Degradation of Ozone-Refractory Organic Phosphates in Wastewater by Ozone and Ozone/Hydrogen Peroxide (Peroxone): The Role of Ozone Consumption by Dissolved Organic Matter

Ozonation is very effective in eliminating micropollutants that react fast with ozone (k > 103 M−1 s−1), but there are also ozone-refractory (k < 10 M−1 s−1) micropollutants such as X-ray contrast media, organic phosphates, and others. Yet, they are degraded upon ozonation to some extent, and this is due to •OH radicals generated in the reaction of ozone with organic matter in wastewater (DOM, determined as DOC). The elimination of tri-n-butyl phosphate (TnBP) and tris-2-chloroisopropyl phosphate (TCPP), added to wastewater in trace amounts, was studied as a function of the ozone dose and found to follow first-order kinetics. TnBP and TCPP concentrations are halved at ozone to DOC ratios of ∼0.25 and ∼1.0, respectively. The •OH rate constant of TCPP was estimated at (7 ± 2) × 108 M−1 s−1 by pulse radiolysis. Addition of 1 mg H2O2/L for increasing the •OH yield had very little effect. This is due to the low rate of reaction of H2O2 with ozone at wastewater conditions (pH 8) that competes unfavorably with the reaction of ozone with wastewater DOC. Simulations based on the reported (Nöthe et al., ES&T 2009, 43, 5990−5995) •OH yield (13%) and •OH scavenger capacity of wastewater (3.2 × 104 (mgC/L)−1 s−1) confirm the experimental data. Based on a typically applied molar ratio of ozone and H2O2 of 2, the contribution of H2O2 addition on the •OH yield is shown to become important only at high ozone doses.