**1. Introduction**

Pesticide may be defined as 'any substance or mixture of substances intended for preventing, destroying, or controlling any pest including vectors of human or animal diseases, unwanted species of plants or animals causing harm during, or otherwise interfering with, the production, processing, storage, or marketing of food, agricultural commodities, wood and wood

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products, or animal feedstuffs, or which may be administered to animals for the control of insects, arachnids or other pests in or on their bodies. The term includes chemicals used as growth regulators, defoliants, desiccants, fruit thinning agents, or agents for preventing the premature fall of fruits, and substances applied to crops either before or after harvest to prevent deterioration during storage or transport' [1]. There are many hundreds of them including herbicides, insecticides, fungicides, rodenticides, nematicides, plant growth regulators and others. Pesticide products contain both 'active' and 'inert' ingredients: active ingredients are the chemicals in pesticide products that kill, control or repel pests. All other ingredients are called 'inert ingredients,' which are important for product performance and usability.

Pesticides have been widely applied to protect agricultural crops since the 1940s, and their use increased steadily during the subsequent decades. The *Green Revolution* was the notable increase in cereal-grain production in many developing countries in the 1960s and 1970s. This tendency resulted from the introduction of hybrid strains of wheat, rice and corn and the adoption of modern agricultural technologies, including irrigation and large doses of agrochemicals, fertilisers and pesticides [2]. However, Rachel Carson (1907–1964) with the publishing of her sensational book *Silent Spring* in 1962 [3] warned of the dangers to all natural systems from the misuse of some pesticides such as DDT (1,1,1-trichloro-2,2-di(4-chlorophenyl) ethane). As a result, a wider audience was warned of the environmental effects of the widespread use of pesticides, and DDT was banned for agricultural use 10 years later in the USA, and the regulation of chemical pesticide use was strengthened. Currently, all pesticides are subject to strict registration as meaning 'the process whereby the responsible national government authority approves the sale and use of a pesticide following the evaluation of comprehensive scientific data demonstrating that the product is effective for the purposes intended and not unduly hazardous to human or animal health or the environment' [4]. In evaluating a pesticide registration application, a wide variety of potential human health and environmental effects associated with their use must be tested. Registrants must generate the necessary scientific information to address concerns corresponding to the identity, composition, potential adverse effects and environmental fate of each pesticide. These data allow evaluating whether a pesticide could harm certain nontarget organisms and endangered species.

Protection of crop losses/yield reduction and increase in food quality are key benefits associated with the use of pesticides in agriculture. However, most organic pesticides characterised as persistent in the environment can bioaccumulate through the food web and can be transported in long distances [5], as evidenced by the accumulation in regions where persistent pesticides have never been used [6]. Persistent organic pollutants (POPs) are *chemical compounds that persist in the environment and adversely affect human health and the environment around the world* [7]. Because they can be transported by wind and water, most POPs can affect human health and wildlife far from where they are applied. They have high persistence in the environment and can accumulate passing from one species to the next through the food chain. To treat this environmental concern, the USA joined forces with 90 other countries and the European Community to sign the agreement United Nations treaty in Stockholm (Sweden, May 2001). The *Stockholm Convention on Persistent Organic Pollutants*, approved by Council Decision 2006/507/ EC [8], entered into force on 17 May 2004. The aim of the Convention was to protect human health and the environment from POPs. Under the treaty, known as the *Stockholm Convention*, countries agreed to reduce or eliminate the production, use and/or release of 12 key POPs ('the dirty dozen', mainly organochlorine (OC) insecticides) and specified under the Convention a scientific review process that has led to the addition of other POP chemicals of global concern. Currently, there are 30 substances catalogued as POPs including mainly pesticides, industrial chemicals and by-products.

products, or animal feedstuffs, or which may be administered to animals for the control of insects, arachnids or other pests in or on their bodies. The term includes chemicals used as growth regulators, defoliants, desiccants, fruit thinning agents, or agents for preventing the premature fall of fruits, and substances applied to crops either before or after harvest to prevent deterioration during storage or transport' [1]. There are many hundreds of them including herbicides, insecticides, fungicides, rodenticides, nematicides, plant growth regulators and others. Pesticide products contain both 'active' and 'inert' ingredients: active ingredients are the chemicals in pesticide products that kill, control or repel pests. All other ingredients are

148 Application of Titanium Dioxide

called 'inert ingredients,' which are important for product performance and usability.

Pesticides have been widely applied to protect agricultural crops since the 1940s, and their use increased steadily during the subsequent decades. The *Green Revolution* was the notable increase in cereal-grain production in many developing countries in the 1960s and 1970s. This tendency resulted from the introduction of hybrid strains of wheat, rice and corn and the adoption of modern agricultural technologies, including irrigation and large doses of agrochemicals, fertilisers and pesticides [2]. However, Rachel Carson (1907–1964) with the publishing of her sensational book *Silent Spring* in 1962 [3] warned of the dangers to all natural systems from the misuse of some pesticides such as DDT (1,1,1-trichloro-2,2-di(4-chlorophenyl) ethane). As a result, a wider audience was warned of the environmental effects of the widespread use of pesticides, and DDT was banned for agricultural use 10 years later in the USA, and the regulation of chemical pesticide use was strengthened. Currently, all pesticides are subject to strict registration as meaning 'the process whereby the responsible national government authority approves the sale and use of a pesticide following the evaluation of comprehensive scientific data demonstrating that the product is effective for the purposes intended and not unduly hazardous to human or animal health or the environment' [4]. In evaluating a pesticide registration application, a wide variety of potential human health and environmental effects associated with their use must be tested. Registrants must generate the necessary scientific information to address concerns corresponding to the identity, composition, potential adverse effects and environmental fate of each pesticide. These data allow evaluating whether a pesticide could harm certain nontarget organisms and endangered species. Protection of crop losses/yield reduction and increase in food quality are key benefits associated with the use of pesticides in agriculture. However, most organic pesticides characterised as persistent in the environment can bioaccumulate through the food web and can be transported in long distances [5], as evidenced by the accumulation in regions where persistent pesticides have never been used [6]. Persistent organic pollutants (POPs) are *chemical compounds that persist in the environment and adversely affect human health and the environment around the world* [7]. Because they can be transported by wind and water, most POPs can affect human health and wildlife far from where they are applied. They have high persistence in the environment and can accumulate passing from one species to the next through the food chain. To treat this environmental concern, the USA joined forces with 90 other countries and the European Community to sign the agreement United Nations treaty in Stockholm (Sweden, May 2001). The *Stockholm Convention on Persistent Organic Pollutants*, approved by Council Decision 2006/507/ EC [8], entered into force on 17 May 2004. The aim of the Convention was to protect human health and the environment from POPs. Under the treaty, known as the *Stockholm Convention*, In addition, many pesticides are endocrine-disrupting chemicals (EDCs), *compounds that alter the normal functioning of the endocrine system of both wildlife and humans increasing incidence of breast cancer, abnormal growth patterns and neurodevelopmental delays in children, as well as changes in immune function* [9–11]. Most of them are organochlorine (OC) pesticides that affect the reproductive function.

Worldwide consumption of pesticides for agricultural use is constantly increasing as increased human population and crop production, and it has undergone significant changes since the 1960s. Nowadays, the worldwide consumption of pesticides is about 2 million tonnes per year, which 45% is used by Europe alone, 25% is consumed in the USA and 30% in the rest of the world. The proportion of herbicides in pesticide consumption increased rapidly, from 20% in 1960 to 47.5% in 2015. However, the proportion of consumption of insecticides (29.5%) and fungicides/bactericides (17.5%) declined despite their sales increased with other accounts for 5.5% only [12]. The rapid increase of herbicide consumption enhanced agricultural intensification and productivity.

The application of chemical pesticides, in particular the organic-synthesised pesticides, has been a significant mark of human civilisation, which greatly protects and facilitates agricultural productivity. Worlwide, insect pests cause an estimated 14% loss, plant pathogens cause a 13% loss, and weeds causes a 13% loss [13]. Pesticides are so indispensable in agricultural production. About one-third of the agricultural products are produced by using pesticides [14]. Without pesticide application, the loss of fruits, vegetables and cereals from pest injury would reach 78, 54 and 32%, respectively [15]. Ideally, a pesticide must be lethal to the targeted pests, but not to nontarget species, including humans. Unfortunately, this is not the case, so the controversy of use and abuse of pesticides is obvious. Consequently, the risks of using pesticides are serious as well [13]. Most pesticides are not spontaneously generated, and they are toxic to humans and the environment in greater or lesser degree [16]. Pesticides and their degraded products can pass into the atmosphere, soils and water, resulting in the accumulation of toxic substances and thus threatening human health and the environment. In addition, accumulated application leads to loss of biodiversity. Because many pesticides are barely degradable, they persist in soil and pollute surface water and groundwater. Depending on their chemical properties, they can bioaccumulate in food chains and consequently affect human health.

For many pesticides, there is evidence of long-term ubiquity in the aquatic environment at the European Union (EU) level, and therefore they need special consideration as regards their impact on the presentation of chemical status under the European Water Framework Directive (EWFD) [17]. The EWFD establishing a framework for community action in the field of water policy lays down a strategy against the pollution of water. Directive 2008/105/EC [18], amended by Directive 2013/39/EU [19] on environmental quality standards in the field of water policy, lays down environmental quality standards in accordance with the EWFD, for the 33 priority substances. The EWFD also sets out general provisions for the protection and conservation of groundwater. The establishment of detailed quality criteria for the assessment of groundwater chemical status in Europe was laid down in the European Groundwater Directive [20]. For this reason, the EU established the following groundwater quality standards: 0.1 μg L−1 for individual pesticide and 0.5 μg L−1 for the sum of all individual pesticides to safeguard people from harmful effects.

During the 1990s of the last century, atrazine (herbicide) and endosulfan (insecticide) were found most often in surface waters in the USA and Australia due to their widespread use. In addition, although in lower proportions, other pesticides such as pronofos, dimethoate, chlordane, diuron, prometryn and/or fluometuron were detected [21]. Studies that are more recent also reported the presence of several pesticides in environmental waters (surface water, groundwater and seawater) close to agricultural lands over the world [22–34]. In addition, different studies have corroborated the presence of some pesticides in drinking water [35–38].

Pesticides are being continuously released into the aquatic environment through anthropogenic activities. Their detection in storm and wastewater effluent has been reported to be a major obstacle as regards wide-ranging acceptance of water recycling [39]. In addition, their variety, toxicity and persistence present a threat to humans through pollution of drinking water resources (e.g. surface water and groundwater). The frequent occurrence of pesticides in surface water and groundwater has prompted the search for suitable methods to destroy them. Although conventional biological treatments of water offer some advantages such as their low cost and easy operation, most studies concerning the treatment of pesticides have concluded that they are not very effective due to their low biodegradability [40, 41]. Other technologies such as adsorption or coagulation merely concentrate pesticides by transferring them in other phases but still remain and not being completely eliminated. To solve this problem, apart from reducing emissions, two main water strategies are followed: (i) chemical treatment of drinking water, surface water and groundwater and (ii) chemical treatment of wastewaters containing biocides and bio-recalcitrant pollutants as pesticides. Chemical treatments of polluted surface water, waste water and groundwater are part of a long-term strategy to improve the quality of water by removing toxic compounds of anthropogenic origin before returning the water to its natural cycle. The Directive 2013/39/EU [19] promotes the preventive action and the polluter pays principle, the identification of pollution causes, dealing with emissions of pollutants at the source, and finally the development of innovative wastewater treatment technologies, avoiding expensive solutions. Therefore, effective, low-cost and robust methods to decontaminate waters are needed, as long as they do not further stress the environment or endanger human health, particularly prior to direct or indirect reuse of reclaimed water. In this context, the development of Solar Chemistry Applications is of special relevance, especially photochemical processes where solar photons are absorbed by reactants and/or a catalyst causing a chemical reaction. Consequently, in recent years there has been growing interest in the use of advanced oxidation processes (AOPs) to remove pesticide residues as alternative to methods that are more conventional because they allow the abatement of them by mineralisation.
