Electrochemical synthesis of peroxomonophosphate using boron-doped diamond anodes

Abstract

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in: http://oatao.univ-toulouse.fr/ Eprints ID : 3043 Abstract A new method for the synthesis of peroxomonophosphate, based on the use of boron-doped diamond electrodes, is described. The amount of oxidant electrogenerated depends on the characteristics of the supporting media (pH and solute concentration) and on the operating conditions (temperature and current density). Results show that the pH, between values of 1 and 5, does not influence either the electrosynthesis of peroxomonophosphate or the chemical stability of the oxidant generated. Conversely, low temperatures are required during the electrosynthesis process to minimize the thermal decomposition of peroxomonophosphate and to guarantee significant oxidant concentration. In addition, a marked influence of both the current density and the initial substrate is observed. This observation can be explained in terms of the contribution of hydroxyl radicals in the oxidation mechanisms that occur on diamond surfaces. In the assays carried out below the water oxidation potential, the generation of hydroxyl radicals did not take place. In these cases, peroxomonophosphate generation occurs through a direct electron transfer and, therefore, at these low current densities lower concentrations are obtained. On the other hand, at higher potentials both direct and hydroxyl radical-mediated mechanisms contribute to the oxidant generation and the process is more efficient. In the same way, the contribution of hydroxyl radicals may also help to explain the significant influence of the substrate concentration. Thus, the coexistence of both phosphate and hydroxyl radicals is required to ensure the generation of significant amounts of peroxomonophosphoric acid.

Synthesis of novel oxidants by electrochemical technology

“…A clear example of this explanation is the lower efficiency of the electrogeneration of oxidants during the electrolysis of phosphate solutions at acidic pH if compared to that obtained at alkaline conditions. According to literature, this is related to the nature of the oxidant electrogenerated in both cases [13,14]. Thus, the most important oxidant reagent produced in the electrolyses of phosphates solutions at alkaline pH is peroxodiphosphate ion (P 2 O 8 4-).…”

“…Thus, several studied have been published in which the generation of, peroxodiphosphate [13], peroxomonophosphate [14], peroxodisulphates [15], ferrates [16][17][18] and oxoanions of chlorine [19][20][21][22] by electrochemical techniques are described.…”

Electrochemical degradation of an anionic surfactant on boron-doped diamond anodes

“…In the former case (advanced oxidation) the oxidation is carried out primarily by hydroxyl radicals while in the other two cases the primary mechanisms is expected to be the chemical oxidation by the oxidant molecule. This suggests that peroxoacids are not good chemical oxidants themselves and that maybe is their activation in the anode to form sulphate or phosphate radicals [31] the responsible of the better results (greater mineralization) obtained in the electrochemical treatment of SDBS with sulphate and phosphate supporting electrolytes as compared to those obtained in NaCl media (in which foreseeable many halogenated organic intermediates could be formed). The absorption spectral changes during the galvanostatic electrolyses of SDBS solutions (0.3 g dm −3 ) at 30 mA cm −2 with NaCl, Na 2 SO 4 and K 3 PO 4 supporting electrolytes are shown in Fig.…”

Electrochemical synthesis of perbromate using conductive-diamond anodes

“…This fact could be related to the complex interaction between hydroxyl radicals and bromate ions which can lead, in case of non optimum ratios, to the decomposition of hydroxyl radicals towards oxygen and to the loss of efficiency of the process. In this point, it is important to state that a similar trend was observed in the formation of monoperoxophosphoric acid using the same technology [21]. This electrosynthesis process was explained by a similar mechanism of that of perbromate, with an important role of the hydroxyl radicals formed in the surface of the conductive-diamond, and it suggests that the important influence of the concentration of raw material can be a characteristic of this technology.…”