Anodic oxidation of surfactants and organic compounds entrapped in micelles - Selective degradation mechanisms and soil washing solution reuse.

Formation of micelles at high surfactant concentration strongly modifies organic pollutant oxidation mechanisms and kinetics during anodic oxidation (AO) using boron doped diamond (BDD) anode. Results presented and discussed in this study emphasized the following mechanisms: (i) micelles act as a protective environment and reduce the availability of target molecules towards BDD(OH); (ii) the use of low current density strongly reduces micelle degradation kinetics due to both steric hindrance phenomenon for oxidation of micelles at the BDD surface and decrease of mediated oxidation in the bulk; (iii) compounds solubilized in surfactant-containing solutions can be either oxidized after degradation of the protective environment formed by micelles or if they are present as free extra-micellar compounds. Therefore, selective degradation of organic compounds entrapped in micelles can be achieved by using low current density and high surfactant concentration. In fact, these operating conditions strongly hinder micelle oxidation, while free (extra-micellar) compounds can still be oxidized. Then, the remaining entrapped compounds can also be continuously released in the aqueous phase, according to the micellar/aqueous phase partitioning coefficient (K). These results have been applied for the treatment of a real polycyclic aromatic hydrocarbon-containing soil washing (SW) solution. After 23 h of treatment at 2.1 mA cm, 83% of phenanthrene, 90% of anthracene, 77% of pyrene and 75% of fluoranthene were degraded and the treated SW solution was reused for an additional SW step with only 5% lower extraction capacity than a fresh TW80 solution. A comparative study highlighted the superiority of this treatment strategy, compared to the use of activated carbon for selective adsorption of polycyclic aromatic hydrocarbons and SW solution reuse.