Sustainable Recovery of Secondary and Critical Raw Materials from Classified Mining Residues Using Mycorrhizal-Assisted Phytoextraction

Round 1

Reviewer 1 Report

The present work is very interesting and well written. The aim of the research is of growing interest today. The experiments have been well chosen, and the conclusions are well summarized. I recommend the work for publication in metals journal.

Author Response

Tables 1a and 1b were modified for a better understanding of results.

Reviewer 2 Report

A document that addresses a very interesting topic for remediation. Opens a possibility to recover various metals. A very clear methodology and consistent results. 

Line 48. What do you mean by means? Media?
Line 58. A new
Line 121. Use past tense and throughout the document
Line 126. Were collected
Line 132. Figure 2 is not clear. Please use a better image.
The authors propose (in point 4) hydrometallurgical techniques for the recovery of SRMs and CRMs. Can the authors explain the various techniques? Are these techniques according to the% presence of the elements?

Author Response

Line 48. What do you mean by means? Media?

Answer: the wording was changed

- Line 58. A new

Answer: the wording was changed

- Line 121. Use past tense and throughout the document

Answer: the wording was changed

- Line 126. Were collected

Answer: the wording was changed

- Line 132. Figure 2 is not clear. Please use a better image.

Answer: Figure 2 was corrected

- The authors propose (in point 4) hydrometallurgical techniques for the recovery of SRMs and CRMs. Can the authors explain the various techniques? Are these techniques according to the% presence of the elements?

Answer: The text in the manuscript was modified as follows:

‘Recovery of SRMs and CRMs from biomass through hydrometallurgy techniques. The leaching extraction is selective and allows the recovery of all the contained elements because specific reagents are used for each of them, applying innovative processes (for example, thiourea process, thiosulfate process, etc.). The metals are purified to a high degree of purity by electrochemical methods.’

Reviewer 3 Report

Sustainable recovery of secondary and critical raw materials from classified mining residues using mycorrhizal-assisted phytoextraction is very interesting paper. Some changes are required.

Line 24, 25: Selected mine residues were treated by MAP at laboratory scale, and the fate of several SRMs (which type of secondary resources) and CRMs such as
Zn, Cr, As, Ni, Cu, Rb, Al, Ba, K, S, Ca, Ga, Ti, P and Sr was investigated (please to write correctly which critical elements; for example Ni is not critical element)

Line 81: CRMs as Ga, Sr, Ti [7,8,1]. Why not cobalt?

 Line 212: Without causing metals leaching (which metals?). 

Line 340. The MAP system showed good extraction capabilities for the SRMs and CRMs under study despite the low biomass obtained at TRL-2 when grown in mining waste substrate.  In which time are good extraction  capabilities?

Conclusion:

Line 474, 475: It is important to monitor pH and Eh (BS ISO 11271:2002) val
ues at various depths of soil, in order to relate the solubilization of the elements with their bioaccumulation. 

What are expected pH and Eh-values? Can you predict it using Porbaix diagrams:

It is also important to monitor temperature in different depths soil?

General question:

What is a selectivity and kinetics of leaching process for Sustainable recovery of secondary and critical raw materials ?

 

Author Response

Line 24, 25: Selected mine residues were treated by MAP at laboratory scale, and the fate of several SRMs (which type of secondary resources) and CRMs such as Zn, Cr, As, Ni, Cu, Rb, Al, Ba, K, S, Ca, Ga, Ti, P and Sr was investigated (please to write correctly which critical elements; for example Ni is not critical element)

Answer: The text in the manuscript was modified

- Line 81: CRMs as Ga, Sr, Ti [7,8,1]. Why not cobalt?

Answer: We did not detect cobalt in the mine soil

- Line 212: Without causing metals leaching (which metals?).

Answer: The text was modified. It refers to not causing carryover / leaching of any of the metals under study in this work.

- Line 340. The MAP system showed good extraction capabilities for the SRMs and CRMs under study despite the low biomass obtained at TRL-2 when grown in mining waste substrate. In which time are good extraction capabilities?

Answer: We consider good extraction when we can obtain the elements in biomass and their TF values are close to or greater than 1. The projection in 1 first cycle of industrial scale (VDM) gives us interesting values to replicate in series and have a commercial impact on the recovery.

- Conclusion: Line 474, 475: It is important to monitor pH and Eh (BS ISO 11271:2002) values at various depths of soil, in order to relate the solubilization of the elements with their bioaccumulation.

What are expected pH and Eh-values? Can you predict it using Porbaix diagrams:

It is also important to monitor temperature in different depths soil?

Answer:

In this work, with the knowledge and instruments used, it is not possible for us to speak of “expected values” of pH and Eh at various depth. The Porbaix diagrams will be of great help to predict the solubilization of the elements, but the depth, the temperature and the interactions between microorganisms and the different elements present must be taken into account. Arbuscular mycorrhizae usually reduce pH and Eh values, and other soil microorganisms play an important role in oxidation-reduction reactions and, therefore, changes the Eh. We think a temperature-dependent "Eh-pH gradient" in depth soil could occurred, but it should be tested. On the other hand, Porbaix diagrams refer to a single element and we have a set of them in our system. Undoubtedly, it will be very interesting to check the comparison between prediction and observed values.

- General question: What is a selectivity and kinetics of leaching process for Sustainable recovery of secondary and critical raw materials?

Answer: For a sustainable recovery of metals from an economic and environmental point of view, the application of hydrometallurgical processes is important, because they allow, through the use of specific innovative reagents and the application of electrochemical techniques, their selective recovery at high degree of purity, which determines an appropriate commercial reuse.

The fast kinetics guarantees the economy of the process because it allows to lower energy costs (therefore environmental) and also of the plant, because smaller capacity reactors can be used.