A comparative study on phosphate removal from water using Phragmites australis biochars loaded with different metal oxides

Metal oxide-loaded biochars are a promising material to remove phosphate from polluted water to ultra-low concentrations. To facilitate preparing the metal oxide-loaded biochar with the best phosphate adsorption performance, five biochars loaded with Al, Ca, Fe, La and Mg oxides, respectively (Al-BC, Ca-BC, Fe-BC, La-BC and Mg-BC) were produced using Phragmites australis pretreated with 0.1 mol AlCl3, CaCl2, FeCl3, LaCl3 and MgCl2, respectively, characterized, and phosphate adsorption kinetics and isotherms of the biochars were determined. The maximum phosphate adsorption capacities (Qm) of the biochars ranked as Al-BC (219.87 mg g−1) > Mg-BC (112.45 mg g−1) > Ca-BC (81.46 mg g−1) > Fe-BC (46.61 mg g−1) > La-BC (38.93 mg g−1). The time to reach the adsorption equilibrium ranked as La-BC (1 h) < Ca-BC (12 h) < Mg-BC (24 h) = Fe-BC (24 h) <Al-BC (greater than 72 h). Qm of Ca-BC, Fe-BC, La-BC and Mg-BC depend on the molar content of metals in the biochars. The small phosphate adsorption rate of Al-BC is due to the slow intra-particle diffusion of phosphate attributed to the undeveloped porosity and dispersed distribution of AlOOH crystals on the Al-BC surface. Mg-BC is suggested for phosphate removal from water considering adsorption rate and capacity. Al-BC is applicable when a long contact time is allowed, e.g. as a capping material to immobilize phosphate in lake sediments.


Recommendation? Major revision is needed (please make suggestions in comments)
Comments to the Author(s) I have carefully read the manuscript RSOS-201789 entitled "A comparative study on phosphate removal from water using phragmites australis biochars loaded with different metal oxides" and find the research work relevant, fitting the scope of the journal and likely to be of interest to a broad readership. The research goals, methodology and results/discussion are well structured, the research hypotheses and scientific interpretations are very well justified, and the research work has sufficient novelty. In my opinion, however, the manuscript needs some major revisions before being published in the Royal Society Open Science. Authors may consider the following recommendations to improve the quality of the manuscript:

1)
Introduction, Page 3, Lines 1-2: A comprehensive study demonstrating the suitability of adsorption to remove and even recover phosphorus from low concentrated secondary effluent wastewater, including a verification of the long-term reusability of the adsorbents, is 2) Introduction, Page 3, Lines 9-10: The following very systematic screening study representing the high phosphorus adsorption and desorption efficiency of various metal (oxy)hydroxides can be added here: "Influence of cation building blocks of metal hydroxide precipitates on their adsorption and desorption capacity for phosphate in wastewater -A screening study". Colloids and Surfaces A: Physicochem. and Eng. Aspects, 488, 145−153. DOI: https://doi.org/10.1016/j.colsurfa.2015.10.017

3)
Page 3, Line 48: Note down also the respective concentration of the biochar in parenthesis: i.e. 1 g biochar in 500 mL KH2PO4 solution corresponds to 2 g/L biochar. The same applies to Page 4, Line 2: 0.08 g biochar in 40 mL KH2PO4 solution corresponds to 2 g/L biochar.

4)
Page 4, Lines 27-28: Authors state that the fitting of the kinetic and isotherm models to the experimental data was performed with Origin 2018 using the non-linear fitting methods, as plotted in Figure 3 and Figure 5. Why didn't authors simply plot the smooth fitting curves based on the actual modeling parameters calculated from the respective model equations (Tables 3 and  4)?

5)
Page 5, Lines 5-6: Authors measured that most of the metal-doped biochars had pHPZC around the neutral range, except for Fe-BC with pHPZC=2.1 and Mg-BC with pHPZC=11.3. They explained this with the high pHPZC~12 of MgO and likely due to the low pHPZC of Fe3O4 (magnetite). In support of this statement, I can recommend the paper by Mandel et al. where the researchers measured the pHPZC of various Mg-based oxyhydroxides which all had pHPZC~12, and they also measured the pHPZC of magnetite (Fe3O4) nanoparticles which showed pHPZC~2: Mandel, K. et al. (2013). "Layered double hydroxide ion exchangers on superparamagnetic microparticles for recovery of phosphate from waste water". Journal of Materials Chemistry A 1 (5), 1840-1848, DOI: https://doi.org/10.1039/C2TA00571A 6) Table 1: There is a typo in the column named "Mg-C". Please, replace with "Mg-BC". 7) Figure 3: It would help the reader if authors include in the figure caption a remark, listing the reaction parameters of the kinetics experiment, i.e. initial P-concentration: 20 mg/L PO4-P; biochar concentration 2 g/L, pH 7, 25°C, etc. The same refers to Figure 5 (isotherms) where authors can list the phosphate and biochar concentration range, contact time, pH, temperature, etc.

8)
Authors performed the study only in a synthetic KH2PO4 solution. Nevertheless, they need to keep in mind that the biochar performance in a real wastewater or water matrix may lead to completely different results. The competing effect of other constituents (dissolved ions, organics, suspended matter, etc.) present in real water / wastewater matrices must be taken into account in future studies.

9)
Have authors checked the chemical and mechanical stability of the developed metalincorporated biochars? They need to prove that none of the metals or other precursors from the Me-BC are leached into the treated effluent, especially if they carry a phosphate ion with them, which would essentially pollute the effluent instead of purifying it.

10)
How do authors imagine the practical engineering application of the metal-loaded biochars, i.e. how would the biochars be harvested / separated from the treated wastewater? The efficiency of the biochar separation will directly affect the effluent quality. If a simple sedimentation is proposed, probably there will be a need to dose flocculants to assure an efficient sedimentation process, but the flocculants may change the properties of the P-rich biochars and make them unsuitable for field applications, e.g. as soil amendment. Generally, in the future the authors need to investigate the plant availability and fertilizer value of the proposed P-loaded biochars. Often times the crystalline metal-phosphate precipitates, e.g. AlPO4, are not suitable as fertilizers and have poor plant availability, which might defeat the original concept of this study to utilize the P-loaded biochars directly on the field! Review form: Reviewer 2

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

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

Comments to the Author(s)
This work actually lots of effective experimental data. Publication of these data in public open access journal media would have good contribution to the related research communities. From the latter positive point, I basically recommend publication of this manuscript in Royal Society Open Science. Please see below necessary revisions. 1) Adsorption results were well reported, but mechanisms are not fully clear. The latter matters would be revealed by appropriate spectral analyses. IR measurement of these bio-derived carbon materials (with metal oxides) with phosphates had better be measured. From the peak shifts, interaction between the materials and phosphates can be discussed.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
In the manuscript entitled "A comparative study on phosphate removal from water using phragmites australis biochars loaded with different metal oxides'', the authors investigated and compared the characteristics and phosphate adsorption kinetics and capacities of the biochars loaded with different metal oxides, and identified possible factors determining the phosphate adsorption capacities and rates of the modified biochars. Overall, most parts of this submission are satisfactory, however, there are some concerns and issues which need be addressed appropriately prior to a possible inclusion in Royal Society Open Science. Detailed comments and suggestions are listed as follows.
On Introduction 1. More better and overall background information for biochars adsorbent should be introduced to the prospective audiences, and the following literature could serve this purpose in some aspects. Wang S, Characterization and Pb(II) removal potential of corn straw-and municipal sludgederived biochars. R. Soc. Open. Sci ,2017,4(9),170402.
Wu SH, Role of biochar on composting of organic wastes and remediation of contaminated soils-a review. Environmental Science and Pollution Research, 2017, 24(20), 16560-16577.
On Materials and Methods 2. Line 28-30 on Page 3, the first sentence was about straws of phragmites australis, the authors should put it on Introduction. 3. Line 50-51 on Page 3, "35 mL samples were taken from each flask at 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 24, 48 and 72 h", the authors should clear present if batch experiment was performed in phosphate adsorption kinetic experiments. If not, the changes of solution volume will affect the accuracy of results. 4. pH was measured in phosphate adsorption kinetic experiments and isotherm experiments, but it was not analyzed in the discussion section. 5. The authors should provide data on reusability and regeneration of the adsorbent, and discuss the disposal of the adsorbent after use.
On Conclusion 6. To match the purpose of the research, the authors should show the results of phosphate adsorption rates of the modified biochars on conclusion. 7. When the modified biochars was used "as a capping material to immobilize phosphate in the sediment of eutrophic lakes". One major issue is regarding the metal ion leaching solubility when evaluating its practical application.

Decision letter (RSOS-201789.R0)
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Dear Dr Wang:
Title: A comparative study on phosphate removal from water using phragmites australis biochars loaded with different metal oxides Manuscript ID: RSOS-201789 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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RSC Subject Editor:
Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Comments to the Author(s) I have carefully read the manuscript RSOS-201789 entitled "A comparative study on phosphate removal from water using phragmites australis biochars loaded with different metal oxides" and find the research work relevant, fitting the scope of the journal and likely to be of interest to a broad readership. The research goals, methodology and results/discussion are well structured, the research hypotheses and scientific interpretations are very well justified, and the research work has sufficient novelty. In my opinion, however, the manuscript needs some major revisions before being published in the Royal Society Open Science. Authors may consider the following recommendations to improve the quality of the manuscript: 1) Introduction, Page 3, Lines 1-2: A comprehensive study demonstrating the suitability of adsorption to remove and even recover phosphorus from low concentrated secondary effluent wastewater, including a verification of the long-term reusability of the adsorbents, is  Table 1: There is a typo in the column named "Mg-C". Please, replace with "Mg-BC". 7) Figure 3: It would help the reader if authors include in the figure caption a remark, listing the reaction parameters of the kinetics experiment, i.e. initial P-concentration: 20 mg/L PO4-P; biochar concentration 2 g/L, pH 7, 25°C, etc. The same refers to Figure 5 (isotherms) where authors can list the phosphate and biochar concentration range, contact time, pH, temperature, etc.
8) Authors performed the study only in a synthetic KH2PO4 solution. Nevertheless, they need to keep in mind that the biochar performance in a real wastewater or water matrix may lead to completely different results. The competing effect of other constituents (dissolved ions, organics, suspended matter, etc.) present in real water / wastewater matrices must be taken into account in future studies. 9) Have authors checked the chemical and mechanical stability of the developed metalincorporated biochars? They need to prove that none of the metals or other precursors from the Me-BC are leached into the treated effluent, especially if they carry a phosphate ion with them, which would essentially pollute the effluent instead of purifying it. 10) How do authors imagine the practical engineering application of the metal-loaded biochars, i.e. how would the biochars be harvested / separated from the treated wastewater? The efficiency of the biochar separation will directly affect the effluent quality. If a simple sedimentation is proposed, probably there will be a need to dose flocculants to assure an efficient sedimentation process, but the flocculants may change the properties of the P-rich biochars and make them unsuitable for field applications, e.g. as soil amendment. Generally, in the future the authors need to investigate the plant availability and fertilizer value of the proposed P-loaded biochars. Often times the crystalline metal-phosphate precipitates, e.g. AlPO4, are not suitable as fertilizers and have poor plant availability, which might defeat the original concept of this study to utilize the Ploaded biochars directly on the field! Reviewer: 2 Comments to the Author(s) This work actually lots of effective experimental data. Publication of these data in public open access journal media would have good contribution to the related research communities. From the latter positive point, I basically recommend publication of this manuscript in Royal Society Open Science. Please see below necessary revisions. 1) Adsorption results were well reported, but mechanisms are not fully clear. The latter matters would be revealed by appropriate spectral analyses. IR measurement of these bio-derived carbon materials (with metal oxides) with phosphates had better be measured. From the peak shifts, interaction between the materials and phosphates can be discussed.

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

Comments to the Author(s)
The revised manuscript RSOS-201789.R1 entitled "A comparative study on phosphate removal from water using phragmites australis biochars loaded with different metal oxides" fulfills all expected revisions supposed to be made and is now of good publishable quality. The authors have carefully addressed every single comment and remark from both reviewers. The answers are elaborate, extensive and provide enough arguments and proof to validate the authors' statements. All necessary changes have been made based on the reviewers' recommendations. In my opinion, in this revised form the manuscript should be accepted for publication in the journal Royal Society Open Science after authors take into consideration one more minor remark: The We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.
Dear Dr Wang: Title: A comparative study on phosphate removal from water using phragmites australis biochars loaded with different metal oxides Manuscript ID: RSOS-201789.R1 Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email.
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Reviewer: 1
Comments to the Author(s) The revised manuscript RSOS-201789.R1 entitled "A comparative study on phosphate removal from water using phragmites australis biochars loaded with different metal oxides" fulfills all expected revisions supposed to be made and is now of good publishable quality. The authors have carefully addressed every single comment and remark from both reviewers. The answers are elaborate, extensive and provide enough arguments and proof to validate the authors' statements. All necessary changes have been made based on the reviewers' recommendations. In my opinion, in this revised form the manuscript should be accepted for publication in the journal Royal Society Open Science after authors take into consideration one more minor remark: The

Decision letter (RSOS-201789.R2)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.
Dear Dr Wang: Title: A comparative study on phosphate removal from water using phragmites australis biochars loaded with different metal oxides Manuscript ID: RSOS-201789.R2 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.
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Response to Referees
Reviewer: 1 1) Introduction, Page 3, Lines 1-2: A comprehensive study demonstrating the suitability of adsorption to remove and even recover phosphorus from low concentrated secondary effluent wastewater, including a verification of the long-term reusability of the adsorbents, is 1 g biochar in 500 mL KH2PO4 solution corresponds to 2 g/L biochar. The same applies to Page 4, Line 2: 0.08 g biochar in 40 mL KH2PO4 solution corresponds to 2 g/L biochar.

Response:
The phrase "(biochar concentration 2 g/L)" is added after "1 g biochar in 500 mL KH2PO4 solution" and "0.08 g biochar and 40 mL KH2PO4 solution" to specify the concentration of the biochar used in the experiments. 4) Page 4, Lines 27-28: Authors state that the fitting of the kinetic and isotherm models to the experimental data was performed with Origin 2018 using the non-linear fitting methods, as plotted in Figure 3 and Figure 5. Why didn't authors simply plot the smooth fitting curves based on the actual modeling parameters calculated from the respective model equations (Tables 3 and 4)?
Response: In order to visually show the differences in the phosphate adsorption kinetics and isotherms of the different materials, the experimental data of the 6 biochars were plotted in one figure, i.e., Figure 3 and Figure 5 respectively. For each material, 4 kinds of kinetic models were fitted to the kinetic data, and 3 kinds of isotherm models were fitted to the isotherm data. If the fitting curves of the kinetic and isotherm models were plotted in the figure, there would be 24 and 18 fitting curves in Figure 3 and Figure 5 respectively. That would be very difficult to show all the information clearly, especially for the biochars with some of the experimental data being close or overlapping, e.g., phosphate adsorption isotherms of Fe-BC and La-BC. From these considerations, the model fitting parameters were given separately in Table 3 and Table 4 to clearly show the model fitting results.  Table 1: There is a typo in the column named "Mg-C". Please, replace with "Mg-BC".

5)
Response: "Mg-C" in the first row of Table 1 is revised to "Mg-BC". The release of other substances, e.g., dissolved organic matter and metal ions, from the metal oxide loaded biochars to the solution was not examined, because the focus of the present study is phosphate. However, we agree with the referee that the stability of the material is important and needs further research. This point is added in the last sentence of the conclusion part of the revised manuscript: "In further studies, effects of various competing constituents in the real polluted water on the phosphate adsorption characteristics of the metal oxide loaded biochars and stability of the biochars in the real polluted water need to be clarified before applying them in real practice.". 10) How do authors imagine the practical engineering application of the metal-loaded biochars, i.e. how would the biochars be harvested / separated from the treated wastewater? The efficiency of the biochar separation will directly affect the effluent quality. If a simple sedimentation is proposed, probably there will be a need to dose flocculants to assure an efficient sedimentation process, but the flocculants may change the properties of the P-rich biochars and make them unsuitable for field applications, e.g. as soil amendment. Generally, in the future the authors need to investigate the plant availability and fertilizer value of the proposed P-loaded biochars. Often times the crystalline metal-phosphate precipitates, e.g.
AlPO4, are not suitable as fertilizers and have poor plant availability, which might defeat the original concept of this study to utilize the P-loaded biochars directly on the field! Response: Biochar particles in the size range of 0.25-0.50 mm were used in the present study to adsorb phosphate from the solution. These particles could quickly settle after shaking, i.e., simple sedimentation was able to separate the biochar particles from the solution. Considering that a certain kind of more efficient mixing, e.g., with a mechanical mixer or effluent circulation, of the biochar particles and the to-be treated water/wastewater is to be used in practical applications, some of the biochar particles might be fragmented and the sedimentation efficiency might deteriorate. Additional operations such as letting the treated effluent pass through sieves with small mesh sizes can be applied. With such methods, the dose of flocculants can be avoided.
Previous research [7-9] has proved that the P-adsorbed MgO loaded biochar promotes the seed germination and growth of plants, confirming that the P-adsorbed MgO loaded biochar can be used as a slow-release fertiliser to improve soil quality and productivity. One conclusion of the present study is that Mg-BC is suggested for phosphate removal from water considering both phosphate adsorption rate and capacity. Therefore, the original concept of this study to utilize the P-loaded biochars on the field as a slow-release fertiliser is realistic. Knowledge of the applicability of P-adsorbed Al-BC, Ca-BC, Fe-BC and La-BC as slow-release fertilisers is however lacking, which will be investigated in our future research. In the manuscript, the sentence "Moreover, the Padsorbed biochar can be reused as a soil amendment to improve soil quality, enabling simultaneous P recovery and waste biomass reuse." (Page 2, Line 4-6) is changed to "Moreover, the P-adsorbed biochar can be reused as soil amendment and slow-release fertiliser to improve soil quality and productivity, enabling simultaneous P recovery and waste biomass reuse.", and the reference at the end of this sentence is changed to [7-9] to be more specific.
Reviewer: 2 1) Adsorption results were well reported, but mechanisms are not fully clear. The latter matters would be revealed by appropriate spectral analyses. IR measurement of these bio-derived carbon materials (with metal oxides) with phosphates had better be measured. From the peak shifts, interaction between the materials and phosphates can be discussed.
Response: FTIR analysis of the six biochars after phosphate adsorption has been additional conducted. The FTIR spectra of each biochar before and after phosphate adsorption are shown in