Photocatalytic effect of zinc oxide and magnetite entrapped calcium alginate beads for azo dye and hexavalent chromium removal from solutions

In this study, the semiconductor powder zinc oxide (ZnO) and magnetite (Fe3O4) powder were either separately or together entrapped in calcium alginate (Ca-Alg) beads to form hybrid photocatalyst. The synergic effect of hybrid ZnO/Fe3O4/Ca-Alg beads was investigated for both photocatalytic oxidation of azo dye Reactive Red 180 (RR 180) and photoreduction of hexavalent chromium (Cr(VI)) in aqueous solutions. The effect of ZnO and Fe3O4 powder amount in Ca-Alg beads, bead loading, and solution pH were systematically evaluated. A maximum (100%) RR180 decolorization (25 mg/L initial dye concentration) and 15.5% Cr(VI) photoreduction (10 mg/L initial Cr(VI) concentration) were obtained by ZnO/Ca-Alg beads (with 15% ZnO powder in Ca-Alg) at 20 g/L bead loading and pH:7.0 in 180 min. However, at the same initial dye and Cr(VI) concentration, 55.2% RR 180 decolorization and 85.4% Cr(VI) reduction were attained by Fe3O4/Ca-Alg beads (with 15% powder in Ca-Alg) at 20 g/L bead loading and pH:4.0 in 180 min. Meanwhile using ZnO/Fe3O4/Ca-Alg beads (with 15% each powder in Ca-Alg) at 20 g/L bead loading; 100% RR180 decolorization at pH:10 in 60 min and 87.2% Cr (VI) photoreduction at pH:4.0 in 180 min were achieved. Hybrid ZnO/Fe3O4/Ca-Alg beads reusability was also tested during 10 cycles and hybrid beads showed good stability for removal of azo dye. Manufactured beads were characterized by Scanning Electron Microscopy/Energy-Dispersive X-Ray (SEM/EDX), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction (XRD).