Green synthesis of di-manganese trioxide mesoporous structure (super nano porous) sheets (Mn2O3-rGO-NS), Graphene oxide nano sheets (GO) and Mn2O3-rGO-NS nano sheets composite (Mn2O3-rGO-NS) were prepared via Thermal decomposition, seconds timescale water electrolytic oxidation and sonication method respectively. The prepared samples were characterized via X-ray diffraction, FESEM, TEM , and FTIR. Average crystallite size was found about 38 and 26 nm for Mn2O3-NS and Mn2O3-rGO-NS, respectively. Mn2O3-rGO–NPs morphology reveal nano porous sheets within average 35 nm and 26 nm in pores diameter and thickness respectively. Mn2O3-NS act distributed on and between graphene sheets creating macro and nano pores. The obtained results showed that the characteristics of the α-Mn2O3-rGO-NS sheets were improved by the addition of rGO sheets. Methyl orange (MO) dye adsorption onto Mn2O3-rGO-NS composite was investigated at various adsorption parameters, including pH, adsorbent quantity, and time. It was discovered that the adsorption behavior complies with the intra-particle diffusion model and the Friedendlich isotherm. Methyl orange in aqueous media may be eliminated 99.3% by Mn2O3-rGO-NS composite.

  • Mesoporous graphene-based composite synthesis for dye removal from aqueous media with high efficiency and capacity. According to thermodynamics and kinetics adsorption is favorable, which opens up the possibility for further trials on various types of pollutant removal studies for the industry.

The primary goal of modern nanotechnology research is the environmentally friendly synthesis of nanomaterials. Green synthesis of nanoparticles is a non-toxic, ecologically beneficial, clean, less expensive, and essentially novel method (Verma et al. 2019; Nakum & Bhattacharya 2022). Removal of dyes by adsorption-technology is a particularly important technique, because of its usability, simplicity, high efficiency, and scale-up over a wide range of concentrations (Kurniawan et al. 2012; Al Jebur & Alwan 2022). Nanomaterials are utilized for water treatment due to being eco-friendly, low-cost, high-performance materials that are well positioned for large-scale water treatment and elucidated in many field studies (Hashem 2014; Hairom et al. 2021; Jain et al. 2021). Metal oxides and their composites are commonly utilized in materials for pollution control and removal. Their various oxidation states, huge surfaces, and diverse electronic configurations give them these features (Liu et al. 2023).

 Tuning synthesis in broad diversity of manganese oxides (such as MnO, MnO2, Mn2O3, Mn3O4) morphology and type appear unique properties, and receive great scientific interest in various applications such as environmental control, batteries, magnetism, and pollution sensing applications (Ahmed et al. 2022a, 2022b). Mn2O3 (p-type semiconducting material) is regarded as likely candidate material as it is non-toxic, low-cost material, has superior structural flexibility, and is environmentally compatible (stable) (Vignesh et al. 2022). Various processes have been carried out to improve the properties of metal oxide nanoparticles by using different methods of preparation or trying to control the particle size or doping with other materials (Stankic et al. 2016; Pradeev Raj et al. 2018; Chavali & Nikolova 2019; Krstić 2021; Pan et al. 2021; Sharma et al. 2022). Mesoporous oxides have drawn considerable interest due to their attractive physical (surface area) and chemical (reactivity) properties, which promised enormous potential for their neutral effect on the environment and their capability for pollutant trapping (Zhang et al. 2020; Yu et al. 2022). Graphene has a hexagonal two-dimensional layer made up of sp2 carbon atoms. It is well known for having a good electron acceptor, excellent charge transport, and a higher surface area, which has good mechanical, thermal, and electronic properties and can be utilized as nanosorbents, is typically composed of one or more atomically layer carbon atoms and it also has a special two-dimensional structure. The adsorption of dyes on a few-layered graphene nanosheets is made possible by Van der Waals forces and stacking interactions. Reduced graphene oxide nanosheets (rGO-NS) can improve metal oxide performance by first incorporating them into composite materials to change their chemical and physical properties (Sundriyal et al. 2018; Cong et al. 2021; Yeon et al. 2022). Strong intermolecular forces between adsorbates are provided by the broad electronic surfaces and high aspect ratios of both rGO-NS layers and single layers. Graphene performs admirably in terms of conductivity, flexibility, and chemical inertness. Lately, graphene-based material is of great scientific interest due to its superior mechanical characteristics, thermochemical stability and conductivity, which is predicted to have various uses (Li et al. 2010; Upadhyay et al. 2013; Kalaiselvi & Chandar 2021). Graphene oxide (GO) is a semiconductor material with a bandgap of 2.2 eV that can be decreased through reduction (Konale et al. 2020). The rGO has a bandgap in the range of 1–1.69 eV (Abid et al. 2018). This material previously was used to tune the optical, physico-chemical, adsorption efficiency, mechanical and promoted properties of metal oxides (Loh et al. 2010; Malefane et al. 2019; Ajala et al. 2022; Chi et al. 2022; Stefan et al. 2022). There are numerous methods for creating nanomaterials, the thermal decomposition process is being considered as a ‘green method’ that does not use or produce harmful chemicals or solvents (Dikshit et al. 2021). In addition, the procedure permits the preparation of a vast quantity of samples in a single batch compatible with ethics of COP 27 regulations (Nassar 2013; Nassar et al. 2013; Stanczyk 2022). In an effort to reduce the production of harmful wastes, this work created impacted nanosheets of Mn2O3-NS and Mn2O3-rGO with thickness less than 100 nm simply by thermal decomposition of metal-citric extract and sonication respectively. The composites were described and their adsorption activity was evaluated. There is not much information in the literature about the study of rGO impact on the structural and adsorption properties of Mn2O3. So, synthesis of Mn2O3-rGO-NS by the green method in two-dimensional morphology and the investigation of adsorption isotherm and kinetics of Mn2O3-rGO-NS are the main objectives of this work.

Chemicals and reagents

All chemicals were used without any further purification. Manganese (II) acetate tetra hydrate (Acros, 99+ %), citrus peel was obtained from a local market in Hail Region, Saudi Arabia. The fruit peel was removed from the pulp. During this process, aluminum foil was used to protect the samples from light. Sodium hydroxide (Fisher chemical, ≥ 97%) graphite rods (99% extra pure) were obtained from LOBA CHEMIE, India. Hydrochloric acid (HCl) (37%), nitric acid (HNO3), sodium hydroxide (NaOH) powder, sulfuric acid (H2SO4), potassium permanganate (KMnO4) and standard Ni (II) solution were purchased from Sigma Aldrich, USA, while hydrogen peroxide (H2O2) and sodium nitrate (NaNO3) were purchased from PIOCHEM, Egypt and ISO-CHEM, France, respectively.

Synthesis of dimanganese trioxide nanosheets (Mn2O3-NS)

For the green synthesis Mn2O3-rGO-NS mesoporous nanosheets, 8 mM of manganese (II) acetate tetra hydrate (C4H16MnO8) was added to 20 mL of Milli Q water and pH was adjusted to 4 (acidic medium) to obtain a flat two-dimensional sheet is consistent with previous reports (Wahab et al. 2009; Nagyné-Kovács et al. 2020; Xie et al. 2022). Using M acetic acid and M NaOH solution for pH optimization. After that, 90 mL of as-prepared citric extract from citrus peels (Fernandes et al. 2022) was then added to C4H16MnO8 mixture and kept under vigorous magnetic stirring for 5 h at 78–80 °C. The visual color change and precipitate (ppt) were formed. The resulting ppt was centrifuged at 10,000 rpm for 20 min and washed several times with Milli Q water. Finally, ppt powder was dried at 85 °C for 6 h then calcined at 700 °C for 2 h. A dark brown residue of Mn2O3-NS was formed.

Graphene oxide (GO) synthesis

The GO was synthesized using Green method via seconds timescale water electrolytic oxidation (Pei et al. 2018).

Synthesis of Mn2O3-rGO nanocomposites

Mn2O3-rGO nanocomposites were prepared via the sonication method (Mohammadi & Entezari 2018) by mixing the GO and Mn2O3- NS (1:4) in 100 mL DI water, then probe sonicated for 30 min in pulsed mode at 600 W (20 KHz high frequency). Finally, homogeneous solutions of the Mn2O3-rGO nanocomposite were obtained and dried in a vacuum oven at 100 °C for 10 h.

Batch adsorption experiments

The amount of Mn2O3-rGO-NS nanoparticles adsorption trials began with 0.1 g (100 mg) achieving 50% removal and increasing directly proportional to the quantity added reaching the maximum removal of 99.3% at 0.5 g (500 mg). pH varies from 3 to 9, and contact time is the time needed to reach the equilibrium between the adsorbent and dyes. In a time range of 0–120 minutes, the impact of contact time on the removal of the dye by Mn2O3-rGO-NS at pH 4, and the concentration of the dye were all studied using a batch technique. All experiments were performed in triplicate to ensure repeatability and accuracy.

Batch adsorption kinetics

Variable amounts of Mn2O3-rGO-NS were stirred into a known amount of MO-dye to study the reaction kinetic isotherms. Measurements were used to determine the moment equilibrium was reached. The mixture was centrifuged at 4,000 rpm. The dye was identified using a UV-visible spectrophotometer at 464 nm. The dye solution's pH was changed using 0.1N NaOH and 0.1N HCl. Kinetic models were used to determine the controlling mechanism of the adsorption process. Three major models, namely pseudo-first-order, pseudo-second-order, and intra-particle diffusion, were used to examine this adsorption. First Lagergren pseudo-first-order model of Revellame et al. (2020) is selected to fit the kinetic data, followed by pseudo-second-order model (Saha & Grappe 2017; Sahoo & Prelot 2020; Krstić 2021). The intra-particle diffusion equation is given as (Sousa et al. 2012; Kuroki et al. 2014; Pan et al. 2017):
where ki (mg/g min0.5) is the intra-particle diffusion rate constant, plots of qt versus t0.5 yield straight lines with intercept (C). The boundary layer thickness is described by the values of the intercept. The larger the intercept, the greater is the boundary layer effect (Lee et al. 2011; Balta et al. 2012; Anastopoulos et al. 2018). qe and qt are the sorption capacities per gram of sorbents (mg/g) at equilibrium and at time t (min), where k1, k2, and ki are the rate constant of the pseudo-first-order, pseudo-second-order and intra particular respectively. For kinetic measurements, 1 L of 50 ppm solutions of MO-Dye were added with 400 mg of Mn2O3-rGO-NS at pH 4 and room temperature. By measuring the absorbance at 464 nm wavelength at various time intervals and using a calibration curve, the corresponding dye concentration (ppm) could then be calculated.

Bath adsorption isotherm

The adsorption isotherm, which is based on the homogeneity or heterogeneity of the adsorbent surface and the interaction between absorbed molecules, theoretically depicts the distribution of dye molecules between the adsorbent and liquid phase. To determine the best model for analysis and optimization of adsorption processes, the equilibrium adsorption data were examined using the most popular adsorption isotherms models, including Langmuir, Freundlich, and Temkin. Four hundred mg of Mn2O3-rGO-NS at pH 4 was added to a 1 L solution of various MO-Dye concentrations (10, 20, 30, 40, 50 and 60 ppm), to evaluate the adsorption isotherms.

Langmuir adsorption isotherm
Langmuir isotherm assumes that during the adsorption process the monolayer is formed at specific homogeneous sites on the surface of the adsorbent, without any interaction between adsorbate molecules The linear form of Langmuir adsorption isotherm is expressed as (Langmuir 1918):
where Ce is the equilibrium concentration, qmax (mg/g) is the maximum adsorption capacity of the adsorbent corresponding to monolayer formation. KL is the Langmuir constant.
Freundlich adsorption isotherm

Freundlich isotherm offers an empirical isotherm that is used to represent multilayer adsorption and assumes that the adsorption occurs on a heterogeneous surface and layers.

The linear form of Freundlich isotherm is given by the following equation (Freundlich 1906, 1910):
where kf is a Freundlich isotherm constant (mg/g), Ce is the equilibrium concentration of adsorbate (mg/L), qe is the amount adsorbed per gram of the adsorbent at equilibrium (mg/g).
Temkin adsorption isotherm
The adsorption of heterogeneous surface energy systems (non-uniform distribution of sorption heat) is described by this Temkin isotherm, it is based on the supposition that the adsorption heat, a function of temperature, decreases linearly with coverage as a result of interactions between the adsorbent and the adsorbate. The Temkin isotherm's linear form can be represented as follows (Temkin & Pyzhev 1940; Johnson & Arnold 1995):

The B is constant which is equal to RT/bT. AT (L/mg) and BT (J/mol) are the Temkin constants. BT is related to the heat of adsorption and AT is the equilibrium binding constant corresponding to the maximum binding energy. R (8.314 J/mol K) is the universal gas constant in energy unit and T (K) is the absolute temperature. The values of AT and BT are calculated from the linear plots of qe versus ln Ce.

Characterization techniques

The phase composition and crystal structure examined via the X-ray diffraction (XRD), model Malvern Panalytical Empyrean. The TEM images of the samples were determined via transmission electron microscope (TEM), model Joel-JEM-2100, operated at 200 kV. The FTIR spectra were collected using a FTIR spectrometer, model Vertex 70-Bruker, Germany (in the range of 400–4,000 cm−1 with a resolution of 4 cm−1). The UV-vis absorption spectra of the samples were measured using a double beam spectrophotometer (Perkin Elmer Lambda 40).

X-ray diffraction

Figure 1 shows the XRD pattern of the as-prepared graphene oxide, Mn2O3- NS and Mn2O3/rGO. In Figure 1, the diffraction pattern of the GO only peak at 2θ = 1.11° indexed for (002) plane confirmed the efficiency of the oxidation reaction with an increase in the inter planar distance of the graphene nano-platelets and it would be clear on FESEM (Castro Neto et al. 2009; Arbuzov et al. 2013; Han et al. 2014; Stobinski et al. 2014; Gascho et al. 2019). Mn2O3-NS pattern profiles indicated the seven characteristic diffraction peaks of the cubic α-Mn2O3 phase with reference code (JCPDS No. 01-081-9976). observed at 23.1, 32.9, 38.2, 45.1, 49.3, 55.2 and 65.7° which are assigned to (211), (222), (400), (332), (431), (440), and (622) planes with D-spacing 1.41, 1.66, 1.84, 2.00, 2.30, 2.70 and 3.80 respectively (Najjar et al. 2019; Mokkath et al. 2021). All diffraction peaks intensities of Mn2O3-NS were decreased after probe-sonication processing and composite forming, indicating that the face-to-face stacking is present because of the formation of Mn2O3-rGO on both sides of the rGO sheets and it would be clear in FESEM images. Previous investigations have shown the disappearance of rGO diffraction peaks for regular stacking exfoliated graphene oxides (Jung et al. 2020) and this is a good indication of rigid bonding.
Figure 1

XRD pattern of graphene oxide (GO), Mn2O3-rGO and Mn2O3-rGO nanoparticles.

Figure 1

XRD pattern of graphene oxide (GO), Mn2O3-rGO and Mn2O3-rGO nanoparticles.

Close modal
The addition of rGO to the Mn2O3-rGO matrix broadens the major peaks while lowering their intensities, indicating that particle size has changed. This broadening is caused by the finite size of the particles and the micro-strain formed in the crystal structure. To confirm this data, the average crystallite size (D(222)) for the maximum peak with (222) plane can be calculated via the Scherer's equation (Nassar 2013):
(1)
where β is the full width at half maximum (FWHM) for (222) plane, θ is the Bragg's angle, and λ is the wavelength of X-ray. The average crystallite size was found to be about 38 and 26 nm for Mn2O3-rGO and Mn2O3-rGO composites, respectively.
One of the main reasons for the strain growth inside the crystal structure is the existence of defects inside the matrix. These defects clearly appeared in the existence of short peaks and disappeared in the presence of rGO sheets in the matrix as shown in the XRD pattern. The micro-strain (ε) and the dislocation density (δ) are important parameters that can be investigated from the XRD pattern via the following relations (Vignesh et al. 2022):
(2)
(3)

The ε and δ values are calculated and tabulated in Table 1.

Table 1

Crystallite size (D), micro-strain (ε), and dislocation density (δ)

SampleD(222) (nm)ε(222), ×10−3δ(222), ×1014 (m−2)
Mn2O3 56 2.3 3.2 
Mn2O3-rGO 43 5.5 
SampleD(222) (nm)ε(222), ×10−3δ(222), ×1014 (m−2)
Mn2O3 56 2.3 3.2 
Mn2O3-rGO 43 5.5 

As shown, ε and δ values were increased when rGO was inserted in the matrix. This result confirms the compression of the material and consequently the particle size decreases as well (Vignesh et al. 2022).

FESEM-TEM analysis

In order to accurately understand the morphology and the internal structure of the Mn2O3-NS, graphene oxide and Mn2O3-rGO-NS composite, FE-SEM at different magnifications and TEM were performed. The Mn2O3-rGO The nano flakes' surface structure was tightly packed and uniformly stacked against one another, with meso pores clearly visible at the top layers (Figure 2(a)–2(c)). Deep scanning by TEM revealed pores diameter within 33 nm and flakes thickness within 26 nm (Figure 2(d)–2(e)). Figure 2(f) of SAED depicts manganese oxide in a polycrystalline structure that exists in 2D-shape (Tsuji & Fujita 2001). A wrinkled sheet-like microstructural structure of synthesized graphene is also observed in Figure 2(h)–2(k) FESEM, with the average length within 10 μm. Stacking composites were created after sonication in Figures 2(l)–2(n) show graphene layers inserted between well-spaced and sequenced Mn2O3 flakes. As high-energy projectiles struck graphite clusters, nanoporous Mn2O3-rGO micro-clusters with higher density than graphene agglomerations exfoliated them to graphene, improving uniformity and mixing of the composites. Collapsing on wrinkled graphene layers created new pores in the composite grain at the micro and nanoscales. Nanoparticle aggregates are primarily caused by the combination of tiny particles with high surface energy.
Figure 2

FESM images of (a, b, e) Mn2O3-NS, TEM images (c, d, g) Mn2O3-NS, FESEM images (h,i,k) of graphene nanosheets and FESEM (l,m,n) Mn2O3/G composite.

Figure 2

FESM images of (a, b, e) Mn2O3-NS, TEM images (c, d, g) Mn2O3-NS, FESEM images (h,i,k) of graphene nanosheets and FESEM (l,m,n) Mn2O3/G composite.

Close modal

FT-IR analysis

In the wavenumber range between 200 and 3,600 cm−1, Figure 3 displays the FTIR spectra of Mn2O3-NS and those that also contain reduced graphene oxide (RGO). The characteristic Mn-O stretching mode of Mn2O3-NS is attributed to the FTIR pattern's peaks located at 516, 576, and 670 cm−1. The bands in the range 1,400–1,640 cm−1 comprise a characteristic stretching vibration mode of water (H2O) absorbed by the samples. The peaks appearing at 1,710 and 1,014 cm−1 are characteristic for the C = C and C-O alkoxide of the rGO, especially the C = C frequency which appears clearly on the composite accompanied by slight red-shift (1,600 cm−1). The observed peaks at 1,049 and 1,090 cm−1, corresponding to C–H vibration, are due to the preparation residuals. When RGO was added, no additional peaks were noticed. The absorbance of OH and C = O in FT-IR chart of rGO deceased in comparison to absorbance of the GO chart due to reduction via sonication.
Figure 3

FTIR of Mn2O3, Mn2O3-rGO, and GO.

Figure 3

FTIR of Mn2O3, Mn2O3-rGO, and GO.

Close modal

Adsorption study

Batch adsorption was studied along with other parameters such as pH, Mn2O3-NS concentration, Methyl Orange starting concentration, and contact time. While maintaining all other variables constant throughout the study (temperature = 25 °C and stirring power = 300 rpm), the study hinges on modifying the parameter to determine its impact on the adsorption process.

Effects of pH on adsorption

The trials were carried out between pH values of 3 and 9, as shown in Figure 4. At pH 4, the highest adsorption (99.3%) was attained. The positively charged Mn2O3-rGO composite will be adsorbed with the sulfonate group of MO-dye at pH 4. The observation that the removal process slows down as pH rises may be explained by the fact that, while there will be fewer positively charged sites on Mn2O3-rGO -NS at higher pH, there will also be a rise in negatively charged sites, which will not favor the adsorption of the MO-dye due to electrostatic repulsion (Coruh et al. 2011; Jahan et al. 2022). In particular, the MO-dye molecules will face competition with the newly formed Mn2O3-rGO hydroxyl in the process of adhering to active sites. This suggests that water will accumulate in layers on top of metal oxide sheets. Surface OH is created when Mn2+ forms hydroxo complexes with OH groups from H2O molecules on the top layer of Mn2O3-rGO surfaces. This interaction will mostly induce Mn-OH to make up the surface layer. Because they are not chemically or physically comparable, the surface hydroxo groups (OH) will dissociate and produce negatively charged O (Bhatnagar et al. 2010; Wang et al. 2020; Nizam et al. 2021). MO-dye at pH 4 will be positively charged and will be adsorbed to the negatively charged surface of Mn2O3-rGO-NS. In addition, the possibility of hydrogen bond formation between dye and rGO substrate and occlusion of the dye into Mn2O3-rGO-NS is supported by the presence of meso-structures on Mn2O3-rGO-NS as indicated from FESEM images.
Figure 4

Relation between percent (%) of removed dye and Mn2O3-rGO –NS at diverse parameters, (a) adsorbent (Mn2O3-rGO-NS) weight (mg), (b) pH, (c) Contact Time and (d) different concentration of MO-dye.

Figure 4

Relation between percent (%) of removed dye and Mn2O3-rGO –NS at diverse parameters, (a) adsorbent (Mn2O3-rGO-NS) weight (mg), (b) pH, (c) Contact Time and (d) different concentration of MO-dye.

Close modal

Effect of weight of Mn2O3-rGO nanoparticles

To evaluate the impact of Mn2O3 -rGO quantities and the removal of MO, interval amounts of 100–500 mg of Mn2O3-rGO at 25 °C and pH 4 were added to 1 L (50 ppm) of MO-dye solutions. The outcomes demonstrate that the Mn2O3-rGO-NS concentration rises, the elimination of MO-dye also rises; 99.3% of the maximum removal was attained with 0.4 g of Mn2O3. The abundance of low energy active sites contributes to the gradual increase in removal efficiency. The remaining active sites will then have more energy, which will lead to a decrease in adsorption capacity that is associated to the development of stacking layers and unequal distribution (Chen et al. 2023).

Effect of contact time

In a time range of 0–120 minutes, the impact of contact time on the removal of the dye by Mn2O3-rGO-NS at pH 4 was examined. Figure 4(c) of the results showed that removal rises with increasing time until it achieves constant adsorption after 40 min. The active sites on the surface of Mn2O3-rGO-NS are predicted to become saturated with the dye after 40 minutes, at which point there will be no active sites left for adsorption.

Effect of the initial concentration of dye

A batch method of dye concentrations (30, 50, 60, 70, 80, and 90 mg/L) was used to study adsorption isotherms and influence of initial on the removal efficiency. The remaining dye in solution can be measured spectrophotometrically at 464 nm. The influence of initial dye concentrations concentration on the removal efficiency of MO-dye was studied as shown in Figure 3(d) using the same weight of Mn2O3-rGO-NS (0.4 gm) at pH 4, keeping all other factors constant. As can be seen in Figure 3(d), the percentage removal of the dye was found to decrease with the increase in initial dye concentration up to 50 mg/L. This suggests that according to active size availability, adsorption energy need, and competition between solvent and dye molecules with adsorbate and one another, saturation occurs.

Adsorption kinetics

Parameters were calculated from Figure 5(a)–5(c), the rate constants (k1, k2, ki, and qe) and related R2 coefficient values are presented in Table 2. The correlation coefficients (R2) are used to describe the applicability of the adsorption kinetics model. The R2 values for the pseudo-first-order model are the lowest among the used models. Moreover, the value of qe (experimental) differs significantly from that of qe (calculated), which indicates the pseudo-first-order model does not work for the adsorption of MO-dye by Mn2O3-rGO-NS. However, the R2 values for the pseudo-second-order kinetic model are higher than 0.91, but the values of (exp) are still different from that of (cal), but more closely than that of the first-order model, which indicates the pseudo-second-order (chemisorption) may be used to explain the adsorption of dye but not the only one. As the above two models cannot give definite mechanisms (Weber & Morris 1962; Crank 1975; McKay et al. 1987). Figure 5(c) shows tje relation between adsorbate species which moving from solution matrix to internal pores and gaps of nanoparticles (McKay et al. 1982, 1984; Sundaram et al. 2011). According to the results obtained in Table 2, we note that the intra-particle diffusion model is the most reliable way to explain this adsorption, since it has the best correlation coefficient (R2 = 0.99). As shown in Figure 5, the fact that the straight lines do not pass through the origin shows that various kinetic models, which may all be active at the same time, may restrict the rate of adsorption and that intra-particle diffusion is not the only rate-limiting process. These findings support the existence of an intra-particle diffusion mechanism as well as surface adsorption (chemisorption) when it comes to the removal of MO-dye by Mn2O3-rGO-NS.
Table 2

Experimental and calculated parameters of kinetics models of adsorption of MO-dyes

Pseudo first order
Pseudo second order
Intra-particle diffusion
qe (exp)qe (cal.)k1R2qe (cal.)k2R2Ck1R2
99.3 215.17 0.194 0.89 181.34 2.7 × 10−4 0.913 –17.03 22.2 0.99 
Pseudo first order
Pseudo second order
Intra-particle diffusion
qe (exp)qe (cal.)k1R2qe (cal.)k2R2Ck1R2
99.3 215.17 0.194 0.89 181.34 2.7 × 10−4 0.913 –17.03 22.2 0.99 
Figure 5

Kinetic models of MO-dye adsorption on Mn2O3-rGO-NS. (a) The pseudo-first-order model, (b) The pseudo-second-order model, (c) The intra-particle diffusion model.

Figure 5

Kinetic models of MO-dye adsorption on Mn2O3-rGO-NS. (a) The pseudo-first-order model, (b) The pseudo-second-order model, (c) The intra-particle diffusion model.

Close modal

Adsorption isotherm

The values of the acquired parameters from the application to the experimental data obtained in the current investigation are shown in Figure 6(a)–6(c) of three models, which are the Freundlich, Langmuir, and Temkin models, respectively. The experimental results were found to fit the Freundlich isotherm model with a regression coefficient R2 (0.98) and fit the Langmuir isotherm model with a lower regression coefficient R2 (0.966), while they exhibit a poor-fitting with Temkin models R2 (0.86). It is obvious from the data depicted that the Freundlich and Langmiur models better described the adsorption process than the Temkin model. The Freundlich model shows that maximum theoretical adsorption capacity is 210 mg/g, which is considered a high value in comparison to other adsorption capacities in published works (38.6 mg/g) (Chakrabarti et al. 2009) and 22.2 mg/g (Abdullah et al. 2021).
Figure 6

Adsorption isotherm for MO-dyes over Mn2O3-rGO-NS. (a) Freundlich isotherm, (b) Langmuir isotherm, (c) Temkin isotherm.

Figure 6

Adsorption isotherm for MO-dyes over Mn2O3-rGO-NS. (a) Freundlich isotherm, (b) Langmuir isotherm, (c) Temkin isotherm.

Close modal

This work opens a green synthesis gate for super nanoporous benign metal oxide (mesoporous structure of α-Mn2O3) with a thickness of less than 26 nm and graphene sheets using thermal decomposition of metal- citrus peels extracts complex and second timescale electrolytic oxidation respectively. Following that, a composite was formed via utilizing ultrasonic radiation. The obtained results showed that the characteristics of the Mn2O3-rGO-NS sheets were improved by the addition of rGO sheets. In comparison to the literature, Mn2O3-rGO-NS adsorption capacity exhibits a high adsorption capacity of 210 mg/g to MO dye when compared to the literature at various adsorption parameters, including pH, adsorbent quantity, and time. It was discovered that the adsorption behavior complies with the intra-particle diffusion model and the Freundlich isotherm and the Langmuir isotherm model with a lower regression coefficient. This technique exhibits good practice for the synthesis of eco-friendly composite materials for the large-scale removal of dye from aqueous systems.

No funding for this research.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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