Ozonation of drinking water: Part I. Oxidation kinetics and product formation

doi:10.1016/S0043-1354(02)00457-8

Water Research

Volume 37, Issue 7, April 2003, Pages 1443-1467

https://doi.org/10.1016/S0043-1354(02)00457-8 Get rights and content

Abstract

The oxidation of organic and inorganic compounds during ozonation can occur via ozone or OH radicals or a combination thereof. The oxidation pathway is determined by the ratio of ozone and OH radical concentrations and the corresponding kinetics. A huge database with several hundred rate constants for ozone and a few thousand rate constants for OH radicals is available. Ozone is an electrophile with a high selectivity. The second-order rate constants for oxidation by ozone vary over 10 orders of magnitude, between <0.1 M⁻¹s⁻¹ and about 7×10⁹ M⁻¹s⁻¹. The reactions of ozone with drinking-water relevant inorganic compounds are typically fast and occur by an oxygen atom transfer reaction. Organic micropollutants are oxidized with ozone selectively. Ozone reacts mainly with double bonds, activated aromatic systems and non-protonated amines. In general, electron-donating groups enhance the oxidation by ozone whereas electron-withdrawing groups reduce the reaction rates. Furthermore, the kinetics of direct ozone reactions depend strongly on the speciation (acid-base, metal complexation). The reaction of OH radicals with the majority of inorganic and organic compounds is nearly diffusion-controlled.

The degree of oxidation by ozone and OH radicals is given by the corresponding kinetics. Product formation from the ozonation of organic micropollutants in aqueous systems has only been established for a few compounds. It is discussed for olefines, amines and aromatic compounds.

Introduction

The present article gives an overview over ozonation of drinking waters with an emphasis on oxidation kinetics and product formation. Disinfection and by-product formation will be discussed in a second article [1].

The application of ozone in drinking-water treatment is widespread throughout the world [2], [3], [4], [5], [6], [7], [8]. The main reasons for the use of ozone are disinfection and oxidation (e.g. taste and odor control, decoloration, elimination of micropollutants, etc.) or a combination of both [9], [10], [11]. Similar to other disinfectants for water treatment (e.g. chlorine or chlorine dioxide), ozone is unstable in water and undergoes reactions with some water matrix components. However, the unique feature of ozone is its decomposition into OH radicals (OH) which are the strongest oxidants in water [12]. Therefore, the assessment of ozonation processes always involves the two species ozone and OH radicals. However, for different applications of ozone the two species are of differing importance. While disinfection occurs dominantly through ozone, oxidation processes may occur through both oxidants, ozone and OH radicals [13], [10], [14]. Ozone is a very selective oxidant; OH radicals react fast with many dissolved compounds and the water matrix. For ozone reactions more than 500 rate constants have been measured [15], [16], [17], [18], [19] for OH radical reactions the database is even larger and contains a few thousand rate constants [20], [21]. In conjunction with the beneficial effects of disinfection and oxidation, undesired by-products can be formed from the reaction of ozone and OH radicals with water matrix components [22], [23], [24]. They include numerous organic and some inorganic species. Because ozonation is usually followed by biological filtration, partly oxidized organic compounds can be mineralized microbiologically. The only ozonation by-product regulated in drinking waters today is bromate, which is formed during ozonation of bromide-containing waters [25], [26]. In contrast to organic oxidation/disinfection by-products (DBPs), bromate is not degraded in a biological filtration process.

Fig. 1 shows a schematic representation of the effects of ozone in drinking-water treatment. Disinfection and oxidation can be achieved simultaneously if ozone reactions are responsible for the oxidation. However, if ozone-resistant compounds have to be oxidized, ozone has to be transformed into OH radicals (see “advanced oxidation processes, AOPs”). This measure has the effect of decreasing the disinfection efficiency. Therefore, optimization of disinfection and oxidation requires careful evaluation of the overall process. If, in addition, by-product formation (e.g. bromate) has to be kept at a minimum, the corresponding formation mechanisms and oxidant concentrations have to be known.

In drinking water, the problem of by-product formation has become even more prominent since the recognition of the importance of microorganisms such as Cryptosporidium parvum oocysts (C. parvum), which are more resistant against disinfection [27]. This requires higher ozone exposures and in turn leads to more by-product formation.

The present paper gives an overview on ozone decomposition in water and its effects on oxidation processes with ozone and secondary oxidants (OH radicals) during drinking-water ozonation and AOPs, including methods of determining oxidant concentrations. A selection of drinking-water relevant micropollutants is presented together with a discussion of their oxidation kinetics and how this information can be used to assess the fate of these compounds during drinking-water ozonation. Furthermore, a state-of-the-art discussion on inorganic and organic oxidation products is provided.

Section snippets

Stability of ozone in water

Ozone is unstable in water. The decay of ozone in natural waters is characterized by a fast initial decrease of ozone, followed by a second phase in which ozone decreases with first-order kinetics. The mechanism and the kinetics of the elementary reactions involved in ozone decomposition have been investigated in numerous studies [12], [28], [29], [30], [31], [32], [33], [34], [35], [36]. Depending on the water quality, the half-life of ozone is in the range of seconds to hours [10], [37]. The

Characterization of ozonation processes

To assess ozonation processes with respect to oxidation by ozone and OH radicals, the concentrations or the exposures to both oxidants have to be known. Ozone concentrations can be easily measured by electrochemical, optical¹ or colorimetric methods ([13] and refs. therein). Standardized procedures for the characterization of ozonation processes with respect to ozone have been described for systems where ozone is

Oxidation of inorganic and organic compounds by ozone

The oxidation of inorganic and organic compounds with ozone occurs via several primary reactions which are discussed in detail below (Scheme 1). The kinetics of the reactions of ozone with inorganic and organic compounds is typically second order, i.e. first order in ozone and first order in the compound. This yields the following rate , , , : $S + O_{3} → products,$ $− d [S]/ d t=k[S][O_{3}]$

For a batch-type or plug-flow reactor this yields: $ln ([S]/[S]_{o})=−k∫[O_{3}] d t$

And the ozonation time required to decrease the

Advanced oxidation processes

Processes which involve the formation of highly reactive OH radicals as an oxidant are generally referred to as AOPs [122], [10]. In drinking-water treatment, they are applied to oxidize ozone-resistant compounds such as pesticides [123], [124], [125], aromatic compounds and chlorinated solvents such as tri- and tetrachloroethene [126], [127]. For complete ozone consumption of a water, the OH radical yield is almost independent of the rate of the ozone decomposition. Therefore, the main

Conclusions

It has been demonstrated that many of the oxidation reactions that occur during ozonation are beneficial to the overall drinking-water quality. These advantages include decoloration of the water, improvement of organoleptic properties and oxidation of micropollutants. During ozonation of drinking waters, the two major oxidants ozone and OH radicals govern the chemical processes. While ozone is a very selective oxidant which reacts quickly with double bonds, activated aromatic compounds and

Acknowledgements

I would like to thank J. Hoigné and C. von Sonntag for fruitful discussions and their substantial input and EAWAG for continuous support. Mike Elovitz and Marc Huber are acknowledged for reviewing the manuscript and Claire Wedema for correcting the English.

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ReviewOzonation of drinking water: Part I. Oxidation kinetics and product formation

Abstract

Introduction

Section snippets

Stability of ozone in water

Characterization of ozonation processes

Oxidation of inorganic and organic compounds by ozone

Advanced oxidation processes

Conclusions

Acknowledgements

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