**4. Conclusions**

This paper represents for the first time the systematic concept of crosslinking epoxy-containing organosilane with branched PEI through a ring-opening reaction, and its further silanization onto silica-coated MNPs for copper removal applications through possible chelation mechanisms via deprotonated amino groups. The synthesized nanoadsorbent was composed of 13 nm-sized magnetic cores, uniformly covered with an approximately 3-nm-thick amorphous silica layer, and additionally silanized with GOPTS-derived bPEI. FTIR analysis revealed the presence of typical functional groups after silanization, as well as the XPS and TGA showed high content of N, suggesting an -NH2 rich adsorbent surface. Moreover, correlations between electrokinetic measurements and pH-potentiometric titrations were in high agreemen<sup>t</sup> with the proposed bonding mechanism. Additionally, the large magnetic response and precisely managed thickness of the surface layers showed quick removal after the finished adsorption process, which is highly appreciated for a successful adsorption process. In order to evaluate the efficiency of the introduced adsorbent, the effect of initial pH and the initial Cu2+ solution concentration were studied, and compared with respect to MNPs@SiO2. Results revealed the importance of surface modification, pH-dependency of the copper adsorption, and the monolayer coverage with large adsorption capacity. Moreover, after reusing the adsorbent, the latter shows high tendency towards Cu removal, even after several cycles. The proposed adsorption mechanism revealed major involvement of the NH/NH2 functional groups by the adsorption of copper by reducing its electronic state by donating free electron pairs.

To conclude, the developed amino-rich nanoadsorbent represents an efficient, environmentally friendly, stable and straightforwardly prepared adsorbent for metal removal, especially in more complicated environmental conditions, such as sludge.

**Author Contributions:** Conceptualization, O.P. and L.F.Z.; methodology, O.P., M.F., L.F.Z., S.G., U.A., I.B.; investigation, O.P.; writing—original draft preparation, O.P., M.F., L.F.Z.; writing—review and editing, O.P., M.F., S.G., U.A., I.B., L.F.Z.; supervision, L.F.Z.

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

**Acknowledgments:** The financial support of the Ministry of Education, Science and Sport of the Republic of Slovenia under Contract C3330-17-529004, and from the Slovenian Research Agency (Research Core Funding Nos. P2-0118, P2–0089 and P2-0032) is gratefully acknowledged. The authors also acknowledge the use of equipment in the Center of Excellence on Nanoscience and Nanotechnology—Nanocenter. We also acknowledge Dr. Matej Braˇciˇc, Anja Petek, Kristina Olovec and Dr. Alenka Vesel for their support regarding the analyses and laboratory work.

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