Facile sol-gel synthesis of calcium oxide nanoparticles for effective removal of Ni(II) from aqueous solution

ABSTRACT In this study, nanoparticles of calcium oxide (CaO-NPs) were prepared via the sol-gel technique and introduced as a highly efficient adsorbent for removing Ni(II) from aqueous media. The prepared adsorbent was characterised via SEM, TEM, FTIR, XRD, and TGA. The pHzpc and surface area were also determined. XRD pattern exposed a polycrystalline nature of the adsorbent. SEM analysis showed that the adsorbent possesses enhanced surface porosity highlighting its potential capacity for adsorption. TEM analysis confirmed the crystallisation of adsorbent with a simple cubic structure. The FTIR spectrum displayed that the adsorbent surface comprises mostly (–OH and C=O) groups which are principally accountable for the adsorption of Ni(II). A series of batch adsorption experiments were done to study the effect of several parameters as pH, adsorbent dose, time of contact, initial concentration of adsorbate, temperature, ionic strength, and presence of interfering ions on the removal of Ni(II) ions from aqueous solutions using CaO-NPs. Maximum removal was obtained with 200 mg of adsorbent after 90 min contact time at pH 7 and 25°C as optimal conditions. The adsorption process was fitted well with Langmuir isotherm model and pseudo-second-order kinetic model, showing that the adsorption of Ni(II) ions on CaO-NPs was monolayer coverage with 166.67 mg/g capacity of adsorption. Thermodynamic studies revealed a spontaneous and endothermic adsorption process. The regeneration study using Na2EDTA indicated that the prepared adsorbent has a high renewal potential without notable loss in its performance. Moreover, the prepared adsorbent proved its good applicability and performance by successful application of the suggested procedure for the recovery of Ni(II) added to some real water samples. Accordingly, the obtained results showed that CaO-NPs can rapidly and efficiently remove Ni(II) from aqueous solutions. Also, CaO-NPs adsorbent is eco-friendly, easy to produce, cost-effective, non-toxic, and with good adsorption-desorption capacity. Therefore, this study will contribute to ecological cleaning and wastewater treatment.


Introduction
Recently, pollution of water considered to be one of the main ecological problems owing to the toxic and hazardous chemicals released from numerous activities [1][2][3].Heavy metals are utilised in several industrial sectors, when these metal ions are discharged in the industrial effluent, they have numerous ecological and harmful impacts on the health of human and plant life [4,5].Ni has numerous industrial uses such as electroplating, paint production, manufacturer of batteries, and enamelling of porcelain [6,7].According to the United States Environmental Protection Agency (EPA) and the World Health Organization (WHO), the maximum allowable concentration of Ni(II) in drinking water is 0.02 mg/L [8].In spite of the widespread use of Ni, it is considered a harmful and risky element that can result in problems to the lungs, kidneys, and stomach [9,10].So, the removal of nickel from aqueous media and industrial effluents is a very urgent issue.Numerous techniques such as ion exchange, flocculation, precipitation, electrochemical, filtration, and reverse osmosis have been suggested by researchers for nickel removal from aqueous media and effluents.Though, most of these techniques require high costs of operation and maintenance and generate toxic sludge [9,11].Also, owing to the high cost, these techniques are not suitable for industries with small scale especially in developing countries.Among all removal processes, adsorption is a promising and effective technique for heavy metal removal from aqueous solutions and industrial effluents [12].The adsorption process has several exceptional features as simplicity to carry out, cost-effectiveness, high efficiency, and flexibility [13,14].Several adsorbents have been suggested for the heavy metal removal from aqueous media, as oxide of metals [15,16], zeolite [17], carbon-based adsorbents [18,19], chitosan [20], functional polymers [21], active carbon, active alumina, and red mud [22].These adsorbents have several disadvantages as the low capacity of adsorption and the slow rate of adsorption therefore the need for suggesting more effective adsorbents is an urgent issue [23][24][25][26].Recently, nanoparticles are extensively utilised as good adsorbents for the removal of metallic pollutants, owing to their unique features as high capacity, simplicity of operation, and quickness of adsorption.So, nanoparticle materials are utilised as efficient adsorbents for heavy metal removal [27][28][29].Nanoparticles of metal oxide have been used as adsorbents for the removal of heavy metals in water and wastewater [30][31][32][33][34]. Calcium oxide is one of the most frequently used nano-sized metal oxides.Nanoparticles of calcium oxide have been manufactured via numerous approaches as ultrasonic-assisted [35], hydrogen plasma-metal reaction [36], biopolymer-assisted [37], microwave-assisted [38], calcination [39], coprecipitation [40], thermal decomposition [41], and green synthesis [42].Those techniques have several disadvantages, as the presence of additives, need high temperature, presence of pressure, time-consuming, not cheap, and complicated steps.The sol-gel technique gets rid of most of the disadvantages of the above-mentioned techniques.The sol-gel is simple, inexpensive, the required equipment is low-priced, proceed at a lower temperature with the absence of pressure.Consequently, it may be a promising technique to prepare nanoparticles of calcium oxide.Therefore, the purpose of this study was to evaluate the preparation, characterisation, adsorption, and desorption properties of calcium oxide nanoparticles (CaO-NPs) as an efficient material for the removal of toxic metals from aqueous solutions.The calcium oxide nanoparticles (CaO-NPs) were prepared by sol-gel method then characterised through Scanning Electron Microscopy (SEM), X-ray Powder Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Brunauer-Emmett-Teller (BET) surface area.The prepared adsorbent material was tested to remove Ni(II) ions from aqueous solution using adsorption technique.The effect of different factors as pH, initial concentration of adsorbate, adsorbent dose, contact time, presence of foreign ions and temperature on the adsorption process were studied.Isotherm, kinetic and thermodynamic studies have also been tested.Also, after adsorption process, the desorption of Ni(II) ions from the CaO-NPs was tested using Na 2 EDTA.Then, the reusability of CaO-NPs for Ni(II) adsorption was investigated.

Chemicals and reagents
Chemicals utilised in this work were of analytical grade, obtained from Sigma-Aldrich (Darmstadt, Germany), and were utilised without any more purification.

Instrumentation
The Ni(II) concentration was determined utilising (iCAP 7400 ICP-OES spectrometer).ICP-OES operational conditions are summarised in Table 1.The magnetic stirrer (VELP, SCIENTIFIC, Italy) was used for the stirring of solutions.GALLENKAMP (Flask Shaker, 5 speeds) was used for shaking.Thermolyne 1900 Hot Plate was used for heating.Analytical balance (Mettler AJ150) was used for weighing.The pH measurements were performed using the pH/mV metre (HI 8519, Hanna instrument, Italy) with a glass electrode.FTIR analysis was proceeding through (Nicolet i10 FTIR spectrometer) in the range of (4000-400 cm −1 ).TEM analysis was obtained using (JEOL 2010 Transmission Electron Microscope) with 200 kV as a voltage of acceleration, and 0.19 nm as a resolution point.SEM analysis was obtained using (JEOL JSM-840A Scanning Electron Microscope) under a high vacuum with 30 kV as a voltage of acceleration.XRD analysis was recorded using (Philips PW 105 diffractometer).The surface area of the prepared adsorbent was carried out by measuring nitrogen adsorption isotherms at −196°C using the high vacuum classical volumetric glass system.Before adsorption measurements, the adsorbent was degassed at 220°C for 90 min, and the experiments were carried out in triplicates and the conventional Brunauer-Emmett-Teller (BET) equation was used to calculate the surface area.Thermal gravimetric analysis (TGA) was achieved by (Thermo-analyser Shimadzu DTG-50, Japan), with a heating temperature range of (20-800°C), a flow rate of 15 mL/min of N 2 atmosphere, and rate of heating 10°C/min.Hydrochloric acid and sodium hydroxide were utilised for the pH adjustment.

Preparation of adsorbent (CaO-NPs)
Calcium oxide nanoparticles (CaO-NPs) were prepared from CaCl 2 by the sol-gel method at room temperature with the absence of pressure which contributing to less energy consumption.The sol-gel preparation method is characterised by costless, absence of additives, short time of preparation, cheap, green, and sustainability [43].Preparation of calcium oxide nanoparticles via the sol-gel technique takes 3 successive steps: (1) hydrolysis to form 'sol', (2) condensation to form 'gel', and (3) drying the resulted gel [44].Firstly, the CaCl 2 homogenous solution was prepared by dissolving a suitable quantity of CaCl 2 in double-distilled water and heated to 40°C.After that, while the CaCl 2 solution was being stirred rapidly, 200 mL of NaOH (0.1 M) was added slowly to convert the solution of CaCl 2 into 'sol' at room temperature.The gradual addition of NaOH led to nucleation with a low rate and stimulated consequent precipitation of Ca(OH) 2 one over one generating a gel with a good crystal-like nature.
The solution which contains Ca(OH) 2 gel was aged for 24 hours at room temperature to get a very well condensation.Next, filtration was done via centrifugation at 2500 rpm to get Ca(OH) 2 gel.After filtration, the Ca(OH) 2 precipitate was washed with double-distilled water then dried in an oven for 24 hours at 60°C [45].To finish, the dried Ca(OH) 2 powder was calcined in a muffle furnace at 500°C for 2 hours.

Batch studies
The effect of several experimental factors as pH, time of contact, adsorbent dose, and initial concentration of adsorbate was tested.For each trial, 100 mL aqueous solution of the metal ion of desired concentration was placed in 250 mL Erlenmeyer flasks.The pH was adjusted to the desired values by 0.1 mol/L of (HCl or NaOH).Then, a known dose of the prepared adsorbent was placed in the reagent flasks.The solutions in the flasks were agitated at room temperature with 250 rpm for the desired equilibrium time.After the completion of the adsorption process, the samples were filtered by Whatman filter papers.The influence of pH was tested in the range (1)(2)(3)(4)(5)(6)(7)(8).The influence of adsorbent dosage was tested in the range (25-250 mg).The influence of time of contact was studied in the range (15-120 min).All trials have proceeded in triplicates and the mean values were utilised for further calculations.The removal percentage (Re %) of Ni(II) and adsorption capacity (q e ) of the prepared adsorbent were determined as follows: where q e (mg/g) capacity of adsorption; C o and C e (mg/L) are the initial and equilibrated concentration of Ni(II) respectively, V (L) is the total volume of Ni(II) solution, and m (g) is the dry mass of the calcium oxide nanoparticle.

Desorption and reusability
After adsorption with Ni(II) ions, calcium oxide nanoparticles were contacted with Na 2 EDTA (0.01 M) as a desorption solution.The adsorption-desorption experiments were conducted as follows, 100 mg of the calcium oxide nanoparticles were placed in 100 mL solution including 50 mg/L of Ni(II) ions at pH 7 and shacked well for 60 min at 25°C.The adsorbent was filtered off and washed with double-distilled water to take away the nonadsorbed Ni(II).After that, 100 mL of 0.01 M of desorption eluent (Na 2 EDTA) was added to the calcium oxide nanoparticles loaded with Ni(II) ions followed by shaking for 30 min as a desorption time, next, the mixture was filtrated off and the Ni(II) concentration was measured in the filtrate.The reusability of the prepared adsorbent was assessed up to 10 cycles of adsorption-desorption.Finally, the desorption percentage (Des.%) was determined as follows: where A d (mg/L) is the concentration of Ni(II) in the desorption solution, A e (mg/L) is the equilibrated concentration of Ni(II), A o (mg/L) is the initial concentration of Ni(II), V d (L) is the desorption solution's volume, and V (L) is the solution volume.

Application on real water samples
Several samples of real water were collected from dissimilar sites in Egypt (Mansoura, Talkha, Gamasa, Ras El-Barr, Damietta, Sharm El-Shiekh, and Alexandria), with 50 cm depth from the higher level, after that, samples of water were filtered over G4 sintered glass.Next, all filtered samples were heated at a temperature less than 90°C for 30 min, allowed to cool and pre-treated as following for determination of Ni(II) content: 500 mL of each sample was poured in a beaker then 10 mL of concentrated nitric acid and 5 mL of 35% (v/v) H 2 O 2 were added for elimination and decomposition of organic compound, finally all treated water samples were wellkept-up in a bottle of polyethylene for future use.The removal percentage of Ni(II) was calculated from equation (3).The physical and chemical characteristics of the real water samples are presented in Table S1.

XRD analysis
XRD analysis was carried out to check if the produced material was CaO-NPs or not and to study the structure of the synthesised CaO-NPs. Figure 1 shows the XRD pattern of CaO-NPs calcinated at 500°C.The main peaks were observed at 2θ = 32.48°,37.28°, and 53.92° that were allocated with (111), (200), and (220) reflection planes for the cubic phase of CaO, respectively [46].The main peak appeared at 2θ = 37.28°, and the narrower spectral width in the XRD diagram indicated a polycrystalline character of the synthesised calcium oxide nanoparticles [43].Therefore, high-quality CaO-NPs were produced.

SEM analysis
The morphology of the prepared CaO-NPs was tested utilising SEM analysis.The SEM image in Figure 2 revealed that the CaO-NPs composed of grains agglomerated to each other to give large agglomerates of small particles.These agglomerated small particles reveal the polycrystalline nature of CaO-NP.Other works confirmed the shape of nanoparticles of calcium oxide [35,47].Also, the SEM image shows that the particle size of the synthesised adsorbent was 10 μm, hence the adsorbent has a good surface porosity.The porosity of CaO-NPs was a consequence of the release of carbon dioxide and water from the internal structure during the calcination of Ca(OH) 2 [43].The presence of the established pores in the microspheres of CaO eases and enables the efficient removal of Ni(II) from aqueous media.So, the prepared adsorbent has a high surface area, uniform shape, and good porosity.

TEM analysis
TEM image of the synthesised CaO-NPs calcinated at 500°C is shown in Figure 3, which confirmed the crystallisation of CaO-NPs with a simple cubic structure.

FTIR spectra
FTIR study allowed us to identify characteristic bands of the prepared adsorbent to understand the possible interactions between the synthesised CaO-NPs and Ni(II) ions.The infrared spectrum of the prepared adsorbent in the region of (4000-500 cm −1 ) is displayed in Figure S1.The bands at 3448 and 3644 cm −1 have resulted as a result of the O-H bonds from H 2 O molecules present on the surface of CaO-NPs [45].Also, both bands could be corresponding to O-H bonds resulted from the remaining OH after calcination of Ca(OH) 2 [36,47,48].Additionally, the sharp band at 3644 cm −1 could be owing to the O-H bond which is characteristic of standard CaO [49].The bands at 2923 and 2517 cm −1 corresponding to vibrations of C-H bond.The band at 1798 cm −1 is standing for C=O vibration [46].The band at 1427 cm −1 , and the sharp band at 876 cm −1 show C-O bond related to the carbonation of nanoparticles of CaO [36,47,48].The sharp band at 714 cm −1 represents a Ca-O bond [46].Therefore, FTIR findings showed that the adsorbent surface comprises mostly (-OH and C=O) groups which are principally accountable for the adsorption of Ni(II).

TGA analysis
The thermogravimetric analysis of the prepared CaO-NPs is presented in Figure S2.It is evident that only the thermal decomposition step was characterised at a temperature above 600°C and this indicates that the prepared adsorbent is highly stable in the studied temperature range up to 600°C.

pH at a zero-point charge (pH zpc )
It is important to find the zero-point charge (pH zpc ) of the prepared adsorbent to examine the behaviour of the adsorbent surface and the effect of pH on the adsorptive removal process.In this work, pH drift way [50] was utilised to detect (pH zpc ).For such a method, 20 mL of NaCl solution (0.05 M) was placed in a sequence of 100 mL flasks.The initial pH (pH i ) of each solution was adjusted in the range of (2-12) by the addition of 0.1 M of (NaOH or HCl).Then, 0.3 g of prepared adsorbent was placed with each solution and the mixtures were agitated for 180 rpm 25°C for 24 hours.The final pH (pH f ) of the supernatant was determined using a pH metre.The change in pH (ΔpH) for each solution was plotted versus initial pH and the intersection point of the curve with abscissa gave the value of pH zpc and it was found to be pH 5.4 as shown in Figure S3.Thus, the CaO-NPs adsorbent works well at the pHs more than 5.4 which is preferable for cationic adsorption.

Determination of surface area
The surface area of CaO-NPs was determined and found to be 71.49m 2 /g.Generally, the more surface area of the adsorbent, the high the adsorption performance.
Based on the obtained results of FTIR, TEM, SEM, XRD, and TGA, we concluded that the calcium oxide nanoparticles were successfully synthesised utilising the suggested procedure of preparation.

Influence of pH
Figure 4 shows the influence of pH on the removal percentage of Ni(II) utilising the CaO-NPs in the pH range of (2-8).As seen, as pH increases from 2 to 6, the percent removal of Ni(II) increased significantly and remained unchanged within the pH range of (6)(7)(8).This can be explained based on the following points, (1) The pH zpc of CaO-NPs was 5.4, (2) At lower pHs (<pH zpc ), the surface of the CaO-NPs is positively charged, so electrostatic repulsion occurred between the adsorbent and the Ni(II) ions, (3) Also, at lower pH, hydrogen ions compete with Ni(II) for binding to the adsorbent and reduce the number of binding sites for uptake of Ni(II) leading to low adsorption capacity [51], (4) At higher pHs (>pH zpc ), the adsorbent is negatively charged and higher removal of Ni(II) was achieved by electrostatic attraction, (5) Also, at higher pHs, there is a low amount of H + ions, leading to a weak competition with Ni(II) leading to a high capacity of adsorption [52], and (6) At pHs more than 8, no experiments were conducted owing to the formation of several hydroxo-species of nickel as Ni(OH) + , Ni(OH) 2 , Ni(OH) 3 − , and Ni(OH) 4 2-[53] so, pH range (1-8) was selected for this study to avoid the precipitation of nickel after pH 8 [54].Subsequently, pH 7 was chosen as the ideal pH for the Ni(II) removal which agreed with the influence of pH on the adsorption of Ni(II) reported in the literature [55].

Influence of adsorbent dose
The influence of adsorbent dose on the removal of Ni(II) from aqueous media was tested by changing the dose from 25 to 250 mg under optimised conditions.It was noted that the percent removal increased as the adsorbent dose increased.The increase in adsorbent dose increased the availability of more active sites for binding, which increased the percent removal of Ni(II) ions.As seen in Figure 5, the percent removal of Ni(II) increased rapidly up with a rise of adsorbent dose and reach a plateau at 200 mg of adsorbent.This indicates that at a higher adsorbent dose more binding sites are available, accordingly, uptake of Ni(II) could be increased until saturation of the binding sites [56].Therefore, 200 mg of adsorbent was chosen as the optimal dose for the Ni(II) removal.

Influence of contact time and kinetic studies
The removal percentage and the capacity of adsorption of Ni(II) on CaO-NPs were measured at different times of contact (15-120 min) and the results are presented in Figure 6.As seen, it was noted that the percent removal of Ni(II) and capacity of adsorption rise rapidly with a rise in the contact time in the first 60 min, then gradually increase until 90 min and finally reach a plateau with no significant increase in the percent removal and the capacity of adsorption beyond 90 min indicating that equilibrium has been reached.Firstly, the binding sites are vacant leading to easily reachable to Ni(II) ions, so the adsorption rate was high.After that, the rate of adsorption becomes lower because of the slow diffusion rate of the adsorbate via the pores [57].Later, the rate of adsorption was reduced because of the blocking of external pores as a result of saturation, consequently, Ni(II) ions needed a long time to reach and penetrate the internal pores [58].Therefore, the contact time of 90 min was selected for further experiments.Adsorption kinetics is very important to investigate the key step of the adsorptive removal process.The experiments of kinetic studies were conducted in a 250 mL Erlenmeyer flask with a 100 mL aqueous solution includes 200 mg of CaO-NPs and 100 mg/L of Ni(II) at room temperature and pH 7. The adsorbate-adsorbent solution was agitated at 250 rpm for times of contact (15, 30, 45, 60, 75, 90, and 120 min).The experimental data were fitted by pseudo-first-order (PFO) and pseudo-second-order (PSO) models [59][60][61], which are represented as follows:  where q t (mg/g) is the adsorbed dose of Ni(II) at time t (min), q e (mg/g) is adsorbed amount of Ni(II) at equilibration, and K 1 (min −1 ) is the rate constant for PFO, K 2 (g/mg.min) is the rate constant for PSO.Figures (7 and 8) represent the linearised plots for both models.The correlation coefficient and parameters for both kinetic models are listed in Table 2. Based on the values of correlation coefficient (R 2 ), the adsorption was found to conform well to the PSO model indicating the chemisorption nature of the adsorption [62] where the adsorption of Ni(II) on CaO-NPs is based on chemical interaction as chemical and ionic bonds, which includes an exchange of electron between adsorbate and adsorbent.In chemisorption, metal ions bind to the surface of the adsorbent by chemical bonding and tend to find sites that maximise the coordination with the surface.This also showed that Ni(II) adsorption onto CaO-NPs may occur through chemisorption including valance forces between Ni(II) ions and CaO-NPs.The obtained result indicates that chemical adsorption is the rate-determining step [63].Furthermore, Morris-Weber's intraparticle diffusion (IPD) model [64] is utilised to discover if intraparticle diffusion is the only rate-determining step.The intraparticle diffusion [64] model can be represented as follows: where I is the intercept and K int (mg/g.min 0.5 ) is the rate constant for IPD. Figure 9 represents the linearised plot for the IPD model.The values of correlation coefficient (R 2 ) and rate constant (K int ) were given in Table 2.In the current work, the non-zero intercept for such a model showed that diffusion is not only the key step for controlling the nickel adsorption on the surface of the adsorbent [65].Generally, the adsorption mechanism follows three stages [66], (a) external mass transfer (external film diffusion stage) when the adsorbate is transported from solution bulk to the external surface of the adsorbent, (b) intraparticle diffusion (pore diffusion stage), where the adsorbate penetrates the inner of particles of the adsorbent, and (c) adsorption of adsorbate at the inner sites of the adsorbent (interaction stage).According to all the found results, the overall adsorption rate of Ni(II) ions by CaO-NPs might be characterised by surface and intraparticle diffusion.

Influence of initial metal ion concentration and isotherm studies
The data depicted in Figure 10 demonstrate the influence of the initial concentration of metal ions on the percent removal and the adsorption capacity of Ni(II) on CaO-NPs.As seen, when the initial concentration of Ni(II) rose from 50 to 500 mg/L, the removal percentage reduced after the initial concentration of 150 mg/L.This could have occurred because, at a lower concentration of metal ion, the number of Ni(II) is low concerning the number of binding sites of the adsorbent which results in highly adsorption rate of Ni(II).But, at a higher concentration of metal ion, the binding sites were saturated, and hence the removal percentage of Ni(II) is reduced.On the contrary, the increase in initial concentrations of Ni(II) rises the interaction between Ni(II) ions and CaO-NPs leading to an increase in the capacity of adsorption.This behaviour could be happened owing to, the increase in the initial concentration of Ni(II) creates the driving force which is needed to reduce the resistance of the metal ion to mass transfer [67].So, 150 mg/L was chosen as the optimum initial concentration of Ni(II) for all further experiments.The equilibrium of the adsorption process was validated mathematically using isotherm models of Langmuir, Freundlich, and Temkin.The three models can relate the adsorption capacity of CaO-NPs (q e ), to the remaining equilibrium concentration of Ni(II) in the solution bulk (C e ).The linearity in plots for the above-mentioned isotherm models was utilised to test the best fitting one and the appropriateness of the model obtained by judging the correlation coefficients.The equilibrium experiments for isotherm studies were tested in a 250 mL Erlenmeyer flask with 100 mL aqueous solution includes 200 mg of CaO-NPs and different initial Ni(II) concentrations of (200, 250, 300, 350, 400, 450, and 500 mg/L) at room temperature and pH 7. The solutions were agitated at 250 rpm for an equilibrium time of 90 min.
Langmuir [68,69] supposes that (1) no interaction between molecules of adsorbate on the surface of the adsorbent, (2) the adsorption of molecules of adsorbate at one active site does not affect the adsorption of other adsorbate molecules in neighbouring active sites, and (3) the attraction between the adsorbate molecules on the surface of the adsorbent and the ones in the solution decreases with distance [70,71].Langmuir model can be represented as follows: where K a is the Langmuir constant which referred to the adsorption energy and q m is the maximum capacity of adsorption (mg/g), C e is the equilibrated concentration (mg/L), q e is the adsorbed dose at equilibrium (mg/g).
Freundlich [72] supposes a multilayer formation of adsorbate on a heterogeneous surface of the adsorbent.Freundlich equation can be represented as follows: where C e is the equilibrated concentration (mg/L), q e is the adsorbed dose at equilibrium (mg/g), (K f and n) are Freundlich isotherm constants related to the adsorption capacity and adsorption intensity, respectively.Figures (11 and 12) represent Langmuir and Freundlich, respectively.The curve fitting results are compiled in Table 3.According to correlation coefficient (R 2 ) values, the model of Langmuir affords a better fit to the obtained results than Freundlich.This could indicate the monolayer adsorption onto the surface of the CaO-NPs [61,73].Also, the adsorption capacity calculated by Langmuir was found to be 166.67 mg/g.
The Temkin [74] model suggests that the heat of adsorption of all molecules will decline linearly with surface coverage and can be represented as follows: where B is related to the heat of adsorption, K T is the equilibrium binding constant.Values of B and K T were calculated from the plot of q e against lnC e as shown in Figure 13 and reported in Table 3, where B = RT/b (b is Temkin constant, R is the universal gas constant 8.314 J/mol.K, and T is absolute temperature 298.15 K).For such a model, the heat of adsorption of all molecules reduces linearly with surface coverage [75] due to interactions between adsorbent and adsorbate with uniform distribution of binding energies till it reaches a maximum value [74].The value of b (kJ/mol) was found to be 147.027which illustrates the electrostatic interaction between Ni(II) ions and CaO-NPs [75].

Influence of temperature and thermodynamic studies
Temperature is also a control parameter in the adsorption process and has a great effect on the removal percentage where the temperature increasing result in an increase in the diffusion rate of the adsorbate and varying the temperature can alter the adsorption capacity of the adsorbent.To test the influence of temperature on the adsorptive removal of Ni(II), thermodynamic studies were performed and the parameters, namely, change in free energy (ΔG o ), change in enthalpy (ΔH o ), and change  where C ads (mg/g) is the adsorbed amount of Ni(II) at equilibration and C eqm (mg/L) is the remaining equilibrium concentration of Ni(II).
Then, ΔGº was calculated as follows: where R (8.314 J/mol.K) is the universal gas constant and T (K) is the absolute temperature.Then, ΔHº and ΔSº were calculated using equation ( 14) by plotting lnK d against 1/T (Van't Hoff plot) [73,77,78] as depicted in Figure 14.
The data represented in Table 4 showed that: (i) the negative values of ΔG° reveal that Ni(II) adsorption by CaO-NPs was thermodynamically favourable and spontaneous [79], (ii) ∆H o has a positive value which indicated that the adsorption process was endothermic, (iii) the endothermic nature is also shown by the increase in the amount of adsorption with rising temperature, and (iv) ∆S o has a positive value which showed that Ni(II) in aqueous solution is in a more random distribution compared with the ordered state of adsorbent surface [80].Also, the value of change in entropy above zero confirms the irreversibility of the adsorption process.

Influence of ionic strength
Data presented in Table S2 shows the effect of the ionic strength on the removal percentage of Ni(II) by CaO-NPs in the presence of different concentrations of NaCl (0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 mol/L).The adsorbent's ability to adsorb Ni(II) is affected by the concentration of NaCl.The decrease in the removal percentage of Ni(II) might be owing to competition between Na + ions with Ni(II) ions for the binding sites on CaO-NPs.

Interference study
To test the selectivity of the proposed procedure and to examine the significance of CaO-NPs for the Ni(II) removal, the influence of several interfering ions (individually or in combination) on removing Ni(II) was investigated under the optimised conditions.The obtained results presented in Table S3 showed that all the investigated interfering ions have no opposing influence on the removal of Ni(II) via the proposed procedure even with relatively high concentrations of interferant compared to that of Ni(II).When interfering ions depress the removal percentage of Ni(II), so excess CaO-NPs will be added to overcome this effect.Accordingly, the suggested procedure might find several uses in treatment processes of water and could be fairly utilised for the Ni(II) removal from numerous complex samples of water.

Desorption and reusability
Desorption and reusability studies help to regenerate the adsorbent, as well as recover Ni(II) from the spent adsorbent along with protecting the environment from solid waste disposal problems.To assess the recovery of Ni(II) and adsorbent reusability, adsorption-desorption experiments were conducted using 100 mL of 0.01 mol/L Na 2 EDTA as a desorption agent.It was found that Na 2 EDTA revealed outstanding desorption efficacy because Na 2 EDTA is a very good chelating agent, which is easy to form stable complexes with transition metals as Ni(II) and can cause more metal ion desorption from the surface of the adsorbent to the solution of the eluent.According to Figure S4, the adsorption efficiency of the recycled adsorbent decreased by only 10% after ten cycles of adsorption-desorption.After desorption, the adsorbed Ni(II) ions were released to renew the binding sites, thus recovering the adsorption capacity of the adsorbent.The outstanding performance of CaO-NPs during adsorption-desorption cycles indicated the high potential for long-term utilisation of the CaO-NPs to remove Ni(II) ions from aqueous solution for 10 times without significant loss in adsorption capacity.

Application on real water samples
For testing the application of the suggested method, a run of tests was achieved to recover 5 and 10 µg/L of Ni(II) added to real water samples.The adsorption trials were proceeded using 50 mL sample solutions at pH 7. The obtained data are presented in Table S4 and showed that the recovery was satisfactory.However, the lower values of some samples were enhanced to about 100% by increasing the CaO-NPs dose.

Analytical characteristics
Under optimal circumstances described for the recommended method, the calibration curve shows good linearity over the range (0.5-20 μg/L).The analytical detection limit of standard aqueous solutions was 1.1 μg/L, with a relative standard deviation equal to 3% (n = 3).The analytical characteristics of this method were listed in Table S5.

Comparison to other adsorbents
The capacity of adsorption of the prepared CaO-NPs for the removal of Ni(II) in the current study was compared with other adsorbents and presented in Table 5.The CaO-NPs have a higher capacity of adsorption for Ni(II) than other adsorbents.Consequently, CaO-NPs is considered to be a promising adsorbent for the treatment of water contaminated with Ni(II).

Suggested adsorption mechanism
Adsorption is a complex process and can proceed through many different mechanisms.
Based on the obtained results for characterisations and batch studies, there are several possible mechanisms for adsorption that can be taken into account, as complexformation, ion exchange, and electrostatic interaction between Ni(II) and surface functional groups of CaO-NPs [54,81].CaO-NPs contain oxygen-containing functional groups as carboxyl (-COOH) and hydroxyl (-OH) groups on their surface which affect the adsorption of Ni(II) ions, where these groups might accept or donate a proton triggering a change in the charge of the adsorbent surface.By increasing pH, carboxyl and hydroxyl groups gradually become de-protonated and negatively charged as follows: Therefore, the strong adsorption force generates a complex between the negatively charged surface and the positively charged Ni(II) ions which can be represented as follows [82,83]: where A − is the anion resulted from (-COOH, or -OH).Figure 15 displays the proposed mechanism for the adsorption of Ni(II) ions onto the surface of CaO-NPs.Adsorbents q m (mg/g)

Conclusions
Synthesis and utilising of nanoparticles of calcium oxide (CaO-NPs) as an efficient adsorbent for removing Ni(II) ions from aqueous media were discussed in the current study.CaO-NPs were synthesised by the method of 'sol-gel'.The synthesised CaO-NPs was utilised as an effective adsorbent for removing Ni(II) from aqueous media via the adsorption technique before its determination via ICP-OES.Characterisation tests confirm that the prepared adsorbent exhibits enhanced cation exchange capacity.The obtained results of batch experiments showed that the adsorptive removal process was pH-dependent and the key step was chemical.The equilibrium data were tested using Langmuir, Freundlich, and Temkin isotherm models.The capacity of adsorption was calculated and equal 166.67 mg/g.Kinetic evaluations showed that the adsorption process obeyed the pseudosecond-order kinetic model.Thermodynamics suggested that the adsorptive removal process was spontaneous and endothermic.The value of entropy change above zero confirms the irreversibility of the adsorption process.Disodium salt of EDTA was used as a desorption agent to study the regeneration of the synthesised adsorbent, and the obtained results revealed that the adsorbent has excellent regenerative power.Finally, the suggested procedure was applied for removing Ni(II) ions from real water samples without any matrix interference.In light of the above results, this study showed that calcium oxide nanoparticles (CaO-NPs) can be utilised as an efficient adsorbent for the removal of Ni(II) from aqueous media.

Figure 4 .
Figure 4.The effect of pH on the removal of Ni(II).(Conditions: adsorbent dose 200 mg; contact time 90 min; contact temperature 25°C; initial metal ion concentration 100 mg/L)

Figure 5 .
Figure 5.The effect of adsorbent dose on the removal of Ni(II).(Conditions: contact time 90 min; pH7; contact temperature 25°C; initial metal ion concentration 100 mg/L)

Figure 7 .
Figure 7. Pseudo-first-order kinetic model for the removal of Ni(II).

Figure 6 .
Figure 6.The effect of contact time on the removal of Ni(II).(Conditions: adsorbent dose 200 mg; pH7; contact temperature 25°C; initial metal ion concentration 100 mg/L)

Figure 8 .
Figure 8. Pseudo-second-order kinetic model for the removal of Ni(II).

5 Figure 9 .
Figure 9. Intra-particle diffusion kinetic model for the removal of Ni(II).

Figure 10 .
Figure 10.Effect of initial metal ion concentration on the removal of Ni(II).(Conditions: adsorbent dose 200 mg; contact time 90 min; pH7; contact temperature 25°C)

Figure 11 .
Figure 11.Langmuir isotherm model for the removal of Ni(II).

Figure 12 .
Figure 12.Freundlich isotherm model for the removal of Ni(II).

Figure 13 .
Figure 13.Temkin isotherm model for the removal of Ni(II).

Figure 15 .
Figure 15.The suggested adsorption mechanism of Ni(II) on CaO-NPs.

Table 1 .
ICP-OES operational conditions for the determination of Ni(II).

Table 2 .
Estimated kinetic models and their parameters of the adsorption process.

Table 3 .
Estimated isotherm models and their parameters of the adsorption process.

Table 4 .
Estimated thermodynamic parameters of the adsorption process.
Figure 14.Plot of lnK d vs. 1/T for the removal of Ni(II).

Table 5 .
Comparison of the adsorption capacity of Ni(II) by various adsorbents.