Kinetics of the electrochemical mineralization of perfluorooctanoic acid on ultrananocrystalline boron doped conductive diamond electrodes

This work deals with the electrochemical degradation and mineralization of perfluorooctanoic acid (PFOA). Model aqueous solutions of PFOA (100 mg/L) were electro-oxidized under galvanostatic conditions in a flow-by undivided cell provided with a tungsten cathode and an anode formed by a commercial ultrananocrystalline boron doped diamond (BDD) coating on a niobium substrate. A systematic experimental study was conducted in order to analyze the influence of the following operation variables: (i) the supporting electrolyte, NaClO4 (1.4 and 8.4 g/L) and Na2SO4 (5 g/L); (ii) the applied current density, japp, in the range 50–200 A/m2 and (iii) the hydrodynamic conditions, in terms of flowrate in the range 0.4 × 10−4–1.7 × 10−4 m3/s and temperature in the range 293–313 K. After 6 h of treatment and at japp 200 A/m2, PFOA removal was higher than 93% and the mineralization ratio, obtained from the decrease of the total organic carbon (TOC) was 95%. The electrochemical generation of hydroxyl radicals in the supporting electrolyte was experimentally measured based on their reaction with dimethyl sulfoxide. The enhanced formation of hydroxyl radicals at higher japp   was related to the faster kinetics of PFOA removal. The fitting of experimental data to the proposed kinetic model provided the first order rate constants of PFOA degradation, kc1 that moved from 2.06 × 10−4 to 15.58 × 10−4 s−1, when japp varied from 50 to 200 A/m2.