Electrochemical oxidation of ionic liquids at highly boron doped diamond electrodes

Abstract

An electrochemical oxidation using a highly boron-doped diamond (BDD) electrode has been tested for the treatment of solutions containing ionic liquids (ILs). The double-sided Si/highly BDD electrodes were synthesized by microwave plasma enhanced chemical vapour deposition (MW PE CVD). Investigation of the electrode surface with scanning electron microscopy (SEM) confirmed that the synthesized layers were continuous and formed densely packed grain structure. The structure of BDD was confirmed by Raman spectra analysis. The effect of IL structure as the kind of electrolyte (Na2SO4 and NaCl) has been investigated. Electrolyses were conducted in an undivided electrolytic cell under galvanostatic conditions. The efficiency of the process has been evaluated in terms of variations of IL concentrations and chemical oxygen demand (COD) removal. Results show that pyridinium ILs were easier removed than imidazolium salts. The intermediates of electrochemical degradation of 1-butyl-3-methylimidazolium salt were detected, and IL degradation pathway was proposed based on the analytical results. It was suggested that center dot OH radicals produced by water electrolysis attacked IL to form its derivatives with keto groups substituted to imidazolium ring. These compounds underwent ring breakage, which led to the formation of aliphatic acids that were eventually mineralized by electrolysis to CO2. Other by-products were obtained by cutting one of the side chains substituted to N atoms in imidazolium ring. Additionally in NaCl electrolyte chloroorganic by-products were identified.