Kinetics and Thermodynamics of Efficient Phosphorus Removal by a Composite Fiber

Round 1

Reviewer 1 Report

This manuscript reports a zeolite composite fiber adsorbent for aqueous phosphorus removal. The authors have conducted a minute investigation on influencing factors during the batch and the fixed-bed adsorption experiments. The phosphorous adsorption behavior of zeolite composite fiber was well analyzed by kinetic and thermodynamic models. Nonetheless, the authors have barely mentioned the characteristics of composite fibers (i.e. morphology, elemental analysis, etc.), which is quite important in terms of material development. Moreover, the adsorption performance of the fiber was not at all compared with other absorbents reported so far, raising a question of how good this material is. There are a few questions need to be addressed as follows.

1. Line 43: Missing content in the sentence “The traditional adsorbents are mostly…”. Plus, the authors have pointed out that current adsorbents have difficulties in separation and recycling. How’s the recyclability of zeolite composite fibers?

2. Line 227: Thermodynamic parameters were listed in the manuscript without showing a proper explanation of what those values mean.

3. Line 306: The authors mentioned that the adsorption efficiency was increased in acidic condition owing to the leaching metal ions (Fe³⁺, Ca²⁺) from the composite fiber. Is this adsorbent stable enough for the repeated use? The best performance was obtained under acidic condition, then there would be a chance of harming materials after a few cycles? Above all, the metal leaching was proven by any analytic tool, such ICP?

4. Line 356: The desorption of phosphorus seemed to be difficult, requiring strong acid for the full regeneration. As asked in the previous question, the composite fibers can be affected under the acidic condition, thus the structural regeneration in sulfuric acid solution might not be the best option. How’s the carbonate solution for the desorption? As shown in Figure 8, carbonate is competing with phosphorous, may be able to facilitate the desorption of phosphorous.

With this stands, this manuscript needs major revision to be published in Applied Sciences. 

Author Response

Response to Reviewer  Comments

P1: This manuscript reports a zeolite composite fiber adsorbent for aqueous phosphorus removal. The authors have conducted a minute investigation on influencing factors during the batch and the fixed-bed adsorption experiments. The phosphorous adsorption behavior of zeolite composite fiber was well analyzed by kinetic and thermodynamic models. Nonetheless, the authors have barely mentioned the characteristics of composite fibers (i.e. morphology, elemental analysis, etc.), which is quite important in terms of material development. Moreover, the adsorption performance of the fiber was not at all compared with other absorbents reported so far, raising a question of how good this material is. There are a few questions need to be addressed as follows.

A1: The preparation and characterization of ZSFB composite fiber are mainly discussed in another paper (in submission). In line 313, we talked about the adsorption performance has met the class A demands of "discharge standard of pollutants for municipal wastewater treatment plant" (GB18918-2002) which is the highest standard in China. But it is more convincing with more comparation with other absorbents.

P2: Line 43: Missing content in the sentence “The traditional adsorbents are mostly…”. Plus, the authors have pointed out that current adsorbents have difficulties in separation and recycling. How’s the recyclability of zeolite composite fibers?

A2: In this paper we mainly investigated the kinetics and thermodynamics of the composite fiber. So I deleted the sentence relating 'reuse' or 'recycle'. The stability of the recyclability of the composite fiber needs to be studied further.

P3: Line 227: Thermodynamic parameters were listed in the manuscript without showing a proper explanation of what those values mean.

A3: I have changed that part and add explanation for each parameter.(In line 103)

P4: Line 306: The authors mentioned that the adsorption efficiency was increased in acidic condition owing to the leaching metal ions (Fe3+, Ca2+) from the composite fiber. Is this adsorbent stable enough for the repeated use? The best performance was obtained under acidic condition, then there would be a chance of harming materials after a few cycles? Above all, the metal leaching was proven by any analytic tool, such ICP?

A4: The mineral compositions of the materials were analyzed by Philips PW1700 X-ray diffractometry (XRD). Data was collected and analyzed by the XRD analyzer's workstation. Adsorption is one of the most efficient techniques for removing heavy metals from the solvent phase in low concentration wastewater [1].The main chemical compositions of the adsorption substrates are shown in Table 1. Among these substrates, zeolite was widely used to remove heavy metals and other cations from aqueous solutions [2-4]. Mombelli et al. conducted leaching test for steel slag in different conditions and used ICP-OES to determine the chemical species concentration. The results show that the modified slag can be considered a safe by-product and no more as hazardous waste.[5]. The components of fly ash used in this experiment didn’t contain much heavy metals. Although these data can somehow reflect the stability of the adsorbents, the stability of the reuse of composite fiber needs to be studied further.

Table 1. The main chemical composition of the adsorption substrate.

P5: Line 356: The desorption of phosphorus seemed to be difficult, requiring strong acid for the full regeneration. As asked in the previous question, the composite fibers can be affected under the acidic condition, thus the structural regeneration in sulfuric acid solution might not be the best option. How’s the carbonate solution for the desorption? As shown in Figure 8, carbonate is competing with phosphorous, may be able to facilitate the desorption of phosphorous.

A5: Desorption in sulfuric acid solution is a single-displacement reaction (strong acid replaces weak acid from its salt, Ca₃(PO₄)₂ + 3H₂SO₄ → 2H₃PO₄ + 3CaSO₄), and phosphoric acid is stronger than carbonic acid, so carbonic acid cannot react with phosphate. The competing relation is because of the Ca⁺ in solution which can combine with CO₃^2- to form calcium carbonate precipitate. In desorption, the calcium phosphate can hardly react with CO₃^2- [6].

References

  1. Burakov, A.E.; Galunin, E.V.; Burakova, I.V.; Kucherova, A.E.; Agarwal, S.; Tkachev, A.G.; Gupta, V.K. Adsorption of heavy metals on conventional and nanostructured materials for wastewater treatment purposes: A review. Ecotoxicology and environmental safety 2018, 148, 702-712.

  2. Babel, S.; Kurniawan, T.A. Low-cost adsorbents for heavy metals uptake from contaminated water: a review. Journal of hazardous materials 2003, 97, 219-243.

  3. Erdem, E.; Karapinar, N.; Donat, R. The removal of heavy metal cations by natural zeolites. Journal of colloid and interface science 2004, 280, 309-314.

  4. Fu, F.; Wang, Q. Removal of heavy metal ions from wastewaters: a review. Journal of environmental management 2011, 92, 407-418.

  5. Mombelli, D.; Mapelli, C.; Barella, S.; Gruttadauria, A.; Le Saout, G.; Garcia-Diaz, E. The efficiency of quartz addition on electric arc furnace (EAF) carbon steel slag stability. Journal of hazardous materials 2014, 279, 586-596.

  6. Ren, W.; Zhou, Z.; Huang, X.; Hu, D.; Jiang, Y.; Dong, F.; Hou, Y.; Shen, X. Optimization of Simultaneous Removal of Phosphorus and Humus from Sludge by Calcium Phosphate Precipitation. Acta Sientiae Circumstantiae 2015, 35, 3545-3551.(in Chinese)

Reviewer 2 Report

- Definitions: Please consider these definitions in your text!

adsorptive: component in fluid phase which can be adsorbed

adsorbate: adsorptive in the adsorbed state

adsorbent: surface of a solid where adsorption takes place

- The Freundlich equation does not assume "multilayer adsorption". The Freundlich equation can be theoretically derived for heterogeneous adsorbents. The eq. shows no Henry law behaviour at low concentration and has no finite limit at sufficiently high concentration. Therefore the F. eq. Is only valid in a limited concentration range. Please check whether your thesis about the Freundlich parameter n  is right (line 91/92). In my opinion it is generally known that with increasing n the isotherm gets more non-linear.

- Thermodynamic parameters:

Please insert the van't Hoff equation and show how you can derive △G, △H and △S form this equation? In Table 2 the values for 298 K and 308 K of  △H and △S are missing.

- "Ammonia nitrogen" has to be substituted by "ammonium" in the whole text!

- In line 360 you write about a precipitation of sulphuric acid and phosphate. How can they precipitate with each other? How does  the phosphate react with sulphuric acid?

- Conclusions: Please describe in the second sentence more precisely "...with initial concentration of ... dosage of...".

- minor changes in the text:

line 18: fiber is a good adsorbent for phosphorus .....remove phosphorous from wastewater... (phosphorous from wastewater not in appears in lines 30, 48, too)

45: Qin et al.

48: The meaning of the last sentence of this paragraph is unclear

72: In equation1 the denominator has to be in parentheses

75,76: Qe indicates the adsorbed amount (mg/g) of the adsorbate per unit mass of adsorbent, Qm represents the adsorbed amount in the saturated state (mg/g),

115: ..factors like dosage of ZSFB fiber, pH value, type of anion in solution and ammonium concentration were considered.

122: added to each group (in lines 131,

126: were taken...

127, 157: and put into

128: Ammonium concentration:

 Problems with formatting: Lines 133, 160/161, Table 2, Table 8

156: H2SO4 solution with

161: calculate the desorbed amount ...

164: wastewater continuously flowed...

179: column is penetrated. In this experiment, ...

205: Figure 2 shows the adsorption isotherms of composite fiber at different temperatures:

208: , and nearly stops, which can be explained by the saturation of the composite fiber.

210: The data were fitted by the Langmuir...

211: The calculated parameters are listed in Table 1.

212: is better fitted

214/215: At 15,.....the saturated adsorption capacity

222: What should an "effortless adsorption" mean?

230: in low concentrated solutions

233: then by linearly fitting....

236: Title of table 2: Thermodynamic parameters of phosphorus adsorption

243: the larger the adsorbed amount

253: wrong verb-substituted; instead:

262: is increased

264: the saturated adsorption capacity...

283: Effect of adsorbent dosage on ....

284: The more of the fiber is dosed,.....

285: (Table 5). When the ......is increased, the usable surface....fiber is increased. 286: More adsorption sites for phosphorus exist,.....

287: active sites and hence removed;

292: positive effect ....

294: , which met the class........

299: Tile of Table 5: Effect of dosage of ZSFB composite fiber on......

313:of the composite fiber.

314:It can be......both acidic and alkaline ......

320: obviously smaller than 88%.

323: in the adsorbent substrate....

324: NH₄⁺ and then combined

340: In Table 6 the results for the ........water are listed.

350: The results show that .....

351: of the adsorbed amount of the composite.....

357/358 and 372 and 391, 406: The results show that ....

370: breakthrough curves for different influent....

372: delete: /L.

373: as the influent flow

380: the larger the flow rate, the higher is the total...... time and the shorter...  

381: when the flow rate is too small , it seems that the wastewater......diffuses and mixes in the......

387, 402, 404, 415: plural s in curves

395: The results indicate that

396: leads to the increase of treated water.

398: (Section 2.3.3-1)? 3.1.3

406: 0.25 mg/L

427: it also shows a new.......for phosphorus.... 

433: in real wastewater

Author Response

Response to Reviewer 2 Comments

P1: The Freundlich equation does not assume "multilayer adsorption". The Freundlich equation can be theoretically derived for heterogeneous adsorbents. The eq. shows no Henry law behaviour at low concentration and has no finite limit at sufficiently high concentration.  Therefore the F. eq. Is only valid in a limited concentration range.1 Please check whether your thesis about the Freundlich parameter n is right (line 91/92). In my opinion it is generally known that with increasing n the isotherm gets more non-linear.

A1: It was a mistake to type "multilayer adsorption" and it should be monolayer (I have mentioned it several times in paper). The range of Freundlich parameter n is right [1,2], I have checked it from the source paper. (In line 87)

P2: Please insert the van't Hoff equation and show how you can derive △G, △H and △S form this equation? In Table 2 the values for 298 K and 308 K of △H and △S are missing.

A2: I have inserted the van't Hoff equation in section 2.1.2 (line 108). And the values in Table 2 was fixed.

P3: In line 360 you write about a precipitation of sulphuric acid and phosphate. How can they precipitate with each other? How does the phosphate react with sulphuric acid?

P3: It is a single-displacement reaction (strong acid replaces weak acid from its salt). In this paper the reaction is calcium phosphate with sulfuric acid (Ca₃(PO₄)₂ + 3H₂SO₄ → 2H₃PO₄ + 3CaSO₄) [3]. It is not stated clearly in the paper, and I have changed the description. (In line 377)

Other minor changes I have checked one by one and modified them follow your instructions.

References

1.Krishna, R.H.; Swamy, A. Physico-Chemical Key Parameters, Langmuir and Freundlich isotherm and Lagergren Rate Constant Studies on the removal of divalent nickel from the aqueous solutions onto powder of calcined brick. Int. J. Eng. Res. Dev 2012, 4, 29-38.

2.Mckay, G.; Blair, H.; Gardner, J. Adsorption of dyes on chitin. I. Equilibrium studies. Journal of applied polymer science 1982, 27, 3043-3057.

3.McKetta Jr, J.J. Encyclopedia of Chemical Processing and Design: Volume 36-Phosphorus to Pipeline Failure: Subsidence Strains; CRC press: 1990.

Round 2

Reviewer 1 Report

The authors have improved the manuscript according to the reviewer's comments and the manuscript can be accepted for the publication in Applied Sciences. 

Author Response

Response to reviewer 1 comments

Comment 1: In this article, the adsorptive removal of P by zeolite-based composite material was investigated. It is generally interesting story.

- Nonetheless, description of adsorption performance metrics needs to be improved significantly.

Pls revise the ms significantly to properly evaluate the results of all exps in a more meaningful way.

- In the abstract, pls provide representative numeric data of performance metrics (e.g., adsorption capacity and partition coefficient).

Response: Thank you very much for providing us very constructive comments and suggestion. We have revised our manuscript very carefully and details on adsorption performance metrics has been added in the revised version. We have also revised the abstract and results and discussion section as per your kind suggestion. Some of the experiments were not carried out in the current study, so honestly we would like to accept that we will consider these points in the future research work.

Comment 2: Pls define what is adsorption mass (mg) and desorption mass (mg) in Tables 6, 7, and 8.

Are they the mass of sorbent (ZSFB) or target (P)? Authors did not specify any information of what are those masses?

Response: Sorry for the unclear description. These are the mass of target (P) (we have added them in line 362). And we also redefine them in Table 6, 7 and 8.

Comment 3: For many adsorption studies, performance is generally assessed and expressed by the equilibrium (or maximum) adsorption capacity.

However, it is not objective to do such judgement based on adsn capacity alone because the maximum adsorption capacity is sensitively affected by the initial loading concn of target pollutant (or more specifically what is left after sorption reaction). If sorbent is exposed to higher concentration of targets, it is apt to exhibit higher adsorption capacity. On the other hand, if the sorbent is exposed to lower levels of target species, it will show lower capacities.

For a given study where all tests were done at the same initial concns, adsn capacity is useful to make comparison in relative sense only under such limited conditions.

However, it is not proper to make comparisons where studies are conducted at all different initial loading concns.

Hence, it is not an objective metric to meaningfully assess the actual performance of sorbents.

The ideal evaluation should be made by the concept of distribution coefficient (DC) or partition coefficient (PC) as explained below. With the concept of PC, the use of adsn capacity will be meaningful instead of using adsn capacity alone.

Response: Thank you very much for this constructive suggestion of the concept of partition coefficient. PC is an objective performance metric to assess the actual performance of sorbents when compare with other sorbents. And per you kind suggestion, most paper just used adsorption capacity of sorbents, which is not objective in comparison with different sorbents in different conditions. Therefore, we just used PC along with adsorption capacity to evaluate the performance of our composite fiber.

Comment 4: Raw isotherm data as well across varying breakthrough levels (not only 100% but some early BT points).

To resolve this problem, pls prepare a new Table to show some specifics of the performance data. Authors show only estimated Qe (max capacity) values in Table 1 or so. Pls show the measured Qe values at 100% BT (I assume that authors already did). In addition, pls show the results at varying BT levels such as 5, 10, and 50% Qt.

BT level as well in addition to 100% of equilibrium which authors did if possible. If authors cannot show the data for all other than 100%, pls try to show the adsn capacity and PC at least at 10% BT point (in addition to 100%). Authors may show all detailed experimental data of adsorption capacity in Table as they are measured in addition to estimated values from model fit.

Response: The specifics of the performance data are shown in Fig. 4, Fig. 5 and Fig. 6 (measured Qe versus time).

Comment 5: Computation of PC

To make a more objective comparison of performance between different sorbents or data obtained under different conditions,pls compute the partition coefficient (PC) or distribution coefficient (DC) and use it for assessing the actual performance.

Pls compare the data with previous study using PC values and expand discussion.

Pls compute PC values and use them along with adsorption capacity to evaluate the performance: Refine Table 5 to show authors' experimental more specifically by including such variables as Initical concn of target, final concn of target, breakthrough (BT: e.g., 10% and 100%), adsorption capacity (mg/g), partition coefficient (mg/g/uM). (Although authors can use 100% BT, 10% BT is more ideal to define the capacity of sorbent. This is because the state of saturation is diagnosed more efficiently in early BT point as explained by Sjulejko et al. (in ref shown below).)

PC can be computed as follows:

PC=Adsorption capacity/Final concentration (or = Adsorption capacity/(Initial concn*removal rate)).

The resulting data can be expressed as "mg/g/uM (<- Refer to refs).

All PC values should be presented in Table and related discussion should also be provided in the main text.

Response: PC are used to compare different sorbents. Table 5 just represent the effect of dosage of ZSFB composite fiber on phosphorus removal. PC were used to in Table 9 (line 429) with relative discussions. In table 5, an important metric is residual phosphorus concentration (line 344) which helps to determine the optimum dosage in sewage with specific phosphorus concentration. This can help to apply the fiber in the real sewage treatment.

Comment 6: For details of PC computation, authors may refer to some of the proper reference if needed.

Authors may or may not necessary cite the following refs but take this concept thoroughly and properly to make a full revision of ms to accommodate the PC to properly judge the performance of materials proposed in this study relative to other materials introduced previously. (Down below are just examples of papers showing the concept of PC. There is no obligation for authors to cite these! Authors may find and use some other references if they can find some other relevant references.)

- Vikrant K., Kim K.-H. (2019) Nanomaterials for the adsorptive treatment of Hg(II) ions from water. Chemical Engineering Journal. 358, 264-282.

- Na C.-J., Yoo M.-J., Tsang D.C.W., Kim H.W., Kim K.-H. (2019) High-performance materials for effective sorptive removal of formaldehyde in air. Journal of Hazardous Materials. 366, 452-465.

-Szulejko J.E., Kim K.-H, Parise J. (2019) Seeking the most powerful and practical real-world sorbents for gaseous benzene as a representative volatile organic compound based on performance metrics. Separation and Purification Technology 212, 980-985

Response: The references are useful and were cited in the paper. Two of the reference papers were studies on sorbents for gaseous benzene and gaseous formaldehyde, the PC equation includes the partial pressure (Pa) of the target gas at the saturation, which cannot be used in our study[1,2].The other reference paper studied the removal of Hg(II) ions by the nanomaterials[3]. The partition coefficient for a solid-liquid adsorption system represents the ratio of analyte concentration in and on the solid adsorbent phase to its concentration in the liquid phase at equilibrium. PC is an objective performance metric to assess the actual performance of adsorbents when we compare the performance of different adsorbents.

Comment 7: Create a new Table in which performance of ZSFB is compared with other sorbents tested previously for the removal of P.

Based on new edition of PC in a new Table, authors are asked to make discussion, especially performance evaluation based on PC values in R & D section.

In the new Table requested above, pls add more data for more meaningful performance metrics by adding several more new columns to care more metrics or parameters such as initial loading concn, final concn, pH, adsn capacity (at 100% BT and low BT (5 or 10%) as well), PC (at 100% BT + at low BT (5 or 10%)), and so on.

Authors should then tell the real-world performance based on PC as a means to explain biased patterns derived by adsorption capacity data.

Especially, the results shown in the updated Table should be evaluated based on PC along with Adsn capacities at the same time to judge the performance by two contrasting criteria (Adsn capacity and PC).

In general, most authors have been comparing to determine the best sorbent simply based on adsn capacity which can lead to extremely biased judgement. Such wrong biased comparison based on Adsn capacity then should be criticized by the new ordering set by more objective criteria of PC!  The wrong order by adsn capacity can be critically assessed by the more objective criteria of PC! So, it will be ideal to assess the performance based on Adsn capacity first. And then expand discussion how those results are biased based on PC values derived newly and shown in revised Table(s).

Response: We add section 3.1.5 (line 388) to compare the performance of ZSFB with other sorbents tested previously for the removal of P. Also new Table is in line 401 with discussion to evaluate the performance of adsorbents by two contrasting criteria (Adsorption capacity and PC).

Comment 8: Authors may also provide discussion wrt Figure of Merit (FoM) of the composites they generated.

Response: We are trully sorry for not provide the Figure of Merit (FoM) of the composite. When we seached ‘adsorption figure of merit’ in google scholar carefully, we just found papers about sensors or dyes adsorption. Can you give us some references to learn how to apply Figure of Merit (FoM) regarding this study?

Comment 9: Provide detailed QA for measurements of target species (three dyes and one drug component) before and after the treatment.

Response: The target specie in this paper is phosphate. We haven’t carry out research on three dyes and one drug component in current study. We would like to consider on this aspect in our future research work.

References

1.         Na, C.-J.; Yoo, M.-J.; Tsang, D.C.; Kim, H.W.; Kim, K.-H. High-performance materials for effective sorptive removal of formaldehyde in air. Journal of hazardous materials 2018.

2.         Szulejko, J.E.; Kim, K.-H.; Parise, J. Seeking the most powerful and practical real-world sorbents for gaseous benzene as a representative volatile organic compound based on performance metrics. Separation and Purification Technology 2019, 212, 980-985.

3.         Vikrant, K.; Kim, K.-H. Nanomaterials for the adsorptive treatment of Hg (II) ions from water. Chemical Engineering Journal 2018.