Investigation of the degradation kinetics of perfluorooctanoic acid (PFOA) has been carried out to calculate rate constants of the main elementary reactions using the multichannel Rice-Ramsperger-Kassel-Marcus theory and canonical variational transition state theory with small-curvature tunneling correction over a temperature range of 200~500 K. The Arrhenius equations of rate constants of elementary reactions are fitted. The decarboxylation is role step in the degradation mechanism of PFOA. For the perfluorinated carboxylic acids from perfluorooctanoic acid to trifluoroacetic acid, the quantitative structure–activity relationship of the decarboxylation was analyzed with the genetic function approximation method and the structure–activity model was constructed. The main parameters governing rate constants of the decarboxylation reaction from the eight-carbon chain to the two-carbon chain were obtained. As the structure–activity model shows, the bond length and energy of C1–C2 (R_C1–C2 and E_C1–C2) are positively correlated to rate constants, while the volume (V), the energy difference between E_HOMO and E_LUMO (ΔE), and the net atomic charges on atom C₂ (Q_C2) are negatively correlated.

Sulfamethoxazole (SMX) is a typical antibiotic which may bring potential risk to human and aquatic ecosystems. In this paper, the OH-initiated degradation mechanisms of sulfamethoxazole are studied at the MPWB1K/6-311+G(3df,2p)// MPWB1K/6-31+G(d,p) level. The thermodynamic parameters and reaction pathways are analyzed in detail. The addition, abstraction and bond-broken reactions are the three main reaction mechanisms, and bond-broken reactions may lead to the direct degradation of SMX. The rate constants are calculated using the transition state theory and canonical variational transition with small-curvature tunneling correction at the temperature range of 200-500 K. The Arrhenius equations of rate constants are fitted. The rate constants of addition and abstraction reactions at 298.15 K are 3.03×10^-23 cm³molecule^-1s^-1 and 6.86×10^-24 cm³molecule^-1s^-1, respectively. The rate constants of bond-broken reactions are much lower than those of the addition and abstraction reactions, but their significance can not be ignored.