Sonophotolytic degradation of phthalate acid esters in water and wastewater: Influence of compound properties and degradation mechanisms

The influence of physicochemical properties on the sonolytic and sonophotolytic degradation of a group of short-chain phthalate acid esters (PAEs) was investigated in this study. A 400kHz ultrasonic system and a photolytic system at 253.7nm were employed separately and together, and both pure water and secondary effluent were used as the water matrices. It was found that PAEs with greater hydrophobicity demonstrated more rapid sonolysis and exhibited greater competitiveness in sonochemical reactions in the presence of other compounds. However, although a greater compound hydrophobicity is beneficial for the sonochemical degradation of PAEs, the observed synergy between ultrasound and UV in the sonophotolytic process is reduced owing to the lower accumulation of H2O2in the aqueous phase. For the sonophotolysis of PAEs in secondary effluent, it was found that PAEs with greater hydrophobicity experienced less inhibition or competition from the background organic substances (expressed as TOC). Identification of prominent degradation intermediates of di-n-butyl phthalate (DBP), as a representative PAE, indicated that hydroxylation of the aromatic ring and butyl-scission of the aliphatic chain are the principal degradation mechanisms. The combined process of US/UV produced a greater degree of DBP mineralization than either US or UV alone (17% TOC reduction within 90min).