Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this work, magnetic CoFe₂O₄ nanoparticles were synthesized and systematically evaluated for their adsorption performance toward PET microplastics. However, the investigation of practical application aspects remains insufficient. Considering the current novelty and completeness of the study, major revision is recommended.

1. Although there have been numerous reports on the use of magnetic adsorbents for microplastic removal in recent years, the introduction section does not adequately review or critically assess the progress in this field. A more thorough literature background would help better position the novelty of the present work.

2.  The rationale for selecting PET as the sole model pollutant is not clearly justified. Moreover, no comparative adsorption data are provided for other types of microplastics (e.g., PE, PP, PS).

3.  The potential interference from coexisting constituents in real water bodies, such as natural organic matter and clay particles, has not been addressed. It is recommended that the authors conduct additional experiments to evaluate the effects of these competing components on PET adsorption performance.

Author Response

Author's Reply to the Review Report (Reviewer 1)

In this work, magnetic CoFe₂O₄ nanoparticles were synthesized and systematically evaluated for their adsorption performance toward PET microplastics. However, the investigation of practical application aspects remains insufficient. Considering the current novelty and completeness of the study, major revision is recommended.

1. Although there have been numerous reports on the use of magnetic adsorbents for microplastic removal in recent years, the introduction section does not adequately review or critically assess the progress in this field. A more thorough literature background would help better position the novelty of the present work.

___________________________________________________________________________

Response 1:

We have expanded the Introduction to explain the environmental relevance and chemical features of PET and to identify the specific mechanistic question posed here. The new PET-focused paragraph discusses drinking water, bottled water, and marine water columns using newly added Refs. [9–11].  Compared with the submitted version, the revised Introduction explicitly includes ligand coverage as a variable and describes the new calculations for water, an acid head-group model, an amine head-group model, and a PET molecular fragment.  The resulting novelty is the investigation of how ligand competition modifies the accessibility of ferrite adsorption sites, combined with new XPS evidence and the existing PET–water framework.  Particularly relevant changes in the revised manuscript include the following paragraphs added to the introduction:

Lines 37 to 49:

“Polyethylene terephthalate (PET) is particularly relevant because of its widespread

use in beverage containers, food packaging, and polyester textiles. PET particles and fibers

have been identified in raw and treated drinking water, bottled water, and marine water

columns [9–11]. For example, Pivokonsky et al. detected PET, polypropylene, and polyethy-

lene among the predominant polymers in drinking-water treatment systems, with residual

microplastics remaining after treatment [9]. PET microplastics have also been reported

throughout different depths of marine water columns, reflecting their environmental persis-

tence and transport beyond surface waters [11]. Compared with nonpolar polyolefins such

as polyethylene and polypropylene, PET contains aromatic terephthalate rings and polar

ester groups. These structural features influence its wettability and interfacial interactions

and provide an intrinsic ultraviolet chromophore that can support spectroscopic quantifica-

tion and electronic-structure analysis. PET is therefore a suitable model for investigating

how polymer functionality affects adsorption at magnetic nanomaterial interfaces.”

Lines 66 to 79:

“Although magnetic microplastic capture has advanced rapidly, most studies em-

phasize removal efficiency and material recovery rather than the molecular competition

between the polymer, interfacial water, and individual adsorption sites. This distinction

is important because adsorption energies calculated for an ideal, ligand-free oxide cannot

be interpreted directly as the behavior of a ligand-capped nanoparticle in water. Density

functional theory (DFT) provides a useful framework for identifying favorable adsorption

geometries, site-dependent energetics, charge redistribution, and magnetic changes on

oxide surfaces. Previous calculations on CoFe₂O₄ have demonstrated that adsorption

depends strongly on surface termination and the coordination environment of Co, Fe, and

O sites [14]. Nevertheless, the interaction of a PET molecular model with CoFe₂O₄ MNPs, including competition with interfacial water and organic dispersing agents [15], remains

insufficiently understood. A computational bare-surface model can therefore provide a

mechanistic limiting case, provided that its differences from the experimentally capped

interface are explicitly acknowledged.”

Lines 91 to 112:

“In this work, PET adsorption on OA/OAm-capped CoFe₂O₄ MNPs (PET+CoFe₂O₄

MNPs) is investigated using complementary experimental and computational approaches.

Specifically, CoFe2 O4 MNPs, PET, and PET+CoFe₂O₄ MNPs are characterized by trans-

mission electron microscopy (TEM), Fourier-transform infrared (FTIR), energy-dispersive

X-ray (EDX), and X-ray photoelectron spectroscopy (XPS), while PET uptake is monitored

by ultraviolet–visible (UV-Vis) spectroscopy. Time-dependent DFT (TDDFT) calculations

are used to interpret the characteristic PET optical absorption and the spatial distribution of

the main excitation at λ ≈ 289 nm. Our spin-polarized DFT calculations model adsorption

on representative sites of the CoFe₂O₄ (001) surface by water, OA, OAm, and PET using

H₂O, propanoic acid (C₃O₂H₅), ethylenamine (C₂NH₇), and the PET monomer (C₁₀O₅H₁₀),

respectively. In so doing, we focus specifically on the more relevant quantum-mechanical

description of the chemical adsorption of the head groups of these long-chain molecules at

a reduced computational cost [23]. A microkinetic treatment is subsequently employed to

evaluate H₂O/OA/OAm/PET coverage as a function of concentration and temperature.

The resulting analysis provides a molecular basis for understanding PET–CoFe₂O₄ interac-

tions and identifies surface-ligand coverage as a central variable that must be controlled in

the design of magnetic adsorbents.”

Reference [9] to [11]

[9] Pivokonsky, M.; Cermakova, L.; Novotna, K.; Peer, P.; Cajthaml, T.; Janda, V. Occurrence of Microplastics in Raw and Treated Drinking Water. Science of the Total Environment 2018, 643, 1644–1651. https://doi.org/10.1016/j.scitotenv.2018.08.102.

[10] Schymanski, D.; Goldbeck, C.; Humpf, H.U.; Fürst, P. Analysis of Microplastics in Water by Micro-Raman Spectroscopy: Release of Plastic Particles from Different Packaging into Mineral Water. Water Research 2018, 129, 154–162. https://doi.org/10.1016/j.watres.2017.11.011.

[11] Liu, M.; Lu, S.; Song, Y.; Lei, L.; Hu, J.; Lv, W.; Zhou, W.; Cao, C.; Shi, H.; Yang, X.; et al. Prevalence of Microplastic Fibers in the Marginal Sea Water Column off Southeast China. Science of the Total Environment 2022, 804, 150138. https://doi.org/10.1016/j.scitotenv.2021.150138.

___________________________________________________________________________

2. The rationale for selecting PET as the sole model pollutant is not clearly justified. Moreover, no comparative adsorption data are provided for other types of microplastics (e.g., PE, PP, PS).

___________________________________________________________________________

Response 2:

We have added a specific rationale for selecting PET. Its use in packaging and polyester textiles, together with its occurrence in drinking-water and marine studies [9–11], makes it environmentally relevant. Chemically, the aromatic rings and ester groups provide both an optical response and defined functional groups for examining interfacial interactions. This allows the experimental UV–Vis measurements to be discussed alongside the calculated electronic excitation and adsorption geometries.

The revised paragraph explains this chemical distinction from nonpolar polyolefins such as PE and PP. The approximately 289 nm band is used within the measured low-concentration calibration range, while the TDDFT calculation provides a qualitative assignment of the aromatic excitation. Neither the existence of an aromatic chromophore nor its calculation makes UV–Vis inherently selective for intact PET particles in a complex water matrix.

Relevant changes to the revised manuscript include:

Lines 37 to 49:

“Polyethylene terephthalate (PET) is particularly relevant because of its widespread

use in beverage containers, food packaging, and polyester textiles. PET particles and fibers

have been identified in raw and treated drinking water, bottled water, and marine water

columns [9–11]. For example, Pivokonsky et al. detected PET, polypropylene, and polyethy-

lene among the predominant polymers in drinking-water treatment systems, with residual

microplastics remaining after treatment [9]. PET microplastics have also been reported

throughout different depths of marine water columns, reflecting their environmental persis-

tence and transport beyond surface waters [11]. Compared with nonpolar polyolefins such

as polyethylene and polypropylene, PET contains aromatic terephthalate rings and polar

ester groups. These structural features influence its wettability and interfacial interactions

and provide an intrinsic ultraviolet chromophore that can support spectroscopic quantifica-

tion and electronic-structure analysis. PET is therefore a suitable model for investigating

how polymer functionality affects adsorption at magnetic nanomaterial interfaces.”

Lines 467 to 469:

“Specifically, we show PET adsorbs

at λ ≈ 289 nm due to a π −π^∗ electronic transition that may be used to characterize the

PET concentration in aqueous solution via the Beer–Lambert law below 300 ppm.”

Reference [9] to [11]

[9] Pivokonsky, M.; Cermakova, L.; Novotna, K.; Peer, P.; Cajthaml, T.; Janda, V. Occurrence of Microplastics in Raw and Treated Drinking Water. Science of the Total Environment 2018, 643, 1644–1651. https://doi.org/10.1016/j.scitotenv.2018.08.102.

[10] Schymanski, D.; Goldbeck, C.; Humpf, H.U.; Fürst, P. Analysis of Microplastics in Water by Micro-Raman Spectroscopy: Release of Plastic Particles from Different Packaging into Mineral Water. Water Research 2018, 129, 154–162.

 https://doi.org/10.1016/j.watres.2017.11.011.

[11] Liu, M.; Lu, S.; Song, Y.; Lei, L.; Hu, J.; Lv, W.; Zhou, W.; Cao, C.; Shi, H.; Yang, X.; et al. Prevalence of Microplastic Fibers in the Marginal Sea Water Column off Southeast China. Science of the Total Environment 2022, 804, 150138.

 https://doi.org/10.1016/j.scitotenv.2021.150138.

___________________________________________________________________________

3. The potential interference from coexisting constituents in real water bodies, such as natural organic matter and clay particles, has not been addressed. It is recommended that the authors conduct additional experiments to evaluate the effects of these competing components on PET adsorption performance.

___________________________________________________________________________

Response 3:

We agree that this is an important limitation for the applicability of our findings.  However, we would argue that our work provides a first step, which should stimulate further studies outside the laboratory.  This is made clear as follows in the conclusion of the revised manuscript:

Lines 479 to 483:

“Our results suggest CoFe₂O₄ MNPs may

prove highly effective for microplastic removal from contaminated water. We believe these

results will stimulate further research into the efficacy of CoFe₂O₄ MNPs for PET removal

that take into account the influence of organic matter, clay particles, and other coexisting

constituents present in real-world water bodies.”

___________________________________________________________________________

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

1. Why was only PET selected for the adsorption study?.
2. The introduction section lists multiple microplastic removal techniques—such as membrane filtration, coagulation, and magnetic separation—but fails to critically evaluate the limitations of each approach.
3. The CoFe₂O₄ magnetic material has been previously synthesized in the literature, which limits its novelty.
4. The absence of Zeta potential and contact angle measurements prevents a direct quantitative assessment of the material's hydrophobicity and surface charge characteristics, which are essential for understanding the adsorption mechanism.

Author Response

Author's Reply to the Review Report (Reviewer 2)

1. Why was only PET selected for the adsorption study?.

___________________________________________________________________________

Response 1:

We have expanded the Introduction to explain that PET combines environmental relevance with chemically identifiable aromatic and ester functionalities. The newly cited drinking-water and marine studies [9–11] motivate its selection. Moreover, its aromatic optical response also permits a connection between the UV–Vis calibration and the calculated excitation, while its oxygen-containing groups allow examination of interactions with exposed metal sites.  Relevant changes to the revised manuscript include:

 

Lines 37 to 49:

 

“Polyethylene terephthalate (PET) is particularly relevant because of its widespread

use in beverage containers, food packaging, and polyester textiles. PET particles and fibers

have been identified in raw and treated drinking water, bottled water, and marine water

columns [9–11]. For example, Pivokonsky et al. detected PET, polypropylene, and polyethy-

lene among the predominant polymers in drinking-water treatment systems, with residual

microplastics remaining after treatment [9]. PET microplastics have also been reported

throughout different depths of marine water columns, reflecting their environmental persis-

tence and transport beyond surface waters [11]. Compared with nonpolar polyolefins such

as polyethylene and polypropylene, PET contains aromatic terephthalate rings and polar

ester groups. These structural features influence its wettability and interfacial interactions

and provide an intrinsic ultraviolet chromophore that can support spectroscopic quantifica-

tion and electronic-structure analysis. PET is therefore a suitable model for investigating

how polymer functionality affects adsorption at magnetic nanomaterial interfaces.”

 

Lines 467 to 469:

“Specifically, we show PET adsorbs

at λ ≈ 289 nm due to a π −π^∗ electronic transition that may be used to characterize the

PET concentration in aqueous solution via the Beer–Lambert law below 300 ppm.”

 

Reference [9] to [11]

 

[9] Pivokonsky, M.; Cermakova, L.; Novotna, K.; Peer, P.; Cajthaml, T.; Janda, V. Occurrence of Microplastics in Raw and Treated Drinking Water. Science of the Total Environment 2018, 643, 1644–1651. https://doi.org/10.1016/j.scitotenv.2018.08.102.

 

[10] Schymanski, D.; Goldbeck, C.; Humpf, H.U.; Fürst, P. Analysis of Microplastics in Water by Micro-Raman Spectroscopy: Release of Plastic Particles from Different Packaging into Mineral Water. Water Research 2018, 129, 154–162. https://doi.org/10.1016/j.watres.2017.11.011.

 

[11] Liu, M.; Lu, S.; Song, Y.; Lei, L.; Hu, J.; Lv, W.; Zhou, W.; Cao, C.; Shi, H.; Yang, X.; et al. Prevalence of Microplastic Fibers in the Marginal Sea Water Column off Southeast China. Science of the Total Environment 2022, 804, 150138. https://doi.org/10.1016/j.scitotenv.2021.150138.

___________________________________________________________________________

 

2. The introduction section lists multiple microplastic removal techniques—such as membrane filtration, coagulation, and magnetic separation—but fails to critically evaluate the limitations of each approach.

___________________________________________________________________________

Response 2:

The revised version of the manuscript includes a reference to a most relevant review article [5] where the critical evaluation of each approach is provided.  Relevant changes to the manuscript include:

 

Lines 31 to 36:

 

“Microplastic removal from water typically involves membrane filtration, coagulation,

flocculation, sedimentation, electrocoagulation, or advanced oxidation [5]. While these

methods can remove significant amounts of microplastics, they face limitations. Common

issues include membrane fouling, high costs, incomplete recovery, and the creation of

secondary waste [5]. As a result, there is ongoing research to develop reusable and efficient

adsorbent materials [5–8].”

 

References [5–8]:

 

[5]. Li, M.; Zhao, Z.; Zhao, Z.; Li, M. Review of Techniques for the Detection, Removal, and Transformation of Environmental Microplastics and Nanoplastics. ACS Applied Materials & Interfaces 2025, 17, 20560–20589. https://doi.org/10.1021/acsami.5c02306.

 

[6]. Shen, M.; Zhang, Y.; Zhu, Y.; Song, B.; Zeng, G.; Hu, D.; Wen, X.; Ren, X.; Zhang, C. Microplastic pollution in freshwater ecosystems and removal technologies. Chemical Engineering Journal 2022, 428, 131928. https://doi.org/10.1016/j.cej.2021.131928.

 

[7]. Briceño, S.; Arevalo-Fester, J.E.; Fierro-Sanchez, I.A. Sustainable Magnetic Nanorobots for Microplastics Remediation. Microplastics 2025, 4, 97. https://doi.org/10.3390/microplastics4040097.

 

[8]. Vargas-Lopez, I.J.; Vera, A.; Rivadeneira, A.; Brämer-Escamilla, W.; González, G.; Briceño, S. Magnetic Ferrotitaniferous Sands for Microplastic Removal. Microplastics 2026, 5, 104. https://doi.org/10.3390/microplastics5020104.

___________________________________________________________________________

 

3. The CoFe₂O₄ magnetic material has been previously synthesized in the literature, which limits its novelty.

___________________________________________________________________________

Response 3:

We agree that neither cobalt ferrite nor its synthesis are new.  Our contribution is instead the analysis, experimentally and theoretically, of PET adsorption on an organic OA/OAm capped CoFe₂O₄ MNPs, and the demonstration by combining high-resolution XPS measurements with DFT calculations that the predominant adsorption mechanism is via charge transfer from the adsorbate to the surface.  This is described as follows in the revised manuscript:

 

Lines 15 to 17:

 

“By combining these techniques, we

show the mechanism of PET adsorption on CoFe₂O₄ MNPs is via charge transfer to the

surface, with an estimated PET uptake on OA/OAm capped CoFe2 O4 MNPs of ∼ 57%”

 

Lines 343 to 350:

 

“The O 1s, C 1s, Co 2p, and Fe 2p binding energies obtained from the Gaussian

decompositions of Figure 5 are provided in Table 1, and compared to those from Refs. [44],

[43], and [46]. Overall, we find our measured binding energies are consistent with those

measured for CoFe₂O₄ nanopowders [44] and PET [43,46]. Most importantly, we observe

a shift of ∼ −1 eV to weaker binding for both Co and Fe octahedral (B) 2p_3/2 levels,

indicative of a charge transfer to the CoFe₂O₄ MNPs upon PET adsorption. Altogether, our

XPS measurements suggest that PET binds strongly to the CoFe₂O₄ MNPs and that the

adsorption mechanism is via charge transfer from the adsorbate to the surface.”

 

Lines 428 to 434:

 

“Figure 11 demonstrates the correlation between adsorption energy Eads and charge

transfer to the surface Qsurf for H₂O, OA, OAm, and PET on Co, Fe, Fe_cus, and O_surf sites

of the CoFe₂O₄ (001) surface. Performing a linear fit, we find Eads ≈ 3.6Q_surf − 0.3 eV, with

a correlation coefficient of R ≈ 0.91, and a standard deviation of σ ≈ ±0.2 eV. Altogether,

this clearly demonstrates that the mechanism of molecular adsorption on the CoFe₂O₄

(001) surface is via charge transfer from adsorbate to surface, as also suggested by our XPS

measurements.”

Figure 11:

 

“Adsorption energy E_ads in eV versus surface charge Q_surf in e/nm² for water (H₂O,

blue circles), OA (C₃O₂H₅, green diamonds), OAm (C₂NH₇, orange triangles), and PET (C₁₀O₅ H₁₀, black squares) on Co, Fe, Fe_cus, and O_surf sites of the CoFe₂O₄ (001) surface, and linear fit E_ads ≈ 3.8Q_surf − 0.3 eV (red line, R ≈ 0.91) with a standard deviation σ ≈ ±0.2 eV (grey region).”

Lines 471 to 475:

“Our XPS measurements and DFT calculations

show the mechanism of PET adsorption on the CoFe₂O₄ (001) surface of MNPs is via charge

transfer from the adsorbate to the substrate, with PET adsorbing on the Fe_cus sites that bind

PET molecules significantly stronger than H₂O or OA in aqueous solution while retaining

their magnetic properties.”

___________________________________________________________________________

4. The absence of Zeta potential and contact angle measurements prevents a direct quantitative assessment of the material's hydrophobicity and surface charge characteristics, which are essential for understanding the adsorption mechanism.

___________________________________________________________________________

Response 4:

Although Zeta potential and contact angle measurements have not been performed in this study, we consider the high-resolution XPS measurements and DFT charge-transfer calculations included in the revised manuscript provide sufficiently strong evidence for the observed charge transfer from the adsorbates to the substrate, and hence the adsorption mechanism.  The revised manuscript includes the following relevant changes:

Lines 15 to 17:

“By combining these techniques, we

show the mechanism of PET adsorption on CoFe₂O₄ MNPs is via charge transfer to the

surface, with an estimated PET uptake on OA/OAm capped CoFe2 O4 MNPs of ∼ 57%”

 

Lines 343 to 350:

“The O 1s, C 1s, Co 2p, and Fe 2p binding energies obtained from the Gaussian

decompositions of Figure 5 are provided in Table 1, and compared to those from Refs. [44],

[43], and [46]. Overall, we find our measured binding energies are consistent with those

measured for CoFe2 O4 nanopowders [44] and PET [43,46]. Most importantly, we observe

a shift of ∼ −1 eV to weaker binding for both Co and Fe octahedral (B) 2p3/2 levels,

indicative of a charge transfer to the CoFe₂O₄ MNPs upon PET adsorption. Altogether, our

XPS measurements suggest that PET binds strongly to the CoFe₂O₄ MNPs and that the

adsorption mechanism is via charge transfer from the adsorbate to the surface.”

Lines 428 to 434:

“Figure 11 demonstrates the correlation between adsorption energy Eads and charge

transfer to the surface Qsurf for H₂O, OA, OAm, and PET on Co, Fe, Fe_cus, and O_surf sites

of the CoFe₂O₄ (001) surface. Performing a linear fit, we find Eads ≈ 3.6Q_surf − 0.3 eV, with

a correlation coefficient of R ≈ 0.91, and a standard deviation of σ ≈ ±0.2 eV. Altogether,

this clearly demonstrates that the mechanism of molecular adsorption on the CoFe₂O₄

(001) surface is via charge transfer from adsorbate to surface, as also suggested by our XPS

measurements.”

Figure 11:

“Adsorption energy E_ads in eV versus surface charge Q_surf in e/nm² for water (H₂O,

blue circles), OA (C₃O₂H₅, green diamonds), OAm (C₂NH₇, orange triangles), and PET (C₁₀O₅ H₁₀, black squares) on Co, Fe, Fe_cus, and O_surf sites of the CoFe₂O₄ (001) surface, and linear fit E_ads ≈ 3.8Q_surf − 0.3 eV (red line, R ≈ 0.91) with a standard deviation σ ≈ ±0.2 eV (grey region).”

 

Lines 471 to 475:

“Our XPS measurements and DFT calculations

show the mechanism of PET adsorption on the CoFe₂O₄ (001) surface of MNPs is via charge

transfer from the adsorbate to the substrate, with PET adsorbing on the Fe_cus sites that bind

PET molecules significantly stronger than H₂O or OA in aqueous solution while retaining

their magnetic properties.”

___________________________________________________________________________

 

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript by Lopez-Vitonera et al. submitted to Microplastics presents an interesting and relevant study on the adsorption of PET onto cobalt ferrite nanoparticles. The work is scientifically novel, written in a clear and measured style, and contains informative illustrations. However, a number of inconsistencies and logical contradictions remain that the authors should resolve before publication. In particular, the following revisions are recommended:

1. Add a “Conclusions” section that clearly summarizes the main findings of the study.

2. The investigation of adsorption mechanisms is stated as a key objective: “Despite interest in magnetic materials for microplastic remediation, the mechanisms underlying PET adsorption onto CoFe2O4 MNPs remain unclear.” For this purpose, the authors provide clearly insufficient experimental data, and the level of analysis of the computational results is inadequate. To allow the reader (and a specialist) to better understand the adsorption mechanism, it is recommended to include a schematic illustration showing the proposed orientation of the adsorbate molecules, the bonds formed with the adsorbent, and the active surface functional groups.

3. The use of FTIR alone to confirm the formation of the target CoFe2O4 structure is clearly insufficient. The authors are advised to employ XRD or SAED/TEM, depending on instrumental availability, to characterize the crystalline phase.

4. Based on FTIR results, the authors claim that the obtained materials contain organic functional groups attributed to oleic acid and oleylamine. At the same time, throughout the text the material is referred to simply as CoFe2O4. It would be more accurate to designate the material as CoFe2O4@oleic acid/oleylamine/oleate or CoFe2O4/oleylamine/oleic acid/oleate, depending on the spatial distribution of the modifiers. This inconsistency should be corrected.

5. For particles modified with oleylamine and oleic acid, SEM-EDS mapping of carbon would be extremely informative to reveal how these modifiers are distributed across the material’s surface.

6. In the DFT modeling, the authors primarily consider PET adsorption on the bare inorganic CoFe2O4 surface. Yet the studied material clearly contains an organic phase (oleic acid/oleylamine/oleate). Why were interactions with this organic phase not taken into account? If it forms a shell around the particles, adsorption processes should predominantly occur within this shell.

7. What is the relevance of the modeling results in this paper if the organic phase significantly affects adsorption, as acknowledged by the authors themselves: “However, as shown by the FTIR measurements of PET and CoFe2O4 MNPs provided in Figure 4, the CoFe2O4 MNPs remain capped by oleic acid/oleylamine. The presence of these groups on the CoFe2O4 MNP surface may significantly hinder PET adsorption, explaining the lower PET adsorption measured experimentally.” How, then, can the validity of the calculations be confirmed if they are not verified experimentally?

8. To characterize the interactions between PET and the studied material, the use of XPS is highly recommended, if instrumentally feasible.

Author Response

Author's Reply to the Review Report (Reviewer 3)

 The manuscript by Lopez-Vitonera et al. submitted to Microplastics presents an interesting and relevant study on the adsorption of PET onto cobalt ferrite nanoparticles. The work is scientifically novel, written in a clear and measured style, and contains informative illustrations. However, a number of inconsistencies and logical contradictions remain that the authors should resolve before publication. In particular, the following revisions are recommended:

1. Add a “Conclusions” section that clearly summarizes the main findings of the study.

___________________________________________________________________________

Response 1:

We apologize for our error in neglecting to include our Conclusions section in the originally submitted version of the manuscript.  The following conclusion section has been included in the revised manuscript:

Lines 466 to 483:

“In this work we have performed a combined theoretical and experimental study of

PET adsorption on CoFe₂O₄ MNPs in aqueous solution. Specifically, we show PET adsorbs

at λ ≈ 289 nm due to a π−π^∗ electronic transition that may be used to characterize the

PET concentration in aqueous solution via the Beer–Lambert law below 300 ppm. Com-

bined with our TEM and EDX characterization of the system, we obtain a PET uptake on

OA/OAm capped CoFe₂O₄ MNPs of ∼57%. Our XPS measurements and DFT calculations

show the mechanism of PET adsorption on the CoFe₂O₄ (001) surface of MNPs is via charge

transfer from the adsorbate to the substrate, with PET adsorbing on the Fe_cus sites that bind

PET molecules significantly stronger than H₂O or OA in aqueous solution while retaining

their magnetic properties. We also employ a microkinetic model based on PET adsorption

on the water-covered CoFe₂O₄ (001) surface to show that PET covers between 65 and 85 %

of the surface at very low concentrations (c_PET∼1 ppm). Moreover, by accounting for

OAm coadsorption, we find PET covers between 55 and 80 % of the surface for PET and

OAm concentrations between 100 and 200 ppm. Our results suggest CoFe₂O₄ MNPs may

prove highly effective for microplastic removal from contaminated water. We believe these

results will stimulate further research into the efficacy of CoFe₂O₄ MNPs for PET removal

that take into account the influence of organic matter, clay particles, and other coexisting

constituents present in real-world water bodies.”

___________________________________________________________________________

2. The investigation of adsorption mechanisms is stated as a key objective: “Despite interest in magnetic materials for microplastic remediation, the mechanisms underlying PET adsorption onto CoFe2O4 MNPs remain unclear.” For this purpose, the authors provide clearly insufficient experimental data, and the level of analysis of the computational results is inadequate. To allow the reader (and a specialist) to better understand the adsorption mechanism, it is recommended to include a schematic illustration showing the proposed orientation of the adsorbate molecules, the bonds formed with the adsorbent, and the active surface functional groups.

___________________________________________________________________________

Response 2:

We thank the reviewer for their comment, and have included the requested schematic as the following graphical abstract included in the revised manuscript.

___________________________________________________________________________

3. The use of FTIR alone to confirm the formation of the target CoFe2O4 structure is clearly insufficient. The authors are advised to employ XRD or SAED/TEM, depending on instrumental availability, to characterize the crystalline phase.

___________________________________________________________________________

Response 3:

XRD or SAED data were not available in the present study. However, in a previous work, we reported the structural properties of the samples using XRD.  This is explained as follows in the revised manuscript:

Lines 124 to 128:

The structural properties of the OA/OAm capped CoFe₂O₄ MNPS have previously

been analyzed using X-ray diffraction (XRD) in Ref. [28]. The peaks corresponding to (220),

(311), (222), (400), (422), (511) and (440) planes, which are indicative of a single phase cubic

mixed type spinel structure, were identified using an XRD card of the Joint Committee on

Powder Diffraction Standards (JCPDS) file no: 22-1086 for CoFe₂O₄ [28].

Reference 28:

[28] Alcalá, O.; Briceño, S.; Brämer-Escamilla, W.; Silva, P. Toroidal cores of MnxCo1–xFe₂O₄/PAA nanocomposites with potential applications in antennas. Mater. Chem. Phys. 2017, 192, 17–21. https://doi.org/10.1016/j.matchemphys.2017.01.035.

___________________________________________________________________________

4. Based on FTIR results, the authors claim that the obtained materials contain organic functional groups attributed to oleic acid and oleylamine. At the same time, throughout the text the material is referred to simply as CoFe2O4. It would be more accurate to designate the material as CoFe2O4@oleic acid/oleylamine/oleate or CoFe2O4/oleylamine/oleic acid/oleate, depending on the spatial distribution of the modifiers. This inconsistency should be corrected.

___________________________________________________________________________

Response 4:

The revised manuscript now explicitly identifies the experimental material as oleic-acid/oleylamine capped, abbreviated OA/OAm-capped, CoFe₂O₄ magnetic nanoparticles (MNPs) in the Abstract, the statement of scope, and the interpretation of the results. The revised FTIR discussion also describes a hybrid interface rather than treating every experimental interaction as adsorption on a completely bare oxide.  Relevant changes throughout the revised manuscript include:

Lines 6 to 7:

“oleic

acid/oleylamine (OA/OAm) capped cobalt ferrite (CoFe₂O₄ ) MNPs”

___________________________________________________________________________

5. For particles modified with oleylamine and oleic acid, SEM-EDS mapping of carbon would be extremely informative to reveal how these modifiers are distributed across the material’s surface.

___________________________________________________________________________

Response 5:

We have added the SEM–EDX instrumentation to the Methods and report a carbon contribution of approximately 7.8 wt.% for the nanoparticle sample. In conjunction with FTIR, this supports the presence of organic-containing material. The revised analysis uses this carbon percentage and the TEM-derived diameter to estimate the number of adsorption site. 

The EDX results shown here exhibit a rather homogeneous distribution of C, O, Fe, and Co atomic species. The following relevant changes have been made to the revised manuscript:

Lines 281 to 287:

“To estimate the OA/OAm coverage of the CoFe₂O₄ MNPs [14], we have performed

EDX measurements of the sample, yielding a carbon contribution of m_C ≈ 7.8 wt.%. This

is rather significant, and clearly demonstrates that the OA/OAm dispersing agents cover

the CoFe₂O₄ MNPs.  Employing the measured CoFe₂O₄ MNP diameter of d ≈ 17 nm and

carbon contribution of m_C ≈ 7.8 wt.%, we find N_sites ≈ 2.2×10¹⁷ . The ratio of adsorption

sites to PET molecules is then 47%, that is, the maximum fraction of PET molecules which

may be adsorbed on CoFe₂O₄ MNP sites.”

___________________________________________________________________________

 

6. In the DFT modeling, the authors primarily consider PET adsorption on the bare inorganic CoFe₂O₄ surface. Yet the studied material clearly contains an organic phase (oleic acid/oleylamine/oleate). Why were interactions with this organic phase not taken into account? If it forms a shell around the particles, adsorption processes should predominantly occur within this shell.

___________________________________________________________________________

Response 6:

We thank the reviewer for their constructive criticism.  To properly include the influence of OA/OAm dispersing agents on PET adsorption on OA/OAm capped CoFe₂O₄ MNPs, we now include adsorption energies for OA and OAm on Co, Fe, Fe_cus, and O_surf sites of the CoFe₂O₄ (001) surface in Figure 10, and have incorporated the presence of OAm molecules adsorbed on the CoFe₂O₄ MNPs in a PET and OAm concentration dependent microkinetic model of PET coverage, as shown in Figure 14. 

Figure 10:

“Adsorption energy E_ads in eV (bars) from aqueous solution (solid lines) or gas phase

(dotted lines) and surface charge Q_surf in e/nm² (symbols) of water (H₂O, blue, circles), OA (C₃O₂H₅, green, diamonds), OAm (C₂NH₇, orange, triangles), and PET (C₁₀O₅H₁₀, black, squares) on Co, Fe, Fe_cus , and O_surf sites of the CoFe₂O₄ (001) surface, as shown above.”

Lines 402 to 416:

“To understand how the presence of organic dispersing agents affect PET adsorption

on CoFe₂O₄ MNPs, we also provide adsorption energies for OA and OAm in Figure 10. On

both Co and Fe sites, we find propanoic acid’s OH group spontaneously dissociates upon

adsorption, with the H atom transfered to an O_surf site, and C₃O₂H₅’s two O atoms both

adsorbing on Co or Fe sites of the CoFe₂O₄ (001) surface. For this reason, OA binds much

stronger than PET to the Co and Fe sites of the CoFe₂O₄ (001) surface. In contrast, on both Fe_cus and O_surf sites, propanoic acid remains intact and binds significantly weaker than

PET. This suggests OA dispersing agents would cover both Co and Fe sites, while leaving

Fe_cus and O_surf available for PET adsorption. Although ethylenamine remains intact upon

adsorption on each site of the CoFe₂O₄ (001) surface, we find OAm binds significantly

stronger than OA, PET, or H₂O on Co sites, binds significantly weaker than PET on both Fe

and O_surf sites, but is isoenergetic to PET on Fe_cus sites, with ∆E_ads ≈ 0.03 eV.  Altogether,

this suggests that, when taking OA/OAm dispersing agents into account, PET adsorption

is primarily on Fe_cus sites of CoFe₂O₄ MNPs, competes with OAm adsorption on the same

sites, and all other sites are covered by H₂O molecules.”

Lines 424 to 427:

“Finally, Figure 10 also provides the surface charge Q_surf in e/nm² of the CoFe₂O₄ (001)

surface upon H₂O, OA, OAm, and PET adsorption. In each case, we find the adsorbate

transfers charge to the surface, with a greater charge transfer typically resulting in a stronger binding to the surface.”

Figure 14:

“Fractional coverage of PET on CoFe₂O₄ (001), θ_PET, versus PET and OAm concentrations,

c_PET and c_OAm, respectively, in parts per million (ppm) at 20^◦C, with the 100 ≤ c_PET , c_OAm ≤ 200 ppm region highlighted (black square).”

Lines 462 to 464:

“In fact, as highlighted in Figure 14, for OAm concentrations cOAm between

100 and 200 ppm, PET coverage range between 55 and 80 %, in excellent agreement with

the measured PET uptake of ∼57%.”

___________________________________________________________________________

7. What is the relevance of the modeling results in this paper if the organic phase significantly affects adsorption, as acknowledged by the authors themselves: “However, as shown by the FTIR measurements of PET and CoFe2O4 MNPs provided in Figure 4, the CoFe2O4 MNPs remain capped by oleic acid/oleylamine. The presence of these groups on the CoFe2O4 MNP surface may significantly hinder PET adsorption, explaining the lower PET adsorption measured experimentally.” How, then, can the validity of the calculations be confirmed if they are not verified experimentally?

___________________________________________________________________________

Response 7:

To address this relevant issue, we have now included model calculations for OA and OAm adsorbed on CoFe₂O₄ MNPs in our revised microkinetic model, and obtain PET uptake values in qualitative and semi-quantitative agreement with our measured value of ~57% once the number of available sites on the  CoFe₂O₄ MNPs per PET is taken into account.  In this way the validity of our calculations Is confirmed and verified experimentally.  Please see our response to comment 6 for a detailed list of relevant changes in the revised manuscript.

___________________________________________________________________________

8. To characterize the interactions between PET and the studied material, the use of XPS is highly recommended, if instrumentally feasible.

___________________________________________________________________________

Response 8:

We have performed high-resolution XPS measurements of the O 1s, C 1s, Co 2p, and Fe 2p spectra for CoFe₂O₄ MNP, PET, and PET+CoFe₂O₄ MNPs in the revised manuscript, as provided in Figure 5 and Table 1.  In so doing, we have provided further evidence of the charge transfer from adsorbed PET to the CoFe₂O₄ MNP surface, and the presence of adsorbed OA, OAm, and PET on the CoFe₂O₄ MNPs.  Relevant changes in the revised manuscript include the following:

Figure 5:

“XPS high-resolution spectra of CoFe₂O₄ MNPs (black diamonds), PET (red squares), and

PET+CoFe₂O₄ MNPs (blue circles), Gaussian fits (thick solid lines) and decompositions (thin solid lines) for (a) O 1s into O–C (green filling), O=C (yellow filling), and O^2– (red filling); (b) C 1s into O=C–O (green filling), C–O (yellow filling), and C–C (red filling) [43]; and (c) Co 2p and (d) Fe 2p into 2 p_1/2 A/B (blue filling), satellite 2 p_3/2 A/B (green filling), 2 p_3/2 A (yellow filling), and 2 p_3/2 B (red filling) sites [44].”

Lines 318 to 327:

“To further probe the nature of the PET–CoFe2 O4 MNP interaction, we provide high-

resolution XPS measurements of the O 1s, C 1s, Co 2p, and Fe 2p core spectra for CoFe₂O₄

MNPs, PET, and PET+CoFe₂O₄ MNPs and their Gaussian decompositions, in Figure 5.

For both the Co 2p and Fe 2p spectra we find the addition of PET to the CoFe₂O₄ MNPs

causes a significant reduction in overall intensity and a shift to weaker binding for the

2p_3/2 peaks, as seen in Figure 5(c) and (d). On the one hand, the observed reduction in

intensity may be attributed to the adsorption of micrometer-long PET molecules screening

the CoFe₂O₄ MNPs, as X-ray photoelectrons typically penetrate tens of nanometers. On

the other hand, the shift to weaker Co and Fe 2p_3/2 binding energies suggests charge is

transferred to metallic sites of the CoFe2 O4 MNPs from PET upon adsorption [45].”

Lines 328 to 342:

“In contrast, for both the O 1s and C 1s spectra we find the addition of PET to the

CoFe₂O₄ MNPs leaves the overall intensity relatively unchanged, with the O 1s main peak

narrowed and the C 1s peaks for PET+CoFe₂O₄ MNPs a convolution of the individual

CoFe₂O₄ MNP and PET spectra, as seen in Figure 5(a) and (b). The observed consistency

in O 1s and C 1s intensities suggests these atomic species are primarily located in the

upper layers of each sample, that is, in OA/OAm layers covering the CoFe₂O₄ MNPs or

in adsorbed PET. The narrowing of the main O 1s peak upon PET adsorption suggests

weaker binding O^2- species of the CoFe₂O₄ surface are shifted to stronger binding, whereas

stronger binding O–C species of adsorbed OA are shifted to weaker binding. Although

this might initially suggest charge is transfered out of the O 1s levels of the CoFe₂O₄  MNPs

into the adsorbed molecules, a more detailed analysis suggests the observed shifts are

related to an improved screening of the O^2- 1s levels by neighbouring negatively charged

Co and Fe species [45]. To provide a more quantitative analysis of the XPS spectra, we have

also performed a Gaussian decomposition of each spectra into specific atomic species with

characteristic binding energies.

Table 1:

“O 1s, C 1s, Co 2p, and Fe 2p XPS binding energies in eV for CoFe₂O₄ MNPs, PET, and

PET+CoFe₂O₄ MNPs from Figure 5 compared to CoFe₂O₄ Powder from Ref. [44] and PET from Refs. [43] and [46].”

 

Lines 343 to 350:

“The O 1s, C 1s, Co 2p, and Fe 2p binding energies obtained from the Gaussian

decompositions of Figure 5 are provided in Table 1, and compared to those from Refs. [44],

[43], and [46]. Overall, we find our measured binding energies are consistent with those

measured for CoFe₂O₄ nanopowders [44] and PET [43,46]. Most importantly, we observe

a shift of ∼ −1 eV to weaker binding for both Co and Fe octahedral (B) 2p_3/2 levels,

indicative of a charge transfer to the CoFe₂O₄ MNPs upon PET adsorption. Altogether, our

XPS measurements suggest that PET binds strongly to the CoFe₂O₄ MNPs and that the

adsorption mechanism is via charge transfer from the adsorbate to the surface.”

___________________________________________________________________________

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The reviewer thanks the authors for their detailed responses. All points raised have been satisfactorily addressed. The addition of a Conclusions section, a schematic illustration of the adsorption mechanism, XPS measurements, EDX carbon mapping, and the inclusion of OA/OAm in the DFT and microkinetic models significantly strengthen the manuscript. The clarification of the material designation as OA/OAm-capped CoFe2O4 MNPs resolves the inconsistency. The reviewer recommends acceptance of the manuscript for publication.

Author Response

We thank the reviewer for their kind response and constructive criticisms which have greatly improved the quality of our manuscript.