Degradation Kinetics and Mechanism of RDX and HMX in TiO₂ Photocatalysis

Abstract

This study was undertaken to examine the photocatalytic degradation of explosives hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) with a circular photocatalytic reactor, using a UV lamp as a light source and TiO₂ as a photocatalyst. The effects of various parameters, such as the RDX or HMX concentration, the amount of TiO₂, and the initial pH, on the photocatalytic degradation rates of explosives were examined. In the presence of both UV light and TiO₂, RDX and HMX were more effectively degraded than with either UV or TiO₂ alone. The degradation rates were found to obey pseudo-first-order kinetics represented by the Langmuir-Hinshelwood model. Increases in the RDX and HMX degradation rates were obtained with decreasing initial concentrations of the explosives. The RDX and HMX degradation rates were higher at pH 7 than at either pH 3 or pH 11. A dose of approximately 0.7 g l⁻¹ of TiO₂ degraded HMX more rapidly than did higher or lower TiO₂ doses. RDX (20 mg l⁻¹) photocatalysis resulted in an approximately 20% decrease in TOC, and HMX (5 mg l⁻¹) photocatalysis resulted in a 60% decrease in TOC within 150 minutes. A trace amount of formate was produced as an intermediate that was further mineralized by RDX or HMX photocatalysis. The nitrogen byproducts from the photocatalysis of RDX and HMX were mainly NO₃ ⁻ with NO₂ ⁻ and NH₄ ⁺. The total nitrogen recovery was about 60% from RDX (20 mg l⁻¹), and 70% from HMX (5 mg l⁻¹), respectively. Finally, a mechanism for RDX/HMX photocatalysis was proposed, along with supporting qualitative and quantitative evidence.

Keywords:

• HEXAHYDRO-1,3,5-TRINITRO-1,3,5-TRIAZINE (RDX)
• OCTAHYDRO-1,3,5,7-TETRANITRO-1,3,5,7-TETRAZOCINE (HMX)
• TIO2
• PHOTOCATALYSIS
• MINERALIZATION