Efficient removal of organic dyes and lead ions with eco-friendly prepared zinc oxide nanoparticles from water

In this research work, different ZnO NPs were prepared via eco-friendly green route using Ziziphus jujuba leaves extract assisted by microwave. Eco-friendly prepared ZnO NPs characterized by different techniques, and the results confirmed the preparation of hexagonal wurtzite ZnO nanoparticles with different particle sizes. The prepared ZnO NPs were then used for the adsorption and removal of two different azo dyes; methyl orange (MO), and methylene blue (MB), as well as toxic Pb(II) ions, from a model solution and real samples. Influence of experimental conditions was explored, and the results showed that most of the MB, MO and Pb(II) could be removed from the model solution within few minutes, at ambient conditions using 15 mg ZnO NPs. The removal of the MB, MO and Pb(II) using ZnO NPs was studied kinetically and thermodynamically, and the results showed that the experimental data were best fitted using the pseudo-second-order kinetic models. The thermodynamics study showed that the process was spontaneous, with exothermic nature. Finally, the prepared ZnO NPs were used for the removal of MB, MO and Pb(II) in real wastewater sample, and high removal efficiency was presented. GMW ZnO NPs were characterized by different techniques: XRD, SEM, and textural properties. The results demonstrated the efficacious preparation of wurtzite ZnO nanoparticles with irregular flake-shape composed of very small particles with an average particle size of and 33 nm. The effect of different environmental conditions on the removal of MB, MO and Pb(II) by the GMW ZnO NPs were studied, and the findings showed that most of the studied pollutants were removed using 15.0 mg GMW ZnO NPs within 30 minutes at pH 5.6 and 298 K. The removal process was explored kinetically, and it was found that the pseudo-second-order model is suitable for the description of the removal process with experimental removal capacities of 5.21, 2.57, and 596 mg GMW ZnO NPs per gram of MB, MO and Pb(II), respectively, indicating the great probability of applying GMW ZnO NPs for environmental remediation. The thermodynamics study presented the spontaneity of the removal process, which is associated with the negative enthalpy, and negative entropy values, showing that the removal of MB, MO and Pb(II) by the GMW ZnO NPs is an enthalpy-driven process. Finally, GMW ZnO NPs were used for removal of MB, MO and Pb(II) from real water samples . The results demonstrated the high efficacy of the GMW ZnO NPs and their possible application for the remediation of polluted environment.

using centrifuge, followed by separation via Whatman™ filter paper # 5 filtration , and the clear filtrate was collected for the analysis of the residual MB, MO and Pb(II). The concentration of the residual MO, and MB dyes in each solution was measured using UVvis spectrophotometer (UV-1650 PC, CPS-240A SHIMADZU) at 464 and 670 nm as shown in Fig. 1, respectively. The residual Pb(II) ion concentration was then measured using Perkin Elmer optima 7000 DV inductively coupled plasma/optical emission spectrometer (ICP-OES). The percent removal (R%) and capacity (qt) were using Equation (1) and Equation (2): where C0 is the initial concentration per (mg/L), Ct is the residual concentration (mg/L) after certain period of time, m is the mass of ZnO NPs(g), and V is solution volume (L).
All the experiments were conducted in triplicates, and the stated values were the average with less than 5% error.

Real samples
Seawater, tap water, and wastewater were used as the real samples. The seawater sample from the Red Sea (Jeddah, Saudi Arabia -latitude 21.518333, longitude 39.150677), tap water sample obtained from the laboratory after the tap water had been allowed to flow for ten minutes. All samples were filtered (45.0 µm Millipore filter paper) and preserved in the dark using Teflon bottles at 278 K.  Fig. 3, which shows that ZnO NPs exists in several sizes and shapes depends on the preparation method. For example, T ZnO NPs sizes and in the form of irregular spherical particles with average particle size of 40 nm, whereas the G ZnO NPs, and MWG ZnO NPs characterized with the irregular flake-shape composed of very small particles with an average particle size of 20 nm, and 33 nm, respectively. The small size of the G ZnO NPs may be attributed to the presence of the Ziziphus jujuba leaves extract which act as capping agent, and prevent the agglomeration of the ZnO NPs. Also, the bigger particle size of the MWG ZnO NPs compared with the G ZnO NPs may be ascribed to the thermally promoted crystallite growth due to the fusion of the small particles as a results of the high temperature.

ZnO NPs characterization
The specific surface areas of the ZnO NPs were calculated from the nitrogen gas adsorption/desorption isotherms at 77 K applying the BET equation, as shown in Fig. 4.
The BET specific surface areas were 12.65, 11.45, and 11.44 m 2 /g for the T ZnO NPs, G ZnO NPs, and MWG ZnO NPs, respectively, indicating surface area slight decrease upon the microwave treatment.

Removal experiment
The

Kinetics and thermodynamics studies
where k1 (min −1 ) is the PFO rate coefficient, and qe and qt represent the amount of MB,

MO and Pb(II) removed/unit mass of MWG ZnO NPs at equilibrium and at any time t,
respectively. The application of the PFO to the removal data at where k2 (g/(mg min)) is the PSO rate coefficient. Applying Equation (4) to the removal data at Fig. 13 and by plotting t/qt versus t, straight lines and appropriate convergence were obtained for MB, MO and Pb(II) with an R 2 values higher than 0.980, as shown in Table 1 and Fig. 15, showing the relevancy of PSO kinetic model for descripting the removal by MWG ZnO NPs from aqueous solution.
The appropriateness of the PSO kinetic model in comparison to the Lagergren PFO kinetic model was validated using two statistical tests, the chi-square test [33]; Equation (5), and the and sum of the squares of errors (SSE) [34]; Equation (6). and washing with deionized water as well as ethanol then deionized water. As it is presented in Fig. 18 This result indicated the applicability of the solid MWG ZnO NPs as a promising and potential adsorbent for the remediation of polluted environment.
The change of Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) are very crucial to understand the thermodynamic feasibility and spontaneity of any chemical or physical process including the environmental remediation. Accordingly, Equations 9-11 were applied to estimate the ΔG, ΔH, and ΔS [36]: RT H R