Degradation of VX Surrogate Profenofos on Surfaces via in Situ Photo-oxidation

Abstract

Surface degradation of profenofos (PF), a VX nerve gas surrogate, was investigated using in situ photo-oxidation that combines simple instrumentation and ambient gases (O₂ and H₂O) as a function of exposure conditions ([O₃], [OH], UV light λ = 185 and/or 254 nm, relative humidity) and PF film surface density (0.38–3.8 g m^–2). PF film 0.38 g m^–2 fully degraded after 60 min of exposure to both 254 and 185 nm UV light in humidified air and high ozone. The observed pseudo-first-order surface reaction rate constant (<i>k</i>_obs = 0.075 ± 0.004 min^–1) and calculated hydroxyl concentration near the film surface ([OH]_g = (9 ± 2) × 10⁷ molecules cm^–3) were used to determine the second-order rate constant for heterogeneous reaction of PF and OH (<i>k</i>^OH_PF = (5 ± 1) × 10^–12 cm³ molec^–1 s^–1). PF degradation in the absence of 185 nm light or without humidity was lower (70% or 90% degradation, respectively). With denser PF films ranging from 2.3 to 3.8 g m^–2, only 80% degradation was achieved until the PF droplet was redissolved in acetonitrile which allowed >95% PF degradation. Surface product analysis indicated limited formation of the nontoxic phosphoric acid ester but the formation of nonvolatile chemicals with increased hydrophilicity and addition of OH