Degradation of VX surrogate profenofos on surfaces via in situ photo-oxidation

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 (k_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 (k^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.