Adsorption kinetics and degradation mechanism study of water persistent insecticide quinalphos: for heterogeneous photocatalysis onto TiO2

摘要

The kinetics of adsorption and photodecomposition of the water polluting insecticide quinalphos (QP), in aqueous suspensions of TiO2 were evaluated under laboratory conditions. Langmuir and Freundlich models were applied to describe the adsorption process and the model proposed by Langmuir-Hinshelwood was used to illustrate the kinetics of the photodecomposition reactions of QP. The adsorption capacity of TiO2 observed to increase from 7.86 to 20.04 mg g(-1) with the increase in initial QP concentration from 5 to 25 mg L-1, respectively. About 45.8% adsorption of 20 mg L-1 QP was observed in just 60 min at pH 6 with 5 g L-1 TiO2 loading and 25 +/- 1 degrees C temperature. TiO2 was found to have a maximum adsorption capacity of 23.7 mg g(-1) for QP. Photocatalytic treatment of 200-mL solution (20 mg L-1) under least harmful UV-A radiations (365 nm/30 W m(-2)) with 0.5 g L-1 TiO2 loading gives nearly 89.67% degradation in 3h. The photodecomposition rate of QP was significantly higher at slightly acidic conditions (pH 6). Increasing light intensity from 20 to 35 W m(-2) and aperture to volume ratio of the photoreaction from 0.515 to 1.327 cm(2) mL(-1) shows a notable increase in the decomposition rate of QP. Formation of two complex intermediates (m/z 282 and 284) and a few smaller compounds (m/z 104, 108, and 149) were detected by the gas chromatography-mass spectrometry analysis of the reaction sample after 3h UV irradiation in the presence of TiO2. These intermediates were further dissociated to simpler hydrocarbons, possibly, by reacting with by reacting with (OH)-O-center dot and O-center dot species.