Effect of lignin structure on adsorbable organic halogens formation in chlorine dioxide bleaching

Adsorbable organic halogens (AOX) are formed in pulp bleaching as a result of the reaction of residual lignin with chlorine dioxide. The natural structure of lignin is very complex and it tends to be damaged by various extraction methods. All the factors can affect the study about the mechanism of AOX formation in the reaction of lignin with chlorine dioxide. Lignin model compounds, with certain structures, can be used to study the role of different lignin structures on AOX formation. The effect of lignin structure on AOX formation was determined by reacting phenolic and non-phenolic lignin model compound with a chlorine dioxide solution. Vanillyl alcohol (VA) and veratryl alcohol (VE) were selected for the phenolic and non-phenolic lignin model compound, respectively. The pattern consumption of lignin model compounds suggests that both VA and VE began reacting with chlorine dioxide within 10 min and then gradually steadied. The volume of AOX produced by VE was significantly higher than that produced by VA for a given initial lignin model compound concentration. In a solution containing a combination of VA and VE in chlorine dioxide, VE was the dominant producer of AOX. This result indicates that the non-phenolic lignin structure was more easily chlorinated, while the phenolic lignin structure was mainly oxidized. In addition, AOX content produced in the combined experiments exceeded the total content of the two separate experiments. It suggested that the combination of phenolic and non-phenolic lignin structure can promote AOX formation.


Recommendation?
Accept with minor revision (please list in comments)

Comments to the Author(s)
The adsorbable organic halogen (AOX) mainly comes from the reaction of residual lignin and chlorinated bleaching agent during bleaching. The application of lignin model compound in pulping is helpful to understand the reaction mechanism of lignin and chlorine dioxide from the molecular level. The pathway to AOX formation by the lignin model compound and chlorine dioxide was speculated, which will provide a theoretical basis for reducing the formation of AOX. I believe the manuscript can be accepted after minor revision. Some specific points are showed below.
2. Introduction. "Many studies have focused on the degradation of lignin model compounds". What is the difference between this study and previous study.
3.2. Experiments. "The reaction mixture was stored in a refrigerator and analyzed as soon as possible". How to ensure the reaction did not continue.
3.4. HPLC Analysis. "using a mobile phase composed of acetonitrile-water (10/90) (vol) and 0.1% acetic acid, a flow rate of 0.3 mL·min-1, a temperature of 50 °C, and UV detection at 280 nm". If there are some references, please insert them. 4.1. Effect of reaction time on AOX formation. "Each individual reaction was performed in a solution with an initial AOX content of 6.2 mmol and concentration of 124 mmol·L-1". Please verify if "AOX content" should be "lignin model compound". 4.1. Effect of reaction time on AOX formation. "The AOX content increased within 10 minutes and ultimately reached 1224 mg·L-1". Please revise the time unit of the manuscript. 4.2. Concentration of lignin model compounds. "The AOX content formed by the reaction of VE with chlorine dioxide solution was 60.9 mg·L-1 creating a total concentration of 12.4 mmol·L-1, which is slightly higher than that formed by VA with chlorine dioxide solution (55.4 mg·L-1)". The author needs further explanation in this result. 4.2. Concentration of lignin model compounds. "The total AOX content of the separate reaction is less than that of combined reaction of VA and VE with chlorine dioxide solution at the same total concentration". The author needs further explanation in this result.

18-Dec-2018
Dear Dr Yao: Title: Effect of lignin structure on AOX formation in chlorine dioxide bleaching Manuscript ID: RSOS-182024 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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2. Introduction. "The chemical structure of lignin is complex, and the lignin macromolecules have a great variety of bonds". Why were phenolic lignin model compound and non-phenolic lignin model compound selected to study? 3.2. Experiments. "Then, the respective lignin model compound was transferred to the chlorine dioxide solution and reacted at 70 °C with magnetic stirring". How many rpm? Please explain the experiment more clearly.
4.1. Effect of reaction time on AOX formation. "This is in agreement with the conclusions of C. Bogatu, who found that the oxidation of phenolic compounds with chlorine dioxide in water solutions contains a very fast step". Please insert references. 4.1. Effect of reaction time on AOX formation. "AOX was also formed rapidly at the beginning of the reaction when VE reacted with chlorine dioxide, reaching 1036 mg•L-1 after 10 minutes". Please revise the time unit of the manuscript. 5. Conclusion. "The AOX content produced by the reaction of VE with chlorine dioxide solution is higher than that of VA with chlorine dioxide solution, although the consumption of VE is smaller than that of VA". The author needs further explanation in "Results and Discussion".

Reviewer: 2
Comments to the Author(s) The adsorbable organic halogen (AOX) mainly comes from the reaction of residual lignin and chlorinated bleaching agent during bleaching. The application of lignin model compound in pulping is helpful to understand the reaction mechanism of lignin and chlorine dioxide from the molecular level. The pathway to AOX formation by the lignin model compound and chlorine dioxide was speculated, which will provide a theoretical basis for reducing the formation of AOX. I believe the manuscript can be accepted after minor revision. Some specific points are showed below.
2. Introduction. "Many studies have focused on the degradation of lignin model compounds". What is the difference between this study and previous study.
3.2. Experiments. "The reaction mixture was stored in a refrigerator and analyzed as soon as possible". How to ensure the reaction did not continue.
3.4. HPLC Analysis. "using a mobile phase composed of acetonitrile-water (10/90) (vol) and 0.1% acetic acid, a flow rate of 0.3 mL·min-1, a temperature of 50 °C, and UV detection at 280 nm". If there are some references, please insert them. 4.1. Effect of reaction time on AOX formation. "Each individual reaction was performed in a solution with an initial AOX content of 6.2 mmol and concentration of 124 mmol·L-1". Please verify if "AOX content" should be "lignin model compound". 4.1. Effect of reaction time on AOX formation. "The AOX content increased within 10 minutes and ultimately reached 1224 mg·L-1". Please revise the time unit of the manuscript. 4.2. Concentration of lignin model compounds. "The AOX content formed by the reaction of VE with chlorine dioxide solution was 60.9 mg·L-1 creating a total concentration of 12.4 mmol·L-1, which is slightly higher than that formed by VA with chlorine dioxide solution (55.4 mg·L-1)". The author needs further explanation in this result.

Concentration of lignin model compounds. "
The total AOX content of the separate reaction is less than that of combined reaction of VA and VE with chlorine dioxide solution at the same total concentration". The author needs further explanation in this result. Thank you for your letter and for the comments concerning our manuscript entitled "Effect of lignin structure on AOX formation in chlorine dioxide bleaching". We have studied your comments carefully and have made corrections which we hope could meet your requirements. All changes have been highlighted in the revised version (red highlighting).

Author's Response to Decision
Questions you put forward are explained as follows: Under the condition of excessive lignin model, less VE was consumed by quantitative chlorine dioxide. This is due to the low reactivity of non-phenolic lignin. The complete reaction of VE requires more chlorine dioxide than VA. VA containing phenolic structure has higher reactivity. Phenolic hydroxyl groups were converted into free radicals for it was easily attacked by chlorine dioxide. And then the oxidation fracture of the benzene ring. Most free radicals are further oxidized by chlorine dioxide to form chlorolipids. It was further decomposed into muconic acid, ortho-quinone, para-quinone and oxacyclopropane structures for its unstable structure.
Oxidative fracture of aromatic structures was the main reaction of VA in chlorine dioxide solution. Therefore, the lower content of AOX was generated from VA. VE has lower reactivity due to the absence of phenolic hydroxyl group. The hydroxymethyl side chain was easier oxidized by chlorine dioxide. Then chlorine substitution was occured on the aromatic-ring. More content of AOX was generated at a lower reaction rate. It has been explained more clearly in the revised version.

Introduction. "Many studies have focused on the degradation of lignin model compounds". What is the difference between this study and previous study.
Previous study mainly focused on the changes of structure and degradation pathway of lignin model compounds with specific structures. In this study, the effect of lignin structures on AOX formation and types of chlorinated products by different structures was studied.

Experiments. "
The reaction mixture was stored in a refrigerator and analyzed as soon as possible". How to ensure the reaction did not continue.
The reaction was terminated by the oxidation of sodium sulfite with residual chlorine dioxide. In addition, the reaction mixture was diluted immediately in deionized water to achieve reaction termination. It has been described more clearly in the revised version. VA containing phenolic structure has higher reactivity.

HPLC
Phenolic hydroxyl groups were converted into free radicals for it was easily attacked by chlorine dioxide. And then the oxidation fracture of the benzene ring. Most free radicals are further oxidized by chlorine dioxide to form chlorolipids. It was further decomposed into muconic acid, ortho-quinone, para-quinone and oxacyclopropane structures for its unstable structure. Oxidative fracture of aromatic structures was the main reaction of VA in chlorine dioxide solution. Therefore, the lower content of AOX was generated from VA. VE has lower reactivity due to the absence of phenolic hydroxyl group.
The hydroxymethyl side chain was easier oxidized by chlorine dioxide. Then chlorine substitution was occured on the aromatic-ring. More content of AOX was generated at a lower reaction rate.

Concentration of lignin model compounds. "The total
AOX content of the separate reaction is less than that of combined reaction of VA and VE with chlorine dioxide solution at the same total concentration". The author needs further explanation in this result.
Under the condition of excessive lignin model, less VE was consumed by quantitative chlorine dioxide. This is due to the low reactivity of non-phenolic lignin. The complete reaction of VE requires more chlorine dioxide than VA. In combination reaction of VA and VE, the dosage of chlorine dioxide reacting with VE increased. The consumption of VE increased comparing to that in separated reaction of VE. Meanwhile, the VA shared similar consumption in separated and combined reaction. Therefore, the total AOX content of the separate reaction is less than that of combined reaction at the same total concentration.
As a whole, issues the reviewer suggested are very pertinent, which are very helpful to modified my entire paper and thank you very much again.