Photocatalytic and Kinetic Study on the Degradation of Three Food Pesticides Using Vanadium-Substituted Polyoxotungstates

Mono-, di-, and tri-vanadium-substituted Keggin-type heteropolyoxoanions [SiW 12 O 40 ] 4− and [PW 12 O 40 ] 4− were evaluated as photocatalysts for the photodegradation of three hazardous food pesticides: atrazine, chlorpyrifos, and dieldrin. Kinetic experiments were performed under UV irradiation at 254 nm. The degradation of each pesticide was assessed by investigating its disappearance with time using high-performance liquid chromatography equipped with an ultra-violet spectrophotometer detector. The photocatalytic degradation of the three pesticides exhibited first-order kinetics. It was found that the introduction of vanadium addenda atoms into the Keggin-type polyoxometalates decrease the degradation rate for the photocatalytic transformation of each pesticide as well as the degradation percentage. This effect was significantly related to the number of vanadium metal ions substituting the tungsten addenda atoms. As a general trend, the photocatalytic efficiency of {XVW 11 } was better than that of {XV 3 W 9 }. Accordingly, a marked drop was noticed in the photocatalytic degradation of atrazine, where 90% was decomposed in the presence of [α-SiW 12 O 40 ] 4− at a rate of 1 mg/L min, whereas the degradation percentage decreased to 55% in the presence of [α-SiVW 11 O 40 ] 5− at a decreased rate of 0.7 mg/L min. Hence, the negative effect on the degradation percentage was evident for the Si-based POMs, which drops from 90 to 38%, 83 to 32%, and 60 to 23% for atrazine, chlorpyrifos, and dieldrin, respectively. Similar effect was observed for the P-based POMs under the studied conditions.