Photocatalytic Enhancement of Metal Ion Release from Oxides in the Presence of Polystyrene: Environmental Implications in Marine Pollution

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear authors

The manuscript is about investigating release behavior of zinc and copper ions from PS/(ZnO or Cu₂O) and PS-free solutions prepared in distillated water. For this purpose, the prepared solutions were exposed to UV irradiations and final formed samples were analyzed by various ICP-MS, SEM, FTIR, and DSC methods.

Please consider following comments:

1-Is concentration of prepared PS-based solutions effective on obtained results? Why did you select this concentration?

2-Why did you select foam form of PS? Other industrial products and sources of PS lead to a huge volume of waste PS-based materials. Why didn’t you select them for your study?

3-Why did you use water as solvent? Is used solvent effective on your results?

4-What is molecular weight of PS for producing selected foam sample? Did you investigate effect of PS molecular weight on the obtained results? In industries, various produced PS-based products have a variety of molecular weights. Which PS products were used for your study? Please explain about it in the text. You just mentioned that used PS has foam form.

5-Are porosity and pore size of utilized PS foam effective on your obtained results?

6-Please write about all specifications of applied PS foam sample in your study.

7-Please add a schematic figure that explain your UV analysis test procedure. How did expose open-air quartz-tubes to UV under stirring?

8-What was the stirring speed? Did you investigate the effect of stirring speed on your obtained results?

9-Is it possible to do the UV analysis under sonication or shaking? It can clear effect of mixing method on the obtained results.     

10-Is the source of used PS-based product important and effective on your achieved results? It means, various sources have different specifications of produced PS pieces. Is your study general and applicable for PS from various sources?

11-There are some spelling and grammatical errors that must be corrected.

Sincerely

 

Author Response

1. Is concentration of prepared PS-based solutions effective on obtained results? Why did you select this concentration?

We selected the ratio between metal oxides and PS weights (1:1) following the literature. See, for example, the cited https://doi.org/10.1016/j.chemosphere.2022.134421.

2. Why did you select foam form of PS? Other industrial products and sources of PS lead to a huge volume of waste PS-based materials. Why didn’t you select them for your study?

We added two new citations and one sentence in the Section Results and Discussion: “Foamed PS accounts for, at least, 23% of global litter in aquatic environments and, in Italy, even 50% of the litter is foamed PS“. In fact, expanded PS accounts for, at least, 23% of plastic litter in aquatic environments, even if it is difficult to have precise and world-consistent data; globally, the analyses reported values from 18% to 60% (see https://doi.org/10.1016/j.envadv.2023.100342). In Italy, even 50-58% of the litter is foamed PS (https://doi.org/10.1016/j.marpolbul.2019.110515), also because of the use of PS boxes during the fishery. Furthermore, PS undergoes to break very easily compared to polyolefin plastics, so being a threat for oceans (https://www.matweb.com/reference/tensilestrength.aspx).

3. Why did you use water as solvent? Is used solvent effective on your results?

The aim of this work is to study the interaction among metal oxides, PS, and aquatic environments. Water, as solvent, is mandatory to understand the implications of the simultaneous presence, under light irradiation, of some plastic and copper / zinc oxides derived from fishery and touristic activities. The interaction among plastics, metal compounds, light in other environments (e.g., soil) could be a great outcome of this manuscript.

4. What is molecular weight of PS for producing selected foam sample? Did you investigate effect of PS molecular weight on the obtained results? In industries, various produced PS-based products have a variety of molecular weights. Which PS products were used for your study? Please explain about it in the text. You just mentioned that used PS has foam form.

We thank the Reviewer for the questions. The PS employed in this study was obtained from post-consumer. Traditional expanded PS is formed of small polystyrene beads and typically exhibits an average molecular weight in the range of approximately 160,000 – 260,000 g·mol⁻¹. To follow the appropriate suggestion, we changed the sentence in the Material and Methods section and we added one citation: “Post-use foamed PS was a gift from a local logistics company; it exhibits high porosity and a broad pore-size distribution, and its typical molecular weight is in the range of 160,000–260,000 g·mol⁻¹”

5. Are porosity and pore size of utilized PS foam effective on your obtained results?

The PS, used in this study, is traditional expanded PS, which is characterized by high porosity and a broad pore-size distribution. In the present work, porosity and pore size were not varied or quantified, as the aim was not to correlate structural parameters of the foam with metal ions dissolution kinetics. The porous structure of PS mainly facilitates light penetration and water accessibility to the polymer surface, supporting the proposed interfacial photochemical mechanism. We added a phrase in Materials and methods section (see 5.).

6. Please write about all specifications of applied PS foam sample in your study.

As above reported (4. and 5.), we completed in the text, as possible, the description of PS sample, a mix of foamed post-use PS, obtained from a logistics company. We selected this because of its similarity with that present as litter in the environment.

7. Please add a schematic figure that explain your UV analysis test procedure. How did expose open-air quartz-tubes to UV under stirring?

Following the suggestion, we inserted images of our set-up as Figure S3 in Supplementary.

8. What was the stirring speed? Did you investigate the effect of stirring speed on your obtained results?

We thank the Reviewer; we added the stirring speed (1200 rpm) in the text. We did not investigate the effect of stirring; we used a fixed value for comparing directly the effect of the irradiation and of the PS presence on the release of ions.

9. Is it possible to do the UV analysis under sonication or shaking? It can clear effect of mixing method on the obtained results.

It is not possible to irradiate the suspension under sonication or shaking, at least with our irradiator and our instruments; it is not ever simple to have instrumentation that is also UV-transparent.

10. Is the source of used PS-based product important and effective on your achieved results? It means, various sources have different specifications of produced PS pieces. Is your study general and applicable for PS from various sources?

We employed a mix of foamed post-use PS, obtained from a logistics company, because this was a real sample that you can find dropped in the environment. This manuscript focuses only on this type of plastic, close to the PS released into seas or rivers.

11. There are some spelling and grammatical errors that must be corrected.

We agree and we carefully corrected them.

Reviewer 2 Report

Comments and Suggestions for Authors

In the present manuscript Authors studied the interaction of transition metal oxides with polystyrene in aqueous suspensions under UV irradiation leading to enhanced metal ion release. The observed phenomenon is interesting, as interaction of two solid state components without strong mechanical impact in aqueous suspension is not expected to be highly pronounced. It should also be noted that both oxides are known as semiconductor materials, and semiconductors are now actively explored as photocatalysts for both water purification and polymer upcycling. So, presented results can be highly important and seem highly novel. However, some issues have to be addressed before publication:

• Polystyrene (PS) can contain multiple additives, which release can be responsible, at least in part, for the observed metal ion release. Moreover, used PS is described as “Post-use”. Were experiments with model pure PS made? It seems like experiment reproduction with “Post-use” PS can be problematic.
• Macroscopic morphology and particle size of PS is not clear from the manuscript.
• Morphology of PS observed by SEM can be affected by sample preparation, including drying procedure. It is not described in the manuscript in detail.
• When comparing PS SEM images before and after irradiation in the presence of metal oxide particles, it is important to add control images (after irradiation and stirring in just water, after stirring with oxide particles in dark).
• Some SEM images in the main file look similar, but SEM imagen in Figure S1 and S2 look strikingly different. Why?
• ZnO powder or Cu2O powder are not characterized in contrast to PS. What are specific surface, morphology, particle size?
• I haven’t found exact experimental procedures (with weights and volumes).

Author Response

1. Polystyrene (PS) can contain multiple additives, which release can be responsible, at least in part, for the observed metal ion release. Moreover, used PS is described as “Post-use”. Were experiments with model pure PS made? It seems like experiment reproduction with “Post-use” PS can be problematic.

We thank the Reviewer for this important comment. The use of post-consumer expanded PS was intentionally chosen to reflect environmentally realistic conditions, particularly fluvial, coastal and marine environments, where aged and weathered PS debris represents the dominant form of plastic pollution. From this perspective, the complexity of post-use PS is not a limitation but an intrinsic feature of the system under investigation. We added two citations and this new sentence: “Foamed PS accounts for, at least, 23% of global litter in aquatic environments and, in Italy, even 50% of the litter is foamed PS“.

We agree that systematic comparisons between pristine PS, additive-free model polymers, and post-consumer materials would be valuable as we have indicated for important direction of future investigations.

2. Macroscopic morphology and particle size of PS is not clear from the manuscript.

We thank the Reviewer for this comment, and we have clarified and deepened the macroscopic morphology and size range of PS fragments, as observed by SEM images. In particular, we added two new parts:

1. “SEM images of PS fragments, before and after 24 h of UV light irradiation, showed only very low morphological degradation and no severe surface changes (Figure 1). The fragments were mainly > 200 mm in size and had lateral dimensions that varied from tens to hundreds of micrometers, rather than discrete spherical or regular particles. In terms of morphology, pristine PS fragments (Figure 1a) showed a sheet-like, lamellar structure characterized by extensive wrinkling, folding, and crumpling. The sheets were not flat but densely bunched, forming a complex three-dimensional topography with pronounced ridges, folds, and crevices. Bright or semi-transparent edges indicated regions where the polymer thickness was extremely low, consistent with foamed PS. After UV irradiation (Figure 1b), no significant fragmentation or erosion was observed; the PS sheets appeared more compact with some small flakes and less translucent, suggesting a partial compression and densification of the lamellar structure.”
2. “The overall macroscopic morphology of PS in the presence of ZnO and Cu₂O remained essentially unchanged (Figures 4, S5, S6, 4, S7 and S8). In all cases, the PS matrix retained its wrinkled and folded sheet-like structure. Metal-containing domains were visible in SEM images due to contrast differences, but they did not correspond to significant changes in the underlying PS morphology. Overall, no major morphological differences were observed, either before or after UV treatment, with the exception of a slight compaction of PS after irradiation.”
3. Morphology of PS observed by SEM can be affected by sample preparation, including drying procedure. It is not described in the manuscript in detail.

We have clarified the SEM sample preparation procedure in the manuscript (Materials and Methods section): “Prior to SEM analysis, PS samples were dried in a laboratory oven at 40 °C to remove residual moisture without inducing thermal deformation or morphological rearrangements. No solvents were used during drying, minimizing the risk of surface collapse or solvent-induced artefacts. After drying, the samples were mounted on aluminum SEM stubs using conductive carbon adhesive tape. To further limit charging effects under the electron beam, a thin carbon coating was deposited by sputtering under vacuum. Carbon sputtering was selected in preference to metal coatings to preserve surface features and avoid masking fine morphological details.”.

4. When comparing PS SEM images before and after irradiation in the presence of metal oxide particles, it is important to add control images (after irradiation and stirring in just water, after stirring with oxide particles in dark).

The control images are shown in the Supplementary as Figures S1-S2 and S5-S8.

5. Some SEM images in the main file look similar, but SEM imagen in Figure S1 and S2 look strikingly different. Why?

We thank the Reviewer for this observation. The images were similar, as we described in the manuscript and above (see, 2.); PS appeared only more compact with some small flakes and less translucent, probably due to the action of the UV light and the continuous stirring.

6. ZnO powder or Cu₂O powder are not characterized in contrast to PS. What are specific surface, morphology, particle size?

ZnO and Cu₂O were commercial ones, purchased from Merck, as reported in manuscript. Their available description reported them as powder. They were not in nanoparticle size but, as usual for powders, with microparticles not controlled in size.

7. I haven’t found exact experimental procedures (with weights and volumes).

The exact procedure was in the Section 2.Materials and Methods, where we had written: “20 mg of ZnO powder or Cu₂O powder were added to 10 mL of distilled water with or without PS (20 mg), and irradiated with UV light (l = 254 nm) for 24 h in open-air quartz-tubes of 15 mL of volume under stirring (1200 rpm).”

Reviewer 3 Report

Comments and Suggestions for Authors

The experimental design is straightforward, and the analytical workflow is appropriate. The dissolution trends observed for ZnO and Cu₂O in the presence of polystyrene (PS) under UV light are consistent with the known photochemical behaviour of these oxides. The authors investigate a well-motivated environmental problem, and all key datasets coherently support the central claim. Overall, the study provides clear and convincing experimental evidence for plastic-mediated enhancement of metal oxide dissolution under irradiation.

Just a few minor suggestions:

• It may simply be a PDF generation issue, but the figures do not appear properly aligned within the text and may need resizing for improved presentation.
• For the consistency and clarity to readers, Figure 1 caption is suggested to be written as: Figure 1. SEM images of PS: (a) before UV irradiation; (b) after UV irradiation (24 h)
• TEM appears in the Abbreviations list but is not mentioned anywhere in the manuscript. Only abbreviations used in the text should be retained.
• Differential scanning calorimetry (DSC) should be defined at its first occurrence in the main text.
• In the Supplementary Information, the subscript formatting for Cu₂O should be corrected in the following captions:

      Figure S6. (a) SEM image of PS after stirring with Cu₂…

      Figure S9. (a) DSC analysis of PS (blue), irradiated PS (green), irradiated PS in the presence of Cu₂O ….

• In the Supplementary material figures, panels appear as individual images and would benefit from being combined into single multi-panel figures for improved presentation.

Author Response

1. It may simply be a PDF generation issue, but the figures do not appear properly aligned within the text and may need resizing for improved presentation.

We thank the Reviewer. We think that it is a pdf generation issue because the images in the manuscript are aligned. We checked deeply the final layout.

2. For the consistency and clarity to readers, Figure 1 caption is suggested to be written as: Figure 1. SEM images of PS: (a) before UV irradiation; (b) after UV irradiation (24 h)

We thank the Reviewer and we corrected all captions.

3. TEM appears in the Abbreviations list but is not mentioned anywhere in the manuscript. Only abbreviations used in the text should be retained.

We canceled TEM from the list.

4. Differential scanning calorimetry (DSC) should be defined at its first occurrence in the main text.

We eliminated the definition of DSC since we inserted it in the Abbreviation list, as done for other techniques.

5. In the Supplementary Information, the subscript formatting for Cu₂O should be corrected in the following captions:

Figure S6. (a) SEM image of PS after stirring with Cu₂…

Figure S9. (a) DSC analysis of PS (blue), irradiated PS (green), irradiated PS in the presence of Cu₂O ….

We thank the Reviewer and we corrected them.

6. In the Supplementary material figures, panels appear as individual images and would benefit from being combined into single multi-panel figures for improved presentation.

We grouped the figures, so the presentation will be improved.

Reviewer 4 Report

Comments and Suggestions for Authors

The author's study indicates that ultraviolet (UV) light can cause the dissolution of zinc oxide and cuprous oxide, resulting in the release of zinc and copper ions. This UV-driven dissolution enhances our understanding of the interactions between plastics and pollutants. The research can assist in anticipating the availability of metals for biological organisms and formulating approaches to reduce the release of pollutants in illuminated marine settings. However, I do have a few suggestions/concerns:

1. The infrared (IR) studies presented in this paper are inconclusive. The carbonyl signal, which typically appears around 1700 cm⁻¹, was not significantly observed in Figures S3A and S3B. These samples, which are the IR spectra of PS stirred in water with Cu₂O or ZnO, are displayed before and after UV irradiation. The carbonyl signals are not evident. This raises questions about whether any significant oxidation is actually occurring. Additionally, the baseline quality is quite poor. Therefore, I recommend that the IR experiments be repeated, with Figures S3A and S3B clearly highlighting the carbonyl signal appearing after UV irradiation. Also, these IR charts should be added to the main MS.
2. Furthermore, could the authors comment on the peak that appeared around 2300 cm⁻¹ in Figure 2 (red)?
3. Have the authors conducted UV-Vis studies? I believe that authors could observe changes in UV-Vis peak absorption before and after prolonged UV exposure to the samples for PS or ZnO/CuO. This experiment could be performed for blank PS (before and after UV irradiation) alongside PS with Cu/Zn salts (before and after irradiation).
4. Lastly, the ICP analysis presented in Table 1 indicated only a slight increase in ion concentration following UV irradiation; however, the study only examined a 24-hour irradiation period. Does the increase in ion concentration reach a threshold, or does it continue to rise with extended UV exposure? A comprehensive study with at least 72 hours of exposure should be conducted to accurately determine ion concentrations.

Author Response

1. The infrared (IR) studies presented in this paper are inconclusive. The carbonyl signal, which typically appears around 1700 cm⁻¹, was not significantly observed in Figures S3A and S3B. These samples, which are the IR spectra of PS stirred in water with Cu₂O or ZnO, are displayed before and after UV irradiation. The carbonyl signals are not evident. This raises questions about whether any significant oxidation is actually occurring. Additionally, the baseline quality is quite poor. Therefore, I recommend that the IR experiments be repeated, with Figures S3A and S3B clearly highlighting the carbonyl signal appearing after UV irradiation. Also, these IR charts should be added to the main MS.

We thank the Reviewer for this detailed comment and for the constructive suggestions. First, we agree that the IR spectra reported in Figures S3A and S3B should be more visible to the reader. In the revised version of the manuscript, these spectra were

 moved from the Supplementary Information to the main text.

Regarding the limited visibility of the carbonyl band around 1700 cm^-1 in the spectra of PS stirred with Cu₂O or ZnO, we would like to clarify that this behavior arises from intrinsic experimental and compositional factors, rather than from the absence of photooxidation. First, the IR spectrum of UV irradiated PS alone reflects a sample composed only of the polymer and, therefore, even a modest degree of surface oxidation leads to a clearly detectable carbonyl signal. In contrast, for the PS samples with metal oxides, the relative amount of PS is significantly lower, as the material analyzed is a heterogeneous mixture of polymer and inorganic oxides. As a consequence, the contribution of oxidized PS functional groups to the overall IR signal is inherently diluted, making the carbonyl band much less intense and, in some cases, only barely discernible. Second, the presence of metal oxides negatively affects spectral quality, particularly the baseline. ZnO and Cu₂O introduce strong scattering and absorption effects in ATR-FTIR measurements, which result in baseline distortion and reduced signal-to-noise ratio. This phenomenon is well known for polymer–inorganic composites and directly impacts the detectability of weak bands such as those associated with early-stage oxidation.

Despite these limitations, small but reproducible differences in transmittance between the spectra recorded before and after UV irradiation are still observable, including subtle changes in the carbonyl region and within the fingerprint region. These variations, although minor, are consistent with superficial photooxidation of PS and align with the trends observed for PS irradiated in the absence of metal oxides.

In the present work, IR data are therefore not intended to provide a quantitative measure of oxidation, but rather to offer supportive qualitative evidence, which must be interpreted together with SEM/EDX, Raman, and DSC results.

2. Furthermore, could the authors comment on the peak that appeared around 2300 cm⁻¹ in Figure 2 (red)?

We thank the Reviewer for drawing attention to this point. The band observed around 2300 cm^-1 in Figure 2 is related to the environmental CO₂ present in the room. This feature is commonly observed in ATR-FTIR measurements, as the technique involves direct contact between the sample and the diamond ATR crystal, which inevitably retains a thin layer of ambient air at the crystal–sample interface.

3. Have the authors conducted UV-Vis studies? I believe that authors could observe changes in UV-Vis peak absorption before and after prolonged UV exposure to the samples for PS or ZnO/CuO. This experiment could be performed for blank PS (before and after UV irradiation) alongside PS with Cu/Zn salts (before and after irradiation).

It is an interesting suggestion but, in our research groups, we do not have an instrument to perform the UV analysis on solid samples.

4. Lastly, the ICP analysis presented in Table 1 indicated only a slight increase in ion concentration following UV irradiation; however, the study only examined a 24-hour irradiation period. Does the increase in ion concentration reach a threshold, or does it continue to rise with extended UV exposure? A comprehensive study with at least 72 hours of exposure should be conducted to accurately determine ion concentrations.

We agree with Reviewer about the comprehensive study and we had monitored the experiments for 72 h; at 48 and at 72 h, we observed by ICP only a slight raising of the ions concentrations and not ever easily reproducible. So, we decided to show only the reliable data, as reported standard deviation certified at 24 h.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear authors

The manuscript can be accepted.

Sincerely

Author Response

We really thank you the Reviewer for his/her work and his/her suggestions.

Reviewer 2 Report

Comments and Suggestions for Authors

As was noted in the previous report, the manuscript is original, interesting and may be very important. The main drawback of the manuscript is the research design, do to which the reproduction of reported results is complicated: the composition and history of PS samples is not known (thus generality of conclusions is not clear), metal oxide powders are not characterized (although are commercially available). The manuscript can be acceptable for publication, but I have one minor remark:

1) The description of conditions of irradiation can be improved: if light power is not known, please, provide the type of source (LED, Hg lamp?), consumed power, and distance from the irradiated sample. 

Author Response

1) The description of conditions of irradiation can be improved: if light power is not known, please, provide the type of source (LED, Hg lamp?), consumed power, and distance from the irradiated sample. 

We really thank you the Reviewer for the suggestions and we improved the description as follows: "A photochemical multiray apparatus (Helios Italquartz; Milano, Italy) was used for photoirradiation of the mixtures; the apparatus is equipped with 10 low-pressure Hg lamps (nominal electrical power consumption: 15 W per lamp, total consumed power 150 W), emitting mainly at 254 nm. Transparent quartz vials (15 mL, placed at around 10 cm of distance from lamps) were used for irradiation of the suspensions."

Reviewer 4 Report

Comments and Suggestions for Authors

I am pleased with the author's revision efforts and believe no further changes are necessary.

Author Response

We really thank you the Reviewer for his/her work and suggestions.