Reaction Mechanisms of Chlorine Dioxide with Phenolic Compounds─Influence of Different Substituents on Stoichiometric Ratios and Intrinsic Formation of Free Available Chlorine

Abstract

Chlorine dioxide (ClO2) is an oxidant applied in water treatment processes that is very effective for disinfection and abatement of inorganic and organic pollutants. Thereby phenol is the most important reaction partner of ClO2 in reactions of natural organic matter (NOM) and in pollutant degradation. It was previously reported that with specific reaction partners (e.g., phenol), free available chlorine (FAC) could form as another byproduct next to chlorite (ClO2 –). This study investigates the impact of different functional groups attached to the aromatic ring of phenol on the formation of inorganic byproducts (i.e., FAC, ClO2 –, chloride, and chlorate) and the overall reaction mechanism. The majority of the investigated compounds reacted with a 2:1 stoichiometry and formed 50% ClO2 – and 50% FAC, regardless of the position and kind of the groups attached to the aromatic ring. The only functional groups strongly influencing the FAC formation in the ClO2 reaction with phenols were hydroxyl- and amino-substituents in ortho- and para-positions, causing 100% ClO2 – and 0% FAC formation. Additionally, this class of compounds showed a pH-dependent stoichiometric ratio due to pH-dependent autoxidation. Overall, FAC is an important secondary oxidant in ClO2 based treatment processes. Synergetic effects in pollutant control and disinfection might be observable; however, the formation of halogenated byproducts needs to be considered as well.

Reaction of methionine with chlorine: kinetics, product formation, and potential use as a scavenger in chlorine dioxide-based systems

“…Detailed information about the impurities of the stock solution can be found in previous work. 8 FAC solutions were prepared daily before every experiment. A 15% FAC solution was 400-fold diluted in pure water and the FAC concentration was measured by direct absorption measurement at λ = 292 nm (ε 292 = 350 M −1 cm −1 ) (note that the concentration is determined by measuring absorption of OCl − thus, pH should be above 10 (pK a = 7.54)).…”

“…While phenol is reported to form 50% FAC and 50% ClO 2 − , 4,5 hydroquinone and DMP are reported to form 100% chlorite. 6,8,9,21,22 These data will later be compared to the chlorine balances measured with the methionine method.…”

“…3 However, it is proven that in its reactions with specific reactive moieties (e.g., phenols) free available chlorine (FAC) is formed as a reaction product. [4][5][6][7][8] Although the most abundant reaction product is chlorite (ClO 2…”

“…4 Glycine has also been used as a selective scavenger. [6][7][8][9] Glycine reacts fast with FAC (k app = 1.5 × 10 5 M −1 s −1 at pH 7) 14 and slow with ClO 2 (k app = <10 −5 M −1 s −1 at pH 7) 15 and is, therefore, a suitable scavenger for intrinsically formed FAC. The reaction of FAC with glycine forms chloro-glycine (Cl-Gly), which can be detected photometrically in the presence of iodide in excess 16 or with IC simultaneous to other chlorine species formed in the reaction (glycine method).…”

“…17 However, it has been shown that Cl-Gly can undergo follow-up reactions with intrinsically formed reactive species (e.g., ortho-benzoquinone) which would result in a wrong interpretation of the FAC yields. 8 All methods mentioned above have the drawback that the second-order reaction rate constant of FAC with the scavenging compound is the limiting factor. Thus, most sulfur-containing compounds can hardly be investigated in terms of FAC formation because the reaction rate of FAC with these sulfurcontaining compounds is several orders of magnitude faster compared to the reaction rate of FAC with the aforementioned scavengers.…”