**4. Conclusions**

The conclusions that can be drawn from the present study are: (a) Biochar magnetization after oxidation results in significantly higher adsorption capacities due to the synergistic effect of the carboxylic moieties, which favor iron oxide deposition on the biochar surface and complex Cu(II) ions even at low pH (pH < 4); (b) The adsorption process follows the pseudo-second order kinetics; and (c) FTIR measurements denote the formation of inner-sphere complexes and XRD data show Cu(II) surface precipitation at pH 6 and increased Cu(II) concentration. Generally, magnetisation of biochar results in better adsorption capacity and desired properties regarding magnetic-based separation.

**Author Contributions:** Conceptualization, I.P.; Data curation, E.N., K.P. and I.P.; Formal analysis, E.N., K.P. and I.P.; Investigation, E.N. and K.P.; Project administration, K.P. and I.P.; Supervision, I.P.; Validation, K.P. and I.P.; Visualization, E.N.; Writing—original draft, I.P.; Writing—review & editing, K.P. and I.A.

**Funding:** This research received no external funding.

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