Design of a RGB-Arduino Device for Monitoring Copper Recovery from PCBs

Round 1

Reviewer 1 Report

In this work, the authors are proposing a color based approach for monitoring the Cu dissolution in bio-processes. I find this idea interesting however there are limitations to its application. I have the following comments:

1. The results seem promising for Cu dissolution in a clear environment with limited noise. The bio-solutions as in bioleaching usually tend to be turbid and would make it difficult to use this type of device. What are your views on this?

2. Most of the write-up is focused on engineering and design of the device. If there is a research component it would add value to highlight it in the text.

3. Bioleaching process usually have multiple metals in the system. There may be a Cobalt compound or other colorful compound in the system. How will it effect the method?

4. Could you please explain the differences in results in Figure 10 and Figure 11?

5. Minor English corrections are needed in the paper. E.g. instead of writing crashed PCBs it should be crushed.

Author Response

First of all, thank you for the opportunity to revise our manuscript. We really appreciate reviewers’ comments, in terms of opportune technical corrections and as clarifying observations. We have addressed their comments and we think that the paper has improved, and it is ready for publication. Please find a summary of our responses to the major points that the reviewer pointed out. Manuscript modifications, resulting from reviewers’ comments, are highlighted in blue. We also attach the certification that the whole manuscript has undergone English language editing by MDPI. The text has been checked for correct use of grammar and common technical terms, and edited to a level suitable for reporting research in a scholarly journal. MDPI uses experienced, native English speaking editors.

Reviewer 1

In this work, the authors are proposing a color based approach for monitoring the Cu dissolution in bio-processes. I find this idea interesting however there are limitations to its application. I have the following comments:

1. The results seem promising for Cu dissolution in a clear environment with limited noise. The bio-solutions as in bioleaching usually tend to be turbid and would make it difficult to use this type of device. What are your views on this?

In the multistep process where the approach is used, biomass, and the corresponding possible turbid associated, is not present because there is a biomass separation step between both operations, as it is indicated in Figure 1. To avoid confusions and clarify this aspect, description of the Figure 1 on page 3 has been rewritten as:

The typical method for biological Cu recovery consists of four steps (depicted in Figure 1), as previously described in [17] and [18]. The first stage consists of creating the leaching agent through the use of bacteria, which oxidizes Fe(II) to Fe(III) under controlled pH and dissolved oxygen concentration conditions in a fixed bed reactor. Acidithiobacillus ferrooxidans, which operates under extremely acidic conditions (e.g., pH of 1.5), is the most commonly utilized bacteria for oxidation of Fe(II) to Fe(III). In the second stage, biomass is recovered from Fe(III) solution produced to avoid the presence of microorganisms in the following steps. Subsequently, the leachate is brought into contact with the e-waste under aggressive acid conditions (pH < 1.5), guaranteeing optimal liquid--solid contact (it is worth noting that, at this point, the e-waste has been previously processed to decrease the size and favor the contact).

The final stage consists of selective recovery of the metal from the resulting solution. The resulting Fe(II) solution is then regenerated biologically and returned to the first stage to produce the Fe(III) needed to continue the process.

It is also noteworthy that the color based approach for monitoring the evolution of the chemical reaction was developed to obtain at real time the efficiency of recovery of copper from e-waste by means of the multi-step process where the biooxidation is the core. The multi-step process is detailed described in page 3. We agree that the title, incorporating the word "biorecovery” can be a little misleading since the approach is used in the chemical step after the biooxidation of the leaching agent occurs. Apart from that, the same approach can be used if the leaching agent has not been produced by the biological pathway. To avoid confusions, not to limiting other inclusive applications, and accordingly to reviewer comment, we have decided to change slightly the title to:

“Design of a RGB-Arduino device for monitoring copper recovery from PCBs”

In the manuscript, this observation has been also included in the Discussion and Conclusions (page 16): 

This same approach can be also used if the leaching agent has not been produced through a biological pathway, as the biological and chemical steps are separated and the described process guarantees that biomass is not present in the solution when put into contact with e-waste. 

2. Most of the write-up is focused on engineering and design of the device. If there is a research component it would add value to highlight it in the text.

We agree with the reviewer that the research component can be highlighted in the text. In this sense the manuscript has been modified in introduction section, specifically on page 4:

The work presented herein details the design of a low-cost device for real-time monitoring of the progress of the chemical reaction involved in the recovery of copper from e-waste through the use of microorganisms, both allowing for improvement of the process and providing a more sustainable approach than conventional ones. The device is both hardware- and software-based: the hardware consists of a color sensor and an Arduino, which carries out pre-processing and transmits the data to a computer. The use of such a system, which allows for data acquisition, signal processing, and color monitoring in real-time, provides a potential tool for use as a research component to improve knowledge regarding the use of bioprocesses for the recovery of metals from e-waste; furthermore, this approach is also applicable to other hydrometallurgic process based on the solubilization of metals through direct contact between e-waste and leaching agents.

It has been also added in discussion and conclusion (page 15):

The designed algorithm could satisfactorily predict the concentrations of both ions, in comparison with the results obtained experimentally, supporting the approach presented in this study. Thus, the proposed approach provides a potential tool for use as a research component to improve knowledge relevant to the use of bioprocesses for the recovery of metals from e-waste, as the amount of data supplied can be notably increased when compared with traditional analytical methods.

3. Bioleaching processes usually have multiple metals in the system. There may be a Cobalt compound or other colorful compound in the system. How will it affect the method?

It is completely true that the presence of other metals that also give color to the solution could affect the measurement. It is for this reason that the procedure contemplates a real solution with all the components that are solubilized when the leaching solution is put in contact with e-waste. To underline that the presence of other metals is also contemplated in the validation of this approach, the chemical characterization of the solution has been added to the manuscript. Al, Li, Co, Ni and Mn have been analyzed by atomic absorption in the same solution where the color sensor device was tested. Al, Li and Co were not detected but the presence of Mn (10 ppm) and Ni (200 ppm) does not disturbed determinations from the proposed system.

In this sense the following clarifications has been added in the Discussion section: “It is noteworthy that the presence of other metals contained in the solution (10 ppm Mn and 200 ppm Ni) does not affect the correspondence with the Cu (II) and Fe(III) concentrations determined by means of the RGB sensor.”

4. Could you please explain the differences in results in Figure 10 and Figure 11?

There is a typographic error in the description capture of the Figure 10. Instead of “Ions concentration analyzed (blue-cross dots) and estimated (red continuous line) with bounded uncertainty (colored red area) using the experiment exp-EC”, it has to indicate “Ions concentration analyzed (blue-cross dots) and estimated (red continuous line) with bounded uncertainty (colored red area) using the experiment metallic copper”. Results for electric cable are shown correctly in Figure 11; this is the difference between both figures. 

On page 12 it is expressed that: “The regression coefficients of equations 7 and 8 were estimated using two experiments  with metallic copper. [...] The Figures 10a and 10b compare the concentrations of Fe(III) and Cu(II) ions (blue-cross dots), respectively, with the simulation results obtained using the estimated coefficients. The red continuous line was computed using the mean value, while the red colored area delimits the lower and upper confidence intervals.”

Moreover, the reference to the figures on page 13 has been corrected: “The results for phone boards are shown in Figures 12a to 12d. Figure 12a shows RGB raw data and Figure 12b shows the corrected data with color emulation.”

5. Minor English corrections are needed in the paper. E.g. instead of writing crashed PCBs it should be crushed.

The authors appreciate the typographical errors detected by the reviewer. The document has been revised again to solve these types of mistakes. The whole manuscript has been sent to Language Editing Services of MPDI to correct errors related with the language. The whole manuscript has undergone English language editing by MDPI. The text has been checked for correct use of grammar and common technical terms, and edited to a level suitable for reporting research in a scholarly journal. MDPI uses experienced, native English speaking editors.

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript can benefit by addressing the following concerns/suggestions:

1. There are misspelled words (e.g., "contais" in line 393, "(p/v)" in page 5). There are incomplete/disorganized sentences (e.g., ""..chelating [27]" in page 5, lines 69 - 72). And there are grammar issues (e.g., lines 153, 238). The manuscript must be revised to address these issues.

2. The title is a little misleading. Although the justification involves biorecovery (oxidation of Fe(II)), the actual work only involved chemical extraction/reaction. The title should be revised to reflect the actual scope of the work.

3. The authors mentioned that their device can monitor the color (and thus the metal concentrations) with high precision. However, their results indicate that this was not the case.

4. I believe that the responses and correlations can be significantly improved if background signals were collected and used as baseline/blank for relating signal and concentrations. This is much similar to the procedure presented in lines 255-256 but using actual reactants (without the addition of Fe(III)).

Author Response

First of all, thank you for the opportunity to revise our manuscript. We really appreciate reviewers’ comments, in terms of opportune technical corrections and as clarifying observations. We have addressed their comments and we think that the paper has improved, and it is ready for publication. Please find a summary of our responses to the major points that the reviewer pointed out. Manuscript modifications, resulting from reviewers’ comments, are highlighted in blue. We also attach the certification that the whole manuscript has undergone English language editing by MDPI. The text has been checked for correct use of grammar and common technical terms, and edited to a level suitable for reporting research in a scholarly journal. MDPI uses experienced, native English speaking editors.

Reviewer 2

This manuscript can benefit by addressing the following concerns/suggestions:

1. There are misspelled words (e.g., "contais" in line 393, "(p/v)" in page 5). There are incomplete/disorganized sentences (e.g., ""..chelating [27]" in page 5, lines 69 - 72). And there are grammar issues (e.g., lines 153, 238). The manuscript must be revised to address these issues.

The authors appreciate the typographical and grammar errors detected by the reviewer. The mentioned sentences have been rewritten:

In this work, Fe(III) concentrations were measured using a UV-Vis spectrometer (Lambda 25, PerkinElmer, United States) following the standard colorimetric method that uses salicylic acid as a chelating [27].

Among these elements, copper is one of the most valuable metals to recover, due to the high demand for it in the fabrication of new electronic devices and components [11].

The whole document has been revised again to solve these types of mistakes. The whole manuscript has been sent to Language Editing Services of MPDI to correct errors related with the language. The whole manuscript has undergone English language editing by MDPI. The text has been checked for correct use of grammar and common technical terms, and edited to a level suitable for reporting research in a scholarly journal. MDPI uses experienced, native English speaking editors.

2. The title is a little misleading. Although the justification involves biorecovery (oxidation of Fe(II)), the actual work only involved chemical extraction/reaction. The title should be revised to reflect the actual scope of the work.

The color based approach for monitoring the evolution of the chemical reaction was developed to obtain at real time the efficiency of recovery of copper from e-waste by means of a multi-step process where the biooxidation is the core. The multi-step process is detailed described in page 3. We agree that the title can be a little misleading since the approach is used in the chemical step after the biooxidation of the leaching agent occurs. Moreover, the same approach can be used if the leaching agent has not been produced by the biological pathway. To avoid confusions, not to limiting other inclusive applications, and accordingly to reviewer comment, we have decided to change slightly the title to:

“Design of a RGB-Arduino device for monitoring copper recovery from PCBs”

In the manuscript, this observation has been included in the Discussion and Conclusions section (page 16):

This same approach can be also used if the leaching agent has not been produced through a biological pathway, as the biological and chemical steps are separated and the described process guarantees that biomass is not present in the solution when put into contact with e-waste.

3. The authors mentioned that their device can monitor the color (and thus the metal concentrations) with high precision. However, their results indicate that this was not the case.

We agree that the expression “high precision” written at the end of the introduction section is not suitable for the accuracy of the results obtained. However, the correspondence obtained between predictions and experimental data is suitable for the purpose of this approach, the monitoring of the evolution of the reaction to develop strategies of control decisions. According to this comment, authors have changed the sentence in page 4:

The work presented herein details the design of a low-cost device for real-time monitoring of the progress of the chemical reaction involved in the recovery of copper from e-waste through the use of microorganisms, both allowing for improvement of the process and providing a more sustainable approach than conventional ones. The device is both hardware- and software-based: the hardware consists of a color sensor and an Arduino, which carries out pre-processing and transmits the data to a computer. The use of such a system, which allows for data acquisition, signal processing, and color monitoring in real-time, provides a potential tool for use as a research component to improve knowledge regarding the use of bioprocesses for the recovery of metals from e-waste; furthermore, this approach is also applicable to other hydrometallurgic process based on the solubilization of metals through direct contact between e-waste and leaching agents.  

4. I believe that the responses and correlations can be significantly improved if background signals were collected and used as baseline/blank for relating signal and concentrations. This is much similar to the procedure presented in lines 255-256 but using actual reactants (without the addition of Fe(III)).

The sign obtained from the light coming from the LED lights is reflected on the color liquid but also on the vessel surface, and also due to the intensity of the light emitted by the LED. The methodology proposed in the present work contemplates the use of the background signals before the addition of Fe(III). For measuring the light reflected by the vessel and also the LED intensity, an experiment without reagents was conducted. This experiment was used for the determination of three weighting factors (equation 6), allowing to adjust the computation of the normalized RGB values. These weighting factors have to be updated when the location of the device changes. Figure 8 evinces the improvement of correspondence between model predictions and experimental results when this correction is performed. It is also noteworthy that the evolution of the reaction is monitored as a relative change of RGB signals from the beginning of the reaction obtaining a good correspondence between concentrations and signals until the total solubilization of copper. 

 

 

Round 2

Reviewer 2 Report

The current version of the manuscript is suitable for publication.