Hyperspectral Imaging Applied to WEEE Plastic Recycling: A Methodological Approach

Round 1

Reviewer 1 Report

Reviewer’s report:

Title: Hyperspectral imaging applied to WEEE plastic recycling: a methodological approach.

In this work, authors reported the possibility to apply hyperspectral imaging technique in the Short-Wave Infrared spectral range to characterize polymeric parts coming from waste from electric and electronic equipment. The manuscript presents data and evaluation which may useful for other researchers who work in the related field. However, the manuscript still needs modification as shown in the comments below.

1. In general the figures presentation has to be improved significantly: (a) The appearance of Fig.1 is fuzzy; typo ‘hypecube’. (b) at Fig 3(b), X and Y lines and labels are unclear and fuzzy. (c) Figs. 5(a) and 5(b) including the labels are unclear and fuzzy. They have to be improved. (d) At Fig. 6, labels of X and Y axes are unclear. (e) At Figs. 9 and 10, the X and Y axes including the labels are unclear and fuzzy. (f) Labels of X and Y axes of Figs. 11 and 12 are fuzzy.

2. Subheading of 3.1.2; 3.2.2; 3.3.2; 3.4.2 are the same. For easy understanding and clear information, it needs to use a specific subheading for each of them (i.e., Result of …?).

3. What’s mean of the higher and lower reflectance as shown in Fig. 5(a) and 5(b)?

4. Why the performance parameters as shown at table 3 are all the same value? What’s mean of 1.000 value?

5. Format of the references should be consistent (i.e., ref 11 use doi but others without doi number).

Comments for author File: Comments.pdf

In general the quality of English is ok although in some parts still need to be polished for better scientific sentences.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The paper of Roberta Palmieri et al. reports an interesting approach based on the utilization of SWIR-HSI techniques for both sorting and/or quality control applications of different plastic streams from waste from electrical and electronic equipment.

The paper is well written, very clear in the exposition and it will be of interest to the readers of Sustainability. According to the reviewer the paper can be accepted for publication in Sustainability.

Some typos should be corrected in the manuscript. See e.g., page 3, line 100: " physical-chemical properties" should be: "physico-chemical properties".

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Dear Author、

This paper is an excellent study suggesting a good way to show how to solve the WEEE.

I have a question.

The four major plastics produced nationwide are PE, PP, PVC and PS.

In this paper, PVC has uses shown in Table 1. In 3.2.2,

“Six polymers in the validation image were correctly detected by the developed PLSDA model with just a small number of misclassified pixels, mostly owing to border-effect.”

But in addition, "Table 3 shows the performance parameters corresponding to the fractionation of plastics" is reported. What about PVC detection?

In your paper, 3.4. Identification of WEEE plastics with brominated flame retardants.

Is it difficult for PVC?

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Hyperspectral imaging applied to WEEE plastic recycling: a methodological approach.

Authors: Giuseppe Bonifazi et al.

Year: 2023

This paper might be accepted after a major revision of the manuscript.

Overall, this work is suitable and relevant to Sustainability. However, the manuscript is in an early stage, lacking a detailed discussion and requiring a significant quality improvement of the figures presented. Thus, to this reviewer, conclusions and statements still need further scientific support.

Some comments on this manuscript are given below.

1.     In general, the quality of all images within the manuscript, especially graphs, is poor. It is impossible to read the labels of the plots. Ironically, we are dealing with a manuscript describing image processing, but we have images that could be further processed or analyzed due to the low resolution.

2.     Please provide suitable references in the following paragraphs:

                                                              i.      Page 3, Line 106. "…HSI enables, inside a WEEE stream, the identification of plastic, the recognition of polymers, and the contaminant detection."

                                                           ii.      Page 6, Line 205. "…corresponding to the first overtone combination band of CH."

3.     There is statistical significance in all case studies. However, please provide support with a chemical rationale for the causes of such statistical significance. For instance, in section 3.1.2, authors find a differential spectral region around 1400-1600 nm, which is notorious in Figure 2. So, If the band is due to C-H overtone combination, we could infer that the "other" are inorganic materials since there are no C-H interactions. Could authors elaborate more on this? In this reviewer's opinion, this explanation is essential because later authors refer to a similar region (Figure 11) but now differentiate between materials with low and high Br content. What is the argument for using similar areas to distinguish different chemical compounds? What would happen if you get a non-plastic material with high content of Br?

4.     Same situation as above, what chemical rationale supports that you can use the analysis shown in Figure 6 to make distinctions between polymers?

5.     On page 9, Line 264, the authors need to provide more detail on how they carried out validation and calibration.

6.     The caption of Figure 8 is not clear.

7.     In section 3.3.2, authors stated that they use higher reflectance to identify contaminants, but could authors elaborate on how this can be affected by plastic additives that contain metals? Will these materials be recognized as contaminants? How was the system trained to deal with this kind of plastic?

8.     Figure 9 deserves further explanation. There are very well-defined spectral features. Chemically, what do we see in such spectra?

9.     There are two Figures 11.

10. Section 3.4.2 requires extensive discussion. There are several figures that the authors practically do not discuss.

11. Finally, it is also recommended to present the MicroXRF results and spectra (mapping if possible) referred to in Section 3.4.1 as supplementary material.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 4 Report

Dear Authors 

I appreciate that you addressed most of my comments. Still my concern is regarding the quality of the Figures. Figures, 5, 6, 9, 10, 12 and 13.

Please make an effort to improve the quality of such images.

Author Response

The authors wish to thank the Editor and the Reviewers for the revisions and the suggestions. The revised text is reported with the “Track Changes” function in the manuscript since the Figures 1, 2, 5, 6, 9, 10, 12 and 13 were modified as requested.  Please see the attachment.

Author Response File: Author Response.pdf