Correspondence/Rebuttal pubs.acs.org/est
Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Comment on “Visible-Light-Driven Photocatalytic Degradation of Organic Water Pollutants Promoted by Sulfite Addition”
I
n a recent article, Deng et al. synthesized a series of visiblelight-active bismuth oxyhalide semiconductors, and subsequently utilized them to activate sulfite for the purpose of water organic pollutants degradation.1 After a variety of hole/ radical quenching experiments, the authors concluded that sulfite radical is the main active species in degrading MO in the sulfite-promoted photodegradation process both aerobically and anaerobically. Under anaerobic conditions, sulfite radical (SO3•−) was supposed to be exclusively obtained without further transformation into peroxomonosulfate radical (SO5•−, eq 2) and sulfate radical (SO4•−, eq 3).2 Moreover, the moderate capability of SO3•− oxidizing organic substrates has been extensively documented3,4 and is consistent with our findings.5 However, we believe the “aerobic” BiOBr/sulfite/ visible light system in their experiments was essentially an anaerobic system, and the authors should create a real aerobic condition and then re-examine the radical contributions before reaching the above conclusion. At first, the authors need to verify whether their “aerobic” BiOBr/sulfite/visible light system was truly aerobic. Sulfite has been renowned as a powerful antioxidant for its strong capability to scavenge dissolved oxygen, and it was found that 10 mM sulfite alone could deplete dissolved oxygen within 2 min even if the reaction solution was open to the air under constant stirring (Figure 1). Moreover, SO3•−, derived after
generated by Cu(II)/sulfite depleted dissolved oxygen within 2 min, slightly faster than sulfite alone, even if atmospheric oxygen was constantly drawn into the solution by stirring (Figure 1). Therefore, we believe the “aerobic” BiOBr/sulfite/ visible light assays conducted by Deng et al., without extra oxygenation, were in fact anaerobic. Cu(II) + SO32 − → Cu(II) − SO32 − → Cu(I) + SO3·− (1) (2)
SO5·− + SO32 − → SO4 2 − + SO4·−
(3)
Furthermore, contrary to the authors’ belief, we reasoned that the possibility of SO3•− oxidizing MO under truly aerobic conditions should be excluded because (1) the reaction of SO3•− with molecular oxygen (k = 1.5 × 109 M−1 s−1) is 100− 1000 times faster than it with AO7 (k = 8.4 × 106 M−1 s−1) − a structural analog of MO,5 or any other reported organic substrate (k < 107 M−1 s−1),3,4 and (2) dissolved oxygen concentration (∼250 μM) is significantly higher than used MO concentration (30 μM) as well. In fact, after SO3•− was generated by hole or hydroxyl radical grabbing electron from sulfite anion as proposed, SO3•− was able to further evolve into SO5•− (eq 2) and SO4•− (eq 3) in the presence of oxygen. Moreover, the strong oxidation capacities of SO5•− and SO4•− have been well studied,5 and we hence believe the radicals involved in MO oxidation are likely SO5•− and SO4•− under truly aerobic conditions. To further validate our speculation, we compared MO degradations by purging nitrogen or oxygen into solutions containing SO3•− produced by Cu(II)/sulfite, and obtained significantly different kinetics performances of MO degradation (Figure 1), indicating the scavenging of SO3•− into SO5•− by oxygen. Consequently, the authors might need to reprobe the contributions of SO5•− and SO4•− in a truly aerobic BiOBr/ sulfite/visible light system with constant aeration of oxygen. The exact portions of individual radical with respect to MO degradation could be discerned by using a differential radicalquenching assay. Simply, two kinetically distinguished radical scavengers (Table 1) are individually spiked into the reaction
Figure 1. Depletion of dissolved oxygen by sulfite or Cu(II)/sulfite in an open system with constant stirring, and degradation of MO by Cu(II)/sulfite by purging N2 or O2 to maintain the anaerobicity or aerobicity. 0.2 mM Cu(II), 10 mM sulfite, 30 μM MO, pH 7.5.
one-electron deprivation of sulfite anion, is an even stronger oxygen depleting agent than its parental sulfite anion. In an aerobic system, SO3•− is rapidly oxidized by dissolved oxygen into SO5•− (eq 2), with an approaching-diffusion rate of 1.5 × 109 M−1 s−1.3,4 Theoretically, SO3•− derived from slight amount of sulfite (0.5 mM) is sufficient to completely deplete dissolved oxygen (∼250 μM under equilibrium).2,6 To verify this, we tracked the consumption of dissolved oxygen by catalytically induced SO3•−. Since we do not have the specifically synthesized BiOBr as theirs, we used 0.2 mM cupric ion (Cu(II)) to initiate the production of SO3•− with the addition of 10 mM sulfite (eq 1).6,7 It was observed that SO3•− © XXXX American Chemical Society
SO3·− + O2 → SO5·−(k = 1.5 × 109M−1s−1)
Table 1. Second-Order Reaction Rate Constants of Scavengers with Sulfate Radical (SO4•−) and Peroxomonosulfate Radical (SO5•−)a rate constant (M−1 s−1)
a
A
scavenger
SO4•−
SO5•−
EtOH (CH3CH2OH) DPA ((C6H5)2NH)
1.6−7.7 × 10 (4) ∼ 8.1× 109 (5) 7
