2.2.4. Characterization Techniques

X-ray powder diffraction measurements were performed using a Bruker D5005 diffractometer (Bruker, Karlsruhe, Germany) with the CuK α radiation, graphite monochromator, and scintillation detector. The patterns were collected in the 7–80◦ two-theta angular range, step size of 0.03◦, and a counting time of 20 s/step. A silicon low-background sample holder was used.

FT-IR spectra were obtained with a Nicolet FT-IR iS10 Spectrometer (Nicolet, Madison, WI, USA) equipped with ATR (attenuated total reflectance) sampling accessory (Smart iTR with ZnSe plate) by co-adding 32 scans in the 4000–650 cm<sup>−</sup><sup>1</sup> range at 4 cm<sup>−</sup><sup>1</sup> resolution.

Thermogravimetric measurements were performed by a TGA Q5000 IR apparatus interfaced with a TA 5000 data station (TA Instruments, Newcastle, DE, USA). The samples were scanned at 10 ◦C min−<sup>1</sup> under nitrogen flow (45 mL min−1) in the 20–850 ◦C temperature range. Each measurement was repeated at least three times.

The specific surface area and porosity were investigated by N2 adsorption using the BET method in a Sorptomatic 1990 Porosimeter (Thermo Electron, Waltham, MA, USA).

SEM measurements were performed using a Zeiss EVO MA10 (Carl Zeiss, Oberkochen, Germany) Microscope, equipped with an Energy Dispersive Detector for the EDS analysis. The SEM images were collected on gold-sputtered samples. HR-SEM images were taken from an FEG-SEM Tescan Mira3 XMU. Samples were mounted onto aluminum stubs using double sided carbon adhesive tape and were then made electrically conductive by coating

in vacuum with a thin layer of Pt. Observations were made at 25 kV with an In-Beam SE detector at a working distance of 3 mm.

TEM micrographs were carried out on a JEOL JEM-1200 EX II (JEOL Ltd., Tokio, Japan) microscope operating at 100 kV high voltage (tungsten filament gun) and equipped with a TEM CCD camera Olympus Mega View III (Olympus soft imaging solutions (OSIS) GmbH, from 2015 EMSIS GmbH, Munster, Germany) with 1376 × 1032 pixel format. The samples were prepared by drop-casting the solution on nickel grids formvar/carbon coated.

Dynamic light scattering (DLS)—Nicomp 380 ZLS (Particle Sizing Systems, Lakeview Blvd. Fremont, CA, USA) was used. For analyses, samples were diluted 1:10 in MilliQ water. The main parameters set up were: channel 10, intensity 100 kHz, temperature 23 ◦C, viscosity 0.933 cPoise, and a liquid index of refraction 1.333. The values considered at the end of the analyses were: mean diameter (nm), standard deviation, and Zeta potential (mV).

To investigate the magnetic behavior of the materials, field dependence of magnetization was investigated using a vibrating sample magnetometer (VSM Model 10–Microsense) equipped with an electromagnetic producing magnetic field in the range ±2 T.

### *2.3. Adsorption Experiments and Analytical Measurements*

### 2.3.1. Adsorption and Kinetic Experiments

OFL adsorption on HNT/Fe3O4-C, HNT/Fe3O4-SG, HNT/Fe3O4-H, and commercial HNT was studied by a batch method. For adsorption equilibrium experiments, 20 mg of each material was suspended in 10 mL of tap water spiked with OFL in the range of 25–200 mg L−1. Flasks were wrapped with aluminum foil to prevent light-induced drug decomposition and shaken for 24 h at room temperature with an orbital shaker. Subsequently, the suspensions were magnetically separated, and the supernatants were filtered (0.22 μm) and analyzed by UV-vis spectrophotometer at 287 nm to determine the antibiotic concentration in solution at equilibrium ( *Ce*). The adsorbed OFL amount at equilibrium (*qe*, mg g<sup>−</sup>1) was calculated by Equation (1):

$$\eta\_{\varepsilon} = \frac{(\mathbb{C}\_0 - \mathbb{C}\_{\varepsilon}) \cdot V}{m} \tag{1}$$

where *C*0 is the initial OFL concentration (mg <sup>L</sup>−1), *Ce* is the drug concentration in solution at equilibrium (mg <sup>L</sup>−1), *V* is the volume of the solution (L), and *m* is the amount of the sorbent material (*g*).

For the kinetic experiments, 20 mg of each material were suspended in 10 mL of 20 mg L−<sup>1</sup> OFL tap water solution. Falcon tubes, wrapped with aluminum foil, were shaken by a roller shaker and, at selected times, the adsorbent was magnetically treated. Then, a few mL of the supernatant were collected, filtered (0.22 μm) in a quartz cuvette, and analyzed by a UV spectrophotometer at 287 nm. The analyzed solution was recovered to keep the suspension volume constant for all experiments. The adsorbed OFL amount at time *t* (*qt*, mg g<sup>−</sup>1) was calculated as (Equation (2)):

$$q\_t = \frac{(\mathbb{C}\_0 - \mathbb{C}\_t) \cdot V}{m} \tag{2}$$

where *C*0 is the initial OFL concentration (mg <sup>L</sup>−1), *Ct* is the drug concentration in solution at time *t* (mg <sup>L</sup>−1), *V* is the volume of the solution (L), and *m* is the amount of the sorbent material (*g*).

All experiments were performed in duplicate. The thermodynamic and kinetic parameters were estimated by dedicated software (OriginPro, Version 2019b. OriginLab Corporation, Northampton, MA, USA).

The well-known Langmuir's and Freundlich's isotherm models were applied to fit the experimental data. The Langmuir model (Equation (3)) describes the adsorption process that takes place on specific homogeneous sites and in a monolayer on the material surface:

$$q\_{\mathfrak{c}} = \frac{q\_{\mathfrak{m}} K\_L \mathbb{C}\_{\mathfrak{c}}}{1 + K\_L \mathbb{C}\_{\mathfrak{c}}} \tag{3}$$

where *KL* is the Langmuir constant and *qm* is the monolayer saturation capacity.

The Freundlich model defines non-ideal adsorption on the heterogeneous surface, and Equation (4) expresses it:

$$q\_{\mathfrak{e}} = \mathbb{K}\_F \mathbb{C}\_{\mathfrak{e}}^{1/n} \tag{4}$$

where *KF* is the empirical constant indicative of adsorption capacity, and *n* is the empirical parameter representing the adsorption intensity.

The time-dependent data were fitted by pseudo-first-order (Equation (5)) and pseudosecond-order kinetic (Equation (6)) models:

$$q\_t = q\_{\ell}(1 - e^{k\_1 t})\tag{5}$$

$$q\_t = \frac{q\_\epsilon^2 k\_2 t}{1 + q\_\ell k\_2 t} \tag{6}$$

where *qt* and *qe* are the drug adsorbed amount at time *t* and equilibrium, respectively, and *k*1 and *k*2 are the pseudo-first-order and the pseudo-second-order rate constants.
