Neonicotinoid insecticides are a group of pesticides used to control insect pests whose main food is crops. The pesticides enter surface or groundwater after application because they are difficult to adsorb to soil particles. They are relatively persistent in water and pose a risk to human and animal health, so they must be removed in an appropriate and safe manner. Advanced oxidation processes are prescribed as an alternative to classical treatment processes, and one of these processes is heterogeneous photocatalysis. The aim of this work is to study the photocatalytic degradation of the neonicotinoid insecticide imidacloprid in an aqueous solution. The experiments were performed in a process vessel with a volume of 1L with recirculation of the reaction mixture with a volume of 500 mL using a UVA LED radiation source at a constant voltage of 32V with a heterogeneous photocatalyst based on perlite as a support for the TiO2 photocatalyst. Before performing the experiment, an experimental design was prepared using the Design-Expert software package. The influence of intensity (5; 6,5; 8 mW/cm^2), recirculation (50; 150; 250 mL/min) and photocatalyst mass (1,5; 3; 4,5 g) on the transformation of the tested pollutant was investigated. The course of degradation of imidacloprid was monitored by high performance liquid chromatography. From the experimental data, the photocatalytic degradation conversions and reaction rate constants were determined according to the proposed Langmuir-Hinshelwood kinetic model, and the root mean square deviation (RMSD) was calculated for each experiment. Good agreement was found between the model and experiment. The experimental test results showed that the most important parameter of photocatalytic degradation under the conditions used in this work is exactly the mass of the photocatalyst. The highest conversion and the highest rate constant of the chemical reaction were observed in the experiments with the highest tested mass of the photocatalyst (4.5 g), the highest recirculation (250 ml/min) at medium radiation intensity (6,5 mW/cm^2, UVA LED 20 W and 30 W). The worst pollutant removal was observed for the experiment with the smallest mass of the photocatalyst (1,5 g), the lowest radiation intensity (5 mW/cm^2, UVA LED 2x20W), but with the medium recirculation value (150 mL/min).