Environmental remediation by an integrated microwave/UV illumination method: VII. Thermal/non-thermal effects in the microwave-assisted photocatalyzed mineralization of bisphenol-A

The photodegradation of the endocrine disruptor 4,4′-isopropyldiphenol (bisphenol-A, BPA; 0.1 mM) was examined in aqueous TiO2 dispersions to assess thermal versus non-thermal effects of the microwave radiation assisted photocatalytic process driven by UV radiation. The effects were examined by comparing the results of the photodegradative process from the hybrid UV/microwave irradiation (PD/MW) with a UV/conventional heating method (PD/TH) and with UV illumination alone (PD). Mechanistic considerations followed from monitoring the process by UV/Vis absorption spectroscopy (cleavage of benzene rings), total organic carbon (TOC) assays, identification of intermediates by electrospray mass spectral techniques, and from calculations of frontier electron densities and partial charges of all non-hydrogen atoms in the BPA skeleton. Near-quantitative mineralization of BPA occurred after only 90 min of irradiation by the PD/MW and PD/TH routes, contrary to the PD route, which accounts for only 67% mineralization under identical conditions. Microwave radiation alone on a solution of BPA (no TiO2) had no effect on the TOC, except for an increase of the pH of the solution (6.7–7.2) and formation of a small quantity of formic acid (ca. 0.005 mM) originating from oxidation of the isopropyl methyl groups after a 30 min irradiation period. For the other three routes, other intermediates were identified in addition to acetic acid and formic acids: 4-hydroxyacetophenone (PD/MW, PD/TH, PD), 4-hydroxybenzaldehyde (PD/MW, PD/TH), 4-hydroxyphenylisopropanol (PD/TH, PD), phenol and hydroquinone (PD/TH), and 3-hydroxy-1,3,5-hexatriene (PD/MW). Mechanistic differences are attributed to adsorption mode variations of the BPA substrate on the TiO2 particle surface.