Adsorption behaviour and mechanism of the PFOS substitute OBS (sodium p-perfluorous nonenoxybenzene sulfonate) on activated carbon

Perfluorooctane sulfonate (PFOS) was listed as a persistent organic pollutant by the Stockholm Convention. As a typical alternative to PFOS, sodium p-perfluorous nonenoxybenzene sulfonate (OBS) has recently been detected in the aquatic environment which has caused great concern. For the first time, the adsorption behaviour and mechanism of OBS on activated carbon (AC) with different physical and chemical properties were investigated. Decreasing the particle size of AC can accelerate its adsorption for OBS, while AC with too small particle size was not conducive to its adsorption capacity due to the destruction of its pore structure during the mechanical crushing process. Intra-particle diffusion had a lesser effect on the adsorption rate of AC with smaller particle size, higher hydrophilicity and larger pore size. Reactivation of AC by KOH can greatly enlarge their pore size and surface area, greatly increasing their adsorption capacities. The adsorption capacity of two kinds of R-GAC exceeded 0.35 mmol g−1, significantly higher than that of other ACs. However, increasing the hydrophilicity of AC would decrease their adsorption capacities. Further investigation indicated that a larger pore size and smaller particle size can greatly enhance the adsorptive removal of OBS on AC in systems with other coexisting PFASs and organic matter due to the reduction of the pore-blocking effect. The spent AC can be successfully regenerated by methanol, and it can be partly regenerated by hot water and NaOH solution. The percentage of regeneration for the spent AC was 70.4% with 90°C water temperature and up to 95% when 5% NaOH was added into the regeneration solution. These findings are very important for developing efficient adsorbents for the removal of these newly emerging PFASs from wastewater and understanding their interfacial behaviour.

3. The overall adsorption process may be jointly controlled by external mass transfer and intraparticle diffusion, hence, the adsorption kinetic data was further analyzed by Boyd model ( Title: Adsorption behavior and mechanism of the PFOS substitute OBS (sodium p-perfluorous nonenoxybenzene sulfonate) on activated carbon Manuscript ID: RSOS-191069 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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• Acknowledgements Please acknowledge anyone who contributed to the study but did not meet the authorship criteria. ********************************************** RSC Associate Editor: Comments to the Author: Please also complete the request of Reviewer 2 as follows: Figure 3. Give 2 sigma error bars in the graph RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) This article investigated the adsorption behavior and mechanism of sodium p-perfluorous nonenoxybenzene sulfonate (OBS) on activated carbons with different physicochemical properties. The removal of OBS from water using activated carbons was attractive to readers. Adsorption kinetics, adsorption isotherms, effects of pH and ionic strength have been examined, and adsorbent regeneration and applicability in co-existing matter systems have also been investigated. The topic is relatively new, which is the first time to examine the adsorptive removal of OBS using activated carbons. This work is important for the practical engineering application of adsorption for removing emerging contaminants. I recommend for publication after minor revision according to the following comments. 1. The abstract could be improved by providing specific information regarding the quantitative data. For example, the adsorption capacity of R-AC? the regeneration percent of hot water and NaOH solution? 2. OBS was a typical alternative to PFOS, and its structural formula should be given, which is very important to know the structural difference compared with PFOS?
3. This paper evaluated the influence of co-existing PFASs on the adsorption of OBS on ACs. OBS in actual water environment always coexists with other PFASs. What is the selectivity of activated carbons for the target OBS? 4. Page 4, line 6, better to use 'Per-and polyfluoroalkyl substances (PFASs)', instead of 'Perfluoroalkyl and polyfluoroalkyl substances'. 5. Page 9, line 21-22, better to give a more detailed description of the analytical methods, e.g., blanks, the LOD of HPLC-UV, the preparation of mobile phase, the sample analysis time, etc. 6. Page 14, line 14, change "previous studies" to " a previous study". 7. Pag15, Lines 28-29 mention two methods previously tested for regeneration of spent AC used to treat PFOS. Better to consider citing additional research related to spent AC that was specifically used to treat PFAS. 8. Fig. 3(a), UAC, not BAC. Comments to the Author(s) The manuscript entitled "Adsorption behavior and mechanism of the PFOS substitute OBS (sodium p-perfluorous nonenoxybenzene sulfonate) on activated carbon deals with removal of emerging environmental contaminant. The manuscript is written systematically. However, I could find some lacunae in the experiment. I recommend the manuscript for publication after addressing the following queries, 1) When you powder GAC to get UAC the specific surface area should increase as particle size is decreasing. But authors have mentioned that the specific surface area decreases which is contradictory to the existing knowledge. Authors should explain or recheck the surface area. 2) Why authors used KOH for R1-GAC and R2 GAC? 3) What are the plausible structure of R1-GAC and R2-GAC? 4) Give the EDX and FTIR data for R1-GAC and R2-GAC, to support the structure and justify higher adsorption rate .

Reviewer: 3
Comments to the Author(s) In this paper, the adsorption behavior of OBS on activated carbon (AC) with different physical and chemical properties were investigated. Further investigation indicated that a larger pore size and smaller particle size can greatly enhance the adsorptive removal of OBS on AC in systems with co-existing other PFASs and organic matter. The regeneration method of AC were studied and summarized systematically. The acceptance of this work is therefore recommended after the major revision addressing the following comments. 1. Author should add the comparsion table and make a comparsion with some other adsorbents 2. The authors should highlight better in which sense their work is novel compared to previous literature. 3. The overall adsorption process may be jointly controlled by external mass transfer and intraparticle diffusion, hence, the adsorption kinetic data was further analyzed by Boyd model ( Comments to the Author(s) This revised manuscript can be accepted in its current form.

Response to reviewer comments
(Manuscript ID: RSOS-191069) The comments of the reviewers are very much appreciated and helped improve the manuscript significantly. We responded to all the comments and made all of the requested changes. Those changes are highlighted with red color in the revised manuscript. In the following section, we explained in details how we responded to each of the comments.

Response to RSC Associate Editor
Comments to the Author: Please also complete the request of Reviewer 2 as follows: Figure 3. Give 2 sigma error bars in the graph Response: Thanks for this good comment. We add 2 sigma error bars in Figure 3.

Response to Reviewer #1
This article investigated the adsorption behavior and mechanism of sodium p-perfluorous nonenoxybenzene sulfonate (OBS) on activated carbons with different physicochemical properties.
The removal of OBS from water using activated carbons was attractive to readers. Adsorption kinetics, adsorption isotherms, effects of pH and ionic strength have been examined, and adsorbent regeneration and applicability in co-existing matter systems have also been investigated.
The topic is relatively new, which is the first time to examine the adsorptive removal of OBS using activated carbons. This work is important for the practical engineering application of 1. The abstract could be improved by providing specific information regarding the quantitative data. For example, the adsorption capacity of R-AC? the regeneration percent of hot water and NaOH solution?
Response: Thanks for this comment. We add some specific information in the abstract.  Table S1 †.

Response to Reviewer #2
The manuscript entitled "Adsorption behavior and mechanism of the PFOS substitute OBS (sodium p-perfluorous nonenoxybenzene sulfonate) on activated carbon deals with removal of emerging environmental contaminant. The manuscript is written systematically. However, I could find some lacunae in the experiment. I recommend the manuscript for publication after addressing the following queries.
1. When you powder GAC to get UAC the specific surface area should increase as particle size is decreasing. But authors have mentioned that the specific surface area decreases which is contradictory to the existing knowledge. Authors should explain or recheck the surface area.
Response: Thank you for this suggestion. We retested and checked the specific surface area results of UAC, and the results were consistent with the previous ones. Indeed, the specific surface area always increase with decreasing the particle size of adsorbent. However, mechanical crushing with too high strength will destruct the pore structure of activated carbon [23]. As shown in Table   1, the specific surface area of UAC is lower than that of GAC, because its pore structure has been destructed during such high strength mechanical crushing process. To make the results more credible, we cited other references to support this statement.

After revision:
Page 9, Line 21: The specific surface area of GAC is little lower than that of PAC but higher than UAC ( Table 1). The specific surface area of UAC is lower, because its pore structure has been destructed during such high strength mechanical crushing process [23].
the general structure of it. R-GAC was impregnated by KOH solution at KOH/C mass and heated at 900 ℃ under N 2 for 1.5 h. After treatment, the number of pores and the pore size of R-GAC greatly increased, shown in Fig. 1 and Fig. S2(b) †. In general, R-GAC also has a heterogeneous pore structure similar to activated carbon, but the pore size and the number of pores of R-GAC were increased after activation. To further explain the structure of R1-GAC and R2-GAC, we added some descriptions of R-GAC compared to GAC in this article.

After revision:
Page 9, Line 11: In general, R-GAC also has a heterogeneous pore structure similar to activated carbon, but the pore size and the number of pores of R-GAC were increased after activation. The enlarged pore sizes are favorable for the diffusion of molecular OBS into the granular adsorbent. analyzer to measure the elemental composition of the prepared ACs and the element composition and proportion of R1-GAC and R2-GAC are showed on Table 1. We also considered using FTIR to analyze the functional group on R-GAC and functional group changes before and after the adsorption of activated carbon (Fig. S3 and Fig.SA), to further illustrate its adsorption mechanism.

Reference
However, no functional group changes were found on the surface of activated carbon before and after adsorption, so it's difficult to use FTIR to justify the high adsorption rate of R-GAC. The higher adsorption rates of R1-GAC and R2-GAC are due to their large pore volume and pore number. Still, to support the structure of R1-GAC and R2-GAC, the FTIR data was used to give a short description in the section of.3.1 Characterization of prepared activated carbon.

Response to Reviewer #3
Comments to the Author(s) In this paper, the adsorption behavior of OBS on activated carbon (AC) with different physical and chemical properties were investigated. Further investigation indicated that a larger pore size and smaller particle size can greatly enhance the adsorptive removal of OBS on AC in systems with co-existing other PFASs and organic matter. The regeneration method of AC were studied and summarized systematically. The acceptance of this work is therefore recommended after the major revision addressing the following comments.

Author should add the comparsion table and make a comparsion with some other adsorbents
Response: Thanks for this suggestion. Because OBS is an emerging alternative to PFOS, its adsorption removal on other adsorbents has not been investigated. Our study is the first one to explore the adsorption removal of OBS on activated carbon, and there is no other literature on OBS adsorption removal for reference. We are so sorry that we cannot make a comparison with some other kinds of adsorbents. This is also a major innovation in our research. We chose the activated carbon adsorbent to study the adsorption removal of OBS because of it's the most commonly used in the water treatment of tap water.
2. The authors should highlight better in which sense their work is novel compared to previous literature.

Response:
Thank you for this good suggestion. OBS has been widely used as a substitute of PFOS, which has been banned, and has the same serious harm to human health, biology and ecological environment. However, as far as we know, there are only two papers discussing OBS removal from water using an oxidation method and aeration-foam collection. Our study is the first one to investigated the adsorption removal of OBS on activated carbon. And we add some information in 1. Introduction to highlight the innovation points of this study.

After revision:
Page 5, Line 21: Our study is the first one to investigate the adsorption behavior and mechanism of OBS on activated carbon. The plots showed a nonlinear segment before sorption equilibrium, suggesting that the rate of adsorption was not only controlled by pore diffusion in the initial period and chemical reaction also controlled the rate of adsorption. Response: Thank you for this suggestion. Our previous research has found that ACs with a biger particle size have longer pores and less exposed sites for OBS adsorption, resulting in a correspondingly slower adsorption process [1]. Due to the big size of GAC and the big molecular size of OBS, it took long time for OBS to diffuse into the long pores of GAC due to the steric hindrance effect in diffusion process. And we add some discussion in 3.2 Adsorption kinetics.

After revision:
Page 9, Line 11: The longer equilibration time for GAC was similar to our previously reported data for the adsorption of organophosphate flame retardants on activated carbon [24]. Due to the big size of GAC and the big molecular size of OBS, it took long time for OBS to diffuse into the long pores of GAC. The ACs with a larger particle size have deeper pores and less exposed sites for OBS adsorption, resulting in a correspondingly slower adsorption process. Response: Thank you for this suggestion. We checked the data and redid the experiment, and sorry for incorrect result of ionic strength experiment. All the data except the ionic strength experiment were measured by HPLC and verified to be correct. In our previous ionic strength experiment, we used the UV spectrophotometer to test the results because it is easy to block the column of HPLC by considering the high concentration of salt ions in the sample. Because the salting-out effect was very serious, the OBS concentration was very low at high salt concentration, and the UV spectrophotometer test results were not accurate, result in giving the wrong experimental results. After inspection and re-experiment, we retested the samples using HPLC-UV and revised in 3.4 Effect of solution pH and ionic strength and Fig. 4(b). Because the OBS concentration after adsorption was around the LOD when the Ca 2+ concentration was 100 mmol/L, we deleted the results of 100 mmol/L Ca 2+ and Na + .

After revision:
Page 14, Line 17: The removal of OBS by PAC increased when increasing the Na + concentration from 0 to 50 mmol/L, and the higher increase in the removal of OBS in the presence of Ca 2+ could be attributed to the stronger salting-out effect of Ca 2+ when compared to Na + [36]. This is consistent with previous work [37] which showed that Ca 2+ showed a stronger influence than Na + on the adsorption of PFASs on PAC. Moreover, the solubility of OBS decreased sharply at high