Identification of Active Radical Species in Alkaline Persulfate Oxidation Chenju Liang*, Jung-Hsuan Lei
ABSTRACT: A proposed mechanism for alkaline activation of persulfate involves generation of sulfate (SO 4 ), hydroxyl (HO), and superoxide radicals (O 2 ). The present study investigated the feasibility of chloroform (CF) degradation using alkaline activated persulfate and identified the active radical species using a radical inhibition technique. 2-propanol (PrOH) (preferentially reacted with HO), phenol (preferentially reacted with both HO and SO 4 ), and carbon tetrachloride (CT) (preferentially reacted with O 2 ) were used to inhibit the degradation of CF, and the extent of inhibited degradation was used to indicate the predominant radical species. Additions of PrOH and phenol appeared to significantly scavenge SO 4 and HO and resulted in inhibited CF degradation. Here, the authors demonstrated that SO 4 and HO were predominant radicals in the alkaline activated persulfate system. The presence of O 2 scavengers (i.e., CT) resulted in a partial inhibition of CF degradation and, hence, one can speculate that O 2 is a minor radical species. Water Environ. Res., 87, 656 (2015).
deduced based on the differences in the reactivities and/or the rates of reactions between NB and the radical species. They reported that both SO 4 and HO were present at pH 9, and that HO was the predominant radical at a more basic pH (i.e., pH . 12). Persulfate anion is the source of HO, SO 4 , or O2 is the predominant oxidant radical at produced. In short, SO 4 acidic pH (e.g., pH , 3); both HO and SO are present at 4 neutral pH; at pH . 12, HO is the predominant radical, and reductant radical O 2 may also be present.
KEYWORDS: sodium persulfate, sulfate radical, remediation, in situ chemical oxidation, advanced oxidation technology, contamination. doi:10.2175/106143015X14338845154986
Introduction A proposed mechanism for alkaline activation of persulfate (S2 O2 8 ) involves generation of sulfate radicals (SO4 ), hydroxyl radicals (HO) (Ko et al., 2012; Liang and Su, 2009), and superoxide radicals (O 2 ) (Furman et al., 2010; Liang and Guo, 2012). Briefly, the alkaline activation of persulfate starts with an initial step of alkaline induced hydrolysis of persulfate to form 2 hydroperoxide (HO 2 ) and sulfate (SO4 ) ions (eq 1). Then, the 0 2 reduction of S2 O8 by HO2 generates SO 4 (E ¼ 2.4 V) (Hayon 0 et al., 1972) and superoxide radicals (O ) (E ¼ 0.33 V) in 2 accordance with eq 2 (Krishna et al., 1992). Therefore, eq 3 presents an overall reaction for the alkaline activated persulfate process. Furthermore, under basic pH, the SO 4 formed reacts with OH– to convert SO 4 to HO in accordance with eq 4 (Hayen et al., 1972; Guan et al., 2011; Liang and Su, 2009; Tsai et al., 2011). Liang and Su (2009) investigated the active radical species generated during thermally activated persulfate oxidation using a chemical probe method. In their study, nitrobenzene (NB) was used as a radical probe that reacted preferentially with HO at a reaction rate constant of (3.0–3.9) 3 109 M1 s1, which is approximately 3000 times greater than that of NB with SO 4 . Therefore, the predominant radical species, HO or SO , were 4 Department of Environmental Engineering, National Chung Hsing University, 250 Kuo-Kuang Rd., Taichung City 402, Taiwan. * Corresponding author: telephone: þ886-4-22856610; fax: þ886-422862587; e-mail address: [email protected]. 656
OH ðhydrolysisÞ
2 þ S2 O2 8 þ 2H2 O HO2 þ 2SO4 þ 3H
ð1Þ
2 2 þ HO 2 þ S2 O8 SO4 þ SO4 þ H þ O2
ð2Þ
2 þ 2S2 O2 8 þ 2H2 OSO4 þ 3SO4 þ 4H þ O2
ð3Þ
2 7 1 1 SO 4 þ OH SO4 þ HOk ¼ 6:5 3 10 M s
ð4Þ
Because multiple reactive species can be generated from alkaline activated persulfate (eqs 1 to 4), reactivities of radical species suitable for degrading different organic contaminants are of interest to environmental remediation. For example, chlorinated solvents with higher carbon oxidation states are prone to degradation via reduction (e.g., more reactive with reductant O 2 ); conversely, compounds with lower carbon oxidation states are prone to degradation via oxidation (e.g., more reactive with HO or SO 4 ). Radical probes that selectively react with specific radicals can also be applied in a radical inhibition technique for identification of radical species by adding specific chemical additives (i.e., radical scavengers) to a reaction to quench radical reactivities. Thereby, the presence of radical scavengers could reduce radical reactivity toward a target compound, and the inhibited degradation of the target compound would be an indirect confirmation of the predominant radical species in a reaction system (Allen et al., 2009; Liang et al., 2007). The inhibition of reactive radicals depends on radical scavengers having markedly different radical scavenging reactivities toward specific radicals such as HO, SO 4 , or O2 . It should be noted that a great excess of radical scavengers (e.g., a molar ratio of radical scavenger/target compound . 10/1 [Espenson, 1981]) should be used to ensure that radicals are scavenged by specific radical scavenger. In this study, the feasibility of chloroform (CF) degradation using persulfate and alkaline activated persulfate was investigated. Chloroform is a hazardous material with a carbon oxidation state of þII. Electrons may be removed or added from the CF molecule and would not be exceptionally resistant to either Water Environment Research, Volume 87, Number 7
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Table 1—Rate constants between selected radical scavengers and possible radicals formed in a persulfate oxidation system. k (M1 s1), Radicals Radical scavenger
SO 4
HO
O 2
References
2-Propanol (PrOH) Phenol
8.3 3 107 8.8 3 109
2.8 3 109 6.6 3 109 (phenol) 6.6 3 109 (phenoxide) n.a.
1.0 3 106 n.a.
(Alfassi et al., 1993; Lindsey and Tarr, 2000) (Chamam et al., 2012; Lindsey and Tarr, 1999; Ziajka and Pasiuk-Bronikowska, 2005) (Haag and Yao, 1992; Teel and Watts, 2002)
Carbon tetrachloride (CT)
, 2 3 106
1.3 3 1010
n.a. ¼ not available.
oxidation or reduction. Oxidation radicals (e.g., HO and SO 4 ) react with organic compounds mainly by three mechanisms: hydrogen abstraction, hydrogen addition, and electron transfer, whereas reduction radicals (e.g., O 2 ) typically undergo displacement reaction by donating electrons. It should be noted that SO 4 is more selective for electron transfer reactions than is HO (Liang et al., 2007). Hence, active radical species can be identified using a radical inhibition technique by differentiating the effects of radical scavengers on the degradation of CF. Examples of the identification of active radical species are as follows:
2-propanol (PrOH) (containing the a-hydrogen, which is preferentially extracted by HO [Lu et al., 1997; Overend and Paraskevopoulos, 1978], at a diffusion rate constant of 340- and 2800-fold greater than those of SO 4 and O2 , respectively); phenol (as dissociated or nondissociated species, which rapidly react with both HO and SO 4 by either OHaddition or by direct electron transfer [Ziajka and PasiukBronikowska, 2005]); and carbon tetrachloride (CT) (with a highest positive carbon oxidation state of IV exhibiting negligible reactivity with oxidizing radicals; preferentially undergoing displacement reaction by accepting electrons from O 2 [Haag and Yao, 1992; Teel and Watts, 2002]). The reaction rate constants between radical scavengers and radical species and associated citations are listed in Table 1.
Material and Methods Chemicals. Sodium persulfate (SPS) (Na2S2O8, .99.0%) and chloroform (CHCl3, 98%) were purchased from Merck. Sodium hydroxide (NaOH, 99%) was purchased from Riedel-de Ha¨en. Carbon tetrachloride (CCl4, 99.7%) was purchased from ALPS Chem. Co. Phenol (C 6 H 6 O, 99.2%) and 2-propanol (CH3CHOHCH3, 99%) were purchased from J.T. Baker. Experimental Procedure. The reaction was carried out in a series of 40-mL amber EPA reaction bottles for 1 and 24 hours at 20 8C in a temperature-controlled chamber. The CF solution was prepared at an initial concentration of 10 mg L1 (0.084 mM) in a 1-L borosilicate bottle. For experiments designed to identify radical species, each radical scavenger was added into CF solution and continuously stirred until dissolved. Thereafter, SPS was added to the borosilicate bottle and mixed for approximately 1 minute (SPS ¼ 0.1 M). Then, the resulting solution was transferred to a series of 40-mL bottles. For the alkaline activated persulfate experiments, 1 mL solution was first removed from each bottle before adding 1 mL of concentrated NaOH stock solution to attain the designated alkaline concenJuly 2015
tration (NaOH ¼ 0.05 M). Zero headspace was maintained in all reaction bottles to prevent possible volatilization of organics. At sampling times (1 and 24 hours), pH and concentrations of CF, radical scavengers, and SPS were determined. All experiments were conducted in triplicate, and averaged data using one standard deviation as an error range were reported. In experiments for scavenging HO, 1000 mg L1 of PrOH was used (molar ratio of PrOH/CF ¼ 198/1); for scavenging HO and SO 4 , low and high concentrations of phenol (100 mg and 10 000 mg L1) were used (molar ratios of phenol/CF ¼ 13 and 127, 1 respectively); for scavenging O of CT was used 2 , 400 mg L (molar ratio of CT/CF ¼ 31/1). A fixed CF concentration (0.084 mM) was used in all experiments. Note that PrOH is miscible with water, and phenol has a water solubility of 8.28 g L1; as such, phenol can be added to water at greater concentrations than CT, which has a water solubility of 800 mg L1. A molar ratio of radical scavenger/CF . 10/1 was used. Control experiments in the absence of radical scavengers and/or SPS were also conducted in parallel. Analysis. A gas chromatograph/flame ionization detector (Agilent 7890A, Santa Clara, California) equipped with a purgeand-trap concentrator (OIA Eclipse 4660, College Station, Texas) was used for analysis of CF and radical scavengers (PrOH, phenol, and CT) (NIEA, 2006). A DB-624 megabore fused-silica capillary column (30 m 3 0.53 mm 3 3.0 lm) was used. The persulfate anion concentration was determined by a spectrophotometer method (Hach DR/2400, Loveland, Colorado) at 400 nm (Liang et al., 2008). Results and Discussion The results of persulfate oxidation of CF with or without addition of radical scavengers (PrOH, phenol, and CT) at reaction times of 1 and 24 hours are shown in Figure 1(a) and (b), respectively. In SPS alone and in SPS/NaOH oxidation systems without addition of radical scavengers, CF degradation at 1 hour reached approximately 10 and 40%, respectively, and complete CF degradations were achieved at 24 hours. Note that the results of control experiments in the absence of SPS show less than 2% loss of CF during 24 hours’ reaction time. Therefore, it can be seen that both oxidation systems are capable of degrading CF, and that alkaline activated persulfate revealed a faster reaction rate. When PrOH was added as the scavenger of HO, significant additional inhibition of CF degradation was observed compared with not adding PrOH. Similar results were observed with addition of phenol, which scavenged HO and SO 4 . However, when a lower phenol concentration (i.e., 100 mg L1) was present in the SPS/NaOH system, little CF degradation (~5%) occurred while accompanying the degradation of phenol at 1 hour of reaction. Note that 657
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Figure 1—Influence of radical scavengers on CF degradation in an alkaline activated persulfate system at (a) 1 hour and (b) 24 hour reaction time (CF ¼ 10 mg L1; PrOH ¼ 1000 mg L1; phenol ¼ 100 and 10 000 mg L1; CT ¼ 400 mg L1; SPS ¼ 0.1 M; NaOH ¼ 0.05 M). no phenol or CF degradations occurred in the SPS only system. Furthermore, it can be seen that CF and phenol were completely degraded at 24 hours in the SPS/NaOH system. Also, it should be noted that simultaneous presence of phenol and phenoxide (upon dissociation at pH . pKa, phenol of 10) would not be degraded by SPS, but instead of being degraded by the SPS/ NaOH system. As seen in Table 1, phenol and phenoxide react with HO with a diffusion limited rate coefficient of 6.6–9.6 3 109 M1 s1. These observations indicated that more HO and SO 4 would be generated in a SPS/NaOH system than in a SPS system. It can also be seen that more persulfate was decomposed in the SPS/NaOH system than in the SPS system (see Table 2). In the alkaline pH condition, SPS would be further hydrolyzed in accordance with eq 1 and, hence, more SPS decomposition was observed. In addition to HO and SO 4 , O2 can potentially be formed and involved in CF degradation in an alkaline activated persulfate system. When CT was added as an O 2 radical scavenger, there were 26 and 28% CF degradations in the SPS 658
and SPS/NaOH systems, respectively. The addition of CT was meant to scavenge O 2 and should have quenched possible reductive reactions by O 2 (see eq 5). Instead, the presence of CT appeared ineffective in quenching CF degradations and resulted in more CF degradation than that of the SPS only system (without addition of radical scavengers) and also in partial inhibition of CF degradation in the SPS/NaOH system. A possible reason for not being able to inhibit CF degradation could be that because HO and SO 4 are major active radical species (as demonstrated in PrOH- and phenol-scavenging experiments), the presence of an O 2 scavenger (i.e., CT) resulted in a partial inhibition of CF degradation in the SPS/ NaOH system. Hence, it can be speculated that O 2 is a minor radical species in the SPS/NaOH system. Moreover, Feng and Lim (2005) have reported that CT may gain electrons while scavenging O 2 and form intermediate radicals (CCl3) (eq 6). Thereafter, CCl3 that has formed can react with CF in accordance with eq 7 and cause CF degradation (Ottolenghi and Stein, 1961). Hence, the presence of excess CT may have Water Environment Research, Volume 87, Number 7
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Table 2—SPS consumption and pH change for different radical scavengers in a persulfate oxidation system. SPS reduction (%)
pH Final Initial (24 hours)
Radical scavenger Propanol (PrOH) (1000 mg L1) Phenol 100 mg L1 10 000 mg L1 Carbon tetrachloride (CT) (400 mg L1) No scavenger (control tests)
SPS SPS/NaOH SPS SPS/NaOH SPS SPS/NaOH SPS SPS/NaOH SPS SPS/NaOH
6.5 13.1 6.5 13.1 6.1 13.1 6.5 13.1 6.5 13.1
2.3 12.8 3.4 12.6 3.2 5.2 3.4 12.8 3.4 12.8
1 hour
24 hours
0.8 3.0 0.8 4.2 0.7 32.5 0.6 2.2 0.6 2.3
5.0 3.1 2.3 8.8 3.0 42.0 1.3 2.7 2.1 3.6
induced occurrence of these reactions and resulted in further CF degradation. O 2 O2 þ e
ð5Þ
CCl4 þ e CCl3 þ Cl
ð6Þ
CCl3 þ CHCl3 C2 Cl6 þ H
ð7Þ
Conclusions The results presented in this study revealed that: (1) both persulfate and alkaline activated persulfate systems are capable of degrading CF; (2) SO 4 and HO are predominant radicals in a alkaline activated persulfate system; (3) O 2 is also present as minor radical species in a SPS/NaOH system. Alkaline activated persulfate is a promising chemical reaction to be used for destroying organic contaminants because of the simultaneous presence of various radical species (SO 4 , HO, and O2 ). Further optimization of alkaline activated persulfate processes used for environmental remediation should take account of these results. Submitted for publication March 31, 2014; revised manuscript submitted September 27, 2014; accepted for publication November 12, 2014. References Alfassi, Z. B.; Padmaja, S.; Neta, P.; Huie, R. E. (1993) Rate Constants for Reactions of Nitrate (NO3–) Radicals with Organic Compounds in Water and Acetonitrile. J. Phys. Chem., 97, 3780–3782. Allen, J. M.; Lucas, S.; Allen, S. K. (2009) Formation of Hydroxyl Radical (OH) in Illuminated Surface Waters Contaminated with Acidic Mine Drainage. Environ. Toxicol. Chem., 15, 107–113. Chamam, M.; F¨oldva´ry, C. M.; Hosseini, A. M.; Tungler, A.; Taka´cs, E.; Wojna´rovits, L. (2012) Mineralization of Aqueous Phenolate Solutions: A Combination of Irradiation Treatment and Wet Oxidation. Radiat. Phys. Chem., 81, 1484–1488. Espenson, J. H. (1981) Chemical Kinetics and Reaction Mechanisms; McGraw-Hill: New York.
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Feng, J.; Lim, T.-T. (2005) Pathways and Kinetics of Carbon Tetrachloride and Chloroform Reductions by Nano-Scale Fe and Fe/Ni Particles: Comparison with Commercial Micro-Scale Fe and Zn. Chemosphere, 59, 1267–1277. Furman, O. S.; Teel, A. L.; Watts, R. J. (2010) Mechanism of Base Activation of Persulfate. Environ. Sci. Technol., 44, 6423–6428. Guan, Y.-H.; Ma, J.; Li, X.-C.; Fang, J.-Y.; Chen, L.-W. (2011) Influence of pH on the Formation of Sulfate and Hydroxyl Radicals in the UV/ Peroxymonosulfate System. Environ. Sci. Technol., 45, 9308–9314. Haag, W. R.; Yao, C. C. D. (1992) Rate Constants for Reaction of Hydroxyl Radicals with Several Drinking Water Contaminants. Environ. Sci. Technol., 26, 1005–1013. Hayon, E.; Treinin, A.; Wilf, J. (1972) Electronic Spectra, Photochemistry, and Autoxidation Mechanism of the Sulfite-Bisulfite-Pyrosulfite Systems. SO2–, SO3–, SO4–, and SO5– Radicals. J. Am. Chem. Soc., 94, 47–57. Ko, S.; Crimi, M.; Marvin, B. K.; Holmes, V.; Huling, S. G. (2012) Comparative Study on Oxidative Treatments of NAPL Containing Chlorinated Ethanes and Ethenes Using Hydrogen Peroxide and Persulfate in Soils. J. Environ. Manage., 108, 42–48. Krishna, M. C.; Grahame, D. A.; Samuni, A.; Mitchell, J. B.; Russo, A. (1992) Oxoammonium Cation Intermediate in the NitroxideCatalyzed Dismutation of Superoxide. Proc. Natl. Acad. Sci., 89, 5537–5541. Liang, C.; Guo, Y.-Y. (2012) Remediation of Diesel Contaminated Soils Using Persulfate Under Alkaline Condition. Water, Air, Soil Pollut., 223, 4605–4614. Liang, C.; Huang, C.-F.; Mohanty, N.; Kurakalva, R. M. (2008) A Rapid Spectrophotometric Determination of Persulfate Anion in ISCO. Chemosphere, 73, 1540–1543. Liang, C.; Su, H.-W. (2009) Identification of Sulfate and Hydroxyl Radicals in Thermally Activated Persulfate. Ind. Eng. Chem. Res., 48, 5558–5562. Liang, C.; Wang, Z.-S.; Bruell, C. J. (2007) Influence of pH On Persulfate Oxidation of TCE at Ambient Temperatures. Chemosphere, 66, 106–113. Lindsey, M. E.; Tarr, M. A. (1999) Inhibition of Hydroxyl Radical Reaction with Aromatics by Dissolved Natural Organic Matter. Environ. Sci. Technol., 34, 444–449. Lindsey, M. E.; Tarr, M. A. (2000) Quantitation of Hydroxyl Radical during Fenton Oxidation Following a Single Addition of Iron and Peroxide. Chemosphere, 41, 409–417. Lu, N.; Kombo, D. C.; Osman, R. (1997) Theoretical Studies of Hydrogen Abstraction from 2-propanol by OH Radical. J. Phys. Chem. A, 101, 926–936. NIEA (2006) Measurement of Volatile Organic Compounds in Water by Purge and Trap Coupled with Capillary Column Gas Chromatography/Mass Spectrometry. Method W785.54B. Ottolenghi, M.; Stein, G. (1961) The Radiation Chemistry of Chloroform. Radiat. Res., 14, 281–290. Overend, R.; Paraskevopoulos, G. (1978) Rates of OH Radical Reactions. 4. Reactions with Methanol, ethanol, 1-propanol, and 2-propanol at 296 K. J. Phys. Chem., 82, 1329–1333. Teel, A. L.; Watts, R. J. (2002) Degradation of Carbon Tetrachloride by Modified Fenton’s Reagent. J. Hazard. Mater., 94, 179–189. Tsai, T. T.; Kao, C. M.; Wang, J. Y. (2011) Remediation of TCEContaminated Groundwater Using Acid/BOF Slag Enhanced Chemical Oxidation. Chemosphere, 83, 687–692. Ziajka, J.; Pasiuk-Bronikowska, W. (2005) Rate Constants for Atmospheric Trace Organics Scavenging SO4– in the Fe-Catalysed Autoxidation of S(IV). Atmos. Environ., 39, 1431–1438.
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ABSTRACT: A proposed mechanism for alkaline activation of persulfate involves generation of sulfate (SO 4 ), hydroxyl (HO), and superoxide radicals (O 2 ). The present study investigated the feasibility of chloroform (CF) degradation using alkaline activated persulfate and identified the active radical species using a radical inhibition technique. 2-propanol (PrOH) (preferentially reacted with HO), phenol (preferentially reacted with both HO and SO 4 ), and carbon tetrachloride (CT) (preferentially reacted with O 2 ) were used to inhibit the degradation of CF, and the extent of inhibited degradation was used to indicate the predominant radical species. Additions of PrOH and phenol appeared to significantly scavenge SO 4 and HO and resulted in inhibited CF degradation. Here, the authors demonstrated that SO 4 and HO were predominant radicals in the alkaline activated persulfate system. The presence of O 2 scavengers (i.e., CT) resulted in a partial inhibition of CF degradation and, hence, one can speculate that O 2 is a minor radical species. Water Environ. Res., 87, 656 (2015).
deduced based on the differences in the reactivities and/or the rates of reactions between NB and the radical species. They reported that both SO 4 and HO were present at pH 9, and that HO was the predominant radical at a more basic pH (i.e., pH . 12). Persulfate anion is the source of HO, SO 4 , or O2 is the predominant oxidant radical at produced. In short, SO 4 acidic pH (e.g., pH , 3); both HO and SO are present at 4 neutral pH; at pH . 12, HO is the predominant radical, and reductant radical O 2 may also be present.
KEYWORDS: sodium persulfate, sulfate radical, remediation, in situ chemical oxidation, advanced oxidation technology, contamination. doi:10.2175/106143015X14338845154986
Introduction A proposed mechanism for alkaline activation of persulfate (S2 O2 8 ) involves generation of sulfate radicals (SO4 ), hydroxyl radicals (HO) (Ko et al., 2012; Liang and Su, 2009), and superoxide radicals (O 2 ) (Furman et al., 2010; Liang and Guo, 2012). Briefly, the alkaline activation of persulfate starts with an initial step of alkaline induced hydrolysis of persulfate to form 2 hydroperoxide (HO 2 ) and sulfate (SO4 ) ions (eq 1). Then, the 0 2 reduction of S2 O8 by HO2 generates SO 4 (E ¼ 2.4 V) (Hayon 0 et al., 1972) and superoxide radicals (O ) (E ¼ 0.33 V) in 2 accordance with eq 2 (Krishna et al., 1992). Therefore, eq 3 presents an overall reaction for the alkaline activated persulfate process. Furthermore, under basic pH, the SO 4 formed reacts with OH– to convert SO 4 to HO in accordance with eq 4 (Hayen et al., 1972; Guan et al., 2011; Liang and Su, 2009; Tsai et al., 2011). Liang and Su (2009) investigated the active radical species generated during thermally activated persulfate oxidation using a chemical probe method. In their study, nitrobenzene (NB) was used as a radical probe that reacted preferentially with HO at a reaction rate constant of (3.0–3.9) 3 109 M1 s1, which is approximately 3000 times greater than that of NB with SO 4 . Therefore, the predominant radical species, HO or SO , were 4 Department of Environmental Engineering, National Chung Hsing University, 250 Kuo-Kuang Rd., Taichung City 402, Taiwan. * Corresponding author: telephone: þ886-4-22856610; fax: þ886-422862587; e-mail address: [email protected]. 656
OH ðhydrolysisÞ
2 þ S2 O2 8 þ 2H2 O HO2 þ 2SO4 þ 3H
ð1Þ
2 2 þ HO 2 þ S2 O8 SO4 þ SO4 þ H þ O2
ð2Þ
2 þ 2S2 O2 8 þ 2H2 OSO4 þ 3SO4 þ 4H þ O2
ð3Þ
2 7 1 1 SO 4 þ OH SO4 þ HOk ¼ 6:5 3 10 M s
ð4Þ
Because multiple reactive species can be generated from alkaline activated persulfate (eqs 1 to 4), reactivities of radical species suitable for degrading different organic contaminants are of interest to environmental remediation. For example, chlorinated solvents with higher carbon oxidation states are prone to degradation via reduction (e.g., more reactive with reductant O 2 ); conversely, compounds with lower carbon oxidation states are prone to degradation via oxidation (e.g., more reactive with HO or SO 4 ). Radical probes that selectively react with specific radicals can also be applied in a radical inhibition technique for identification of radical species by adding specific chemical additives (i.e., radical scavengers) to a reaction to quench radical reactivities. Thereby, the presence of radical scavengers could reduce radical reactivity toward a target compound, and the inhibited degradation of the target compound would be an indirect confirmation of the predominant radical species in a reaction system (Allen et al., 2009; Liang et al., 2007). The inhibition of reactive radicals depends on radical scavengers having markedly different radical scavenging reactivities toward specific radicals such as HO, SO 4 , or O2 . It should be noted that a great excess of radical scavengers (e.g., a molar ratio of radical scavenger/target compound . 10/1 [Espenson, 1981]) should be used to ensure that radicals are scavenged by specific radical scavenger. In this study, the feasibility of chloroform (CF) degradation using persulfate and alkaline activated persulfate was investigated. Chloroform is a hazardous material with a carbon oxidation state of þII. Electrons may be removed or added from the CF molecule and would not be exceptionally resistant to either Water Environment Research, Volume 87, Number 7
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Table 1—Rate constants between selected radical scavengers and possible radicals formed in a persulfate oxidation system. k (M1 s1), Radicals Radical scavenger
SO 4
HO
O 2
References
2-Propanol (PrOH) Phenol
8.3 3 107 8.8 3 109
2.8 3 109 6.6 3 109 (phenol) 6.6 3 109 (phenoxide) n.a.
1.0 3 106 n.a.
(Alfassi et al., 1993; Lindsey and Tarr, 2000) (Chamam et al., 2012; Lindsey and Tarr, 1999; Ziajka and Pasiuk-Bronikowska, 2005) (Haag and Yao, 1992; Teel and Watts, 2002)
Carbon tetrachloride (CT)
, 2 3 106
1.3 3 1010
n.a. ¼ not available.
oxidation or reduction. Oxidation radicals (e.g., HO and SO 4 ) react with organic compounds mainly by three mechanisms: hydrogen abstraction, hydrogen addition, and electron transfer, whereas reduction radicals (e.g., O 2 ) typically undergo displacement reaction by donating electrons. It should be noted that SO 4 is more selective for electron transfer reactions than is HO (Liang et al., 2007). Hence, active radical species can be identified using a radical inhibition technique by differentiating the effects of radical scavengers on the degradation of CF. Examples of the identification of active radical species are as follows:
2-propanol (PrOH) (containing the a-hydrogen, which is preferentially extracted by HO [Lu et al., 1997; Overend and Paraskevopoulos, 1978], at a diffusion rate constant of 340- and 2800-fold greater than those of SO 4 and O2 , respectively); phenol (as dissociated or nondissociated species, which rapidly react with both HO and SO 4 by either OHaddition or by direct electron transfer [Ziajka and PasiukBronikowska, 2005]); and carbon tetrachloride (CT) (with a highest positive carbon oxidation state of IV exhibiting negligible reactivity with oxidizing radicals; preferentially undergoing displacement reaction by accepting electrons from O 2 [Haag and Yao, 1992; Teel and Watts, 2002]). The reaction rate constants between radical scavengers and radical species and associated citations are listed in Table 1.
Material and Methods Chemicals. Sodium persulfate (SPS) (Na2S2O8, .99.0%) and chloroform (CHCl3, 98%) were purchased from Merck. Sodium hydroxide (NaOH, 99%) was purchased from Riedel-de Ha¨en. Carbon tetrachloride (CCl4, 99.7%) was purchased from ALPS Chem. Co. Phenol (C 6 H 6 O, 99.2%) and 2-propanol (CH3CHOHCH3, 99%) were purchased from J.T. Baker. Experimental Procedure. The reaction was carried out in a series of 40-mL amber EPA reaction bottles for 1 and 24 hours at 20 8C in a temperature-controlled chamber. The CF solution was prepared at an initial concentration of 10 mg L1 (0.084 mM) in a 1-L borosilicate bottle. For experiments designed to identify radical species, each radical scavenger was added into CF solution and continuously stirred until dissolved. Thereafter, SPS was added to the borosilicate bottle and mixed for approximately 1 minute (SPS ¼ 0.1 M). Then, the resulting solution was transferred to a series of 40-mL bottles. For the alkaline activated persulfate experiments, 1 mL solution was first removed from each bottle before adding 1 mL of concentrated NaOH stock solution to attain the designated alkaline concenJuly 2015
tration (NaOH ¼ 0.05 M). Zero headspace was maintained in all reaction bottles to prevent possible volatilization of organics. At sampling times (1 and 24 hours), pH and concentrations of CF, radical scavengers, and SPS were determined. All experiments were conducted in triplicate, and averaged data using one standard deviation as an error range were reported. In experiments for scavenging HO, 1000 mg L1 of PrOH was used (molar ratio of PrOH/CF ¼ 198/1); for scavenging HO and SO 4 , low and high concentrations of phenol (100 mg and 10 000 mg L1) were used (molar ratios of phenol/CF ¼ 13 and 127, 1 respectively); for scavenging O of CT was used 2 , 400 mg L (molar ratio of CT/CF ¼ 31/1). A fixed CF concentration (0.084 mM) was used in all experiments. Note that PrOH is miscible with water, and phenol has a water solubility of 8.28 g L1; as such, phenol can be added to water at greater concentrations than CT, which has a water solubility of 800 mg L1. A molar ratio of radical scavenger/CF . 10/1 was used. Control experiments in the absence of radical scavengers and/or SPS were also conducted in parallel. Analysis. A gas chromatograph/flame ionization detector (Agilent 7890A, Santa Clara, California) equipped with a purgeand-trap concentrator (OIA Eclipse 4660, College Station, Texas) was used for analysis of CF and radical scavengers (PrOH, phenol, and CT) (NIEA, 2006). A DB-624 megabore fused-silica capillary column (30 m 3 0.53 mm 3 3.0 lm) was used. The persulfate anion concentration was determined by a spectrophotometer method (Hach DR/2400, Loveland, Colorado) at 400 nm (Liang et al., 2008). Results and Discussion The results of persulfate oxidation of CF with or without addition of radical scavengers (PrOH, phenol, and CT) at reaction times of 1 and 24 hours are shown in Figure 1(a) and (b), respectively. In SPS alone and in SPS/NaOH oxidation systems without addition of radical scavengers, CF degradation at 1 hour reached approximately 10 and 40%, respectively, and complete CF degradations were achieved at 24 hours. Note that the results of control experiments in the absence of SPS show less than 2% loss of CF during 24 hours’ reaction time. Therefore, it can be seen that both oxidation systems are capable of degrading CF, and that alkaline activated persulfate revealed a faster reaction rate. When PrOH was added as the scavenger of HO, significant additional inhibition of CF degradation was observed compared with not adding PrOH. Similar results were observed with addition of phenol, which scavenged HO and SO 4 . However, when a lower phenol concentration (i.e., 100 mg L1) was present in the SPS/NaOH system, little CF degradation (~5%) occurred while accompanying the degradation of phenol at 1 hour of reaction. Note that 657
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Figure 1—Influence of radical scavengers on CF degradation in an alkaline activated persulfate system at (a) 1 hour and (b) 24 hour reaction time (CF ¼ 10 mg L1; PrOH ¼ 1000 mg L1; phenol ¼ 100 and 10 000 mg L1; CT ¼ 400 mg L1; SPS ¼ 0.1 M; NaOH ¼ 0.05 M). no phenol or CF degradations occurred in the SPS only system. Furthermore, it can be seen that CF and phenol were completely degraded at 24 hours in the SPS/NaOH system. Also, it should be noted that simultaneous presence of phenol and phenoxide (upon dissociation at pH . pKa, phenol of 10) would not be degraded by SPS, but instead of being degraded by the SPS/ NaOH system. As seen in Table 1, phenol and phenoxide react with HO with a diffusion limited rate coefficient of 6.6–9.6 3 109 M1 s1. These observations indicated that more HO and SO 4 would be generated in a SPS/NaOH system than in a SPS system. It can also be seen that more persulfate was decomposed in the SPS/NaOH system than in the SPS system (see Table 2). In the alkaline pH condition, SPS would be further hydrolyzed in accordance with eq 1 and, hence, more SPS decomposition was observed. In addition to HO and SO 4 , O2 can potentially be formed and involved in CF degradation in an alkaline activated persulfate system. When CT was added as an O 2 radical scavenger, there were 26 and 28% CF degradations in the SPS 658
and SPS/NaOH systems, respectively. The addition of CT was meant to scavenge O 2 and should have quenched possible reductive reactions by O 2 (see eq 5). Instead, the presence of CT appeared ineffective in quenching CF degradations and resulted in more CF degradation than that of the SPS only system (without addition of radical scavengers) and also in partial inhibition of CF degradation in the SPS/NaOH system. A possible reason for not being able to inhibit CF degradation could be that because HO and SO 4 are major active radical species (as demonstrated in PrOH- and phenol-scavenging experiments), the presence of an O 2 scavenger (i.e., CT) resulted in a partial inhibition of CF degradation in the SPS/ NaOH system. Hence, it can be speculated that O 2 is a minor radical species in the SPS/NaOH system. Moreover, Feng and Lim (2005) have reported that CT may gain electrons while scavenging O 2 and form intermediate radicals (CCl3) (eq 6). Thereafter, CCl3 that has formed can react with CF in accordance with eq 7 and cause CF degradation (Ottolenghi and Stein, 1961). Hence, the presence of excess CT may have Water Environment Research, Volume 87, Number 7
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Table 2—SPS consumption and pH change for different radical scavengers in a persulfate oxidation system. SPS reduction (%)
pH Final Initial (24 hours)
Radical scavenger Propanol (PrOH) (1000 mg L1) Phenol 100 mg L1 10 000 mg L1 Carbon tetrachloride (CT) (400 mg L1) No scavenger (control tests)
SPS SPS/NaOH SPS SPS/NaOH SPS SPS/NaOH SPS SPS/NaOH SPS SPS/NaOH
6.5 13.1 6.5 13.1 6.1 13.1 6.5 13.1 6.5 13.1
2.3 12.8 3.4 12.6 3.2 5.2 3.4 12.8 3.4 12.8
1 hour
24 hours
0.8 3.0 0.8 4.2 0.7 32.5 0.6 2.2 0.6 2.3
5.0 3.1 2.3 8.8 3.0 42.0 1.3 2.7 2.1 3.6
induced occurrence of these reactions and resulted in further CF degradation. O 2 O2 þ e
ð5Þ
CCl4 þ e CCl3 þ Cl
ð6Þ
CCl3 þ CHCl3 C2 Cl6 þ H
ð7Þ
Conclusions The results presented in this study revealed that: (1) both persulfate and alkaline activated persulfate systems are capable of degrading CF; (2) SO 4 and HO are predominant radicals in a alkaline activated persulfate system; (3) O 2 is also present as minor radical species in a SPS/NaOH system. Alkaline activated persulfate is a promising chemical reaction to be used for destroying organic contaminants because of the simultaneous presence of various radical species (SO 4 , HO, and O2 ). Further optimization of alkaline activated persulfate processes used for environmental remediation should take account of these results. Submitted for publication March 31, 2014; revised manuscript submitted September 27, 2014; accepted for publication November 12, 2014. References Alfassi, Z. B.; Padmaja, S.; Neta, P.; Huie, R. E. (1993) Rate Constants for Reactions of Nitrate (NO3–) Radicals with Organic Compounds in Water and Acetonitrile. J. Phys. Chem., 97, 3780–3782. Allen, J. M.; Lucas, S.; Allen, S. K. (2009) Formation of Hydroxyl Radical (OH) in Illuminated Surface Waters Contaminated with Acidic Mine Drainage. Environ. Toxicol. Chem., 15, 107–113. Chamam, M.; F¨oldva´ry, C. M.; Hosseini, A. M.; Tungler, A.; Taka´cs, E.; Wojna´rovits, L. (2012) Mineralization of Aqueous Phenolate Solutions: A Combination of Irradiation Treatment and Wet Oxidation. Radiat. Phys. Chem., 81, 1484–1488. Espenson, J. H. (1981) Chemical Kinetics and Reaction Mechanisms; McGraw-Hill: New York.
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Feng, J.; Lim, T.-T. (2005) Pathways and Kinetics of Carbon Tetrachloride and Chloroform Reductions by Nano-Scale Fe and Fe/Ni Particles: Comparison with Commercial Micro-Scale Fe and Zn. Chemosphere, 59, 1267–1277. Furman, O. S.; Teel, A. L.; Watts, R. J. (2010) Mechanism of Base Activation of Persulfate. Environ. Sci. Technol., 44, 6423–6428. Guan, Y.-H.; Ma, J.; Li, X.-C.; Fang, J.-Y.; Chen, L.-W. (2011) Influence of pH on the Formation of Sulfate and Hydroxyl Radicals in the UV/ Peroxymonosulfate System. Environ. Sci. Technol., 45, 9308–9314. Haag, W. R.; Yao, C. C. D. (1992) Rate Constants for Reaction of Hydroxyl Radicals with Several Drinking Water Contaminants. Environ. Sci. Technol., 26, 1005–1013. Hayon, E.; Treinin, A.; Wilf, J. (1972) Electronic Spectra, Photochemistry, and Autoxidation Mechanism of the Sulfite-Bisulfite-Pyrosulfite Systems. SO2–, SO3–, SO4–, and SO5– Radicals. J. Am. Chem. Soc., 94, 47–57. Ko, S.; Crimi, M.; Marvin, B. K.; Holmes, V.; Huling, S. G. (2012) Comparative Study on Oxidative Treatments of NAPL Containing Chlorinated Ethanes and Ethenes Using Hydrogen Peroxide and Persulfate in Soils. J. Environ. Manage., 108, 42–48. Krishna, M. C.; Grahame, D. A.; Samuni, A.; Mitchell, J. B.; Russo, A. (1992) Oxoammonium Cation Intermediate in the NitroxideCatalyzed Dismutation of Superoxide. Proc. Natl. Acad. Sci., 89, 5537–5541. Liang, C.; Guo, Y.-Y. (2012) Remediation of Diesel Contaminated Soils Using Persulfate Under Alkaline Condition. Water, Air, Soil Pollut., 223, 4605–4614. Liang, C.; Huang, C.-F.; Mohanty, N.; Kurakalva, R. M. (2008) A Rapid Spectrophotometric Determination of Persulfate Anion in ISCO. Chemosphere, 73, 1540–1543. Liang, C.; Su, H.-W. (2009) Identification of Sulfate and Hydroxyl Radicals in Thermally Activated Persulfate. Ind. Eng. Chem. Res., 48, 5558–5562. Liang, C.; Wang, Z.-S.; Bruell, C. J. (2007) Influence of pH On Persulfate Oxidation of TCE at Ambient Temperatures. Chemosphere, 66, 106–113. Lindsey, M. E.; Tarr, M. A. (1999) Inhibition of Hydroxyl Radical Reaction with Aromatics by Dissolved Natural Organic Matter. Environ. Sci. Technol., 34, 444–449. Lindsey, M. E.; Tarr, M. A. (2000) Quantitation of Hydroxyl Radical during Fenton Oxidation Following a Single Addition of Iron and Peroxide. Chemosphere, 41, 409–417. Lu, N.; Kombo, D. C.; Osman, R. (1997) Theoretical Studies of Hydrogen Abstraction from 2-propanol by OH Radical. J. Phys. Chem. A, 101, 926–936. NIEA (2006) Measurement of Volatile Organic Compounds in Water by Purge and Trap Coupled with Capillary Column Gas Chromatography/Mass Spectrometry. Method W785.54B. Ottolenghi, M.; Stein, G. (1961) The Radiation Chemistry of Chloroform. Radiat. Res., 14, 281–290. Overend, R.; Paraskevopoulos, G. (1978) Rates of OH Radical Reactions. 4. Reactions with Methanol, ethanol, 1-propanol, and 2-propanol at 296 K. J. Phys. Chem., 82, 1329–1333. Teel, A. L.; Watts, R. J. (2002) Degradation of Carbon Tetrachloride by Modified Fenton’s Reagent. J. Hazard. Mater., 94, 179–189. Tsai, T. T.; Kao, C. M.; Wang, J. Y. (2011) Remediation of TCEContaminated Groundwater Using Acid/BOF Slag Enhanced Chemical Oxidation. Chemosphere, 83, 687–692. Ziajka, J.; Pasiuk-Bronikowska, W. (2005) Rate Constants for Atmospheric Trace Organics Scavenging SO4– in the Fe-Catalysed Autoxidation of S(IV). Atmos. Environ., 39, 1431–1438.
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