*Article* **Fe3O4-Halloysite Nanotube Composites as Sustainable Adsorbents: Efficiency in Ofloxacin Removal from Polluted Waters and Ecotoxicity**

**Doretta Capsoni 1,2, Paola Lucini 1,2, Debora Maria Conti 1,2, Michela Bianchi 1, Federica Maraschi 1, Beatrice De Felice 3, Giovanna Bruni 1,2, Maryam Abdolrahimi 4,5, Davide Peddis 4,6, Marco Parolini 3, Silvia Pisani 7 and Michela Sturini 1,2,\***

	- Department of Environmental Science and Policy, University of Milan, 20133 Milan, Italy
	- 7 Department of Otorhinolaryngology, Fondazione IRCCS Policlinico San Matteo, 27100 Pavia, Italy
	- Correspondence: michela.sturini@unipv.it; Tel.: +39-0382-987347

**Abstract:** The present work aimed at decorating halloysite nanotubes (HNT) with magnetic Fe3O4 nanoparticles through different synthetic routes (co-precipitation, hydrothermal, and sol-gel) to test the efficiency of three magnetic composites (HNT/Fe3O4) to remove the antibiotic ofloxacin (OFL) from waters. The chemical–physical features of the obtained materials were characterized through the application of diverse techniques (XRPD, FT-IR spectroscopy, SEM, EDS, and TEM microscopy, thermogravimetric analysis, and magnetization measurements), while ecotoxicity was assessed through a standard test on the freshwater organism *Daphnia magna*. Independently of the synthesis procedure, the magnetic composites were successfully obtained. The Fe3O4 is nanometric (about 10 nm) and the weight percentage is sample-dependent. It decorates the HNT's surface and also forms aggregates linking the nanotubes in Fe3O4-rich samples. Thermodynamic and kinetic experiments showed different adsorption capacities of OFL, ranging from 23 to 45 mg g<sup>−</sup>1. The kinetic process occurred within a few minutes, independently of the composite. The capability of the three HNT/Fe3O4 in removing the OFL was confirmed under realistic conditions, when OFL was added to tap, river, and effluent waters at μg L−<sup>1</sup> concentration. No acute toxicity of the composites was observed on freshwater organisms. Despite the good results obtained for all the composites, the sample by co-precipitation is the most performant as it: (i) is easily magnetically separated from the media after the use; (ii) does not undergo any degradation after three adsorption cycles; (iii) is synthetized through a low-cost procedure. These features make this material an excellent candidate for removal of OFL from water.

**Keywords:** magnetite-halloysite composites; magnetic sorbent materials; fluoroquinolone antibiotic; adsorption; wastewater treatment; magnetic remediation; emerging contaminants; ecotoxicity
