Degradation of dimethyl phthalate by activating peroxymonosulfate using nanoscale zero valent tungsten: Mechanism and degradation pathway

The oxidation capacity and mechanisms involved in activation of peroxomonosulfate (PMS) by nanoscale zero valent tungsten (nZVW) were investigated with dimethyl phthalate (DMP) as a targeted contaminant. DMP removal was completely achieved by hydroxyl radical (OH) and sulfate radical (SO4−) at 50 min in the nZVW/PMS system, and structural and componential changes of nZVW powder before and after reaction were not obvious. Low valence tungsten species were released during nZVW corrosion in oxygenated water via heterogeneous reactions, which was accelerated by PMS. The homogeneous reactions were mainly responsible for radical generation: 1) the stepwise oxidation of nZVW to produce H2O2, which was then activated by low valence tungsten species to generate OH; 2) the activation of PMS by low valence tungsten species to produce SO4− and OH. However, OH− reduced the reactivity of low valence tungsten species due to the formation of tungstate, higher pH thus partly inhibited the DMP removal. Two stages of DMP removal were found, and DMP degraded dramatically in the second stage. The degradation pathways of DMP were proposed, involving the formation of aromatic esters (monomethyl phthalate, methyl benzoate, and methyl salicylate), aromatic acids (phthalic acid, salicylic acid, and benzoic acid), and phenols (catechol, phenol, and hydroquinone) in turn. These results help to comprehend the application of nZVW in reactive radicals-based oxidation processes and the reactivity of low valence tungsten species in water treatment field.