Sono-activated persulfate oxidation of bisphenol A: Kinetics, pathways and the controversial role of temperature

The sonochemical degradation of bisphenol A (BPA) in the presence of sodium persulfate (SPS) was investigated at 20 kHz. Experiments were performed at 110-450 mu g/L BPA and 1-100 mg/L SPS concentration and an ultrasound power density between 4 and 60 W/L. The liquid bulk temperature was either kept constant at 30 degrees C or left uncontrolled and gradually increased up to 80 degrees C. BPA degradation occurs due to the combined action of hydroxyl radicals generated by the ultrasound and sulfate radicals generated by SPS sonolysis and this was confirmed with experiments with methanol and t-butanol as radical scavengers. Considerable thermal SPS activation can also occur at increased temperatures (60-80 degrees C) although the enhanced formation of sulfate radicals is compensated by a decrease in sonochemical activity. Kinetics can be approached by a pseudo-first order expression with the apparent rate constant decreasing with increasing BPA concentration; this implies that the actual order is below first. The rate increases with increasing power density and SPS concentration and decreases at alkaline conditions (i.e. pH = 9) and in the presence of inorganic and organic constituents (e.g. bicarbonate, humic acid etc) typically found in environmental matrices. Liquid chromatography time of flight mass spectrometry (LC-TOF-MS) analysis revealed the formation of twelve transformation by-products that accompany BPA sonodegradation in the presence of SPS. Based on their evolution profiles, a reaction mechanism is proposed consisting of two major pathways: (i) hydroxylation that can occur mainly in the aromatic ring through HO center dot, SO4 center dot- radical attack and to a lesser extent in methyl groups, through hydrogen abstraction by HO center dot radicals, with subsequent O-2 addition, and (ii) scission of the bond between the isopropylidene carbon and the phenyl group. (C) 2015 Elsevier B.V. All rights reserved.