A review on effect of operational parameters for the degradation of azo dyes by some advanced oxidation processes

Azo dyes are widely used in industries such as textiles and paper, but their complex molecular structures make them resistant to conventional wastewater treatments. Advanced oxidation processes (AOPs) have emerged as efficient alternatives by generating highly reactive radicals for effective degradation. These processes are based on the generation of highly reactive oxidative species, such as hydroxyl radicals and sulphate radicals, for the destruction of the toxic organic pollutants into carbon dioxide, water, and inorganic salts. This study reviews AOPs such as Fenton and Fenton-like processes, UV photolysis, and photo-Fenton methods for azo dye degradation. The Fenton process (${\mathrm{Fe}}^{2+}/H_2{O}_2$) produces hydroxyl radicals at an optimal pH of ∼3, while the Fenton-type process (${\mathrm{Fe}}^{2+}$/$S_2{O}_8^{2-}$) generates sulfate radicals with higher oxidation potential (2.5–3.1 V). UV-assisted processes enhance oxidation by accelerating radical formation. Photo-Fenton and photo-Fenton-type processes integrate UV light to improve ${\mathrm{Fe}}^{2+}$ regeneration, minimizing sludge formation and increasing efficiency. The efficiency of this process does not depend on a single parameter but on numerous parameters such as the concentration of the oxidant, catalyst doses, pH, treatment period, etc., to attain maximum removal of contaminants. Therefore, it becomes very challenging for the researchers to design an effective and efficient process under suitable operational parameters for the treatment processes. Excess oxidants cause radical scavenging, while inorganic anions (${\mathrm{Cl}}^{-}$, ${\mathrm{CO}}_3^{2-}$, ${\mathrm{HCO}}_3^{-}$ and ${\mathrm{PO}}_4^{3-}$) can inhibit degradation. Temperature (∼25–30°C) affects reaction rates, while excess ${\mathrm{Fe}}^{2+}$ can lead to sludge formation. Given their economic feasibility and environmental sustainability, AOPs offer a promising solution for mitigating dye pollution and improving wastewater treatment.