Electrochemical Mineralization (Define Mineralization) The electrochemical method for the mineralization of organic pollutants is a new technology that has attracted a great deal of attention recently. This technology is interesting for the treatment of dilute wastewater . COD < 5 g/ L and it is in competition with the process of chemical oxidation using strong oxidants. The main advantage of this technology is that chemicals are not used. In fact, only electrical energy is consumed for the mineralization of organic pollutants.

Electrochemical Advanced Oxidation Process (EAOP). In Class 1 electrodes (called active electrodes) hydroxyl radical interact with the electrode surface and oxidation of organics is done by a direct transfer of electrons from this electrode surface and not by the action of hydroxyl radicals. As a consequence, in some cases the oxidation of the electrode and very low efficiencies are obtained. In other cases, it results in the formation of which attack chemically the organic pollutant with very different results in speciation and efficiency.

Class 2 electrodes (also called non-active), hydroxyl radicals do not interact with the anode surface but directly with organics in an electrochemical reaction zone very narrow in the nearness of the electrode surface (because average lifetime of hydroxyl radicals is very short). The very small width of this zone allows researchers to consider this process as an "almost" direct electrochemical process, although it is clearly a mediated electrochemical oxidation

The unique property of non reactive electrodes is the production of oxidants directly from the water. This is the basis of the Electrochemical Advanced Oxidation Process (EAOP). EAOP is a very effective addition to common treatment methods and an environmentally friendly process for reducing organic pollutants in water. The Electrochemical Advanced Oxidation Process EAOP produces highly oxidative hydroxyl radicals OH* by current-controlled electrolysis in aqueous solutions. The redox potential can go up to 2,8 eV (Diamond electrodes).

Several anodes (marked into Class 1 )produce a soft oxidation of the organics, with the formation of polymers and many refractory species as final products of the electrolytic process. On the contrary, the oxidation conditions produced during the bulk electrolysis of organics with other electrodes (Class 2) organics were easily mineralized (transformed into carbon dioxide), with no production of polymers. Following these ideas, we chose water as the complete atom source to electrochemically generate the most reactive •OH to initiate oxidative reactions for environmental protection.

In general, anodic oxidation reactions are accompanied by transfer of oxygen from water to the reaction products. This is the so-called EOTR. A typical example of EOTR is the EM of acetic acid In this anodic reaction, water is the source of oxygen atoms for the complete oxidation of acetic acid to CO2 . However, in order to achieve the EOTR, water should be activated. Depending on electrode material, there are two possibilities for the electrochemical activation of water in acid media : (1) by dissociative adsorption of water in the potential region of the thermodynamic stability of water. (2) by electrolytic discharge of water at potentials above its thermodynamic stability.

Activation of water by dissociative adsorption H2O + M → M-OH + M-H M-H → H+ + e- H2O + M → M-OH + H+ + e- This reaction takes place at lower potential of water oxidation (1.23 V vs SHE). Activation of water by electrolytic discharge HOH → H+ + OH- →OH. + e- H2O + M → M(OH. ) + H+ + e- M(OH. ) → M + ½ O2 + H+ + e-

The EOTR between an organic compound R (supposed none adsorbed on the anode) and the hydroxyl radicals (loosely adsorbed on the anode) takes place close to the anode’s surface: R(aq) + M(OH. )n/2 → M + Oxidation products + n/2 H+ + n/2 e- Influence of Anode Material on the Reactivity of Electrolytic Hydroxyl Radicals. Cf. Table

Table: Oxidation power of the anode material in acid media

Oxidation Mechanism direct electrolysis via hydroxyl radicals produced by the discharge of the water.

δ BDD ANODE ANODE M + H2O M.........OH. + H+ + e- O2 Quasi (weakly) adsorbed OH. Priority in presence of R Reacts with R δ In the absence of R 2OH. H2O2 H2O2 O2 + 2H+ + 2e- O2 + 2H2O

BDD profile Direction of potential shift OH. formation O2 1.3V vs SHE 2.5V vs SHE

Example of Mineralization of Formic Acid HCOOH+HO• →CO2 +H2O + H+ + e− CH3OH + HO• →• CH2OH + H2O   •CH2OH + HO• → HCHO + H2O HCHO+HO• →HCOOH + H+ + e−

Acetic acid is one of the most refractory organic compounds towards oxidation. In fact, the oxidation of acetic acid with O2 (air) is difficult even at a high temperature (300◦C) and pressure (100 bar) in the wet air oxidation process (WAO). As a consequence, acetic acid appears as a final product of oxidation of many organic compounds.

CH3COOH + 2H2O → 2CO2 + 8H+ + 8e− 0.11 V/SHE H2O → HO• + H+ + e− in strongly acidic media the electro-oxidation of acetic acid is more difficult as mainly un-dissociated molecules are present in the solution . On boron-doped diamond (BDD) electrodes, the onset potential for oxygen evolution reaction (OER) is about 2.3 V vs. SHE. Although very high, as compared with other conventional electrode materials, this potential corresponds to a thermodynamic potential of formation of free hydroxyl radicals (2.38 V vs. SHE) in acid aqueous solution . CH3COOH + 2H2O → 2CO2 + 8H+ + 8e− 0.11 V/SHE H2O → HO• + H+ + e−   CH3COOHads + HO• → CH3COO• + H2O   CH3COO• → CH3. + CO2
   CH3. + HO• → CH3OH HO•
+ CH3OH → CO2 + H2O 2CH3. → C2H6   CH3COO• + CH3. → CH3COOCH3

3 R1COO− + 3 R2COO− → R1−R1 + R1−R2 + R2−R2 + 6 CO2 + 6 e− On platinum electrodes, anodic oxidation of acetate ions results in Kolbe reaction   2CH3COO− → C2H6 + 2CO2 + 2e− CH3COO− → CH3COO• + e− CH3COO• → CH3• + CO2 2CH3• →C2H6

These quasi-free hydroxyl radicals are very reactive and can result in the mineralization of the organic compounds : R(aq) + BDD(OH. )n/2 → BDD + mineralization products + n/2 H+ + n/2 e- ( Boron-doped diamond-based anode (BDD) is a typical high oxidation poweranode (2004 )). Furthermore, BDD anodes have a high overpotential for the oxygen evolution reaction compared with the platinum anode (Cf.Figure). This high overpotential for oxygen evolution at BDD electrodes is certainly related to the weak BDD– hydroxyl radical interaction, what results in the formation of H2O2 near to the electrode’s surface ,which is further oxidized at the BDD anode : 2OH. → H2O2 → 2H+ + 2e- + O2

Cyclic voltammograms of BDD and platinum electrodes