Experimental Design Modeling of the Effect of Hexagonal Wurtzite-ZnO Synthesis Conditions on Its Characteristics and Performance as a Cationic and Anionic Adsorbent

Abstract

Surface composite design was used to study the effect of the ZnO synthesis conditions on its adsorption of methyl orange (MO) and methylene blue (MB). The ZnO was prepared via hydrothermal treatment under different conditions including temperature (T), precursor concentration (C), pH, and reaction time (t). Models were built using four Design expert-11 software-based responses: the point of zero charge (pHzc), MO and MB removal efficiencies (R_MO, R_MB), MO and MB adsorption capacities (q_MO, q_MB), and hydrodynamic diameter of ZnO particles (D_h). ZnO was characterized by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, UV/VIS spectroscopy, thermal gravimetric analysis (TGA), and dynamic light scattering (DLS). The formation of ZnO was confirmed by the XRD, UV, and FTIR spectra. Results showed a very high efficiency for most of the samples for adsorption of MB, and more than 90% removal efficiency was achieved by 8 samples among 33 samples. For MO, more than 90% removal efficiency was achieved by 2 samples among 33 samples. Overall, 26 of 31 samples showed higher MB adsorption capacity than that of MO. R_MB was found to depend only on the synthesis temperature while R_MO depends on temperature, pH, and reaction time. pHzc was found to be affected by the synthesis pH only while D_h depends on the synthesis pH and precursor concentration.

Keywords: ZnO; adsorption; methyl orange; methylene blue; nanoparticles; surface composite design.

Figure 1

(a) XRD pattern of sample 31 prepared under the conditions of C = 0.2 M, pH = 7, T = 100 °C, and t = 1 h. (b) Thermal gravimetric analysis (TGA) and DTA analysis results of sample 10 prepared under the conditions of C = 0.4 M, pH = 7, T = 100 °C, and t = 2 h. (c) UV-VIS spectrum of sample 1 prepared under the conditions of C = 0.3 M, pH = 11, T = 150 °C, and t = 1.5 h. (d) FTIR spectrum for the range of 4000–400 cm⁻¹ of sample 1 prepared under the conditions of C = 0.2 M, pH = 7, T = 100 °C, and t = 1 h. (e) Nitrogen adsorption desorption isotherms for samples 30 and 34. (f) Pore size distributions for samples 30 and 34.

Figure 2

FE-SEM images of ZnO samples prepared under different conditions: (a) C= 0.1 M, pH = 9, T = 150 °C, and t = 1.5 h; (b) C= 0.5 M, pH = 9, T = 150 °C, and t = 1.5 h; (c) C= 0.3 M, pH = 11, T = 150 °C, and t = 1.5 h; (d) C = 0.3M, pH = 9, T = 50 °C, and t = 1.5 h; (e) C = 0.3 M, pH = 9, T = 150 °C, and t = 1.5 h; (f) C = 0.3 M, pH = 7, T = 150 °C, and t = 1.5 h. (g) TEM images of ZnO sample prepared under the conditions of C = 0.3 M, pH = 9, T = 150 °C, and t = 1.5 h; (h) TEM images of ZnO sample prepared under the conditions of C = 0.3 M, pH = 9, T = 150 °C, and t = 1.5 h.

Figure 2

FE-SEM images of ZnO samples prepared under different conditions: (a) C= 0.1 M, pH = 9, T = 150 °C, and t = 1.5 h; (b) C= 0.5 M, pH = 9, T = 150 °C, and t = 1.5 h; (c) C= 0.3 M, pH = 11, T = 150 °C, and t = 1.5 h; (d) C = 0.3M, pH = 9, T = 50 °C, and t = 1.5 h; (e) C = 0.3 M, pH = 9, T = 150 °C, and t = 1.5 h; (f) C = 0.3 M, pH = 7, T = 150 °C, and t = 1.5 h. (g) TEM images of ZnO sample prepared under the conditions of C = 0.3 M, pH = 9, T = 150 °C, and t = 1.5 h; (h) TEM images of ZnO sample prepared under the conditions of C = 0.3 M, pH = 9, T = 150 °C, and t = 1.5 h.

Figure 3

ZnO particle size distribution obtained from dynamic light scattering (DLS) for samples 1, 7, 8, 9, 15, 21, and 21 (a), samples 3, 4, and 22–25 (b), samples 5, 7, and 30 (c), samples 8 and 14 (d). Details of each sample synthesis conditions can be found in Table 2.

Figure 4

Model adequacy tests for D_h response: Normal probability of the externally studentized residuals (a), Predicted versus externally studentized residuals (b).

Figure 5

Model adequacy tests for pH_ZC response: Normal probability of the externally studentized residuals (a), Predicted versus externally studentized residuals (b).

Figure 6

(a) Removal efficiencies (R) and (b) adsorption capacities (q) of MO and MB by adsorption on the 34 prepared ZnO samples.

Figure 7

Model adequacy tests for R_MO response: Normal probability of the externally studentized residuals (a), Predicted versus externally studentized residuals (b). Model adequacy tests for R_MB response: Normal probability of the externally studentized residuals (c), Predicted versus externally studentized residuals (d).