Hydroxyl radical oxidation of cyclic methylsiloxanes D4 ∼ D6 in aqueous phase

Cyclic methylsiloxanes (CMS) were listed as candidates of substances of very high concerns in 2018 by the REACH. These compounds can enter environmental waters, and potentially cause harmful effects to aquatic organisms and human beings. Until now, reaction mechanisms of these pollutants with hydroxyl radicals (HO) in aqueous phase were unknown. In this study, reaction mechanisms of three typical CMS (D4 ∼ D6) with HO in aqueous phase were investigated by employing both UV/H₂O₂ experiments and density functional theoretical calculations. Bimolecular reaction rate constants (k_HO·) of D4 ∼ D6 with HO were determined as k_HO·(D4) = 8k_HO·(D5) = 12k_HO·(D6) = 6.6 × 10⁸ L mol^-1 s^-1. Half-lives of HO oxiding D4 ∼ D6 ranged from 12 to 140 days at [HO] = 10^-15 mol L^-1 in sunlit surface water, and were comparable to (D4, D5) or much shorter (D6) than hydrolytic half-lives. The reactivity to HO decreased with the increasing size of siloxane ring in aqueous phase, in an order totally opposite to that in gaseous phase. Calculation results indicated that HO oxidation of the three CMS proceeded spontaneously through an exothermic H-abstraction process at the first step. Water molecules participated into H-abstraction of CMS and caused energy barrier of D5 higher than that of D4. Thus, H-bonds formed by water molecules were responsible for the reverse reactivity of CMS in aqueous phase. This work provided basic evidences suggesting environmental persistence of CMS in aqueous phase completely different from that in gaseous phase.