CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Comment on “Effect of Dissolved Organic Matter on the Transformation of Contaminants Induced by Excited Triplet States and the Hydroxyl Radical” he recent paper of Wenk et al.1 caught our attentions. In this and the authors’ previous studies,2 they observed “a marked inhibition by DOM of triplet-induced oxidation” for several pollutants, and proposed that the inhibition was caused by DOM reducing the oxidation intermediates (P 3 +) back to the parent pollutant (P). The mechanism understanding is of low credibility. As were also mentioned by Wenk et al.,1 DOM exhibits multiple effects on the photodegradation of organic pollutants. The effects depend on DOM origins,3 pollutant reactivities,4 and aquatic (experimental) conditions.5 Wenk et al.1 did not provide evidence to exclude other effects of DOM, for example, reduction of the excited triplet states (3Sens*) by DOM or DOM radicals.6 Even though DOM can reduce P 3 +, the reaction cannot be regarded as the only or dominant pathway responsible for the inhibiting effects of DOM on the tripletinduced transformation. As P 3 + was oxidized from P by 3Sens*,1 the oxidizing ability of 3 Sens* should be stronger than P 3 +. The oxidation of DOM by 3 Sens* is thermodynamically more favorable than by P 3 +. The reactions responsible for the inhibiting effects of DOM on the triplet-induced oxidation are described as eqs 14.
T
3
Sens þ DOM f Sensred þ DOM 3 þ
ð1Þ
3
Sens þ P f Sensred þ P 3 þ
ð2Þ
P 3 þ f Pox
ð3Þ
P 3 þ þ DOM f P þ DOM 3 þ
ð4Þ
We evaluated the electron donating abilities for the electron donors investigated by Wenk et al.,1 employing the density functional theory (DFT) calculation described previously.7 An SRFA average structure model, representing the average property of SRFA,8 was employed to represent SRFA. The calculated vertical ionization potential values (eV) are 5.297 (N,Ndimethylaniline) < 5.732 (SRFA) < 6.055 (trimethoprim) ≈ 6.142 (4-methylphenol) < 6.248 (sulfamethoxazole). The computed energies of the highest occupied molecular orbital (eV) are 0.233 (sulfamethoxazole) < 0.227 (4-methylphenol) ≈ 0.226 (trimethoprim) < 0.213 (SRFA) < 0.199 (N,Ndimethylaniline). These results indicate that SRFA has moderate electron donating ability among the five electron donors. For other DOM, no concrete structures are available for the quantum chemical computation. Nevertheless, the reactivity of redoxactive groups in DOM of various origins was found to be fairly constant in the reduction of chlorinated pollutants,9 although the redox property varies with origins of DOM.10 Accordingly, DOM competes with pollutant P in the reactions with 3Sens*, leading to the decrease of the oxidation rate of P. Additionally, for pollutants with lower reducing abilities than those studied by r 2011 American Chemical Society
Wenk et al.,1 the competition of DOM for 3Sens* becomes more significant. The statement that “Triplet state quenching would result in a uniform decrease in oxidation rate for any target contaminant” is subjective and needs experimental evidence. The DOM induced decrease of oxidation rate for P can be expressed by eqs 58. For eq 1: k1ðSensÞ ¼ kDOM ½DOM½3 Sens
ð5Þ
For eq 2: kO ¼ k2ðSensÞ ¼ kp ½P½3 Sens
ð6Þ
steady-state : ½3 Sens ¼ ðksens -kQ Þ=ðkP ½P þ kDOM ½DOMÞ
ð7Þ kO -kO 0 ¼ ðksens kQ Þ=ð1 þ kp ½P=kDOM ½DOMÞ
ð8Þ
where k1(Sens*) and k2(Sens*) are consumption rates of 3Sens* reacting with DOM and P, respectively; kP and kDOM are secondorder rate constants of P and DOM reacting with 3Sens*, respectively; ksens, the generation rate of 3Sens*, is constant under steady-state irradiation;1 kQ, the physical quenching rate of 3 Sens*, varies little under low [DOM]; kO and kO0 are oxidation rates of P in the absence and presence of DOM, respectively. As indicated by eq 8, for a specific pollutant, DOM with different kDOM values lead to different decrease of kO; and for a specific DOM, pollutants with different kp values lead to different decrease of kO too. Thus, triplet state quenching could not result in a uniform decrease in oxidation rate of target contaminants. We estimate the [3Sens*] value under the experimental conditions of Wenk et al.1 by eq 9. ½3 Sens ¼ kobs =kp
ð9Þ
where kobs (109 M1 s1. Thus, [DOM]max is calculated to be
