Adsorption Mechanisms, Kinetics and Photoactivities of Green Synthesized Hydroxyapatite Supported Zno and La-Zno Catalysts

Abstract

In this study, hydroxyapatite (HAP) is synthesized by a co-precipitation method from the waste eggshell, utilized as a support for ZnO (ZnO-HAP) and La-ZnO (La-ZnO-HAP) and employed to degrade methyl orange (MO) and methylene blue (MB) dyes under UV irradiation. Water vapor adsorption as relative humidity (84 % RH) on the HAP structure and the as-prepared catalysts are also examined. The characteristic ZnO and HAP reflections are detected in X-ray diffraction (XRD) analysis of ZnO-HAP, La-ZnO-HAP and humidified samples. ZnO and La-ZnO existence is also verified by scanning electron microscopy-energy dispersive X-ray (SEM-EDX) analysis, UV-Vis diffuse reflectance (UV-Vis DRS) spectra and X-ray photoelectron spectra (XPS). ZnO and La-ZnO loading on the HAP induce the formation of mesoporous structures with high surface areas. Dark adsorption capacities and photoactivities of the as-prepared samples are explored regarding electrostatic interactions, Lewis acid-base interactions and hydrogen bonding for both MO and MB. In particular, La-ZnO-HAP 500 degrees C and La-ZnO-HAP 500 degrees C (RH) exhibit improved adsorption abilities and photoactivities. The pseudo-second order model describes the kinetic behavior of all samples under dark conditions. Unhumidified samples follow Langmuir isotherm while Freundlich isotherm better fits humidified ones. Under irradiation, the Langmuir-Hinshelwood kinetic model describes the photoactivities of all samples. The four recycling tests confirm the stabilities of HAP 100 degrees C, HAP 500 degrees C, ZnO-HAP 500 degrees C and La-ZnO-HAP 500 degrees C. This study suggests that the ZnO or La-ZnO loaded HAP catalysts prepared in the presence and absence of humid conditions are considered promising materials for environmental remediation.