Synthesis of a novel amphoteric copolymer and its application as a dispersant for coal water slurry preparation

In this work, a novel amphoteric copolymer named Poly(sodium p-styrenesulfonate–co-acrylic acid-co-diallyldimethylammonium chloride) (P(SS-co-AA-co-DMDAAC)) was synthesized via free radical polymerization. Afterwards, P(SS-co-AA-co-DMDAAC) was explored for use as a dispersant in coal water slurry (CWS) preparation. The structure of P(SS-co-AA-co-DMDAAC) was verified by Fourier transform infrared spectroscopy and nuclear magnetic resonance. The synthetic conditions were optimized as the feed ratio of AA to SS was 1 : 1 (for Yulin coal) or 1.5 : 1 (for Yili coal), and DMDAAC dosage was 4.0 wt% (for Yulin coal) and 6.0 wt% (for Yili coal) toward total monomers. The performances of P(SS-co-AA-co-DMDAAC) as a dispersant for CWS were evaluated by various technologies, such as apparent viscosity, zeta potential, static stability and contact angle measurements. The results revealed that the optimized dosage of P(SS-co-AA-co-DMDAAC) in CWS preparation was 0.3 and 0.4 wt% for Yulin coal and Yili coal respectively. In this optimum condition, CWS prepared using P(SS-co-AA-co-DMDAAC) as dispersant showed a typical shear thinning behaviour and excellent stability, which are desired in industries. The rheological models also confirmed the pseudo-plastic characteristics of CWS. Finally, compared with the widely used anionic dispersant naphthalene sulphonate formaldehyde condensate (NSF) and poly(sodium p-styrenesulfonate) (PSS), P(SS-co-AA-co-DMDAAC) developed in this work exhibited better slurry making performance. The introduction of cationic functional groups promoted the adsorption of the dispersant, which further enhanced the electrostatic repulsion and steric hindrance among coal particles. Accordingly, the viscosity of CWS decreased and static stability enhanced.

Q5. Please check the format of reference 10.

Decision letter (RSOS-201480.R0)
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Dear Dr Du: Title: Synthesis of a novel amphoteric copolymer and its application as a dispersant for coal water slurry preparation Manuscript ID: RSOS-201480 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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Reviewer: 2
Comments to the Author(s) This manuscript investigated the preparation of a novel amphoteric copolymer via free radical polymerization and it can improve the dispersion and stability of coal water slurry. The novel dispersant of amphoteric copolymer shows a promising application in industrial. This manuscript could be accepted after minor revisions.
Q1. Why Yili coal and Yulin coal were selected for comparison in the manuscript? What are the characteristics of these two coal samples? Q2. In abstract, the first letter in the full names of the instruments should be lowercase. Q3. In 2.3.1, the sample given is a coal sample, but the result of the detection (in Figure 2

Recommendation?
Accept as is

Comments to the Author(s)
Authors have made appropriate modifications in the revised manuscript and response to the previous comments. It could be published in Royal Society Open Science.

Decision letter (RSOS-201480.R1)
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Dear Dr Du:
Title: Synthesis of a novel amphoteric copolymer and its application as a dispersant for coal water slurry preparation Manuscript ID: RSOS-201480.R1 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The comments of the reviewer(s) who reviewed your manuscript are included at the end of this email.

Response to Reviewers Comments (RSOS-201480)
Dear Editor, Thank you for your letter and for the reviewers' comments concerning our manuscript entitled "Synthesis of a novel amphoteric copolymer and its application as a dispersant for coal water slurry preparation" (RSOS-201480). Those comments are all valuable for improving our paper. We have studied the comments and revised our manuscript carefully before submitting the final version which we hope to meet with approval. All the revised portions are marked in red in the revised manuscript. The authors' response to the reviewer's comments and the main corrections in the revised manuscript are as following: Response to the Reviewer A Comment 1: The language needs a lot of improvements.
Reply: Thanks for your comments and questions. We have checked the language of this manuscript carefully and rewrite the sentences and paragraphs. The changes of language are highlighted in red in the revised manuscript. The language in the manuscript had been polished by a professional institution, and the polished proof is shown in Figure R6.

Comment 2:
Contrast test that conducted with common CWS dispersant(s) is needed. This manuscript did not provide enough information.
Reply: Thanks for your comments and questions. We have added a common CWS dispersant naphthalene sulfonic formaldehyde condensate (NSF) into the contrast test. The measurements of maximum concentration, contact angle, adsorption amount and adsorption layer thickness were conducted, and these results and discussions were also added to the revised manuscript.
It is shown in Figure R1 that the CWS prepared using P(SS-co-AA-co-DMDAAC) has a higher maximum concentration than that of PSS and NSF. This may attributes to the  The result of contact angle is displayed in Figure R2. Compared with the contact angle of distilled water, the contact angle of P(SS-co-AA-co-DMDAAC) significantly decreases from 67° to 40° for Yili coal and from 79° to 29° for Yulin coal, which are even lower than the that of NSF solution and PSS solution. It indicates that P(SS-co-AA-co-DMDAAC) solution could wet the coal surface faster than PSS and NSF, exhibiting better wettability to coal surface. It can be explained by the introduction of cationic groups and COOgroups which enhances the mutual attraction between P(SS-co-AA-co-DMDAAC) and coal surface. This speculation is further confirmed by the adsorption amount measurement.   Then the suspension was filtered to remove water and dispersant that did not been adsorbed.
After that, the coal sample was dried at 105 °C to constant weight and then attached to the sample plate for XPS analysis. Due to the fact that coal has Si element while NSF, PSS and P(SS-co-AA-co-DMDAAC) has no Si element, Si 2p was selected as the character element for the calculation of adsorption layer thickness. The photoelectron intensity of Si 2p will decay after passing through the adsorption layer. When the dispersant is adsorbed on the coal surface, the Si peak obtained belongs to the Si atoms under the adsorption layer. Thus, the decay of photoelectron intensity will be used to evaluate the thickness of layer. The thickness of adsorption layer was calculated by the semi-rational equations listed below.
where Id is the intensity of the photoelectron transmitted through the adsorption layer, I0 is the incident photoelectron intensity, d is the thickness of adsorption layer (nm), λ(Ek) is the average depth (nm) at which the light electron escaped, Ek is the light electron kinetic energy and Eb is the atomic binding energy of Si. The value of I0 and Id can be obtained according to the area of the Si 2p photoelectron in the XPS spectrum. The XPS analysis results are shown in Figure R5, and the thickness of adsorption layer formed by dispersants are listed in Table R1.   Figure R5 and Table R1 that the integral area of Si 2p peak decreased after the adsorption of dispersant, and area of the coal adsorbed P(SS-co-AA-co-DMDAAC) has the minimum value. The thickness of adsorption layer ranges from 0.76 to 2.11 nm for Yulin coal while it ranges from 0.52 to 1.83 nm for Yili coal. It increases in the order NSF < PSS < P(SSco-AA-co-DMDAAC). This is consistent with the result of adsorption amount test, verifying that P(SS-co-AA-co-DMDAAC) exhibit better adsorb ability than PSS and NSF no matter on Yulin coal or Yili coal.

Comment 4:
The analysis on the mechanism in Section 3.5 is too idealistic. There is lack of data support for the microscopic adsorption behavior of the synthesized dispersant. Besides, wetting dispersion effect and steric hindrance effect are also the key mechanisms for CWS, which were not discussed in this manuscript. It should be noted that steric hindrance is recognized as the most important mechanism, while there is opposite result reported against electrostatic repulsion effect.
Reply: Thanks for your comments and questions. As you mentioned in comments, the wetting effect and steric hindrance effect are also important mechanism which significantly affect dispersion. In order to provide detailed information about the adsorption, the contact angle, adsorption amount and adsorption layer thickness measurements were conducted to evaluate wetting effect. The results showed that the amphoteric structure effectively improved the adsorption of P(SS-co-AA-co-DMDAAC) as compared to anionic dispersants such as NSF and PSS. In the adsorption process of P(SS-co-AA-co-DMDAAC), the N + groups could be anchored to negatively charged area by electrostatic attraction, and meanwhile, the COOgroups could combine with cations through stable chelate adsorption. Such interactions make the P(SS-co-AA-co-DMDAAC) molecules being adsorbed intensely on the coal surface, which are beneficial to wetting. As a result, the P(SS-co-AA-co-DMDAAC) exhibit lower contact angle, larger adsorption amount and thicker adsorption layer as compare to NSF and PSS.
In addition, the COOand SO3 -groups of P(SS-co-AA-co-DMDAAC) which are hydrophilic, could combine with the water molecules which further form hydration film on the outside of the dispersant-coal composite particles [1]. This hydration film is considered to generate steric hindrance among particles [2,3]. Due to the synergistic effect of electrostatic repulsion, steric hindrance and wetting dispersion effect, the agglomeration of coal particles is suppressed. As the consequence, CWS prepared using P(SS-co-AA-co-DMDAAC) as dispersant exhibits low viscosity and superior stability. The discussions of wetting and steric hindrance were added to the section 3.5 in revised manuscript. Besides, the figure of schematic illustration of adsorption and dispersion mechanism was modified. What are the characteristics of these two coal samples?
Reply: Thanks for your comments and questions. The structure of the copolymer P(SS-co-AAco-DMDAAC) was designed according to the surface characteristics of low-rank coal.
However, the composition of coal is complex and the property differences still exist among different low-rank coal. These property differences can also affect the optimal composition and adsorption ability of dispersant. Thus, in order to reliably evaluate the performances of P(SS-co-AA-co-DMDAAC) for used as a dispersant and its adaptive ability to different types of low-rank coal, we chose Yili coal and Yulin coal. The coal rank of Yulin coal is relatively higher than that of Yili coal. According to the proximate and ultimate analysis result, there are more multivalent cations and hydrophilic oxygen groups on the surface of Yili coal than that of Yulin coal. Thus, the surface of Yili coal is more hydrophilic than the surface of Yulin coal, which is also verified by the moisture content and contact angle.

Comment 2:
In abstract, the first letter in the full names of the instruments should be lowercase.
Reply: Thanks for your comments and questions. The appropriate changes have been made in the revised manuscript and highlighted in red. Comment 3: In 2.3.1, the sample given is a coal sample, but the result of the detection (in Figure 2) is the structure analysis of the dispersant P(SS-co-AA-co-DMDAAC).

Reply:
We apologise for the clerical error in the manuscript. In 2.3.1, the sample prepared for FTIR test consists of 5 mg P(SS-co-AA-co-DMDAAC) and 500 mg KBr. The appropriate change has been made in the revised manuscript and marked in red.

Comment 4:
The optimal dosage of dispersant for Yili coal and Yulin coal were different. It is recommended to explain this result in 3.3.1.
Reply: Thanks for your comments and questions. We have added the explanation of this result in 3.3.1. The difference of optimal dispersant dosage for Yulin coal and Yili coal mainly attributes to the surface characteristics. Compared with Yulin coal, Yili coal has higher ash content, which means that more cations like Ca 2+ , Mg 2+ exists on the surface of Yili coal.
During the adsorption process of P(SS-co-AA-co-DMDAAC), more anionic groups of dispersant were combined with cations on the surface of Yili coal, and accordingly, anionic groups which orient to the water and generate electrostatic repulsion decreased. Therefore, more dispersant molecules are required to maintain the coal particles dispersed.