Azo dye removal using photochemical advanced oxidation processes and photocatalysts synthesized by sol-gel method

Advanced oxidation processes (AOP) are used as an effective option for the removal of reactive azo dyes used in the textile industry, especially due to their high solubility and low degradability. In the first part of the study, the dye removal efficiency of UV-based homogeneous advanced oxidation processes, ultraviolet (UV) irradiation, UV/hydrogen peroxide (UV/H2O2), UV/persulfate (UV/S2O8-2) and UV/peroxymonosulfate (UV/HSO5-) processes effect was investigated. The effects of oxidant concentration, initial solution pH, initial RO122 concentration and solution temperature on RO122 removal in UV/oxidant processes were investigated. 19.5% RO122 removal was achieved after 120 minutes under UV irradiation (254 nm) only. RO122 removal decreased at pH 9 in UV/oxidant processes. Experimental results revealed that RO122 removal followed pseudo-first-order kinetics well. It was determined that there was a linear relationship between the initial oxidant concentration and the pseudo-first order kinetic rate constant (k1). For the three UV/oxidant-based processes, the oxidant concentration was 50 mg/L, the RO122 removal efficiency at 20°C and pH 5 in the order UV/H2O2 > UV/HSO5- > UV/S2O8-2. At any specific oxidant concentration, the UV/H2O2 process had higher RO122 removal than UV/S2O82- and UV/HSO5-. While the UV/H2O2 process required 90 minutes reaction duration at 50 mg/L H2O2 concentration to provide 95% RO122 removal, it required 60 minutes, 45 minutes and 30 minutes reaction duration at 100, 150 and 200 mg/L H2O2 concentrations, respectively. RO122 removal rate increased with increasing initial oxidant concentration and temperature, and decreased with increasing initial RO122 concentration. In the second part, ZnO produced by sol-gel method and Zr doped ZnO (10% Zr/ZnO, 5% Zr/ZnO, 2.5% Zr/ZnO, 1.25% Zr/ZnO) photocatalysts were prepared and photocatalytic removals of RO122 were investigated. In the removal study using 25 mg/L RO122 concentration and 0.6 g photocatalyst, 10% Zr doped ZnO photocatalyst was determined as the photocatalyst with the highest removal rate.