**5. Conclusions**

The spruce bark BBCs were produced using ZnCl2 and KOH as the activation agents. The BBC characteristics were strongly dependent on the type of activating agent. KOH-BBC had a higher SBET (1067.2 m<sup>2</sup> g<sup>−</sup>1) and a larger pore volume (0.5584 cm<sup>3</sup> g<sup>−</sup>1) than ZnCl2- BBC. However, the KOH-BBC had a more developed aromatic structure. KOH treatment generated a BBC with a more well-developed porosity and a higher number of mesopores than ZnCl2-BBC. Additionally, KOH-BBC had a less hydrophobic surface and a higher H and O content than ZnCl2-BBC. However, the carbon yield for KOH-activation was 63% lower than for ZnCl2-activation. For both dyes' adsorption on both BBCs, the general-order model and the Liu model exhibited the best fitness for adsorption kinetics and equilibrium, respectively. The equilibrium Qmax at 22 ◦C was for RO-16 on KOH-BBC and ZnCl2-BBC 354.8 and 90.1 mg g<sup>−</sup>1, respectively, and for RB-4 582.5 and 332.9 mg g<sup>−</sup>1. Based on characterization and experimental data, it was suggested that electrostatic interactions and hydrogen bonds between BBCs and RO-16 and RB-4 dyes played the most important role in the adsorption process. In an analysis of removing two synthetic effluents, both BBCs had good outcomes in the percentage; the BBC made with KOH had much better performances. We have shown that efficient and low-priced BBCs can be produced from Norway spruce bark through simple activation procedures. These results call for further studies on underlying mechanisms and how to optimize the treatment procedures for different applications.

**Author Contributions:** Conceptualization, G.S.d.R.; investigation G.S.d.R.; formal analysis, G.S.d.R. and T.N.P.; data curation, G.S.d.R.; writing—original draft preparation, G.S.d.R.; funding acquisition, S.H.L.; writing—review and editing, S.H.L., M.T., H.P.d.O., E.C.L. and G.L.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Treesearch Postdoctoral program, Bio4Energy—a Strategic Research Environment appointed by the Swedish government, and the Swedish University of Agricultural Sciences.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Lima thanks CNPq, CAPES, and FAPERGS for supporting his researches. The authors are grateful to ChemAxon for giving us an academic research license for the Marvin Sketch software, Version 21.3.0 (http://www.chemaxon.com), 2021 used for molecule physicalchemical properties.

**Conflicts of Interest:** The authors declare no conflict of interest.
