The Performance of Carbonate-Modified Nonionic Surfactants in Microplastic Flotation

Round 1

Reviewer 1 Report

* The conditions of flotation tests should be given in more detail.

 * The knowledge of the parameters in the flotation tests and the effects of these parameters on the test results should be examined.

 * Adsorption analyzes should be given in detail.

Author Response

Reviewer #1

* Flotation conditions should be given in more detail.

We thank the reviewer for the comments and modified the manuscript in the corresponding sections.

We extended the description of the flotation conditions as follows:

The foam collected at the column outlet was stored for about 2 hours awaiting the collapse of the foam. The PET particles were filtered, washed, and dried. The weight was determined together with the filter paper with a known weight.

We also included the pore size of the sinter plate of 40 to100 mm in the description of the device in section 2.2.3.

 * The knowledge of the parameters in the flotation tests and the effects of these parameters on the test results should be examined.

For the discussion of the effects of flotation conditions, we referred to a previous study that we do not want to repeat here. We included the following sentence in section 2.2.4  and refer to the previous paper:

The parameters of the flotation tests in the applied device were optimized in a previous study. The effects of the operation conditions on the test results are discussed in detail in ref.16 . The optimal test conditions for an analysis of the performance of the surfactants were transferred to this study.

 * Adsorption analyzes should be given in detail.

We extended the paragraph on the adsorption analysis of the surfactant as follows (see section 3.2):

From the slope of the surface tension reduction as a function of the concentration, the headgroup area per surfactant molecule was determined via the analysis of Gibb’s isotherm, and the values are given in Table 3. For the CO₂-modified C₁₂ surfactants, the values are in the range of 0.55-1.07 nm², which are very well in agreement with the value of 0.66 nm² reported for pure C₁₂E₈ at 25 °C. Only for C₁₂(CO₂)_3.1EO_8.2OH a lower value of 0.55 nm² was determined, which indicates an essentially lower tendency for hydration with an increasing number of CO₂ units. This observation is supported by the investigation of the HLB values of these surfactants, where a CO₂ unit contributes less to the HLB value than an EO group. This reduced hydration of the surfactant head groups results in a closer packing of the molecules at interfaces allowing stronger interactions with hydrophobic materials, as will be seen in the following section. More details of the adsorption analysis are given in ref.23.

 

Reviewer 2 Report

Dear Authors,

here are my comments/editing notes

- the entire manuscript should be checked against the publisher's guidelines

- Fig. 1 curves A and B are not marked,

- all letter abbreviations used in the manuscript should be explained

- whether the publisher accepts the unit: mol/L or rather the concentration should be given in the unit: mol/dm3 - this and other units used should be verified

- chapter 2.2.3, should be redrafted, there are repetitions of information

- the time form used should be uniform throughout the manuscript

- Fig.3, shows the points which are experimental values. They should be connected by a theoretical curve or determined, e.g. by the method of least squares, experimental curve - to be completed

- Fig. 4 , reset the maximum values (or not?) for recovery and EF, if so, the axis descriptions should be corrected

- fig. 6 and fig. 7 and fig. 8 show correlations, but the equations of the curves and the correlation coefficient are not given - this should be supplemented

- there are no conclusions that will be the answer to the initial questions - they should be completed

Best Regards, 

reviewer

Author Response

Reviewer #2

We thank reviewer #2 for the helpful comments and respond to them as follows:

- the entire manuscript should be checked against the publisher's guidelines  Done

- Fig. 1 curves A and B are not marked,
Curve A and B are labeled now

- all letter abbreviations used in the manuscript should be explained 

All abbreviation are explained and spelled out where they are first mentioned in the text

- whether the publisher accepts the unit: mol/L or rather the concentration should be given in the unit: mol/dm3 - this and other units used should be verified

- chapter 2.2.3, should be redrafted, there are repetitions of information

Chapter 2.2.3 is rewritten. Essentially one redundant sentence was removed and others rephrased

- the time form used should be uniform throughout the manuscript

The time forms were checked and changed

- Fig.3, shows the points which are experimental values. They should be connected by a theoretical curve or determined, e.g. by the method of least squares, experimental curve - to be completed

A theoretical curve is included in Fig 3.

- Fig. 4, reset the maximum values (or not?) for recovery and EF, if so, the axis descriptions should be corrected

Fig. 4 has been re-scaled.

- fig. 6 and fig. 7 and fig. 8 show correlations, but the equations of the curves and the correlation coefficient are not given - this should be supplemented

The equations and correlation coefficients are included in the figures

- there are no conclusions that will be the answer to the initial questions - they should be completed

The last paragraph of the discussion is separated as a conclusion and extended by answers to the initial questions. The following sentences are included in the revision:

5. Conclusion

As a conclusion and answer to the initial questions, we can state that..

 

The diffusion coefficient of the surfactant monomer, the half-life time of the foam, and the contact angle of the surfactant solutions are important descriptors that control the performance of the flotation. Since these three parameters depend in different ways on the carbonate content of the surfactant headgroup, a compromise needs to be identified for every material. Due to the weaker impact of the contact angle on the removal, a priority can be given to the diffusivity and the foam stability, which both increase with an increasing number of CO₂ units per head group.

Round 2

Reviewer 2 Report

Dear Authors,

thank you for responding to my review comments. The changes made greatly improved the manuscript.

I have one more suggestion, namely: Fig.4 contains two graphs, which in their current form are small and in normal scale (without using magnification) they are difficult to read. I propose to split the graphs into two separate images and increase their size. All graphs in the manuscript should be of similar size. However, I leave it to the Authors' decision, because it is more convenient to compare the obtained results when the graphs are next to each other, although it is necessary to enlarge the text image.

Note: Equation 6 allows you to calculate a dimensionless value, while in the graphs, the EF factor is expressed as a percentage. It needs to be unified.

king regards

Reviewer #2

Author Response

Dear editors and referees,

thank you for the careful check of our manuscript. We agree that Fig. 4 shows room for improvement. We split it up into four graphs now and rescaled the axis for the enrichment factors. The unit of EF is also corrected. It is given as a dimensionless number.

Best regards
Reinhard Schomäcker