Electrochemical oxidation of phenol in simulated petrochemical wastewaters using boron-doped diamond electrodes: Investigation of effect of ammonia, carbon nitride and sulfur presence

Abstract

Presence of high concentrations of inorganic pollutants in phenolic wastewater effluents from refinery and petrochemical industries renders improper discharge of such wastewaters liable to leading to detrimental environmental consequences and also posing greater challenge for effective phenol oxidation and overall decontamination process. This dissertation thoroughly addressed such issues considering electrochemical oxidation of phenol in simulated petrochemical wastewaters containing NH4+, CN− and S2− in different single, binary, ternary and quaternary mix matrixes using Boron Doped Diamond (BDD) anodes. Statistical experimental design was employed to model and optimize the experimental operating conditions through Response Surface Methodology (RSM) modeling technique. Phenol, TOC and COD decay profiles excellently fitted experimentally verified second order quadratic models having all investigated influencing parameters statistically significant. The overall decontamination process was mainly kinetically controlled process dominated by heterogeneous and irreversible direct oxidation reactions on the BDD anode surface mainly mediated by electro-generated OH* with mass transfer effects as the rate limiting step. At pH < pKa> the pKa and low phenol concentration, accumulation of intermediary byproducts and polymerization hindered phenol oxidation. Erroneous predictions of phenol oxidation kinetics from existing COD evolution based models due to presence of the inorganic species were corrected with modified models based on TOC decay data. With the exception of NH 4+ ions, the degradation kinetics for phenol as well as the inorganic species in the different mix matrixes were consistently pseudo-first order kinetics with the kinetic and hydrodynamic constants for phenol, TOC and COD been reduced mainly as result of decline in the BDD anodes' activities. Comprehensive analyses of oxidation byproducts suggested that the mechanism for phenol oxidation was primarily initiated by speciation of the phenol molecules that led to the generation of aromatic intermediates via hydroxylation which were further oxidized and converted to short chain aliphatic acids before finally converting to CO2. Presence of the inorganic species didn't hamper the phenol oxidation ability of the BDD anode, though reaching steady state of complete decontamination was significantly delayed according to the inorganic species presence in solution.