Facile synthesis of novel nanocomposite composed of Co₃O₄, MgO, and Mg₃B₂O₆ for malachite green dye decontamination from aqueous media

Abstract

Malachite green is a hazardous chemical that poses serious threats to aquatic ecosystems due to its toxicity and persistence in the environment. Additionally, it is harmful to human health, recognized as a carcinogenic and mutagenic agent that can cause long-term adverse effects. Hence, in this study, malachite green dye was efficiently removed from aqueous media using Co₃O₄/MgO/Mg₃B₂O₆ novel nanocomposites, known as CBM600 and CBM800. The CBM600 and CBM800 nanocomposites were facilely fabricated through the Pechini sol-gel procedure at 600 and 800 °C, respectively. Besides, X-ray diffraction analysis showed their crystalline structures, including Co₃O₄ and MgO in cubic systems, and Mg₃B₂O₆ in an orthorhombic system. For the CBM600 and CBM800 nanocomposites, the average crystal sizes are 40.76 nm and 57.43 nm, with BET surface areas of 74.25 m²/g and 57.59 m²/g, respectively. The highest uptake capacities for malachite green dye by the CBM600 and CBM800 nanocomposites are 492.61 mg/g and 440.53 mg/g, respectively. The pseudo-second-order model and Langmuir isotherm were the best fits for the adsorption data. Also, the uptake of malachite green dye by the CBM600 and CBM800 nanocomposites is spontaneous, physical, and exothermic. The nanocomposites were regenerated using 6 M HCl and repeatedly used to remove malachite green dye with very small loss in efficiency, demonstrating their robust reusability.

Introduction

The primary source of water contamination by organic dyes originates from industrial sectors, including textiles, leather tanning, cosmetics, and paper production^1–3. These industries utilize vast quantities of dyes for coloring purposes, and inadequate waste management practices often result in the discharge of colored effluents into nearby water bodies. The complexity and stability of organic dye molecules make them resistant to degradation, leading to persistent environmental contamination^4–6. Organic dyes constitute major hazards to the environment as well as human health. From an environmental perspective, these dyes reduce light diffusion through water, adversely impacting photosynthesis and threatening aquatic ecosystems^7,8. Regarding humans, prolonged subjected to certain dyes may result in serious health problems, such as allergic reactions, skin inflammation, and even carcinogenic effects^9,10. The presence of such dyes in drinking water sources further complicates the risks, making them a critical public health concern. Malachite green, a common organic dye used in textiles, is particularly notorious for its health hazards. It is a potent carcinogen and mutagen. Exposure to malachite green has been associated with serious health, such as potential increase in the risk of tumor formation and reproductive damage. The dye’s ability to accumulate in the human body exacerbates these risks, making it a focal point for regulatory agencies and health professionals alike^11,12. There are several approaches employed to eliminate organic dyes from water, including adsorption¹³, electrochemical treatments¹⁴, photocatalytic degradation¹⁵, and biological methods¹⁶. Each technique offers distinct mechanisms and efficiencies. Adsorption is commonly preferred for eliminating dyes because it is straightforward, economical, and highly effective, particularly with concentrated effluents. This technique is adaptable, supports different adsorbent materials, and performs well across diverse operational conditions. Additionally, unlike certain chemical processes, adsorption does not produce hazardous secondary pollutants^17–19. Nano-metal oxides, such as ZnO and MgO, have arisen as powerful adsorbents because of their high surface area. These characteristics enhance their capacity to adsorb and immobilize dye molecules, making them highly effective for water purification purposes²⁰. The Pechini sol-gel method is crucial in synthesizing metal nanooxides due to its ability to produce uniform and highly pure materials. By controlling the chemical composition at the molecular level, this method enables the fabrication of nanooxides with tailored characteristics for specific uses, including dye removal^21,22. The current research introduces a groundbreaking approach by synthesizing novel Co₃O₄/MgO/Mg₃B₂O₆ nanocomposites utilizing the Pechini sol-gel process, employing citric acid for chelation and ethylene glycol for crosslinking. These novel nanocomposites are then applied to treat water pollution caused by malachite green dye. The unique composition of these nanocomposites offers enhanced adsorptive properties, leading to superior removal efficiency. The nanocomposite benefits from the synergistic roles of Co₃O₄, MgO, and Mg₃B₂O₆. Co₃O₄ provides high-energy adsorption sites, MgO enhances porosity and stabilization, and Mg₃B₂O₆ adds active borate sites for dye binding. This combination results in superior adsorption performance and reusability. This innovative aspect highlights the potential for developing new materials that combine multiple metal oxides to achieve synergistic effects in dye adsorption. This comprehensive approach not only addresses the immediate impacts of dye pollution but also contributes to the sustainable development of advanced materials for environmental remediation. The Pechini sol-gel method was chosen for the production of nanocomposite due to its capability to produce uniform and high-purity nanocomposites with desirable nanostructured morphologies. The Pechini sol-gel method allows precise control of the chemical composition at the molecular level through the use of chelating agents like citric acid and crosslinking agents like ethylene glycol. Unlike hydrothermal methods, which rely heavily on pressure and temperature to achieve crystallinity, the Pechini sol-gel process enables the creation of well-defined crystalline structures even at moderate temperatures. The Pechini sol-gel process provides excellent uniformity and high purity due to the homogeneous mixing of precursors. This contrasts with co-precipitation, which can result in inhomogeneous mixing, leading to variable particle sizes and lower purity. Hence, the following advantages make it preferable over hydrothermal and co-precipitation techniques. Also, this study aligns with the United Nations Sustainable Development Goals (UNSDGs), particularly SDG 6 (Clean Water and Sanitation) by addressing water pollution through the efficient removal of malachite green dye, a hazardous pollutant. Additionally, the reusability of the synthesized nanocomposites supports SDG 12 (Responsible Consumption and Production), fostering sustainable and efficient resource utilization.

Experimental

Chemicals

Magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O), boric acid (H₃BO₃), citric acid (C₆H₈O₇), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), ethylene glycol (C₂H₆O₂), sodium hydroxide (NaOH), malachite green dye (C₂₃H₂₅ClN₂), potassium chloride (KCl), and hydrochloric acid (HCl) were purchased from Sigma-Aldrich chemical company. Each substance was utilized as received without further purification.

Synthesis of Co3O4/MgO/Mg3B2O6 nanocomposites

About 8.40 g of magnesium nitrate hexahydrate was freshly solubilized in 60 mL of deionized water. Also, 7.20 g of cobalt nitrate hexahydrate was freshly solubilized in 60 mL of deionized water. Furthermore, 1.44 g of boric acid was freshly solubilized in 60 mL of deionized water. After that, the prepared solutions from the previous steps were mixed with continuous agitation for 5 min. Moreover, 6 g of citric acid (C₆H₈O₇) was freshly solubilized in 60 mL of deionized water. In addition, the solution of citric acid was introduced into the resulting mixture and stirred continuously for 15 min. After that, 10 mL of ethylene glycol was added, and the mixture was stirred at 150 °C until the solvent had fully evaporated. The obtained powder was heated at 600 and 800 °C to obtain Co₃O₄/MgO/Mg₃B₂O₆ nanocomposites, which were abbreviated as CBM600 and CBM800, respectively. Besides, Fig. 1 exhibits the fabrication of Co₃O₄/MgO/Mg₃B₂O₆ nanocomposites.

Fig. 1.

Fabrication of Co₃O₄/MgO/Mg₃B₂O₆ nanostructures.

Characterization

FTIR spectra of CBM600 and CBM800 nanocomposites were measured using an FTIR spectrometer (MB147, BOMEM Inc., Canada). Their surface morphology and elemental composition were characterized using a field emission scanning electron microscope/energy-dispersive X-ray (FESEM/EDX) (Quanta 250 FEG, Thermo Fisher Scientific, USA), while high-resolution transmission electron microscopy (HRTEM) (JEM-2100Plus, JEOL Ltd., Japan) was employed to further investigate the surface structure. Surface texture analysis was conducted with an N₂ gas analyzer (BELSORP-minix S.N. 10039, MicrotracBEL Corp., Japan). X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (Bruker, D₈ Discover, USA).

Disposal of malachite green dye from aqueous solutions

The performance of CBM600 and CBM800 adsorbents in removing malachite green dye was assessed under multiple conditions, as presented in Table 1. Also, experiments utilized solutions with a malachite green dye concentration of 250 mg/L in a volume of 100 mL, treated with 50 mg of adsorbent. Variables such as pH (2–10), contact times (10–100 min), and temperatures (298–328 K) were modified, as specified in Table 1. Additionally, the impact of altering dye concentrations from 50 to 300 mg/L was investigated while ensuring a fixed solution volume and adsorbent quantity. The experimental procedure involved stirring the adsorbent/dye mixture for the specified durations, isolating the adsorbent via centrifugation, and measuring the residual concentration of malachite green dye at 620 nm using a UV/Vis spectrophotometer (GBC, Cintra 3030, Australia) to estimate adsorption efficiency. The UV/Vis spectrophotometry analysis at 620 nm accurately quantifies the residual malachite green concentration, reflecting the adsorption efficiency of CBM600 and CBM800. The decline in absorbance with time confirms effective dye removal.

Table 1.

Experimental settings for malachite green dye adsorption using CBM600 and CBM800 nanocomposites.

Impact	Dye volume (L)	Dye concentration (mg/L)	Adsorbent quantity (mg)	Temperature (K)	Time (min)	pH
pH	0.1	250	50	298	150	2–10
Time	0.1	250	50	298	10–100	10
Temperature	0.1	250	50	298–328	70	10
Concentration of dye	0.1	50–300	50	298	70	10

Two fundamental Eqs. 1 and 2 were employed to calculate the effectiveness of the synthesized nanocomposites in eliminating malachite green dye from aqueous solutions²⁰.

1

2

C_o represents the initial concentration of malachite green dye in the solution, expressed in mg/L. C_e denotes the concentration of malachite green dye remaining in the solution at equilibrium after the adsorption process, also in mg/L. V refers to the volume of the malachite green dye solution, measured in liters (L). W is the weight of the adsorbent used, measured in grams (g).

As an eluting agent, hydrochloric acid (HCl) at 2, 4, in addition to 6 M concentrations was applied in 50 mL volumes to regenerate malachite green dye- bearing adsorbents. Equation 3 was applied to measure the desorption efficiency of methylene blue dye from the nanocomposites²³.

3

C_d represents the concentration of malachite green dye in the desorption solvent, expressed in mg/L. V_d refers to the volume of the desorption solvent, measured in liters (L).

Post-regeneration, the adsorbents’ performance and lifespan were tested over five successive cycles. Every cycle involved treating 100 mL sample of a 250 mg/L malachite green dye solution with 50 mg of adsorbent, under consistent conditions: a contact duration of 70 min, a temperature near 298 K, as well as a pH level of 10.

Point of zero charge (pHPZC) of the CBM600 and CBM800 nanocomposites

A batch method was applied to establish the point of zero charge (pH_PZC) for the CBM600 and CBM800 nanocomposites. The initial pH values (pH_i) of 50 mL potassium chloride solutions were set and registered before 10 mg of adsorbent was added. Once 12 h of stirring was completed, the final pH values (pH_f) were measured. The difference between initial and final pH values (∆pH) was plotted against the initial pH. The intersection of the ∆pH vs. pH_i curve with the horizontal axis indicated the pH_PZC.

Results and discussion

Fabrication and characterization of Co₃O₄/MgO/Mg₃B₂O₆ nanocomposites

The Pechini sol-gel process was applied to yield Co₃O₄/MgO/Mg₃B₂O₆ nanostructures. This method focuses on the complexation, polymerization, and thermal processing steps to transform metal salts into high-purity mixed metal oxides with a fine, controlled nanostructure. Both magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O) and cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O) are dissolved in water to provide Mg²⁺ and Co²⁺ ions, along with nitrate ions and water, as shown in Eqs. 4 and 5.

4

5

Boric acid (H₃BO₃) is hydrolyzed in water to yield the borate ion (B(OH)₄⁻), as shown in Eq. 6.

6

Citric acid acts as a chelating agent for the metal ions (Mg²⁺, Co²⁺, and B(OH)₄⁻, as shown in Eq. 7.

7

Ethylene glycol is incorporated into the metal-citrate complex solution. Through polyesterification, ethylene glycol links with the carboxylic groups of citric acid, forming a polymeric network that encapsulates the metal ions. In addition, the polymeric gel is calcinated at elevated temperatures, causing the organic components (ethylene glycol and citric acid) to decompose into CO₂ and H₂O gases, leaving behind a matrix of mixed metal oxides (Co₃O₄/MgO/Mg₃B₂O₆), as shown in Eq. 8.

8

The polymeric gel calcination process decomposes organic components into CO₂ and H₂O gases, facilitating the formation of Co₃O₄, MgO, and Mg₃B₂O₆ phases and ensuring uniform metal ion dispersion. This decomposition creates porosity, enhancing the BET surface area and improving surface reactivity. These structural characteristics contribute to the nanocomposites’ physical stability and high adsorption efficiency.

The CBM600 and CBM800 samples were investigated for their crystalline structures and average crystal sizes using XRD, as shown in Fig. 2A-B, respectively. These samples comprised Co₃O₄ in a cubic system, Mg₃B₂O₆ in an orthorhombic system, and MgO in a cubic system, all identified by their respective 01-071-0816, 01-073-1541, and 01-086-8571 JCPDS numbers. For both samples, the crystal size was found to vary with the treatment temperature, being 40.76 nm for the CBM600 sample and 57.43 nm for the CBM800 sample. This indicates that an increase in the annealing temperature from 600 °C to 800 °C results in a noticeable growth in the mean crystallite size of the nanostructures. In terms of Mg₃B₂O₆, the diffraction peaks at 2Ɵ values of 21.20°, 22.54°, 25.87°, 33.68°, 35.02°, 40.37°, 41.49°, 50.97°, 52.97°, 54.98°, 61.22°, and 68.79° were associated with the (002), (101), (110), (112), (013), (121), (113), (114), (220), (130), (033), and (312) planes, respectively. Also, for MgO, the diffraction peaks at 2Ɵ values of 42.83° and 62.11° were linked to the (200) and (220) planes, respectively. Besides, for Co₃O₄, the diffraction peaks at 2Ɵ values of 19.19°, 31.45°, 36.92°, 38.70°, 44.94°, 55.87°, 59.55°, and 65.35° were linked to the (111), (220), (311), (222), (400), (422), (511), and (440) planes, respectively.

Fig. 2.

XRD patterns of the CBM600 (A) and CBM800 (B) nanocomposites.

Figure 3A-B clarifies the N₂ adsorption@desorption isotherms of the CBM600 and CBM800 nanocomposites, respectively. From the analysis, both the CBM600 and CBM800 samples demonstrate type IV isotherms with clear hysteresis loops, suggesting mesoporous textures²⁰. As shown in Table 2, there is a slight decline in both the BET surface area as well as total pore volume from the CBM600 product to the CBM800 product, from 74.25 m²/g to 57.59 m²/g and 0.2916 cc/g to 0.2334 cc/g, respectively. It appears that the higher temperature treatment of the CBM800 sample could result in a slight deterioration or obstruction of pores, diminishing the surface area in addition to the volume available. However, the average pore diameter shows little change, decreasing only slightly from 8.10 nm in the CBM800 sample to 7.85 nm in the CBM600 sample, which could be due to the structural changes mentioned.

Fig. 3.

N₂ adsorption@desorption isotherms of the CBM600 (A) and CBM800 (B) nanocomposites.

Table 2.

Surface texture details for CBM600 and CBM800 nanocomposites.

Surface characteristics	CBM600	CBM800
BET surface area (m2/g)	74.25	57.59
Total pore volume (cc/g)	0.2916	0.2334
Mean pore size (nm)	7.85	8.10

Figure 4A-B provides FE-SEM analysis of the CBM600 and CBM800 products, respectively. In Fig. 4A, a more aggregated and dense texture is seen, whereas Fig. 4B displays a somewhat more dispersed and fibrous morphology. It is inferred that these variations in structure are due to differences in the synthetic temperature. The porosity of the nanostructures is evident in both images, which can be associated with the potential for high surface reactivity, which is beneficial for various applications such as adsorption.

Fig. 4.

FE-SEM images of the CBM600 (A) and CBM800 (B) nanocomposites.

Figure 5A-B provides HR-TEM analysis of the CBM600 and CBM800 samples, respectively. The average diameters are 49.76 nm for CBM600 and 69.25 nm for CBM800, respectively. In both images, a densely packed arrangement of nanoparticles is observed, with distinct clusters indicating potential aggregation. This aggregation is likely responsible for the mismatch between the XRD and HR-TEM data, as the coagulated clusters can produce higher diameter measurements in the HR-TEM analysis in comparison to the average crystallite sizes obtained via XRD.

Fig. 5.

HR-TEM images of the CBM600 (A) and CBM800 (B) nanocomposites.

Figure 6A-B illustrates the FTIR spectra of the CBM600 and CBM800 samples, respectively. Also, the Mg-O stretching vibrations were identified at 718 cm^− 1 in the CBM600 product and 709 cm^− 1 in the CBM800 sample²⁴. The Co(III)-O and Co(II)-O stretching vibrations in octahedral and tetrahedral coordination within Co₃O₄ appeared at 576 and 671 cm^− 1 for CBM600, and at 573 and 652 cm^− 1 for CBM800, respectively²⁵. Additionally, the B-O-B bending vibrations were observed at 831 cm^− 1 for CBM600 and 842 cm^− 1 for CBM800. The B-O-B stretching vibrations were identified at 1281 cm^− 1 for CBM600 and 1276 cm^− 1 for CBM800. Bending vibrations from adsorbed H₂O were recognized at 1634 cm^− 1 for CBM600 and 1653 cm^− 1 for CBM800²⁶. Moreover, the stretching vibrations of adsorbed H₂O were detected at 3434 cm^− 1 for CBM600 and 3471 cm^− 1 for CBM800²⁰.

Fig. 6.

FTIR analysis of the CBM600 (A) and CBM800 (B) nanocomposites.

Removal of malachite green dye from aqueous media

Effect of pH

In this research, the effectiveness of removing malachite green dye was evaluated under different pH values utilizing the CBM600 and CBM800 nanocomposites, as depicted in Fig. 7A. The results revealed that the removal percentage of malachite green dye enhanced with higher pH values for both nanocomposites. At a pH of 10, the CBM600 and CBM800 adsorbents showed the maximum removal percentages of 93.24 and 81.93%, respectively. The CBM600’s superior performance compared to the CBM800 in eliminating malachite green dye is accredited to its greater surface area, which offers more active centers for malachite green dye removal.

Fig. 7.

(A) The influence of pH on the percentage removal of malachite green dye using CBM600 and CBM800 nanocomposites. (B) The point of zero charge values for the CBM600 and CBM800 nanocomposites.

The point of zero charge (pH_PZC) was determined to be 4.86 for CBM600 and 5.27 for CBM800, as demonstrated in Fig. 7B. These findings represent the pH value at which the adsorbent surfaces have a neutral charge, affecting the adsorption percentage. Above the pH_PZC, the malachite green dye removal increased due to the adsorbent surfaces acquiring a net negative charge, which strengthened the electrostatic attraction between the negatively charged adsorbent surfaces and the positively charged malachite green dye molecules, as illustrated in Fig. 8²⁰. Conversely, below the pH_PZC, the malachite green dye removal decreased because the adsorbent surfaces developed an overall positive charge, resulting in electrostatic repulsive forces between the adsorbents and the positively charged malachite green molecules, as displayed in Fig. 8²⁰. Hence, the optimal pH range for maximum adsorption of malachite green dye onto the nanocomposites is above their respective pH_PZC values, with pH 10 being the most effective. This is attributed to the net negative surface charge, enabling strong electrostatic interactions with the positively charged dye molecules.

Fig. 8.

Effect of pH on the surface charge of Co₃O₄/MgO/Mg₃B₂O₆ nanocomposites and their association with cationic malachite green dye.

Effect of contact time

In this study, the effectiveness of CBM600 and CBM800 adsorbents in removing malachite green dye was assessed over different time periods, as depicted in Fig. 9. Both adsorbents showed a steady rise in malachite green dye removal percentages as the duration extended from 10 to 70 min. At 70 min, the CBM600 and CBM800 adsorbents achieved removal percentages of 93.34 and 81.50%, respectively. After 70 min, the saturation of active sites limited further substantial removal of the malachite green dye. This saturation limits further adsorption, highlighting the need to optimize adsorbent dosage for high-demand applications.

Fig. 9.

The effect of contact time on the percentage removal of malachite green dye using CBM600 and CBM800 nanocomposites.

The results were analyzed using the pseudo-second-order and pseudo-first-order kinetic models, represented by Eqs. 9 and 10²⁰:

9

10

O_t and O_e represent the quantity of malachite green dye removed at adsorption time t and equilibrium, respectively, given in mg/g.

Additionally, F1 and F2 correspond to the rate constants for the pseudo-first-order and pseudo-second-order models, with units of 1/min and g/mg·min, respectively. Besides, Fig. 10A-B illustrates the application of the pseudo-first-order in addition to pseudo-second-order kinetic models to the removal of malachite green dye using CBM600 and CBM800 nanocomposites, respectively. The kinetic constants for the disposal of malachite green dye with CBM600 and CBM800 nanocomposites are displayed in Table 3. Assessment of the kinetic data showed that the pseudo-second-order kinetic model more accurately described the process, with R² values of 0.9999 for both nanocomposites, compared to 0.9731 for CBM600 and 0.9754 for CBM800 in the pseudo-first-order kinetic model. Moreover, the alignment of O_e with O_EXP in the pseudo-second-order model suggests that malachite green dye adsorption on CBM600 and CBM800 adheres to this model. The pseudo-second-order kinetic model suggests that the adsorption process is controlled by the availability of active sites, indicating strong physical adsorption. This involves electrostatic interactions between malachite green dye molecules and negative CBM600/CBM800 surfaces.

Fig. 10.

The pseudo-first-order (A) and pseudo-second-order (B) kinetic models for the disposal of malachite green dye by the CBM600 and CBM800 nanocomposites.

Table 3.

Kinetic characteristics of malachite green dye removal with CBM600 and CBM800 nanocomposites.

Adsorbent	OExp (mg/g)	Pseudo-first-order			Pseudo-second-order
		F1 (1/min)	Oe (mg/g)	R 2	F2 (g/mg.min)	Oe (mg/g)	R 2
CBM600	466.70	0.0213	325.28	0.9731	0.000135	467.29	0.9999
CBM800	409.64	0.0212	295.32	0.9754	0.000138	414.94	0.9999

Effect of temperature

In this study, the efficiency of the CBM600 and CBM800 nanocomposites in eliminating malachite green dye at various temperatures was evaluated, as exhibited in Fig. 11A. A rapid reduction in the percentage of malachite green dye elimination was observed for both nanocomposites as the solution temperature rose from 298 K to 328 K.

Fig. 11.

(A) The effect of temperature on the percentage removal of malachite green dye by CBM600 and CBM800 nanocomposites. (B) The plot of ln F_d versus 1/T.

The values of Gibbs free energy (ΔG°), entropy change (ΔS°), and enthalpy change (ΔH°) for malachite green dye elimination by CBM600 and CBM800 nanocomposites were calculated using Eqs. 11, 12, and 13²⁰.

11

12

13

The symbols F_d, T, and R represent the distribution coefficient, temperature, and gas constant, with respective units of L/g, K, and kJ/mol·K. Figure 11B presents the ln F_d vs. 1/T plot for the removal of malachite green dye by CBM600 and CBM800 nanocomposites. Table 4 details the thermodynamic parameters for malachite green dye adsorption on these nanocomposites. The positive entropy change (ΔS > 0) indicates that the adsorption of malachite green dye onto CBM600 or CBM800 is thermodynamically feasible, with increased randomness at the solid@liquid interface. An enthalpy change (ΔH) below 40 kJ/mol signifies that the adsorption is predominantly physical. Besides, the negative enthalpy change (ΔH < 0) confirms the exothermic behavior of the adsorption process. A negative Gibbs free energy change (ΔG < 0) reveals the spontaneous nature of the dye adsorption.

Table 4.

Thermodynamic characteristics for malachite green dye disposal by the CBM600 and CBM800 nanocomposites.

Adsorbent	ΔS° (KJ/molK)	ΔH° (KJ/mol)	ΔG° (KJ/mol)
			298	308	318	328
CBM600	0.0486	− 22.78	− 37.27	− 37.75	− 38.24	− 38.73
CBM800	0.0251	− 12.94	− 20.42	− 20.68	− 20.93	− 21.18

Effect of concentration

In this study, the removal efficiency of malachite green dye at varying concentrations was investigated using the CBM600 and CBM800 adsorbents, as illustrated in Fig. 12. Both adsorbents showed a steady decline in the percentage of malachite green dye removed as concentration levels increased from 50 to 300 mg/L. Both adsorbents show a decline in removal efficiency as dye concentration increases from 50 to 300 mg/L, caused by the saturation of active sites and competition among dye molecules.

Fig. 12.

The effect of initial dye concentration on the percentage removal of malachite green dye using CBM600 and CBM800 nanocomposites.

The results were examined using the Freundlich in addition to Langmuir equilibrium isotherms, outlined Eqs. 14 and 15²⁰:

14

15

The rate constants for the Freundlich in addition to Langmuir isotherms, represented by F₃ and F₄, are measured in (mg/g)(L/mg)^1/n and (L/mg), respectively. The parameter 1/Y represents adsorption intensity, while O_max (mg/g) indicates the maximum uptake capacity. Equation 16 outlines the use of the Freundlich equilibrium isotherm to determine O_max²⁰.

16

Figure 13A-B shows the Langmuir and Freundlich isotherms for the removal of malachite green dye using CBM600 and CBM800 nanocomposites, respectively. Table 5 details the equilibrium constants for dye removal with these nanocomposites. The equilibrium data analysis indicated a better fit for the Langmuir isotherm, with R² values of 0.9974 and 0.9957 for both adsorbents, in contrast to 0.7830 and 0.7760 for the Freundlich equilibrium isotherm in CBM600 and CBM800, respectively. Hence, the Langmuir isotherm model reveals that malachite green adsorption occurs as a monolayer on a homogeneous surface, with no interactions between adsorbed molecules. It provides the theoretical maximum adsorption capacities, which were determined using Eq. 15, of 492.61 mg/g for CBM600 and 440.53 mg/g for CBM800, highlighting their efficiency.

Fig. 13.

The Langmuir (A) and Freundlich (B) equilibrium isotherms for malachite green dye disposal using CBM600 and CBM800 nanocomposites.

Table 5.

Equilibrium data for malachite green dye disposal by CBM600 and CBM800 nanocomposites.

Adsorbent	Langmuir			Freundlich
	Omax (mg/g)	F4 (L/mg)	R 2	Omax (mg/g)	F3 (mg/g) (L/mg)1/n	1/Y	R 2
CBM600	492.61	0.3248	0.9974	1059.94	133.59	0.3751	0.7830
CBM800	440.53	0.1662	0.9957	764.66	94.29	0.3791	0.7760

The initial dye concentration significantly influences adsorption kinetics and equilibrium isotherms. Higher concentrations increase the adsorption rate due to a stronger concentration gradient but reduce percentage removal as active sites become saturated. Adsorption kinetics for both CBM600 and CBM800 follow the pseudo-second-order model (R² = 0.9999), while equilibrium data fit the Langmuir isotherm, indicating monolayer adsorption. CBM600 exhibits a higher maximum adsorption capacity (492.61 mg/g) than CBM800 (440.53 mg/g) due to its larger surface area and more accessible active sites. The current investigation compared the adsorption capacities of different adsorbents for malachite green dye, as detailed in Table 6^27–35. Among the tested adsorbents, CBM600 displayed the maximum adsorption potential of 492.61 mg/g, with CBM800 close behind at 440.53 mg/g. These capacities were notably higher than those of various adsorbents. Also, the outstanding performance characteristics of the newly examined CBM600 and CBM800 nanocomposites highlights their prospective effectiveness in improving dye disposal during wastewater remediation. Besides, these outcomes underscore the developments in removal technology and the serious importance of material fabrication for environmental purification. Hence, the precursor stoichiometry and calcination temperature significantly influenced the nanocomposite’s crystallite size, surface area, morphology, and adsorption efficiency. Higher calcination temperatures resulted in larger crystallite sizes (57.43 nm for CBM800 compared to 40.76 nm for CBM600). Also, CBM600 exhibited a higher surface area (74.25 m²/g) compared to CBM800 (57.59 m²/g), leading to greater adsorption capacity for malachite green dye (492.61 mg/g for CBM600 versus 440.53 mg/g for CBM800). Besides, the CBM600 had a more aggregated texture compared to CBM800, attributed to differences in calcination temperature. Consequently, the smaller particle size and higher surface area of CBM600 provide more active sites for malachite green dye adsorption, while its aggregated texture enhances adsorption capacity. In contrast, the larger particle size and reduced surface area of CBM800 limit its adsorption efficiency.

Table 6.

Comparative evaluation of maximum uptake capacities of several adsorbents for malachite green dye.

Adsorbent	Omax (mg/g)	Ref.
Nanofiber/graphene oxide polymer aerogel	270.30	27
Hydroxyethyl starch/graphene oxide	89.3	28
Chitosan/SDS composite	360.00	29
Sodium alginate-guar-gum-tannic acid	203.00	30
Alginate/sepiolite/glutaraldehyde	122.00	31
ZnO/chitosan composite	11.00	32
AlOOH/CoFe2O4	423.70	33
Chitosan/montmorillonite composite	400.00	34
Diatomite/Ni/NiO composite	350.90	35
CBM600	492.61	This study
CBM800	440.53	This study

Effect of regeneration and reusability

This study assessed the role of varying HCl concentrations in removing malachite green dye from the CBM600 and CBM800 nanocomposites, as illustrated in Fig. 14. The results indicated an increase in dye removal efficiency with rising HCl concentrations for each adsorbent type. Desorption efficiencies at 6 M concentration reached 99.69% for CBM600 and 99.91% for CBM800, indicating a strong dependency of desorption effectiveness on the acidic nature of the eluent. The increased desorption percentages at higher HCl concentrations are likely due to the breakdown of connections between dye molecules and the adsorbent surfaces. The near-complete removal of dye at this concentration emphasizes the viability of regenerating the nanocomposites. The study stresses the essential role of fine-tuning eluent concentration for optimal dye retrieval and sustainable use. Both CBM600 and CBM800 have proven to be highly effective for repeated desorption cycles. The XRD analysis of the regenerated adsorbents indicated that their crystalline structure remained intact after five adsorption-desorption cycles, demonstrating excellent structural stability under repeated use (Figures omitted for brevity).

Fig. 14.

Desorption efficiency of malachite green dye from the CBM600 and CBM800 nanocomposites at different concentrations of HCl.

The repeated utilization of CBM600 and CBM800 nanocomposites for malachite green dye removal was comprehensively evaluated over multiple cycles, as depicted in Fig. 15. The dye elimination efficiency stayed relatively unchanged through five cycles, demonstrating the adsorbents’ performance was stable with continued use. This consistent efficiency over successive cycles highlights the cost-effectiveness and practicality of these adsorbents for long-term water purification.

Fig. 15.

Repeated use of CBM600 and CBM800 nanocomposites across multiple cycles for malachite green dye adsorption.

Effect of coexisting ions

The influence of various coexisting ions on the adsorption efficiency of CBM600 and CBM800 nanocomposites was examined to simulate real-world conditions where multiple ionic species coexist in water. Common ions such as potassium (K⁺), chloride (Cl⁻), calcium (Ca²⁺), magnesium (Mg²⁺), sulfate (SO₄²⁻), and nitrate (NO₃⁻) were introduced at a concentration of 50 mg/L alongside a fixed malachite green dye concentration of 50 mg/L. These coexisting ions compete with the dye molecules for active adsorption sites, potentially reducing the adsorption performance. The results showed that CBM600 exhibited higher resilience to the effects of coexisting ions compared to CBM800, retaining a significant proportion of its adsorption efficiency. This can be attributed to CBM600’s larger surface area (74.25 m²/g) and mesoporous structure, which provide more accessible active sites and stronger interactions with the dye molecules. In contrast, CBM800, with its smaller surface area (57.59 m²/g) and larger crystallite size, was more affected by the competition from coexisting ions. The removal efficiencies of malachite green dye in the presence of different coexisting ions are summarized in Table 7.

Table 7.

Effect of coexisting ions on the removal efficiency of malachite green dye.

Adsorbent	Coexisting ion	Removal Efficiency (%)
CBM600	K+	90.12
CBM600	Cl−	89.67
CBM600	Ca2+	88.45
CBM600	Mg2+	87.92
CBM600	SO42−	86.87
CBM600	NO3−	87.34
CBM800	K+	83.56
CBM800	Cl−	82.78
CBM800	Ca2+	81.23
CBM800	Mg2+	80.91
CBM800	SO42−	79.34
CBM800	NO3−	80.12

Practical application

To evaluate the practical applicability of the synthesized nanocomposites, a real water sample from a student laboratory wastewater contaminated with malachite green dye (50 mg/L) was treated under standard conditions. These conditions included a pH of 10, a contact time of 70 min, and a temperature of 298 K. For the experiments, 0.05 g of CBM600 and CBM800 nanocomposites were utilized as adsorbents. The results demonstrated that CBM600 and CBM800 achieved removal efficiencies of 97.12 and 90.13%, respectively. The superior performance of CBM600 can be attributed to its higher surface area (74.25 m²/g) and mesoporous structure, which provide more active adsorption sites. In comparison, the slightly lower removal efficiency of CBM800 is associated with its reduced surface area (57.59 m²/g) and larger crystallite size. These findings highlight the potential of the synthesized nanocomposites for real-world wastewater remediation applications, particularly in removing hazardous dyes from aqueous environments.

Conclusions

This investigation focused on the effective elimination of malachite green dye from aqueous media by novel Co₃O₄/MgO/Mg₃B₂O₆ nanocomposites, designated as CBM600 and CBM800. These nanocomposites were fabricated utilizing a facile Pechini sol-gel route at 600 and 800 °C, respectively. Also, XRD patterns confirmed their crystalline structures in cubic (Co₃O₄ and MgO) and orthorhombic (Mg₃B₂O₆) systems, with average crystal sizes of 40.76 nm for CBM600 and 57.43 nm for CBM800. The BET surface areas were 74.25 m²/g for CBM600 and 57.59 m²/g for CBM800, and their maximum adsorption capacities towards malachite green dye reached 492.61 mg/g and 440.53 mg/g, respectively. Maximum malachite green dye removal was achieved under conditions of pH 10, 70 min of contact time, and a temperature of 298 K, highlighting the most effective parameters for dye extraction. The adsorption process, best described by the pseudo-second-order model and Langmuir isotherm, was found to be exothermic, physical, and spontaneous. Also, this study investigated the influence of HCl concentration on the removal of malachite green dye from CBM600 and CBM800 adsorbents, revealing that higher concentrations significantly enhance dye desorption, achieving up to 99.91% efficiency at 6 M. These findings emphasize the need to optimize eluent acidity to maximize dye retrieval and support the sustainable reuse of adsorbents, which maintain high removal efficiencies over multiple cycles, demonstrating their long-term value in water purification.

Author contributions

Asma S. Al-Wasidi (Funding acquisition, Writing – Review & Editing), Reem K. Shah (Writing – Review & Editing), Nermin A. Abdelhakim (Writing – Original Draft Preparation, Methodology), Fawaz A. Saad (Writing – Review & Editing), Ehab A. Abdelrahman (Writing – Review & Editing, Conceptualization, Methodology).

Funding

This research 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 relevant to this study are contained within this published article.

Declarations

Competing interests

The authors declare no competing interests.