Adsorption of Contaminants of Emerging Concern (CECs) with Varying Hydrophobicity on Macro- and Microplastic Polyvinyl Chloride, Polyethylene, and Polystyrene: Kinetics and Potential Mechanisms

Round 1

Reviewer 1 Report

The paper entitled ‘Adsorption of Contaminants of Emerging Concern (CECs) with Varying Hydrophobicity on Macro- and Microplastic Polyvinyl Chloride, Polyethylene, and Polystyrene: Kinetics and Potential Mechanisms’ presented an interesting work related to the adsorption of multiple CECs on micro- and macroplastics. Microplastics are rising increasing concern and their interactions with other co-existed pollutants are of great importance for understanding their environmental behavior. Some points are to clarified.

1. As the authors mentioned, some CECs are retained on glass serum bottles. Have the authors tested other containers? e.g., Plastics tubes?

2. In section 2.1 (Line 115-125), the authors mentioned that the sizes of different microplastics are not identical. Then, I doubt whether the comparison among different microplastics meaningful (Line 404-407), as both microplastics types and sizes are different.

3. Section 3.6 is NOT related to the main content in this work, which I suggest to be removed.

4. Mechanism analysis is kind of weak in the present work. Maybe some material characterization approaches can be applied? Also, a schematic diagram will help for a clear view of the mechanisms.

Author Response

The paper entitled ‘Adsorption of Contaminants of Emerging Concern (CECs) with Varying Hydrophobicity on Macro- and Microplastic Polyvinyl Chloride, Polyethylene, and Polystyrene: Kinetics and Potential Mechanisms’ presented an interesting work related to the adsorption of multiple CECs on micro- and macroplastics. Microplastics are rising increasing concern and their interactions with other co-existed pollutants are of great importance for understanding their environmental behavior. Some points are to clarified.

Response:

We thank the reviewer for taking the time to evaluate our work, and we have addressed the points laid out by the reviewer below.

1. As the authors mentioned, some CECs are retained on glass serum bottles. Have the authors tested other containers? e.g., Plastics tubes?

Response 1:

We thank the reviewer for the comment.  We wanted to eliminate other potential plastics from interfering the adsorption process as much as we could, so we have avoided contact with other plastics during the experiment and sample processing.  In addition, we listed all the plastic materials that we have used in the study in case of any inadvertent interference from the listed plastics for the purpose of reproducibility.  We suspect that the majority of the adsorption studies in literature used glass containers because they had a similar line of justification.  Although we did not test plastic tubes in this study and the glass serum bottles retained some of the chemicals used in this study, our literature search has shown that polypropylene (the material of which plastic tubes are usually made; i.e., Wang et al., 2015, Chemosphere) may retain chemicals as well (Fig. 2). Therefore, we have instead addressed the issue of chemical retention by the glass serum bottle with controls without plastics and the statistical test (Student’s t-test) to determine whether the chemicals were removed by the plastic being tested or by the glass serum bottle.  We have made an effort to clarify this more in the manuscript in the Materials and Methods subsection “CECs concentration calculation, kinetic modeling, and statistical analysis”:

“Since we found that the amber glass serum bottles could sorb some of the CECs, we also performed the Student’s t-test to determine how significantly different the CEC concentration detected in the plastics was from that detected in the controls, thus differentiation the adsorption due to the glass serum bottle and due to the plastics. We decided to continue using glass serum bottles because usually adsorption studies on plastics are conducted in glass containers, and only few reported using PP tubes. Moreover, we found that chemicals may also sorb onto PP (Fig. 2). Therefore, we decided to address the glass sorption with statistical analysis.”

2. In section 2.1 (Line 115-125), the authors mentioned that the sizes of different microplastics are not identical. Then, I doubt whether the comparison among different microplastics meaningful (Line 404-407), as both microplastics types and sizes are different.

Response 2:

We thank the reviewer for their comment on the differences in types and size of microplastics.  Unfortunately, we determined that it was very cost-prohibitive and time-consuming to produce a specific size fraction at the quantity required for our study. And the manufacturing method also limited the types of microplastics that were able to be produced.  Further, different types of plastics have different surface areas and pore volumes even with the same microplastic sizes.  We thus addressed the adsorption with different sizes and different pore volumes by plotting adsorption concentration against specific surface area and pore volume (SI Fig. 11).  Additional text has been added to the section to discuss our results in the context of different microplastic types and size fractions in the Materials and Methods subsection “Kinetics and adsorption mechanisms”:

“We examined the potential mechanisms from the perspective of chemical hydrophobicity and from the plastic surface properties. Although the plastic sizes were not the same within same size category, (i.e., macro- or microplastics in this study), we measured specific surface area and pore volume using the nitrogen adsorption BET method and mercury intrusion porosimetry (SI Table 3) to aid our analysis (SI Fig. 13).”

We have also included a discussion of our results on the sorption concentrations normalized to surface area and pore volume for the different microplastic types in the Results and Discussion subsection “Kinetics and adsorption mechanisms”:

“There are several possible adsorption mechanisms outlined by Yu et al. (2021) ²⁶. Based on the plastic types and the structure of the CECs, adsorption mechanisms including cation ligand, electrostatic, and hydrogen bonding interactions were not likely involved. The adsorption of DCF by plastics generally increased with specific surface area (SI Fig. 13; micro-PE had a high adsorbed concentration despite aggregating and floating on the surface and despite a specific surface area of only 0.07 m² g^-1) suggesting adsorption mechanisms generally more consistent with hydrophobic and van der Waals interactions.”

3. Section 3.6 is NOT related to the main content in this work, which I suggest to be removed.

Response 3:

We thank the reviewer for the suggestion to remove the section regarding the discussion about specific surface area and pore volume measurements.  We agree that it was awkwardly placed and discussed.  We think it may be useful to discuss the different specific surface area and pore volumes that the readers may encounter in different studies, since there have been many studies utilizing different methods to measure specific surface area and pore volume.  We felt it is important to report adsorption concentration relative to the plastic mass (as many of the previous adsorption studies have done in general) and to report adsorption concentration relative to the surface area, since as the adsorbent becomes increasingly small, the specific surface area becomes increasingly important to adsorption. We thus reorganized, revised, and included the following text in the Results and Discussion subsection “Kinetics and adsorption mechanisms”:

“Since the nitrogen adsorption BET method only worked for micro-PVC, micro-PE, and macro-PS, the alternate method, mercury intrusion, was used for the other plastics. However, we recognize that these methods may not be available to many researchers, and our measurements may not be comparable to other studies that opted to use other methods of measurement, such as calculating the surface area based on assumptions (the geometry of the plastic particles based on observations and theory, e.g., scalene ellipsoid, Legendre ellipse with ellipticity and roundness correction, cylinder) ^{52, 63, 80, 94, 95} and relying on manufactured dimensions and microscopy measurements ^{80, 96}. Chinaglia et al. (2018) have found that the surface area measured by the BET method was 3-5 times higher than the calculated surface area due to the visually invisible small pores ⁸⁰. Even with the BET method measurements, it may still underestimate the true surface area and porosity of the material due to capillary condensation ^{70, 97}. We recognize that different methods may produce substantially different measurements due to the barriers of instrument access ⁸⁰ and cost.”

4. Mechanism analysis is kind of weak in the present work. Maybe some material characterization approaches can be applied? Also, a schematic diagram will help for a clear view of the mechanisms.

Response 4:

We thank the reviewer for the comment on the mechanism analysis.  We agree and acknowledge that our experiments were not designed to explain the mechanisms occurring in our study fully since the experiments were designed mainly to determine the kinetics of the adsorption, and the following text reflect our acknowledgement of this limitation in the Results and Discussion subsection “Kinetics and adsorption mechanisms”:

“Finally, there is also a possibility for pore-filling interaction to occur, however, one way to assess the pore-filling interaction is to examine the curve of adsorption potential density vs. adsorption volume ⁸⁶, and the experimental setup in this study was not suitable to determine whether pore-filling was occurring.”

Most of our mechanism analysis was based on theory with support from our material characterization.  The material characterization of plastic specific surface area and pore volume was performed and clarified with the new subheading “Material characterization: Specific surface area and pore size distribution” in the Materials and Methods section.  Additionally, we used SEM to capture images of the plastics to understand the size and surface of the plastics (SI Fig. 10).  We also thank the reviewer for the suggestion for a schematic.  We included a schematic of the relevant mechanisms for the ease of 

Author Response File: Author Response.pdf

Reviewer 2 Report

Summary and general comments

In this study, the authors investigate the adsorptivity of several pharmaceutical drugs onto the microplastic and macroplastic. This research contributes knowledge to the understanding of possible adsorption of pharmaceutical drugs to microplastics, as the pairing of microplastic with the drugs can lead to possible enhancement of toxicity.

Overall, the experimental work is sound and interesting to read. The proposed experimental work is suitable for the study of drug adsorption onto the microplastic, and the scientific interpretations are adequate to support the conclusion. The limitation of the study (BET surface area) is also discussed. However, numerous minor errors were observed, and there are some sentences in the methodology which are not clear.

Specific comments

1.       Line 167-168. The sentence is unclear whether the six CECs are mixed in a single solution or prepared separately.

2.       Is there any reason for the differences in CECs initial concentration? Are these due to the solubility of the CECs? Do mention the initial pH of the CECs.

3.       Line 179. How was the mixture stirred, and at what speed, using what machine?

4.       Inconsistencies with the in-text citations: It was observed quite a few of the mixed formats. As authors used square-bracket [number] throughout the manuscript, however, the superscripted format was observed on lines 171, 215, 420, and also the author (year) format at line 411 onwards. If the author’s name is mentioned, there is no need to include the year. Do correct for all to keep the in-text formatting consistent.

5.       Section 2.1.3 Authors need to include the equation of Chi-square.

6.       Section 2.1.3 is there any reason not to use Langmuir isotherm to describe the adsorption characteristics?

7.       Authors need to be careful with the assigned abbreviation. Ibuprofen is abbreviated as IBU in the introduction and method sections; however, from section 3 onwards, it was changed to IBP, which was observed in the text, table and figure.

8.       Full-stop is not required for the values in Table 1 and should be removed.

9.       In the description of Table 1, the pseudo first-order rate constant symbol should be corrected as k1, instead of k.

Author Response

In this study, the authors investigate the adsorptivity of several pharmaceutical drugs onto the microplastic and macroplastic. This research contributes knowledge to the understanding of possible adsorption of pharmaceutical drugs to microplastics, as the pairing of microplastic with the drugs can lead to possible enhancement of toxicity.

Overall, the experimental work is sound and interesting to read. The proposed experimental work is suitable for the study of drug adsorption onto the microplastic, and the scientific interpretations are adequate to support the conclusion. The limitation of the study (BET surface area) is also discussed. However, numerous minor errors were observed, and there are some sentences in the methodology which are not clear.

Response:

We thank the reviewer for taking the time to evaluate our work.  Below we have improved and clarified the points raised by the reviewer.

1. Line 167-168. The sentence is unclear whether the six CECs are mixed in a single solution or prepared separately.

Response 1:

We thank the reviewer for the suggestion to explain our study more clearly.  We have amended the sentence to clarify that all the CECs were mixed in a single working solution for the adsorption experiment in Materials and Methods subsection “Adsorption experimental setup”:

“The working solution used in this experiment contained a mix of 40.1 mg L^-1 DCF, 8.15 mg L^-1 ATN, 18.38 mg L^-1 IBU, 0.65 mg L^-1 ACE, 3.65 mg L^-1 BPA, and 10.34 mg L^-1 MBT in Milli-Q water.”

2. Is there any reason for the differences in CECs initial concentration? Are these due to the solubility of the CECs? Do mention the initial pH of the CECs.

Response 2:

We thank the reviewer for the comment about the different starting concentration for each of the CECs.  The concentrations were determined indeed due to their solubility in water.  Other minor considerations and adjustments were made for the extraction procedure and GC/MS analysis after several trials of preliminary tests.  We have amended the Materials and Methods subsection “Adsorption experimental setup” with more details about the different concentrations:

“The different concentration of the CECs was due to their solubility, and after preliminary experiments, adjusted according to the extraction procedures and GC/MS analysis.”

3. Line 179. How was the mixture stirred, and at what speed, using what machine?

Response 3:

We thank the reviewer for pointing out the unclear stirring method.  We included the shaker model in Materials and Methods subsection “Materials”:

“All experiment bottles were continuously stirred on the Thermo Scientific MaxQ SHKA2508 Dual Action Orbital Reciprocating Shaker”

And we amended the text to include the speed of shaking at around 180 rpm in the Materials and Methods subsection “”:

“All samples were continuously stirred on the shaker at about 180 rpm, except the Day 0 samples which immediately went through the extraction procedure.”

4. Inconsistencies with the in-text citations: It was observed quite a few of the mixed formats. As authors used square-bracket[number] throughout the manuscript, however, the superscripted format was observed on lines 171, 215, 420, and also the author(year) format at line 411 onwards. If the author’s name is mentioned, there is no need to include the year. Do correct for all to keep the in-text formatting consistent.

Response 4:

We thank the reviewer for noticing the inconsistency of the reference format.  When we submitted the manuscript in MS Word, all the references were in superscript.  However, after the submission and MDPI formatting, some of the references were converted to square brackets and some remained superscripts.  We will make an effort to alert this format change to the journal before the manuscript is finalized.

5. Section 2.1.3 Authors need to include the equation of Chi-square.

Response 5:

We thank the reviewer for the suggestion to include the chi-squared calculation.  We have amended the Methods and Materials subsection “CECs concentration calculation, kinetic modeling, and statistical analysis” with the equation for chi-squared calculation:

“Then the chi-squared test was performed on the fit and the data to determine the significance of the fit using the pseudo first-order model

where  is chi-squared value that is to be compared to a chi-squared distribution for a specific degree of freedom, , of the dataset,  is the i-th data point in the dataset, and  is the i-th equivalent point predicted by the model.”

6. Section 2.1.3 is there any reason not to use Langmuir isotherm to describe the adsorption characteristics?

Response 6:

We thank the reviewer for providing an opportunity for us to explain our choice of using the Freundlich isotherm.  This choice was made mainly because one the assumptions of Langmuir isotherm is single-layer adsorption, which we were not sure if it was the case for our chemicals onto the plastics.  Specifically, π-π interaction would allow layering of ringed chemicals, and this interaction was determined to be possible with PS from other previous studies (Hüffer and Hofmann, 2016; Rochman et al., 2013; Liu et al., 2016).  Another minor reason for Freundlich isotherm was that most of the isotherm studies reported Freundlich isotherm constants (Fig. 2), and the readers may be interested in comparing the other reported values with our isotherm values.  We have included the following text at the end of the Material and Methods subsection “CECs concentration calculation, kinetic modeling, and statistical analysis” to help clarify our choice:

“We opted for Freundlich isotherm instead of Langmuir due to one of the assumptions of Langmuir being single-layer adsorption, which may not apply in possible π-π interaction in PS.”

Author Response File: Author Response.pdf

Reviewer 3 Report

This manuscript evaluates the adsorption of CECs on microplastics commonly present in aquatic environments. Generally speaking, the MS is interesting, well organized, and the content is worth to be published. Only minor changes are necessary:

-          Please include keywords

-          Please be sure that all figures and tables (including the ones present in the supplementary material) are mentioned in the text. Also, they should be presented in the same order they are mentioned.

-          In section 2.1.1, please mention that the LODs and LOQs are presented in Table S1.

-          Please do not use ppm. The units should be presented in mg/L or µg/mL.

-          Please explain how the CECs concentrations were selected (section 2.1.2). 

Author Response

This manuscript evaluates the adsorption of CECs on microplastics commonly present in aquatic environments. Generally speaking, the MS is interesting, well organized, and the content is worth to be published. Only minor changes are necessary:

Response:

We thank the reviewer for taking the time to evaluate our manuscript and allowing us to improve it.  Below we have made an effort to clarify the issues pointed out by the reviewer.

-          Please include keywords

Response 1:

We thank the reviewer for pointing out the lack of keywords.  We have now included the keywords.

-          Please be sure that all figures and tables (including the ones present in the supplementary material) are mentioned in the text. Also, they should be presented in the same order they are mentioned.

Response 2:

We thank the reviewer for the suggestion to improve our organization of the tables and figures.  We have now reorganized the figures and tables in the order as they appeared in the text.

-          In section 2.1.1, please mention that the LODs and LOQs are presented in Table S1.

Response 3:

We thank the reviewer for the suggestion.  We have included LODs and LOQs in the text when we refer to SI Table 1 in the Materials and Methods subsection “Gas chromatography/mass spectrometry (GC/MS)”:

“We analyzed the samples with a Shimadzu gas chromatography-mass spectrometry (GC/MS, QP2010 SE; capillary column SH-Rxi™-5Sil MS, L 30m, ID 0.25 mm, DF 0.25 um) in the selected ion mode (SIM; masses, limits of detection [LODs], and limits of quantitation [LOQs] are listed in SI Table 1), an AOC-20s autosampler, an AOC-20i auto injector (Columbia, MD, USA), and with column helium flow at 1.00 ml min^-1.”

-          Please do not use ppm. The units should be presented in mg/L or µg/mL.

Response 4:

We thank the reviewer for the suggestion for a better unit reporting practice.  In the manuscript we have corrected all the mention of ppm to mg L^-1.  As much as we could, we changed ppm to mg L^‑1 in the SI text and tables.  However, since ppm was used to compare mass fraction remaining in the solution (w/v) and on the plastic (w/w) in the SI, we kept the ppm units in the SI figures.  The non-SI figures and tables were not affected because the unit of ppm was not used in those non-SI figures and tables.  We changed the texts in the Materials and Methods subsection “Adsorption experimental setup”:

“The working solution used in this experiment contained a mix of 40.1 mg L^-1 DCF, 8.15 mg L^-1 ATN, 18.38 mg L^-1 IBU, 0.65 mg L^-1 ACE, 3.65 mg L^-1 BPA, and 10.34 mg L^-1 MBT in Milli-Q water.”

“An aliquot of 500 µl of each extract was transferred to a GC vial with 10 μl of 200 mg L^-1 anthracene-d₁₀ added as internal standard ⁷⁵.”

And we clarified the choice of reporting in ppm (w/v) or (w/w) in the first paragraph of the Materials and Methods subsection “CECs concentration calculation, kinetic modeling, and statistical analysis”:

“The concentration obtained from GC/MS of a liquid sample was multiplied by the concentrating factor of 20 (original 100 ml concentrated to final 5 ml) to calculate for the original concentration in the original solution, reported as ppm w/v in SI figures.  The concentration obtained from a plastic sample was first multiplied by the volume of the eluent extract of 5 ml and divided by the original mass of the plastic, reported as ppm w/w in SI figures.”

-          Please explain how the CECs concentrations were selected (section 2.1.2).

Response 5:

We thank the reviewer for the comment about the selection of concentration for each of the CECs.  This comment was also raised by another reviewer.  The concentrations were determined mainly because of their solubility in water.  Other minor considerations and adjustments were made for the extraction procedure and GC/MS analysis after several trials of preliminary tests.  We have amended the Materials and Methods subsection “Adsorption experimental setup” with more details about the different concentrations:

“The different concentration of the CECs was due to their solubility, and after preliminary experiments, adjusted according to the extraction procedures and GC/MS analysis.”

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

This manuscript is fine for publication.

Reviewer 2 Report

The quality of the manuscript has improved after the revision. All previously raised issues were clarified / corrected. I support this manuscript for publication in this journal