TREATMENT OF CHLOROPHENOLS IN INDUSTRIAL EFFLUENTS USING ADVANCED OXIDATION PROCESSES (AOPs)

Abstract

Chlorophenols are the ubiquitous compounds representing one of the most abundant families of toxic pollutants emerging from various industrial manufacturing units. The toxicity of these chloroderivatives is proportional to the number and position of chlorine atoms on the benzene ring. In the aquatic environment, these pollutants accumulate in the tissues of living organisms, primarily in fishes, inducing mortality at an early embryonic stage. Contemplating the behaviour of such xenobiotics and their prevalence in different environmental components, it is crucial to remove/degrade the chlorophenol from contaminated environment. Advanced Oxidation Processes (AOPs) are effective in degrading such organics with enhanced rate and efficiency. Different processes such as sonication, photocatalysis, and Fenton’s treatment are discussed for the degradation of chlorophenols (CPs). Furthermore, a statistical approach, Response Surface Methodology was employed to validate the optimum conditions of regulating parameters of different processes for maximum contaminant removal. The degradation of 4-CP using photocatalysis under optimised conditions (Nano-TiO2= 0.1g/l; H2O2=10.0mM; pH= 5.0) resulted in complete (~98%) removal in 3.5 hours whereas, in the case of Photo-Fenton’s process, at optimised condition (pH= 3.0; Fe2+= 0.7mM; H2O2= 7.0mM), 100% removal was achieved in 6 minutes. ANOVA results displayed high regression and fitting values between experimentally observed data against RSM-predicted values. The removal efficiency of different integrated processes employed towards the degradation of 4-CP reported the Fenton’s and Fenton-integrated processes as more efficient ones towards effective removal of 4-CP. In the case of 2,4-DCP, Complete degradation of 2,4-DCP was reported using AOPs- Photocatalysis (in 210 min) and Photo-Fenton (in 5 min) treatment. Under optimised conditions, with a photocatalyst dose of 0.2 g/L, oxidant concentration of 10.0 mM and pH 5.0, complete removal of 2,4-dichlorophenol (2,4-DCP) was observed in 210 minutes in photocatalytic treatment. In the case of the photo-Fenton process, at an H2O2 dose of 5.0 mM and Fe2+ concentration of 0.5 mM, the organic pollutant was eliminated within 5 minutes of reaction time under acidic conditions (pH 3.0). The RSM model reported the perfect fit of experimental data with the predicted response. For the obtained optimised conditions, sonication v and solar energy-driven processes were incorporated to study enhanced mineralisation. The solar-assisted Fenton process was found to be the most efficient with the maximum degree of mineralisation (90%) of 2,4-DCP; and cost-efective ($0.01/litre for 100 mg/L 2,4-DCP) treatment among different hybrid oxidation processes. Photocatalytic degradation of 2,4,6-Trichlorophenol using nano-TiO2 exhibited higher potential towards the removal with nearly complete degradation within 3.5 h, as compared to analytical grade TiO2. Under the optimal dose of 250 mg/L of nano-TiO2, around 97% removal was observed. No effect of addition of oxidant was observed in case of removal of 2,4,6-TCP. In case of Photo-Fenton’s process, under optimized conditions (at pH= 3.0, Fe2+= 0.5 mM, and H2O2= 10.0 mM), complete degradation of TCP was attained in 6 minutes with 50% mineralisation. However, Solar-Fenton reported complete removal and nearly complete mineralisation (98%) of 2,4,6-TCP within 4 minutes. Degradation of Mixed-CPs under optimised conditions obtained for individual chlorophenols was further investigated. In photocatalysis degradation, rapid and complete removal of mixed-CPs was reported at pH 6.0, TiO2 dose of 0.25g/L, and H2O2 concentration of 10.0mM within 270 minutes. In the case of Photo-Fenton’s process, complete removal was observed in 12 minutes at pH 3.0, Fe(II) 0.5mM, and H2O2 10.0mM. Different integrated-AOPs employed towards removal of CPs at optimised conditions reported that Solar-derived processes exhibiting enhanced degradation rates. The work thus provides insight into harnessing the naturally available solar energy, reducing the overall treatment cost, and opting for a sustainable treatment method towards degradation of organics. Toxicity analysis of treated effluent using Eichhornia crassipes resulted in the death of the plant inferring the presence of such intermediary by-products which hamper the growth and metabolic activities of the plants. Toxicity analysis study suggested the invasive macrophyte is intolerant to oxidative stress conditions even when cultured in treated effluent thus suggesting the need for further treatment of the AOP-treated wastewater.