**4. Conclusions**

In the present work, magnetic halloysite nanotubes were successfully synthesized by three different approaches: co-precipitation, hydrothermal, and sol-gel method. The applied characterization techniques demonstrate that the nanometric-sized Fe3O4 (diameter of about 10 nm) were formed and connected to the HNT particles. Magnetic phase abundance depended on the synthetic route and was evaluated by EDS and TGA analyses, as well as by magnetization data. Thermodynamic and kinetic experiments suggested

that HNT/Fe3O4 composites can be considered as performing materials for ofloxacin adsorption. All the investigated samples were able to quantitatively reduce the antibiotic concentration under realistic conditions and, more interestingly, the sample obtained by the co-precipitation synthetic approach—the most cost-effective—was also easily magnetically removed from the media after treatment and reused for three cycles with no degradation. The ecotoxicity test performed on the freshwater organism *D. magna* completed the characterization of this adsorbent material and confirmed that it might be safely applied in water depuration processes.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano12234330/s1, Figure S1: SEM images of the commercial halloysite at (a) 9 kX and (b) 200 kX.; Figure S2: SEM images of the HNT/Fe3O4 composites. (a,b): HNT/Fe3O4-C sample; (c) and (d): HNT/Fe3O4-H sample; (e,f): HNT/Fe3O4-SG sample. Magnification: 9 kX (left) and 200 kX (right); Figure S3: (a) investigated area and distribution maps of (b) Al, (c) Fe and (d) Si elements of the HNT/Fe3O4-C sample; Figure S4: (a) investigated area and distribution maps of (b) Al, (c) Fe and (d) Si elements of the HNT/Fe3O4-H sample; Figure S5: (a) investigated area and distribution maps of (b) Al, (c) Fe and (d) Si elements of the HNT/Fe3O4-SG sample; Figure S6: X-ray diffraction pattern of the HNT/Fe3O4-C sample as-prepared (black line) and after three cycles of OFL recover (red line); Table S1: Mean particle size and intensity determined by DLS analysis; Table S2: Physico-chemical characterization of tap and river water samples, and WWTP effluent.

**Author Contributions:** Conceptualization, D.C., M.S. and F.M.; formal analysis, D.M.C., P.L. and F.M.; investigation, P.L., D.M.C., M.B., B.D.F., M.A., G.B., S.P. and F.M.; resources, D.C.; writing—original draft preparation, M.S., D.C., D.P. and M.P.; writing—review and editing, D.C., M.S., D.P. and M.P.; visualization, D.M.C., P.L., M.A. and B.D.F., Project administration, D.C. and M.S. All authors have read and agreed to the published version of the manuscript.

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

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors are grateful to Massimo Boiocchi for the support in the TEM analysis performed at the Centro Grandi Strumenti, University of Pavia, to Vittorio Berbenni for TGA, surface area and porosity measurements, and Alessandro Girella for the support in the HR-SEM analysis performed at the Arvedi Laboratory, CISRiC (Centro Interdipartimentale di Studi e Ricerche per la Conservazione del Patrimonio Culturale), University of Pavia.

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