## 4 Conclusions  
Biogenic calcium phosphate has been obtained from fish discards, giving them an additional application as a cheap raw material that does not have any negative effect on the environment. The powders obtained consisted of a biphasic material whose major component was B-type carbonated hydroxyapatite and a minor amount of beta-tricalcium phosphate.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{2}{c}{**Calcination**} & \multirow{2}{*}{**a (Å)**} & \multirow{2}{*}{**c (Å)**} & \multirow{2}{*}{**c/a**} & \multicolumn{2}{c}{**Unit Cell**} & \multirow{2}{*}{**Crystallite**} \\  & **Temperature (\({}^{\circ}\)C)** & & & & & **Volume (Å \({}^{3}\))** & **Size (nm)** & **\%CO\({}_{3}\)** \\ \hline Scorp & 750 & 9.427 & 6.899 & 0.732 & 530.95 & 50 \(\pm\) 8 & 2.37 \\ \hline Trach & 750 & 9.427 & 6.899 & 0.732 & 530.95 & 50 \(\pm\) 8 & 2.37 \\ \hline Salmo & 750 & 9.420 & 6.892 & 0.732 & 529.63 & 35 \(\pm\) 4 & 2.08 \\ \hline Scorp & 950 & 9.427 & 6.899 & 0.732 & 530.95 & 59 \(\pm\) 10 & 2.37 \\ \hline Trach & 950 & 9.427 & 6.899 & 0.732 & 530.95 & 59 \(\pm\) 10 & 2.37 \\ \hline Salmo & 950 & 9.405 & 6.879 & 0.731 & 527.02 & 59 \(\pm\) 10 & 1.49 \\ \hline Synthetic HA & - & 9.418 & 6.884 & 0.731 & 527.80 & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 5: Cell parameters, unit cell volume, crystallite size and carbonate content of powders obtained from fish bones at different calcination temperatures.  
The amount of \(\beta\)-TCP within the biphasic HA-TCP biphasic material was increased by turning up the temperature of the thermal treatment from 750 \({}^{\circ}\)C to 950 \({}^{\circ}\)C. Ions of Na, Mg, Sr, and K were also detected, replacing Ca ions and slightly modifying the unit cell parameters with respect to the synthetic hydroxyapatite ones. Additionally, the concentration of heavy metals (Hg, Cd, and Pb) in the obtained powders was below the maximum permitted by the ASTM for organic bone implant materials.  
The biphasic HA-TCP material has a promising range of applications in the biomedical field based on its great similarity to calcium phosphates found in human bones in terms of crystallite size and carbonate content. The presence of Na, Mg, Sr, and K ions is usual in biological apatites, and is very beneficial, since they contribute to bone metabolism and cell adhesion.  
M.F.-A.: formal analysis, investigation, writing--original draft preparation; I.A.-O.: formal analysis, investigation, writing--original draft preparation; P.M.-F.: formal analysis, investigation, writing--original draft preparation; J.A.V.: conceptualization, resources, writing--review and editing, supervision, funding acquisition; M.B.: conceptualization, methodology, data curation, writing--review and editing; R.C.: validation, formal analysis; J.P.: conceptualization, writing--review and editing, supervision, funding acquisition. All authors have read and agreed to the published version of the manuscript.  
This work was partially supported by the EU research project Bluehuman (EAPA_151/2016 Interreg Atlantic Area), and by Xunta de Galicia (ED431C 2019/23, ED481D 2017/010, ED481B 2016/047-0). REVAL-Lab also wants to thank to GAIN (EU, Horizon 2020 Framework Research and Innovation Programme under GA n. 773330) and Xunta de Galicia (Grupos de Potencial Crecimiento, IN607B 2018/19) for the financial support of this investigation.  
The technical staff from CACTI (University of Vigo) 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.