Studies On Electrochemical Treatment Application Of Mixed Metal Anodes For The Removal Of Reactive Black-5 (RB-5) Dye From Aqueous Solution

Abstract

The present study is mainly related to the removal of selected dyes from synthetic wastewater which was further extended for the treatment of actual industrial wastewater. For electrochemical treatment of wastewater, a critical task was performed to develop mixed metal oxide coated electrodes with a high activity for oxidation of pollutants, longer service life and lower cost. As a part of the electrode development process, various DSAs were prepared by standard thermal decomposition (STD) and polymeric precursor thermal decomposition (PPTD) methods using Ti substrates coated with mixture of transition and noble metal oxides. For comparison of preparation methods, few electrodes were prepared by plasma and electroplating techniques. Screening study of about thirty electrodes carried out for electrolysis of selected dye compound (RB-5) proved that mixed metal oxide coated anodes performed better compared to electrodes coated with single, binary or ternary metal oxides. It was also observed that presence of NaCl in acidic pH of the electrolyte was the most favourable condition for removal of dyes from synthetic wastewater compared to neutral and alkaline pH. Based on screening study, pairs of electrodes containing Ru-Sn-Sb-Ni, Ru-Sn-Sb-Ir, Ru-Sn-Sb-Ta and Ru-Sn-Sb-Pt were prepared by two different thermal decomposition methods. Indigenously prepared electrodes were further applied on synthetic wastewaters containing 100 and 780 mg/L RB-5 dye compounds for colour, COD and TOC reduction. Additionally, the performance of prepared electrodes was compared with commercially proven Nb/BDD and Si/BDD anodes with respect to instantaneous current efficiency, energy consumption, mineralization current efficiency, mass transfer co-efficient and kinetic rate constants calculated for batch experiments. The results of electro-oxidation and accelerated life test showed that although removal efficiency obtained for electrodes prepared by PPTD route was higher than that of electrodes prepared by STD route, the service life was found much higher for electrodes prepared by STD route. It was further observed that due to different mechanism for electro-oxidation of dye followed by BDD electrodes, oxidation efficiency was highest for BDD anodes followed by Ti/Pt-RuO2-SnO2-Sb2O5 anode of STD route compared to rest of the anodes employed for the studies. Addition to batch process, suitability of three tanks in series model was tested as a continuous process using all mixed metal oxide coated anodes as well as BDDs for removal of RB-5. In these continuous processes, the performance of anodes was measured in terms of COD, colour and TOC, as was done for batch process. All the parameters calculated for batch process was again calculated for continuous process and some of them were compared with results of batch process. In this comparison, continuous process was found efficient for higher dye removal with lower energy requirement. Among all, an electrode prepared form the mixture of Pt-Ru-Sn-Sb was proved the most efficient electrode for removal of dye and again tested at variable flow rates in continuous process. The final selection of optimum electrode was made considering removal efficiency of dye in batch process, the accelerated service life, preparation cost and energy consumption in batch process. The proved Ti/Pt-RuO2-SnO2-Sb2O5 as an optimum electrode was applied for the treatment of actual industrial wastewater containing phenol and ammonia compounds. In industrial wastewater treatment, 97, 98 and 44% colour, COD and TOC removal was obtained on Ti/Pt-RuO2-SnO2-Sb2O5 anode in batch reactor. Finally, the performance of all electrodes was explained and compared based on the results obtained after characterization performed for scanning electron microscope (SEM), X-ray energy dispersive spectrometry (EDX), X-ray diffraction (XRD) and cyclic Voltammetry (CV) techniques. A clear morphological difference was observed in the results of SEM analysis of anodes prepared by STD and PPTD routs. The plate type structure of oxides formed on Ti substrate proved more efficient for removal of dye on PPTD electrodes compared to mud type morphology of STD anodes due to prevention of loss of Sn in PPTD coatings. The higher Sn content in all PPTD coating as observed from EDX analysis showed higher O2 evolution potential in CV analysis. But, due to higher crystalline form of oxide layers, lower Sn and higher Ru metal compositions found in EDX results, higher service life was observed for all electrodes prepared by STD route compared to electrodes prepared by PPTD route.