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

Abstract

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°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 3 h. 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 cm² 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 3 h UV irradiation in the presence of TiO₂. These intermediates were further dissociated to simpler hydrocarbons, possibly, by reacting with OH and O species.

Keywords:

• Quinalphos
• Titanium dioxide
• Catalytic adsorption
• Slurry reactor
• Photodegradation

Acknowledgments

We would like to acknowledge the following for support and help: the instrumental facilities provided by Central Instrumentation Laboratory (CIL), Punjab University, Chandigarh. Markfed Unit, Mohali (Punjab) for providing the sample of insecticide (QP). Aeroxide (China) for providing free sample of P25-TiO₂.