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Photocatalytic degradation of a typical agricultural chemical: metalaxyl in water using ­TiO2 under solar irradiation Jahida Binte Islam1 · Mai Furukawa1 · Ikki Tateishi2 · Hideyuki Katsumata1 · Satoshi Kaneco1,2 Received: 24 January 2020 / Accepted: 7 April 2020 © Springer Nature Switzerland AG 2020

Abstract The photodegradation and mineralization of the metalaxyl [methyl N-(2,6-dimethyl-phenyl)-N-(methoxyacetyl)-alaninate], which is a popular benzenoid fungicide, was conducted in the presence of ­TiO2 photocatalyst under solar irradiation. An initial metalaxyl concentration of 50 ppm was completely degraded in presence of ­TiO2 after 30 min irradiation, while no degradation was observed in absence of ­TiO2 under solar irradiation. The effect of different parameters, such as amount of ­TiO2, initial pH, light intensity, reaction temperature and irradiation time, on the photocatalytic degradation of metalaxyl was evaluated. The drop of total organic carbon as a consequence of mineralization of metalaxyl was detected during the photocatalytic process. The kinetics of photocatalytic degradation followed a pseudo-first order law according to Langmuir–Hinshelwood model, and the rate constant was 0.105 min−1. Ammonium ion and CO2 were speculated as the end-products after completing degradation of metalaxyl. The five types of intermediate products were identified by GC–MS during the decomposition of metalaxyl. In order to investigate the degradation pathway of metalaxyl, the point charge and frontier electron density at each atom on the molecule were determined by using MOPAC stimulation. The degradation mechanism was proposed from the identified intermediates. The solar photocatalytic degradation method can become an effective technique for the treatment of metalaxyl-polluted water. Keywords  Metalaxyl · Photodegradation · TiO2 photocatalyst · Point charge · Frontier electron density · GC–MS

1 Introduction Metalaxyl is a fundamental benzenoid fungicide, and can be immensely applied to the plant diseases caused by Oomycete fungi [1]. Its IUPAC name is methyl N-(2, 6-dimethyl-phenyl)-N-(methoxyacetyl)-alaninate and molecular weight is 279.33 g/mol. The molecular structure of metalaxyl is shown in Fig. 1. This fungicide is very active

across soil-borne and foliaceous diseases. It is continuously used in agricultural purpose because of its great tolerance for light, temperature and pH of soil [2]. Metalaxyl is frequently sprayed on plant after dilution [3]. It has long half-life in soil and it is greatly soluble in water (solubility: 8400 mg/L) which can issue a great threat of environment [4, 5]. The excessive application of metalaxyl can stimulate the contamination of soil and ground water, owing to high

All experiments were conducted at Mie University. Any opinions, findings, conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the view of the supporting organizations. Electronic supplementary material  The online version of this article (https​://doi.org/10.1