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Remediation and Control Technologies
Reductive Hexachloroethane Degradation by S2O8•- with Thermal Activation of Persulfate under Anaerobic Conditions Changyin Zhu, Fengxiao Zhu, Cun Liu, Ning Chen, Dongmei Zhou, Guodong Fang, and Juan Gao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06279 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018
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Reductive Hexachloroethane Degradation by S2O8•− with Thermal Activation of
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Persulfate under Anaerobic Conditions
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Changyin Zhua,b, Fengxiao Zhua, Cun Liua, Ning Chena,b, Dongmei Zhoua, Guodong
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Fang a,*, Juan Gaoa,*
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a
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, PR China
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b
University of Chinese Academy of Sciences, Beijing 100049, PR China
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*
Corresponding author.
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E-mail address:
[email protected] (G.D. Fang)
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[email protected] (J. Gao).
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ABSTRACT
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Despite that persulfate radical (S2O8•−) is an important radical species formed from the
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persulfate (PS) activation process, its reactivity toward contaminant degradation has
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rarely been explored. In this study, we found that S2O8•− efficiently degrades the
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contaminant hexachloroethane (HCA) under anaerobic conditions, whereas HCA
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degradation is negligible in the presence of oxygen. We observed dechlorination
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products such as pentachloroethane, tetrachloroethylene, and Cl− during HCA
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degradation, which suggest that HCA degradation is mainly a reductive process under
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anaerobic conditions. Using free radical quenching and electron paramagnetic
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resonance (EPR) experiments, we confirmed that S2O8•− forms from the reaction
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between sulfate radical (SO4•−) and S2O82−, which are the dominant reactive species in
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HCA degradation. Density functional theory (DFT) calculations were used to
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elucidate the pathways of HCA degradation and S2O8•− radical decomposition. Further
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investigation showed that S2O8•− can efficiently degrade HCA and DDTs in soil via
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reduction during the thermal activation of PS under anaerobic conditions. The finding
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of this study provide a novel strategy for the reductive degradation of contaminant
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when PS-based in-situ chemical oxidation used in the remediation of soil and
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groundwater, particularly those contaminated with highly halogenated compounds.
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INTRODUCTION
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In situ chemical oxidation based on persulfate (PS) has recently been widely
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used for the remediation of contaminated soil and groundwater.1 PS can be activated
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by heat,2-5 ultraviolet light,6 bases7,
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radical (SO4•−, E0 of 2.5–3.1 V).13 Its degradation of a wide range of contaminants,
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such as phenyls,14 phenols,15 dyes,16 and pharmaceuticals,17 have a second order
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reaction rate constant in the range of 108–109 M−1 s−1. However, highly halogenated
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contaminants are hard degraded by SO4•−,18,
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polybrominated diphenyl ethers,21 and hexachloroethane (HCA).22 SO4•− is an
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electrophilic radical that tends to attack molecules with high electron densities,23 but
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halogenated organic contaminants are susceptible to attack by nucleophilic radicals.24
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and transition metals9-12 to produce sulfate
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including perfluorooctanoic acid,20
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In addition to SO4•− and hydroxyl radical (HO•, Eq.1), persulfate radical (S2O8•−)
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is also an important reactive species generated from the reaction between SO4•− or
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HO• and PS ion via one electron transfer processes (Eqs. 2 and 3) in the PS activation
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system.25-30 SO4 •- +H2 O/OH- →HO• +SO4 2-
(1)
SO4 • +S2 O8 2 →S2 O8 • +SO4 2 k=6.3×105 M-1 s-1 HO• +S2 O8 2 →S2 O8 • +OH k=1.4×107 M-1 s-1
(2) (3)
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For example, Clarke et al.31 provided the direct evidence for S2O8•− formation in
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aqueous solution from the pulse radiolysis of PS under anaerobic condition using
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transient absorption spectra method. Liu et al.30 proposed that S2O8•− formation (Eq. 4)
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continuously regenerates Fe(II) (Eq. 5) during PS activation with iron oxides, and 3
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S2O8•− is similar to superoxide radical anion (O2•−) and capable of reducing Fe(III)
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(Eq. 6).
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≡Fe(III)+S2 O8 2 →≡Fe(II)+S2 O8 • (4) ≡Fe(III)+S2 O8 • +2H2 O→≡FeII+2HSO5 +2H+ Fe3+ +O2 • →Fe2+ +O2
(5) (6)
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O2•− radicals are known to be produced in H2O2-based Fenton or Fenton-like
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systems (Eqs. 7–10), through a similar pathway to S2O8•− (Eq. 10). O2•− can both
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accept and donate an electron, and act as both reducing and oxidizing agents.32-34 O2•−
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accepts an electron to yield H2O2, while loses an electron to reductively degrade
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highly halogenated organic compounds.24, 32 Fe2+ +H2 O2 →Fe3+ +HO• +OH
Fe3+ +H2 O2 →Fe2+ +HO2 •+H+
(8)
HO2 • ↔O2 • +H+ 72
(7)
(9)
HO• +H2 O2 →H2 O+O2 • +H+ (10)
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Therefore, as an analog to O2•−, it was hypothesized that S2O8•− can also act as
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both reducing and oxidizing agents. For target pollutant easily to be oxidized, it would
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be oxidatively degraded by S2O8•−; on the contrary, the pollutant would be reductively
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degraded by S2O8•− if it was recalcitrant to be oxidized. Although the formation of
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S2O8•− has been well established, the reactivity of S2O8•− toward contaminants during
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PS activation is largely unknown.
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The main objective of this study was to investigate whether S2O8•− can
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reductively degrade halogenated organic contaminants in PS activation processes 4
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following the pathways similar to those of O2•−. The main objectives of this study
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were 1) to determine the potential contribution of S2O8•− to contaminant degradation
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in the thermally activated PS process and 2) to elucidate the possible pathways of
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S2O8•− formation and the controlling factors in these reactions, including temperature,
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reagent concentration, and solution chemistry. We used HCA as a model contaminant
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because it is recalcitrant to oxidizing radicals but easily undergoes reductive
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degradation via electron transfer.22 Quenching studies and electron paramagnetic
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resonance (EPR) were conducted to identify the key radical species contributing to
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HCA degradation. Density functional theory (DFT) calculations were used to
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elucidate the pathways of HCA degradation and DMPO-S2O8 adduct decomposition.
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Furthermore,
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o,p’-dichlorodiphenyltrichloroethane (DDT) in soil slurry using S2O8•− produced from
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the thermal activation of PS in order to determine the potential application of S2O8•−
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for reductively degrading halogenated organic contaminants.
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EXPERIMENTAL SECTION
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Chemicals. Chemicals used in this study are presented in the Supporting Information
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(SI, Text S1).
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Experiment procedures. All experiments were conducted in 9.0 mL brown serum
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bottles sealed with Teflon Mininerts and containing 9.0 mL of the reaction solutions.
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The headspace was excluded to avoid HCA evaporation. Deionized water was purged
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with argon for 30 min until a dissolved oxygen concentration of
