Treatment of synthetic soil washing solutions containing phenanthrene and cyclodextrin by electro-oxidation. Influence of anode materials on toxicity removal and biodegradability enhancement

Electrochemical advanced oxidation processes were applied to treatment of highly loaded synthetic soil washing solution. Phenanthrene (PHE) and hydroxypropyl-beta-cyclodextrin (HPCD) were chosen respectively as a polycyclic aromatic hydrocarbon (PAH) representative and as a solubilizing agent. Different anode materials such as platinum (Pt), dimensionally stable anode (DSA;) and boron-doped diamond (BDD) were employed to carry out the treatment in a electrocatalytic way. Two electrochemical processes were compared: electro-Fenton (EF) and anodic oxidation (AO) with BUD as an anode. Toxicity (Microtox (R)) and biodegradability (BOD5/COD) of treated solutions during the treatment were assessed. Pt anode was found to be the most efficient one in degradation of PHE, while BDD anode showed better ability to degrade HPCD and to mineralize the solution. This confirms the different ways to treat the effluent, which are related to the O-2 evolution overpotential of selected anode: mainly electro-oxidation in the case of Pt and DSA, mineralization (conversion of organics to CO2) mechanism in AO with BDD anode and paired electrocatalysis in the case of EF with BDD anode. Toxicity and biodegradability assays corroborate these mechanisms. After a complete degradation of PHE and HPCD corresponding to 60% mineralization of solution with EF and AO treatments using BDD as anode, the toxicity starts to decrease and the biodegradability reaches its maximal value (100%). In these conditions, the complete mineralization needs longer treatment times. But in this study electrochemical treatment is conceived to enhance the biodegradability for a biological post-treatment in a reasonably short time. Therefore six factors including energy consumptions values were compared after reaching a biodegradability of 33% and after reaching complete mineralization. Performing electrochemical treatment until reaching a BOD5/COD ratio of 33% appears to be the best option, since EF and AO displayed most of the time a similar behaviour. (C) 2014 Elsevier B.V. All rights reserved.