## 4 Conclusions  
Carbonate deposition from carbonated water at atmospheric pressure and room temperature can be a safe and technically simple technique highly efficient for surface modification of biomedical Mg and alloys by rapid deposition of carbonate clusters and nanoparticles in non-equilibrium conditions from a saturated carbonated aqueous solution containing calcium ions.  
The coating of Mg by a compact crystalline layer of magnesium calcite has been presented. The mechanism of calcite formation involves the rapid deposition of carbonates groups that substitute hydroxyl groups on the surface of Mg, a key step for the formation of the coating, as non-soluble hydroxycarbonates inhibit the formation of Mg(OH)\({}_{2}\) at the Mg surface, impeding the growth of a hydroxide layer resulting from the natural oxidation of Mg in water, hydroxide anions being replaced rapidly by carbonate groups at the oxidized metal surface. We have named this stage surface conversion to hydroxycarbonates. The substitution of soluble hydroxides with insoluble carbonates is promoted by the increased pH associated with the oxidation of the Mg surface in an aqueous solution.  
Carbonatation or deposition of a surface layer of amorphous calcium carbonate nanoparticles from the solution, due to reduced solubility when gaseous CO\({}_{2}\)(g) is released, takes place after approximately 15 min of immersion in carbonated water. After 30 min of immersion, crystallization of calcite is demonstrated and growth by nanoparticle attachment due to the incorporation of amorphous carbonate from the solution, which seems to be the precursor for the crystallization of calcite, leading to the formation of a continuous layer of euhedral calcite crystals that cover the whole disk surface after one hour of immersion. SEM and AFM observations confirm that the calcite coating is continuous and that lateral growth of the calcite nuclei leads to bi-dimensional coalescence to a polycrystalline coating layer. The full surface coverage of the disk, by the formation of a dense coating of intersecting crystals, results in a characteristic texture of crystalline steps between neighboring calcite crystals that confers hydrophobicity to the calcite coating.  
We conclude that carbonate films grown by green chemistry on magnesium have a high potential to be used as a non-toxic protective barrier to modulate the physico-chemical properties of the magnesium surface, for example, in the physiological environment, with possible application in biodegradable implants, in where biocompatibility assessment is required.  
Conceptualization and methodology J.M.C.M. and M.P.; formal analysis and investigation, J.M.C.M., M.P., M.A. and I.C.G.; writing--original draft preparation, review and editing J.M.C.M., M.A. and M.P.; project administration and funding acquisition, J.M.C.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Executive Unit for the Financing of Higher Education, Research, Development, and Innovation (UEFISCDI) Exploratory Research Project PN-III-P4-ID-PCE-2020-0992 (PCE-234/2021).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The paper was carried out within the research program \"Science of Surfaces and Thin Layers\" of the \"[PERSON]\" Institute of Physical Chemistry. The support of the Romanian Government that allowed for the acquisition of the research infrastructure under POSCCE O 2.2.1 project INFRANANANOCHEM--No. 19/01.03.2009 9 is gratefully acknowledged.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.