Abatement of Nitrophenol in Aqueous Solution by HOCl and UV/HOCl Processes: Kinetics, Mechanisms, and Formation of Chlorinated Nitrogenous Byproducts
, Jean-Marc Chovelon 2, Jing Chen 1, Canlan Jiang 1 and Yuefei Ji 1,*Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis manuscript by Li et al. “Abatement of nitrophenol in aqueous solution by HOCl and UV/HOCl processes: Kinetics, mechanisms, and formation of chlorinated nitrogenous byproducts” investigated the elimination of 4-nitrophenol (4NP) and the generation of chlorinated byproducts in HOCl and UV/HOCl treatment processes. The authors found that 2-chloro-4-nitrophenol (2C4NP) and 2,6-dichloro-4-nitrophenol (26DC4NP) were generated consecutively in HOCl process; but their formation was less in the UV/HOCl process. Trichloronitromethane (TCNM) was only found in the UV/HOCl process, and its production increased with increasing HOCl dosage. Transformation pathways were proposed for 4NP transformation in UV/HOCl process, and wastewater matrix effect was evaluated. Overall, results of this work are helpful to advance the understanding of the transformation of nitrophenolic compounds and assess the formation potential of chlorinated byproducts in HOCl and UV/HOCl disinfection processes. The manuscript was prepared in a good manner and the experiments were well designed. Some new data was obtained, and technical aspect looks good. Some comments are proposed as follows to help the authors improve the quality of their manuscript.
1. In the introduction section, the authors should add some discussion to reveal the advantages of UV/HOCl as compared to other UV-based advanced oxidation processes typically known to be used in water and wastewater treatment, such as UV/H2O2 and UV/persulfate processes.
2. Line 41, as the authors mentioned that the photolysis of hypochlorous acid/hypochlorite ion (HOCl/OCl-) can directly generate reactive oxidizing species including chlorine (Cl•) and hydroxyl radicals (HO•). What is the quantum yield of the UV/HOCl process? And factors influencing such process should be mentioned, for example if the UV/HOCl process is wavelength-dependent? This information should be supplemented to support related discussion.
3. Line 44, the authors mentioned that Cl• is a more selectively radical as compared to HO•. thus, what about the second-order rate constants between Cl• and organic contaminants typically encountered in wastewater? Some related references should be supplemented to fortify discussion.
4. Line 53-65, the introduction of the formation of chlorophenols during HOCl treatment of phenolic compounds has been provided, but there is still a lack of information regarding the formation of chlorophenols during UV/HOCl. Among the latter, the mechanism accounting for the formation of chlorophenols during HOCl process is perhaps different with those of HOCl (i.e., nucleophilic substitution). Please discuss in detail.
5. One of the purposes of this study is to investigate the effects of water matrixe on the performance of HOCl and UV/HOCl processes. Please discuss in the introduction section how the water matrix could influence the performance of HOCl and UV/HOCl processes. This is important for the readers to better understand the experimental design and purpose of this work.
6. Line 105, Please explain if the dosage of HOCl, i.e., 10 uM, is representative of the level typically used for wastewater treatment.
7. Line 128-129, it is interesting to see that direct photolysis of 4NP was very weak over the time period investigated. Please explain why the direct photolysis of 4NP was very weak under UV254 irradiation. What is the light-absorbing ability of 4NP and the quantum yield under UV254?
8. Line 301-303, please supplement that the reactivity of 2C4NP with HOCl is less as compared to the parent 4NP because the electron-withdrawing effect of Cl substituent. Therefore, it is not easy for HOCl alone to oxidatively cleave the benzyl ring as compared to more reactive radicals.
9. Line 309, the authors concluded that the formation potential of TCNM increased significantly with the increase of HOCl dosage based on their experimental results. Please discuss in more detail what is the implications of this result for water and wastewater treatment used UV/HOCl as treatment unit？
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsIn this work, the authors studied the kinetics, mechanisms, and formation of three chlorinated nitrogenous byproducts in chlorination or UV-chlorination of nitrophenol. This is a well-designed delicate study with reasonable findings. Below are several minor comments and suggestions for the authors’ consideration.
It seems to me that there is no need to use abbreviations such as 4NP, 2C4NP and 26DC4NP, which may affect the readability of the paper. Please use their full names directly.
The initial concentration of 4NP was 10 μM, which is higher than the environmental concentrations. My understanding is that this concentration was intentionally used to amplify the reactions for better detection of transformation products. It would be better if the authors could briefly justify this in the main text.
The resolution of Figure 1 is low. Please try to improve it.
The sizes of the dot for radicals were not consistent. Please check them throughout the manuscript.
Halogenated nitrophenols are one group of highly toxic halogenated DBPs. Previous toxicological studies have shown that aromatic DBPs generally exhibit substantially higher developmental toxicity and growth inhibition (ES&T 55, 5906; 47, 10868; WR 65, 64), and nitrogenous DBPs generally exhibit substantially higher toxicity than carbon-based DBPs. With studies indicating that the one and two-carbon-atom DBPs of current interest account for only ~16% of disinfected water cytotoxicity, there is a need to identify toxicity drivers within the higher-molecular-weight (more than two carbon atoms) DBP fraction (e.g., Mitch et al., Nat. Water 1, 336). It would be more meaningful if the authors could also provide some discussion on the toxicity of those high-molecular-weight aromatic nitrogenous DBPs.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
