Photocatalytic degradation of aqueous methylene blue using ca-alginate supported ZnO nanoparticles: point of zero charge role in adsorption and photodegradation

Abstract

A novel insoluble Ca-Alginate created from soluble Na-Alginate was used as a support substrate for ZnO nanoparticles producing ZnO@Ca-Alginate composite photocatalyst. Fourier Transform Infrared (FT-IR), Ultraviolet-Visible (UV-Vis), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDX), and X-ray Diffraction (XRD) analysis techniques were used in the characterization of the prepared ZnO@Ca-Alginate. The ZnO@Ca-Alginate was tested for its potential use in the photodegradation of Methylene Blue (MB) from an aqueous solution under solar-simulated light. This composite photocatalyst efficiency in MB removal was compared with naked ZnO potential considering different conditions and parameters (e.g. pH, MB concentration, amount of photocatalyst, and irradiation time). The MB concentrations were identified using UV-vis spectrophotometric methods. While, high-performance liquid chromatography (HPLC), Total organic carbon (TOC) analysis, and other elemental analyses were used to confirm the MB complete mineralization. The MB photodegradation results were performed by using UV-vis analysis., the results showed that up to 95% of MB (40 mL, 40 ppm) was removed within 30 min of irradiation using either ZnO@Ca-Alginate or naked ZnO. The pH and the zero-charge point (Pzc) values play a main role in the adsorption and photodegradation results. The Pzc values for Ca-Alginate, ZnO, and Zn@Ca-Alginate were 6.5, 8.8, and 6.8 respectively. The prepared composite catalyst showed a maximum adsorption and photodegradation in a basic to slightly basic medium, the MB completely removed at pH of 7.7 within an hour of irradiation. The complete miniralzation of MB at the end of the photodegrdation process was confirmed. Here it is proved that the ZnO@Ca-Alginate photocatalyst can be recovered and reused without any significant decrease in its effectiveness.

Keywords: Organic contaminants; Photodegradation; Supported photocatalyst; Surface charge; Water purification.