**3. Results and Discussion**

First, structure, morphology, composition, magnetic behavior, adsorption capacity, and adsorption kinetics of the magnetic HNT composites and the commercial HNT were investigated. Then the materials were tested under environmental conditions to remove the antibiotic OFL chosen as being representative of emerging contaminants. In addition, their potential ecotoxic effects, along with reusability, were evaluated.

### *3.1. Morphological, Structural, and Magnetic Characterization*

Figure 1a shows the XRPD pattern of the commercial halloysite. It compares to those reported in the literature [27,30,43,44] and deposited in JCPDS database (PDF# 028-1487). The peak detected at about 12◦ corresponds to the *d001* basal spacing of 7.35 Å, peculiar of the anhydrous form (halloysite-(7 Å)). The (002) reflection is observed at about 24◦. The peaks at 20◦ and 62.8◦ are typical of halloysites with nanotubular morphology [44,45]. No peaks are detected at about 8.8◦, assigned to the *d001* basal spacing of the di-hydrated halloysite (halloysite-(10 Å)). This is consistent with the easy loss of the interlayer water molecules near room temperature [46]. The very sharp reflections observed at 10.1, 26.6, and 27.3◦ are attributed respectively to the small amount of kaolinite 1A (PDF# 074-1786), quartz (PDF# 046-1045), and rutile (PDF# 021-1276); these impurity phases are often detected in halloysite clay minerals.

**Figure 1.** XRPD patterns of (**a**) commercial HNT, (**b**) Fe3O4, and (**c**) HNT/Fe3O4 composites, Fe3O4 and HNT.

Figure 1b displays the XRPD pattern of the Fe3O4 samples obtained by the three synthetic routes. The 2-theta reflection positions fairly agree with those expected for the magnetite structure (PDF# 088-0315). The iron oxide phase has been successfully synthesized, and no impurity phases are detected within the detection limit of the technique. The iron oxide samples are nanocrystalline: a crystallite size of 10, 13, and 8 nm was calculated for Fe3O4-C, Fe3O4-SG, and Fe3O4-H samples by applying the Scherrer equation to the 311 reflection.

Figure 1c displays the diffraction pattern of the magnetite–halloysite composites. The diffraction patterns of the commercial halloysite and the Fe3O4-C sample (chosen as reference for the magnetic phase), are also shown for comparison. The three composite samples display the peaks of both the magnetite and the halloysite phases, thus confirming the successful formation of the magnetite–halloysite adduct. An investigation of the magnetite crystallite size in the composites by applying the Scherrer equation could not be carried out, due to the strong overlap of the 311 reflection of the magnetite phase to the broad peaks of halloysite in the 33–40◦ 2 theta range. Nonetheless, the comparable peaks broadening of the magnetite phase in the composites and in the Fe3O4 samples suggests nanocrystalline magnetite is obtained also in the HNT/Fe3O4 samples. The peaks' intensity of halloysite and magnetite in the composite samples returns an idea on the phases amount in each sample. The peaks' intensity of halloysite decreases and Fe3O4 increases progressively from HNT/Fe3O4-SG to HNT/Fe3O4-H and HNT/Fe3O4-C, suggesting that the magnetite and halloysite amounts in the composite samples depend on the synthesis route.

The FT-IR spectra of the commercial halloysite and the HNT/Fe3O4 composites are shown in Figure 2. The spectrum of the commercial HNT well compares to the literature ones [27,29,30,43]. The bands centered at about 3622 and 3707 cm<sup>−</sup><sup>1</sup> are attributed to the stretching vibrations of the Al-OH of the HNT inner surface, while the small peaks at about 3545 and 1641 cm<sup>−</sup><sup>1</sup> to the stretching and banding of the H2O molecules in the interlayer. This result puts into evidence the possible presence of small amount of the hydrated form (halloysite-(10 Å)) in the commercial halloysite, below the detection limit of XRPD. The bands at about 1031, 794, and 689 cm<sup>−</sup><sup>1</sup> are attributed to the Si-O stretching modes, the one at about 918 cm<sup>−</sup><sup>1</sup> to the Al-OH ones. In the FT-IR spectra of the HNT/Fe3O4 composites (Figure 2b), all the halloysite bands are detected. As for the Fe3O4 phase, only one broad band centered at about 3435 cm<sup>−</sup><sup>1</sup> attributed to OH-bending of hydroxyl groups was observed [43]. This broad band was not detected in the HNT/Fe3O4-SG sample, displaying a high amount of halloysite and a few magnetites (see XRPD results).

**Figure 2.** FT-IR spectra of (**a**) the commercial halloysite, and (**b**) the HNT/Fe3O4 composites.

The SEM images of the commercial HNT are shown in Figure S1a,b. The sample displayed 2–10 μm agglomerates of nanotubular particles, better highlighted in TEM micrographs (Figure 3a,b). The nanotubes exhibited an external diameter of 60–70 nm, a lumen of 20–30 nm, and variable length, from a few hundred nanometers to 1–2 μm. The

DLS results showed a bimodal particle size distribution. The mean particle size is reported in Table S1.

**Figure 3.** TEM images of the commercial halloysite sample at magnifications of (**a**) 75 kX and (**b**) 100 kX.

Figure S2 shows the SEM micrographs of the HNT/Fe3O4 composites synthesized by co-precipitation (Figure S2a,b), hydrothermal (Figure S2c,d), and sol-gel (Figure S2e,f) routes. All the composites displayed micrometric nanotubular particles, whose morphology well compares to the HNT sample one (Figure S1a,b). In addition, nanometric rounded aggregates, possibly due to the magnetite phase, were observed on the nanotubes surface and between the nanotubes, interconnecting them; they were mainly detected in the HNT/Fe3O4-C sample (Figure S2a,b) which was richer in magnetite, as suggested by XRPD and FT-IR results.

Figure 4 displays the TEM images of the HNT/Fe3O4 composites and Fe3O4 samples synthesized by co-precipitation (Figure 4a–c), hydrothermal (Figure 4d–f), and sol-gel (Figure 4g–i) routes. Independent of the applied synthesis, both rounded Fe3O4 nanometric particles and halloysite nanotubes were observed in the HNT/Fe3O4 composites. Noteworthy, the Fe3O4 amount was high in the HNT/Fe3O4-C sample (Figure 4a,b); it covered the nanotubes' surface, but also formed aggregates linking the nanotubes. This was also slightly observed in the HNT/Fe3O4-H sample. The Fe3O4 agglomerates were mainly observed on the tips of nanotubes. As reported by Tian et al. [30], the synthetic strategy based on the use of glucose in the first step favored the formation of carbon/organic groups on the HNT surface and on the tip of nanotubes, acting as nucleation centers for the Fe3O4 nanoparticles. As for the HNT/Fe3O4-SG sample, it displayed a lower amount of magnetite (see XRPD and FT-IR results), and the Fe3O4 nanoparticles only decorated the nanotubes' surface. The size and shape of the magnetite nanoparticles in the composites (about 10 nm) well compared to the Fe3O4 samples (Figure 4c,f,i) for the Fe3O4-C, Fe3O4-H, and Fe3O4-SG respectively), and fairly agreed with the crystallite size evaluated by XRPD data. In both the magnetite and composite samples, the Fe3O4 nanoparticles aggregate; particle size distribution was evaluated by DLS analysis and reported in Table S1. The Fe3O4-C sample displayed wide particle size distribution. The HNT/Fe3O4-C and NHT/Fe3O4-SG samples displayed particle size >900 nm, slightly similar to the larger ones of the commercial halloysite. Instead, the HNT/Fe3O4-H composite displayed lower particle size. To better characterize the tendency of particles to aggregate and to investigate particles' surface charge changes, zeta-potential was evaluated. Commercial HNT exhibits a negative zetapotential of −31.77 mV; this value confirms that the outer nanotube surface is negatively charged and is in good agreemen<sup>t</sup> with the literature data [47].

**Figure 4.** TEM images of the HNT/Fe3O4 and magnetite samples. HNT/Fe3O4-C at magnifications of (**a**) 150 kX and (**b**) 50 kX: Fe3O4-C (**c**) at 150 kX; HNT/Fe3O4-H at (**d**) 100 kX and (**e**) 200 kX; Fe3O4-H (**f**) at 200 kX; HNT/Fe3O4-SG at (**g**) 50 kX and (**h**) 200 kX; Fe3O4-SG (**i**) at 150 kX.

The Fe3O4-C sample (chosen as reference of the magnetite samples) exhibits a zetapotential of −7.16 mV, comparable to the literature values [48]; this value is not sufficient to achieve a stable suspension, and justifies particle aggregation (see TEM and DLS results).

Zeta-potential values of −36.36, −12.89 and −112.02 mV are obtained for HNT/Fe3O4- C, HNT/Fe3O4-SG and HNT/Fe3O4-H composites. The sample prepared by the hydrothermal process displays the most negative zeta-potential value; this may be due to the carbonaceous component (see TEM results and Section 3.2.) and explains the improved stability of the suspension and the lower mean particle size, as shown by DLS results.

The EDS analysis was applied to display the distribution map of halloysite and magnetite in each composite sample and to evaluate the weight percentage. Figures S3–S5 show the distribution maps of Al, Fe, and Si for the HNT/Fe3O4-C, HNT/Fe3O4-H, and HNT/Fe3O4-SG samples. Independently of the synthetic route, Al and Si were detected in the same areas. The Fe distribution was rather homogeneous in the sol-gel and hydrothermal samples (Figures S4 and S5, respectively), but also in some regions in which Fe prevails were detected. In the co-precipitation composite, Fe prevailed in areas poor in Al and Si, thus confirming the presence of magnetite aggregates connecting the halloysite particles.

From the EDS analysis, the Al, Si, and Fe atomic percentages were evaluated. Al:Si:Fe molar ratios of 5.25:5.15:20.33, 5.11:5.03:4.62, and 12.36:13.35:3.46 were obtained for the HNT/Fe3O4-C, HNT/Fe3O4-H, and HNT/Fe3O4-SG samples, respectively. According to the halloysite chemical formula, equimolar values of Al and Si were detected in each sample. The molar ratios obtained by EDS were used to calculate halloysite and magnetite weight percentage in each composite: the results are shown in Table 2.


**Table 2.** Halloysite and magnetite weight percentages evaluated by EDS, TGA, and magnetization data.

The halloysite amount in the HNT/Fe3O4 composites was also calculated by thermogravimetric analyses. The thermograms of commercial HNT and composites are shown in Figure 5. The halloysite TG curve (Figure 5a) well compared to the literature data [27]. The mass loss detected at low temperature (below 250 ◦C) was ascribed to the release of physisorbed water molecules. The steep mass loss observed at about 450 ◦C gave more insight, as it is due to the dehydroxylation process of the structural Al-OH groups of the aluminosilicate layers. A weight loss of 13.95% was calculated from halloysite stoichiometry. The mass loss detected in the commercial HNT was about 14.60%, in fair agreemen<sup>t</sup> with the calculated value. Figure 5b–d show the thermograms of the HNT/Fe3O4-C, HNT/Fe3O4-H, and HNT/Fe3O4-SG samples, respectively. Different mass losses were detected at low temperature (below 250 ◦C), depending on the amount of the physisorbed water, then a sample-dependent steep mass loss occurs at about 450 ◦C. As reported by Xie et al. [27], this mass loss can be compared to the HNT sample one (14.60%) to evaluate the halloysite weight percentage in each composite. The results are reported in Table 2; the halloysite weight percentages well compared to the values obtained by EDS analysis.

**Figure 5.** TGA curves of (**a**) commercial HNT (**b**) HNT/Fe3O4-C, (**c**) HNT/Fe3O4-H, and (**d**) HNT/Fe3O4-SG samples.

Field dependence of magnetization was investigated for all the samples at 300 K (Figure 6a,b).

**Figure 6.** Field dependence of magnetization at 300 K for the (**a**) bare Fe3O4 nanoparticles synthetized by coprecipitation (Fe3O4-C) sol-gel (Fe3O4-SG) and hydrothermal methods (Fe3O4-H) and (**b**) HNTs/Fe3O4 nanocomposites. The insets are zoom of the coercive field region.

For bare nanoparticles prepared with co-precipitation and sol-gel synthesis methods (Figure 6a), negligible value of reduced remanence magnetization (Mr/Ms) and small value of coercivity were obtained (Table 3), suggesting that at 300 K most of the nanoparticles were in a superparamagnetic state and just a small fraction of nanoparticles showed a quasi-static behavior. While the zero coercivity in the nanoparticles synthesized with the hydrothermal procedure indicated that all nanoparticles were in a supermagnetic state. Fe3O4-C and Fe3O4-SG samples showed a weak non-saturating character at high field, with respect to the Fe3O4-H sample. Due to the small difference in size between the samples, a non-saturating character showed by samples prepared by sol-gel and coprecipitation techniques can be ascribed to an increase in surface anisotropy, probably due to the presence of magnetic disorder (i.e., canted spin) [49,50] at the particles' surface. This hypothesis was also confirmed by the decrease in MS in SG and C samples. All the HNT nanocomposites showed a decrease in MS with respect to bare nanoparticles in qualitative agreemen<sup>t</sup> with TGA and EDS measurements. This behavior confirmed that the amount of magnetic phase decreases along the order Fe3O4-C, Fe3O4-SG, and Fe3O4-H. From a quantitative point of view, if the agreemen<sup>t</sup> among magnetization measurements, TGA and EDS, was pretty good for Fe3O4-SG and Fe3O4-H, a difference was observed for Fe3O4-C nanocomposite. In particular, the particles prepared by co-precipitation looked to decrease their MS when prepared as nanocomposites. This can be ascribed to a decrease in nanoparticles' crystallinity that can be observed in the co-precipitation synthesis with respect to hydrothermal and sol-gel syntheses [51,52].


**Table 3.** Saturation magnetization MS, reduce remanence magnetization (Mr/MS) and coercive field (μ0HC) of Fe3O4-C, Fe3O4-SG, Fe3O4-H, HNT/Fe3O4-C, HNT/Fe3O4-SG, and HNT/Fe3O4-H samples.

It is well known that the adsorption capacity of the materials is strictly related to their specific surface area [53]. The BET method was applied to investigate the specific surface area of the commercial halloysite and the three HNT/Fe3O4 composites. The values of 58.20, 57.66, 52.15, and 54.56 m<sup>2</sup> g<sup>−</sup><sup>1</sup> were obtained for the commercial HNT, HNT/Fe3O4-C, HNT/Fe3O4-H, and HNT/Fe3O4-SG samples, respectively. The pore specific volume was also evaluated, and values of 0.19, 0.26, 0.16, and 0.27 cm<sup>3</sup> g<sup>−</sup><sup>1</sup> were obtained. These results sugges<sup>t</sup> that the deposition of the magnetite nanoparticles on the nanotubular halloysite surface did not affect the halloysite surface area and pore volumes. The obtained values fairly agreed with the literature data for halloysite nanotubes (surface areas: 22.1–81.6 m<sup>2</sup> g<sup>−</sup>1; pore volumes: 0.09–0.25 cm<sup>3</sup> g<sup>−</sup>1) [22].
