title: Fundamental reaction mechanisms of chlorine dioxide during water treatment - Reactions with phenols and biomolecules during inactivation mechanisms
Creators: name: Jütte, Mischa
Publisher: name: Verein zur Förderung des Institutes IWAR der Technischen Universität Darmstadt
Year: 2023

description: Two key elements of drinking water treatment are disinfection and pollution control. For this purpose, different chemical oxidants are used, for instance, chlorine (free available chlorine (FAC)), ozone (O₃), or chlorine dioxide (ClO₂). The presented work investigated the reaction mechanisms of ClO₂ during drinking water treatment. ClO₂ reacts mainly with activated aromatic compounds (e.g., phenols, anilines) and forms chlorite as major by-product (drinking water standard, 200 µg L⁻¹, Germany). It is increasingly implemented in drinking water treatment as a substitute for chlorination to avoid the formation of a halogenated disinfection by-product (DBP). However, recently it has been shown that FAC also forms in reactions of ClO₂ as a by-product. This results in a combined oxidation with ClO₂ and FAC, and both oxidants can work together synergistically in disinfection and pollutant degradation but may also form two sets of DBPs. The present study focuses on the intrinsic formation of FAC and other inorganic by-products (chloride, chlorite, and chlorate) in the ClO₂ reactions with phenols as representatives for reactive sites in natural organic matter (NOM) and biomolecules (amino acids). Furthermore, the contribution of FAC to disinfection in a ClO₂ water treatment model system has been investigated. The reaction of ClO₂ with amino acids was studied in the context of disinfection mechanisms. Thereby amino acids may be an important reaction partner for reaction with microbial cells during the disinfection. Therefore, reactions of ClO₂ with tyrosine and tryptophan were investigated regarding reaction kinetics and the formation of different chlorine species (FAC, chlorite, chloride, chlorate). Tyrosine and tryptophan displayed a very high reactivity towards ClO₂ (kapp = 3.16 × 10⁴ M⁻¹ s⁻¹ and 1.81 × 10⁴ M⁻¹ s⁻¹ at pH 7), and it seems likely that these represent a possible point of primary reaction of ClO₂ in microbial cells. Both investigated amino acids showed a significant formation of FAC (tyrosine ≈ 50 %, tryptophan ≈ 36 % of dosed ClO₂ concentration). Thereby FAC may serve as an additional reactive species contributing to cell inactivation. Since amino acids are the building blocks of peptides and proteins, it is possible that the reaction of ClO₂ with cell proteins during disinfection is not only causing the inactivation of the corresponding proteins but also forms FAC, which can cause further cell damage and may enhance the total cell inactivation. In ClO₂ based treatment ClO₂ is mainly consumed by NOM. The strong depletion can be explained by the different phenolic moieties, which show high reactivity towards ClO₂. Recently, it has been shown that the reaction of ClO₂ with NOM is forming 25 % FAC. Since phenol, the major reactive side in NOM, itself forms 50 % FAC in the reaction with ClO₂; it might be possible that the presence of different functional groups attached to the phenolic ring is causing a change in the reaction mechanism regarding the formation of inorganic chlorine species. Therefore, the yields of different chlorine species (chlorine balance) of different phenolic compounds with different substituents (e.g., alkyl, hydroxyl, or methoxy groups) in ortho-, meta-, and para-position were investigated. It could be shown that most substituents do not particularly affect the chlorine balance. However, para-substituted phenols seem to form ortho-benzoquinone, which is very reactive and causes a change in the chlorine balance over time (reduced FAC yields and increased chloride yields). This might explain the different reported yields of FAC in the literature. The substituents which strongly affect the chlorine balance of phenol are hydroxyl and amino groups in ortho- and para-position, which results in 100 % yields of chlorite and total hampering of FAC formation. The exact reason for this observation requires further investigation. Glycine has been frequently used to determine intrinsic FAC in ClO₂ reactions with phenols which have a low reaction kinetics with FAC (kapp = 10² M⁻¹ s⁻¹, at pH 7). Thus, FAC can be successfully scavenged by glycine, which reacts several orders of magnitude faster with FAC (kapp = 1.5 × 10⁵ M⁻¹ s⁻¹ at pH 7). The ensuing product of this reaction (chloro-glycine) can be determined to quantify FAC formation. However, if the compound under study reacts fast with FAC (e.g., cysteine kapp = 6.2 × 10⁷ M-¹ s-¹ at pH 7) glycine may not be able to quantitatively scavenge FAC resulting in an underestimation of intrinsic FAC. Examples of compounds with such high reaction kinetics with FAC are thiols (e.g., Glutathione (GSH)), which react fast with both oxidants ClO₂ and FAC (kapp ≥ 10⁷ M⁻¹ s⁻¹). The reaction of GSH with FAC is two orders of magnitudes faster than the reaction of FAC with glycine. Therefore, a new method was developed using methionine as a selective scavenger. Methionine is a sulfide-containing amino acid, which reacts fast with FAC (kapp = 6.8 × 10⁸ M⁻¹ s⁻¹ at pH 7) and forms chloride and methionine sulfoxide (MSO) in equal parts. The yields of chloride and MSO can be used to quantify the FAC yields. The reaction of methionine with ClO₂ was determined to be kapp = 10⁻² M⁻¹ s⁻¹ at pH 7. The method was successfully applied to qualitatively state that FAC is formed in the reaction of ClO₂ with the tripeptide GSH. However, in some cases, MSO formation was observed from a yet unknown source, which requires further investigation. Finally, the intrinsic FAC participation during ClO₂-based disinfection was investigated. First, a novel concept has been developed to determine different levels of microbial cell inactivation, which is based on the extension of the lag phase (initial growth phase preceding the exponential growth). Thereby an increase of the Escherichia coli inactivation results in a prolongation of the lag phase. Since the growth can be monitored online by an increase in optical density, this method is fast and enables the simultaneous measurement of several samples. With this method, it was possible to show that in ClO₂-based disinfection processes, the intrinsic formation of FAC may be very important. This was shown in experiments of E. coli elimination in the presence of NOM. The addition of methionine as a fast-reacting FAC-scavenger fully suppressed the inactivation of E. coli. This indicates that the observed E. coli inactivation on ClO₂-based processes with high loads of NOM may be mainly driven by FAC. Furthermore, it was shown that disinfection in the presence of NOM is pH-dependent (pH 6.5 &gt; 7.5 &gt; 8.5). This can be explained by the depletion of ClO₂, which is accelerated at higher pH values due to the dissociation of the phenolic moieties (pKa: 10) of the NOM (note that the deprotonated phenolate species reacts more than five orders of magnitude faster with ClO₂ compared to protonated phenol). With an increasing consumption rate of ClO₂, less ClO₂ will be available for disinfection. Additionally, the speciation of FAC (HOCl) might be responsible for the observed stronger inactivation at lower pH since HOCl is a stronger disinfectant than OCl⁻ (pKa: 7.54).