Improvement in iron activation ability of alachlor Fenton-like oxidation by ascorbic acid

The addition of a strong reductant, L-ascorbic acid (AA), to Fe3+/hydrogen peroxide (H2O2) improved alachlor degradation substantially. AA in the form of ascorbate monoanion undergoes a two-step oxidation to yield dehydroascorbic acid through the formation of intermediate semidehydroascorbate and ascorbyl radicals that are responsible for the autocatalytic transformation of Fe3+ to Fe2+. The ability of AA to reduce Fe3+ to Fe2+ and propagate the formation of hydroxyl radicals was confirmed by the indirect measuring of hydroxyl radicals. Hydroxyl radicals production and alachlor degradation by H2O2/AA resulted. Alachlor degradation rate by Fe3+/H2O2/AA was similar to that obtained by Fe2+/H2O2, although a much lower alachlor degradation and mineralization by Fe3+/H2O2 was achieved. The kinetics of alachlor removal by Fe2+/H2O2 and Fe3+/H2O2/AA were characterized by a rapid decay (0-1 min) because of a higher formation of hydroxyl radicals and then by a slower decay (1-120 min). The determination of a second-order reaction rate constant of hydroxyl radicals with alachlor relied on the competitive reactions of deoxyribose and alachlor with hydroxyl radicals. A rate constant of 6.9 x 10(9) M-1 s(-1) (correlation coefficient of 0.997) implies that the reaction rate is close to that of diffusion-controlled reactions in water, and indicates that the hydroxyl radicals play a predominant role during oxidation. Phenylacetamide compounds that were primary by-products of alachlor degradation allowed for the prediction of the general reaction pathway. This study highlights the ability of AA to reduce Fe3+ making a presumption for non-regenerated ferric oxyhydroxide sludge reuse for activation of H2O2 oxidation in wastewater treatment at acidic pH. (C) 2015 Elsevier B.V. All rights reserved.