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

Wenjing Han; Hongyu Sun; Siyu Zhang; Qing Zhao; Xuejiao Zhang; Yuqin Ma; Jingwen Chen; Haibo Li

doi:10.1016/j.chemosphere.2019.125200

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

Chemosphere. 2020 Mar:242:125200. doi: 10.1016/j.chemosphere.2019.125200. Epub 2019 Oct 24.

Authors

Wenjing Han¹, Hongyu Sun², Siyu Zhang³, Qing Zhao⁴, Xuejiao Zhang⁴, Yuqin Ma⁵, Jingwen Chen⁶, Haibo Li²

Affiliations

¹ Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, 110016, China; Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian, 116024, China.
² School of Resources and Civil Engineering, Northeastern University, Shenyang, 110819, China.
³ Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, 110016, China. Electronic address: syzhang@iae.ac.cn.
⁴ Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, 110016, China.
⁵ School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun, 130022, China.
⁶ Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian, 116024, China.

PMID: 31683163
DOI: 10.1016/j.chemosphere.2019.125200

Abstract

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.

Keywords: Cyclic methylsiloxanes; Density functional calculation; H-abstraction; Hydroxyl radicals; Photooxidation mechanism.

MeSH terms

Density Functional Theory
Hydrogen Peroxide / chemistry
Hydroxyl Radical / chemistry*
Kinetics
Oxidation-Reduction
Siloxanes / chemistry*
Structure-Activity Relationship
Ultraviolet Rays
Water / chemistry*
Water Pollutants, Chemical / analysis
Water Pollutants, Chemical / chemistry*

Substances

Siloxanes
Water Pollutants, Chemical
Water
Hydroxyl Radical
Hydrogen Peroxide