Comparison of Photocatalyticand Adsorption Propertiesof ZnS@ZnO, CdS@ZnO, and PbS@ZnO Nanocomposites to Select the BestMaterial for the Bifunctional Removal of Methylene Blue

In this study, photocatalytic- and adsorption-based removal processes were conducted, which are frequently preferred in wastewater treatment due to their ease of control and high removal efficiency. An innovative method aimed at wastewater treatment was developed by combining the advantages of these two distinct approaches within the same material. The study synthesized ZnO, ZnS, CdS, PbS, and their composite structures (ZnS@ZnO, CdS@ZnO, and PbS@ZnO) using a hydrothermal synthesis method. Characterization of the samples was performed through field emission-scanning electron microscopy (FE-SEM), FE-SEM-energy dispersive X-ray (FE-SEM-EDX), X-ray diffraction (XRD), Raman spectroscopy, and Fourier-transform infrared spectroscopy (FTIR) measurement. Additionally, the optical properties of all samples (absorption spectra and band gap) were investigated by using absorbance measurements obtained from ultraviolet (UV)–visible absorption spectroscopy. Although ZnO nanoparticles are among the materials with high photocatalytic properties (exhibiting a photodegradation efficiency of 95.8% in a short duration of 90 min), their adsorption properties are low. Therefore, with the aim of enhancing both the low adsorption values and the photocatalytic properties of pure metal sulfides (ZnS, CdS, PbS), nanocomposites ZnS@ZnO, CdS@ZnO, and PbS@ZnO with different morphologies were synthesized, and their photocatalytic and adsorption-based removal performances on methylene blue (MB) dye were investigated. FE-SEM images indicated that ZnS nanoparticles exhibit a spherical morphology, CdS nanoparticles have a flower-like morphology, and PbS nanoparticles display a dendritic-like structure. The results obtained from experimental studies demonstrated that the highest efficiency in both photocatalytic- and adsorption-based removal was achieved with the ZnS@ZnO nanocomposite. The degradation rates of MB were found to be 95.3, 90.5, and 89.4% for the heterojunction composites ZnS@ZnO, CdS@ZnO, and PbS@ZnO, respectively, over a time range of 0–480 min. The optimal amount of photocatalyst that could effectively degrade MB was determined to be 100 mg, and the reusability studies revealed that the ability of the ZnS@ZnO semiconductor heterojunction photocatalyst to decompose MB into simpler molecules was limited after the fourth cycle. The adsorption-based removal rates were 96.0, 30.5, and 19.4% for the heterojunction composites ZnS@ZnO, CdS@ZnO, and PbS@ZnO, respectively. Finally, parameters influencing the adsorption-based removal of MB, such as pH, mass, and contact time, were examined, indicating that the adsorption capacity of ZnS@ZnO remained unchanged after reaching a value of 40 mg·g^–1.