Anodic Oxidation of Phenol by Mixed-Metal Oxide Electrodes: Identification of Transformation By-Products and Toxicity Assessment

Abstract

This study examined the oxidation of phenol by electrochemical processes using different mixed-metal oxide (MMO) anodes, namely, Ti/Pt, Ti/IrO 2 -Ta 2 O 5 , Ti/Ta 2 O 5 -SnO 2 -IrO 2 , Ti/RuO 2 and Ti/IrO 2 -RuO 2 . First, the effectiveness of different anodes by phenol oxidation was evaluated with the help of such parameters as cyclic voltammetry (CV), current efficiencies and degradation rates. The Ti/Pt anode was found to be the most effective of all types of anodes, as almost complete phenol degradation was gained with its use. Then, optimum operational conditions, applied current density and initial pH, were determined using Ti/Pt as the anode. By-products of phenol degradation (benzoquinone, hydroquinone, pyrocatechol, oxalic and maleic acid) arising gradually during the reaction were determined via high-performance liquid chromatography (HPLC) for all selected anodes. There was a considerable difference between the anodes with regard to degradation efficiency and concentrations of formed by-products. The toxicity was evaluated by measuring the natural luminescence of the bacteria, Vibrio fischeri, for all selected anodes. According to the toxicity assessment, a number of toxic by-products were formed from phenol degradation for all selected anodes. However, their toxicity decreased at the end of electrolysis on the Ti/Pt anode.

Combined Analytical Study on Chemical Transformations and Detoxification of Model Phenolic Pollutants during Various Advanced Oxidation Treatment Processes

“…In contrast, an MMO anode provided only less than 10% removal ( Figure 2 ). Data are comparable to the literature [ 17 , 40 , 44 ].…”

“…On the other hand, slower EO in Na 2 SO 4 provided more acceptable chemical transformations. Non-chlorinated, hydroxylated and/or highly oxidized aromatics were preferentially formed ( Table 2 ), such as hydroquinone, catechol, p- benzoquinone, and organic acids [ 40 , 44 ], as well as some coupling products. On the long run, aromatic ring might have been opened.…”

“…In addition, probably relatively greater amount of p- benzoquinone was generated by MMO than by BDD, as estimated from peak areas in HPLC–DAD. Hydroquinone, catechol, p- benzoquinone, and organic acids (e.g., fumaric, oxalic, maleic) formation were in majority identified by target analysis with HPLC–UV also in the literature [ 17 , 40 , 42 , 44 ].…”

“…One such opportunity has been seen in electrochemical oxidation (EO), by which degradation is achieved at mild conditions during electrolysis [ 7 , 13 ]. Lab-scale optimizations of parameters—using boron-doped diamond [ 40 , 41 , 42 ] or metal-oxide [ 41 , 42 , 43 , 44 ] anodes in different supporting electrolytes—to reach optimal phenol removal efficiency versus energy consumption are being widely reported. Meanwhile, monitoring the evolution and toxicity of formed TPs is highly disregarded.…”

“…Meanwhile, monitoring the evolution and toxicity of formed TPs is highly disregarded. Significant research on this matter (PHN only) has been, for example, performed by Jiang et al [ 40 ], Barışçı et al [ 44 ], Amado-Piña et al [ 17 ], and Xing et al [ 41 ], again mainly using HPLC–UV.…”