# Efficient Removal of Cd (II) from Aquatic Media by Heteronanostructure MgO@TiO2@g-C3N4

Authors: A. Modwi; M. Ismail; H. Idriss; M. A. Ben Aissa; L. Khezami; M. Bououdina
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

MgO@TiO2@g-C3N4 heteronanostructure was synthesized using a simple ultrasonication technique and assessed potentially to remove Cd (II) from aqueous environments. X-ray diffraction analysis confirms composite formation with mean crystallite size in the range of 4-17 nm while transmission electron microscopy analysis reveals nanosheet-like nanoparticles with the homogeneous elemental distribution. N2 adsorption-desorption measurements indicate the formation of a mesoporous structure with a BET surface area of about 107 m2/g. Fourier-transformed infrared elucidates the presence of O–H, amino groups, triazine, Mg–O, and Ti–O vibrations modes. At the same time, X-ray photoelectron spectroscopy analysis manifests the presence of Mg, O, N, Ti, and C elements. For aqueous Cd (II) ions, the MgO@TiO2@g-C3N4 nanostructure displays a superior adsorption efficiency, reaching 99.94% Cd (II) elimination with an optimum adsorption capacity of 515.86 mg/g in a short duration of 16 min. This study demonstrates the capability of using the MgO@TiO2@g-C3N4 nanostructure as an efficient and reusable adsorbent for the uptake of Cd (II) ions in wastewater treatment and potentially for the removal of other heavy metal ions.


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## Body

## 1. Introduction

Over the years, heavy metals have been extensively employed in a range of manufacturing processes, including mineral extraction and tannery, metal processing, metal metallurgy, and dyeing [1]. When heavy metals are used in excess, they contribute considerably to worldwide environmental concerns. The occurrence of heavy metal in industrial wastewater poses a serious hazard to human health, animals, and ecosystem [2]. The problem is that heavy metals are resistant to degradation in nature and rapidly concentrate in organisms, raising the possibility of severe risks to human health due to biomagnification in the food chain [3]. Therefore, the elimination of heavy metals and inorganic pollutants is of vital significance to environmental and human health [4].Cadmium pollution in industrial sewage is a serious ecological issue that threatens both humans and animals from water safety and environmental point of view [5]. Cadmium is utilized in a variety of industrial activities, including electrochemical, metallurgical processing, battery manufacturing, smelting, alloy manufacturing, and mining sewage. Furthermore, cadmium can cause severe kidney problems by impairing their ability to remove acids from the blood, also causes bones osteomalacia and osteoporosis, and is a major cause of heart disease [6]. Low levels of heavy metals such as cadmium and lead commonly occur in the soil owing to the decomposition of old materials; however, high levels of these metals are caused by anthropogenic emissions, which negatively impact the ecosystems [7].Several conventional methods have been adopted for heavy metal remediation from aquatic environments such as membrane filtration, precipitation, reverse osmosis, ion exchange, ultrafiltration, and biosorption processes [8–10]. Nevertheless, most of these techniques are expensive and generate secondary wastes. Nonetheless, adsorption has been utilized as a powerful and reasonable method for removing organic and inorganic pollutants from wastewater [9]. This is a cost-effective approach that also provides several advantages resulting from the large variety of adsorbents that are currently accessible. Many substances were used for this purpose including charcoal, clays, natural and artificial zeolites, and kaolinite [11, 12]. Nanomaterials are very attractive and explored on a large scale in the treatment of organic and inorganic pollutants in the aquatic system [13, 14]. Specifically, the properties of metallic oxide nanoparticles such as MgO, TiO₂, ZnO, and CuO are widely used in water purification owing to their novel physicochemical proprieties besides being tuned upon the chosen chemical composition, by doping with different materials to achieve particular requirements and applications [15–18].On the other hand, nanocomposites play an important role in environmental remediation as scavengers of water contaminants via adsorption or ion exchange [19, 20]. Tremendous efforts have been devoted in order to obtain nanocomposites with distinctive characteristics through alloying and doping [21, 22]. Nanocomposites such as RuO2-ZnO, MgO@g-C3N4, ZnO-doped TiO2/rGO, and TiO demonstrated many interesting properties, including small particle size, porous structure, a larger surface area, and the possibility of adjusting the bandgap [23–25].This study is aimed at producing low-cost and high-performance nanocomposite as a straightforward solution to address Cd (II) contamination issues in aquatic systems. A ternary MgO@TiO2@g-C3N4 nanosorbent is prepared and assessed for the removal of Cd (II) from aqueous solution. The effectiveness of the removal parameters such as the initial Cd (II) concentration, pH, and contact time in terms of adsorption capability has been evaluated. Additionally, the adsorption isotherms and kinetic studies and recyclability have been discussed. FTIR analysis elucidates a plausible mechanism for removing Cd (II) onto MgO@TiO2@g-C3N4 nanostructure surface.


## 2. Experimental Section

### 2.1. Preparation of MgO@TiO2@g-C3N4 Nanostructures

The g-C3N4 powder was prepared through urea pyrolysis degradation. 4.5 g of urea compound was placed in a covered pot and tempered at 550°C for 120 minutes at room temperature heating rate of 10°C/min. The raw yellow g-C3N4 obtained was then cooled, grinded, and collected in a container. Magnesium oxide (MgO) powder was synthesized by thermal decomposition of Mg carbonate salts (MgCO3) at 800°C for 60 min, whereas TiO2 nanopowder was purchased from Sigma Aldrich.Magnesium and titanium oxide graphitic carbon nitride nanostructure (MgO@TiO2@g-C3N4) was prepared by utilizing a conventional ultrasonication process. 1.84 g of g-C3N4 was sonicated for 15 min in 125 mL methanol. A 400 mg of MgO and TiO2 powdered mixture was added to the g-C3N4 methanolic solution and sonicated for an additional 45 min. The resulting yellowish solution was then evaporated for 24 h at 85°C. The final product MgO@TiO2@g-C3N4 was annealed for 60 min at 180°C.


### 2.2. Characterizations

Rigaku D/max-RA powder diffractometer equipped with Cu-Kα radiation source (λ=1.5418 Å) was used to analyze the crystal structure and to determine structural and microstructural parameters of MgTiO3@g-C3N4 nanopowder. The Brunauer–Emmett–Teller (BET) surface area of the nanomaterial was determined by recording N2 adsorption/desorption at 196°C on a Micrometrics ASAP 2020 analyzer. A Hitachi H-800 transmission electron microscope (TEM) equipped with electron dispersive X-ray (EDX) spectroscopy was used for morphological observations and elemental chemical analysis. The chemical surface properties of the as-prepared nanopowder were investigated by means of X-ray photoelectron spectroscopy (XPS) using Perkin Elmer PHI 550 ESCA/SAM equipped with a monochromatized Al-Kα X-ray source (hν=1486.6 eV) and a hemispherical electron analyzer. To elucidate the possible adsorption mechanism, Fourier-transformed infrared (FTIR) spectra of MgO@TiO3@g-C3N4 nanopowder before and after Cd (II) removal were recorded by using KBr pellet technique on a Nicolet Nexus 870 FTIR spectrometer.


### 2.3. Cd (II) Adsorption Experiments

Batch experiments were performed to assess the adsorption isotherms of Cd (II) on MgO@TiO3@g-C3N4 heteronanostructure. A 10 mg of MgO@TiO3@g-C3N4 sorbent was introduced in 50 mL glass bottles with initial concentrations of the Cd (II) ranging from 5 to 200 ppm. The mixture suspensions were magnetically stirred for 1440 min. Upon attaining equilibrium with the aqueous phase, the nanopowder was centrifuged and the residual Cd (II) concentrations in the aliquot were determined by atomic absorption spectroscopy (AAS) (Hitachi Z-8100, Japan).The amount of adsorbed Cd (II) ions at any timet (min) and subsequently the equilibrium value of qt and qe (in mg/g) were calculated by using the following equations:
(1)qt=VC0−Ctm,qe=VC0−Cem,where V is the solution volume (L), C0, Ce, and Ct are the initial concentration, equilibrium concentration, and concentration at any time t of Cd (II) metal ions (mg/L) in solution, and m is the mass of the adsorbent (g). All the measurements were reproduced twice, and only the average values were reported in the manuscript. The ultimate attained deviation was found to be smaller than 5%. All the experimental data were plotted and modelled using the Origin 2019b software. All dynamic and equilibrium graphs were illustrated with error bars (i.e., Figures 1–4). Moreover, all statistical data obtained from Origin are presented in Table 1.Figure 1
Variation of removal rate as function of Cd (II) initial concentration by MgO@TiO2@g-C3N4 nanopowder.Figure 2
Experimental Cd (II) adsorption by MgO@TiO2@g-C3N4 nanopowder fitted with nonlinear Langmuir and Freundlich models.
(a)(b)Figure 3
Effect of contact time on Cd (II) removal by MgO@TiO2@g-C3N4 nanopowder.Figure 4
(a) PFO and (b) PSO kinetics; (c) IPD models for Cd (II) adsorption by MgO@TiO2@g-C3N4 nanopowder.
(a)(b)(c)Table 1
Parameters calculated from kinetics models for Cd (II) adsorption onto MgO@TiO2@g-C3N4 heteronanostructure.
Model nameParametersVariable unitComputed valuesPFOqemg/g148.91±0.5395k1L/min0.4471±0.019R2-0.9232PSOqecalculatedmg/g150.66±0.6748qeexperimentalmg/g150.31±0.4748k2mg/mg.min0.01168±0.0010R2-0.9650IPDC1-132.20±3.2632kdif1mg/mg.min3.118±0.84255R2-0.9341C2-149.53±0.0033kdif2mg/mg.min0.0039±0.00014R2-0.8101


## 2.1. Preparation of MgO@TiO2@g-C3N4 Nanostructures

The g-C3N4 powder was prepared through urea pyrolysis degradation. 4.5 g of urea compound was placed in a covered pot and tempered at 550°C for 120 minutes at room temperature heating rate of 10°C/min. The raw yellow g-C3N4 obtained was then cooled, grinded, and collected in a container. Magnesium oxide (MgO) powder was synthesized by thermal decomposition of Mg carbonate salts (MgCO3) at 800°C for 60 min, whereas TiO2 nanopowder was purchased from Sigma Aldrich.Magnesium and titanium oxide graphitic carbon nitride nanostructure (MgO@TiO2@g-C3N4) was prepared by utilizing a conventional ultrasonication process. 1.84 g of g-C3N4 was sonicated for 15 min in 125 mL methanol. A 400 mg of MgO and TiO2 powdered mixture was added to the g-C3N4 methanolic solution and sonicated for an additional 45 min. The resulting yellowish solution was then evaporated for 24 h at 85°C. The final product MgO@TiO2@g-C3N4 was annealed for 60 min at 180°C.


## 2.2. Characterizations

Rigaku D/max-RA powder diffractometer equipped with Cu-Kα radiation source (λ=1.5418 Å) was used to analyze the crystal structure and to determine structural and microstructural parameters of MgTiO3@g-C3N4 nanopowder. The Brunauer–Emmett–Teller (BET) surface area of the nanomaterial was determined by recording N2 adsorption/desorption at 196°C on a Micrometrics ASAP 2020 analyzer. A Hitachi H-800 transmission electron microscope (TEM) equipped with electron dispersive X-ray (EDX) spectroscopy was used for morphological observations and elemental chemical analysis. The chemical surface properties of the as-prepared nanopowder were investigated by means of X-ray photoelectron spectroscopy (XPS) using Perkin Elmer PHI 550 ESCA/SAM equipped with a monochromatized Al-Kα X-ray source (hν=1486.6 eV) and a hemispherical electron analyzer. To elucidate the possible adsorption mechanism, Fourier-transformed infrared (FTIR) spectra of MgO@TiO3@g-C3N4 nanopowder before and after Cd (II) removal were recorded by using KBr pellet technique on a Nicolet Nexus 870 FTIR spectrometer.


## 2.3. Cd (II) Adsorption Experiments

Batch experiments were performed to assess the adsorption isotherms of Cd (II) on MgO@TiO3@g-C3N4 heteronanostructure. A 10 mg of MgO@TiO3@g-C3N4 sorbent was introduced in 50 mL glass bottles with initial concentrations of the Cd (II) ranging from 5 to 200 ppm. The mixture suspensions were magnetically stirred for 1440 min. Upon attaining equilibrium with the aqueous phase, the nanopowder was centrifuged and the residual Cd (II) concentrations in the aliquot were determined by atomic absorption spectroscopy (AAS) (Hitachi Z-8100, Japan).The amount of adsorbed Cd (II) ions at any timet (min) and subsequently the equilibrium value of qt and qe (in mg/g) were calculated by using the following equations:
(1)qt=VC0−Ctm,qe=VC0−Cem,where V is the solution volume (L), C0, Ce, and Ct are the initial concentration, equilibrium concentration, and concentration at any time t of Cd (II) metal ions (mg/L) in solution, and m is the mass of the adsorbent (g). All the measurements were reproduced twice, and only the average values were reported in the manuscript. The ultimate attained deviation was found to be smaller than 5%. All the experimental data were plotted and modelled using the Origin 2019b software. All dynamic and equilibrium graphs were illustrated with error bars (i.e., Figures 1–4). Moreover, all statistical data obtained from Origin are presented in Table 1.Figure 1
Variation of removal rate as function of Cd (II) initial concentration by MgO@TiO2@g-C3N4 nanopowder.Figure 2
Experimental Cd (II) adsorption by MgO@TiO2@g-C3N4 nanopowder fitted with nonlinear Langmuir and Freundlich models.
(a)(b)Figure 3
Effect of contact time on Cd (II) removal by MgO@TiO2@g-C3N4 nanopowder.Figure 4
(a) PFO and (b) PSO kinetics; (c) IPD models for Cd (II) adsorption by MgO@TiO2@g-C3N4 nanopowder.
(a)(b)(c)Table 1
Parameters calculated from kinetics models for Cd (II) adsorption onto MgO@TiO2@g-C3N4 heteronanostructure.
Model nameParametersVariable unitComputed valuesPFOqemg/g148.91±0.5395k1L/min0.4471±0.019R2-0.9232PSOqecalculatedmg/g150.66±0.6748qeexperimentalmg/g150.31±0.4748k2mg/mg.min0.01168±0.0010R2-0.9650IPDC1-132.20±3.2632kdif1mg/mg.min3.118±0.84255R2-0.9341C2-149.53±0.0033kdif2mg/mg.min0.0039±0.00014R2-0.8101


## 3. Results and Discussions

### 3.1. Characterization of MgO@TiO3@g-C3N4

#### 3.1.1. Structural Analysis

The XRD pattern of MgTiO3@g-C3N4 nanopowder, as shown in Figure 5, displays well-defined diffraction peaks with relative broadening and intensity, manifesting the formation of well-nanocrystallized phase(s).Figure 5
XRD patter of MgTiO3@g-C3N4 nanopowder.Peaks’ identification using High Score program indicates the presence of three phases, i.e., MgO, anatase TiO2, and g-C3N4. The characteristic diffraction peaks at 2θ=12.7° and 27.38° for g-C3N4 appear to correlate with the in-plane structural packing pattern (100) and interlayer stacking plane (002) of the hexagonal structure (JCPDS card No. 87-1526) [26], whereas the peaks located at 2θ of 25.07°, 37.59°, 47.05°, 53.91°, 61.99°, 69.09, and 74.53° represent the distinctive reflections (101), (004), (200), (105), (204), (116), and (220) of the anatase phase TiO2 (JCPDS card No. 021-1272) [27]. The remaining peaks observed at 2θ of 36.61°, 42.53°, 61.99°, and 78.16° are assigned to the MgO cubic structure’s (111), (200), (220), and (222) planes [28].The crystallite size (D) has been calculated using the well-known Scherrer equation:
(2)D=Kλβcosθ,where K is constant depending on particle’s shape (K=0.9 for spherical morphology), λ is the wavelength of the X-ray source (1.5418 Å), β is the full width at half maximum of the chosen reflection (in rad), and θ is the diffraction of the chosen reflection.For anatase TiO2 (I41/amd, space group No. 141), MgO with rock-salt type structure (Fm-3 m, space group No. 225), and g-C3N4 (Fd-3 m, space group No. 227) with a tetragonal, cubic, and cubic crystal structures, respectively, the lattice parameters are calculated by means of the following equations:
(3)1d2=h2+k2a2+l2c2,(4)1d2=h2+k2+l2a2,where d is the interplanar spacing and h,k,l are Miller indices of the corresponding diffraction peak. The calculated crystallite size of the lattice parameters of TiO2, MgO, and g-C3N4 phases are reported in Table 2. It can be noticed that the crystallite size for all phases is within the nanoscale regime, i.e., 4–17 nm. The as obtained heterostructured composite with high surface area would be suitable for the adsorption of heavy metal ions. The computed lattice parameters are found to deviate from the bulk compounds, i.e., 3.6600 Å and 9.7600 Å for anatase TiO2 [29], 4.2110 Å for MgO [30], and 6.7824 Å for g-C3N4 [31]. This can be associated with the occurrence of the dissolution of Mg ions within TiO2 host lattice by occupying Ti ionic sites followed by its expansion since Mg2+ (0.72 Å) has a larger ionic radius compared to that of Ti4+ (0.61 Å).Table 2
Microstructural (crystallite size) and structural (lattice parameters and unit cell volume) parameters of TiO2, MgO, and g-C3N4 phases.
PhasesCrystallite size (nm)Lattice parameters (unit A (angstrom))TiO211.34a=3.8196  c=9.7815MgO17.09a=4.2424g-C3N44.08a=6.5093


#### 3.1.2. Morphology and Chemical Composition

As shown in Figures6(a) and 6(b), TEM images of the synthesized MgO@TiO2@g-C3N4 powder exhibit characteristic two-dimensional nanosheet-like nanoparticles with a corrugated thickness of around 27 nm. As can be seen, the integrated MgO and TiO2 nanoparticles in the MgO@TiO2@g-C3N4 nanostructure have an average particle size of about 20-50 nm. The EDX spectrum (Figure 6(c)) displays typical characteristic peaks of Ti, Mg, N, O, and C, hence confirming the purity of the prepared composite. The inset table in Figure 6(c) indicates the corresponding chemical composition of the synthesized MgO@TiO2@g-C3N4. Furthermore, the elemental mapping of C, N, O, Mg, and Ti, as shown in Figures 6(e)–6(i), clearly evidenced the homogeneous distribution of the elements over the analyzed particles (Figure 6(d)).Figure 6
TEM images of MgO@TiO2@g-C3N4 nanopowder with (a, b) different magnification, (c) EDX for element composition, and (d–i) elemental mapping of C, N, O, Mg, and Ti, respectively.
(a)(b)(c)(d)(e)(f)(g)(h)(i)


#### 3.1.3. BET Analysis

A good adsorbent, for instance MgO@TiO3@g-C3N4 nanostructure, must have a significant number of available adsorption sites. In other words, the material’s surface area, pore volume, and size should be enough. The nitrogen adsorption-desorption analysis (Figure 7 inset pore size) reveals that MgO@TiO2@g-C3N4 nanostructure is a mesoporous material with an IUPAC type IV adsorption isotherm. The isotherm is coupled with a hysteresis of type H1, implying a restricted distribution of uniform mesoporous and limited networking effects [32, 33]. The TEM image (Figure 7) confirms the presence of pores besides the appearance of large aggregates generated by the interconnection of MgO and TiO2 nanoparticles with g-C3N4 nanosheets. The surface area, total porous volume, and average pore size of MgO@TiO2@g-C3N4 nanostructure are 107 m2/g, 0.254 cc/g, and 1.57 nm, respectively. With a large surface area and porosity, the adsorbent is predicted to expose a vast number of adsorption sites, resulting in a high adsorption capacity.Figure 7
N2 adsorption/desorption isotherm (inset pore size) for MgO@TiO2@g-C3N4 nanopowder.


#### 3.1.4. XPS Analysis

XPS analysis is used to determine the chemical composition and the valences of the elements on the nanoadsorbent surface. Figures8(a)–8(f) depict the XPS survey results for MgO@TiO2@g-C3N4, and Mg, O, N, Ti, and C elements are detected in the whole XPS scan spectrum. The XPS spectrum of Mg-2p (Figure 8(b)) displays a single peak at 51 eV characteristic of MgO compound [34]. The XPS spectrum of O-1s (Figure 8(c)) manifests a single peak at 532 eV associated to O-1s [35, 36]. For the XPS spectrum of N-1s (Figure 8(d)), two peaks are detected, i.e., for the amino groups (401 eV) and bridging N atoms (399 eV) [37]. Similarly, the XPS spectrum of Ti-2p (Figure 8(e)) displays two peaks at 459 and 464.3 eV, corresponding to the characteristic peaks of Ti-2p3/2 and Ti-2p1/2, respectively [38]. Further, three peaks located at 285, 287, and 289 eV are observed in the C-1s XPS spectrum (Figure 8(f)) and attributed to C=C, C–C, and O–C bands, respectively [39]. It is clearly observed from the above XPS analysis that only Mg, Ti, C, N, and O, elements are present, thereby no presence of any impurity within the as-fabricated MgO@TiO2@g-C3N4 composite.Figure 8
XPS spectra of (a) MgO@TiO2@g-C3N4 full survey and individual spectra of (b) Mg-2p, (c) O-s, (d) N-1s, (e) Ti-2p, and (f) C-1s.
(a)(b)(c)(d)(e)(f)


### 3.2. Adsorption Ability of MgO@TiO2@g-C3N4 Nanocomposite

#### 3.2.1. Influence of Initial Cd (II) Concentration

The impact of Cd (II) concentration has been tested in the range (5–200 mg/L) under the optimized operational parameters, i.e., MgO@TiO3@g-C3N4 dose (10 mg), pH 7, a fixed volume of Cd (II) solution (25 mL), at room temperature, for a contact time of 24 hrs. The percentage and the optimum adsorption capacity of Cd (II) onto MgO@TiO3@g-C3N4 sorbent are shown in Figure 1. It can be observed that as the Cd (II) concentration increases, the adsorbed quantity increases gradually from 12.49 to 515.86 mg/g and the removal percentage remains very high within the range 99.94-98.75%. In this case, the fundamental driving force that increases the initial concentration of Cd (II) overcomes any barrier to Cd (II) migration from the solution. The resulting fractional adsorption becomes concentration-dependent.


#### 3.2.2. Adsorption Isotherms Modeling

The adsorption isotherm of Cd (II) onto MgO@TiO2@g-C3N4 nanopowder has been examined by introducing a certain amount of the adsorbent (dose of 400 mg/L) to the Cd (II) solution (5 to 200 mg/L) for a duration of 1440 min at room temperature and pH 7 (see Figure 2). The equilibrium concentration of Cd (II) is found to be 0.033 mg/L compared to the initial concentration of 5 mg/L, resulting in a removal efficiency greater than 99.34%. This finding indicates that MgO@TiO2@g-C3N4 is an excellent adsorbent for Cd (II) removal.To determine the adsorption behavior of the adsorbent, the experimental data has been plotted using the Freundlich (Equation (5)) and Langmuir (Equation (6)) mathematical equations:
(5)qe=KFCe1/n,(6)qe=QmaxKLCe1+KLCe.The constantsK and n of the Freundlich model can be related to the strength of the adsorptive bond and the bond distribution, respectively [37]. In the Langmuir model (Eq. (3)), qmax is the solid-phase concentration corresponding to the complete monolayer coverage of adsorption sites [38]. KL is the constant related to the free energy of adsorption. The Langmuir isotherm is the quickest, easiest, and the most effective isotherm for describing the chemical adsorption process because it is based on the following assumptions: the adsorbent configuration is homogeneous; the adsorbed molecules or atoms are held in discrete and fixed locations; each site can compensate only one molecule or atom; the energy of adsorption is consistent across all sites; and no interaction between neighboring adsorbent surface and adsorbed molecules occurs. In comparison, the Freundlich model addresses heterogeneous systems and reversible adsorption, whereas the monolayer adsorption is not restricted. Figures 2(a) and 2(b) depict linear plots of experimental data generated using the Langmuir and Freundlich models. The Langmuir model fits the data better, with a regression coefficient R2=0.991, compared to the Freundlich model’s R2=0.963. According to the Langmuir model, the maximum adsorption capacity of the studied MgO@TiO2@g-C3N4 is found to be 529 mg/g. This relatively high capacity can be attributed to its enhanced surface area and porous nanostructure with relatively large pore size. This result elucidates that Cd (II) ions are adsorbed as a monolayer over MgO@TiO3@g-C3N4 surface through chemisorption being the dominant process.


#### 3.2.3. Effect of Equilibrium Contact Duration on Cd (II) Removal

Figure3 illustrates the effect of contact time on Cd (II) removal. The adsorption of Cd (II) ions onto the sorbent MgO@TiO2@g-C3N4 nanostructure has been investigated over a stirring time in the range 5–1440 min at an initial Cd (II) concentration of 45 ppm. The removal of Cd (II) as a function of contact time approaches equilibrium in less than 16 min. Due to the numerous active sites on the sorbent MgO@TiO2@g-C3N4 surface, the amount of adsorbed Cd (II) reaches approximately 150 mg/g during this short time interval. During the equilibrium period, the concentration of the active site naturally reduces, and the removal rate becomes relatively constant to reach an equilibrium state. With a large surface area and porosity, the adsorbent is predicted to expose a large number of adsorption sites, resulting in a high adsorption capacity.


#### 3.2.4. Adsorption Kinetic Investigations

Adsorption kinetic parameters are fundamental in predicting an adsorbent’s efficiency. The adsorption of Cd (II) ions onto MgO@TiO2@g-C3N4 nanostructures has been evaluated by employing pseudofirst-order (PFO), pseudosecond-order (PSO) kinetic, and intraparticle diffusion (IPD) models as displayed in Figures 4(a)–4(c). The corresponding nonlinear mathematical equations are as follows:
(7)qt=qe1−e−k1t,(8)qt=tk2qe2k2qet+1,(9)qt=kdift+C.Equation (7) describes the PFO model of Lagergren [39] with a rate constant k1 (1/min), and qe is the maximum amount of Cd (II) metal ions removed at equilibrium. For PSO law (Equation (8)), k2 stands for the PSO rate constant (g/mg.min) [40]. The initial rate of sorption, h0=k2.qe2, is assessed from PSO kinetic model equation. The half-sorption time, t1/2=1/k2.qe, is known as the time required for the adsorption to remove up to half as much Cd (II) metal ions as its equilibrium value.Concerning the IPD mechanism model (Equation (9)) [41], C represents the intercept that avails more details about the thickness of the edge layer. The IPD constant, kdif value (in mg/g.min1/2), for the used adsorbent, is estimated from the slope of the plot (Figure 4(c)). The validity of these models is examined based on the regression coefficient R2. The parameters computed from the fitting of the experimental data of Cd (II) adsorption by the kinetic models are given in Table 1. It can be noted that Cd (II) kinetic adsorption data are successfully fitted by the PSO model, since the value of R2=0.97, implying the occurrence of Cd (II) chemisorption onto MgO@TiO2@g-C3N4 nanostructures [40]. Further, the calculated value of qe=150.66 mg/g is identical to the measured value qe=150.31 mg/g. In the literature, the PSO model has recently been found to accurately predict the retention of metals on activated carbon [41].It is known that PSO kinetic model may be used to deduce all stages of the sorption process, including exterior film diffusion, sorption, and internal particle diffusion. Nonetheless, this model cannot account for the particular adsorption mechanism [42]. Therefore, the obtained data have been analyzed using the intraparticle diffusion kinetic model (Figure 4(c)). Numerous investigations demonstrated that the intraparticle diffusion graph may exhibit multilinearity, implying the possible occurrence of two or more steps during the adsorption process [43]. The findings indicate that most data points have two straight lines, and that the plots do not cross through the origin (plots not shown).Thekdif2 value for MgO@TiO2@g-C3N4 nanostructure is found to be lower than the kdif1 value, implying that intraparticle diffusion is the rate-limiting step for Cd (II) adsorption onto the hybrid composite. Also, it is important to mention that the slight deflection of the straight lines from the beginning implies that intraparticle diffusion may not be the only rate-limiting step in the adsorption of Cd (II) ions onto MgO@TiO2@g-C3N4 nanostructure; other possible surface adsorption processes also contribute to the overall sorption rate [44, 45].


#### 3.2.5. Influence of pH on Cd (II) Removal

The pH value is known to be crucial in better understanding the rate of surface reactions between adsorbate (Cd (II)) and MgO@TiO2@g-C3N4 adsorbent. The effect of pH on Cd (II) adsorption ability has been investigated in the pH range 1.0-8.0, with the optimum adsorption capacity obtained at pH 5 as shown in Figure 9. Below pH 5, significant protonation occurs on the adsorbent surface, resulting in weak adsorbate-adsorbent interactions. Similar findings were reported in analogous studies [46, 47]. Metal ions such as Cd (II) have a pH dependence on their solubility. At lower pH, Cd (II) is extremely soluble as Cd (II) free ions and Cd (OH)+. Furthermore, at pH value greater than 7, Cd (II) ions will precipitate as metal hydroxide Cd (OH)2 [48]. It can be concluded that the quantitative removal capacity increases with the increment of the pH solution until the pH value is equal to 7.Figure 9
Effect of pH on Cd (II) removal by MgO@TiO2@g-C3N4 nanopowder.


#### 3.2.6. Cd (II) Adsorption Mechanism onto MgO@TiO2@g-C3N4 Nanomaterial

The FTIR analysis help to clarify the adsorption mechanism of Cd (II) onto MgO@TiO2@g-C3N4 nanopowder [49]. The FTIR spectra of MgO@TiO2@g-C3N4 before and after Cd (II) ions adsorption, as shown in Figure 10(a), reveal considerable changes. The O–H and the terminal amino group stretching modes situated between 3000 and 3500 cm-1 are shifted to lower frequencies after adsorption. Similarly, the triazine ring mode peak at 891 cm-1 also shifts slightly to 883 cm-1. In addition, both Mg–O and Ti–O stretching vibration modes [50, 51] have mostly changed position indicating the interaction between Cd (II) and oxygen atoms of MgO@TiO2@g-C3N4 by π - π stacking bridging [52]. This result indicates that functional groups of MgO@TiO2@g-C3N4 (OH, CN, and N-H) and π delocalized electron systems of the triazine ring (C3N3) most probably are involved in the removal of Cd (II). Shen et al. reported that the triazine ring serves as a Lewis base and contributes to the Cd (II) elimination process as confirmed by FTIR and XPS analyses [53]. Based on the above FTIR observations of, a plausible sorption mechanism of Cd (II) onto MgO@TiO2@g-C3N4 is proposed as shown in Figure 10(b).Figure 10
FTIR spectra of (a) heteronanostructure MgO@TiO2@g-C3N4 before and after Cd adsorption; (b) proposed sorption mechanism.
(a)(b)


#### 3.2.7. Recovery and Reusability of MgO@TiO2@g-C3N4 Nanomaterial

The strategies employed to remove harmful pollutants such as heavy metal ions from diverse effluents are considered on various measures such as efficiency, suitability, stability, cost, and their reusability in large-scale industrial applications. The recycling of nanoadsorbents has become a cost-effective process. Accordingly, the regeneration of the MgO@TiO2@g-C3N4 nanostructure from the reaction mixture has been investigated by adopting the same approach used by Khezami et al. [13]. In such a process, succeeding the removal process, the nanostructural material was separated from the mixture by filtration, washed with deionized water, oven-dried, and then soaked with a sodium hydroxide solution. The alkaline desorbing agent, NaOH, is proved effective in desorbing Cd (II) ions from the surface of the adsorbent. The adsorption-desorption procedure has been replicated for four consecutive cycles to test the reusability of the MgO@TiO2@g-C3N4 nanostructure for the removal of Cd (II) metal ions. The below expression then estimates the metal recovery:
(10)Desorptionefficiency%=AmountofCdIIionsdesorbedAmountofCdIIionsadsorbed×100%.As shown in Figure11, MgO@TiO2@g-C3N4 nanostructure visibly exhibits a higher constancy. It is also important to note that the synthesized heteronanostructured by a simple ultrasonication technique can be recovered and reused for four adsorption-desorption cycles.Figure 11
Desorption efficiency % of Cd (II) metal ions from MgO@TiO2@g-C3N4 nanostructure by NaOH.


### 3.3. Comparison of MgO@TiO2@g-C3N4 Nanosorbent with Other Adsorbents

According to the results given in Table3, the adsorption performance of MgO@TiO2@g-C3N4 nanostructure toward aqueous Cd (II) has been further assessed in comparison to other previously adsorbents reported in the literature. Primarily, the MgO@TiO2@g-C3N4 exhibits a shorter equilibrium duration, i.e., only 16 min, hence demonstrating rapid kinetics in the removal of Cd (II) from the aqueous solution. Further, the MgO@TiO2@g-C3N4 heteronanostructure has a larger adsorption capacity compared to other nanostructures, i.e., 516 mg/g compared to 26–342 mg/g. The high performance of MgO@TiO2@g-C3N4 nanosorbent for the removal of Cd (II) from aqueous solution is associated to its interesting nanostructure with a larger surface area and large pore size, making it a very promising and potentially an efficient adsorbent for the removal of other heavy metal ions.Table 3
Observation parameter adsorption of the MgO@TiO2@g-C3N4 compared with the various adsorbent nanomaterials.
AdsorbentsOptimal pHEquilibrium time (min)Removal (%)Qmax (mg/g)RefsDate Palm Biochar5.0-86.8043.58[45]Fe3O4@SiO2 nanoparticles6.06074.0026.10[54]Phosphorus-containing iron ore6.09099.80-[55]LDH@GO-SH5.060-102.77[56]Silicates on diatomite5.5120-29.00[57]Calcium carbonate/Nano-TiO25.08100103.6[58]Hydrated manganese dioxide3.06045.44342.3[59]MgO@TiO2@g-C3N4 heteronanostructure5.01699.94515.8This study


## 3.1. Characterization of MgO@TiO3@g-C3N4

### 3.1.1. Structural Analysis

The XRD pattern of MgTiO3@g-C3N4 nanopowder, as shown in Figure 5, displays well-defined diffraction peaks with relative broadening and intensity, manifesting the formation of well-nanocrystallized phase(s).Figure 5
XRD patter of MgTiO3@g-C3N4 nanopowder.Peaks’ identification using High Score program indicates the presence of three phases, i.e., MgO, anatase TiO2, and g-C3N4. The characteristic diffraction peaks at 2θ=12.7° and 27.38° for g-C3N4 appear to correlate with the in-plane structural packing pattern (100) and interlayer stacking plane (002) of the hexagonal structure (JCPDS card No. 87-1526) [26], whereas the peaks located at 2θ of 25.07°, 37.59°, 47.05°, 53.91°, 61.99°, 69.09, and 74.53° represent the distinctive reflections (101), (004), (200), (105), (204), (116), and (220) of the anatase phase TiO2 (JCPDS card No. 021-1272) [27]. The remaining peaks observed at 2θ of 36.61°, 42.53°, 61.99°, and 78.16° are assigned to the MgO cubic structure’s (111), (200), (220), and (222) planes [28].The crystallite size (D) has been calculated using the well-known Scherrer equation:
(2)D=Kλβcosθ,where K is constant depending on particle’s shape (K=0.9 for spherical morphology), λ is the wavelength of the X-ray source (1.5418 Å), β is the full width at half maximum of the chosen reflection (in rad), and θ is the diffraction of the chosen reflection.For anatase TiO2 (I41/amd, space group No. 141), MgO with rock-salt type structure (Fm-3 m, space group No. 225), and g-C3N4 (Fd-3 m, space group No. 227) with a tetragonal, cubic, and cubic crystal structures, respectively, the lattice parameters are calculated by means of the following equations:
(3)1d2=h2+k2a2+l2c2,(4)1d2=h2+k2+l2a2,where d is the interplanar spacing and h,k,l are Miller indices of the corresponding diffraction peak. The calculated crystallite size of the lattice parameters of TiO2, MgO, and g-C3N4 phases are reported in Table 2. It can be noticed that the crystallite size for all phases is within the nanoscale regime, i.e., 4–17 nm. The as obtained heterostructured composite with high surface area would be suitable for the adsorption of heavy metal ions. The computed lattice parameters are found to deviate from the bulk compounds, i.e., 3.6600 Å and 9.7600 Å for anatase TiO2 [29], 4.2110 Å for MgO [30], and 6.7824 Å for g-C3N4 [31]. This can be associated with the occurrence of the dissolution of Mg ions within TiO2 host lattice by occupying Ti ionic sites followed by its expansion since Mg2+ (0.72 Å) has a larger ionic radius compared to that of Ti4+ (0.61 Å).Table 2
Microstructural (crystallite size) and structural (lattice parameters and unit cell volume) parameters of TiO2, MgO, and g-C3N4 phases.
PhasesCrystallite size (nm)Lattice parameters (unit A (angstrom))TiO211.34a=3.8196  c=9.7815MgO17.09a=4.2424g-C3N44.08a=6.5093


### 3.1.2. Morphology and Chemical Composition

As shown in Figures6(a) and 6(b), TEM images of the synthesized MgO@TiO2@g-C3N4 powder exhibit characteristic two-dimensional nanosheet-like nanoparticles with a corrugated thickness of around 27 nm. As can be seen, the integrated MgO and TiO2 nanoparticles in the MgO@TiO2@g-C3N4 nanostructure have an average particle size of about 20-50 nm. The EDX spectrum (Figure 6(c)) displays typical characteristic peaks of Ti, Mg, N, O, and C, hence confirming the purity of the prepared composite. The inset table in Figure 6(c) indicates the corresponding chemical composition of the synthesized MgO@TiO2@g-C3N4. Furthermore, the elemental mapping of C, N, O, Mg, and Ti, as shown in Figures 6(e)–6(i), clearly evidenced the homogeneous distribution of the elements over the analyzed particles (Figure 6(d)).Figure 6
TEM images of MgO@TiO2@g-C3N4 nanopowder with (a, b) different magnification, (c) EDX for element composition, and (d–i) elemental mapping of C, N, O, Mg, and Ti, respectively.
(a)(b)(c)(d)(e)(f)(g)(h)(i)


### 3.1.3. BET Analysis

A good adsorbent, for instance MgO@TiO3@g-C3N4 nanostructure, must have a significant number of available adsorption sites. In other words, the material’s surface area, pore volume, and size should be enough. The nitrogen adsorption-desorption analysis (Figure 7 inset pore size) reveals that MgO@TiO2@g-C3N4 nanostructure is a mesoporous material with an IUPAC type IV adsorption isotherm. The isotherm is coupled with a hysteresis of type H1, implying a restricted distribution of uniform mesoporous and limited networking effects [32, 33]. The TEM image (Figure 7) confirms the presence of pores besides the appearance of large aggregates generated by the interconnection of MgO and TiO2 nanoparticles with g-C3N4 nanosheets. The surface area, total porous volume, and average pore size of MgO@TiO2@g-C3N4 nanostructure are 107 m2/g, 0.254 cc/g, and 1.57 nm, respectively. With a large surface area and porosity, the adsorbent is predicted to expose a vast number of adsorption sites, resulting in a high adsorption capacity.Figure 7
N2 adsorption/desorption isotherm (inset pore size) for MgO@TiO2@g-C3N4 nanopowder.


### 3.1.4. XPS Analysis

XPS analysis is used to determine the chemical composition and the valences of the elements on the nanoadsorbent surface. Figures8(a)–8(f) depict the XPS survey results for MgO@TiO2@g-C3N4, and Mg, O, N, Ti, and C elements are detected in the whole XPS scan spectrum. The XPS spectrum of Mg-2p (Figure 8(b)) displays a single peak at 51 eV characteristic of MgO compound [34]. The XPS spectrum of O-1s (Figure 8(c)) manifests a single peak at 532 eV associated to O-1s [35, 36]. For the XPS spectrum of N-1s (Figure 8(d)), two peaks are detected, i.e., for the amino groups (401 eV) and bridging N atoms (399 eV) [37]. Similarly, the XPS spectrum of Ti-2p (Figure 8(e)) displays two peaks at 459 and 464.3 eV, corresponding to the characteristic peaks of Ti-2p3/2 and Ti-2p1/2, respectively [38]. Further, three peaks located at 285, 287, and 289 eV are observed in the C-1s XPS spectrum (Figure 8(f)) and attributed to C=C, C–C, and O–C bands, respectively [39]. It is clearly observed from the above XPS analysis that only Mg, Ti, C, N, and O, elements are present, thereby no presence of any impurity within the as-fabricated MgO@TiO2@g-C3N4 composite.Figure 8
XPS spectra of (a) MgO@TiO2@g-C3N4 full survey and individual spectra of (b) Mg-2p, (c) O-s, (d) N-1s, (e) Ti-2p, and (f) C-1s.
(a)(b)(c)(d)(e)(f)


## 3.1.1. Structural Analysis

The XRD pattern of MgTiO3@g-C3N4 nanopowder, as shown in Figure 5, displays well-defined diffraction peaks with relative broadening and intensity, manifesting the formation of well-nanocrystallized phase(s).Figure 5
XRD patter of MgTiO3@g-C3N4 nanopowder.Peaks’ identification using High Score program indicates the presence of three phases, i.e., MgO, anatase TiO2, and g-C3N4. The characteristic diffraction peaks at 2θ=12.7° and 27.38° for g-C3N4 appear to correlate with the in-plane structural packing pattern (100) and interlayer stacking plane (002) of the hexagonal structure (JCPDS card No. 87-1526) [26], whereas the peaks located at 2θ of 25.07°, 37.59°, 47.05°, 53.91°, 61.99°, 69.09, and 74.53° represent the distinctive reflections (101), (004), (200), (105), (204), (116), and (220) of the anatase phase TiO2 (JCPDS card No. 021-1272) [27]. The remaining peaks observed at 2θ of 36.61°, 42.53°, 61.99°, and 78.16° are assigned to the MgO cubic structure’s (111), (200), (220), and (222) planes [28].The crystallite size (D) has been calculated using the well-known Scherrer equation:
(2)D=Kλβcosθ,where K is constant depending on particle’s shape (K=0.9 for spherical morphology), λ is the wavelength of the X-ray source (1.5418 Å), β is the full width at half maximum of the chosen reflection (in rad), and θ is the diffraction of the chosen reflection.For anatase TiO2 (I41/amd, space group No. 141), MgO with rock-salt type structure (Fm-3 m, space group No. 225), and g-C3N4 (Fd-3 m, space group No. 227) with a tetragonal, cubic, and cubic crystal structures, respectively, the lattice parameters are calculated by means of the following equations:
(3)1d2=h2+k2a2+l2c2,(4)1d2=h2+k2+l2a2,where d is the interplanar spacing and h,k,l are Miller indices of the corresponding diffraction peak. The calculated crystallite size of the lattice parameters of TiO2, MgO, and g-C3N4 phases are reported in Table 2. It can be noticed that the crystallite size for all phases is within the nanoscale regime, i.e., 4–17 nm. The as obtained heterostructured composite with high surface area would be suitable for the adsorption of heavy metal ions. The computed lattice parameters are found to deviate from the bulk compounds, i.e., 3.6600 Å and 9.7600 Å for anatase TiO2 [29], 4.2110 Å for MgO [30], and 6.7824 Å for g-C3N4 [31]. This can be associated with the occurrence of the dissolution of Mg ions within TiO2 host lattice by occupying Ti ionic sites followed by its expansion since Mg2+ (0.72 Å) has a larger ionic radius compared to that of Ti4+ (0.61 Å).Table 2
Microstructural (crystallite size) and structural (lattice parameters and unit cell volume) parameters of TiO2, MgO, and g-C3N4 phases.
PhasesCrystallite size (nm)Lattice parameters (unit A (angstrom))TiO211.34a=3.8196  c=9.7815MgO17.09a=4.2424g-C3N44.08a=6.5093


## 3.1.2. Morphology and Chemical Composition

As shown in Figures6(a) and 6(b), TEM images of the synthesized MgO@TiO2@g-C3N4 powder exhibit characteristic two-dimensional nanosheet-like nanoparticles with a corrugated thickness of around 27 nm. As can be seen, the integrated MgO and TiO2 nanoparticles in the MgO@TiO2@g-C3N4 nanostructure have an average particle size of about 20-50 nm. The EDX spectrum (Figure 6(c)) displays typical characteristic peaks of Ti, Mg, N, O, and C, hence confirming the purity of the prepared composite. The inset table in Figure 6(c) indicates the corresponding chemical composition of the synthesized MgO@TiO2@g-C3N4. Furthermore, the elemental mapping of C, N, O, Mg, and Ti, as shown in Figures 6(e)–6(i), clearly evidenced the homogeneous distribution of the elements over the analyzed particles (Figure 6(d)).Figure 6
TEM images of MgO@TiO2@g-C3N4 nanopowder with (a, b) different magnification, (c) EDX for element composition, and (d–i) elemental mapping of C, N, O, Mg, and Ti, respectively.
(a)(b)(c)(d)(e)(f)(g)(h)(i)


## 3.1.3. BET Analysis

A good adsorbent, for instance MgO@TiO3@g-C3N4 nanostructure, must have a significant number of available adsorption sites. In other words, the material’s surface area, pore volume, and size should be enough. The nitrogen adsorption-desorption analysis (Figure 7 inset pore size) reveals that MgO@TiO2@g-C3N4 nanostructure is a mesoporous material with an IUPAC type IV adsorption isotherm. The isotherm is coupled with a hysteresis of type H1, implying a restricted distribution of uniform mesoporous and limited networking effects [32, 33]. The TEM image (Figure 7) confirms the presence of pores besides the appearance of large aggregates generated by the interconnection of MgO and TiO2 nanoparticles with g-C3N4 nanosheets. The surface area, total porous volume, and average pore size of MgO@TiO2@g-C3N4 nanostructure are 107 m2/g, 0.254 cc/g, and 1.57 nm, respectively. With a large surface area and porosity, the adsorbent is predicted to expose a vast number of adsorption sites, resulting in a high adsorption capacity.Figure 7
N2 adsorption/desorption isotherm (inset pore size) for MgO@TiO2@g-C3N4 nanopowder.


## 3.1.4. XPS Analysis

XPS analysis is used to determine the chemical composition and the valences of the elements on the nanoadsorbent surface. Figures8(a)–8(f) depict the XPS survey results for MgO@TiO2@g-C3N4, and Mg, O, N, Ti, and C elements are detected in the whole XPS scan spectrum. The XPS spectrum of Mg-2p (Figure 8(b)) displays a single peak at 51 eV characteristic of MgO compound [34]. The XPS spectrum of O-1s (Figure 8(c)) manifests a single peak at 532 eV associated to O-1s [35, 36]. For the XPS spectrum of N-1s (Figure 8(d)), two peaks are detected, i.e., for the amino groups (401 eV) and bridging N atoms (399 eV) [37]. Similarly, the XPS spectrum of Ti-2p (Figure 8(e)) displays two peaks at 459 and 464.3 eV, corresponding to the characteristic peaks of Ti-2p3/2 and Ti-2p1/2, respectively [38]. Further, three peaks located at 285, 287, and 289 eV are observed in the C-1s XPS spectrum (Figure 8(f)) and attributed to C=C, C–C, and O–C bands, respectively [39]. It is clearly observed from the above XPS analysis that only Mg, Ti, C, N, and O, elements are present, thereby no presence of any impurity within the as-fabricated MgO@TiO2@g-C3N4 composite.Figure 8
XPS spectra of (a) MgO@TiO2@g-C3N4 full survey and individual spectra of (b) Mg-2p, (c) O-s, (d) N-1s, (e) Ti-2p, and (f) C-1s.
(a)(b)(c)(d)(e)(f)


## 3.2. Adsorption Ability of MgO@TiO2@g-C3N4 Nanocomposite

### 3.2.1. Influence of Initial Cd (II) Concentration

The impact of Cd (II) concentration has been tested in the range (5–200 mg/L) under the optimized operational parameters, i.e., MgO@TiO3@g-C3N4 dose (10 mg), pH 7, a fixed volume of Cd (II) solution (25 mL), at room temperature, for a contact time of 24 hrs. The percentage and the optimum adsorption capacity of Cd (II) onto MgO@TiO3@g-C3N4 sorbent are shown in Figure 1. It can be observed that as the Cd (II) concentration increases, the adsorbed quantity increases gradually from 12.49 to 515.86 mg/g and the removal percentage remains very high within the range 99.94-98.75%. In this case, the fundamental driving force that increases the initial concentration of Cd (II) overcomes any barrier to Cd (II) migration from the solution. The resulting fractional adsorption becomes concentration-dependent.


### 3.2.2. Adsorption Isotherms Modeling

The adsorption isotherm of Cd (II) onto MgO@TiO2@g-C3N4 nanopowder has been examined by introducing a certain amount of the adsorbent (dose of 400 mg/L) to the Cd (II) solution (5 to 200 mg/L) for a duration of 1440 min at room temperature and pH 7 (see Figure 2). The equilibrium concentration of Cd (II) is found to be 0.033 mg/L compared to the initial concentration of 5 mg/L, resulting in a removal efficiency greater than 99.34%. This finding indicates that MgO@TiO2@g-C3N4 is an excellent adsorbent for Cd (II) removal.To determine the adsorption behavior of the adsorbent, the experimental data has been plotted using the Freundlich (Equation (5)) and Langmuir (Equation (6)) mathematical equations:
(5)qe=KFCe1/n,(6)qe=QmaxKLCe1+KLCe.The constantsK and n of the Freundlich model can be related to the strength of the adsorptive bond and the bond distribution, respectively [37]. In the Langmuir model (Eq. (3)), qmax is the solid-phase concentration corresponding to the complete monolayer coverage of adsorption sites [38]. KL is the constant related to the free energy of adsorption. The Langmuir isotherm is the quickest, easiest, and the most effective isotherm for describing the chemical adsorption process because it is based on the following assumptions: the adsorbent configuration is homogeneous; the adsorbed molecules or atoms are held in discrete and fixed locations; each site can compensate only one molecule or atom; the energy of adsorption is consistent across all sites; and no interaction between neighboring adsorbent surface and adsorbed molecules occurs. In comparison, the Freundlich model addresses heterogeneous systems and reversible adsorption, whereas the monolayer adsorption is not restricted. Figures 2(a) and 2(b) depict linear plots of experimental data generated using the Langmuir and Freundlich models. The Langmuir model fits the data better, with a regression coefficient R2=0.991, compared to the Freundlich model’s R2=0.963. According to the Langmuir model, the maximum adsorption capacity of the studied MgO@TiO2@g-C3N4 is found to be 529 mg/g. This relatively high capacity can be attributed to its enhanced surface area and porous nanostructure with relatively large pore size. This result elucidates that Cd (II) ions are adsorbed as a monolayer over MgO@TiO3@g-C3N4 surface through chemisorption being the dominant process.


### 3.2.3. Effect of Equilibrium Contact Duration on Cd (II) Removal

Figure3 illustrates the effect of contact time on Cd (II) removal. The adsorption of Cd (II) ions onto the sorbent MgO@TiO2@g-C3N4 nanostructure has been investigated over a stirring time in the range 5–1440 min at an initial Cd (II) concentration of 45 ppm. The removal of Cd (II) as a function of contact time approaches equilibrium in less than 16 min. Due to the numerous active sites on the sorbent MgO@TiO2@g-C3N4 surface, the amount of adsorbed Cd (II) reaches approximately 150 mg/g during this short time interval. During the equilibrium period, the concentration of the active site naturally reduces, and the removal rate becomes relatively constant to reach an equilibrium state. With a large surface area and porosity, the adsorbent is predicted to expose a large number of adsorption sites, resulting in a high adsorption capacity.


### 3.2.4. Adsorption Kinetic Investigations

Adsorption kinetic parameters are fundamental in predicting an adsorbent’s efficiency. The adsorption of Cd (II) ions onto MgO@TiO2@g-C3N4 nanostructures has been evaluated by employing pseudofirst-order (PFO), pseudosecond-order (PSO) kinetic, and intraparticle diffusion (IPD) models as displayed in Figures 4(a)–4(c). The corresponding nonlinear mathematical equations are as follows:
(7)qt=qe1−e−k1t,(8)qt=tk2qe2k2qet+1,(9)qt=kdift+C.Equation (7) describes the PFO model of Lagergren [39] with a rate constant k1 (1/min), and qe is the maximum amount of Cd (II) metal ions removed at equilibrium. For PSO law (Equation (8)), k2 stands for the PSO rate constant (g/mg.min) [40]. The initial rate of sorption, h0=k2.qe2, is assessed from PSO kinetic model equation. The half-sorption time, t1/2=1/k2.qe, is known as the time required for the adsorption to remove up to half as much Cd (II) metal ions as its equilibrium value.Concerning the IPD mechanism model (Equation (9)) [41], C represents the intercept that avails more details about the thickness of the edge layer. The IPD constant, kdif value (in mg/g.min1/2), for the used adsorbent, is estimated from the slope of the plot (Figure 4(c)). The validity of these models is examined based on the regression coefficient R2. The parameters computed from the fitting of the experimental data of Cd (II) adsorption by the kinetic models are given in Table 1. It can be noted that Cd (II) kinetic adsorption data are successfully fitted by the PSO model, since the value of R2=0.97, implying the occurrence of Cd (II) chemisorption onto MgO@TiO2@g-C3N4 nanostructures [40]. Further, the calculated value of qe=150.66 mg/g is identical to the measured value qe=150.31 mg/g. In the literature, the PSO model has recently been found to accurately predict the retention of metals on activated carbon [41].It is known that PSO kinetic model may be used to deduce all stages of the sorption process, including exterior film diffusion, sorption, and internal particle diffusion. Nonetheless, this model cannot account for the particular adsorption mechanism [42]. Therefore, the obtained data have been analyzed using the intraparticle diffusion kinetic model (Figure 4(c)). Numerous investigations demonstrated that the intraparticle diffusion graph may exhibit multilinearity, implying the possible occurrence of two or more steps during the adsorption process [43]. The findings indicate that most data points have two straight lines, and that the plots do not cross through the origin (plots not shown).Thekdif2 value for MgO@TiO2@g-C3N4 nanostructure is found to be lower than the kdif1 value, implying that intraparticle diffusion is the rate-limiting step for Cd (II) adsorption onto the hybrid composite. Also, it is important to mention that the slight deflection of the straight lines from the beginning implies that intraparticle diffusion may not be the only rate-limiting step in the adsorption of Cd (II) ions onto MgO@TiO2@g-C3N4 nanostructure; other possible surface adsorption processes also contribute to the overall sorption rate [44, 45].


### 3.2.5. Influence of pH on Cd (II) Removal

The pH value is known to be crucial in better understanding the rate of surface reactions between adsorbate (Cd (II)) and MgO@TiO2@g-C3N4 adsorbent. The effect of pH on Cd (II) adsorption ability has been investigated in the pH range 1.0-8.0, with the optimum adsorption capacity obtained at pH 5 as shown in Figure 9. Below pH 5, significant protonation occurs on the adsorbent surface, resulting in weak adsorbate-adsorbent interactions. Similar findings were reported in analogous studies [46, 47]. Metal ions such as Cd (II) have a pH dependence on their solubility. At lower pH, Cd (II) is extremely soluble as Cd (II) free ions and Cd (OH)+. Furthermore, at pH value greater than 7, Cd (II) ions will precipitate as metal hydroxide Cd (OH)2 [48]. It can be concluded that the quantitative removal capacity increases with the increment of the pH solution until the pH value is equal to 7.Figure 9
Effect of pH on Cd (II) removal by MgO@TiO2@g-C3N4 nanopowder.


### 3.2.6. Cd (II) Adsorption Mechanism onto MgO@TiO2@g-C3N4 Nanomaterial

The FTIR analysis help to clarify the adsorption mechanism of Cd (II) onto MgO@TiO2@g-C3N4 nanopowder [49]. The FTIR spectra of MgO@TiO2@g-C3N4 before and after Cd (II) ions adsorption, as shown in Figure 10(a), reveal considerable changes. The O–H and the terminal amino group stretching modes situated between 3000 and 3500 cm-1 are shifted to lower frequencies after adsorption. Similarly, the triazine ring mode peak at 891 cm-1 also shifts slightly to 883 cm-1. In addition, both Mg–O and Ti–O stretching vibration modes [50, 51] have mostly changed position indicating the interaction between Cd (II) and oxygen atoms of MgO@TiO2@g-C3N4 by π - π stacking bridging [52]. This result indicates that functional groups of MgO@TiO2@g-C3N4 (OH, CN, and N-H) and π delocalized electron systems of the triazine ring (C3N3) most probably are involved in the removal of Cd (II). Shen et al. reported that the triazine ring serves as a Lewis base and contributes to the Cd (II) elimination process as confirmed by FTIR and XPS analyses [53]. Based on the above FTIR observations of, a plausible sorption mechanism of Cd (II) onto MgO@TiO2@g-C3N4 is proposed as shown in Figure 10(b).Figure 10
FTIR spectra of (a) heteronanostructure MgO@TiO2@g-C3N4 before and after Cd adsorption; (b) proposed sorption mechanism.
(a)(b)


### 3.2.7. Recovery and Reusability of MgO@TiO2@g-C3N4 Nanomaterial

The strategies employed to remove harmful pollutants such as heavy metal ions from diverse effluents are considered on various measures such as efficiency, suitability, stability, cost, and their reusability in large-scale industrial applications. The recycling of nanoadsorbents has become a cost-effective process. Accordingly, the regeneration of the MgO@TiO2@g-C3N4 nanostructure from the reaction mixture has been investigated by adopting the same approach used by Khezami et al. [13]. In such a process, succeeding the removal process, the nanostructural material was separated from the mixture by filtration, washed with deionized water, oven-dried, and then soaked with a sodium hydroxide solution. The alkaline desorbing agent, NaOH, is proved effective in desorbing Cd (II) ions from the surface of the adsorbent. The adsorption-desorption procedure has been replicated for four consecutive cycles to test the reusability of the MgO@TiO2@g-C3N4 nanostructure for the removal of Cd (II) metal ions. The below expression then estimates the metal recovery:
(10)Desorptionefficiency%=AmountofCdIIionsdesorbedAmountofCdIIionsadsorbed×100%.As shown in Figure11, MgO@TiO2@g-C3N4 nanostructure visibly exhibits a higher constancy. It is also important to note that the synthesized heteronanostructured by a simple ultrasonication technique can be recovered and reused for four adsorption-desorption cycles.Figure 11
Desorption efficiency % of Cd (II) metal ions from MgO@TiO2@g-C3N4 nanostructure by NaOH.


## 3.2.1. Influence of Initial Cd (II) Concentration

The impact of Cd (II) concentration has been tested in the range (5–200 mg/L) under the optimized operational parameters, i.e., MgO@TiO3@g-C3N4 dose (10 mg), pH 7, a fixed volume of Cd (II) solution (25 mL), at room temperature, for a contact time of 24 hrs. The percentage and the optimum adsorption capacity of Cd (II) onto MgO@TiO3@g-C3N4 sorbent are shown in Figure 1. It can be observed that as the Cd (II) concentration increases, the adsorbed quantity increases gradually from 12.49 to 515.86 mg/g and the removal percentage remains very high within the range 99.94-98.75%. In this case, the fundamental driving force that increases the initial concentration of Cd (II) overcomes any barrier to Cd (II) migration from the solution. The resulting fractional adsorption becomes concentration-dependent.


## 3.2.2. Adsorption Isotherms Modeling

The adsorption isotherm of Cd (II) onto MgO@TiO2@g-C3N4 nanopowder has been examined by introducing a certain amount of the adsorbent (dose of 400 mg/L) to the Cd (II) solution (5 to 200 mg/L) for a duration of 1440 min at room temperature and pH 7 (see Figure 2). The equilibrium concentration of Cd (II) is found to be 0.033 mg/L compared to the initial concentration of 5 mg/L, resulting in a removal efficiency greater than 99.34%. This finding indicates that MgO@TiO2@g-C3N4 is an excellent adsorbent for Cd (II) removal.To determine the adsorption behavior of the adsorbent, the experimental data has been plotted using the Freundlich (Equation (5)) and Langmuir (Equation (6)) mathematical equations:
(5)qe=KFCe1/n,(6)qe=QmaxKLCe1+KLCe.The constantsK and n of the Freundlich model can be related to the strength of the adsorptive bond and the bond distribution, respectively [37]. In the Langmuir model (Eq. (3)), qmax is the solid-phase concentration corresponding to the complete monolayer coverage of adsorption sites [38]. KL is the constant related to the free energy of adsorption. The Langmuir isotherm is the quickest, easiest, and the most effective isotherm for describing the chemical adsorption process because it is based on the following assumptions: the adsorbent configuration is homogeneous; the adsorbed molecules or atoms are held in discrete and fixed locations; each site can compensate only one molecule or atom; the energy of adsorption is consistent across all sites; and no interaction between neighboring adsorbent surface and adsorbed molecules occurs. In comparison, the Freundlich model addresses heterogeneous systems and reversible adsorption, whereas the monolayer adsorption is not restricted. Figures 2(a) and 2(b) depict linear plots of experimental data generated using the Langmuir and Freundlich models. The Langmuir model fits the data better, with a regression coefficient R2=0.991, compared to the Freundlich model’s R2=0.963. According to the Langmuir model, the maximum adsorption capacity of the studied MgO@TiO2@g-C3N4 is found to be 529 mg/g. This relatively high capacity can be attributed to its enhanced surface area and porous nanostructure with relatively large pore size. This result elucidates that Cd (II) ions are adsorbed as a monolayer over MgO@TiO3@g-C3N4 surface through chemisorption being the dominant process.


## 3.2.3. Effect of Equilibrium Contact Duration on Cd (II) Removal

Figure3 illustrates the effect of contact time on Cd (II) removal. The adsorption of Cd (II) ions onto the sorbent MgO@TiO2@g-C3N4 nanostructure has been investigated over a stirring time in the range 5–1440 min at an initial Cd (II) concentration of 45 ppm. The removal of Cd (II) as a function of contact time approaches equilibrium in less than 16 min. Due to the numerous active sites on the sorbent MgO@TiO2@g-C3N4 surface, the amount of adsorbed Cd (II) reaches approximately 150 mg/g during this short time interval. During the equilibrium period, the concentration of the active site naturally reduces, and the removal rate becomes relatively constant to reach an equilibrium state. With a large surface area and porosity, the adsorbent is predicted to expose a large number of adsorption sites, resulting in a high adsorption capacity.


## 3.2.4. Adsorption Kinetic Investigations

Adsorption kinetic parameters are fundamental in predicting an adsorbent’s efficiency. The adsorption of Cd (II) ions onto MgO@TiO2@g-C3N4 nanostructures has been evaluated by employing pseudofirst-order (PFO), pseudosecond-order (PSO) kinetic, and intraparticle diffusion (IPD) models as displayed in Figures 4(a)–4(c). The corresponding nonlinear mathematical equations are as follows:
(7)qt=qe1−e−k1t,(8)qt=tk2qe2k2qet+1,(9)qt=kdift+C.Equation (7) describes the PFO model of Lagergren [39] with a rate constant k1 (1/min), and qe is the maximum amount of Cd (II) metal ions removed at equilibrium. For PSO law (Equation (8)), k2 stands for the PSO rate constant (g/mg.min) [40]. The initial rate of sorption, h0=k2.qe2, is assessed from PSO kinetic model equation. The half-sorption time, t1/2=1/k2.qe, is known as the time required for the adsorption to remove up to half as much Cd (II) metal ions as its equilibrium value.Concerning the IPD mechanism model (Equation (9)) [41], C represents the intercept that avails more details about the thickness of the edge layer. The IPD constant, kdif value (in mg/g.min1/2), for the used adsorbent, is estimated from the slope of the plot (Figure 4(c)). The validity of these models is examined based on the regression coefficient R2. The parameters computed from the fitting of the experimental data of Cd (II) adsorption by the kinetic models are given in Table 1. It can be noted that Cd (II) kinetic adsorption data are successfully fitted by the PSO model, since the value of R2=0.97, implying the occurrence of Cd (II) chemisorption onto MgO@TiO2@g-C3N4 nanostructures [40]. Further, the calculated value of qe=150.66 mg/g is identical to the measured value qe=150.31 mg/g. In the literature, the PSO model has recently been found to accurately predict the retention of metals on activated carbon [41].It is known that PSO kinetic model may be used to deduce all stages of the sorption process, including exterior film diffusion, sorption, and internal particle diffusion. Nonetheless, this model cannot account for the particular adsorption mechanism [42]. Therefore, the obtained data have been analyzed using the intraparticle diffusion kinetic model (Figure 4(c)). Numerous investigations demonstrated that the intraparticle diffusion graph may exhibit multilinearity, implying the possible occurrence of two or more steps during the adsorption process [43]. The findings indicate that most data points have two straight lines, and that the plots do not cross through the origin (plots not shown).Thekdif2 value for MgO@TiO2@g-C3N4 nanostructure is found to be lower than the kdif1 value, implying that intraparticle diffusion is the rate-limiting step for Cd (II) adsorption onto the hybrid composite. Also, it is important to mention that the slight deflection of the straight lines from the beginning implies that intraparticle diffusion may not be the only rate-limiting step in the adsorption of Cd (II) ions onto MgO@TiO2@g-C3N4 nanostructure; other possible surface adsorption processes also contribute to the overall sorption rate [44, 45].


## 3.2.5. Influence of pH on Cd (II) Removal

The pH value is known to be crucial in better understanding the rate of surface reactions between adsorbate (Cd (II)) and MgO@TiO2@g-C3N4 adsorbent. The effect of pH on Cd (II) adsorption ability has been investigated in the pH range 1.0-8.0, with the optimum adsorption capacity obtained at pH 5 as shown in Figure 9. Below pH 5, significant protonation occurs on the adsorbent surface, resulting in weak adsorbate-adsorbent interactions. Similar findings were reported in analogous studies [46, 47]. Metal ions such as Cd (II) have a pH dependence on their solubility. At lower pH, Cd (II) is extremely soluble as Cd (II) free ions and Cd (OH)+. Furthermore, at pH value greater than 7, Cd (II) ions will precipitate as metal hydroxide Cd (OH)2 [48]. It can be concluded that the quantitative removal capacity increases with the increment of the pH solution until the pH value is equal to 7.Figure 9
Effect of pH on Cd (II) removal by MgO@TiO2@g-C3N4 nanopowder.


## 3.2.6. Cd (II) Adsorption Mechanism onto MgO@TiO2@g-C3N4 Nanomaterial

The FTIR analysis help to clarify the adsorption mechanism of Cd (II) onto MgO@TiO2@g-C3N4 nanopowder [49]. The FTIR spectra of MgO@TiO2@g-C3N4 before and after Cd (II) ions adsorption, as shown in Figure 10(a), reveal considerable changes. The O–H and the terminal amino group stretching modes situated between 3000 and 3500 cm-1 are shifted to lower frequencies after adsorption. Similarly, the triazine ring mode peak at 891 cm-1 also shifts slightly to 883 cm-1. In addition, both Mg–O and Ti–O stretching vibration modes [50, 51] have mostly changed position indicating the interaction between Cd (II) and oxygen atoms of MgO@TiO2@g-C3N4 by π - π stacking bridging [52]. This result indicates that functional groups of MgO@TiO2@g-C3N4 (OH, CN, and N-H) and π delocalized electron systems of the triazine ring (C3N3) most probably are involved in the removal of Cd (II). Shen et al. reported that the triazine ring serves as a Lewis base and contributes to the Cd (II) elimination process as confirmed by FTIR and XPS analyses [53]. Based on the above FTIR observations of, a plausible sorption mechanism of Cd (II) onto MgO@TiO2@g-C3N4 is proposed as shown in Figure 10(b).Figure 10
FTIR spectra of (a) heteronanostructure MgO@TiO2@g-C3N4 before and after Cd adsorption; (b) proposed sorption mechanism.
(a)(b)


## 3.2.7. Recovery and Reusability of MgO@TiO2@g-C3N4 Nanomaterial

The strategies employed to remove harmful pollutants such as heavy metal ions from diverse effluents are considered on various measures such as efficiency, suitability, stability, cost, and their reusability in large-scale industrial applications. The recycling of nanoadsorbents has become a cost-effective process. Accordingly, the regeneration of the MgO@TiO2@g-C3N4 nanostructure from the reaction mixture has been investigated by adopting the same approach used by Khezami et al. [13]. In such a process, succeeding the removal process, the nanostructural material was separated from the mixture by filtration, washed with deionized water, oven-dried, and then soaked with a sodium hydroxide solution. The alkaline desorbing agent, NaOH, is proved effective in desorbing Cd (II) ions from the surface of the adsorbent. The adsorption-desorption procedure has been replicated for four consecutive cycles to test the reusability of the MgO@TiO2@g-C3N4 nanostructure for the removal of Cd (II) metal ions. The below expression then estimates the metal recovery:
(10)Desorptionefficiency%=AmountofCdIIionsdesorbedAmountofCdIIionsadsorbed×100%.As shown in Figure11, MgO@TiO2@g-C3N4 nanostructure visibly exhibits a higher constancy. It is also important to note that the synthesized heteronanostructured by a simple ultrasonication technique can be recovered and reused for four adsorption-desorption cycles.Figure 11
Desorption efficiency % of Cd (II) metal ions from MgO@TiO2@g-C3N4 nanostructure by NaOH.


## 3.3. Comparison of MgO@TiO2@g-C3N4 Nanosorbent with Other Adsorbents

According to the results given in Table3, the adsorption performance of MgO@TiO2@g-C3N4 nanostructure toward aqueous Cd (II) has been further assessed in comparison to other previously adsorbents reported in the literature. Primarily, the MgO@TiO2@g-C3N4 exhibits a shorter equilibrium duration, i.e., only 16 min, hence demonstrating rapid kinetics in the removal of Cd (II) from the aqueous solution. Further, the MgO@TiO2@g-C3N4 heteronanostructure has a larger adsorption capacity compared to other nanostructures, i.e., 516 mg/g compared to 26–342 mg/g. The high performance of MgO@TiO2@g-C3N4 nanosorbent for the removal of Cd (II) from aqueous solution is associated to its interesting nanostructure with a larger surface area and large pore size, making it a very promising and potentially an efficient adsorbent for the removal of other heavy metal ions.Table 3
Observation parameter adsorption of the MgO@TiO2@g-C3N4 compared with the various adsorbent nanomaterials.
AdsorbentsOptimal pHEquilibrium time (min)Removal (%)Qmax (mg/g)RefsDate Palm Biochar5.0-86.8043.58[45]Fe3O4@SiO2 nanoparticles6.06074.0026.10[54]Phosphorus-containing iron ore6.09099.80-[55]LDH@GO-SH5.060-102.77[56]Silicates on diatomite5.5120-29.00[57]Calcium carbonate/Nano-TiO25.08100103.6[58]Hydrated manganese dioxide3.06045.44342.3[59]MgO@TiO2@g-C3N4 heteronanostructure5.01699.94515.8This study


## 4. Conclusion

A new mesoporous MgO@TiO2@g-C3N4 nanosorbent fabricated by ultrasonication demonstrated a high adsorption capacity for the removal of Cd (II) from an aqueous solution. This was associated with its unique nanostructure composed of three main phases, high surface area, and large pore size. Furthermore, equilibrium and kinetic modelling established that the adsorption of Cd (II) ions occurred by chemisorption and is spontaneous. The FTIR analysis indicates that the functional groups of MgO@TiO2@g-C3N4 (OH, CN, and N-H) and πdelocalized electron systems of the triazine ring (C3N3) are involved in the removal of Cd (II). This research generated mesoporous MgO@TiO2@g-C3N4 heteronanostructure potentially for the removal of heavy metal ions from aqueous wastewater. The desorption studies with the concentrated NaOH method revealed that MgO@TiO2@g-C3N4 nanostructure could be recovered and reused effectively.



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