DOI: 10.14457/tu.the.2019.53
title: Photocatalytic degradation of glyphosate pesticide in the presence of TiO2 pellet for sustainable agriculture

description: In Thailand, glyphosate herbicide has been used intensively to prevent food crops from weeds and grass in agriculture, contributing to the contamination in the environment especially in natural water resources. To decontaminate glyphosate in water, TiO2 photocatalysis has been an effective process to remove glyphosate in water. Therefore, the present study aims to examine the photocatalytic degradation of glyphosate herbicide by using TiO2 pellet. Firstly, the effect of UV types on photocatalytic degradation of glyphosate was studied by using TiO2 powder. The removal efficiency of glyphosate by using UVA and UVC irradiation showed a similar result in the range of 92.67% to 99.03 %. Considering the User's Safety, UVA irradiation was applied in this experiment. In order to solve the separation problem of TiO2 powder (Degussa -P25) from treated water, clay TiO2 pellets were prepared through the calcination temperature at 600 oC for 2 h with range of 0, 5, 10, 20, 30, 40 and 50wt% of TiO2 (CT0, CT5, CT10, CT20, CT30, CT40 and CT50 pellets). As shown in this work, the TiO2/clay ratio can affect to the stability of the pellet in water. The stability of the clay TiO2 pellets in water decreases with the increasing clay content. CT0, CT5, CT10, CT20 pellets were unstable in water, whereas CT30, CT40 and CT50 pellets can be immersed in water. Then, these pellets were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) and Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) and Atomic force microscope (AFM). Next, CT30, CT40 and CT50 pellets was preliminarily applied for the photocatalytic degradation by using methylene blue (MB) as a model organic pollutant. As a result, CT30, CT40 and CT50 pellets can remove MB (10 mgL-1) in water under UVA light (1700 µW/cm2) within 120 mins. CT30 pellets showed the highest photocatalytic activity for MB, with approximately 79.01%. This is due to the roughest surface of CT30 pellets as shown in SEM and AFM images. A rough surface might provide more contact area between reactive oxygen species (ROS) and adsorbed MB on the surface of clay TiO2 pellets. Additionally, the decrease of MB under the dark condition was mainly adsorbed by clay. The enhance adsorption provided by the clay in CT30 pellets could be direct more contaminants to the surface of clay TiO2 pellet for photocatalysis. In order to decontaminate glyphosate in water, the photocatalytic degradation of glyphosate by using CT30, CT40 and CT50 pellets also was studied. CT30, CT40 and CT50 pellets can effectively remove glyphosate (1 mgL-1) under UVA light (2497 µw/cm2) within 240 mins, with approximately 98% of removal efficiency. Also, the photocatalytic degradation rate constant of CT30, CT40 and CT50 pellets were quite similar. Therefore, considering the high-performance removal in both MB (cationic molecule) and glyphosate (anionic molecule), CT30 pellets showed optimum weight percentage of TiO2 and clay in this experiment. Then, the removal efficiency of glyphosate by using CT30 pellets were continue compared with commercial PE-TiO2 pellets within 240 mins under UVA light. Results showed that removal efficiency of glyphosate was reached 99.15% by using CT30 pellets, while the removal efficiency of glyphosate by using PE-TiO2 pellets was lower, with 74.36%. This is probably due to mixture of anatase-rutile in CT30 pellets, leading to improve electron–hole separation. For PE-TiO2 pellets, only anatase phase was observed. Also, CT30 pellets showed a larger surface area than PE-TiO2 pellets. Lastly, photocatalytic degradation of glyphosate by using TiO2 powder found only PO_4^(3-), while photocatalytic degradation of the glyphosate by using CT30 pellets found both AMPA and PO_4^(3-). This difference is might be due to the lower photocatalytic activity of CT30 pellets than TiO2 powder, resulting in AMPA is not rapidly decomposed. Due to AMPA are more toxic and longer half-life than glyphosate, complete degradation of AMPA is essential. With increasing reaction time into 300 mins, thereby, AMPA can be completely disappeared during the photocatalytic reaction by using CT30 pellets. Consequently, CT30 pellets were simply prepared and easily removed from water. Also, glyphosate was easily degraded by using CT30 pellets under UVA light.