On the selection of the anode material for the electrochemical removal of methylparaben from different aqueous media

Abstract

Parabens are widely used industrial preservatives, routinely found in wastewater along with major inorganic ions like sulfate and chloride. This work investigates the oxidation ability of three electrochemical processes in tank reactors equipped with an air-diffusion cathode to electrogenerate H2O2 on site, namely electro-oxidation (EO-H2O2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF), to degrade aqueous solutions of methylparaben (MeP) at pH 3.0. Their performance using boron-doped diamond (BDD), Pt or two kinds of dimensionally stable anodes (DSA ® ) has been compared from the analysis of mineralization profiles and decay kinetics in the presence of sulfate and/or chloride ions. The use of BDD ensured the overall mineralization in all three processes according to the sequence: PEF > EF > EO-H2O2, thanks to the contribution of BDD( • OH), • OH and UVA light. Pt and DSA ® became an interesting alternative in PEF, with slower organic matter removal but similar final mineralization percentages, being much less powerful than BDD in EO-H2O2. The presence of Cl − was beneficial in the latter process, due to the formation of active chlorine as an additional oxidant that caused a much faster decay of MeP.Conversely, it became significantly detrimental in EF due to the partial destruction of H2O2 and• OH in the bulk by active chlorine and Cl − , respectively. The oxidation power of PEF was so high that similar fast, complex decay kinetics was found in all media regardless of the anode, although the mineralization was decelerated owing to the accumulation of chlorinated by-products. GC-MS and HPLC analysis allowed the identification of up to seven aromatic MeP derivatives in sulfate + chloride mixtures, including three non-chlorinated compounds also found in pure sulfate medium. These molecules were gradually transformed into oxalic acid, along with four chlorinated aliphatic carboxylic acids in Cl − -containing media.

Alachlor dechlorination prior to an electro-Fenton process: Influence on the biodegradability of the treated solution

“…A maximum accumulated H2O2 concentration of 5.92 ± 0.33 mmol L -1 was obtained at a current of -200 mA after 3 h of electrolysis, as shown in Figure 1(a). According to equation (2), a more negative current is expected to improve the production of H2O2 [34] at the cathode and the production of hydroxyl radicals at the Pt anode (Pt( • OH)) [35,36].…”

Assessment of 4‐Aminoantipyrine Degradation and Mineralization by Photoelectro‐Fenton with a Boron‐Doped Diamond Anode: Optimization, Treatment in Municipal Secondary Effluent, and Toxicity

“…Carbonaceous cathodes are known to possess the highest electrocatalytic ability for the promotion of Reaction (1), thus yielding an excellent current efficiency. Among these materials, carbon nanotubes, [24,25] carbon sponge, [26] and carbon or graphite felt, [27][28][29][30] as well as carbon-PTFE fitted into air-diffusion devices, [31][32][33][34][35][36] are the most widely employed. On the other hand, heterogeneous * OH can be formed on the anode surface from water discharge at high current.…”

“…On the other hand, heterogeneous * OH can be formed on the anode surface from water discharge at high current. The most powerful anodes are the BDD thin films due to their large O 2 -evolution overpotential and small interaction with heterogeneous * OH, yielding highly reactive BDD( * OH) as shown in Reaction (3): [17,21,34] BDD þ…”

“…Apart from the oxidation with homogeneous and heterogeneous * OH formed from Reaction (2) and (3), respectively, in an undivided cell, the UVA radiation in PEF allows the additional destruction of organic matter by different ways: (i) generation of larger * OH amounts via Reaction (4), with Fe 2 + regeneration, upon [Fe(OH)] 2 + photoreduction, (ii) direct photolysis of photosensitive pollutants, and (iii) photodecomposition of Fe(III) complexes with some N-derivatives and carboxylic acids according to the general Reaction (5). [16,19,21,[31][32][33][34][35][36]…”

Facile crosslinking of poly(vinylpyrrolidone) by electro-oxidation with IrO2-based anode under potentiostatic conditions

“…This means that it mainly exists as a superoxide (MO) and pre-eminently favors a milder reactivity with organic molecules [29,30]. These kinds of anodes are made of metal and mixed-metal oxides [30,31], being IrO 2 -based anodes preferred for O 2 evolution with M( • OH ) as intermediate [32][33][34][35][36][37][38]. As a result, the attack of this species on PVP may be more controlled.…”