Photodegradation of Methylene Blue and Methyl Orange using Metal Oxide Photocatalysts

Abstract

Dyes are important in textile, food and cosmetic industry. Unfortunately, some dyes are highly toxic even in the presence of a tiny amount. Photocatalyst is a catalyst that drives chemical reactions under light irradiation and produces a strong oxidizing agent to treat the dye. There are several factors that affect the rate of degradation of dye such as the pH, type and amount of photocatalyst and concentration of dye. Out of all metal oxides used in this study, nickel ferrite (NiFe₂O₄) and zinc ferrite (ZnFe₂O₄) are catalytically inert while zinc oxide (ZnO) and titanium dioxide (TiO₂) are photocatalytically active and thus used in the subsequent studies. Photodegradation of methylene blue (MB) and methyl orange (MO) using ZnO and TiO₂ was taken at 90 minutes to show the effect of pH. At pH 5, TiO₂ and ZnO showed 67% and 97%, and at pH 9, both photocatalyst have 97% photodegradation rate of MB, respectively. For photodegradation of MO, at pH 5 both photocatalysts have similar photodegradation rate. At pH 9, TiO₂ and ZnO has a 78% and 95% photodegradation rate of MO. The rate of was found to increase proportionally with the amount of photocatalyst used but at high concentration, the degradation rate was saturated. The optimized condition for photodegradation of MB is at pH 9 and MO at pH 5 using 30 mg photocatalysts (ZnO or TiO₂) owing to the cationic and anionic character of the dyes. The crystal structure of TiO₂ and ZnO are tetragonal anatase and hexagonal wurtzite, respectively, as verified by XRD. FESEM images revealed that ZnO consists of nanoparticles and nanoplatelets with average size of 45.5 ± 14.1 nm and TiO₂ shows average size of 42.5 ± 10.5 nm. The band gap of TiO₂ and ZnO are 3.46 and 3.26 eV, respectively, as calculated from the UV-visible diffuse reflectance spectroscopy (UV-vis DRS). The X-ray photoelectron spectra for TiO₂ confirm the presence of Ti⁴⁺ and O^2- states.