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Ind. Eng. Chem. Res. 2009, 48, 5558–5562
Identification of Sulfate and Hydroxyl Radicals in Thermally Activated Persulfate Chenju Liang* and Hsin-Wey Su Department of EnVironmental Engineering, National Chung Hsing UniVersity, 250 Kuo-kuang Road, Taichung 402, Taiwan
Thermal activation can induce persulfate (S2O82-) degradation to form sulfate radicals (SO4-•) that can undergo radical interconversion to form hydroxyl radicals (HO•) under alkaline conditions. The radicals SO4-•/HO• can be present either individually or simultaneously in the persulfate oxidation system. To identify the active radical species, a chemical probe method was developed. An excess of probe compounds was added to the system, and differences between the reactivity of the probes and the potential radical species were observed. The usage of various probes, including tert-butyl alcohol, phenol, and nitrobenzene (NB), for simultaneously identifying SO4-•/HO• was investigated. NB can only react with radicals: it cannot react with persulfate. The reaction rate of NB with HO• is 3000-3900 times greater than that of NB with SO4-•, which is a good candidate for use as a probe for differentiating between SO4-•/HO• reactivity. Furthermore, the effects of pH on the formation of SO4-•/HO• were demonstrated by the degradation kinetics of NB at varying pH values. The results indicated that SO4-• is the predominant radical at pH 8.5) can induce the mechanism of SO4-• interconversion to HO• (see eq 2) in the persulfate activation system. In addition, it has also been shown that sulfate radicals can react with water at all pHs to produce HO•, in accordance with eq 3. The rate constants for eqs 2 and 3 are on the orders of (6.5 ( 1.0) × 107 M-1 s-1 (ref 6) and k[H2O] < 2 × 10-3 s-1 (ref 5), respectively. However, Norman et al.4 reported that the reaction rate constant of eq 3 is low, in comparison to those achieved for SO4-• reactions with organic compounds and eq 2 is mainly in control of radical interconversion. Alkaline pH: All pHs:
SO4 •+OH
SO4 •+H2O
f
f
SO24
SO24
+ HO• +
+ HO•+H
(2) (3)
Because of a higher redox potential and the less-selective contaminant destruction mechanisms of HO•, the formation of HO• presents the potential to increase the contaminant degradation rate and the possibility of reactions with a wider variety of * To whom correspondence should be addressed. Tel.: +886-422856610. Fax: +886-4-22862587. E-mail address: cliang@ dragon.nchu.edu.tw.
compounds. Moreover, when comparing the use of peroxide and persulfate as sources of HO•, persulfate is more stable than peroxide in aquatic systems (e.g., in the presence of alkalinity carbonate species8). Therefore, for ISCO remediation of subsurface contamination, it is recognized that the persulfate anion can move greater distances down-gradient from injection wells in the subsurface and hence persulfate is a better choice than peroxide. Although the formation of radical species (e.g., HO•) can be recorded using an ESR, it is still uncertain how active the radicals are during the oxidation of organic compounds.9 Identification of HO• yields, using the chemical probe method, have been intensively investigated in the Fenton oxidation processes.10-12 The chemical probe method was developed based on the use of an excess of probe compounds and ensuring that all of the radicals were scavenged by the probe compound. Generally, for a probe compound with radicals to be considered to be effective for instantaneous scavenging, a rate constant of >109 M-1 s-1 for the scavenging reaction must be achieved. Anipsitakis and Dionysious13 studied the transformation of 2,4dichlorophenol by the interaction of metals with oxidants (e.g., ferrous ion activated persulfate according to eq 4) and successfully employed the use of chemical probe compounds to quench reactions in conjunction with competition kinetics (i.e., differences in the reactivity between the potential radical species formed and the chemical probe additive) for identifying the primary radical species formed (e.g., SO4-• or HO•). 22+ 3+ f SOS2O28 +Fe 4 •+SO4 +Fe
(4)
A literature survey on the reaction rate constants between SO4-• or HO• and chemical probe compounds is provided in Table 1. As shown in the table, anisole, benzoic acid, benzene and phenol can react with both SO4-• and HO• at high rates (i.e., rate constant >109 M-1 s-1) and, hence, these four compounds are suitable for scavenging both radicals. Phenol has the highest water solubility (i.e., 8.28 g/L) and, as such, can be added in water at greater concentrations than the other compounds and can be used to effectively quench radical activity. Moreover, the reactivities of the two radicals are
10.1021/ie9002848 CCC: $40.75 2009 American Chemical Society Published on Web 05/06/2009
Ind. Eng. Chem. Res., Vol. 48, No. 11, 2009 Table 1. Second-Order Rate Constants for Reactions of Selected Chemical Probes with Hydroxyl and Sulfate Radicals Reaction Rate Constant (M-1 s-1) radical probe anisole benzoic acid benzene ethanol methanol nitrobenzene, NB propanol phenol tert-butyl alcohol, TBA
SO4-• 4.9 × 109 1.2 × 109 (2.4-3) × 109 (1.6-7.7) × 107 3.2 × 106 99.5%) and sodium hydroxide (99.0%) were purchased from Riedel-deHae¨n; nitrobenzene (>99.0%) and sodium persulfate (99.0%) were purchased from Merck; phenol (99.2%) was purchased from J.T. Baker; sulfuric acid (95∼97%) was purchased from Fluka; and methanol (99.9%) and potassium iodide (99.5%) were purchased from ECHO Chemical. 2.2. Experimental Methodology. Solutions that contain chemical probes (i.e., NB, TBA, and phenol) at a fixed initial concentration of 5 mM were used to react with three persulfate concentrations (i.e., 50, 80, and 100 mM). The pH of the solution was initially adjusted to pH 7 using NaOH or H2SO4 in all experiments. The experimental procedure used was in accordance with the method of Liang et al.14 Experiments were conducted in a series of 40-mL amber screw cap borosilicate glass bottles that were placed in a temperature-controlled water bath (Firstek Co., Model B206) at 70 °C. Reaction bottles for analysis were removed from the water bath at the proposed intervals, immediately placed in an ice bath (4 °C) to quench the reaction by chilling and set aside for analysis. Experiments to identify the predominant radical species were conducted in a 1.3-L heavy-wall plain pressure reaction flask (ACE Glass) kept at a temperature of 70 °C. The top of the flask was covered with a flat Teflon reaction head that was sealed with a stainless-steel clamp. The pH electrode was inserted through Teflon-lined septum ports on the top Teflon cover. The pH of the solution was maintained at the designated pH value (i.e., 2, 4, 7, 9, and 12) during the course of the reaction with a pH controller (Suntex pH/ORP controller, Model PC-310) by pumping H2SO4 or NaOH at a rate of 1 mL/min through two Teflon tubes inserted in a port on the top cover of the flask. The pH was also monitored continuously using the data acquisition software (Eutech Instruments, Model CyberComm 5000), and the data was recorded at every two seconds and exhibited a stable pH variation within 0.2 pH units. A
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syringe filled with RO purified water (50 mL) was inserted through a septum port to replace water removed from the reactor when taking samples, and zero headspace was maintained, in accordance with the procedure outlined by Dai and Reitsma.15 Note that seven samples (5 mL each removed by a gas-tight syringe (SGE Co.) at each sampling interval) were taken for chemical probe and S2O82- analysis. The acid and base solutions that were used to maintain the solution pH in the reaction flask were less than a total volume of 30 mL of acid or base injected in each experiment; therefore, the variation was limited to 1 indicates the formation of HO• and the increased ratio results from the increased concentration of HO•. The results of the NB degradation tests show that, at a higher pH (i.e., pH >9), relatively more HO• was being generated. Accordingly, NB is a suitable chemical probe for differentiating between HO• and SO4-•. Acknowledgment This study was funded by the National Science Council (NSC) of Taiwan (under Project No. 96-2221-E-005-017-MY3). Literature Cited Figure 2. Degradation of NB with 70 °C thermally activated persulfate at different pH values. Inset figure indicates the degradation of persulfate. Error bars denote one standard deviation error for four replicate data at each sampling interval. Table 3. Summary of Results from Thermally Activated Persulfate Oxidation of NB at Various pH Valuesa kobs,PS pH
(× 10-3 min-1)
2 4 7 9 12
1.67 ( 0.03 1.94 ( 0.06 1.76 ( 0.03 1.87 ( 0.02 4.15 ( 0.23
kobs,NB R2
(× 10-3 min-1)
0.997 4.34 ( 0.08 0.993 4.37 ( 0.08 0.998 4.02 ( 0.10 0.999 5.03 ( 0.13 0.976 21.18 ( 0.78
kobs,NB R2 0.997 0.998 0.995 0.995 0.993
kobs,NB predominant (at pH 2) radical species 1.00 1.01 0.93 1.16 4.88
SO4-• SO4-• SO4-• SO4-•/HO• HO•
a Note: The error ranges represent the standard deviation of linear regression analysis.
are independent of pH, with the exception of the basic pH condition (e.g., pH 12). Moreover, SO4-• produced in the reaction may also be scavenged by SO4-• itself and/or persulfate anions, according to eqs 8 and 9. Also, HO• in the persulfate system can react with persulfate anions, in accordance with eq 10. Therefore, based on the reported rate constants, the presence of HO · may induce more persulfate degradation. Hence, faster persulfate degradation occurred at pH 12. Therefore, the results of NB degradation with thermally activated persulfate would confirm the generation of predominant HO• in a basic solution.23,24,25 2SO4 •+SO4 • f S2O8
(k ) 4 × 108 M-1 s-1)
(8)
22SO4 •+S2O8 f SO4 +S2O8 •
(k ) 6.1 × 105 M-1 s-1) (9)
HO•+S2O28 f OH +S2O8 •
(k ) 1.2 × 107 M-1 s-1) (10)
4. Conclusions The use of a chemical probe method is a simple and quick way to assess the formation of HO• and SO4-• in systems that employ persulfate activation. This study demonstrated that tertbutyl alcohol (TBA) can directly react with persulfate, which is the source of HO• and SO4-• radicals. It is also suggested that TBA should not be used as a chemical probe. Phenol degradation in the thermal persulfate activation systems shows that phenol reacts with radicals rather than the persulfate and it was observed that radical scavenging rates increased with
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ReceiVed for reView February 19, 2009 ReVised manuscript receiVed April 21, 2009 Accepted April 23, 2009 IE9002848
