Removal of Pb(II) by Adsorption of HCO–(Fe₃O₄)_x Composite Adsorbent: Efficacy and Mechanism

Round 1

Reviewer 1 Report

Referee report on manuscript “Removal of Pb (II) by adsorption of HCO-(Fe3O4)x composite adsorbent: efficacy and mechanism”

This is a fairly good and necessary article, and it can be recommended for publication, but only after clarification / improvement of some ambiguities.

1.     Why Fe3O4 and not Fe2O3?

2.     To attract more readers, more recent introductory information about Fe3O4 oxides would be useful to provide. See, for example, few recent MDPI papers and references therein:

Serga, V et al. Impact of Gadolinium on the Structure and Magnetic Properties of Nanocrystalline Powders of Iron Oxides Produced by the Extraction-Pyrolytic Method. Materials 2020, 13, 4147. https://doi.org/10.3390/ma13184147

Gu, B.; et al. Fabrication of UV-Stable Perovskite Solar Cells with Compact Fe₂O₃ Electron Transport Layer by FeCl₃ Solution and Fe₃O₄ Nanoparticles. Nanomaterials 2022, 12, 4415. https://doi.org/10.3390/nano12244415

Nordin, A.H.; et al. The State of the Art of Natural Polymer Functionalized Fe₃O₄ Magnetic Nanoparticle Composites for Drug Delivery Applications: A Review. Gels 2023, 9, 121. https://doi.org/10.3390/gels9020121

3.     Fig.7. - This figure needs to be improved, because small details (measurement legend) are not visible.

4.     Furthermore, considering the above-mentioned SEM data, how stable are these images over time, are there any dynamics and are there any aging effects?

5.     More information on porosity would be helpful.

6.     Paragraph 3.7.2. Accuracy in hundredths of a percent seems excessive and requires serious justification, especially when the sample is morphologically heterogeneous.

7.     Data in Table 3 needs error bars.

8.     Paragraph 3.7.3. X-ray diffraction analysis. Can this XRD pattern be compared with literature data?

9.     Furthermore can these data be supplemented by Raman spectra.

10.  In the conclusions, it is necessary to clearly formulate what new data about the studied material were obtained in this work?

In general, the manuscript is interesting and can be considered for publication after constructive reflection on the above comments.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript brings results of synthesis of a cerium oxide-Fe₃O₄ composite and its applicability as adsorbent for Pb(II) removal from aqueous systems. Even though the results seems to indicate that Pb(II) polluted waters can be successfully treated by this approach, however, they are some important key points that make this manuscript not acceptable for publication in Water:

General comments:

1) The manuscript is a too routine work on adsorption. The overall scientific content and degree of novelty are not high enough. The manuscript contains very little new information. The authors just tried to replicate the previous study co-authored by one of the authors of this manuscript (Renjian Deng): Optimal Synthesis and Mechanistic Performance of HCO-Doped-(Fe3O4)x Adsorbent for Sb(III)/Sb(V) Removal From Water. Preprints 2020. They changed the pollutant from Sb(III)/Sb(VI) to Pb(II). Unfortunately for them, they have forgotten in the present manuscript some ”traces” of Sb(III)/Sb(VI), which, obviously, is totally not acceptable at this level:

Lines 251-254: ”As shown in Tab.2, the correlation R2 of the pesudo-second-order model fitting HCO-(Fe3O4)x adsorption of Sb(III) and Sb(V) is higher than that of the correlation pseudo-first-order model fitting, so the kinetic process of HCO-(Fe3O4)x adsorption of Sb(III) and Sb(V) is appropriate to be fitted”

Lines 362-363: ”In addition, the literature reports that the kinetic process of antimony adsorption by iron matrix adsorbents can be fitted with a variety of models

2) Only two isotherm models (Langmuir, Freundlich) and two kinetic models (pseudo-1 and 2) were tested? At least 2 others (e.g., Temkin, Dubinin–Radushkevich, Elovich, Weber-Morris etc.) should have been employed in order to have a reliable adsorption study.

3) Interpretation of the results is very often speculative, not supported by appropriate data. See the below specific comments.

4) The rationale of the selection of 20-100 mg/L Pb(II) concentration must be explained in the manuscript. Is this concentration range relevant for wastewaters? Or for contaminated natural waters? Some references must be added to support explanation.

5) The authors seem to forget the fact that Pb(II) is a cation, not an anion like Sb(V). Or, maybe the authors are not aware of the fact that adsorption of anions and cations do not occur on the same type of surface centers (see the below specific comments). In either case, this is not acceptable at this level.

6) The authors seem not knowing what the doping process is. This is not acceptable at this level. See the below specific comments.

7) The English of manuscript needs a thorough revision for grammatical errors and typos. In its current state, the manuscript does not meet the journal's required standard.

Specific comments:

Lines 11-12: ” A new HCO-(Fe3O4)x composite adsorbent prepared by co-precipitation method of Fe3O4 doping in cerium-rich grinding and polishing sludge was used to remove Pb from water”

First: the HCO-(Fe3O4)x composite is not new; it was previously reported by other studies.

Second: precipitation of such high amounts of Fe3O4 on Ce-sludge surface cannot be considered doping.

Lines 40-41: ” among which adsorption is characterized by low cost, short reaction time, large adsorption capacity and simple operation”

Adsorbents are not always cheap, the adsorption time is not always short (depending on many parameters) and not all adsorbents have a large adsorption capacity (this is what you are saying just 2 lines below: ”However, the adsorption capacity of Pb(II) is low”). This must be rewritten.

Lines 74-75: ” The stirring speed (150 rpm) was controlled at (25±1)℃”

The name (brand) of the apparatus used to stir the flasks and control the temperature must be given.

Line 85: ”qe is the adsorption capacity of the adsorbent for heavy metal”.

You must add: ”at equilibrium”

Lines 86-87: ” V is the volume of the solution after fixing, mL”

If concentration dimension is mg/L, then dimension of volume must be L!

Line 88: Section ”(2) pH effect experiment”

You must give the Pb(II) concentration.

Lines 89-90: ” the effects of the initial pH of the solution at 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 and 9.0”

It is difficult to believe that you can adjust the pH of 100 mL solution Pb(II) at those exact values using only 1 M HCl or 1 M NaOH.

Line 92: Section ”(3) Experiment on the effect of coexisting anions”

First: Pb(II) is an cation. Thus, the effect of coexisting anions on Pb(II) adsorption is less important. You should have studied the competitive effect of coexisting cations.

Second: You must give the concentration of Pb(II) and of all studied ions.

Line 92: ” The effect of chlorine ions”

It is chloride, not chlorine

Lines 173-174: Figure 1 ”The initial concentration was [Pb]=20 m2·g-1”

This is a strange dimension for Pb concentration in solution. The same at caption of Figure 2.

Lines 184-186: ”Related studies[36,38] showed that the zero-point potential (pHpzc) of HCO-(Fe3O4)x is between 4 and 5, and thus the pH=4-5 has the best removal effect on Pb(II)”

In reference 36 it is written: ”Non-doped Fe3O4 was found to be an isoelectric point of about 4.5 and the value increased 6.5–7.3 when Ce doped”. Therefore, 6.5–7.3 is far away from pH 4-5.

Reference 38 is a strange study which cannot be found with a Google search. Therefore, it should be removed from references. What is it? An article, a scientific report, a PhD/bachelor/master thesis?

To sum up, the assumption that pHpzc of HCO-(Fe3O4)x lies between 4 and 5 is wrong. You should experimentally measure the pHpzc of both Ce-sludge and HCO-(Fe3O4)x!

Lines 186-188: ” when the pH increases to 6-9, Pb ions in solution mainly exist in various forms such as Pb2+, Pb(OH)+, and Pb(OH)2[39], and the increasing OH- in solution will compete with Pb(II) for active sites.

This is totally not true and wrong. HO- is an anion and Pb2+ a cation. They do not compete for the same active sites. Cations are adsorbed on anionic centers and anions are adsorbed on cationic centers.

Line 196: ”In natural waters, a wide variety of anions are usually present, which affect the removal of heavy metals”

This is true if the heavy metal is an anion (e.g., HCrO4-). But, in natural waters, a wide variety of cations are also present, and they have the most important influence on the adsorption process of cationic heavy metals (like Pb(II)). This is why you must study the influence of other cations, not of anions!

Lines 202-205: ”It can be seen that Cl- and SiO32- had a negative effect on the adsorption and removal of Pb(II), and the reason for this is that Cl- and SiO32- competed with Pb(II) for the active sites on the adsorbent surface during the adsorption process”

This is a nonsense. As I said before, there is no competition between the Pb cation and the studied anions for the active sites.

Lines 254-261: ”is appropriate to be fitted by the pseudo second-order model, which implies that electron sharing or electron transfer is the main source of adsorption kinetics and chemisorption is dominant………supporting that the kinetic process of Pb(II) adsorption by HCO-(Fe3O4)x follows the quasi-secondary kinetic model, i.e., the whole adsorption process of Pb(II) is controlled by the chemical reaction at the solid-liquid interface”

This is just a speculation. Adsorption mechanisms cannot be directly assigned based on observing simple kinetic experiments or by fitting kinetic models. Please read the following article: Tran et al. (2017) Mistakes and inconsistencies regarding adsorption of contaminants from aqueous solutions: A critical review. Water research.

Lines 268-270: ” The surface of the unmodified treated polished sludge was relatively smooth and porous (Fig.7(a)), whereas the HCO-(Fe3O4)x prepared with different Ce/Fe moles had a rough surface and showed many tiny pores”

SEM figures are too small and blurred. It is impossible to discern the pores mentioned by the authors. Authors must replace them with bigger and clearer figures (1 new figure ≈ the total size of all 4 existing figures) taken at the SAME magnification level!

Lines 271-273: ”At the same time, irregular spherical particles appear on the surface of the adsorbent and belong to amorphous iron oxide crystals such as FeCe2O4, FeOOH, Fe3O4 and Fe2O3”

This is pure speculation. You cannot reach to such conclusions from SEM images.

Lines 304-305: ” …indicating that the doping of Fe3O4 alters”

As I said, precipitation of Fe3O4 is such high amounts onto CeO2 cannot be considered doping.

Lines 304-305: ”It can be assumed from the JADE software analysis that HCO-(Fe3O4)x underwent a complex reaction between Fe3O4 and CeO2 in HCO during the preparation process”

This reaction, even if theoretically possible, is very slow, because takes place between two solids; therefore, it is insignificant.

Line 311: section ” 3.7.4. HCO-(Fe3O4)x before and after adsorption of XPS analysis”

This is a study investigating Pb(II) retaining on HCO-(Fe3O4), not synthesis of HCO-(Fe3O4). Where is the XPS data regarding the adsorbed Pb? In this section the most important information was not provided: that regarding the redox oxidation state of Pb existent at surface of the exhausted adsorbent. The reduction potential of Pb2+/Pb0 is -0.13 V (https://www.gov.nl.ca/education/files/k12_evaluation_chem3202_standardreductionpotentials.pdf). The reduction potential of solid Fe(II) in Fe3O4 can range between -0.48 and +0.05 V, as magnetite ratio x = Fe2+/Fe3+ varies from fully stoichiometric (x = 0.50) to highly oxidized (x = 0.31) (Gorski et al., 2010. Redox Behavior of Magnetite: Implications for Contaminant Reduction). Therefore, reduction of Pb(II) to Pb(0) could, theoretically, take place to a certain extent. The XPS data regarding the adsorbed Pb must be added and discussed.

Lines 359-361: ”The quasi-secondary kinetic model provides a good fit for the kinetics of Pb(II) adsorption by HCO-(Fe3O4)x, implying that the Pb(II) ion is controlled by the chemical reactions occurring at the solid-liquid interface during the adsorption process”.

As I said before, this is just a speculation. Adsorption mechanisms cannot be directly assigned based on fitting kinetic models.

Line 371: ” Ligand exchange and complexation reactions play a major role in this process[54].”

 This is another speculation. What experimental data can sustain this statement?

Lines 372-373: ” The HCO-(Fe3O4)x synthesis was prepared with the doping of Fe3O4 to produce the FeCe2O4 compound[33],Which carries a more negative charge than polished sludge.”

This is another speculation. In order to say that HCO-(Fe3O4)x carries a more negative charge than polished sludge, you must compare the pHpzc of HCO-(Fe3O4)x and polished sludge. Where in the manuscript can we find this data?

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

This manuscript demonstrated effects of HCO-Fe₃O₄ composite adsorbents with different Ce/Fe molar ratios on removal of Pb(II) in aqueous solution under  various condition. The manuscript is well-organized to describe the structure of adsorbent and efficiency on removal of Pb(II) ions. However, I think the manuscript missed several experimental of removal of Pb(II) ions under couple of conditions. I strongly suggest that the authors address the following comments to improve the paper before publication in this journal.

1. If authors can add the schematic illustration about structure of adsorbent and mechanism for removal of Pb(II) ions using the adsorbents prepared in this manuscript, it would be very helpful for the readers to understand this paper quickly.

2. There are representative experimentals for adsorption study of various materials, such as metal ions, surfactant, or organic pollutant. However, the authors missed the results on several studies under various condition. So, I strongly suggest the additional studies as follow; 1) different concentration of adsorbents, 2) removal efficiency of adsorbents (which can be only one type of adsorbent with best ratio of Ce and Fe) adsorption time at various amount of adsorbents, and 3) removal efficiency under various initial concentration of Pb(II) ions in aqueous solution.

3. Would authors please add more explanation of the reason for use of coexisting anions in Section 3.3, even though authors added reference numbers in this paragraph? It would be helpful to explain in detail the reasons for the ions added to the experiment (eg specific waste or specific environment).

4. In the study on adsorption isothermal model of Pb(II), it showed pretty high R² values of Freundlich model and fitting lines of Freundlich model were shown to be well-matched, and to be well-distinguished from Langmuir model. But, In the study on dynamical model in the Section 3.6, the fitting lines, Pseudo-first and second order models showed no significant difference as shown in Figure 6, and similar R² values in Table 2. Would authors please explain about reason for that to us??

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

After successful revision, this manuscript can be accepted.

Reviewer 2 Report

Authors presented an improved version of their manuscript and they have responded to some of my earlier comments

Reviewer 3 Report

Authors have made some efforts to address previous comments. The paper is now likely publishable.