Boron doped diamond (BDD) electrode has been widely used in wastewater treatment, due to its wide potential windows, low background current, high stability and corrosion resistance [1]. Typical BDD electrode is always referred to microcrystalline diamond, which is mainly composed of sp3 carbon and has little sp2 carbon in the film [2]. Thus the electrochemical oxidation ability of this type of BDD electrode can be attributed to the effect of sp3 carbon. When the diamond grain size reduces to the degree of nanometer, its characteristics will certainly change. Nanocrystalline diamond film can be divided into two parts, named grain and grain boundary, grain is made of sp3 diamond, while grain boundary is mainly consist of sp2 carbon, including graphite, hydrocarbon and amorphous carbon et al [3]. The unique structure of the nanocrystalline diamond film makes it has many excellent properties, including high hardness, low surface roughness, high thermal conductivity and electron emission et al. Recently, many researchers try to focus on the study of the role of sp3/sp2 ratio on the influence of electrochemical oxidation efficiency. And they do have some discoveries. Araújo et al. studied the influence of sp3/sp2 ratio on the performance of electrochemical oxidation of Rhodamine B [4]. They found that higher diamond content in the film favored the greater TOC and COD removal rates as well as the oxidation of organic compounds to CO2. Souza et al. studied the effect of the sp3/sp2 ratio on the electrochemical oxidation of 2,4-dichlorophenoxyacetic acid [5]. They found that higher sp3/sp2 ratio would lead to a more rapid and efficient oxidation. Besides, they also found that with the increase of sp3 content in the film, the maximum concentration of chlorinated compounds would decrease. It has shown that sp2 content in the diamond film has significant influence on the performance of electrochemical oxidation. Thus, in this paper, comparative study of electrochemical oxidation of artificial humidity condensate and artificial human urine using boron doped nanocrystalline diamond electrode (BDNCD) and boron doped microcrystalline diamond electrode (BDMCD) respectively was carried out. TOC removal efficiency as well as the variation of the organic and inorganic compounds are investigated. Finally, the accelerated lifetime test was measured to comparison the lifetime of both electrodes.

The results showed that the degradation rates were in good agreement with the pseudo first order kinetic model, with the degradation rate constant of 1.33h-1 and 1.02h-1 for BDMCD and BDNCD anode respectively. The degradation rate in BDMCD anode was 1.3 times faster than BDNCD anode, this was mainly due to different carbon phases in BDMCD and BDNCD electrode. BDMCD film was mostly composed of sp3 carbon, the concentration of sp2 carbon was negligible, while BDNCD film had a relatively large amount of sp2 carbon in grain boundary together with sp3 carbon in grain. It was found that NO2- and NO3- were the main intermediate products, and NO3- was the final product for two kinds of electrodes. Compared with BDMCD system, the oxidation of nitrogen compounds also show a lagging behind. In summary, both electrode had the same mineralization efficiency in the enough treatment time, but BDMCD anode has a higher degradation rate. That means high sp³ content in the film favors high degradation rate. The comparative study of the accelerated lifetime test of both electrodes showed that the lifetime of BDNCD electrode was 32.8% higher than BDMCD electrode. So it means high sp² content in the film will lead to a higher lifetime of diamond electrode due the low surface roughness of electrode.

Reference

[1] Panizza M, Cerisola G., Journal of Electroanalytical Chemistry, 2010, 638(1): 28–32.

[2] Guo L, Chen G., Diamond and Related Materials, 2007, 16(8): 1530–1540.

[3] Williams O A., Diamond and Related Materials, 2011, 20(5-6): 621–640

[4] De Araújo D M, Cañizares P, Martínez-Huitle C A, et al., Electrochemistry Communications, 2014, 47: 37–40.

[5] Souza F L, Saéz C, Lanza M R V, et al., Electrochimica Acta, 2016, 187: 119–124.