Atmospheric Oxidation of Piperazine by OH has a Low Potential To Form Carcinogenic Compounds

Abstract

Piperazine [HN(CH 2 CH 2 ) 2 NH, PZ] is widely recognized as an efficient solvent for carbon capture (CC). We present the first determination of the rate coefficient, k, and the branching ratios for the reaction of OH with PZ in the gas phase using the technique of pulsed laser photolysis with detection of OH by laser-induced fluorescence giving k 298 K of (2.38 ± 0.28) × 10 −10 cm 3 molecule −1 s −1 . The reaction has a negative temperature dependence parametrized as k OH+PZ = (2.37 ± 0.03) × 10 −10 (T/298) −(1.76±0.08) . The high rate coefficient suggests that gas phase processing in the atmosphere will compete with uptake onto aerosols. The branching ratios, abstraction from C−H versus N−H, have been determined by analysis of OH temporal profiles obtained in the presence of O 2 /NO. The result (r N−H = 0.09 ± 0.06) shows that the potential for forming the carcinogenic nitrosamines or nitramines from PZ oxidation is smaller than for the oxidation of the benchmark CC solvent monoethanolamine (MEA).

An experimental investigation into the atmospheric degradation of piperazine

“…N-nitrosopiperazine and N-nitropiperazine were identified and confirmed using isotopically labelled nitric oxide, with a maximum yield of nitrosamine of less than 5% observed in any samples. This is lower than, and therefore consistent with, the branching ratio determined for the aminyl radical for piperazine of 0.09 ± 0.05 (Onel et al, 2014). Using known reaction pathways for amine photooxidation (Nielsen et al, 2012b), the other photooxidation products observed in this work are likely only explained by subsequent reaction of piperazine degradation products.…”

“…The reaction rate of PZ with OH has been calculated recently at 2.38 Â 10 À10 cm 3 molecule À1 s À1 at 298 K (Onel et al, 2014), which is considerably faster than the reaction rate constants determined for other amines with the OH radical. As a diamine with four carbon centres adjacent to nitrogen atoms, it is expected that its gas phase reaction products would also react rapidly with OH.…”

“…The atmospheric chemistry of PZ is not currently well understood, with two experimental studies having been reported (Nielsen et al, 2012a;Onel et al, 2014). Products which were observed from the degradation of PZ include tetrahydropyrazine, 1-nitrosopiperazine (NPZ), 1-nitropiperazine (Nitro-PZ), pyrazine, ammonia and formaldehyde.…”

Branching ratios for the reactions of OH with ethanol amines used in carbon capture and the potential impact on carcinogen formation in the emission plume from a carbon capture plant

“…The concentrations of MEA and MMEA were determined using the technique developed previously using in situ absorption measurements at 185 nm. 16,18 Absorptions were converted into concentrations using the absorption cross sections of amines, s 185 nm, MEA = (8.53 AE 0.24) Â 10 À18 cm 2 molecule À1 , 16 and s 185 nm, MMEA = (9.71 AE 0.37) Â 10 À18 cm 2 molecule À1 (this work).…”

“…12 Previous studies of MEA, PZ and methylamines have shown that, once these amines are released into the atmosphere, their gas-phase processing, primarily initiated by OH radical reactions, will compete with heterogeneous uptake (lifetimes range from 15 min -1 hour at peak OH concentrations, typically 5 Â 10 6 molecule cm À3 ). [13][14][15][16][17][18] The atmospheric removal of amines by reaction with Cl atoms in gas-phase has been found to be a minor sink for amines (lifetime of B20 days for typical peak Cl concentrations, around 2 Â 10 3 atom cm À3 ). 19,20 A major concern with PCCC is the impact of carcinogenic nitrosamines (R 2 N-NO) and nitramines (R 2 N-NO 2 ), formed in the gas-phase processing of amines, on human health.…”