Kinetics and Mechanism of •OH Mediated Degradation of Dimethyl Phthalate in Aqueous Solution: Experimental and Theoretical Studies

The hydroxyl radical ((OH)-O-center dot) is one of the main oxidative species in aqueous phase advanced oxidation processes, and its initial reactions with organic pollutants are important to understand the transformation and fate of organics in water environments. Insights into the kinetics and mechanism of (OH)-O-center dot mediated degradation of the model environmental endocrine disruptor, dimethyl phthalate (DMP), have been obtained using radiolysis experiments and computational methods. The bimolecular rate constant for the (OH)-O-center dot reaction with DMP was determined to be (3.2 +/- 0.1) x 10(9) M(-1)s(-1). The possible reaction mechanisms of radical adduct formation (RAF), hydrogen atom transfer (HAT), and single electron transfer (SET) were considered. By comparing the experimental absorption spectra with the computational results, it was concluded that the RAF and HAT were the dominant reaction pathways, and OH-adducts ((DMPOH1)-D-center dot, (DMPOH2)-D-center dot) and methyl type radicals (DMP)-D-center dot(-H)alpha were identified as dominated intermediates. Computational results confirmed the identification of transient species with maximum absorption around 260 nm as (DMPOH1)-D-center dot and (DMP)-D-center dot(-H)alpha, and these radical intermediates then converted to monohydroxylated dimethyl phthalates and monomethyl phthalates. Experimental and computational analyses which elucidated the mechanism of (OH)-O-center dot-mediated degradation of DMP are discussed in detail.