Introduction
The increasing presence of synthetic organic dyes in aquatic systems has become a critical environmental concern due to their extensive use in various industrial processes, for example, textiles, leather, paper, plastics, and cosmetics1, 2–3. These dyes are often discharged as untreated or partially treated effluents into water bodies, where they contribute significantly to water contamination. The complex molecular structures of organic dyes impart excellent structural durability and low susceptibility to biological decomposition, resulting in considerable difficulty during their separation from aqueous effluents4,5. In particular, the persistence of such pollutants in aquatic environments severely disrupts ecological balance and threatens the availability of clean water resources6,7. The release of synthetic organic dyes into natural aquatic systems constitutes alarming threat to both the human health and environment. Dyes interfere with light diffusion within aquatic environments, affecting photosynthetic activity and disturbing aquatic ecosystems8,9. Moreover, many dyes and their degradation byproducts exhibit toxic, mutagenic, and carcinogenic effects10,11. Prolonged exposure to contaminated water may lead to skin irritation, respiratory problems, and severe organ damage in humans12,13. Among these dyes, malachite green has drawn particular attention due to its widespread application and pronounced toxicity14. Malachite green is considered a cationic triphenylmethane dye frequently employed in biological staining, textiles, and aquaculture. Despite its effectiveness in these applications, malachite green is known for its toxicological profile, which includes teratogenic, mutagenic, and carcinogenic effects15. Once released into water bodies, it can accumulate in aquatic organisms, enter the food chain, and ultimately affect human health. Its persistence in the environment and resistance to natural degradation mechanisms make it a high-priority contaminant16. To mitigate the environmental impact of such dyes, several treatment technologies have been explored. These include membrane filtration17coagulation-flocculation18electrodialysis19photocatalytic degradation20bioremediation21and adsorption22. Each method has distinct advantages and limitations. For instance, membrane filtration offers high removal efficiency but is often hindered by fouling and operational costs23. Coagulation-flocculation is simple but generates substantial sludge24. Photocatalytic degradation and bioremediation are environmentally friendly but typically require long reaction times or specific operational conditions25,26. Among these approaches, adsorption has emerged as a particularly promising technique due to its simplicity, cost-effectiveness, scalability, and high removal efficiency even at low contaminant concentrations27. Unlike many chemical or biological treatments, adsorption does not involve harmful byproducts or secondary pollution. The ability to regenerate and reuse adsorbents further enhances its appeal for sustainable wastewater management28. In recent years, metal oxide nanoparticles and their composite materials have gained substantial interest as proficient adsorbents for organic dye remediation29, 30–31. Their high surface area, tunable surface properties, and enhanced reactivity make them suitable for capturing a wide range of pollutants. When these nanoparticles are combined with organic components or other metal oxides, the resulting composites often exhibit synergistic effects, improving adsorption performance and stability under various environmental conditions32, 33, 34–35. The Pechini sol-gel method has proven to be a powerful technique for synthesizing metal oxide nanoparticles with controlled morphology, homogeneity, and compositional uniformity. Unlike traditional sol-gel or co-precipitation methods, the Pechini route ensures molecular-level mixing of precursors and allows for the production of complex multi-component oxide systems at relatively low calcination temperatures. This method enables the precise engineering of adsorbent materials tailored for specific applications in environmental remediation36. Several adsorbents have been reported in the literature regarding the remediation of malachite green dye, including bentonite with a maximum adsorption capacity of 78.39 mg/g37bentonite/cobalt nanoparticles at 145.09 mg/g37sulfur-doped biochar at 30.18 mg/g38meta-kaolin-based geopolymer at 185.18 mg/g39and graphene oxide/cellulose/copper composite at 207.10 mg/g40. Despite their relative success, these materials exhibit limitations such as low thermal stability, poor reusability, limited adsorption capacity, or sensitivity to pH fluctuations. This situation underscores the need for the development of more efficient and robust adsorbents. The present study addresses this need by simply synthesizing novel Pb2(CrO4)O/MgO/MgCrO4/C nanocomposites by adopting a polymer-assisted sol-gel synthesis pathway followed by calcination at 600 and 800 oC. Additionally, the impact of calcination temperature on structure and adsorption performance toward malachite green dye was systematically investigated by comparing materials synthesized at 600 and 800 oC. Unlike some previous adsorbents in the literature37, 38, 39–40this work combines detailed structural and morphological analyses to demonstrate the high efficiency of these nanocomposites in removing malachite green dye from aqueous media. The integration of Pb2(CrO4)O contributes to strong electrostatic interactions with cationic dye molecules due to its high anionic charge density41. MgO enhances structural stability and surface basicity, which favors dye adsorption42. MgCrO4 introduces additional active sites through its dual metal centers43while the carbon content derived from organic precursors increases surface area and facilitates dye binding through π-π interactions44, 45–46. Together, these components synergistically improve the adsorption capacity and functional performance of the resulting nanocomposites, making them promising candidates for the effective removal of malachite green dye from contaminated water.
Materials and methods
Materials
All chemicals utilized in this investigation were of analytical grade with a purity ≥ 99% except hydrochloric acid (HCl, 36%) and supplied by Sigma-Aldrich. Ethylene glycol (C2H6O2) as well as citric acid (C6H8O7) were served as organic precursors in the fabrication process. Magnesium nitrate hexahydrate (Mg(NO3)2.6H2O), chromium nitrate nonahydrate (Cr(NO3)3.9H2O), and lead nitrate (Pb(NO3)2) were employed as metal precursors. Hydrochloric acid (HCl) as well as sodium hydroxide (NaOH) were utilzed to regulate the solution pH. In addition, potassium chloride (KCl) was served as an electrolytic stabilizer for the estimation of the point of zero charge. Malachite green dye (C23H25ClN2) was selected as the target pollutant to evaluate the adsorption performance of the prepared nanocomposites.
Synthesis of Pb2(CrO4)O/MgO/MgCrO4/C nanocomposites
The synthesis of Pb2(CrO4)O/MgO/MgCrO4/C nanocomposites was carried out through a Pechini-type sol-gel route, as schematically represented in Fig. 1. The procedure commenced through dissolving 35 g of citric acid in 100 mL of fresh distilled water to prepare the organic complexing agent. Separately, 20 g of Mg(NO3)2.6H2O, 20 g of Cr(NO3)3.9H2O, and 20 g of Pb(NO3)2 were completely solubilized in 200 mL of fresh distilled water. Subsequently, the citric acid solution was slowly introduced into the metal nitrate solution while preserving constant agitating for 30 min to ensure homogeneous mixing and chelation. Thereafter, 15 mL of ethylene glycol was incorporated into the reaction system as a polymerizing agent, and the obtained mixture was agitated consistently at 200 oC until all liquid content had evaporated, forming a dry gel. The resulting powders were then subjected to calcination at 600 and 800 oC for 3 h in air, using a heating rate of 5 oC/min, to yield MP600 and MP800, respectively.
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Fig. 1
Schematic representation of the Pechini sol-gel synthesis route for Pb2(CrO4)O/MgO/MgCrO4/C nanocomposites.
Instrumentation
The crystalline structures of all generated products were investigated utilizing an X-ray diffraction diffractometer (XRD) from Bruker (USA) of D8 Advance model. Also, the surface morphology in addition to elemental composition were investigated utilizing a field emission scanning electron microscope coupled with energy dispersive X-ray spectroscopy (FE-SEM/EDX) from Thermo Fisher Scientific (USA) with Quanta 250 FEG model. Detailed internal structures and particle sizes were investigated by a high-resolution transmission electron microscope (HR-TEM) from (JEOL Ltd., Japan) with JEM-2100Plus model. The surface textures of the samples were determined using Brunauer–Emmett–Teller (BET) analysis performed on a Micromeritics ASAP 2020 instrument. Fourier-transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS10, USA) was conducted in the range of 4000–400 cm− 1 to identify functional groups and chemical bonding present in the samples. The concentrations of malachite green dye in aqueous solution were examined utilizing a UV-Vis spectrophotometer from (GBC, Australia) with Cintra 3030 model.
Uptake behavior of malachite green dye from water-based solutions
The remediation experiments for malachite green dye were performed under various experimental settings, as summarized in Table 1. In addition, to investigate the effect of pH on dye remediation, 150 mL of an aqueous solution containing 300 mg/L of malachite green dye in addition to 0.1 g of the adsorbent was prepared and subjected to stirring at 298 K for 300 min at different pH values ranging from 2 to 10 with the aid of magnetic agitation. For examining the influence of contact duration, the same solution composition and temperature were used, and the samples were stirred for varying durations from 10 to 100 min at a fixed pH of 10 to assess the adsorption kinetics. The influence of temperature on dye removal was examined by preparing 150 mL of a 300 mg/L dye solution with 0.1 g of the adsorbent and adjusting the temperature within the range of 298 to 328 K. In addition, the stirring time was selected as 50 min for MP600 in addition to 70 min for MP800 at pH 10. To evaluate the effect of initial malachite green dye concentration, 150 mL of malachite green dye solutions in the concentration interval of 100–400 mg/L were treated with 0.1 g of the adsorbent at 298 K and pH 10. The contact time was 50 min regarding MP600 in addition to 70 min regarding MP800. After each experiment, the solid phase was isolated via centrifugation, and the residual concentration of malachite green dye after adsorption was determined in the supernatant with the aid of a UV–Vis spectrophotometer at a wavelength of 617 nm. The dye removal percentage (% R) and the adsorption capacity (Q) were determined based on Eqs. (1) and (2), respectively47.
1
2
Co is the initial dye concentration (mg/L), Ce is the equilibrium dye concentration (mg/L), V is the volume of the solution (L), and W is the mass of the adsorbent (g).
All batch adsorption experiments were performed in duplicate, and the average values are reported. The relevant figures include error bars representing the range between the two measurements to indicate experimental reproducibility.
It should be noted that the contact times used for MP600 (50 min) and MP800 (70 min) were determined based on the time each material required to reach adsorption equilibrium during preliminary kinetic studies. MP600 exhibited a faster adsorption rate and reached equilibrium within 50 min, while MP800 required 70 min to reach its equilibrium point. These differences are attributed to variations in surface properties and textural characteristics between the two samples. Therefore, each material was evaluated at its respective equilibrium time to ensure a consistent and unbiased comparison of their adsorption efficiencies. All subsequent studies of influencing parameters, such as pH, initial dye concentration, and temperature, were conducted using these equilibrium times to reflect the optimal performance conditions of each sample.
Table 1. Experimental conditions applied to evaluate the effects of pH, contact time, temperature, and initial malachite green dye concentration on the adsorption performance of MP600 and MP800 nanocomposites.
Impact | Experimental conditions | |||||
|---|---|---|---|---|---|---|
V (mL) | Co (mg/L) | W (g) | T (K) | t (min) | pH | |
pH | 150 | 300 | 0.1 | 298 | 300 | 2–10 |
Time | 150 | 300 | 0.1 | 298 | 10–100 | 10 |
Temperature | 150 | 300 | 0.1 | 298–328 | 50 (MP600) 70 (MP800) | 10 |
Concentration of malachite green dye | 150 | 100–400 | 0.1 | 298 | 50 (MP600) 70 (MP800) | 10 |
To evaluate the regeneration performance of the adsorbents, desorption experiments were conducted using hydrochloric acid solutions of varying concentrations. Malachite green-loaded MP600 and MP800 nanocomposites were treated with 100 mL of HCl at concentrations of 1 M, 1.5 M, and 2 M. Each suspension was agitated consistently over a period of 100 min to facilitate desorption of the dye from the adsorbent surface. After treatment, the mixtures were centrifuged to separate the solids, and the dye concentration in the supernatant was measured using a UV-Vis spectrophotometer at the maximum wavelength of malachite green dye. The percentage of malachite green desorbed (% D) was determined according to Eq. (3)36.
3
Cd is the concentration of malachite green dye in the desorption medium (mg/L), and Vd is the volume of the desorption medium (L).
The reusability of MP600 and MP800 nanocomposites was evaluated through five successive adsorption–desorption runs to evaluate their regeneration efficiency and long-term performance. In each cycle 150 mL of malachite green dye solution with an initial concentration of 300 mg/L was treated with 0.1 g of the adsorbent at 298 K and pH 10. The contact time was maintained at 50 min regarding MP600 in addition to 70 min regarding MP800. After the remediation step, the dye-loaded adsorbent was isolated and subjected to desorption using 100 mL of 2 M hydrochloric acid. The regenerated adsorbent was washed thoroughly with distilled water and reused under the same conditions for the subsequent cycle. This procedure was repeated for five cycles to determine the stability and reusability of the nanocomposites in dye removal applications.
A binary adsorption study was conducted to evaluate the impact of interfering ions on malachite green dye adsorption using MP600 and MP700 nanocomposites. The experiments were performed at pH 10 and 298 K with an initial dye concentration of 300 mg/L maintaining a 1:1 molar ratio of malachite green dye to each competing ion. The interfering ions included Na+, K+, Mg2+, Ca2+, Cl−, NO3−, and SO42−, representing common cations and anions found in aqueous environments. Batch adsorption experiments were conducted by adding 0.1 g of each adsorbent to 150 mL of dye solution containing the competing ion. The mixtures were stirred for 50 min for MP600 and 70 min for MP800 until equilibrium was achieved. The final dye concentration was measured using UV-Vis spectrophotometer and adsorption capacities were determined based on mass balance calculations.
Evaluation of the pHPZC of the synthesized adsorbent materials
The pHPZC values of MP600 and MP800 nanocomposites were evaluated through batch equilibrium experiments, where potassium chloride (KCl) functioned as the inert electrolyte. To investigate surface charge behavior, 0.01 M KCl solutions (50 mL each) were formulated and their initial pH (pHI) levels were tuned across the 2–12 range using either 0.1 M HCl or 0.1 M NaOH. A predetermined quantity of 0.05 g of each nanocomposite was incorporated into the solutions, and the blends were agitated for one day at 298 K to allow for pH equilibration. After equilibration, each solution’s final pH (pHF) was measured, and the shift in pH (△pH) was computed in accordance with Eq. (4)35.
4
The values of △pH were plotted against the corresponding initial pH values, and the pHPZC was identified as the point where △pH equals zero.
Results and discussion
Characterization results
XRD
Figure 2 illustrates the X-ray diffraction (XRD) patterns of synthesized nanocomposite samples MP600 (Fig. 2A) and MP800 (Fig. 2B). The identified peaks in both samples matched the standard diffraction patterns of Pb2(CrO4)O, MgO, and MgCrO4, confirming the successful formation of the composite. Hence, both MP600 and MP800 exhibited identical phase compositions, with all peaks corresponding to Pb2(CrO4)O, MgO, and MgCrO4 observed in both samples. Pb2(CrO4)O was indexed to a monoclinic crystal structure (JCPDS-00-029-0768), exhibiting characteristic diffraction peaks at 2θ values of 13.95, 14.90, 17.09, 20.02, 23.60, 25.17, 26.34, 27.59, 30.00, 31.48, 33.78, 34.73, 35.99, 37.88, 38.90, 39.87, 41.13, 43.96, 45.53, 46.58, 48.68, 51.62, 52.99, 54.03, 55.08, 56.45, 57.29, 58.75, 61.07, 63.27, and 65.57, corresponding to Miller indices of (-101), (101), (110), (011), (-301), (211), (310), (-202), (-112), (020), (-411), (220), (411), (312), (510), (402), (013), (222), (303), (-611), (-422), (422), (330), (620), (-132), (323), (-712), (431), (-811), (721), and (040), respectively. MgO exhibited a cubic structure (JCPDS-00-045-0946), identified through characteristic peaks at 2θ angles of 43.01, 62.30, in addition to 78.90, with corresponding Miller indices values of (200), (220), in addition to (222). The orthorhombic MgCrO4 phase (JCPDS-01-072-1606) presented diffraction peaks at 2θ of 68.51, 70.83, 72.72, 73.76, 75.33, and 77.43, associated with Miller indices (400), (204), (134), (243), (153), and (333), respectively. The average crystallite sizes were 62.4 nm for MP600 and 73.6 nm for MP800. This increase in crystallite size for MP800 relative to MP600 is attributed to enhanced crystallinity and particle growth facilitated by the higher calcination temperature, resulting in improved crystal structures.
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Fig. 2
XRD patterns of (A) MP600 and (B) MP800 samples.
EDS and mapping
Figure 3A-B displays the EDS spectra of the MP600 and MP800 products, respectively. The detected elements, as clarified in Table 2, include carbon (C), oxygen (O), magnesium (Mg), chromium (Cr), and lead (Pb), which are consistent with the expected composition of the synthesized nanocomposites. The presence of carbon in both samples is associated with the decomposition residues of the organic precursors, namely ethylene glycol and citric acid, utilized during synthesis.
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Fig. 3
EDS patterns of (A) MP600 and (B) MP800 samples.
Table 2. Elemental analysis of MP600 and MP800 products.
Product | Atomic percentages | ||||
|---|---|---|---|---|---|
% C | % O | % Mg | % Cr | % Pb | |
MP600 | 11.9 | 47.9 | 18.9 | 7.6 | 13.7 |
MP800 | 9.8 | 54.1 | 23.2 | 5.6 | 7.3 |
Figure 4A-B presents the elemental mapping images of MP600 and MP800 nanocomposites, respectively, which reveal the spatial distribution of the constituent elements, including carbon, oxygen, magnesium, chromium, and lead. The uniform and widespread presence of these elements across the examined regions confirms their successful incorporation within the composite matrices. MP600 displays slightly more localized elemental clusters, particularly for magnesium and chromium, whereas MP800 shows a more homogeneous and evenly dispersed elemental pattern. The elemental uniformity observed in MP800 suggests enhanced mixing and diffusion at higher calcination temperatures, contributing to the improved structural integrity of the nanocomposite. These findings support the effective formation of multiphase composites and indicate that the synthesis conditions directly influence the compositional uniformity of the final materials.
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Fig. 4
Elemental mapping images of (A) MP600 and (B) MP800 samples.
FE-SEM
Figure 5 presents the FE-SEM analysis of the MP600 (Fig. 5A) in addition to MP800 (Fig. 5B) products, revealing significant morphological distinctions between the two samples. The MP600 sample in Fig. 5A exhibits an irregular and highly agglomerated structure composed of poorly defined particles with a broad size distribution and a rough surface texture. In contrast, the MP800 sample shown in Fig. 5B displays a more uniform morphology characterized by well-defined quasi-spherical to polyhedral nanoparticles with smooth surfaces and reduced aggregation. These morphological differences are primarily attributed to the effect of calcination temperature. The higher temperature used for MP800 promotes enhanced crystallinity, grain growth, and thermal decomposition of residual organics, resulting in better particle definition and dispersion. The presence of distinct shapes in MP800, such as nearly spherical and faceted polyhedral forms, suggests the thermally induced transformation toward a more thermodynamically stable structure. Overall, the morphological evolution from disordered aggregates in MP600 to well-shaped nanoparticles in MP800 confirms that thermal treatment plays a crucial role in determining the structural integrity and homogeneity of the synthesized nanomaterials.
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Fig. 5
FE-SEM images of (A) MP600 and (B) MP800 samples.
HR-TEM
Figures 6 and 7 provide valuable insights into the morphological and size distribution characteristics of the MP600 and MP800 nanocomposites. The HR-TEM images in Fig. 6A show that the MP600 sample consists of nearly spherical and highly agglomerated nanoparticles with indistinct boundaries, while the MP800 sample in Fig. 6B exhibits a more diverse morphology comprising well-resolved spherical to polygonal nanoparticles with clearer edges and reduced agglomeration. This evolution in particle shape and definition is attributed to the effect of calcination temperature, where the higher temperature used for MP800 promotes improved crystallinity and particle growth. Figure 7 displays the particle size distribution histograms for both samples. The MP600 histogram in Fig. 7A shows a relatively narrow distribution centered around smaller sizes, while the MP800 histogram in Fig. 7B reveals a broader size distribution extending to larger diameters. The average particle size is 16.8 nm for MP600 and 26.8 nm for MP800, indicating that higher thermal treatment leads to particle coalescence and growth. The broader size range and larger average diameter observed in MP800 result from the improved atomic transport and sintering influences that occur under high-temperature conditions, facilitating the formation of greater and more defined nanoparticles. These findings confirm that the calcination temperature plays a critical role in controlling the particle size, shape, and dispersion of the synthesized nanomaterials.
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Fig. 6
HR-TEM images of (A) MP600 and (B) MP800 products.
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Fig. 7
HR-TEM histograms of (A) MP600 and (B) MP800 products.
The crystalline nature of the synthesized nanocomposites was further confirmed by selected area electron diffraction (SAED) analysis. As shown in Fig. 8, the distinct and continuous diffraction rings observed for both MP600 and MP800 indicate their polycrystalline structure, consistent with the XRD results.
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Fig. 8
SAED patterns of (A) MP600 and (B) MP800 products.
N2 adsorption-desorption analysis
Figure 9 displays the nitrogen adsorption–desorption isotherms for the MP600 and MP800 nanocomposites, and the corresponding surface texture parameters are summarized in Table 3. Both samples exhibit isotherms that resemble Type IV behavior according to the IUPAC classification, which is indicative of mesoporous materials35. The observed hysteresis loops further confirm the presence of mesopores, suggesting that capillary condensation occurs within the pore structures. As shown in Table 3, MP600 demonstrates a higher BET surface area of 75.62 m2/g compared to 52.64 m2/g for MP800. In addition, MP600 exhibits a greater total pore volume of 0.2641 cm3/g, whereas MP800 shows a lower value of 0.1785 cm3/g. These differences indicate that MP600 possesses a more porous structure with greater adsorption potential. The average pore sizes of 6.98 nm for MP600 and 7.12 nm for MP800 confirm the mesoporous nature of both samples, since their pore diameters fall within the 2–50 nm range typically associated with mesoporous materials. The decrease in BET surface area and pore volume observed for MP800 can be attributed to enhanced grain growth and partial pore collapse during calcination at the higher temperature of 800 oC. This thermal effect leads to densification and reduced porosity, which explains the overall decline in surface textural properties compared to MP600.
Table 3. Surface textures of MP600 and MP800 products.
Product | BET surface area (m2/g) | Total pore volume (cm3/g) | Average pore size (nm) |
|---|---|---|---|
MP600 | 75.62 | 0.2641 | 6.98 |
MP800 | 52.64 | 0.1785 | 7.12 |
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Fig. 9
N2 adsorption/desorption isotherms of MP600 and MP800 products.
FTIR
The FTIR spectra of MP600 and MP800 nanocomposites displayed in Fig. 10A-B confirm the presence of distinct functional groups and bonding environments influenced by the synthesis and calcination conditions. Both samples exhibit a broad absorption band at 3433 cm− 1 for MP600 and 3450 cm− 1 for MP800, which corresponds to O–H stretching vibrations from surface-adsorbed water or residual hydroxyl groups. The absorption bands at 1610 cm− 1 for MP600 and 1617 cm− 1 for MP800 are attributed to the bending vibrations of adsorbed H2O molecules. A noticeable band appears at 1415 cm− 1 for MP600 and shifts slightly to 1420 cm− 1 for MP800, which can be ascribed to the stretching vibrations of C = O groups originating from residual organic products used during the Pechini sol-gel synthesis48,49. Prominent bands at 860 cm− 1 for MP600 and 864 cm− 1 for MP800 are linked to Cr–O stretching vibrations, indicating the successful formation of chromate phases50. The absorption features at 580 cm− 1 for MP600 and 587 cm− 1 for MP800 are assigned to Mg–O stretching vibrations, confirming the incorporation of magnesium oxide51. Lastly, the bands detected at 440 cm− 1 for MP600 and 445 cm− 1 for MP800 correspond to Pb–O vibrations, verifying the presence of lead oxide components52,53. These spectral features collectively demonstrate the formation of well-structured multi-metal oxide nanocomposites and reflect the influence of calcination temperature on their chemical structure.
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Fig. 10
FTIR spectra of (A) MP600 and (B) MP800 products.
Optimization
Influence of dye pH variation on adsorption performance
Adsorption efficiency increased steadily from pH 2 to 10, with MP600 consistently exhibiting higher removal efficiency than MP800 across all pH values tested, as shown in Fig. 11. Also, the adsorption performance of MP600 and MP800 nanocomposites significantly improves as the pH increases, especially notable at pH 10 compared to pH 2, with MP600 demonstrating greater dye removal efficiency (92.56%) than MP800 (77.88%). Furthermore, inductively coupled plasma (ICP) instrument was conducted to determine the concentrations of lead, magnesium, and chromium in the filtrates. The elements were not detected, indicating that no leaching occurred from the nanocomposites, and thus they can be considered environmentally safe.
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Fig. 11
Influence of initial pH on the remediation efficiency of malachite green dye utilizing MP600 and MP800 products.
Influence of time variation on adsorption performance
As illustrated in Fig. 12, the remediation efficiency of malachite green dye increases rapidly with time for both MP600 and MP800 nanocomposites, followed by a plateau that marks the equilibrium state. For the MP800 sample, the percentage removal rises from 32.81% at 10 min to 77.75% at 70 min, indicating a gradual approach to saturation. In contrast, MP600 demonstrates a much faster adsorption process, reaching 92.45% removal within 50 min compared to 53.39% at 10 min. The higher removal rate and shorter equilibrium time observed for MP600 suggest a more favorable surface chemistry and larger number of accessible active sites. Beyond the equilibrium times of 50 min for MP600 and 70 min for MP800, no further increase in dye removal was detected, confirming that the adsorbent surfaces had become saturated and that all active binding sites were occupied. The comparison clearly indicates that MP600 outperforms MP800 in both adsorption rate and overall removal efficiency under identical experimental conditions.
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Fig. 12
Variation in malachite green dye remediation efficiency over time for MP600 and MP800 products.
Influence of temperature variation on adsorption performance
As shown in Fig. 13, the remediation efficiency of malachite green dye by both MP600 and MP800 nanocomposites decreases upon elevation of temperature, indicating that the uptake of adsorbate is characterized by an exothermic nature. At 298 K, MP600 achieved a removal percentage of 92.45%, while MP800 reached 77.75%, reflecting the elevated adsorption potential demonstrated by MP600 during low-temperature processing. When the solution temperature increased to 328 K, the remediation efficiency declined to 85.58% regarding MP600 in addition to 68.73% regarding MP800, confirming that higher thermal energy reduces the interaction between the dye entities and the external surface of the adsorbent. The more significant reduction in removal percentage observed for MP800 highlights its comparatively lower thermal stability and weaker binding affinity toward malachite green dye compared to MP600. These results suggest that both nanocomposites perform more effectively at lower temperatures, with MP600 consistently demonstrating higher adsorption efficiency across the studied thermal range.
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Fig. 13
Effect of temperature on the remediation efficiency of malachite green dye using MP600 and MP800 products.
Influence of concentration variation on adsorption performance
As illustrated in Fig. 14, the remediation efficiency of malachite green dye by both MP600 and MP800 nanocomposites decreases progressively upon elevation of initial concentration. In the presence of 100 mg/L dye solution, MP600 gets a remediation efficiency of 99.22%, whereas MP800 gets 96.95%, demonstrating high remediation efficiency under dilute conditions. However, upon increasing the dye concentration to 400 mg/L, the remediation efficiency drops to 69.93% regarding MP600 in addition to 58.96% regarding MP800 owing to the full occupancy of available active positions on the adsorbent surfaces. The comparison reveals that MP600 consistently outperforms MP800 across the tested concentration range, likely because of its elevated surface area in addition to better number of accessible remediation positions. This behavior confirms that the adsorption process is strongly influenced by the initial dye concentration and that the adsorption efficiency becomes limited at higher concentrations as the surface sites approach saturation.
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Fig. 14
Influence of primary malachite green dye concentration on the remediation efficiency using MP600 and MP800 products.
Adsorption isotherms
The adsorption equilibrium data for malachite green dye removal by MP600 and MP800 nanocomposites were interpreted through the application of Langmuir in addition to Freundlich isotherm equations as depicted in Fig. 15A-B, respectively.
The linear form of the Langmuir equation is expressed by Eq. (5)54.
5
K3 is the Langmuir constant related to adsorption affinity, whereas Qmax is the theoretical maximum adsorption capacity.
The linear form of the Freundlich equation is expressed by Eq. (6)54.
6
K₄ denotes the Freundlich constant, which reflects the adsorption capacity of the material, whereas 1/n denotes the heterogeneity factor.
Qmax was calculated using Co = 400 mg/L, the maximum initial concentration tested, as applied in Eq. (7)35.
7
As shown in Fig. 15 and confirmed by the isotherm parameters in Table 4, the adsorption of malachite green dye onto both MP600 and MP800 nanocomposites follows the Langmuir model more closely than the Freundlich model, as supported by the higher correlation coefficients R2 of 0.9998 in addition to 0.9994, respectively. These values indicate that the adsorption process primarily takes place through single-layer adsorption on a uniform surface and that MP600 exhibits a greater adsorption capacity and stronger interaction with the dye molecules compared to MP800.
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Fig. 15
Adsorption isotherm modeling of malachite green dye on MP600 and MP800 products. (A) Linearized Langmuir isotherm plots indicating monolayer adsorption behavior. (B) Freundlich isotherm plots demonstrating multilayer adsorption on heterogeneous surface.
Table 4. Langmuir and Freundlich isotherm parameters for the adsorption of malachite green dye onto MP600 and MP800 products.
Sample | Langmuir | Freundlich | |||||
|---|---|---|---|---|---|---|---|
Qmax (mg/g) | R2 | K3 (L/mg) | K4 (mg/g)(L/mg)1/n | Qmax (mg/g) | 1/n | R2 | |
MP600 | 425.53 | 0.9998 | 0.5919 | 189.49 | 598.42 | 0.2016 | 0.8087 |
MP800 | 362.32 | 0.9994 | 0.2857 | 145.11 | 457.15 | 0.2012 | 0.7198 |
Kinetics
The kinetic profile of malachite green dye adsorption onto MP600 and MP800 products was analyzed by applying the pseudo-1st -order in addition to pseudo-2nd -order rate models, as outlined in Eqs. (8) and (9)53.
8
9
where, Qe and Qt represent the adsorption capacities at equilibrium and at time t, respectively, and K1 is the pseudo-1st -order rate constant. Also, K2 is the pseudo-2nd -order rate constant.
The kinetic plots shown in Fig. 16A-B reveal linear relationships consistent with these models. Based on the comparison between the experimental equilibrium adsorption capacity (QExp) and the calculated values in Table 5, along with the correlation coefficient (R2), it is evident that the pseudo-1st -order model provides a better fit to the experimental data for both MP600 and MP800. This indicates that the adsorption process is predominantly governed by a physical interaction mechanism rather than chemisorption, and that the adsorbents exhibit rapid uptake kinetics consistent with pseudo-1st -order behavior.
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Fig. 16
Kinetic assessment of malachite green dye remediation utilizing MP600 and MP800 products. (A) Pseudo-1st -order kinetic plot. (B) Pseudo-2nd -order kinetic plot.
Table 5. Kinetic constants for the remediation of malachite green dye utilizing MP600 and MP800 products.
Sample | QExp (mg/g) | Pseudo-1st -order | Pseudo-2nd -order | ||||
|---|---|---|---|---|---|---|---|
K1 (1/min) | R2 | Qe (mg/g) | K2 (g/mg.min) | R2 | Qe (mg/g) | ||
MP600 | 416.03 | 0.0860 | 0.9999 | 415.37 | 0.000175 | 0.9961 | 518.13 |
MP800 | 349.89 | 0.0549 | 0.9999 | 350.40 | 0.000115 | 0.9954 | 452.49 |
Thermodynamic study
The thermodynamic behavior of malachite green dye adsorption onto MP600 and MP800 nanocomposites was evaluated using Eqs. (10), (11), and (12)35.
10
11
12
where, Kd refers to the distribution coefficient, △So refers the entropy change, R refers to the universal gas constant, △Ho refers to the enthalpy change, and T represents the absolute temperature in Kelvin. Also, △Go represents the Gibbs free energy.
As shown in Fig. 17 and supported by the values presented in Table 6, both MP600 and MP800 exhibit negative values of △Ho, confirming that the adsorption process is exothermic. The values of △Go are also negative at all studied temperatures, indicating that the adsorption is spontaneous. Additionally, the positive values of △So for both samples suggest that the process is thermodynamically feasible due to the enhanced molecular disorder at the liquid-solid boundary. Since the enthalpy changes are less than 40 kJ/mol, the adsorption mechanism can be classified as physical rather than chemical. Collectively, these results demonstrate that the remediation of malachite green utilizing MP600 and MP800 products is spontaneous, exothermic, physical in nature, and thermodynamically favorable under the studied conditions.
[See PDF for image]
Fig. 17
Van’t Hoff plots for the adsorption of malachite green dye onto MP600 and MP800 products.
Table 6. Thermodynamic parameters for the remediation of malachite green dye utilizing MP600 and MP800 products.
Samples | △So (kJ/mol.K) | △Ho (kJ/mol) | △Go (kJ/mol) | |||
|---|---|---|---|---|---|---|
298 | 308 | 318 | 328 | |||
MP600 | 0.0418 | -19.73 | -32.19 | -32.61 | -33.03 | -33.45 |
MP800 | 0.0282 | -12.57 | -20.98 | -21.26 | -21.54 | -21.82 |
Comparison with other adsorbents
As presented in Table 7, the maximum adsorption capacities (Qmax) of a variety of adsorbents for malachite green dye removal vary significantly depending on their composition and structural characteristics37, 38, 39–40,55, 56, 57, 58, 59–60. Conventional materials such as activated charcoal and sulfur-doped biochar exhibit relatively low capacities of 27.00 and 30.18 mg/g, respectively, while more advanced composites such as alginate hydrogel and carbon nanotubes/TiO2/chitosan composites achieve higher values of 322.58 and 269.98 mg/g. Notably, the MP600 and MP800 nanocomposites produced in this study exhibit high adsorption capacities of 425.53 and 362.32 mg/g, respectively, compared to the other adsorbents listed in Table 7. This exceptional adsorption capacity can be attributed to the high surface area, enhanced porosity, and optimized surface functionality of the nanocomposites prepared via the Pechini sol-gel method. Additionally, the presence of metal oxide components in MP600 and MP800 facilitates strong electrostatic interactions and dye-binding affinity, which collectively contribute to their enhanced adsorption efficiency. The comparison demonstrates that the structural and compositional design of MP600 and MP800 offers a significant advantage over other conventional and composite adsorbents reported in the literature.
Table 7. Comparison of synthesized adsorbents with maximum adsorption capacities (Qmax) of various adsorbents reported in the literature for malachite green dye removal.
Adsorbent | Qmax (mg/g) | Time (min) | Ref |
|---|---|---|---|
Bentonite | 78.39 | 180 | 37 |
Bentonite/cobalt nanoparticles | 145.09 | 150 | 37 |
Sulfur-doped biochar | 30.18 | 120 | 38 |
Meta-kaolin based geopolymer | 185.18 | 240 | 39 |
Activated Charcoal | 27.00 | 180 | 55 |
ZIF-8@Fe/Ni nanoparticles | 151.52 | 90 | 56 |
Graphene oxide/cellulose/copper composite | 207.10 | 60 | 40 |
Carbon nanotubes/TiO2/chitosan composite | 269.98 | 180 | 57 |
Alginate hydrogel | 322.58 | 60 | 58 |
Graphene/Fe3O4/polyaniline composite | 196.10 | 120 | 59 |
SiO2/MgO composite | 115.64 | 150 | 60 |
MP600 | 425.53 | 50 | This study |
MP800 | 362.32 | 70 | This study |
To further validate the relationship between material characteristics and adsorption performance, statistical analysis was performed using SPSS software. Pearson correlation coefficients (r) were calculated between the maximum adsorption capacities (Qmax) of MP600 and MP800 and their corresponding structural/morphological parameters, including average crystallite size (XRD), particle size (HR-TEM), and selected elemental contents (EDX). A strong negative correlation was observed between particle size and Qmax (r = − 0.995), while a moderate negative correlation existed between crystallite size and Qmax (r = − 0.940). Conversely, carbon content exhibited a positive correlation with Qmax (r = + 0.976), suggesting that smaller, carbon-rich particles enhance adsorption capacity. These findings statistically reinforce the hypothesis that reduced particle size and higher carbon incorporation favor higher dye adsorption performance.
Proposed mechanism
The observed behavior aligns closely with the results depicted in Fig. 18, where the measured points of zero charge (pHPZC) for MP600 and MP800 were 6.61 and 7.25, respectively. When the solution pH is below these pHPZC values, such as at pH 2, both nanocomposites exhibit electrostatically positive interfaces, causing charge-based repulsion between the adsorbent surface and the positively charged malachite green dye ions, thereby reducing adsorption significantly, as illustrated schematically in Fig. 1936. Conversely, under pH conditions exceeding their point of zero charge (pHPZC), particularly at pH 10, the surface region of both MP600 and MP800 becomes negatively charged, enhancing the surface–dye attraction driven by charge complementarity, as depicted schematically in Fig. 1936, resulting in substantially higher removal efficiencies. Overall, the high adsorption efficiency exhibited by MP600 relative to MP800 at higher pH may be related to its smaller particle size (16.8 nm vs. 26.8 nm for MP800), which can enhance the accessibility of dye molecules to the adsorbent surface.
[See PDF for image]
Fig. 18
Determination of the point of zero charge (pHPZC) for MP600 and MP800 nanocomposites through analysis of the pH drift method.
[See PDF for image]
Fig. 19
Schematic illustration of the adsorption mechanism of malachite green dye onto MP600 and MP800 nanocomposites.
To confirm the adsorption of the dye, FTIR analysis was performed for the MP600 sample, as an illustrative example, after adsorption of malachite green dye. The FTIR spectrum of MP600 after adsorption of malachite green dye, as shown in Fig. 20, displays distinct changes in band positions and the appearance of new peaks that confirm successful adsorption of the dye molecules onto the surface of the adsorbent. A broad absorption band is observed at 3438 cm− 1, slightly shifted from 3433 cm− 1 before adsorption, indicating O–H stretching vibrations associated with surface hydroxyl groups and adsorbed water48. This shift to a higher wavenumber suggests the formation of hydrogen bonding interactions between the dye molecules and hydroxyl groups present on the surface of MP600, as shown in Fig. 13. New bands appearing at 3056 and 2914 cm− 1 are attributed to aromatic and aliphatic C–H stretching vibrations, respectively, arising from the malachite green structure. The absorption band at 1630 cm− 1 is assigned to the bending vibration of adsorbed water as well as the C = C stretching mode of the aromatic ring in the dye. Additional peaks at 1588 and 1483 cm− 1 correspond to aromatic ring vibrations and C = N stretching of the dye molecules. The band at 1421 cm− 1, originally present in MP600, remains visible, indicating that the structure of the adsorbent is retained after adsorption. New peaks at 1219 and 1153 cm− 1 are assigned to C–N and C–C stretching vibrations, further confirming the presence of malachite green on the surface49. The characteristic metal–oxygen bands remain identifiable at 859, 580, and 441 cm− 1, corresponding to Cr–O, Mg–O, and Pb–O vibrations, respectively, confirming that the structural integrity of the nanocomposite was maintained after adsorption. These spectral features collectively support the successful adsorption of malachite green dye onto MP600 and suggest that hydrogen bonding played a role in the dye–surface interaction.
[See PDF for image]
Fig. 20
FTIR spectrum of MP600 sample after adsorption of malachite green dye.
To further confirm the adsorption of the dye, EDX analysis was performed for the MP600 sample, as an illustrative example, after adsorption of malachite green dye, as shown in Fig. 21.
The EDX analysis of MP600 before adsorption (Fig. 3A) revealed the atomic percentages of the main elements as follows: C (11.9%), O (47.9%), Mg (18.9%), Cr (7.6%), and Pb (13.7%). After the adsorption of malachite green dye (Fig. 21), the EDX spectrum exhibited a noticeable change in the elemental composition, with the atomic percentages recorded as: C (23.8%), N (6.2%), O (39.0%), Mg (13.0%), Cr (5.5%), and Pb (12.5%). The significant increase in the carbon content and the appearance of a new nitrogen peak provides strong evidence for the successful adsorption of malachite green dye onto the MP600 surface. These changes are attributed to the organic structure of the dye, which is rich in carbon and contains nitrogen atoms, thereby confirming its presence on the adsorbent surface.
[See PDF for image]
Fig. 21
EDS pattern of MP600 after adsorption of malachite green dye.
Regeneration/reuse studies
As illustrated in Fig. 22, the desorption efficiency of malachite green dye from MP600 and MP800 nanocomposites increases with the concentration of hydrochloric acid, indicating strong pH-dependent desorption behavior. Desorption efficiency increased from 86.16% at 1 M to 99.74% at 2 M for MP600, and from 79.27% at 1 M to 99.79% at 2 M for MP800, with intermediate efficiencies observed at 1.5 M. This remarkable desorption performance is attributed to the high concentration of hydrogen ions in the acidic medium, which effectively protonates the adsorbent surface and disrupts the electrostatic interactions between the cationic dye molecules and the negatively charged active sites. The high desorption percentages confirm that HCl is an effective desorbing agent and that the interaction between malachite green dye and the nanocomposite surfaces is predominantly electrostatic and reversible, making the materials highly suitable for regeneration and repeated use in adsorption applications.
[See PDF for image]
Fig. 22
Effect of HCl concentration on the desorption efficiency of malachite green dye from MP600 and MP800 nanocomposites.
As shown in Fig. 23, the remediation efficiency of malachite green dye by MP600 and MP800 nanocomposites remains relatively high over five successive adsorption–desorption runs, indicating strong reusability potential. In the initial cycle, MP600 exhibits a remediation efficiency of 92.45%, while MP800 gets 77.75%. After five cycles, the efficiencies slightly decrease to 88.16% for MP600 and 72.48% for MP800. This gradual reduction in performance may result from minor loss of active sites or structural changes during repeated use, yet both materials retain the majority of their adsorption performance. The consistently higher performance of MP600 compared to MP800 across all cycles reflects its superior structural stability and higher affinity for malachite green dye. The findings demonstrate that both products, predominantly MP600, demonstrate outstanding regeneration capability and are well-suited for repeated adsorption applications.
[See PDF for image]
Fig. 23
Assessment of the reusability behavior of MP600 and MP800 products through five adsorption–desorption repetitions.
Interference
The influence of various common cations and anions on the adsorption efficiency of MP600 and MP800 nanocomposites for malachite green dye was examined under binary conditions, as presented in Table 8. In the absence of interfering ions, MP600 and MP800 exhibited maximum adsorption capacities of 416.03 mg/g and 349.89 mg/g, respectively. The introduction of Na⁺ and K⁺ ions resulted in moderate declines in adsorption performance, with MP600 capacities decreasing to 403.25 mg/g and 400.80 mg/g and MP800 capacities reducing to 338.40 mg/g and 336.20 mg/g, respectively. The presence of divalent cations such as Mg²⁺ and Ca²⁺ caused the most significant reductions, lowering MP600 adsorption to 362.10 mg/g and 356.40 mg/g, and MP800 to 305.10 mg/g and 298.75 mg/g, respectively, likely due to stronger competition for active binding sites. In contrast, anions including Cl⁻, NO₃⁻, and SO₄²⁻ exhibited only slight interference, with minimal decreases in dye uptake. These results indicate that although the nanocomposites maintain effective performance in the presence of typical background ions, multivalent cations such as Ca²⁺ and Mg²⁺ can considerably hinder adsorption, highlighting the importance of evaluating adsorbent selectivity under realistic environmental conditions.
Table 8. Effect of interfering ions on adsorption of malachite green dye using MP600 and MP800 nanocomposites.
Interfering ions | Q of MP600 (mg/g) | Q of MP800 (mg/g) | Reduction in Q of MP600 (mg/g) | Reduction in Q of MP800 (mg/g) |
|---|---|---|---|---|
None (Control) | 416.03 | 349.89 | ---- | ---- |
Na⁺ | 403.25 | 338.40 | 12.78 | 11.49 |
K⁺ | 400.80 | 336.20 | 15.23 | 13.69 |
Mg²⁺ | 362.10 | 305.10 | 53.93 | 44.79 |
Ca²⁺ | 356.40 | 298.75 | 59.63 | 51.14 |
Cl⁻ | 410.50 | 345.20 | 5.53 | 4.69 |
NO₃⁻ | 409.10 | 344.10 | 6.93 | 5.79 |
SO₄²⁻ | 407.80 | 342.60 | 8.23 | 7.29 |
Conclusions
In summary, novel Pb2(CrO4)O/MgO/MgCrO4/C nanocomposites (MP600 and MP800) were successfully synthesized via the Pechini sol–gel method and demonstrated excellent performance in removing malachite green dye from aqueous media. MP600, with a smaller particle size and higher surface area, showed superior adsorption capacity (425.53 mg/g) compared to MP800 (362.32 mg/g). The adsorption process was spontaneous, exothermic, and followed pseudo-first-order kinetics and the Langmuir isotherm, indicating monolayer physical adsorption. Both materials exhibited outstanding desorption efficiency (> 99%) and retained high removal capacity over five reuse cycles, confirming their reusability. The synergy between the composite components contributed to enhanced adsorption performance. These nanocomposites demonstrate high adsorption capacity and reusability for malachite green dye removal under laboratory conditions, warranting further investigation for practical wastewater treatment applications.
Acknowledgements
This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant number IMSIU-DDRSP2502).
Author contributions
Ehab A. Abdelrahman (Conceptualization, Methodology, Writing – Review & Editing, Funding acquisition); Abdulrahman G. Alhamzani (Writing—original draft preparation); Mortaga M. Abou-Krisha (Supervision, Writing—review and editing); Reem K. Shah (Methodology, Writing—review and editing); Huda M. Alamri (Writing – Review & Editing).
Funding
This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant number IMSIU-DDRSP2502).
Data availability
The datasets generated and/or analysed during the current study are available in the [Data-of-MP600-and-MP800-nanocomposites] repository, [https://github.com/Ehab1986539/Data-of-MP600-and-MP800-nanocomposites].
Declarations
Competing interests
The authors declare no competing interests.
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Abstract
In this study, novel nanocomposites composed of Pb2(CrO4)O, MgO, MgCrO4, and carbon (MP600 and MP800) were synthesized via the Pechini sol–gel method at 600 and 800 oC, respectively. The materials were characterized using XRD, FTIR, HR-TEM, FE-SEM, BET, and EDX techniques. XRD analysis revealed average crystallite sizes of 62.4 nm for MP600 and 73.6 nm for MP800. HR-TEM images showed that MP600 consisted of smaller, more uniform spherical particles, while MP800 exhibited larger and slightly aggregated structures. The nanocomposites were applied for malachite green dye removal from aqueous media under optimized conditions of pH 10, contact time of 50 min for MP600 and 70 min for MP800, and temperature of 298 K. MP600 exhibited a higher adsorption capacity (425.53 mg/g) than MP800 (362.32 mg/g). The adsorption followed the pseudo-second-order kinetic model and fitted the Langmuir isotherm well. Thermodynamic results indicated that the process was spontaneous and exothermic. MP600 also showed excellent regeneration efficiency over multiple cycles, highlighting its potential as a promising, cost-effective adsorbent.
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Details
1 Imam Mohammad Ibn Saud Islamic University (IMSIU), Department of Chemistry, College of Science, Riyadh, Saudi Arabia (GRID:grid.440750.2) (ISNI:0000 0001 2243 1790)
2 Umm Al-Qura University, Department of Chemistry, Faculty of Science, Makkah, Saudi Arabia (GRID:grid.412832.e) (ISNI:0000 0000 9137 6644)
3 University of Jeddah, Department of Chemistry, College of Science, Jeddah, Saudi Arabia (GRID:grid.460099.2) (ISNI:0000 0004 4912 2893)