## 1 Introduction  
Extensive agricultural production implies continuous use of agrochemicals to guarantee pest control and maximize crop yields. Since the 1970s, pesticides have been detected in environmental matrices such as soil, water and in organisms at practically all levels of the food chain due to their bioaccumulative and biomagnifiable characteristics. Even more relevant are their carcinogenic, mutagenic and teratogenic effects [1,2]. Pesticides are classified based on their chemical structure and functional groups as carbamates, organochlorines, organophosphates, pyrethroids and bipyridyls. Currently, organophosphate pesticides are the most widely used insecticides in tropical regions, representing health risks since their mechanism of action inhibits the cholinesterase enzyme [3]. Around seventy percent of the extracted water volume is used for agriculture, hence this economic activity is considered the largest water consumer. In developing countries this volume reaches up to 95%, because irrigation practices do not allow optimal water use [4]. Consequently, a considerable volume of cropland wastewater (CropWW) is generated, which is characterized by low biodegradability and a high concentration of nutrients [5; 6]. The CropWW is transported through the infrastructure of agricultural drains to surface water bodies, sometimes ending in the aquifer that supplies water for the population. In both cases, the composition and volume of CropWW discharged generates diffuse discharges difficult to control which causes pollution problems, eutrophication and health risks. Cereals are one of the primary sources of food around the world. In 2018/19 agricultural cycle, their production amounted to 2653 million tons with an estimated increase of 2.3% for the following cycle [7]. In tropical regions, maize requires between 500 and 800 mm of water layer per cycle [8]. During the cultivation stage, malathion is intensively applied for insect control which is subsequently released into the environment through agricultural drains. Despite its half-life of less than one week, this pesticide has been quantified in agricultural drains and in the vicinity of water bodies and river mouths of neighboring water bodies in concentrations ranging from 0.005 to 3.3 ug/L [9; 10]. Chemical formula of malathion is C\({}_{10}\)H\({}_{19}\)O\({}_{6}\)PS\({}_{2}\), and it presents chronic and environmental toxicity characteristics; its LC\({}_{50}\) for rats by inhalation is 43.8 mg/m\({}^{3}\)/4 h (PubChemChem, 2019) [3]. This pesticide class is not as persistent as organochlorines, but its degradation byproducts, such as malaoxon, have a higher bioactivity and toxicity [11; 12]. Moreover, CropWWs have a high load of nitrogen and phosphorus from fertilizers, thus causing eutrophication problems in agricultural basins [13]. Therefore, implementation of effective and feasible CropWW treatment is crucial to mitigate water bodies' pollution. Ref. [14] reported removal efficiencies of chlororyrifos, lambda-cyhalothrin and diazinon of 73.3%, 68.9%, 49.1% and 84%, respectively, as chemical oxygen demand (COD) through a photocatalytic process of wastewater from an agrochemical and pesticides company. The authors reported 120 min as optimal irradiation time, and H\({}_{2}\)O\({}_{2}\) and TiO\({}_{2}\) concentrations of 1.5 g/L and 2 g/L, respectively. Ref. [15] measured atrazine degradation (compound with toxicity and application volume similar to malathion) by a UV/ H\({}_{2}\)O\({}_{2}\) system, reaching a complete removal after 30 min of irradiation with 20 mg/L H\({}_{2}\)O\({}_{2}\). However, the concentration of non-biodegradable byproducts increased over time. Ref. [16] tested organophosphate pesticide removal by different advanced oxidation processes (AOP), such as UV/H\({}_{2}\)O\({}_{2}\), photo-Fenton and ozonation, using concentrations up to 12 and 11 mg/L of malathion and parathion. All treatments tested removed around 100% of both pesticides at 90 min of operation.