Abstract
This lecture presents the fundamentals and some interesting applications of the solar photoelectro-Fenton (SPEF) method for the destruction of organics in wastewaters. This electrochemical advanced oxidation method is an environmentally friendly technique where hydrogen peroxide is continuously supplied to an acidic contaminated solution from the two-electron reduction of oxygen injected to a gas-diffusion cathode: O2(g) + 2 H+ + 2 e ?? H2O2 (1) The oxidizing power of electrogenerated H2O2 is then strongly enhanced by adding a small quantity of Fe2+ catalyst to the solution to give Fe3+ and hydroxyl radical (OH) by the well-known Fentons reaction: Fe2+ + H2O2 ? Fe3+ + OH + OH- (2) . An undivided cell is used to oxidize the pollutants by both, OH formed from reaction (2) and M(OH) produced from water oxidation at a high O2-overvoltage anode by reaction (3): M(H2O) ? M(OH) + H+ + e- (3) The SPEF process also involves the additional irradiation of the treated solution with sunlight to favor: (i) the photoreduction of Fe(OH)2+, which is the predominant Fe3+ species in acid medium, to Fe2+ and more OH by photo-Fenton reaction (4), and (ii) the photolysis of complexes formed between Fe(III) and final carboxylic acids such as shown for oxalic acid via reaction (5): Fe(OH)2+ + hv ? Fe2+ + OH (4) Fe(C2O4)n(3-2n) + hv ? 2 Fe2+ + (2n-1) C2O42- + 2 CO2 (5) Oxalic acid is formed during the oxidation of most aromatics and the fast photodecarboxylation of Fe(III)-oxalate complexes (Fe(C2O4)+, Fe(C2O4)2- and Fe(C2O4)33-) favors their decontamination. Exemples on the good oxidation ability of SPEF are presented for solutions of: (i) salicylic acid and the aminoacid a-methylphenylglycine using small electrolytic cells with a Pt or boron-doped diamond (BDD) anode and and O2-diffusion cathode, all them of 3 cm2 area, and (ii) the herbicide mecoprop and o-, m- and p- cresols using a flow plant of 2.5 l with a filter-press cell containing BDD and O2-diffusion electrodes of 20 cm2 area, coupled to a solar photoreactor with 600 ml of irradiation volume. Treated solutions were prepared with 0.05 M Na2SO4 and 0.25-1.0 mM Fe2+ at pH 3.0 and electrolyses were carried out by applying a constant current density between 25 and 150 mA cm-2. Comparative trials with electro-Fenton (EF) in the dark were also made to confirm the synergistic effect of sunlight during the SPEF process. While in the EF method a slow, but complete mineralization of all contaminants is found using a BDD anode due to the efficient oxidizing action of homogeneous OH and BDD(OH), the SPEF treatment yields a much faster decontamination with a Pt or BDD anode because of the efficient photodecomposition of Fe(III) complexes with UVA irradiation supplied by solar light. The efficiency of all degradation processes increases strongly with rising pollutant concentration and decreasing current density. The decay kinetics for all initial pollutants and the evolution of their aromatic by?products were followed by reversed-phase HPLC chromatography, whereas generated carboxylic acids were identified and quantified by ion-exclusion HPLC chromatography. Detection of reaction intermediates allows the proposal of a plausible sequence for the mineralization of each initial pollutant. In all cases the ultimate product is oxalic acid, which forms Fe(III)-oxalate complexes that can be destroyed with BDD(OH) in EF, but much more rapidly photolyzed to CO2 in SPEF. Other final acids like acetic or oxamic are also formed, undergoing slower destruction. The treatment of 128 mg l-1 of all cresols with 0.5 mM Fe2+ by SPEF in the flow plant leads to an energy cost for total mineralization as low as 6.6 kWh m-3 at 25 mA cm-2, showing the viability of this procedure for its possible application to wastewater remediation at industrial scale.