Correspondence/Rebuttal pubs.acs.org/est
Comment on “Synergetic Transformations of Multiple Pollutants Driven by Cr(VI)-Sulfite Reactions” and 2). Reactions between As(III) and •OH and SO4•− are shown in eqs 3 and 4, respectively. All of the rate constants are adapted from the article of Jiang et al.1 Equations 5 and 6, which are based on the competition kinetics method,11 may be used to estimate the efficiency of TBA in inhibiting As(III) oxidation. Here, kobs,TBA=0 and kobs,TBA respectively represent pseudo-first-order rate constants for the reaction of As(III) with • OH or SO4•− in the absence and presence of As(III) at various
recent article by Jiang et al.1 entitled “Synergetic transformations of multiple pollutants driven by Cr(VI)sulfite reactions” reports the efficient oxidation of contaminants (e.g., As(III)) in the Cr(VI)−S(IV) system. They ascribed the oxidation in aqueous solutions at acid pH to SO4•− and •OH. They determined the contributions of SO4•− and •OH to As(III) oxidation by using tert-butanol (TBA) as scavenger for the radical •OH. Although the performance of the Cr(VI)− S(IV) system is unquestionable, we disagree with the interpretation regarding the role of •OH in As(III) oxidation. Two arguments may be presented against such interpretation of • OH formation and its role. First, qualitative characterization of • OH by electron spin resonance (ESR) spectroscopy or use of coumarin as chemical probe in the Cr(VI)−S(IV) system in the presence of SO4•− is not valid. Second, quantitative differentiation between SO4•− and •OH cannot be done because of simultaneous scavenging of SO 4•− by TBA at excess concentrations. Radical chain reactions among •SO3−, SO5•−, and SO4•− during sulfite and bisulfite autoxidation in aqueous solutions have been proposed.2,3 Jiang et al.1 used the spin trap, 5,5dimethyl-1-pyrroline-N-oxide (DMPO), in ESR experiments on the Cr(VI)−S(IV) system. Signals in ESR spectroscopy are time-dependent.4,5 Jiang et al.1 indicated that they started ESR spectroscopy immediately after in situ addition of S(IV). The conventional accumulative scan time for the analysis was ∼1 min in order to obtain sufficiently stable and strong signals. The ESR spectra in their article suggest adduct formation within ∼1 min of the reaction. Unfortunately, neither DMPO/•OSO3− nor DMPO/•OOSO3− adduct is stable enough during ESR within 1 min. Either DMPO/•OSO3− or even DMPO/•OOSO3− adduct reacts with H2O/OH− quickly,6 forming the more stable DMPO/•OH adduct.3,4 Hence, the apparent production of DMPO/•OH adduct seen in Figure 3(b) in the article of Jiang et al.1 only reflects the decomposition of some of the DMPO/•OSO3− adduct.3−6 Jiang et al.1 interpreted a false-positive signal for the DMPO/•OH adduct as evidence of •OH formation in the Cr(VI)−S(IV) system at low pH. A cited study by Zou et al. (ref 25),7 indicates the transformation of DMPO/•OSO3− to DMPO/•OH via nucleophilic substitution. In addition to ESR, Jiang et al.1 used fluorescence spectrometry to qualitatively characterize •OH formation based on the reaction between coumarin and •OH resulting in the production of 7-hydroxycoumarin (coumarin/•OH adduct).8,9 Unfortunately, this method cannot be used in a system producing both •OH and SO4•− because the identical product, 7-hydroxycoumarin, forms via reaction between coumarin and SO4•−.10 Therefore, fluorescence signals in Figure 3(c) in the article of Jiang et al.1 cannot be regarded as evidence of •OH formation in the Cr(VI)−S(IV) system. TBA is a good probe for differentiating SO4•− and •OH because of the large difference between rate constants for scavenging reactions for •OH and SO4•− (ca. 1000-fold; eqs 1
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© XXXX American Chemical Society
concentrations of TBA, and
[TBA]0 [As(III)]0
is the ratio between initial
concentrations of TBA and As(III). The efficiency of inhibition by TBA (ρ) on As(III) oxidation by •OH or SO4•− is described in eq 7, which clearly shows a function of ρ versus [TBA]0 . Figure 1 shows the calculated inhibition efficiency at
[As(III)]0 [TBA]0 [As(III)]0
Figure 1. Plots of the calculated efficiency of inhibition by TBA (ρ) on As(III) oxidation by •OH (●) or SO4•− (▼, ▲) versus [TBA]0 ratio, [As(III)]0
at ratios of 200−3200. kOH,TBA = 5.7 × 108 M−1 s−1, kSO−4 ,TBA = 6.5 × 105 M−1 s−1, and kSO−4 ,As(III) = 8.0 × 108 M−1 s−1 (▼) or 1.0 × 1010 M−1 s−1 (▲). The appropriate ρ values for •OH and
[TBA]0 ratio might be 400, corresponding to [As(III)]0 •− SO4 of 96.4% and 2.5%−24.5%, respectively.
ratios in the range of 200−3200. Jiang et al.1 used a
[TBA]0 [As(III)]0 •
ratio of 3200, which corresponds to 99.5% inhibition of OH; this could cause inhibition of SO4•− at efficiencies of 17.2% (kSO−4 ,As(III) = 1.0 × 1010 M−1 s−1) to 72.1% (kSO−4 ,As(III) = 8.0 × 108 M−1 s−1). This result may lead to an overestimation of the contribution efficiency of •OH in As(III) oxidation by 21− 260%. TBA + •OH → product 1 k OH,TBA = (3.8 − 7.6) × 108M−1s−1 (1)
A
DOI: 10.1021/acs.est.6b00205 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
(4) Harbouar, J.; Hair, M. L. Spin trapping of the •CO2− radical in aqueous medium. Can. J. Chem. 1979, 57, 1150−1152. (5) Taniguchi, H.; Madden, K. P. An in situ radiolysis time-resolved ESR study of the kinetics of spin trapping by 5,5-Dimethyl-1-pyrrolineN-oxide. J. Am. Chem. Soc. 1999, 121, 11875−11879. (6) Mottley, C.; Mason, R. P. Sulfate anion free radical formation by the peroxidation of (bi)sulfite and its reaction with hydroxyl radical scavengers. Arch. Biochem. Biophys. 1988, 267 (2), 681−689. (7) Zou, J.; Ma, J.; Chen, L.; Li, X.; Guan, Y.; Xie, P.; Pan, C. Rapid acceleration of ferrous iron/peroxymonosulfate oxidation of organic pollutants by promoting Fe(III)/Fe(II) cycle with hydroxylamine. Environ. Sci. Technol. 2013, 47 (20), 11685−11691. (8) Ishibashi, K. I.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Detection of active oxidative species in TiO2 photocatalysis using the fluorescence technique. Electrochem. Commun. 2000, 2 (3), 207−210. (9) Kim, D. H.; Lee, J.; Ryu, J.; Kim, K.; Choi, W. Arsenite oxidation initiated by the UV photolysis of nitrite and nitrate. Environ. Sci. Technol. 2014, 48 (7), 4030−4037. (10) Singh, T. S.; Madhava Rao, B. S.; Mohan, H.; Mittal, J. P. A pulse radiolysis study of coumarin and its derivatives. J. Photochem. Photobiol., A 2002, 153, 163−171. (11) Matta, R.; Tlili, S.; Chiron, S.; Barbati, S. Removal of carbamazepine from urban wastewater by sulfate radical oxidation. Environ. Chem. Lett. 2011, 9, 347−353.
TBA + SO4•− → product 2 k SO4−,TBA = (4.0 − 9.1) × 105M−1s−1 (2)
As(III) + •OH → product 3 k OH,As(III) = 8.5 × 109 M−1s−1
(3)
As(III) + SO4•− → product 4 k SO−4 ,As(III) > 8.0 × 108 M−1s−1
kobs,TBA = 0 kobs,TBA kobs,TBA = 0 kobs,TBA
−1=
−1=
kHO,TBA kHO,As(III) k SO−4 ,TBA k
×
×
SO−4 ,As(III)
(4)
[TBA]0 [As(III)]0
(5)
[TBA]0 [As(III)]0
(6)
⎛ kobs,TBA = 0 − kobs,TBA ⎞ ⎟⎟ × 100 ρ(%) = ⎜⎜ kobs,TBA = 0 ⎝ ⎠ ⎛ kobs,TBA ⎞ ⎟⎟ × 100 = ⎜⎜1 − kobs,TBA = 0 ⎠ ⎝
(7)
In conclusion, ESR using DMPO as spin trap or fluorescence spectrometry using coumarin as probe is not valid for qualitative characterization of •OH formation in systems involving multiple sulfur-derived radicals (e.g., Cr(VI)−S(IV) system). The concentration of TBA should not be too high for it to be suitable for differentiation between •OH and SO4•−. We agree with the performance of the Cr(VI)−S(IV) system in contaminant removal, but we disagree with the evidence presented regarding •OH formation and its role in As(III) oxidation.
Yingtan Yu Yanan Yuan Shaojie Yang Feng Wu*
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Department of Environmental Science, Hubei Key Lab of Biomass Resource Chemistry and Environmental Biotechnology, School of Resources and Environmental Science, Wuhan University, 430079, P. R. China
AUTHOR INFORMATION
Corresponding Author
*Phone: (86)-27-68778511; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Jiang, B.; Liu, Y.; Zheng, J.; Tan, M.; Wang, Z.; Wu, M. Synergetic transformations of multiple pollutants driven by Cr(VI)sulfite reactions. Environ. Sci. Technol. 2015, 49 (20), 12363−12371. (2) Kuo, D. T. F.; Kirk, D. W.; Jia, C. Q. The chemistry of aqueous S(IV)-Fe-O2 system: state of the art. J. Sulfur Chem. 2006, 27, 461− 530. (3) Ranguelova, K.; Rice, A. B.; Khajo, A.; Triquigneaux, M.; Garantziotis, S.; Magliozzo, R. S.; Mason, R. P. Formation of reactive sulfite-derived free radicals by the activation of human neutrophils: An ESR study. Free Radical Biol. Med. 2012, 52 (8), 1264−1271. B
DOI: 10.1021/acs.est.6b00205 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
