The efficiency and mechanisms of catalytic ozonation

Abstract

Catalytic ozonation has recently gained significant attention as an effective process used for the removal of organics from water. Unfortunately, despite increasing research efforts in the field of catalytic ozonation and the introduction of new catalysts, the mechanisms of catalytic processes are still largely unknown. An understanding of the mechanisms of catalytic ozonation is vital in order to introduce this technique in water treatment at an industrial scale. Therefore, the main aim of this paper is to provide a short overview of catalytic processes, their recent advances and to identify major directions taken to understand mechanisms governing catalytic processes utilised in water and wastewater treatment. Catalytic ozonation is considered to belong to Advanced Oxidation Processes. However, the results published by several research groups indicate that not all catalytic processes utilise the power of hydroxyl radicals.

Introduction

Ozone is used as an effective oxidant in water and wastewater treatment and also air purification. The effective usage of ozone in all the above mentioned fields is vital and therefore extensive research aiming at the utilisation of catalytic ozonation has been undertaken. Two comprehensive papers were published by Oyama [1], [2] on the application of catalytic ozonation in air purification. Several applications of ozone in water treatment technology are subjects of discussion in many books [3], [4], [5], [6], [7]. It is a well known fact that ozonation can proceed via two routes: direct molecular ozone reactions and/or indirect pathway leading to ozone decomposition and the generation of hydroxyl radicals (OH). Reactions of ozone with organic matter usually lead to the formation of aldehydes and carboxylic acids, both of which do not react with ozone. This is an important limitation of ozonation as total mineralization of organic matter is not achieved. Additionally, oxidative reactions with ozone are relatively slow and selective. Therefore, if the ozonation reaction is not satisfactorily fast, radical processes have to be implemented. Ozone decomposition in water is strongly pH dependent and occurs faster with an increase of pH. Reactions of radicals with organic and inorganic molecules are, as opposed to direct molecular ozone reactions, fast and non-selective. Recombination of hydroxyl radicals might also take place and might lead as a result to inactive moieties. Termination of the radical chain reaction leads to a decrease of ozone oxidative power. In contrast to ozonation alone, catalytic ozonation allows for the effective formation of hydroxyl radicals also at a low pH. Catalytic ozonation utilises catalysts in order to achieve controlled decomposition of ozone and hydroxyl radicals’ formation. Therefore catalytic ozonation can be placed in the group of methods called advanced oxidation processes, although there are some exceptions observed. Catalytic ozonation should provide fast degradation of organic pollutants and also more effective mineralization of both micropollutants and natural organic matter.

The main aims of this paper are to provide a short overview of catalytic processes and to indicate new possibilities for effective water purification and wastewater treatment. This manuscript focuses on recent advances in the field of catalytic ozonation. In order to obtain additional information on catalytic ozonation, the reader is referred to a book written by Beltrán [8] and other reviews earlier written by Legube [9], Pirkanniemi and Sillanpää [10] and Kasprzyk-Hordern et al. [11]. Unfortunately, despite increasing research efforts in the field of catalytic ozonation, the mechanisms of catalytic processes are still largely unknown. Several catalysts have been proven to be effective in the enhancement of ozonation efficiency but the mechanism of such catalytic processes is still to be resolved. An understanding of the mechanisms of hydroxyl radicals’ formation during catalytic ozonation is vital in order to introduce this technique to water treatment at an industrial scale.

Catalytic ozonation processes utilise several phenomena and can be divided into the following sub-techniques, which facilitate ozone decomposition and hydroxyl radicals’ formation:

• Homogeneous catalytic ozonation: ozone decomposition is catalysed by transition metal ions.

• Heterogeneous catalytic ozonation: ozone decomposition is catalysed by solid catalysts.

Because catalytic ozonation should be characterised by higher efficiency of organic contaminants removal than ozonation alone, the organic contaminants that are usually chosen for study are those which can be only removed via radical mechanisms. Among them are: oxalic, pyruvic, p-chlorobenzoic and 1,3,6-naphtalenetrisulfonic acid, nitro- and chlorinated aromatic compounds.

Another group of methods enhancing the ozonation process take advantage of the higher solubility and stability of ozone in non-polar media. These methods called two-phase ozonation can be undertaken in liquid–liquid or liquid–solid systems. Despite the fact that these methods aim at molecular ozone stabilisation, it has been shown that such processes can also support processes occurring via radical mechanisms [12].

The literature concerning catalytic ozonation indicates several controversies in this field. Frequently, the same catalysts, studied by different research groups, lead to different, sometimes contradictory results. This aspect of research in the field of catalytic ozonation is particularly emphasised in this manuscript. The verification of mechanisms governing catalytic ozonation seems to be particularly problematic, as the usage of catalysts in aqueous solutions will lead to competition between water, ozone and organic compounds for catalytic (adsorptive) active sites.

Several research groups have reported the high efficiency of catalytic ozonation in the removal of organic contaminants in water. There is, however, limited understanding of this technique as far as mechanisms of the process are concerned. There are therefore several questions that need answering in order to understand the processes occurring during catalytic ozonation and to apply this emerging chemical oxidation technique on a technological scale:

• It is not clear whether catalysts cause ozone decomposition leading to hydroxyl radicals formation (some published reports suggest mechanisms that are different from a radical pathway leading to hydroxyl radicals formation).

• It is not clear how ozone is decomposed on the surface of catalysts (several mechanisms are proposed and the major question that needs answering is whether hydroxyl radicals are formed directly due to ozone decomposition on the surface of catalyst or indirectly, as a result of secondary reactions).

• How do natural water constituents such as sulfates, phosphates, carbonates, bicarbonates and fluorides influence the catalytic ozonation process, e.g. catalytic activity?

• What is the long-term activity of catalysts?

• Which factors influence catalytic activity (why are some materials active and some not)?