# Green Ca-Loaded MgO Nanoparticles as an Efficient Adsorbent for Organic Hazardous Dyes

Authors: Noha Al-Awaji; Moufida Boukriba; Abuzar Albadri; Mohamed Ali Ben Aissa; Mohamed Bououdina; Abueliz Modwi
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

Textile industry wastewater is a principal source of environmental contamination, posing serious health and environmental risks; consequently, appropriate treatments are necessary before ultimate discharge to ensure environmental and public health safety. In this study, 10% Ca-doped MgO (Ca0.10Mg0.90O) has been prepared via a green and ecologically benign approach using aqueous solution of gum arabic (GA) and then used to eliminate Congo red (CR) from aqueous solution. The XRD analysis reveals a size reduction of MgO nanoparticles from 11 to 6 nm after doping with a surface area reaching 50.1 m2·g-1 for the synthesized heterostructure. An excellent adsorption efficiency of 97% has been achieved under the following optimal operational conditions, i.e., adsorbent dosage of Ca0.10Mg0.90O (0.400 g/L), contact time of 120 min, and solution pH of 7.00. As indicated by pH, isotherm, and FTIR examinations, the CR sorption process is related to chemisorption and hydrogen bonding. The findings indicate that the synthesized nanomaterial is a suitable candidate for dyes containing effluent treatment due to its facile green synthetic approach using a natural product GA, low-cost, mesoporous nature, and high uptake capacity under neutral solution pH.


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## Body

## 1. Introduction

Water is regarded as a vital resource for the survival of life on earth. The textile industry has contaminated clean water supplies for decades [1]. The use of synthetic dyes throughout the industrial processes results in massive wastewater effluents discharged into the environment on a daily basis [2]. Synthetic organic dyes, such as Congo red (CR), are resistant to the biodegradation, and thus, they are hazardous to the environment and human health [3–6]. Congo red dye (C32H22N6Na2O6S2) is a class of dyes mostly utilized to color industrial items and hence discharged in significant amounts into water resources [7]. CR dyes cause serious environmental and health concerns; the most prevalent are cancer, gene mutation, and lung and kidney diseases [8].In this context, numerous viable solutions have been developed for treating CR-contaminated waters and minimizing their further discharge into water resources [9–11]. Diverse physical and chemical processes for the removal of organic dyes have been proposed, including Fenton oxidation [12], biological approaches [13], membrane separation [14], electrochemical oxidation [15], photocatalysis [16], and electrocoagulation. Nonetheless, adsorption methods have gained considerable attention, thanks to their simplicity, low cost, and effectiveness [6, 17].Nanostructured materials exhibit appreciably novel and improved chemical, biological, and physical properties [18], which can be tuned by chemical composition (doping, composites), particle’s shape and size, surface treatment, and functionalization. The observed outstanding and unique properties at the nanoscale are associated with quantum confinement, surface plasmonic resonance, and surface-to-volume ratio (surface area). Hence, nanoparticles have found widespread applications in various fields, including photocatalysis [19], wastewater treatment [20], and air purification [21].Magnesium oxide (MgO) with its rock-salt cubic structure (Fm-3 m space group no. 225) possesses interesting properties such as good thermal conductivity, transparency to infrared, chemical stability, and excellent corrosion resistance [22]. Besides, it offers broad range of usage in fuel cells, supercapacitors, paints, batteries, ceramics, catalysts, and electronics [23, 24]. MgO nanoparticles have also been tested as an effective adsorbent to remove heavy metals and organic dyes from contaminated waters [25, 26].MgO was synthesized using different methods, such as sonochemical route [27], chemical vapor deposition [28], precipitation [29], hydrothermal [30], microwave [31], combustion [32], and carbothermic reduction [33]. Sol-gel is a simple and common method already adopted to synthesize MgO nanostructures, since it manifests a high yield with minimal temperature requirement [34]. Furthermore, doping of metal oxide nanomaterials has been reported as an effective approach for the removal of dyes from contaminated waters by adsorption or photodegradation. Indeed, it was found that doping modifies and tunes markedly nanomaterial’s properties, notably the porosity and surface area, and consequently improves the sorption capacity and kinetics. Due to the existence of polysaccharides (galactosyl, arabinosyl, rhamnosyl, glucuronosyl, and 4-O-methyl-glucuronosyl) which are competent reducing agents, gum arabic (GA) was employed as a reducing dispersing agent to prepare nanostructures with enhanced properties to achieve high performance in diverse applications, specifically the elimination of hazardous organic and inorganic pollutants from contaminated wastewaters [35, 36].Nowadays, green technologies attract great interest for environmental protection. In this study, aqueous solution of gum arabic has been used as a natural stabilizing and capping agent for the green synthesis of Ca-loaded MgO (Ca0.10Mg0.90O) nanoparticles and then evaluated for the adsorption of Congo red dye. Microstructural, structural, and surface area and pore size and functional groups are checked by SEM, XRD, BET, and FTIR analyses. The influence of operational parameters, such as dye dose, contact time, and pH on sorption capability, has been examined. Besides, kinetic and adsorption isotherm experiments were conducted. The possible mechanism of CR dye removal by Ca-doped MgO nanoparticles was proposed.


## 2. Experimental Procedures

### 2.1. Chemicals

Magnesium carbonate dehydrate (≥99.0%), calcium carbonate (≥98.0%), sodium chloride (NaCl, ≥99%), sodium hydroxide (NaOH, ≥99%), hydrochloric acid (HCl, 37%), Congo red (CR, ≥ 97.0%), basic fuchsin (BF, ≥85%), malachite green (MG, ≥90%), crystal violet (CV, ≥90%), methyl orange (MO, ≥85%), and methylene blue (MB, ≥82%) purchased from Merck Company were used without further purification. Gum arabic (Acacia senegal) with the required concentrations (5 to 100 ppm) was obtained by diluting CR stock solution (200 ppm).


### 2.2. Material’s Preparation

The Ca-doped MgO 10% (Ca0.10Mg0.90O) nanoparticles were synthesized via the sol-gel method. First, 21 g of magnesium carbonate dehydrate (MgCO3) and 2.5 g of calcium carbonate (CaCO3) were dissolved in 500 mL of distilled water (DW). Then, 125 mL of NaOH ethanolic solution (0.02 M) was added dropwise while sonicating for 30 min. After that, 25 mL of gum arabic (Acacia senegal) solution (5 g in 100 mL of DW) was added and sonicated for 15 min. The mixture was left 24 h; subsequently, the as-formed white gel was washed thoroughly with DW and dried at 90°C for 5 h. The formed product was then calcined at 700°C for 120 min. The same procedure was repeated by taking similar quantities of the reagents without adding CaCO3 to synthesize MgO nanoparticles.


### 2.3. Characterizations

Scanning electron microscopy (SEM) was used to examine the morphology of the as-prepared powders using a JEOL JEM-6700F apparatus equipped with electron dispersive X-ray spectroscopy (EDS) to determine the elemental chemical composition. X-ray diffraction (XRD) was used to check the phase purity and the crystal structure using a Rigaku Mini Flex 600 (Tokyo, Japan) diffractometer equipped with a CuKα radiation source (λ=1.5418Å). A Micrometrics ASAP 2020 analyzer was used to determine the surface area and pore size of the fabricated nanopowders. The Brunauer-Emmett-Teller (BET) formula and Lippens and de Boer’s t-plot approach were used to determined nanopowders’ surface area and porosity. A JASCO FT-IR spectrometer was used to study the vibration modes of Ca0.10Mg0.90O nanoparticles before and after CR dye adsorption.


### 2.4. Adsorption and Kinetic Studies

The sorption process was utilized to estimate the CR dye removal rate by Ca0.10Mg0.90O nanopowders. For the kinetic and contact time experiments, 150 mL of CR dye solution with 150 ppm initial concentration was mixed with 60 mg of Ca0.10Mg0.90O2 nanopowders. A small suspension volume was removed at defined periods of time and then centrifuged to estimate the remaining CR dye quantity using a LABOMED UVS-2800 spectrophotometer. The residual CR concentration was obtained at a maximum wavelength of 497 nm, and the quantity of CR dye extracted per gram of sorbent Ca0.10Mg0.90O2 at any interval (min) is calculated using the following formula [37]:
(1)Qt=Ci−CtVw,where Qt (in mg·g−1) is the capacity of CR dye removed per mass of sorbent (w (g)) at each time t (min), Ci and Ct represent the concentrations of CR dye (in ppm) before and after adding CR ultimately at any time, and V is the CR dye volume (L).The equilibrium removal experiment was carried out in a 25 mL glass vial containing 10 mg of sorbent material, and 25 mL of CR dye with varying concentrations (25, 50, 100, 150, 250, and 300 ppm) was enrolled under a magnetic agitation for 1440 min. Following the removal equilibrium study, the mixture was then centrifuged and separated, whereby the remaining CR concentration was calculated as follows [38]:
(2)Qe=Ci−CeVw,where Qe (mg·g−1) is the capacity of CR dye eliminated through the mass of sorbent nanopowders and Ce is the CR dye concentration at equilibrium in mg·L−1. After the sorption experiment, the utilized Ca0.10Mg0.90O nanopowders have been recovered by filtration and subsequently calcined at 773 K for 2 h. After that, the collected Ca0.10Mg0.90O nanopowders have been utilized for additional adsorption tests.


## 2.1. Chemicals

Magnesium carbonate dehydrate (≥99.0%), calcium carbonate (≥98.0%), sodium chloride (NaCl, ≥99%), sodium hydroxide (NaOH, ≥99%), hydrochloric acid (HCl, 37%), Congo red (CR, ≥ 97.0%), basic fuchsin (BF, ≥85%), malachite green (MG, ≥90%), crystal violet (CV, ≥90%), methyl orange (MO, ≥85%), and methylene blue (MB, ≥82%) purchased from Merck Company were used without further purification. Gum arabic (Acacia senegal) with the required concentrations (5 to 100 ppm) was obtained by diluting CR stock solution (200 ppm).


## 2.2. Material’s Preparation

The Ca-doped MgO 10% (Ca0.10Mg0.90O) nanoparticles were synthesized via the sol-gel method. First, 21 g of magnesium carbonate dehydrate (MgCO3) and 2.5 g of calcium carbonate (CaCO3) were dissolved in 500 mL of distilled water (DW). Then, 125 mL of NaOH ethanolic solution (0.02 M) was added dropwise while sonicating for 30 min. After that, 25 mL of gum arabic (Acacia senegal) solution (5 g in 100 mL of DW) was added and sonicated for 15 min. The mixture was left 24 h; subsequently, the as-formed white gel was washed thoroughly with DW and dried at 90°C for 5 h. The formed product was then calcined at 700°C for 120 min. The same procedure was repeated by taking similar quantities of the reagents without adding CaCO3 to synthesize MgO nanoparticles.


## 2.3. Characterizations

Scanning electron microscopy (SEM) was used to examine the morphology of the as-prepared powders using a JEOL JEM-6700F apparatus equipped with electron dispersive X-ray spectroscopy (EDS) to determine the elemental chemical composition. X-ray diffraction (XRD) was used to check the phase purity and the crystal structure using a Rigaku Mini Flex 600 (Tokyo, Japan) diffractometer equipped with a CuKα radiation source (λ=1.5418Å). A Micrometrics ASAP 2020 analyzer was used to determine the surface area and pore size of the fabricated nanopowders. The Brunauer-Emmett-Teller (BET) formula and Lippens and de Boer’s t-plot approach were used to determined nanopowders’ surface area and porosity. A JASCO FT-IR spectrometer was used to study the vibration modes of Ca0.10Mg0.90O nanoparticles before and after CR dye adsorption.


## 2.4. Adsorption and Kinetic Studies

The sorption process was utilized to estimate the CR dye removal rate by Ca0.10Mg0.90O nanopowders. For the kinetic and contact time experiments, 150 mL of CR dye solution with 150 ppm initial concentration was mixed with 60 mg of Ca0.10Mg0.90O2 nanopowders. A small suspension volume was removed at defined periods of time and then centrifuged to estimate the remaining CR dye quantity using a LABOMED UVS-2800 spectrophotometer. The residual CR concentration was obtained at a maximum wavelength of 497 nm, and the quantity of CR dye extracted per gram of sorbent Ca0.10Mg0.90O2 at any interval (min) is calculated using the following formula [37]:
(1)Qt=Ci−CtVw,where Qt (in mg·g−1) is the capacity of CR dye removed per mass of sorbent (w (g)) at each time t (min), Ci and Ct represent the concentrations of CR dye (in ppm) before and after adding CR ultimately at any time, and V is the CR dye volume (L).The equilibrium removal experiment was carried out in a 25 mL glass vial containing 10 mg of sorbent material, and 25 mL of CR dye with varying concentrations (25, 50, 100, 150, 250, and 300 ppm) was enrolled under a magnetic agitation for 1440 min. Following the removal equilibrium study, the mixture was then centrifuged and separated, whereby the remaining CR concentration was calculated as follows [38]:
(2)Qe=Ci−CeVw,where Qe (mg·g−1) is the capacity of CR dye eliminated through the mass of sorbent nanopowders and Ce is the CR dye concentration at equilibrium in mg·L−1. After the sorption experiment, the utilized Ca0.10Mg0.90O nanopowders have been recovered by filtration and subsequently calcined at 773 K for 2 h. After that, the collected Ca0.10Mg0.90O nanopowders have been utilized for additional adsorption tests.


## 3. Results and Discussion

### 3.1. Ca0.10Mg0.90O Nanoparticle Description

MgO and Ca0.10Mg0.90O SEM images are presented in Figure 1. SEM images of MgO (Figure 1(a)) and Ca-doped MgO (Figure 1(b)) show irregular spherical crystallized particles with a meandiameter≈100nm. EDX spectra display intense signals at 1.2 and 0.5 keV related to Mg and O for MgO (Figure 1(c)) and 3.7, 1.2, and 0.5 keV corresponding to Ca, Mg, and O for Ca0.10Mg0.90O (Figure 1(d)). This confirms the formation of Ca-doped MgO nanoparticles without the presence of any impurities. The weight proportions of the elements determined from the EDX analysis provided as an inset in Figures 1(c) and 1(d) are consistent with the starting chemical composition.Figure 1
SEM images and corresponding EDX spectrum of MgO (a–c) and Ca0.10Mg0.90O nanoparticles (b–d).
(a)(b)(c)(d)The phase purity of MgO and Ca0.10Mg0.90O nanopowders was determined by XRD analysis (see Figure 2). The principal peaks located at 2θ=36.9°, 42.9°, 62.2°, 74.6°, and 78.6° correspond, respectively, to (111), (200), (220), (311), and (222) reflections of MgO face-centered cubic-FCC phase (Fm3¯m, 225, a=4.213Å) with a rock salt-type structure [39], in agreement with JCPDS No. 87-0653. Two minor peaks located at 2θ=36.9° and 42.9° are associated with organic molecules of the gum arabic extract [40].Figure 2
XRD patterns of MgO and Ca0.10Mg0.90O nanopowders.For Ca0.10Mg0.90O, the XRD pattern manifests similar diffraction peaks with a slight shift towards higher 2θ angles, indicating a contraction of the unit cell of MgO crystal structure. Meanwhile, two additional peaks located at 2θ=32.3° and 53.9° are indexed as (110) and (433) reflections, respectively, belonging to the CaO phase with a cubic structure (Fm3¯m, 225, a=4.801Å) [41] .The calculated values of the crystallite size are 10.93 nm for pure MgO and then reduced by almost half with 10% Ca doping reaching 5.98 nm. The obtained smaller particle size can contribute to the increased total surface area per unit volume and enhance the diffusion of dye molecules from the bulk solution toward the solid phase. Because the active sites exposed to the surrounding are greatly increased with reduced particle size, besides the interior surface of particles may be available for adsorption in comparison to larger size particles [42]. However, both values of the microstrain are found negative, i.e., −2.4×10−3% and −5.6×10−3% for MgO and Ca0.10Mg0.90O, respectively. The as-obtained compressive microstrain (negative values) may be explained by the existence of oxygen and magnesium vacancies inside the crystal structure of MgO.The lattice parametera of the MgO phase has been calculated using the main diffraction peak (200) by using the following expression:
(3)a=λ2sinθh2+k2+l2,whered is the interplanar distance, θ is the diffraction angle, λ the wavelength of the X-ray source (1.5418 Å), and h,k,l are Miller indices of the diffraction peak. The calculated lattice constants for MgO and Ca0.10Mg0.90O are found to be 4.5337 and 4.5304 Å, respectively. It is noted that the calculated MgO lattice parameter is lower than the rock salt bulk phase value reported in the literature [43], which may be due to insufficient oxygen/magnesium interstitials, self-interstitials substituting oxygen with magnesium, and di-interstitials.Furthermore, it is important to mention that the incorporation of Ca ions into MgO host lattice by occupying Mg sites would be accompanied by an expansion of the unit cell, since Ca2+ possesses higher ionic radius (0.99 Å) compared to Mg2+ (0.65 Å). However, it is noticed that the value of the lattice parameter of Ca0.10Mg0.90O (4.5304 Å) is slightly lower than that of pure MgO (4.5337 Å). This discrepancy can be explained by lattice distortion upon partial substitution of much larger Ca ions replacing smaller Mg ions and the possibility of the formation of oxygen vacancies during the synthesis process.N2 adsorption-desorption curves have been recorded to examine the nature of the microstructure and determine the surface area and mean pore size of Ca0.10Mg0.90O (see Figure 3). The isotherm (Figure 3(a)) manifests type V, suggesting that the presence of mesopores in phase change like pore condensation could occur [44]. The Ca0.10Mg0.90O BET surface area is found to be around 50.1 m2·g-1 with a pore distribution and volume of 11.79 nm and 0.277 cm3 g-1, respectively (Figure 3(b)).Figure 3
(a) Nitrogen adsorption-desorption isotherm and (b) pore size distribution of Ca0.10Mg0.90O nanopowders.
(a)(b)FTIR analysis has been performed to determine the chemical bonds and composition (purity) of the prepared compounds (see Figure4). FTIR spectra display a broad band at 3465 cm-1 assigned to the stretching vibration of the O–H group [45], whereas the band at 1639 cm-1 is ascribed to the adsorbed water molecule’s -OH stretching mode. In addition, the three bands observed at 1524, 1406, and 1056 cm-1 are related to a unidentate carbonate adsorbed on the surface of nanoparticles [46]. The characteristic band located at 420 cm-1 is assigned to Mg–O vibration mode [47], hence confirming the formation of the MgO phase. Furthermore, the appearance of a band at 871 cm-1 is associated with CaO stretching [48], hence asserting the incorporation of Ca within the MgO host lattice, in accordance with XRD analysis.Figure 4
FTIR spectra of MgO and Ca0.10Mg0.90O nanopowders.


### 3.2. Adsorption Measurements

#### 3.2.1. Effect of pH, Initial CR Concentration, and Contact Time

The pH value plays a critical role in the adsorption mechanism because it influences the sorbent’s surface charge hence enabling the adsorption of the dye molecules. The adsorption of CR onto Ca0.10Mg0.90O has been evaluated at pH value varying in the range 3 to 11 (see Figure 5(a)). It is noted that the adsorption efficiency increases markedly when the pH value increases in the acidic medium range (3 to 7) and then decreases at basic pH values. The zero-point charge experiment has been conducted to better elucidate the adsorption mechanism in terms of surface charge effect. From Figure 5(b), the point zero charges (pHPZC) of Ca0.10Mg0.90O have been estimated; i.e., its value is found to be 10. Thus, its surface is positively charged at pH below 10 and negatively charged above pH 10. Hence, it can be deduced that when the pH is increased, the Ca0.10Mg0.90O surface becomes negatively charged, resulting in an electrostatic repulsion between the Ca0.10Mg0.90O surface and the anionic CR dye molecules, and consequently, a reduced adsorption rate is attained. The decrease in the adsorption capacity of CR in the alkaline pH might be attributed to the excess OH− and the anionic CR (−SO3−) competition [49] as well as the hydrolysis of MgO in basic medium.Figure 5
(a) Influence of pH on CR % removal for initial dye concentration of 100 ppm, (b) plot for the pHPZC for Ca0.10Mg0.90O, (c) impact of the initial CR concentration at pH=7, and (d) contact time on the sorption rate onto Ca0.10Mg0.90O nanoparticles for initial dye concentration of 150 ppm and pH=7.
(a)(b)(c)(d)In addition, the influence of CR dye concentration on the amount of dye adsorbed is examined. Figure5(c) indicates that when the concentration of CR dye is raised gradually from 25 up to 300 mg. L-1, the CR adsorption rate increases significantly and linearly from 63.2 up to 769.2 mg·g-1, respectively. Herein, it can be observed that increasing the initial concentration of CR generates an efficient force that overcomes any resistance to CR migration.


### 3.3. Contact Time and Kinetic Studies

The effect of equilibrium time for the CR adsorption onto Ca0.10Mg0.90O nanoparticles has been examined for a fixed CR initial concentration of 150 ppm over an agitation time in the range 5-1440 min (see Figure 5(d)). The elimination of the CR is enhanced with the contact time and reaches the equilibrium within 120 min. The adsorption rate is found to increase significantly during the first few min of the process (h0=61.6mg·g−1·min−1), due to the accessibility of many active sites available onto Ca0.10Mg0.90O nanoparticles’ surface. Eventually, the concentration of the active sites falls once approaching the equilibrium resulting in a lower sorption rate; hence, the CR molecules’ removal remains almost unchanged.A kinetic study for the CR elimination by Ca0.10Mg0.90O nanoparticles has been carried out for a sorbent dose of 500 mg/L and CR0=150ppm, while varying the contact time at 5, 10, 20, 40, 80, and 120 min. Four models, namely, pseudo-first-order (PFO), pseudo-second-order (PSO), intraparticle diffusion, and Elovich models, have been adopted to elucidate the adsorption kinetics; as shown in Figure 6, the relevant equations and the obtained results (the kinetic parameters k1 and k2 and the correlation coefficient R2) are given in Table 1. It can be noted that the R2 value obtained from the PSO plots (R2=0.9998) is greater than that obtained from the PFO model (R2=0.8615), manifesting that the kinetics and the sorption process fit extremely well with the PSO model. Moreover, the adsorption capacity assessed using the PSO curve (qm=370mg·g−1) is very close with the experimental value (qm=365mg·g−1).Figure 6
PFO (a), PSO (b), Elovich models (c) for CR sorption onto Ca0.10Mg0.90O nanoparticles, and (d) intraparticle diffusion plots for CR adsorption by Ca0.10Mg0.90O nanoparticles for initial dye concentration of 150 ppm and pH=7.
(a)(b)(c)(d)Table 1
Kinetic models for CR sorption onto Ca0.10Mg0.90O nanoparticles.
Kinetic modelKinetic equationParametersValuesPFO [53]lnqe−qt=lnqe−k1tqe (mg/g)166±1.32K1 (min-1)13.7×10−3±0.19R20.97PSO [53]tqt=1k2qe2+1qetqm (exp) (mg/g)130.7±1.19qm (cal) (mg/g)370K2 (g/mg·min)0.45×10−3±0.39h0 (mg·g-1·min-1)61.6t1/2 (min)0.166R20.99Elovich [54]qt=1βlnαβ+1βlntβ0.032±0.58α8192±0.62R20.92Intraparticle diffusion [54]qt=kdift1/2+CKdif1 (mg·min1/2/g)15.22C1184.85R20.98Kdif2 (mg·min1/2/g)0.58C2349.57R20.55Through the intraparticle transport/diffusion mechanism, the CR dye molecules may be transported from the solution to the surface of Ca0.10Mg0.90O nanoparticles [50] (see Figure 6(d)). It can be observed that the adsorption process occurs upon two distinct stages [51]. The first stage reflects the transport of CR molecules from the bulk solution through the boundary layer to the surface of Ca0.10Mg0.90O nanoparticles [52], whereas the second stage manifests the equilibrium state in which intraparticle diffusion starts to diminish due to low CR concentration [52].


### 3.4. Adsorption Equilibrium Study

One of the furthermost important characteristics to evaluate the adsorption ability of Ca0.10Mg0.90O nanoparticles is the CR maximum quantity (Qmax). The adsorption data has been fitted using different isotherms to estimate the uptake capacity of Ca0.10Mg0.90O nanoparticles. Dubinin-Radushkevich, Freundlich-Langmuir, and Temkin’s adsorption models are examined at equilibrium under isothermal conditions. The corresponding formulas of the used isotherm models and the computed parameters are given in Table 2.Table 2
Isotherm models for CR sorption onto Ca0.10Mg0.90O nanoparticles.
Equilibrium modelLinear formParametersValuesLangmuir [55]Ceqe=1qmKL+Ceqmqm (mg/g)526±4.94KL (mg/g)0.528±1.21×10−3RL (L/mg)0.006R20.98Freundlich [56]lnqe=lnKF+1nlnCen0.752±0.123KF (L/mg)35.95±2.86R20.96Dubinin-Radushkevich [57]qe=RTblnKT+RTblnCeqm (mg/g)457±17.61β×10−9 (mol/kJ)27.84±1.13×10−5E (KJ/mol)17.43±11.12R20.95Temkin [58]lnqe=lnqm−Kε2B (J/mol)6.418±5.21KT (L/mg)4.29±0.145R20.92The isotherm graphs for CR elimination by Ca0.10Mg0.90O nanoparticles are illustrated in Figure 7. It can be observed from the fitting curves and the data reported in Table 2 that the Langmuir isotherm model has the highest R2 (0.98). The equilibrium value of RL is approximately 0.006, indicating a favorable equilibrium adsorption. These results demonstrate that the Langmuir isotherm model provides the most accurate match to the experimental data. According to the Langmuir isotherm model, the maximum sorption capacity of Ca0.10Mg0.90O nanoparticles for CR dye is 526 mg×g-1 (Table 2).Figure 7
Adsorption of CR equilibrium data fitted using (a) nonlinear models and (b) Langmuir, (c) Freundlich, (d) Temkin, and (e) Dubinin-Radushkevich models atpH=7.
(a)(b)(c)(d)(e)


### 3.5. Comparison Study

The ability of Ca0.10Mg0.90O nanoparticles to adsorb CR dye molecules has been compared to that of other sorbents previously reported in the literature. As seen in Table 3, the produced Ca0.10Mg0.90O nanoparticles exhibit a greater capacity for CR adsorption than the previously reported sorbents, indicating its potential as a promising nanosorbent for the elimination of hazardous organic dyes from wastewaters. Indeed, Sarojini et al. successfully obtained the polypyrrole-iron oxide for CR dye adsorption from aqueous solutions [59]. The achieved capacity of 500 mg/g is slightly lower compared to the value obtained in this study. Lei et al. investigated the elimination of CR from an aqueous solution by adsorption onto porous ZnO microspheres [60]. The maximum adsorption capacity was found to be 334 mg/g, which is very lower than that of Ca0.10Mg0.90O nanopowders.Table 3
Sorption capacity of various sorbents for the CR removal as reported in the literature.
AdsorbentsAdsorption capacity (mg/g)Time (min)ReferenceMgO nanoplates13130[61]Hierarchical hollow Fe2O316025[62]Co3O4@Fe3O4 hollow spheres12520[63]Jujuba seeds56100[64]Porous alumina37030[65]Zinc curcumin oxide9470[66]Porous ZnO microspheres334180[60]Fe3O4@bacteria32030[67]Polypyrrole-iron oxide50040[59]Ca0.10Mg0.90O nanoparticles526110Present study


### 3.6. Adsorption Mechanism

To elucidate the adsorption mechanism, FTIR measurements of Ca0.10Mg0.90O nanopowders have been recorded before and after CR adsorption in addition to the CR dye (see Figure 8(a)). The corresponding bands for Ca0.10Mg0.90O, Ca0.10Mg0.90O@CR, and CR are given in Table 4.Figure 8
(a) FTIR spectra of Ca0.10Mg0.90O, CR, and Ca0.10Mg0.90O@CR and (b) the proposed sorption mechanism of CR onto Ca0.10Mg0.90O.
(a)(b)Table 4
FTIR characteristic bands for Ca0.10Mg0.90O, Ca0.10Mg0.90O@CR, and CR.
Vibration modesCR (cm–1)Ca0.10Mg0.90O (cm–1)Ca0.10Mg0.90O@CR (cm–1)OH stretching—34603452N–H stretching3475—3454N=N stretching1594—1581C=C stretching1445—1420C−N bonding1363—1359SO3- stretching1220, 1178, and 1119—1212, 1136, and 1061Ring vibrations833—831As seen in Figure8(a) and Table 4, the characteristic bands of CR dye are also present in the spectrum of Ca0.10Mg0.90O after adsorption alongside some changes in the position and the relative intensity compared to the free CR bands. After CR adsorption, the stretching O–H band shifts slightly to 3452 cm-1 due to the hydrogen bond formed between the amine groups of CR and the OH groups of moisture present at Ca0.10Mg0.90O nanoparticles’ surface [68]. Additionally, the SO3-stretching bands shift to lower wavenumbers, indicating the occurrence of a bonding between Ca0.10Mg0.90O particles’ surface functional groups and CR dye molecules. The adsorption mechanism can be predicted by the energy value (E) in the D-R isothermal model. If the value of E>8kJ·mol−1, the adsorption mechanism is principally chemical. If the value of E<8kJ·mol−1, it indicates that the physical forces greatly affect the adsorption process [69]. The calculated energy value is found to be 17.43 kJ/mol, hence suggesting that CR molecules are chemisorbed-adsorbed onto the Ca0.10Mg0.90O surface. Rahali et al. established theoretically and experimentally that CR is chemosorbed onto Ba-doped ZnO [7]. The possible sorption mechanism of CR onto Ca0.10Mg0.90O nanoparticles’ surface is illustrated in Figure 8(b).


### 3.7. Reusability Study of the Sorbent Ca0.10Mg0.90O

The regeneration and the reusability of an adsorbent are key features when considering scale-up and industrial applications. In this regard, it is necessary to test the cyclic viability of Ca0.10Mg0.90O nanoparticles. The reusability performance results, as shown in Figure 9, indicate that Ca0.10Mg0.90O nanoparticles demonstrate effective four adsorption/desorption cycles for CR removal with a mean value of 89%.Figure 9
Reusability study of Ca0.10Mg0.90O nanopowders.


### 3.8. Adsorption Ability of Ca0.10Mg0.90O for Other Organic Contaminants

The high adsorption capacity of Ca0.10Mg0.90O for CR dye has been also examined and compared to other organic dyes (BF, CV, MG, MO, and MB), as illustrated in Figure 10 under the previous optimal operating conditions and using 50 ppm dye solution. It is readily apparent that Ca0.10Mg0.90O nanoparticles demonstrate a high selectivity for BF, CV MG, and CR and low removal rate for both MB and MO dyes, i.e., 99.9, 99.8, 99.6, 99.4, 58.6, and 37.3% toward CR, MG, BF, CV, MO, and MB, respectively. It can be concluded that the as-prepared Ca0.10Mg0.90O is an efficient sorbent for the removal of BF, MG, CV, and CR dyes from wastewater.Figure 10
Adsorption ability of Ca0.10Mg0.90O nanopowders.


## 3.1. Ca0.10Mg0.90O Nanoparticle Description

MgO and Ca0.10Mg0.90O SEM images are presented in Figure 1. SEM images of MgO (Figure 1(a)) and Ca-doped MgO (Figure 1(b)) show irregular spherical crystallized particles with a meandiameter≈100nm. EDX spectra display intense signals at 1.2 and 0.5 keV related to Mg and O for MgO (Figure 1(c)) and 3.7, 1.2, and 0.5 keV corresponding to Ca, Mg, and O for Ca0.10Mg0.90O (Figure 1(d)). This confirms the formation of Ca-doped MgO nanoparticles without the presence of any impurities. The weight proportions of the elements determined from the EDX analysis provided as an inset in Figures 1(c) and 1(d) are consistent with the starting chemical composition.Figure 1
SEM images and corresponding EDX spectrum of MgO (a–c) and Ca0.10Mg0.90O nanoparticles (b–d).
(a)(b)(c)(d)The phase purity of MgO and Ca0.10Mg0.90O nanopowders was determined by XRD analysis (see Figure 2). The principal peaks located at 2θ=36.9°, 42.9°, 62.2°, 74.6°, and 78.6° correspond, respectively, to (111), (200), (220), (311), and (222) reflections of MgO face-centered cubic-FCC phase (Fm3¯m, 225, a=4.213Å) with a rock salt-type structure [39], in agreement with JCPDS No. 87-0653. Two minor peaks located at 2θ=36.9° and 42.9° are associated with organic molecules of the gum arabic extract [40].Figure 2
XRD patterns of MgO and Ca0.10Mg0.90O nanopowders.For Ca0.10Mg0.90O, the XRD pattern manifests similar diffraction peaks with a slight shift towards higher 2θ angles, indicating a contraction of the unit cell of MgO crystal structure. Meanwhile, two additional peaks located at 2θ=32.3° and 53.9° are indexed as (110) and (433) reflections, respectively, belonging to the CaO phase with a cubic structure (Fm3¯m, 225, a=4.801Å) [41] .The calculated values of the crystallite size are 10.93 nm for pure MgO and then reduced by almost half with 10% Ca doping reaching 5.98 nm. The obtained smaller particle size can contribute to the increased total surface area per unit volume and enhance the diffusion of dye molecules from the bulk solution toward the solid phase. Because the active sites exposed to the surrounding are greatly increased with reduced particle size, besides the interior surface of particles may be available for adsorption in comparison to larger size particles [42]. However, both values of the microstrain are found negative, i.e., −2.4×10−3% and −5.6×10−3% for MgO and Ca0.10Mg0.90O, respectively. The as-obtained compressive microstrain (negative values) may be explained by the existence of oxygen and magnesium vacancies inside the crystal structure of MgO.The lattice parametera of the MgO phase has been calculated using the main diffraction peak (200) by using the following expression:
(3)a=λ2sinθh2+k2+l2,whered is the interplanar distance, θ is the diffraction angle, λ the wavelength of the X-ray source (1.5418 Å), and h,k,l are Miller indices of the diffraction peak. The calculated lattice constants for MgO and Ca0.10Mg0.90O are found to be 4.5337 and 4.5304 Å, respectively. It is noted that the calculated MgO lattice parameter is lower than the rock salt bulk phase value reported in the literature [43], which may be due to insufficient oxygen/magnesium interstitials, self-interstitials substituting oxygen with magnesium, and di-interstitials.Furthermore, it is important to mention that the incorporation of Ca ions into MgO host lattice by occupying Mg sites would be accompanied by an expansion of the unit cell, since Ca2+ possesses higher ionic radius (0.99 Å) compared to Mg2+ (0.65 Å). However, it is noticed that the value of the lattice parameter of Ca0.10Mg0.90O (4.5304 Å) is slightly lower than that of pure MgO (4.5337 Å). This discrepancy can be explained by lattice distortion upon partial substitution of much larger Ca ions replacing smaller Mg ions and the possibility of the formation of oxygen vacancies during the synthesis process.N2 adsorption-desorption curves have been recorded to examine the nature of the microstructure and determine the surface area and mean pore size of Ca0.10Mg0.90O (see Figure 3). The isotherm (Figure 3(a)) manifests type V, suggesting that the presence of mesopores in phase change like pore condensation could occur [44]. The Ca0.10Mg0.90O BET surface area is found to be around 50.1 m2·g-1 with a pore distribution and volume of 11.79 nm and 0.277 cm3 g-1, respectively (Figure 3(b)).Figure 3
(a) Nitrogen adsorption-desorption isotherm and (b) pore size distribution of Ca0.10Mg0.90O nanopowders.
(a)(b)FTIR analysis has been performed to determine the chemical bonds and composition (purity) of the prepared compounds (see Figure4). FTIR spectra display a broad band at 3465 cm-1 assigned to the stretching vibration of the O–H group [45], whereas the band at 1639 cm-1 is ascribed to the adsorbed water molecule’s -OH stretching mode. In addition, the three bands observed at 1524, 1406, and 1056 cm-1 are related to a unidentate carbonate adsorbed on the surface of nanoparticles [46]. The characteristic band located at 420 cm-1 is assigned to Mg–O vibration mode [47], hence confirming the formation of the MgO phase. Furthermore, the appearance of a band at 871 cm-1 is associated with CaO stretching [48], hence asserting the incorporation of Ca within the MgO host lattice, in accordance with XRD analysis.Figure 4
FTIR spectra of MgO and Ca0.10Mg0.90O nanopowders.


## 3.2. Adsorption Measurements

### 3.2.1. Effect of pH, Initial CR Concentration, and Contact Time

The pH value plays a critical role in the adsorption mechanism because it influences the sorbent’s surface charge hence enabling the adsorption of the dye molecules. The adsorption of CR onto Ca0.10Mg0.90O has been evaluated at pH value varying in the range 3 to 11 (see Figure 5(a)). It is noted that the adsorption efficiency increases markedly when the pH value increases in the acidic medium range (3 to 7) and then decreases at basic pH values. The zero-point charge experiment has been conducted to better elucidate the adsorption mechanism in terms of surface charge effect. From Figure 5(b), the point zero charges (pHPZC) of Ca0.10Mg0.90O have been estimated; i.e., its value is found to be 10. Thus, its surface is positively charged at pH below 10 and negatively charged above pH 10. Hence, it can be deduced that when the pH is increased, the Ca0.10Mg0.90O surface becomes negatively charged, resulting in an electrostatic repulsion between the Ca0.10Mg0.90O surface and the anionic CR dye molecules, and consequently, a reduced adsorption rate is attained. The decrease in the adsorption capacity of CR in the alkaline pH might be attributed to the excess OH− and the anionic CR (−SO3−) competition [49] as well as the hydrolysis of MgO in basic medium.Figure 5
(a) Influence of pH on CR % removal for initial dye concentration of 100 ppm, (b) plot for the pHPZC for Ca0.10Mg0.90O, (c) impact of the initial CR concentration at pH=7, and (d) contact time on the sorption rate onto Ca0.10Mg0.90O nanoparticles for initial dye concentration of 150 ppm and pH=7.
(a)(b)(c)(d)In addition, the influence of CR dye concentration on the amount of dye adsorbed is examined. Figure5(c) indicates that when the concentration of CR dye is raised gradually from 25 up to 300 mg. L-1, the CR adsorption rate increases significantly and linearly from 63.2 up to 769.2 mg·g-1, respectively. Herein, it can be observed that increasing the initial concentration of CR generates an efficient force that overcomes any resistance to CR migration.


## 3.2.1. Effect of pH, Initial CR Concentration, and Contact Time

The pH value plays a critical role in the adsorption mechanism because it influences the sorbent’s surface charge hence enabling the adsorption of the dye molecules. The adsorption of CR onto Ca0.10Mg0.90O has been evaluated at pH value varying in the range 3 to 11 (see Figure 5(a)). It is noted that the adsorption efficiency increases markedly when the pH value increases in the acidic medium range (3 to 7) and then decreases at basic pH values. The zero-point charge experiment has been conducted to better elucidate the adsorption mechanism in terms of surface charge effect. From Figure 5(b), the point zero charges (pHPZC) of Ca0.10Mg0.90O have been estimated; i.e., its value is found to be 10. Thus, its surface is positively charged at pH below 10 and negatively charged above pH 10. Hence, it can be deduced that when the pH is increased, the Ca0.10Mg0.90O surface becomes negatively charged, resulting in an electrostatic repulsion between the Ca0.10Mg0.90O surface and the anionic CR dye molecules, and consequently, a reduced adsorption rate is attained. The decrease in the adsorption capacity of CR in the alkaline pH might be attributed to the excess OH− and the anionic CR (−SO3−) competition [49] as well as the hydrolysis of MgO in basic medium.Figure 5
(a) Influence of pH on CR % removal for initial dye concentration of 100 ppm, (b) plot for the pHPZC for Ca0.10Mg0.90O, (c) impact of the initial CR concentration at pH=7, and (d) contact time on the sorption rate onto Ca0.10Mg0.90O nanoparticles for initial dye concentration of 150 ppm and pH=7.
(a)(b)(c)(d)In addition, the influence of CR dye concentration on the amount of dye adsorbed is examined. Figure5(c) indicates that when the concentration of CR dye is raised gradually from 25 up to 300 mg. L-1, the CR adsorption rate increases significantly and linearly from 63.2 up to 769.2 mg·g-1, respectively. Herein, it can be observed that increasing the initial concentration of CR generates an efficient force that overcomes any resistance to CR migration.


## 3.3. Contact Time and Kinetic Studies

The effect of equilibrium time for the CR adsorption onto Ca0.10Mg0.90O nanoparticles has been examined for a fixed CR initial concentration of 150 ppm over an agitation time in the range 5-1440 min (see Figure 5(d)). The elimination of the CR is enhanced with the contact time and reaches the equilibrium within 120 min. The adsorption rate is found to increase significantly during the first few min of the process (h0=61.6mg·g−1·min−1), due to the accessibility of many active sites available onto Ca0.10Mg0.90O nanoparticles’ surface. Eventually, the concentration of the active sites falls once approaching the equilibrium resulting in a lower sorption rate; hence, the CR molecules’ removal remains almost unchanged.A kinetic study for the CR elimination by Ca0.10Mg0.90O nanoparticles has been carried out for a sorbent dose of 500 mg/L and CR0=150ppm, while varying the contact time at 5, 10, 20, 40, 80, and 120 min. Four models, namely, pseudo-first-order (PFO), pseudo-second-order (PSO), intraparticle diffusion, and Elovich models, have been adopted to elucidate the adsorption kinetics; as shown in Figure 6, the relevant equations and the obtained results (the kinetic parameters k1 and k2 and the correlation coefficient R2) are given in Table 1. It can be noted that the R2 value obtained from the PSO plots (R2=0.9998) is greater than that obtained from the PFO model (R2=0.8615), manifesting that the kinetics and the sorption process fit extremely well with the PSO model. Moreover, the adsorption capacity assessed using the PSO curve (qm=370mg·g−1) is very close with the experimental value (qm=365mg·g−1).Figure 6
PFO (a), PSO (b), Elovich models (c) for CR sorption onto Ca0.10Mg0.90O nanoparticles, and (d) intraparticle diffusion plots for CR adsorption by Ca0.10Mg0.90O nanoparticles for initial dye concentration of 150 ppm and pH=7.
(a)(b)(c)(d)Table 1
Kinetic models for CR sorption onto Ca0.10Mg0.90O nanoparticles.
Kinetic modelKinetic equationParametersValuesPFO [53]lnqe−qt=lnqe−k1tqe (mg/g)166±1.32K1 (min-1)13.7×10−3±0.19R20.97PSO [53]tqt=1k2qe2+1qetqm (exp) (mg/g)130.7±1.19qm (cal) (mg/g)370K2 (g/mg·min)0.45×10−3±0.39h0 (mg·g-1·min-1)61.6t1/2 (min)0.166R20.99Elovich [54]qt=1βlnαβ+1βlntβ0.032±0.58α8192±0.62R20.92Intraparticle diffusion [54]qt=kdift1/2+CKdif1 (mg·min1/2/g)15.22C1184.85R20.98Kdif2 (mg·min1/2/g)0.58C2349.57R20.55Through the intraparticle transport/diffusion mechanism, the CR dye molecules may be transported from the solution to the surface of Ca0.10Mg0.90O nanoparticles [50] (see Figure 6(d)). It can be observed that the adsorption process occurs upon two distinct stages [51]. The first stage reflects the transport of CR molecules from the bulk solution through the boundary layer to the surface of Ca0.10Mg0.90O nanoparticles [52], whereas the second stage manifests the equilibrium state in which intraparticle diffusion starts to diminish due to low CR concentration [52].


## 3.4. Adsorption Equilibrium Study

One of the furthermost important characteristics to evaluate the adsorption ability of Ca0.10Mg0.90O nanoparticles is the CR maximum quantity (Qmax). The adsorption data has been fitted using different isotherms to estimate the uptake capacity of Ca0.10Mg0.90O nanoparticles. Dubinin-Radushkevich, Freundlich-Langmuir, and Temkin’s adsorption models are examined at equilibrium under isothermal conditions. The corresponding formulas of the used isotherm models and the computed parameters are given in Table 2.Table 2
Isotherm models for CR sorption onto Ca0.10Mg0.90O nanoparticles.
Equilibrium modelLinear formParametersValuesLangmuir [55]Ceqe=1qmKL+Ceqmqm (mg/g)526±4.94KL (mg/g)0.528±1.21×10−3RL (L/mg)0.006R20.98Freundlich [56]lnqe=lnKF+1nlnCen0.752±0.123KF (L/mg)35.95±2.86R20.96Dubinin-Radushkevich [57]qe=RTblnKT+RTblnCeqm (mg/g)457±17.61β×10−9 (mol/kJ)27.84±1.13×10−5E (KJ/mol)17.43±11.12R20.95Temkin [58]lnqe=lnqm−Kε2B (J/mol)6.418±5.21KT (L/mg)4.29±0.145R20.92The isotherm graphs for CR elimination by Ca0.10Mg0.90O nanoparticles are illustrated in Figure 7. It can be observed from the fitting curves and the data reported in Table 2 that the Langmuir isotherm model has the highest R2 (0.98). The equilibrium value of RL is approximately 0.006, indicating a favorable equilibrium adsorption. These results demonstrate that the Langmuir isotherm model provides the most accurate match to the experimental data. According to the Langmuir isotherm model, the maximum sorption capacity of Ca0.10Mg0.90O nanoparticles for CR dye is 526 mg×g-1 (Table 2).Figure 7
Adsorption of CR equilibrium data fitted using (a) nonlinear models and (b) Langmuir, (c) Freundlich, (d) Temkin, and (e) Dubinin-Radushkevich models atpH=7.
(a)(b)(c)(d)(e)


## 3.5. Comparison Study

The ability of Ca0.10Mg0.90O nanoparticles to adsorb CR dye molecules has been compared to that of other sorbents previously reported in the literature. As seen in Table 3, the produced Ca0.10Mg0.90O nanoparticles exhibit a greater capacity for CR adsorption than the previously reported sorbents, indicating its potential as a promising nanosorbent for the elimination of hazardous organic dyes from wastewaters. Indeed, Sarojini et al. successfully obtained the polypyrrole-iron oxide for CR dye adsorption from aqueous solutions [59]. The achieved capacity of 500 mg/g is slightly lower compared to the value obtained in this study. Lei et al. investigated the elimination of CR from an aqueous solution by adsorption onto porous ZnO microspheres [60]. The maximum adsorption capacity was found to be 334 mg/g, which is very lower than that of Ca0.10Mg0.90O nanopowders.Table 3
Sorption capacity of various sorbents for the CR removal as reported in the literature.
AdsorbentsAdsorption capacity (mg/g)Time (min)ReferenceMgO nanoplates13130[61]Hierarchical hollow Fe2O316025[62]Co3O4@Fe3O4 hollow spheres12520[63]Jujuba seeds56100[64]Porous alumina37030[65]Zinc curcumin oxide9470[66]Porous ZnO microspheres334180[60]Fe3O4@bacteria32030[67]Polypyrrole-iron oxide50040[59]Ca0.10Mg0.90O nanoparticles526110Present study


## 3.6. Adsorption Mechanism

To elucidate the adsorption mechanism, FTIR measurements of Ca0.10Mg0.90O nanopowders have been recorded before and after CR adsorption in addition to the CR dye (see Figure 8(a)). The corresponding bands for Ca0.10Mg0.90O, Ca0.10Mg0.90O@CR, and CR are given in Table 4.Figure 8
(a) FTIR spectra of Ca0.10Mg0.90O, CR, and Ca0.10Mg0.90O@CR and (b) the proposed sorption mechanism of CR onto Ca0.10Mg0.90O.
(a)(b)Table 4
FTIR characteristic bands for Ca0.10Mg0.90O, Ca0.10Mg0.90O@CR, and CR.
Vibration modesCR (cm–1)Ca0.10Mg0.90O (cm–1)Ca0.10Mg0.90O@CR (cm–1)OH stretching—34603452N–H stretching3475—3454N=N stretching1594—1581C=C stretching1445—1420C−N bonding1363—1359SO3- stretching1220, 1178, and 1119—1212, 1136, and 1061Ring vibrations833—831As seen in Figure8(a) and Table 4, the characteristic bands of CR dye are also present in the spectrum of Ca0.10Mg0.90O after adsorption alongside some changes in the position and the relative intensity compared to the free CR bands. After CR adsorption, the stretching O–H band shifts slightly to 3452 cm-1 due to the hydrogen bond formed between the amine groups of CR and the OH groups of moisture present at Ca0.10Mg0.90O nanoparticles’ surface [68]. Additionally, the SO3-stretching bands shift to lower wavenumbers, indicating the occurrence of a bonding between Ca0.10Mg0.90O particles’ surface functional groups and CR dye molecules. The adsorption mechanism can be predicted by the energy value (E) in the D-R isothermal model. If the value of E>8kJ·mol−1, the adsorption mechanism is principally chemical. If the value of E<8kJ·mol−1, it indicates that the physical forces greatly affect the adsorption process [69]. The calculated energy value is found to be 17.43 kJ/mol, hence suggesting that CR molecules are chemisorbed-adsorbed onto the Ca0.10Mg0.90O surface. Rahali et al. established theoretically and experimentally that CR is chemosorbed onto Ba-doped ZnO [7]. The possible sorption mechanism of CR onto Ca0.10Mg0.90O nanoparticles’ surface is illustrated in Figure 8(b).


## 3.7. Reusability Study of the Sorbent Ca0.10Mg0.90O

The regeneration and the reusability of an adsorbent are key features when considering scale-up and industrial applications. In this regard, it is necessary to test the cyclic viability of Ca0.10Mg0.90O nanoparticles. The reusability performance results, as shown in Figure 9, indicate that Ca0.10Mg0.90O nanoparticles demonstrate effective four adsorption/desorption cycles for CR removal with a mean value of 89%.Figure 9
Reusability study of Ca0.10Mg0.90O nanopowders.


## 3.8. Adsorption Ability of Ca0.10Mg0.90O for Other Organic Contaminants

The high adsorption capacity of Ca0.10Mg0.90O for CR dye has been also examined and compared to other organic dyes (BF, CV, MG, MO, and MB), as illustrated in Figure 10 under the previous optimal operating conditions and using 50 ppm dye solution. It is readily apparent that Ca0.10Mg0.90O nanoparticles demonstrate a high selectivity for BF, CV MG, and CR and low removal rate for both MB and MO dyes, i.e., 99.9, 99.8, 99.6, 99.4, 58.6, and 37.3% toward CR, MG, BF, CV, MO, and MB, respectively. It can be concluded that the as-prepared Ca0.10Mg0.90O is an efficient sorbent for the removal of BF, MG, CV, and CR dyes from wastewater.Figure 10
Adsorption ability of Ca0.10Mg0.90O nanopowders.


## 4. Conclusion

A Ca-doped MgO (Ca0.10Mg0.90O) nanosorbent was successfully prepared by a green and ecologically benign approach using aqueous solution of natural gum arabic (GA) and exhibited a high efficacy for the removal of CR dye. Batch experiments revealed that the removal of CR dye by Ca0.10Mg0.90O was pH-dependent, with the highest adsorption capacity of 526 mg/g achieved at pH=7. Equilibrium and kinetic modelling of the experimental data indicated that the sorption of CR by Ca0.10Mg0.90O followed the pseudo-first-order kinetics and Langmuir adsorption isotherm models. Based on pH (zero-point charge) and FTIR measurements, the CR adsorption mechanism occurred through the chemisorption and hydrogen bonding. The investigation of Ca0.10Mg0.90O nanopowder as adsorbents for different dyes (MB, MO, CV, BF, and MG) revealed the overall significant potential of the nanosorbent for hazardous dyes’ elimination from wastewaters.



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