09 g of H₃BO₃ was solubilized in another 50 mL of distilled water. Next, 5.45 g of Zn(NO₃)₂·6H₂O was solubilized in 50 mL of distilled water. The solutions from the first two steps were then incorporated into the third solution with persistent stirring for 5 min. Following this, 4.54 g of citric acid was solubilized in 50 mL of distilled water, and this solution was incorporated to the mixture with persistent agitating for 15 min. Subsequently, 5 mL of ethylene glycol was incorporated into the mixture, which was persistently stirred at 120 °C until complete evaporation of the solvent occurred. Finally, the resultant powder was calcined at 650 and 850 °C to obtain ZnO/Mg₃B₂O₆ nanostructures, abbreviated as ZM650 and ZM850, respectively. Figure 2 illustrates the synthesis process of ZnO/Mg₃B₂O₆ nanostructures.

Fig. 2.

Synthesis process of ZnO/Mg₃B₂O₆ nanostructures.

Instrumentation

The structural properties of the ZM650 and ZM850 samples were analyzed using various advanced characterization techniques. X-ray diffraction (XRD) was performed using an X’Pert PRO diffractometer from PANalytical, Netherlands. Fourier transform infrared spectroscopy (FTIR) was accomplished with a Nicolet IS10 spectrometer from Thermo Fisher Scientific, USA. High-resolution transmission electron microscopy (HR-TEM) images were acquired through a JEM-2100 microscope from JEOL Ltd., Japan. In addition, the specific surface area measurements were conducted using the NOVA2000 series instrument from Quantachrome, USA. Field emission scanning electron microscopy (FE-SEM) images were acquired through a JSM-6510LV microscope from JEOL Ltd., Japan.

Removal of basic yellow 28 dye from aquatic environments

Table 1 illustrates the experimental conditions for removing the basic yellow 28 dye. Also, a UV-visible spectrophotometer (V-670, Jasco, Japan) was employed to determine the concentration of basic yellow 28 dye before and after adsorption at 438 nm, which is the maximum absorption wavelength of basic yellow 28 dye³⁷. Besides, the removal percentage of basic yellow 28 dye (% R) in conjunction with the adsorption capacity of the adsorbent (Q) were calculated through the application of Eqs. 1 and 2 ³⁸.

1

2

Table 1.

Experimental conditions for studying the decontamination of basic yellow 28 dye utilizing ZM650 or ZM850 adsorbents.

Effect	Volume of dye (mL)	Concentration of dye (mg/L)	Quantity of adsorbent (g)	Temperature (K)	Contact time (min)	pH
pH	100	200	0.05	298	360	2–10
Time	100	200	0.05	298	10–100	10
Temperature	100	200	0.05	298–328	70	10
Concentration of dye	100	50–300	0.05	298	70	10

C_o is the beginning concentration of basic yellow 28 dye (mg/L) whereas C_e is the final concentration of basic yellow 28 dye (mg/L). In addition, V corresponds to the Volume of the basic yellow 28 dye solution (L) whilst W corresponds to the weight of the adsorbent (g).

The desorption experiments for the regeneration of a basic yellow 28 dye-laden adsorbent were conducted using different concentrations of hydrochloric acid (HCl) as the eluting agent. The concentrations used were 2, 4, and 6 M. For each experiment, a fixed volume of 50 mL of the eluting agent was used. The desorption percentage of basic yellow 28 dye (% D) was calculated using Eq. 3 ³⁹.

3

C_d is the concentration of the basic yellow 28 dye in the desorbing solution (mg/L), whereas V_d is the volume of the desorbing solution (L).

The reusability experiments for the adsorbent were conducted over five cycles to evaluate its efficiency in adsorbing basic yellow 28 dye. A dye solution with a concentration of 200 mg/L was prepared, and a volume of 100 mL was used for each cycle. The amount of adsorbent used in each experiment was 0.05 g. The experiments were conducted at 25 ^oC and a pH of 10. The interaction time for each cycle was 70 min.

Assessment of point of zero charge (pHPZC) for the ZM650 and ZM850 products

The pH_PZC for the ZM650 and ZM850 materials was determined using the batch adsorption method with KCl as the supporting electrolyte²⁷. In this method, a group of 50 mL KCl solutions of different initial pH values (pH_I) were prepared by adjusting with either 0.1 M sodium hydroxide or 0.1 M hydrochloric acid solution. A constant amount (0.10 g) of the ZM650 or ZM850 adsorbent was incorporated into each KCl solution, and the blends were allowed to equilibrate by stirring for 24 h. After equilibrium, the final pH (pH_F) of each solution was measured. The difference in pH (ΔpH) was calculated for each solution. The point of zero charge (pH_PZC) was determined by plotting ΔpH versus pH_I and identifying the pH value at which ΔpH equals zero.

Results and discussion

Production and identification of ZnO/Mg3B2O6 nanostructures

Figure 3 illustrates the stepwise process of synthesizing ZnO/Mg₃B₂O₆ nanostructures by the Pechini sol-gel strategy. Initially, magnesium nitrate hexahydrate and zinc nitrate hexahydrate dissociate in water, releasing Mg²⁺ and Zn²⁺ ions, along with nitrate ions and water molecules. Concurrently, boric acid reacts with water to form B(OH)_4- and H⁺ ions. These metal ions (Mg²⁺ and Zn²⁺) then interact with B(OH)_4- and citric acid to form a metal-citrate complex. This complex undergoes polyesterification with ethylene glycol, resulting in a polymeric network. Upon heating, this network decomposes to produce ZnO and Mg₃B₂O₆ nanostructures, releasing CO₂ and H₂O as byproducts.

Fig. 3.

Synthesis of ZnO/Mg₃B₂O₆ nanostructures.

The XRD patterns for the ZM650 and ZM850 samples, depicted in Fig. 4A-B, reveal the presence of ZnO and Mg₃B₂O₆ phases. Both samples exhibit characteristic peaks corresponding to the hexagonal system of ZnO (JCPDS No. 01-071-6424) and the orthorhombic system of Mg₃B₂O₆ (JCPDS No. 00-038-1475). The lattice parameters of ZnO are a = b = 3.25 Å and c = 5.20 Å. Besides, the lattice parameters of Mg₃B₂O₆ are a = 5.40 Å, b = 8.42 Å, and C = 4.51 Å. These values are consistent with previously reported data for ZnO and Mg₃B₂O₆ phases, indicating the successful formation of the nanostructures^36,40. The identified diffraction peaks for ZnO in the ZM650 and ZM850 samples are located at 31.68°, 34.51°, 36.19°, 47.53°, 56.45°, 62.96°, 64.41°, 67.99°, and 69.04°. The relevant Miller index values of ZnO are (100), (002), (101), (102), (110), (103), (200), (112), and (201), respectively. The identified diffraction peaks for Mg₃B₂O₆ in the ZM650 and ZM850 samples are observed at 21.09°, 25.81°, 33.58°, 38.72°, 40.29°, 41.44°, 42.69°, 50.67°, 52.88°, and 54.88°. The relevant Miller index values of Mg₃B₂O₆ are (020), (101), (121), (201), (211), (131), (040), (141), (202), and (132), respectively. The average crystallographic sizes of the ZM650 and ZM850 products are 39.28 nm and 51.03 nm, respectively. The sample synthesized at 850 °C (ZM850) has a larger average crystallite size compared to the one synthesized at 650 °C (ZM650) due to the higher temperature promoting increased mobility and diffusion of the compounds (ZnO and Mg₃B₂O₆) during the Pechini sol-gel process. This results in enhanced grain growth and the formation of larger crystallites. At elevated temperatures, the compounds have more energy to overcome activation barriers, allowing them to migrate more easily and coalesce into larger crystalline structures. This process leads to the observed increase in crystallite size in the ZM850 sample.

Fig. 4.

XRD patterns of the ZM650 (A) and ZM850 (B) products.

The nitrogen adsorption-desorption curves of the ZM650 and ZM850 products are illustrated in Fig. 5. The isotherms show that the volume adsorbed by ZM650 is greater than that adsorbed by ZM850 across all relative pressures. Table 2 presents the BET surface profiles of the ZM650 and ZM850 samples. The ZM650 sample exhibits an overall pore volume of 0.2385 cc/g in addition to a BET surface area of 70.71 m²/g, while the ZM850 sample has an overall pore volume of 0.1801 cc/g in addition to a BET surface area of 48.13 m²/g. The average pore size of ZM650 is 6.53 nm, compared to 7.48 nm for ZM850. This difference can be explained by the inverse relationship between crystallite size and surface area. As the crystallite size decreases, the surface area increases, causing an elevated BET surface area in addition to total pore volume. Therefore, ZM650, with its smaller crystal size, exhibits a larger surface area and pore volume compared to ZM850. In terms of the type of pores, the adsorption-desorption isotherms indicate the presence of mesopores in both samples. The steep increase in volume at higher relative pressures suggests capillary condensation within the mesopores. In addition, the pores are classified as mesopores because the average pore sizes for both ZM650 and ZM850 samples are greater than 2 nm²⁷.

Fig. 5.

Nitrogen adsorption-desorption isotherms of the ZM650 and ZM850 products.

Table 2.

BET surface profiles of the ZM650 and ZM850 products.

Surface profiles	ZM650	ZM850
Overall pore volume (cc/g)	0.2385	0.1801
BET surface area (m2/g)	70.71	48.13
Average pore size (nm)	6.53	7.48

Figure 6 presents the FE-SEM images of two samples, ZM650 (Fig. 6A) and ZM850 (Fig. 5B). Figure 6B illustrates the morphology of the ZM650 sample, showcasing a dense aggregation of rod-like and granular particles. The particles appear to be uniformly distributed with a high degree of surface roughness, indicating potential active sites for adsorption processes. The rods, with a length in the micrometer range, are interspersed among the granular particles, forming a complex, three-dimensional network. Figure 5B, depicting the ZM850 sample, reveals a similar morphological structure with notable differences. The image shows larger clusters of granular particles, suggesting a higher degree of agglomeration compared to ZM650. The presence of both rod-like and granular particles is evident, but the granular particles dominate the microstructure, potentially contributing to a different surface area and porosity profile than ZM650. The distribution of these particles is less uniform, with distinct regions of dense aggregation.

Fig. 6.

FE-SEM pictures of the ZM650 (A) and ZM850 (B) products.

The HR-TEM images in Fig. 7A-B depict the morphology of the ZM650 and ZM850 samples, respectively. The particles in both images exhibit irregular shapes with a tendency to aggregate, forming larger clusters. The average particle diameter of the ZM650 sample is 135.78 nm, whereas the ZM850 sample has an average particle diameter of 224.93 nm. It is important to note that the particle diameters observed from HR-TEM are larger compared to those measured by XRD. This discrepancy is attributed to the particles’ tendency to combine or aggregate with each other, forming larger structures visible in the HR-TEM images. This aggregation leads to the observed larger particle sizes in the HR-TEM analysis compared to the XRD results.

Fig. 7.

HR-TEM pictures of the ZM650 (A) and ZM850 (B) nanostructures.

Figure 8 presents the FTIR examination of the ZM650 (Fig. 8A) and ZM850 (Fig. 8B) samples. The spectra indicate several significant peaks that are correlated between the two products. For ZM650, the peak at 459 cm⁻¹ corresponds to the stretching vibration mode of Zn-O, while for ZM850, this vibration is observed at 468 cm^{−1 41}. The stretching vibration of Mg-O appears at 727 cm⁻¹ for ZM650 and at 746 cm⁻¹ for ZM850 ⁴². Both samples exhibit a peak at 1049 cm⁻¹, which is attributed to the symmetric stretching vibration of B-O-B. The asymmetric stretching vibration of B-O-B is found at 1247 cm⁻¹ for ZM650 and at 1219 cm⁻¹ for ZM850 ⁴³. The bending vibration of adsorbed H₂O is noted at 1629 cm⁻¹ in ZM650 and at 1634 cm⁻¹ in ZM850. Finally, the stretching vibration of adsorbed H₂O is observed at 3428 cm⁻¹ in ZM650 and at 3427 cm⁻¹ in ZM850 ²⁷.

Fig. 8.

FTIR examination of the ZM650 (A) and ZM850 (B) nanostructures.

Figure 9 illustrates the Energy-Dispersive X-ray (EDX) patterns of the ZM650 (Fig. 9A) and ZM850 (Fig. 9B) products. The EDX analysis confirms the elemental composition of both samples, showing the presence of zinc (Zn), magnesium (Mg), oxygen (O), and boron (B). In the case of ZM650, the elemental weight percentages are 43.20% for Zn, 26.80% for Mg, 28.15% for O, and 1.85% for B. For ZM850, the corresponding weight percentages are 43.30% for Zn, 25.60% for Mg, 29.80% for O, and 1.30% for B. These results indicate that the elemental distribution is consistent across both samples, with only slight variations in the weight percentages of Mg, O, and B. The presence of these elements confirms the successful synthesis of ZnO/Mg₃B₂O₆ nanostructures in both ZM650 and ZM850.

Fig. 9.

EDX patterns of the ZM650 (A) and ZM850 (B) products.

Decontamination of basic yellow 28 dye from aquatic environments

Effect of pH

The adsorption efficiency of the ZM650 and ZM850 samples in removing basic yellow 28 dye from aqueous solutions was evaluated with respect to pH, as outlined in Fig. 10A. The percentage removal of basic yellow 28 dye increased with increasing pH for both samples. At pH 2, the ZM650 sample removed 2.23% of the dye, while the ZM850 sample removed 1.34%. As the pH increased to 10, the % removal for ZM650 rose to 92.50%, and for ZM850, it increased to 72.39%.

Fig. 10.

(A) Removal percentage (% R) of basic yellow 28 dye as a function of pH for ZM650 and ZM850 samples. (B) pH_PZC of the ZM650 and ZM850 samples.

Mechanism of adsorption

The point of zero charge (pH_PZC) is a critical parameter influencing adsorption processes. As depicted in Fig. 10B, the pH_PZC for ZM650 is 6.60, while for ZM850, it is 5.64. At pH values below the pH_PZC, the surface of the nanostructures is positively charged, prompting electrostatic repulsion with the cationic basic yellow 28 dye molecules, resulting in lower adsorption efficiency, as outlined in Fig. 11 ²⁷. As the pH increases above the pH_PZC, the surface becomes negatively charged, enhancing the electrostatic attraction between the basic yellow 28 dye molecules and the nanostructures, thereby increasing the adsorption efficiency, as outlined in Fig. 11 ²⁷.

Fig. 11.

Graphic representation of the interaction between ZnO/Mg₃B₂O₆ nanostructures and cationic basic yellow 28 dye.

Figure 6A-B shows the FE-SEM images of the ZM650 and ZM850 products before adsorption, respectively. In Fig. 6A, the ZM650 sample exhibits a rough surface morphology with a dense aggregation of small granular particles distributed across the surface. In contrast, Fig. 6B presents the ZM850 sample, which shows a less uniform surface with more significant clusters of particles. Figure 12 illustrates the FE-SEM images of the ZM650 and ZM850 products after adsorption, respectively. Figure 12A depicts the ZM650 sample after adsorption, where the surface morphology remains relatively intact, though with some visible changes indicating the adsorption of basic yellow 28 dye molecules. In Fig. 12B, the ZM850 sample shows similar surface features post-adsorption, but with a more pronounced clustering of particles, suggesting the adsorption process has slightly modified the surface structure of the material. Both images confirm that the ZnO/Mg₃B₂O₆ nanostructures retain their structural integrity post-adsorption, demonstrating their stability and reusability.

Fig. 12.

FE-SEM pictures of the ZM650 (A) and ZM850 (B) products after adsorption of dye.

XRD analysis was performed on the ZM650 and ZM850 nanostructures both before the adsorption of basic yellow 28 dye, as shown in Fig. 4A-B, respectively. Also, XRD analysis was performed on the ZM650 and ZM850 nanostructures both after the adsorption of basic yellow 28 dye, as shown in Fig. 13A-B, respectively. The XRD patterns (Fig. 13) reveal no significant shifts in peak positions and slight changes in intensity, confirming that the crystallinity and structure of the ZnO and Mg₃B₂O₆ phases remain stable post-adsorption. The presence of all characteristic peaks after adsorption indicates that the adsorption process does not alter the structural integrity of the nanostructures, confirming their suitability for repeated use in water remediation.

Fig. 13.

XRD patterns of the ZM650 (A) and ZM850 (B) products after adsorption of dye.

Effect of contact time

Figure 14 demonstrates the removal percentage (% R) of basic yellow 28 dye as a function of time for ZM650 and ZM850 samples. Initially, the removal percentage of basic yellow 28 dye increases rapidly, with ZM650 showing a higher removal percentage compared to ZM850. At 70 min, the decontamination percentage of basic yellow 28 dye using ZM650 reaches 92.38%, while ZM850 reaches 72.07%. After 70 min, the % R plateaus for both samples, indicating that equilibrium is achieved due to the saturation of the adsorbents.

Fig. 14.

Removal percentage (% R) of basic yellow 28 dye as a function of time for ZM650 and ZM850 samples.

In Fig. 15, the linear kinetic modeling of basic yellow 28 dye adsorption onto ZM650 and ZM850 is presented using both pseudo-first-order (Fig. 15A) and pseudo-second-order (Fig. 15B) models. In addition, the pseudo-first-order model is represented by Eq. 4 ⁴⁴.

4

Fig. 15.

Linear kinetic modeling of basic yellow 28 dye adsorption onto ZM650 and ZM850 adsorbents: (A) pseudo-first-order model and (B) pseudo-second-order model.

Also, the pseudo-second-order model is represented by Eq. 5 ⁴⁴.

5

Based on the values of Q_Exp and R² in Table 3, the adsorption process for both ZM650 and ZM850 follows the linear pseudo-second-order model. The R² values for the linear pseudo-second-order model are 0.9999 for both adsorbents, indicating a very high degree of correlation. Additionally, the Q_e values obtained from the linear pseudo-second-order model (366.30 mg/g for ZM650 and 288.18 mg/g for ZM850) are very close to the experimental values (Q_Exp) of 369.52 mg/g for ZM650 and 288.26 mg/g for ZM850, further confirming the suitability of this model. The values of the residual sum of squares (RSS) are critical in evaluating the fit of the kinetic models used to describe the adsorption process. In Table 3, the RSS values for the pseudo-second-order model are significantly lower for both ZM650 and ZM850 adsorbents when compared to the pseudo-first-order model. The lower RSS values for the pseudo-second-order model demonstrate that it provides a more accurate description of the adsorption kinetics compared to the pseudo-first-order model.

Table 3.

Linear kinetic parameters for the adsorption of basic yellow 28 dye onto ZM650 and ZM850.

Adsorbent	QExp (mg/g)	Pseudo-first-order				Pseudo-second-order
		K1 (1/min)	R 2	Qe (mg/g)	RSS	K2 (g/mg.min)	R 2	Qe (mg/g)	RSS
ZM650	369.52	0.02632	0.9417	157.45	0.0112	0.0004544	0.9999	366.30	1.62 × 107−
ZM850	288.26	0.02206	0.9682	191.94	0.0042	0.0002455	0.9999	288.18	2.15 × 107−

In Fig. 16, the nonlinear kinetic modeling of basic yellow 28 dye adsorption onto ZM650 and ZM850 is presented using both pseudo-first-order (Fig. 16A) and pseudo-second-order (Fig. 16B) models. In addition, the linear pseudo-first-order model is represented by Eq. 6 ⁴⁵.

6

Fig. 16.

Nonlinear kinetic modeling of basic yellow 28 dye adsorption onto ZM650 and ZM850 adsorbents: (A) pseudo-first-order model and (B) pseudo-second-order model.

Also, the linear pseudo-second-order model is represented by Eq. 7 ⁴⁵.

7

Based on the values of Q_Exp and R² in Table 4, the adsorption process for both ZM650 and ZM850 follows the nonlinear pseudo-second-order model. The R² values for the nonlinear pseudo-second-order model are 0.9999 for both adsorbents, indicating a very high degree of correlation. Additionally, the Q_e values obtained from the nonlinear pseudo-second-order model (367.06 mg/g for ZM650 and 288.29 mg/g for ZM850) are very close to the experimental values (Q_Exp) of 369.52 mg/g for ZM650 and 288.26 mg/g for ZM850, further confirming the suitability of this model. The values of χ² in Table 4 show a significant decrease for the pseudo-second-order model compared to the pseudo-first-order model, indicating a much better fit for the pseudo-second-order model. The lower χ² values, combined with the high R² values, confirm that the adsorption process is best described by the nonlinear pseudo-second-order model.

Table 4.

Nonlinear kinetic parameters for the adsorption of basic yellow 28 dye onto ZM650 and ZM850.

Adsorbent	QExp (mg/g)	Pseudo-first-order				Pseudo-second-order
		K1 (1/min)	R 2	Qe (mg/g)	χ2	K2 (g/mg.min)	R 2	Qe (mg/g)	χ2
ZM650	369.52	0.1135	0.94821	324.73	79.36	4.51 × 104−	0.9999	367.06	0.2296
ZM850	288.26	0.0665	0.9845	232.51	27.97	2.45 × 104−	0.9999	288.29	0.0620

Effect of temperature

The temperature-dependent removal percentages (% R) of basic yellow 28 dye using ZM650 and ZM850 adsorbents were investigated. As shown in Fig. 17A, the % R of basic yellow 28 dye decreases with increasing temperature for both adsorbents. Specifically, at 298 K, ZM650 achieved a removal percentage of 92.38%, which steadily decreased to 84.14% at 328 K. Similarly, ZM850 showed a removal percentage of 72.07% at 298 K, which dropped to 62.90% at 328 K. These trends indicate a decline in adsorption efficiency with rising temperatures, suggesting that the adsorption process is exothermic.

Fig. 17.

(A) Effect of temperature on the removal percentage (% R) of basic yellow 28 dye by ZM650 and ZM850 adsorbents, and (B) Van’t Hoff plot.

In Fig. 17B, the Van’t Hoff plot, which plots lnK_d against 1/T, displays a linear relationship for both adsorbents. The slope and intercept of these lines were applied to assess the thermodynamic constants. The calculations of the thermodynamic parameters were based on Eqs. 8–10 ²⁷. Besides, the results are tabulated in Table 5.

8

9

10

Table 5.

Thermodynamic parameters for the adsorption process of basic yellow 28 dye on ZM650 and ZM850 adsorbents at different temperatures.

Adsorbent	△So (KJ/molK)	△Ho (KJ/mol)	△Go (KJ/mol)
			298	308	318	328
ZM650	0.04766	-22.17	-36.37	-36.85	-37.33	-37.80
ZM850	0.02412	-11.32	-18.51	-18.75	-18.99	-19.24

The thermodynamic parameters reveals that the decontamination process for both ZM650 and ZM850 is characterized by a negative enthalpy change (△H), indicating an exothermic nature (△H < 0). For ZM650, △H is -22.17 kJ/mol, while for ZM850, it is -11.32 kJ/mol, both values being less than 40 kJ/mol, which confirms that the decontamination process is physical. In addition, the Gibbs free energy change (△G) for both adsorbents is negative across all temperatures, affirming the spontaneity of the decontamination process (△G < 0). Moreover, the positive entropy change (△S) values, 0.04766 kJ/molK for ZM650 and 0.02412 kJ/molK for ZM850, indicate the feasibility and increased randomness at the solid-solution interface during the decontamination process (△S > 0)²⁷.

Effect of concentration

The removal efficiency of basic yellow 28 dye (% R) for the ZM650 and ZM850 adsorbents with respect to initial dye concentration is depicted in Fig. 18. At the initial concentration of 50 mg/L, ZM650 and ZM850 showed removal efficiencies of 97.80 and 93.54%, respectively. As the concentration was raised to 300 mg/L, the removal efficiencies were 62.01% for ZM650 and 48.35% for ZM850. The removal efficiency of basic yellow 28 dye decreases with increasing dye concentrations due to the saturation of adsorption sites on the adsorbent surface. As the concentration increases, there is increased competition among dye molecules for the limited available sites.

Fig. 18.

Effect of initial basic yellow 28 dye concentration on the removal efficiency (% R) of ZM650 and ZM850 adsorbents.

Figure 19A-B illustrates the linear equilibrium modeling of basic yellow 28 dye adsorption onto ZM650 and ZM850 adsorbents using the Langmuir and Freundlich equations, respectively. The linear Langmuir isotherm is described by Eq. 11 ⁴⁴.

11

Fig. 19.

Linear equilibrium modeling of basic yellow 28 dye adsorption onto ZM650 and ZM850 adsorbents: (A) Langmuir isotherm and (B) Freundlich isotherm.

The linear Freundlich isotherm is described by Eq. 12 ⁴⁴.

12

where, Q_max can be calculated by Eq. 13 ⁴⁴.

13

The adsorption follows the Langmuir isotherm based on the higher R² values obtained from the Langmuir isotherm in comparison to the Freundlich isotherm, as shown in Table 6. This suggests that the adsorption process is more consistent with monolayer coverage of dye molecules on a homogeneous adsorbent surface for both ZM650 and ZM850. Also, the RSS values for the Langmuir isotherm are significantly lower than those for the Freundlich isotherm, indicating a better fit for the Langmuir model.

Table 6.

Linear equilibrium constants for the adsorption of basic yellow 28 dye onto ZM650 and ZM850.

Adsorbent	Langmuir				Freundlich
	Qmax (mg/g)	R 2	K3 (L/mg)	RSS	K4 (mg/g)(L/mg)1/n	Qmax (mg/g)	1/n	R 2	RSS
ZM650	381.68	0.9993	0.4352	3.92 × 105−	133.38	533.98	0.2618	0.7320	0.3046
ZM850	303.03	0.9987	0.1878	2.08 × 104−	85.37	370.66	0.2771	0.7731	0.1860

Figure 20A-B illustrates the nonlinear equilibrium modeling of basic yellow 28 dye adsorption onto ZM650 and ZM850 adsorbents using the Langmuir and Freundlich equations, respectively. The nonlinear Langmuir isotherm is described by Eq. 14 ⁴⁵.

14

Fig. 20.

Nonlinear equilibrium modeling of basic yellow 28 dye adsorption onto ZM650 and ZM850 adsorbents: (A) Langmuir isotherm and (B) Freundlich isotherm.

The nonlinear Freundlich isotherm is described by Eq. 15 ⁴⁵.

15

The χ² values for both the Langmuir and Freundlich nonlinear models are large, indicating poor model fits.

Table 7.

Nonlinear equilibrium constants for the adsorption of basic yellow 28 dye onto ZM650 and ZM850.

Adsorbent	Langmuir				Freundlich
	Qmax (mg/g)	R 2	K3 (L/mg)	χ2	K4 (mg/g)(L/mg)1/n	Qmax (mg/g)	1/n	R 2	χ2
ZM650	391.75	0.9552	0.3681	584.48	169.88	458.18	0.1873	0.7059	3834.65
ZM850	313.98	0.9641	0.1491	233.83	112,19	336.06	0.2073	0.7422	168.04

Table 8 presents the maximum adsorption capacities (Q_max) of various adsorbents for the removal of basic yellow 28 dye, highlighting the performance of the newly synthesized ZM650 and ZM850 adsorbents^46–49. Clinoptilolite and amberlite XAD-4 exhibit relatively low adsorption capacities, with Q_max values of 56.70 mg/g and 14.90 mg/g, respectively. Bentonite demonstrates a significantly higher capacity of 256.40 mg/g, while poly(methacrylic acid) hydrogels and Silybum marianum stem show Q_max values of 157.00 mg/g in addition to 271.73 mg/g, respectively. Notably, the newly synthesized ZM650 and ZM850 adsorbents surpass these values, achieving remarkable Q_max values of 381.68 mg/g and 303.03 mg/g, respectively, indicating their superior efficacy in removing basic yellow 28 dye from aqueous media.

Table 8.

Maximum adsorption capacities (Q_max) of various adsorbents for basic yellow 28 dye removal compared to those of the newly synthesized ZM650 and ZM850 adsorbents.

Adsorbent	Qmax (mg/g)	Ref
Clinoptilolite	56.70	46
Amberlite XAD-4.	14.90	46
Bentonite	256.40	47
Poly(methacrylic acid) hydrogels	157.00	48
Silybum marianum stem	271.73	49
ZM650	381.68	This study
ZM850	303.03	This study

Effect of regeneration and reusability

The desorption efficiency of basic yellow 28 dye from ZM650 and ZM850 samples was evaluated at varying concentrations of HCl, as shown in Fig. 21. At 2 M HCl, ZM650 exhibited a desorption efficiency of approximately 64.98%, whereas ZM850 demonstrated a slightly higher efficiency at 70.19%. As the concentration of HCl increased to 6 M, the desorption efficiency significantly improved for both samples, with ZM650 reaching 99.71% and ZM850 achieving 99.64%. These results indicate that the desorption efficiency for both ZM650 and ZM850 samples increases with the concentration of HCl. Hydrochloric acid (HCl) is highly effective in desorption processes due to its strong acidic nature, which can break the interactions between the dye molecules and the adsorbent surface. In the case of basic yellow 28 dye, the presence of HCl leads to protonation of the dye molecules, reducing their affinity for the adsorbent. This results in the dye molecules being released back into the solution.

Fig. 21.

Desorption percentage (% D) of basic yellow 28 dye from ZM650 and ZM850 as a function of HCl concentration.

ZM650 and ZM850 adsorbents demonstrate distinct reusability performances over five adsorption-desorption cycles, which can be observed by analyzing their percentage removal (% R) efficiencies, as shown in Fig. 22. Initially, ZM650 exhibits a higher removal efficiency of 92.38% compared to 72.07% for ZM850. As the cycles progress, the efficiency of ZM650 slightly decreases but remains higher than that of ZM850 throughout all cycles. After the first cycle, ZM650 achieves 91.59% removal efficiency, while ZM850 shows 71.89%. By the second cycle, ZM650’s efficiency is 90.70%, whereas ZM850 records 70.33%. In the third cycle, the removal efficiency for ZM650 is 89.16%, and for ZM850, it is 68.26%. During the fourth cycle, ZM650’s efficiency is 87.40%, compared to 66.61% for ZM850. Finally, in the fifth cycle, ZM650 maintains a removal efficiency of 86.56%, while ZM850 reaches 65.23%. Overall, ZM650 consistently outperforms ZM850 in removal efficiency across all cycles, indicating its superior reusability for adsorption-desorption processes.

Fig. 22.

Reusability performance of ZM650 and ZM850 adsorbents over five adsorption-desorption cycles, demonstrating the percentage removal (%R) efficiency.

Conclusions

The research successfully demonstrated the synthesis and characterization of ZnO/Mg₃B₂O₆ nanostructures via the Pechini sol-gel strategy, highlighting their potential for removing basic yellow 28 dye from aqueous solutions. The nanostructures synthesized at 650 °C (ZM650) and 850 °C (ZM850) showed distinct properties, with ZM650 having a higher BET surface area and smaller average crystallite size compared to ZM850. The adsorption efficiency was influenced by pH, contact time, temperature, and initial dye concentration, with optimal removal observed at higher pH values and lower temperatures, indicating an exothermic and spontaneous process. Kinetic studies confirmed that the adsorption followed the pseudo-second-order model. Also, the equilibrium data aligned well with the Langmuir isotherm, suggesting monolayer adsorption on a homogeneous surface. The ZM650 exhibited superior adsorption capacity and reusability over multiple cycles compared to the ZM850, making it a more effective adsorbent for basic yellow 28 dye removal. The study underscores the promise of ZnO/Mg₃B₂O₆ nanostructures as efficient, reusable, and sustainable adsorbents for treating basic yellow 28 dye-contaminated water, contributing significantly to environmental remediation efforts.

Author contributions

Asma S. Al-Wasidi (Funding acquisition, Writing – Review & Editing), Gharieb S. El-Sayyad (Methodology), Fawaz A. Saad (Writing – Review & Editing, Visualization), Reem K. Shah (Visualization, Writing – Review & Editing), Ehab A. Abdelrahman (Methodology, Writing – Review & Editing, Conceptualization).

Funding

This work was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R35), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Data availability

All data generated or analyzed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.

Ethics approval

Not applicable.

Consent to participate

Not applicable.

Consent for publication

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