Heterogeneous catalytic wet peroxide oxidation of paraquat in the presence of modified activated carbon

The catalytic wet peroxide oxidation (CWPO) method was applied to the degradation of paraquat, a widely used and highly toxic herbicide. The effect of ammonia pre-treatment and iron impregnation on Fenton-driven reaction efficiency was first investigated. Iron amendment and acidic oxygen-surface groups were found to promote the catalytic activity towards paraquat degradation. The effects of three significant operational parameters (hydrogen peroxide concentration, catalyst dosage and temperature) on catalytic performance of the most promising catalyst (AC-Fe) were then investigated. The best degradation yield (71.4% of COD abatement in a 20 mg L−1 solution) was obtained with 12.5 mmol L−1 H2O2, 1 g L−1 catalyst dosage and T = 70 °C, although ambient temperature could be preferred for application convenience.A nearly complete degradation of paraquat and its intermediates (∼92% of COD removal) was achieved after 12 h treatment, when proceeding at ambient temperature via regular sequential additions of the spent amount of H2O2 in the presence of optimal catalyst dosage. A reasonably good stability of the catalyst was demonstrated during consecutive re-use of the same recovered sample, in long-term CWPO runs. Only a slight decrease of catalytic performance (from 92% to 85% of COD removal) was observed after five consecutive cycles, probably due to high stability of the supported iron species.Time evolutions of aromatic intermediates and short-chain carboxylic acids were recorded using HPLC analysis, and clearly indicated that CWPO is capable of effectively degrading the paraquat herbicide.