Removal of Procion Red MX-5B dye from wastewater by conductive-diamond electrochemical oxidation

Abstract

In this work, the removal of Procion Red MX-5B dye by electrochemical oxidation with boron doped diamond (BDD) anodes was investigated. The impact of current density, flow rate, initial pH, and supporting electrolyte was evaluated on dye and organic matter removal. Furthermore, the use of dimensionally stable anodes (DSA) was tested to evaluate process performance. Results show that after 240 min, it is possible to achieve full dye and COD (Chemical Oxygen Demand) removal, regardless of applied current density. This is due to the generation of powerful oxidants – i.e. hydroxyl radicals and peroxodisulfate–, which attack the organic matter in the wastewater, promoting its complete degradation. However, process efficiency increases when using lower current densities (10 mA cm−2): electric charges of about 5 Ah dm−3 are sufficient to fully remove both dye and COD, while charges higher than 15 Ah dm−3 are required when working at higher current densities (>30 mA cm−2). This fact is related to the production of large amounts of hydroxyl radicals, which are wasted in other reactions at higher current densities. On the other hand, higher flow rates (300 dm3 h−1) promote Procion Red MX-5B and organic matter degradation, due to improved mass transfer within the system. Regarding the impact of initial pH on dye removal, no significant differences were observed. Conversely, COD is clearly affected by this parameter: it is only possible to fully remove the organic matter when working at natural pH.Finally, with DSA anodes, higher dye removal efficiencies are attained than with BDD electrodes, when 100 mg dm−3 chlorides are added to the supporting electrolyte. Likewise, higher chloride concentration (100–1000 mg dm−3) was observed to enhance process efficiency when using DSA as anode material. However, during electrolysis with both BDD and DSA, chloride ions in the supporting electrolyte promote the production of intermediate organochlorinated compounds. Therefore, under these conditions, no full organic matter removal can be achieved, regardless of the anode material employed.