Zinc Oxide Nanoparticles Preparation for Pd2+ Ions Adsorption From Aqueous Wastewaters: A Green Technique

ZnO nanoparticles (NPs) were easily synthesized using zinc nitrate through centaurea cyanus extract (as a reducing agent) at ambient conditions. XRD results demonstrated that ZnO NPs have a high-crystalline hexagonal structure with an average size of 48 nm in diameter. FT-IR spectral analysis indicated an active contribution of centaurea cyanus-derived biomolecules in zinc ions bioreduction. According to SEM analysis, ZnO NPs were properly dispersed and had a hexagonal shape. Batch experiments were performed to investigate the impact of several process parameters such as initial pH of solution, adsorption dosage, Pd 2+ ions initial concentration and contact time on the Pd 2+ ions adsorption from the solution. The Freundlich isothermal model could excellently legitimize a multilayer adsorption. Furthermore, the adsorption process followed a pseudo-second-order reaction kinetic. The maximum adsorption (99.24%) was experimentally found at pH of 5.5, adsorption dosage of 1.63 g.L -1 , Pd 2+ ions initial concentration of 77.5 mg.L -1 and contact time of 91.25 min.


Introduction
Palladium and its compounds are mostly used in the catalytic converters. They can convert around 90% of the harmful gases such as hydrocarbons, carbon monoxide, and nitrogen dioxide into the less noxious substances such as nitrogen, carbon dioxide and water vapor [1,2]. Palladium is also used in electronics, dentistry, medicine, hydrogen puri cation, chemical applications, groundwater treatment, and jewelry.
A large number of carbon-carbon bonding reactions in organic chemistry are facilitated by palladium compound catalysts such as Lindlar's palladium [7]. Furthermore, palladium is an excellent electrocatalyst for oxidation of primary alcohols in alkaline media. Palladium can also be applied in the homogeneous catalysis, used in combination with a variety of ligands [8]. Palladium is used in small amounts (about 0.5%) in some alloys of dental amalgam to decrease corrosion and increase the metallic luster [9]. Palladium is also produced in nuclear ssion reactors and can be extracted from spent nuclear fuel [10]. Several modern industries such as batteries, pesticides, fertilizers, paper and chemical ones produce wastewaters containing some heavy metals.
Wang et al. prepared the plate-like nanostructured ZnO nanoparticles by solvothermal method. The prepared nanoparticles had a particle size of 5-20 nm and surface area 147 m 2 /g. The nanoparticles of ZnO tested for the removal of Cu(II) and the results showed high adsorption capacity up to 1600 mg/g [25]. Ma  Centaurea cyanusas a wildling plant in North America, Australia and Asia (Iran) was successfully applied as a reducing agent during the nanoparticles preparation [32].
According to the literature, there were no previous researches on the Centaurea cyanus extract as an inexpensive and biocompatible reducing agent for the Zinc oxide nanoparticles preparation. Therefore, Zinc oxide nanoparticles (ZnO NPs) were synthesized through Centaurea cyanus extract. Then, they were applied in the pd 2+ ions adsorption from a wastewater. The impact of several variables such as the initial concentration of pd 2+ in the solution, pH, adsorbent dosage, and time on pd 2+ ions adsorption was statistically and experimentally studied and optimized. Finally, the kinetic and equilibrium of pd 2+ ions adsorption on the synthesized nanoparticles were investigated. 2.2. Synthesis and preparation of ZnO NPs 14.9 g of the zinc nitrate salt was dissolved in 500 ml of distilled water. It was then added to 500 ml of the Centaurea cyanus extract. The solution was mixed at the ambient temperature [33]. Rapid formation of a white sediment in the solution indicates the ZnO NPs synthesis. The synthesized nanoparticles were then separated from the solution at 4500 rpm. The sediment was puri ed by several re-dispersions in deionized water and centrifuged. It was nally dried at 80°C through an oven.

Characterization of zinc oxide nanoparticles
The ZnO NPs were characterized by X-ray diffraction (XRD) through a X́ pert PROMPD X-ray diffractometer (Philips, Netherland). The functional groups of ZnO NPs were analyzed through a FT-IR (ALPHA BRUKER, US). Fourier Transform Scanning Electron microscopy (FT-SEM) was used to study the surface morphology of the ZnO NPs (MIRAІІІ TESCAN, Czech Republic). Energy dispersive X-ray analysis (EDAX) was used to reveal the phase of Zn and O presented in the samples. This con rms the elemental composition of ZnO.

Adsorption process
The Pd 2+ ions were adsorbed by ZnO nanoparticles in the batch systems. The experiments were designed using Design Expert software (version 11). The applied pH range was at 1-7 (because a red sediment is rapidly formed at the bottom of beaker at pH of 7 while a good results are usually obtained through the acidic conditions. The other conditions ranges are extracted from the literature for heavy metals adsorption [32]) at ambient temperature with a shaking of 200 rpm (without any vortex observation). The samples were withdrawn from the shaker at each contact time and the consumed adsorbents were extracted from the solutions by centrifugation at 4500 rpm (Sigma, Osterode am Harz, Germany) with whatman lter paper. The absorbance of solution was measured by monitoring the absorbance through an Atomic Adsorption apparatus (Shimadzu-AA-6800). Effect of pH was carefully studied by adjusting the pH of palladium solutions by 0.1 N HCl and NaOH solutions.
The amount of Pd 2+ ions adsorbed by the adsorbent was calculated using the following equation: (1) where, C 0 and C e (mg.L − 1 ) are initial and nal concentrations of palladium before and after adsorption process. The equilibrium adsorption of palladium can be calculated by: (2) where, q e (mg.g − 1 ) is the equilibrium amount of palladium adsorbed on the nanoparticles, v (L) is the volume of solution and w (g) is the mass of adsorbent.

Response surface methodology (RSM)
Central composite design (CCD) is applied under RSM technique through the DoE software. Four independent variables such as pH, contact time, adsorbent dosage and initial concentration of solution (based on Pd 2+ ions) are carefully studied on the Pd 2+ ions removal as a dependent variable (response). These vary at ve different levels (-2, -1, 0, + 1, +2) as shown in Table 1.
where, D is the crystallite size of the particle, K represents the Scherrer constant (K is a dimensionless shape factor with a value close to unity), which is equal to 0.9, λ is the wavelength of light used for diffraction (λ = 1.54 A°) and β is the FWHM (full width at half maximum) of the diffraction peak and θ is the angle of re ection [34]. Therefore, the average size of ZnO nanoparticles was found at 48 nm. Figure  1 shows XRD pattern of the synthesized ZnO nanoparticles through the centaurea cyanus extract.

SEM analysis
SEM image is used to observe the surface morphology and structure. Figure 2(a) shows the SEM image of ZnO nanoparticles. It shows a hexagonal uniform morphology of nanoparticles. Furthermore, a compact structure is seen after the adsorption process [Figures 2(b) and 2(c) show ZnO nanoparticles morphology before and after adsorption, respectively]. The observed agglomeration is due to polarity and electrostatic attraction of ZnO nanoparticles [35].

FTIR analysis
Fourier transform infrared (FTIR) spectroscopy is a characterization technique for the detection of functional groups in compounds. The phase formation of the ZnO powders is further characterized by the FTIR spectroscopy. Figure 3 shows 3.4. Energy-dispersive X-ray spectroscopy (EDX) Figure 4 shows the elemental composition analysis of the ZnO NPs by the EDX. The EDX spectra con rms Zn and O elements presence with high purities (69.2% and 30.8%) in the synthesized ZnO NPs.

Experiments design
The experiments design was used to determine the individual and interactive effects of the process variables. The result of the experiments design was illustrated in Table 2.   The in uence of solution pH is one of the important parameters on the Pd 2+ adsorption. In fact, pH affects both degree of ionizations of some adsorbates and surface charge of adsorbents [46,47]. The removal percentage increased at high a pH (acidic condition) and a high adsorbent dosage [47]. Figure 5(b) shows the interactive effects of pH and contact time on the Pd 2+ removal percentage. An initial rapid rate of adsorption ( rst 33 min) was observed while it gradually increased up to the equilibrium conditions. The initial rapid trend is due existing high numbers of un lled sites in the adsorbent [48]. The result showed that increase in pH led to an increase in adsorption e ciency. The interactive effects of contact time and adsorbent dosage are shown in Fig. 5(d). The highest percentage of adsorption is observed at the lowest contact time when the highest adsorbent dosage is used. The results show that an increase in the adsorbent dosage led to increase in the Pd 2+ ions removal. Its reason is due to increasing adsorbate molecules diffusion across the external boundary layers.

Three dimensional plots for the regression model
According to the analysis by software (con rmed by the experiments), the operating parameters effect follows the following trend as: pH > adsorbent dosage > contact time > solution concentration 3.7. Response surface modeling Central composite design (CCD) as a module of the Design-Expert software under response surface methodology (RSM) was applied to model this process. High correlation coe cient (R 2 ) and low standard as the best model. The relationship between the response and independent variables is shown through a polynomial equation (Eq. 4) obtained by the software: R%=79.51 + 19.05A-7.60B + 13.02C + 2.66D + 569.44AB-567.66AC + 567.05AD-568.44BC + 568.71BD-568.40CD-5.13A 2 -5.63B 2 -6.09C 2 + 2.28D 2 (4) where, A, B, C and D are pH, time (min), adsorbent dosage (g/40mL) and Pd 2+ ions initial concentration in the solution (mg.L -1 ), respectively. The above coded equation is used to predict the Pd 2+ ions removal from the solution. High and low levels of the factors are coded as + 1 and − 1, respectively.
The deviations were together studied to select the most appropriate model for this process. The quadratic model with a standard deviation of 2.18 and correlation coe cient (R 2 ) of 1.00 was suggested.

Analysis of variance (ANOVA)
The analysis of variance data are given in Table 3. This is used to evaluate the signi cance of the quadratic model parameters. They would be signi cant if the p-value 0.05 [49]. Moreover, a higher Fvalue indicates greater signi cance of each term on the response [50].

Adsorbent reusability
The stability and durability of ZnO NPs was examined in ve sequential cycles at the optimum conditions. After each cycle, the ZnO NPs were separated from the solution by centrifugation. They were then washed twice by HNO 3 and dried at room temperature [51]. According to Fig. 6, the adsorbent activity was conserved more than 99% by 5 cycles.

Adsorption isotherm
The isotherms equations and their parameters were extracted from the literature [32]. As shown in Table   4, each equation data were calculated. R 2 for Langmuir, Freundlich and Temkin isotherm models were at 0.4345, 1.0 and 0.9805, respectively. Therefore, the Freundlich model (as a non-ideal and multilayer adsorption) was the best one compared with the others. The kinetic parameters were extracted from the literature [32]. The tting data for the pseudo-rst-order and pseudo-second-order models are shown in Table 5. R 2 data for the pseudo-rst-order and pseudosecond-order models were around 0.9319 and 1.0, respectively. Therefore, the adsorption of Pd 2+ ions onto the synthesized ZnO NPs was properly tted with the pseudo-second-order model. This shows that Pd 2+ ions adsorption onto the ZnO NPs should be a chemisorption process. The pseudo-second order model is based on the adsorption capacity of the solid phases.

Conclusions
In the present study, ZnO nanoparticles were synthesized by centaurea cyanus aqueous extract as a reducing and stabilizing agent. A white color sediment in the solution was observed and several tests such as FTIR, XRD, SEM and EDX were carried out. It was con rmed that ZnO NPs were properly produced. They were then applied to remove Pd 2+ ions (as a symbol of heavy metals) from an aqueous solution (as symbol of wastewater). The optimum conditions were statistically obtained and validated by an experiment. The Pd 2+ ions percentage removal increased with increasing adsorbent dosage, reducing pH and contact time. The lm diffusion as the most probable rate-controlling step of the adsorption mechanism was investigated. The Freundlich isotherm model and pseudo-second order kinetic model could respectively legitimize the phase equilibria and chemisorption of Pd 2+ ions adsorption. Figure 1 XRD pattern of the synthesized ZnO nanoparticles through the centaurea cyanus extract  Various parameters effect on the removal percentage