*2.5. In Batch Desorption Experiments*

After the Kp adsorption from water, NaCl (0.25 M) was selected, with regard to a green economy and cleaner production, as the best salt to induce the release of the adsorbed Kp. With the same approach adopted for Kp adsorption, the UV–Vis investigation was used to assess the amount of desorbed pollutant. After the Kp adsorption, the adsorbent was washed with fresh water to remove the non-adsorbed Kp swollen in the NaCl solution for release. The effect of the contact time was evaluated, and 30 min was found to be suitable for Kp recovery.

## *2.6. Adsorption Kinetics*

Information about the kinetics of the adsorption process between Kp and the chitosan film was inferred by adopting both pseudo first-order and pseudo second-order kinetic models. The following linearized equations for the pseudo first-order (Equation (2)) and pseudo second-order (Equation (3)) models were adopted [13,14,16]:

$$\ln\left(\mathbf{q}\_{\varepsilon} - \mathbf{q}\_{t}\right) = \ln\left(\mathbf{q}\_{\varepsilon}\right) - \mathbf{K}\_{1} \times \text{t},\tag{2}$$

$$\frac{\mathbf{t}}{\mathbf{q}\_{\text{t}}} = \frac{1}{\mathbf{K}\_{2}\mathbf{q}\_{\text{e}}^{2}} + \frac{1}{\mathbf{q}\_{\text{e}}} \times \mathbf{t}\_{\text{v}} \tag{3}$$

where qe and qt represent the adsorption capacities at equilibrium and at time t, respectively (mg·g<sup>−</sup>1), and k1 (min<sup>−</sup>1) and k2 (g·mg−1·min<sup>−</sup>1) are the rate constants of the pseudo first-order and second-order models, respectively.

#### *2.7. Thermodynamic Studies*

Free energy (ΔG◦), entropy (ΔS◦), and enthalpy (ΔH◦) were calculated [27] for the Kp adsorption onto the chitosan film at three selected temperatures of 298, 288, and 278 K. The value of ΔG◦ was inferred using Equation (4).

$$
\Delta \mathbf{G}^{\diamond} = -\mathbf{R} \mathbf{T} \ln \mathbf{K}\_{\text{eq}\_{\text{V}}} \tag{4}
$$

where *R* is the universal gas constant (8.314 J/mol·K), *T* is the temperature (K), and Keq represents the equilibrium constant that was calculated as qe/Ce, where qe is the sorption capacity (mg·g−1) at equilibrium, and Ce is the equilibrium concentration (mg·g−1). The values of <sup>Δ</sup>*H*◦ and <sup>Δ</sup>*S*◦ were determined by combining Equation (4) with Equation (5), thereby obtaining Equation (6).

$$
\Delta \mathbf{G}^{\diamond} = \Delta \mathbf{H}^{\diamond} - \mathbf{T} \Delta \mathbf{S}^{\diamond}.\tag{5}
$$

$$
\ln \text{K}\_{\text{eq}} = -\frac{\Delta \text{H}^{\circ}}{RT} + \frac{\Delta \text{S}^{\circ}}{R} \,. \tag{6}
$$

#### *2.8. Determination of Chitosan Film Zero-Point Charge*

The zero-point charge pH (pHZPC) of the chitosan adsorbent was evaluated by using the pH drift method [14]. Firstly, 30 mL of NaCl solution with a concentration of 5.0 <sup>×</sup> <sup>10</sup>−<sup>2</sup> M was used at different pH values ranging from 2 to 12 (pHi). Concentrated HCl and NaOH solutions were used for this purpose. The pHi values of these solutions were measured, and 25 mg of adsorbent was subsequently introduced. These solutions were stirred at 298 K for 48 h. The final pH (pHF) values were measured. By reporting the pHi versus pHF values, the value of pHZPC was inferred at the cross-section of the latter curve with the line of pHi versus pHi. All experiments were performed in triplicate, calculating the relative standard deviations.

#### **3. Results and Discussion**

The UV–Vis absorption spectrum of Kp was used to monitor its removal from contaminated water. As a first step, an aqueous solution purposely spiked with the drug was investigated, and the Kp spectroscopic main signal at 260 nm (Figure 1A) was followed.

By adopting 150 mg of adsorbent, in the presence of 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M Kp, the adsorption was followed until 60 min. The main process of Kp removal was observed in the first 15 min, exhibiting an efficiency of 55%; by extending the contact time, the effect resulted less pronounced with respect to the beginning of the process, and, in 60 min, almost 85% of the NSAID was eliminated from the water (Figure 1A). The results suggested the probable key role of the Kp concentration gradient (ΔC) between the bulk of the solution and the adsorbent surface [28]. At the beginning of the adsorption process, the ΔC was high enough to induce the diffusion of the NSAID from the bulk of the solution at the surface of the adsorbent; on the other hand, upon extending the contact time, the ΔC was reduced, thereby slowing

down the Kp adsorption. Furthermore, at the beginning of the process, the presence of a large number of free sites on the adsorbent surface for Kp adsorption also favored the NSAID removal. However, upon extending the contact time, the number of available free sites to host Kp decreased, reducing the relative Kp adsorption (Figure 1A) [13,14,16,29].

**Figure 1.** Ultraviolet–visible light (UV–Vis) spectra of a 1 <sup>×</sup> 10−<sup>5</sup> M ketoprofen (Kp) solution, pH 5, collected at different contact times, in the presence of 150 mg of adsorbent (**A**); Kp adsorption percentage (1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M, pH 5) calculated at different contact times in the presence of 200 mg and 35 mg of adsorbent (**B**); adsorption capacities, qt, referring to different amounts of chitosan (200, 150, and 35 mg) in contact with a 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M Kp solution, pH 5 (**C**).

Moreover, this behavior could be attributed to the presence of repulsive forces between free Kp molecules in solution and those adsorbed, which further hindered the drug adsorption [30].

After these assessments, the effects of several parameters affecting the NSAID removal from water were investigated, while also evaluating the effects on the adsorption capacities.

## *3.1. E*ff*ects of Adsorbent Dosage and Kp Concentration*

To study the effect of the CH amount on the Kp removal from water, thus evidencing the role of active sites hosting Kp, two other CH films with different weights (200 and 35 mg) were compared under the same experimental conditions (Figure 1B), i.e., a 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M Kp solution at pH 5 (pH of the Kp solution soon after adsorbent addition).

The Kp removal efficiencies were calculated, and adsorption percentages of 65% and 8% were determined in the first 15 min when using the greatest and the smallest amounts of adsorbent, respectively; as shown in Figure 1B, these efficiencies were increased upon extending the contact time to 120 min, obtaining 90% Kp removal if in presence of the largest amount of CH.

This result confirmed the previously mentioned importance of free active sites able to host the NSAID [13,14,16,29], which increase in number upon increasing the available surface of the adsorbent. Overall, in Figure 1C, the influence of the chitosan amount on Kp adsorption was evaluated by reporting the qt values (Equation (1)) as a function of the three investigated CH weights.

The obtained results indicated that, by increasing the adsorbent amount, the relative adsorption of Kp molecules increased (see the plateaus on the graph), while the adsorption capacity decreased. This suggests that, despite the great efficacy, by using a large amount of adsorbent, the adsorption sites remained partially unsaturated during the sorption process, reducing the qt values as a whole [13,14]. In particular, as already observed by monitoring the Kp UV–Vis absorption spectrum (Figure 1A), at the beginning of the adsorption process (Figure 1C), the presence of a large quantity of free sites for Kp adsorption and the high ΔC increased the qt values. On the contrary, by extending the contact time, the free sites and ΔC decreased, reducing the Kp adsorption overall, leading to a plateau. All of these findings agree with the results obtained upon varying the Kp concentrations as described below.

**Figure 2.** Percentage of Kp adsorption (**A**) and adsorption capacities, qt (**B**), referring to different Kp concentrations of 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M and 5 <sup>×</sup> <sup>10</sup>−<sup>6</sup> M, pH 5, in the presence of 150 mg of adsorbent.

For this purpose, the concentration of Kp was changed (1 <sup>×</sup> 10−<sup>5</sup> M and 5 <sup>×</sup> 10−<sup>6</sup> M), fixing the chitosan weight at 150 mg, as shown in Figure 2A. In both cases, the Kp removal showed great variation in the first minute of contact, showing that, upon diluting the Kp solution, the pollutant removal percentage decreased. Not surprisingly, the dilution of Kp solutions (with a dilution factor of 1:2) reduced the percentage of Kp removal from 65% to 25% in the first 15 min and from 85% to 50% after 120 min. Accordingly, the associated qt values were calculated, as reported in Figure 2B, showing higher qt values for the concentrated Kp solution. Moreover, in the latter case, the relative maximum adsorption capacities were quickly obtained after a few min, as compared with the diluted Kp solution.
