**1. Introduction**

In the current scenario of water shortage, there is an urgen<sup>t</sup> need to favor water loops. For this purpose, preserving and guaranteeing water quality is mandatory, as reclaimed water can be directly reused and re-enter natural water bodies [1]. A critical aspect of water quality is represented by xenobiotics, such as heavy metals, dyes, pesticides, etc., detected in natural water bodies, also at trace levels, because of their recalcitrance in conventional wastewater treatment plants (WWTPs) [2]. In particular, pharmaceuticals and

**Citation:** Capsoni, D.; Lucini, P.; Conti, D.M.; Bianchi, M.; Maraschi, F.; De Felice, B.; Bruni, G.; Abdolrahimi, M.; Peddis, D.; Parolini, M.; et al. Fe3O4-Halloysite Nanotube Composites as Sustainable Adsorbents: Efficiency in Ofloxacin Removal from Polluted Waters and Ecotoxicity. *Nanomaterials* **2022**, *12*, 4330. https://doi.org/10.3390/ nano12234330

Academic Editor: Vivian Hsiu-Chuan Liao

Received: 4 November 2022 Accepted: 2 December 2022 Published: 6 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

personal care products (PPCPs) have attracted the attention of the scientific community and civil society because of their widespread diffusion in the environment and their potential toxicity towards humans and ecosystems [3,4]. Although the current levels of PPCPs in aquatic ecosystems can be considered as low, they pose a severe threat for aquatic organisms because of their high biological activity and peculiar mechanism(s) of toxic action [4]. Among PPCPs, a remarkable concern is due to antibiotics whose presence in water ecosystems has been identified to affect natural microbial communities and to stimulate multi-resistant bacteria and antibiotic resistance genes, which pose serious risks to human and veterinary health [4,5]. To tackle the rising threats induced by the release of antibiotics, a recent action plan has proposed developing innovative strategies to reduce the diffusion of these emerging contaminants [6]. Over the last years, many research efforts have been made to develop sustainable and low-cost processes, easily implementable to conventional WWTPs and efficient in antibiotic removal from wastewater [7]. In this context, adsorption is a convenient method in terms of low energy consumption, reuse of the adsorbent material, no production of toxic by-products, and reduced waste production after treatment [8,9].

Many materials, both bare and functionalized, have been tested for water and wastewater decontamination, including activated carbon, nanomaterials, biopolymers, clays, agriculture and industrial wastes, and other natural sorbents [10–13].

The use of natural sorbents in the adsorption process [14–16] offers even more advantages, as they are abundant, low-cost, non-toxic, easy to modify, and competitive in water remediation compared to most conventional adsorbents [8,12,14].

Nanoclay materials surely fit the advantages mentioned above as sorbents to remove various pollutants, such as heavy metals, pesticides and antibiotics [17,18]. Among nanoclays, those displaying a tubular structure are even more intriguing, due to their additional properties related to the nanoscale dimension, cylindrical hollow form, and porosity. The halloysite nanotubes (HNTs) pertain to these nanoclays. Halloysites are aluminosilicates belonging to the kaolin group, with the chemical formula Al2Si2O5(OH)4 · *n* H2O. Two halloysite forms are reported in the literature, depending on the moles of hydrating molecules and the *d001* basal spacing: halloysite-(10 Å) is the di-hydrated form [19], and halloysite- (7 Å) is the anhydrous one. The latter form is the most common, due to the easy release of the halloysite water molecules at ambient conditions [20]. The halloysite structure is based on corner-sharing SiO4 tetrahedra sheets connected via oxygens to edge-sharing AlO6 octahedra ones [21,22]. The mismatch of the larger SiO4 tetrahedra and the smaller AlO6 octahedra accounts for the local stress on the atomic scale of the aluminosilicate layer, inducing its wrapping and the nanotubes' morphology [22]. The nanotube typically displays lengths of 0.4–1 μm, an outer diameter of 20–200 nm, and an inner lumen diameter of 10–70 nm [23]. The siloxane (Si-O-Si) groups form the negatively charged outer surface, and the aluminol groups (-OH and Al-OH) form the positively charged inner one [24,25]. The peculiar physical and chemical features reported above make the HNTs suitable candidates for applications in various fields, including controlled drug release, nanotemplating, and adsorption. They are also employed as catalyst support and nanocomposites [26].

It is well known that the separation of the nanosorbent phase after pollutants removal is not a trivial challenge. A feasible and low-cost approach is to decorate the adsorbent material with magnetic nanoparticles to make it easily magnetically recovered. Some examples on the synthesis of halloysite–magnetite composites by co-precipitation, thermal decomposition, and solvothermal approaches are reported in the literature [27–30], and these materials are not ye<sup>t</sup> investigated for water depollution.

Another key point to optimize before the application of nanomaterials in water remediation processes concerns the investigation of potential environmental and human risks associated with their use. The characterization of nanomaterials should have to include not only the assessment of any transformation occurring in environmental media, from its inclusion into a polluted site to the removal (or degradation) after the remediation of the targe<sup>t</sup> pollutant [31], but also the potential toxicity towards aquatic organisms. Ecotoxicology

can provide useful tools to assess the risk related to nanomaterials and to select eco-friendly and sustainable ones for water remediation [32,33]. The application of standard and/or novel ecotoxicological tests completes the characterization of nanomaterials through the identification of possible toxicological targets and sheds light on the mechanism(s) of toxic action in aquatic species at different levels of the ecological hierarchy [34].

In the present study, we synthesized HNT/Fe3O4 nanocomposites by using three different approaches: co-precipitation, sol-gel, and hydrothermal. Each material was characterized by FT-IR spectroscopy, X-ray powder diffraction (XRPD), scanning electron and transmission electron microscopy (SEM and TEM), energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), and magnetization measurements. Moreover, the magnetite and halloysite amount in each sample was evaluated by EDS, TGA, and magnetization data. Lastly, potential ecotoxicity of these materials towards aquatic organisms was tested on the freshwater Cladoceran *Daphnia magna* according to the *Daphnia* sp. Acute Immobilization Test, OECD 202 guideline (OECD, 2004). Adsorption properties and mechanism of each nanocomposite were investigated, and compared with the commercial halloysite. The antibiotic ofloxacin (OFL) was chosen as the target molecule to assess the adsorption efficiency of HNT/Fe3O4 nanocomposites for different reasons: (i) it is a very useful antibacterial agen<sup>t</sup> belonging to the last class of antibiotics; (ii) it is largely detected in wastewaters and surface waters [3]; (iii) it is a recalcitrant to biological degradation [35]; (iv) it maintains a certain antibacterial activity after the first steps of its degradation [35]; (v) it has been used in our previous studies regarding both fluoroquinolones' environmental fate and their removal by adsorption processes [36–41]. The suitability of three materials for OFL removal under environmental conditions, i.e., tap and river waters, and wastewater treatment plant (WWTP) effluent, was also verified.
