Studies on Electrochemical Treatment of Reactive Black-5 Dye and Urea Solutions using Indigenously Developed and Commercial Electrodes

Abstract

The present study aims at removal of organic model pollutants – Reactive Black-5 (RB- 5) dye and urea from aqueous solutions using advanced electrochemical oxidation technology. In this context, mixed metal oxide (MMO) electrodes, popularly known as dimensionally stable anodes (DSA), were indigenously developed using the standard thermal decomposition technique in three phases by involving catalytic coatings comprising of single to quinary metal oxide combinations over Ti substrate. These anodes were prepared specifically catering to selective catalytic organics oxidation, high service life and stability, efficient process economics in terms of energy efficacy as well as affordability in view of practical implementation. Initial studies were directed towards deciding favourable operating parameters affecting electrooxidation like pH, current density and electrolyte concentration employing Taguchi L27 array design and its analysis via MINITAB 17 statistical software. Extended electrooxidation of synthetic solutions containing 1000 mg/L RB-5 dye and 300 mg/L urea was independently conducted under acidic pH and using NaCl electrolyte in both batch and continuous reactor operation modes. Amongst all DSAs tested for RB-5, quinary-DSAs performed best in terms of higher COD, TOC and colour abatements. Hence, quinary-DSAs were further adopted for investigating batch electrooxidation of urea solutions in terms of TN abatement. The electrochemical treatment performance of quinary-DSAs was further compared with that of commercial Boron Doped Diamond (Si/BDD) electrodes with two different boron doping levels of 100 ppm and 10000 ppm boron (B) respectively. Further, using the acquired data on COD, TOC, colour and TN removal, overall treatment efficacy of various anodes was also assessed in terms of associated electrochemical process parameters such as current efficiency (CE), mineralization current efficiency (MCE), specific energy consumption, electrochemical oxidation index (EOI), kinetics of pollutant degradation as well as mass transfer coefficients (MTC). In case of RB-5, the results of batch studies revealed highest oxidation efficiency exhibited by BDD anodes followed by Ti/Ta2O5-RuO2-B2O3-SnO2-Sb2O5 electrode amongst the quinary-DSAs employed. DSA and BDD anodes follow different oxidation pathways and hence the oxidation via hydroxyl radicals (OH) as well as electrogenerated active chlorine species resulted in enhanced COD and TOC elimination at BDD electrodes. Almost complete decolourization for RB-5 with no sludge generation was achieved with all tested anodes. In case of urea, TN removal efficiency observed using both DSA and BDD electrodes was almost equal of the order of 97%, rendering DSAs better suited for urea elimination compared to BDD anodes. Based on the results of batch studies, selected quinary-DSAs were subjected to accelerated service life tests in extreme environment (0.5 M NaCl at 1000 mA/cm2) in order to predict their probable lifetime. Selected quinary-DSAs and BDD electrodes were further characterized in terms of SEM, EDAX, XRD and CV in order to understand the electrochemical interactions between the model pollutants and the electrode surface. Selected treated effluent samples were analysed for mass fragments using LC-MS technique. Based on their batch performance in RB-5 and urea removal, selected electrodes were tested in continuous mode using three-tanks-in-series model. This arrangement confirmed an obvious improvement in oxidation efficiency for RB-5 specifically compared to its batch results. Supported by these observations, an attempt was made to develop decision matrix for selecting optimum electrode for both RB-5 and urea electrooxidation by considering various important electrode attributes. In order to consider the practical applicability, studies on electrooxidation of mixed solution of RB- 5 and urea as well as three real industry effluents using Ti/Ta2O5-RuO2-B2O3-SnO2-Sb2O5 as optimum anode amongst tested DSAs were also undertaken and interpreted.