Slurry and Fixed Bed Photo Catalytic Degradation of Herbicide 4-Chloro Phenoxy Acetic Acid Using Doped/Undoped Catalyst

Abstract

The introduction of chlorinated aromatic compounds as herbicides into the environment is a matter of great concern due to their toxicity. The chemical resistance of these compounds makes their removal from contaminated waters a difficult task. In this regard, photo catalytic reactions provide an auspicious solution to this problem. The present study focuses on the degradation of a herbicide 4-chlorophenxyacetic acid in a shallow pond slurry reactor through heterogeneous photocatalysis using TiO2 as a photo catalyst in suspended as well as supported form. The influence of various parameters like dose of photo catalyst, oxidant, A/V ratio of the reactor, pH, and UV intensity have been studied under UV-A (365 nm) irradiations. The degradation rate was found to follow the pseudo first order rate kinetics. The optimized conditions for better degradation in case of slurry photocatalysis were TiO2 dose 0.625 g L-1, oxidant dose 0.125mL per 200mL, UV intensity 25 Wm-2. All experiments were conducted at solution’s natural pH i.e. 3.5. For fixed-bed studies, TiO2 coated clay beads were used and 92% degradation of 4-CPA was achieved after 3 h of treatment using these catalyst immobilized beads. A significant amount of degradation was achieved when experiments were performed in natural solar light for the degradation of concerned pollutant under optimized conditions. Efforts were also made to dope the catalyst with certain metals like iron and copper. The degradation rate for 4-CPA was fastened and enhanced when doped catalyst was used as around 94% degradation occurred while 85% degradation was attained in case of un-doped catalyst after three hours of photo catalytic treatment. The mineralization of 4-CPA was confirmed by reduction in COD (85.7%) along with the generation of chloride ions (75%), after 2 h of irradiation as well as elimination of the parent compound peak in HPLC chromatograms . Scale–up of the studied photo-oxidation process can lead to its feasibility at pilot-scale in near future for the treatment and removal of recalcitrant compounds like 4-chlorophenxyacetic acid present in water bodies.