Reaction pathways and mechanisms of photodegradation of pesticides

Abstract

The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processes involved, the nature of reactive intermediates and final products. Potential use of photochemical processes in advanced oxidation methods for water treatment is also discussed. Processes considered include direct photolysis leading to homolysis or heterolysis of the pesticide, photosensitized photodegradation by singlet oxygen and a variety of metal complexes, photolysis in heterogeneous media and degradation by reaction with intermediates generated by photolytic or radiolytic means.

Introduction

Due to the world-wide general application of intensive agricultural methods during the last few decades and to the large-scale development of the agrochemical industry, the variety and quantities of agrochemicals present in continental and marine natural waters has dramatically increased. Most pesticides are resistant to chemical and/or photochemical degradation under typical environmental conditions [1]. In recent years, the scientific community has shown a great concern about the possible adverse effects that the presence of these pesticides in water and food [2] may have for human health and for the equilibrium of ecosystems [3], [4], [5], [6], [7]. Such concern, which has recently been highlighted [8], [9], is supported by results from major monitoring studies already performed over 20 years ago [10], and confirmed by more recent investigations. Among the possible chronic effects of these compounds are carcinogenesis [11], [12], neurotoxicity [13], effects on reproduction [14] and cell development effects, particularly in the early stages of life [15]. With increasing global demand for vegetables, the situation does not look likely to improve. In fact, the current situation might worsen with the appearance of new and more sophisticated substances.

The critical nature of this environmental problem has prompted the development of faster and more accurate methods for the characterisation and quantification of the pesticides dispersed in the environment. These have generally been very successful. However, until now no completely efficient methods have been developed for remediation of contaminated waters [16].

Advanced oxidation processes (AOPs) are at present considered to have considerable potential in this area and a general survey of these can be found in the review by Legrini et al. [17] The most efficient of these used nowadays involve the use of UV irradiation with light of an appropriate wavelength [18], [19], [20], [21]. The method is based on the ability of UV radiation to attack and damage the DNA of undesirable microorganisms. As a consequence, photochemical processes may take place, in which different transient species are generated: e⁻_aq (photoionization), radicals generated by bond homolysis or bond heterolysis, etc, as well as a number of photophysical processes (fluorescence, phosphorescence, etc.) Alternatively, UV-sensitive materials may be added to give rise to photosensitized oxidation, that allow the use of wavelengths that are not absorbed by the pesticides [17]. Hydroxyl radical (HO^⋅) might also be generated by various different ways, giving rise to induced photodegradation.

The regulations for the control of the quality of water for human use in the European Union clearly establishes the maximum admissible values of different substances in solution [22]. Among these, pesticides are identified as toxic, with very low concentrations of them in general being allowed (a maximum permissible concentration for a particular pesticide of 0.1 ppb and of 0.5 ppb for the total load of all plaguicides). In addition, these regulations also establish maximum concentrations for the products of degradation of pesticides [23], [22]. No maximum concentration levels have been established in the USA, the regulations depending on the particular compounds, based on the toxicological evidences available for them [24].

The literature reports on pesticide photodegradation products is relatively abundant. However, little information is available on the reaction mechanisms involved in the photolysis of pesticides under typical environmental conditions [1]. The environmental photochemistry of herbicides was reviewed several years ago [25]. Similarly, the kinetics and mechanisms of photodegradation of chlorophenol pesticides have recently been discussed [26].

Considering the environmental relevance and importance of this problem, we review the state-of-the-art situation on the photodegradation of pesticides. These will be classified in terms of the main structural groups, following a chronological order within each group. We shall concentrate on the behaviour in solution and on solid supports. The photochemical decomposition of pesticides and herbicides on plants, on model plant systems and on soil is of fundamental importance to the situation in vivo. Some studies in this area, notably on model plant cuticles, are presented in Section 2. However, much less detailed mechanistic information is available in this case, and we feel, that as a first approximation, it is not unreasonable to extrapolate the behaviour from solution or solid supports to these systems. Most papers reviewed are from the last decade, although some previous articles have also been included to facilitate understanding of mechanistic pathways. Although the production and use of some of the pesticides discussed has been discontinued, we have still considered them, because of their possible persistence in the environment. In identifying products, we report a systematic name in all cases, as some of the common commercial names have changed through the years (CAS names are given in those cases where no systematic name has been found). In addition, we note that several excellent compendia are available compiling the different names and properties of the various different compounds [27], [28], [29], [30], [31]. We have chosen to classify the photodegradation studies into four broad categories: direct photodegradation, photosensitized degradation, photocatalyzed degradation and degradation by reaction with hydroxyl radical. Within each group these are discussed in terms of general classes of compounds. However, in some cases these categories overlap. In such situations, we have tried to fit the work to the field that is the authors’ main target area. Finally, our aim in this review is to compile and annotate the main reaction types and mechanisms. We do not attempt to evaluate or authoritatively criticise the cited literature, but hope that this review will stimulate more detailed analyses of the data in each of the particular areas discussed.