Overlooked Role of Peroxides as Free Radical Precursors in Advanced Oxidation Processes

Abstract

Research efforts on advanced oxidation processes (AOPs) have long been focused on the fundamental chemistry of activation processes and free radical reactions. Little attention has been paid to the chemistry of the precursor oxidants. Herein, we found that the precursor oxidants could lead to quite different outcomes. A counterintuitive result was observed in the photoreduction of bromate/iodate: the combination of H 2 O 2 and UV enhanced the reduction of bromate/iodate, whereas the addition of persulfate to the UV system led to an inhibitory effect. Thermodynamic and kinetic evidence suggests that the reduction of bromate in UV/H 2 O 2 was attributable to the direct reaction between HOBr and H 2 O 2 . Both experimental determination and kinetic simulation demonstrate that the reaction between HOBr and H 2 O 2 dominated over the • OH-mediated reactions. These results suggest that H 2 O 2 possesses some particular redox properties that distinguish it from other peroxides. The prototypical UV/H 2 O 2 process is not always an AOP: it can also be an enhanced reduction process for chemicals with intermediates that are reducible by H 2 O 2 . Considering the increasing interest in persulfate-based AOPs, the results of this study identify some novel advantages of the classical H 2 O 2 -based AOPs.

Mechanisms of Advanced Oxidation Processes, the Principle of Detailed Balancing, and Specifics of the UV/Chloramine Process

“…For this reason, it may be desirable to repair the illegal loops by adding the requisite rate constants rather than removing others. If a minimal mechanism is desired, unnecessary steps can be identified readily by use of sensitivity analyses as has been described recently in a study on H 2 O 2 -based advanced oxidation processes …”

“…If a minimal mechanism is desired, unnecessary steps can be identified readily by use of sensitivity analyses as has been described recently in a study on H 2 O 2 -based advanced oxidation processes. 51 The high concentrations of NH 2 • , NHCl • , and NO • produced in the simulations described above are unrealistic. These excessively high concentrations arise because the model lacks certain important reactions of these species.…”

Kinetics and Mechanism Investigation of Selective Arsenite Oxidation by Reactive Iodine Species in Hydrogen Peroxide and Iodide (H2O2/I–) System

“…− to analyze HIO 36,37 could not be used here as H 2 O 2 would react with the excess I − , resulting in inaccurate quantification.…”

“…Concentrations of I − and possibly generated IO 3 − were analyzed using a high performance liquid chromatography (HPLC, Agilent Technologies, 1260 Infinity, U.S.) method at a wavelength of 226 nm, which has already been well established and applied (SI Text S4). 36,37 The concentrations of As(V) and chromate (Cr(VI)) were determined by colorimetric method with a detection limit of 0.03 μM and 0.02 μM, respectively, and specific details were provided in SI Text S5. The effect of Cr(VI) was investigated since it commonly coexists with As(III) in some mine drainage wastewater.…”

“…For the reactions without reported k 2 values, a one-at-a-time (OAT) sensitivity analysis method was used by iteration. 36 Some reactions could be excluded when the simulation results kept unchanged whatever k 2 was input while some others could be excluded according to scavenger results. The remained reactions (Table 1) were then used to determine the best simulation reaction pathway by correction coefficients (r) analysis.…”