Electrooxidation of cardanol on mixed metal oxide (RuO2-TiO2 and IrO2-RuO2-TiO2) coated titanium anodes: insights into recalcitrant phenolic compounds

Abstract

In Brazil, the cashew-nut industry has an important role in national economy, but the lack of suitable treatment of the residues generated by this activity represents a serious risk to the environment and biodiversity. Cardanol (CDN) is a highly toxic and recalcitrant phenolic compound, and it is the major component from this activity. This work evaluated, for first time, the performance of mixed metal oxide DSA®-type anodes (Ti/TiO2-RuO2 and Ti/TiO2-RuO2-IrO2) to develop a suitable electrochemical treatment for the cashew-nut industry residues, using CDN as model molecule. Results clearly showed that for both Ti/TiO2-RuO2 and Ti/TiO2-RuO2-IrO2 active anodes, an increase on the current densities (j) enhanced the percentage of total organic carbon (TOC) removal as a function of the electrolysis time (t). The complete TOC removal was efficiently achieved with Ti/TiO2-RuO2 (j≥40mAcm−2 and t≥5.0h) at 25°C, although Ti/TiO2-RuO2-IrO2 ternary anode has also showed suitable performance for this purpose (j=60mAcm−2 and t≥4.0h). UV-visible spectrophotometric data showed similarity between the CDN electrooxidation kinetics for both anodes, independently of the applied current density. Analyses performed by gas chromatography coupled to mass spectrometry confirmed the complete mineralization of CDN and its intermediates, mainly at higher densities. A positive effect in the degradation performances was achieved when an increase on the temperature was attained, at both anodes; despite the satisfactory results achieved at 25°C, favor an affordable cost.