A Bimetallic Fe-Mn Oxide-Activated Oxone for In Situ Chemical Oxidation (ISCO) of Trichloroethylene in Groundwater: Efficiency, Sustained Activity, and Mechanism Investigation Xueying Yang, Jingsheng Cai, Xiaoning Wang, Yifan Li, Zhangxiong Wu, Winston Duo Wu, Xiao Dong Chen, Jingyu Sun, Sheng-Peng Sun,* and Zhaohui Wang Cite This: Environ. Sci. Technol. 2020, 54, 3714-3724 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Bimetallic Fe-Mn oxide (BFMO) has been regarded as a promising activator of peroxysulfate (PS), the sustained activity and durability of BFMO for long-term activation of PS in situ, however, is unclear for groundwater remediation. A BFMO (i.e., Mn 1.5 FeO 6.35 ) was prepared and explored for PS-based in situ chemical oxidation (ISCO) of trichloroethylene (TCE) in sand columns with simulated/actual groundwater (SGW/AGW). The sustained activity of BFMO, oxidant utilization efficiency, and postreaction characterization were particularly investigated. Elec- tron spin resonance (ESR) and radical scavenging tests implied that sulfate radicals (SO 4 •- ) and hydroxyl radicals (HO • ) played major roles in degrading TCE, whereas singlet oxygen ( 1 O 2 ) contributed less to TCE degradation by BFMO-activated Oxone. Fast degradation and almost complete dechlorination of TCE in AGW were obtained, with reaction stoichiometry efficiencies (RSE) of ΔTCE/ΔOxone at 3-5%, much higher than those reported RSE values in H 2 O 2 -based ISCO (≤0.28%). HCO 3 - did not show detrimental effect on TCE degradation, and effects of natural organic matters (NOM) were negligible at high Oxone dosage. Postreaction characterizations displayed that the BFMO was remarkably stable with sustained activity for Oxone activation after 115 days of continuous-flow test, which therefore can be promising catalyst for Oxone-based ISCO for TCE-contaminated groundwater remediation. ■ INTRODUCTION Trichloroethylene (TCE) is one of the most frequently detected organic groundwater contaminants. 1 Over the past decades, biotic and abiotic approaches have been pursued to remediate TCE-contaminated groundwater. 2-10 In situ chem- ical oxidation (ISCO) is deemed as one of the most attractive approaches for this purpose. 11,12 As an alternative oxidant for ISCO, the usage of peroxysulfate (PS), reference to peroxymonosulfate (HSO 5 - , available as Oxone) and/or peroxydisulfate (S 2 O 8 2- ), is becoming a promising option. Compared to the commonly used H 2 O 2 , PS is more stable and capable of dispersing a larger distance in the subsurface. 13-16 In particular, sulfate radicals (SO 4 •- , E (SO 4 •- /SO 4 2- ) o = 2.5-3.1 V vs NHE) 17 and hydroxyl radicals (HO • , E (HO • /H 2 O) o = 2.73 V vs NHE) generated from the activation of PS have been proven to effectively degrade most of refractory organic contaminants (RfOCs), 18,19 including TCE as well. 20-22 The rate constants of SO 4 •- and HO • reacting with TCE have been reported as k TCE,SO 4 •- = 1.8 × 10 9 M -1 s -1 and k TCE,HO • = 4.2 × 10 9 M -1 s -1 , respectively. 23,24 Various methods toward the activation of PS have been pursued, mainly including thermolysis, photolysis, chemicals, and electricity. 19,25-30 Heterogenous activation of PS by iron oxide/mineral is deemed as one of the most attractive candidates because of its environmentally benign character- istics and particularly being able to work under neutral pH conditions, which are essentially required for ISCO. However, the slow rate of surface iron (≡Fe III /≡Fe II ) redox cycle limits the catalytic activity of most iron-bearing solid catalysts. 31 Since the reducing potential of PS (V HSO 5 - /SO 4 2- = 1.4-1.8 V and V S 2 O 8 2- /SO 4 2- = 2.01 V vs NHE) 19 are higher than that of Fe III (V Fe III / Fe II ) = 0.77 V vs NHE), 32 the reduction of ≡Fe III by PS is thermodynamically unfavorable. To this end, strong reducing agent (hydroxylamine), 31,33,34 UV irradiation, 35 and electric- Received: January 9, 2020 Revised: February 11, 2020 Accepted: February 18, 2020 Published: February 18, 2020 Article pubs.acs.org/est © 2020 American Chemical Society 3714 https://dx.doi.org/10.1021/acs.est.0c00151 Environ. Sci. Technol. 2020, 54, 3714-3724 Downloaded via UNIV OF NEWCASTLE on March 20, 2020 at 02:27:00 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.