An Easy Synthesis for Preparing Bio-Based Hybrid Adsorbent Useful for Fast Adsorption of Polar Pollutants

For the first time, γ-Al2O3 and Bio-Based Substances (BBS) hybrids (A-BBS) were prepared through a simple electrostatic interaction occurring between alumina, used as a support, and BBS (Bio-Based Substance from composted biowastes) carrying positive and negative charges, respectively. We evaluated the optimal amount of BBS to be immobilized on the support and the stability of the resulting A-BBS in order to use this novel hybrid material as an adsorbent for the removal of polar pollutants. Characterization was carried out by X-Ray Diffraction (XRD) for evaluating the crystal structure of the support, Fourier transform infrared spectroscopy (FT-IR) to evidence the presence of BBS on the hybrid material, thermogravimetric analysis (TGA) to measure the thermal stability of the hybrid materials and quantify the BBS amount immobilized on the support, N2 adsorption at 77 K for the evaluation of the surface area and porosity of the systems, Zeta potential measurements to evaluate the effect of BBS immobilization on the surface charge of the particles and choose the substrates possibly interacting with them. Firstly, we tested the adsorption capability of three samples differently coated with BBS toward cationic species considering various adsorbate/adsorbent ratio. Crystal Violet (CV) was chosen as model pollutant to compare the performance of the hybrid materials with those of other materials described in the literature. The adsorption data were modeled by Langmuir and Freundlich adsorption isotherms. Then, we studied the adsorption capability of the developed material towards molecules with different structures; for this purpose, two contaminants of emerging concerns (carbamazepine and atenolol) were tested. The results indicate that A-BBS could be applied in wastewater treatment for the removal of a significant amount of polar species. In addition, a comparison with literature data concerning CV adsorption was carried out in order to evaluate the environmental impact of synthetic routes used to prepare different adsorbents.


Introduction
The removal of pollutants from urban and industrial wastewaters is one of the most important issues to be solved by modern research. In fact, water pollution has increased in the last decades by the virtue of the disposal of industrial effluents enriched with toxic species [1] which has hazardous effects on flora, fauna, and humans. Moreover, the growth of the world population results in a limitation of water supplies and scarcity of water resources, therefore the necessity of clean water has become fundamental for our society [1,2]. * The related amounts are not q m but refer to the following experimental conditions: In Ref. [49]: 500 mg of adsorbent in 523.3 mL of 1.5 × 10 −2 M dye in water for a contact time of 48 h, Ref. [52]: 30 mg of adsorbent was poured in 5 mL of 0.1 mM of dye under stirring for 420 min.

Materials
γ-alumina was kindly supplied by Centro Ricerche FIAT (Orbassano (TO), Italy), the X-ray diffraction pattern confirms the expected crystalline structure of the material consistent with the reference card 01-075-0921 related to γ-Al 2 O 3 . As a simple electrostatic interaction of the support with the BBS molecules is not expected to modify the crystalline structure of the support, the hybrid sample diffractograms are not shown.
BBS were extracted from composted organic refuses (from urban public park trimming and home gardening residues) aged for more than 180 days supplied by ACEA Pinerolese Industriale (Pinerolo (TO), Italy) [23]. The extraction procedure was described elsewhere [53].
Crystal violet (CV) was purchased from Merck (Milan, Italy) and used without any further treatment to prepare solution in ultrapure MilliQ water for adsorption tests.
Carbamazepine and Atenolol were provided by Sigma-Aldrich (Milano, Italy) in analytical purity ≥99.0% and used in adsorption experiments.
All aqueous solutions for High-Performance Liquid Chromatography (HPLC) analysis were prepared using ultrapure water Millipore Milli-QTM (resistivity >18 MΩ). All chemicals were used without further purification.

Preparation of Hybrid Materials
The γ-Al 2 O 3 particles were used as support for different amounts of BBS immobilized at their surface by simple electrostatic interaction occurring between the two components carrying opposite surface charges.
Hybrid materials were prepared by mixing 1 g of γ-Al 2 O 3 in 20 mL of distilled water containing 0.1, 0.2, and 0.4 g of BBS under stirring for 24 h at 25 • C. The pH of solution was about 6.5 during the preparation. The samples were washed with 10 mL of distilled water for 10 min and every time centrifuged at 4000 rpm for 10 min. The washing solution was tested using UV-Vis spectrophotometer to evidence the presence of leached BBS molecules. The procedure was carried out several times, till the washing solution did not evidence the presence of BBS in the UV-Vis spectra. Drying process was performed in the oven at 40 • C for 24 h. Hybrid samples were named A-BBS0.1, A-BBS0.2, and A-BBS0.4, the reference pure alumina sample was indicated as A.

Characterization Methods
X-ray diffraction (XRD) analyses of alumina support was obtained using a X'Pert PRO MPD diffractometer from PANalytical (Royston, UK), equipped with Cu anode and working at 45 kV and 40 mA in a Bragg-Brentano geometry. In this study, the flat sample-holder configuration was used.
Fourier transform infrared (FTIR) spectra were recorded in transmission mode by means of a Bruker Vector 22 spectrophotometer equipped with Globar source, DTGS detector (Billerica, Massachusetts, USA), and working with 128 scans at 4 cm −1 resolution in the 4000-400 cm −1 range. Samples were dispersed in KBr (approximatively, sample: KBr weight ratio was 0.045).
Nitrogen adsorption-desorption experiments were carried out using an ASAP 2010 Micromeritics volumetric apparatus (Norcross, GA, USA). Before the measurements, the samples were outgassed at 40 • C for 24 h. Specific surface areas (SSA) were calculated using the Brunauer, Emmett and Teller (BET) method. Pore volumes (PV) and Pore Size Distribution (PSD) were determined by the Barrett, Joyner, and Halenda method [54] applied to the isotherm desorption branch.
Zeta potential measurements were performed on the instrument Zetasizer ZS90 by Malvern (Malvern, UK). 10 mg of BBS and hybrid materials were suspended in 20 mL of deionized water under constant stirring (400 rpm) for 15 min. The zeta potential measurements were performed starting from the natural pH of the suspension then decreasing it point by point by addition of 0.1 M HCl and successively increasing it with 0.1 M NaOH. A digital pH meter (Metrohm, model 827 pH lab, swiss mode, Herisau, Switzerland) was used to measure the pH of the solution.
Thermo-gravimetric analysis (TGA) was carried out using a TA Q600 (New Castle, DE, USA). Thermal analyses were performed with a heating ramp of 10 • C/min from RT to 600 • C under air in order to quantify the amount of BBS immobilized on γ-Al 2 O 3 particles.

Analytical Instruments
UV-Vis spectrophotometer (Varian Cary 300 Scans, Agilent, Santa Clara, CA, USA) was used to determine the adsorption of CV (maximum absorbance at 584 nm).
A Merck-Hitachi liquid chromatographer (Knauer, Berlin, Germany) equipped with Rheodyne injector L-6200 and L-6200A pumps for high-pressure gradients, L-4200 UV-Vis detector, and a LiChrocart RP-C18 column (Merck, Milano, Italy, 12.5 cm × 0.4 cm) was used to determine the concentration of atenolol and carbamazepine during the experiments. The detection wavelength was set at 224 nm for atenolol and 284 nm for carbamazepine). Isocratic elution (1 mL min −1 flow rate) Nanomaterials 2019, 9, 731 5 of 19 was carried out with 60% of phosphate buffer 1 × 10 −2 M at pH 2.8 and 40% acetonitrile and retention times were 5 min.

Kinetic of CV Adsorption
The kinetic of the adsorption was followed contacting 10 ppm of CV with 20 mg of adsorbing hybrid materials (total volume 10 mL) at pH 6.5 (therefore, a CV:adsorbent ratio of 1:2 wt was applied to all the preliminary measurements). The mixture was stirred vigorously under isolated orbital mixing plate (rotation at 1000 rpm) keeping the temperature at 15 • C. The adsorption phenomenon was followed measuring every 10 min the absorbance of the dye in the supernatant after centrifugation (at 4000 rpm for 10 min) by means of a UV-Vis spectrophotometer. Although an equilibrium time of less than or equal to 10 min was evidenced, all the experiments were carried out leaving adsorbent and adsorptive in contact for 30 min. All the measurements (data not showed) were carried out in duplicate and the average data are reported.

CV Adsorption Study and Model Application
The adsorption experiments performed on the hybrid absorbents were carried out at 15 • C and pH = 6.5 (natural value of the suspensions in CV) modifying the relative amount of CV and adsorbent in order to explore a wide Ce range (10 to 100 mg of dye were mixed with 20 mg of adsorbents in 10 mL).
Adsorption studies were carried out using 20 mg of adsorbents in contact with 10 batches containing 10 mL of different concentration of CV aquatic solution (from 10 to 100 ppm). The batches were sealed and placed in a shaker for 30 min at 15 • C and pH = 6.5 to obtain the measurement of the adsorption capacity. The experiments were performed in duplicate and average values were reported.
The adsorption capacity was calculated by using Equation (1): where C 0 (mg/L) is the initial dye concentration and C e (mg/L) is the concentration of dye at equilibrium, V (L) is the volume of dye concentration, W (g) is the mass of the adsorbent, and q e (mg/g) is the amount of dye CV adsorbed. The percentage removal of CV was calculated from the formulae given below: Freundlich and Langmuir models were applied to the experimental data.

Freundlich Model
The Freundlich model considers an adsorption taking place on a heterogeneous surface. The isotherm model can be represented by the following equation: where C e is the adsorbate equilibrium concentration expressed in mg/L, q e is the amount of adsorbate in the adsorbent at equilibrium expressed in mg/g, K F is the Freundlich constant representing the affinity of the adsorptive towards the adsorbing material, 1/n is the Freundlich constant, representing the degree of affinity adsorptive/adsorbing material and indicating how much the adsorption process is favored (n < 1 indicates a poor adsorption and the desorption as the favored process, 1 < n < 2 indicates a good equilibrium between adsorption/desorption, 2 < n < 10 represents a very good adsorption going towards an irreversible phenomenon [55]). K F and n can be determined from the linearized plot ln q e vs. C e (not reported for the sake of brevity).

Langmuir model
The Langmuir model assumes the adsorption reaches a monolayer of coverage [56]. The general equation is where C e is the adsorbate equilibrium concentration expressed in mg/L, C e is the adsorbate initial concentration expressed in mg/L, q e is the amount of adsorbate in the adsorbent at equilibrium expressed in mg/g, q o is the monolayer coverage capacities expressed in mg/g, K L is the Langmuir constant indicating the ratio of reagents and products at the equilibrium.
In the linearized form Equation (4) becomes The plot reporting 1 q e vs 1 C e allows to obtain q 0 and K L . The value, indicates if the adsorption is unfavored (if R L > 1), if it shows a linear trend (if R L = 1) or if the adsorption is favored (if 0 < R L < 1). R L = 0 indicates that the reaction is irreversible.

Contaminants of Emerging Concern (CECs) Adsorption Study
Kinetic studies were performed using 10 ppm of Atenolol and Carbamazepine in contact with 20 mg of A-BBS0.4 in a volume of 10 mL, at 20 • C and pH 6.5. Adsorption studies were performed using different amounts of Atenolol and Carbamazepine in contact with 20 mg of A-BBS0.4 in duplicate. The mixtures were kept under stirring at 20 • C, pH 6.5 and the residual amount of contaminants was measured at 30 min-contact time in order to obtain the adsorption isotherms.

FTIR Spectroscopy
The IR spectra of alumina before and after BBS immobilization were collected in order to assure BBS presence on hybrid alumina systems. The recorded spectra, together with the reference BBS spectrum, are reported in Figure 1. The pure alumina sample shows the presence of atmospheric water molecules interacting with the surface and producing a signal at 1630 cm −1 (δ HOH vibration) and a large absorption around 3500 cm −1 (ν OH vibrations), whereas the intense signal at low wavenumbers (<1000 cm −1 ) is due to alumina framework vibrations. Pure BBS shows again an intense signal at around 3500 cm −1 due to OH groups and atmospheric moisture interacting with the surface (ν OH vibrations), a large signal at 1600 cm −1 due to both carbonyl (C=O) stretching and vibration of water molecules adsorbed at the surface (δ HOH signal), and other two signals at 1400 and 1000 cm −1 due to carboxylic acid/C-H bending and OCO vibrations, respectively. Another very weak signal can be observed at around 3000 cm −1 due to CH stretching vibrations. surface (νOH vibrations), a large signal at 1600 cm −1 due to both carbonyl (C=O) stretching and vibration of water molecules adsorbed at the surface (δHOH signal), and other two signals at 1400 and 1000 cm −1 due to carboxylic acid/C-H bending and OCO vibrations, respectively. Another very weak signal can be observed at around 3000 cm −1 due to CH stretching vibrations. Specific signals due to BBS (evidenced in the Figure by vertical lines) are visible in the spectra of the hybrid materials at around 1600, 1400, and 1000 cm −1 , whereas the less intense signals at around 3000 are not visible due to the small amount of BBS immobilized on the support. In any case, the presence of these absorptions confirms that alumina functionalization was carried out and, moreover, their increasing intensity suggests an increase of the amount of the functionalizing phase on the A support increasing the amount of BBS used during the material preparation. No modification of the signal positions was observed with respect to pure BBS (see curve A in the Figure), as expected for a simple electrostatic interaction occurring between alumina surface and BBS molecules. Moreover, it is possible to evidence an important contribute of the hydration layer on the hybrid samples surface, as witnessed by the growth of the signal at 1600 (δHOH vibration) and 3500 cm −1 (νOH), given the polar nature of BBS molecules leading to a stronger interaction with atmospheric moisture.

Gas-Volumetric N2 Adsorption at 77K
The adsorption/desorption isotherms of all hybrid materials and reference non-modified sample are of the IV type (IUPAC classification) indicating that all materials are mesoporous ( Figure  2A).
As it can be seen, the hysteresis loops of all the samples are very similar, although the height of the step decreases by increasing the amount of BBS, indicating a minor amount of mesopores after functionalization of the support. Nevertheless, the amount of BBS immobilized seems to be not enough to modify the featuring morphology of the material. The values of BET specific surface area of the materials are reported in Table 2 together with the total amount of mesopores, evaluated by BJH model on the desorption branch of the isotherms, whereas the pore size distribution curves are reported in Figure 2B. The specific surface area of the samples decreases increasing the amount of Specific signals due to BBS (evidenced in the Figure by vertical lines) are visible in the spectra of the hybrid materials at around 1600, 1400, and 1000 cm −1 , whereas the less intense signals at around 3000 are not visible due to the small amount of BBS immobilized on the support. In any case, the presence of these absorptions confirms that alumina functionalization was carried out and, moreover, their increasing intensity suggests an increase of the amount of the functionalizing phase on the A support increasing the amount of BBS used during the material preparation. No modification of the signal positions was observed with respect to pure BBS (see curve A in the Figure), as expected for a simple electrostatic interaction occurring between alumina surface and BBS molecules. Moreover, it is possible to evidence an important contribute of the hydration layer on the hybrid samples surface, as witnessed by the growth of the signal at 1600 (δ HOH vibration) and 3500 cm −1 (ν OH ), given the polar nature of BBS molecules leading to a stronger interaction with atmospheric moisture.

Gas-Volumetric N2 Adsorption at 77K
The adsorption/desorption isotherms of all hybrid materials and reference non-modified sample are of the IV type (IUPAC classification) indicating that all materials are mesoporous ( Figure 2A).
As it can be seen, the hysteresis loops of all the samples are very similar, although the height of the step decreases by increasing the amount of BBS, indicating a minor amount of mesopores after functionalization of the support. Nevertheless, the amount of BBS immobilized seems to be not enough to modify the featuring morphology of the material. The values of BET specific surface area of the materials are reported in Table 2 together with the total amount of mesopores, evaluated by BJH model on the desorption branch of the isotherms, whereas the pore size distribution curves are reported in Figure 2B. The specific surface area of the samples decreases increasing the amount of BBS, although to a very limited extent. This suggests that BBS is not immobilized at the external surface of the particles, since they do not act as aggregating agent between the single alumina particles, as the loss of specific surface area of the material after functionalization should be much higher. This indicates that BBS are allocated essentially into the mesopore empty space, probably because in that space the electrostatic interaction of the support with the molecule can be stronger. The pore size distribution curves allow obtaining other important details concerning the BBS-solid interaction. All samples show two families of mesopores, the first one quite narrow centered at 37 Å of width and another one very wide covering the values 20-180 Å of width. Both families of pores are affected by support functionalization because the large mesopores decrease in amount and width, the relative maximum shifting from 80 to 60 Å of width, whereas the smaller ones increase in intensity after functionalization. This feature suggests that the smallest pores are not involved in the functionalization, probably because of the limited dimensions, whereas BBS are placed into the largest pores, which are those decreasing in amount and size after functionalization causing the increase of the amount of the smallest ones. Also the fact that the hystheresis loops of the isotherms do not change shape after functionalization confirms again that BBS molecules enter into the pores sticking at the internal walls of the solid (causing a decrease of their size) rather than fixing at their entrance (causing the modification of the pore shape which is not observed indeed).
Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials BBS, although to a very limited extent. This suggests that BBS is not immobilized at the external surface of the particles, since they do not act as aggregating agent between the single alumina particles, as the loss of specific surface area of the material after functionalization should be much higher. This indicates that BBS are allocated essentially into the mesopore empty space, probably because in that space the electrostatic interaction of the support with the molecule can be stronger. The pore size distribution curves allow obtaining other important details concerning the BBS-solid interaction. All samples show two families of mesopores, the first one quite narrow centered at 37 Å of width and another one very wide covering the values 20-180 Å of width. Both families of pores are affected by support functionalization because the large mesopores decrease in amount and width, the relative maximum shifting from 80 to 60 Å of width, whereas the smaller ones increase in intensity after functionalization. This feature suggests that the smallest pores are not involved in the functionalization, probably because of the limited dimensions, whereas BBS are placed into the largest pores, which are those decreasing in amount and size after functionalization causing the increase of the amount of the smallest ones. Also the fact that the hystheresis loops of the isotherms do not change shape after functionalization confirms again that BBS molecules enter into the pores sticking at the internal walls of the solid (causing a decrease of their size) rather than fixing at their entrance (causing the modification of the pore shape which is not observed indeed).    Figure 3, upper section, shows the results of TGA analysis on sample A before and after immobilization of BBS in the temperature range 40-650 • C under air, Figure 3, lower section, reports the curve related to BBS residual weight registered in the same conditions. As reported in the literature [57], BBS evidences two main regions of weight loss, the first in the range 40-220 • C related to physisorbed and chemisorbed water elimination, the second, occurring in two steps, in the range 220-650 • C related to the oxidation of BBS. Analogously, all the curves of A and hybrid materials present two important weight losses. The foster falls in the range 40-220 • C and is due to the removal of adsorbed water molecules: this contribution increases increasing the amount of BBS on the hybrid samples as a consequence of the increased interaction with atmospheric moisture given by the increased polarity of the materials. The latter falls in the range 220-650 • C and deserves some comments. Alumina sample possesses surface OH groups that can be partially eliminated via condensation reaction in form of water molecules at high temperature: the weight loss observed in the range 220-650 • C (1.1%) can be assessed to this phenomenon. Otherwise, the hybrid materials treated at high temperature can experience both the OH elimination via condensation reaction and the elimination of organic moieties through an oxidation reaction which forms CO 2 and H 2 O. From the comparison of the four curves, it is possible to quantify the amount of the three contributions, namely adsorbed water molecules, amount of OH eliminated via condensation reaction and organic content, for the four samples as reported in Table 3.   Figure 3, upper section, shows the results of TGA analysis on sample A before and after immobilization of BBS in the temperature range 40-650 °C under air, Figure 3, lower section, reports the curve related to BBS residual weight registered in the same conditions. As reported in the literature [57], BBS evidences two main regions of weight loss, the first in the range 40-220 °C related to physisorbed and chemisorbed water elimination, the second, occurring in two steps, in the range 220-650 °C related to the oxidation of BBS. Analogously, all the curves of A and hybrid materials present two important weight losses. The foster falls in the range 40-220 °C and is due to the removal of adsorbed water molecules: this contribution increases increasing the amount of BBS on the hybrid samples as a consequence of the increased interaction with atmospheric moisture given by the increased polarity of the materials. The latter falls in the range 220-650 °C and deserves some comments. Alumina sample possesses surface OH groups that can be partially eliminated via condensation reaction in form of water molecules at high temperature: the weight loss observed in the range 220-650 °C (1.1%) can be assessed to this phenomenon. Otherwise, the hybrid materials treated at high temperature can experience both the OH elimination via condensation reaction and the elimination of organic moieties through an oxidation reaction which forms CO2 and H2O. From the comparison of the four curves, it is possible to quantify the amount of the three contributions, namely adsorbed water molecules, amount of OH eliminated via condensation reaction and organic content, for the four samples as reported in Table 3.    The amount of BBS actually loaded onto the support is obtained by calculating the percentage of weight loss for A-BBS0.1, A-BBS0.2, and A-BBS0.4 in the range of 220 to 650 °C and subtracting that observed for A support in the same range. The amount of organic matter actually immobilized on hybrid samples depends on the concentration of the BBS solution used for the functionalization: in these experiments the maximum loading has been reached by A-BBS0.4 with 7.2% of organic matter immobilized.

Zeta Potential
The zeta potential measurements have been used in order to evaluate the surface charge of the support and its modification after BBS functionalization. This is a very important indication, dealing with adsorption, in order to forecast the type of substrate that can be subjected to efficient interaction with the materials. Figure 4 shows the trend of zeta potential in the pH range 3-11. From the measured trends it is possible to conclude that reference A material possesses a positive surface charge in the range 4-7.9 and this makes easy an interaction of the solid with negatively charged substrates. At neutral pH, in particular, BBS molecules bring a negative charge given essentially by dissociated COOH and phenolic Ph-OH groups, therefore the interaction with the alumina positive solid surface occurs very easily. Consequently, the isoelectric point (IEP) of hybrid materials shifts to lower pH increasing the BBS loading.  The amount of BBS actually loaded onto the support is obtained by calculating the percentage of weight loss for A-BBS0.1, A-BBS0.2, and A-BBS0.4 in the range of 220 to 650 • C and subtracting that observed for A support in the same range. The amount of organic matter actually immobilized on hybrid samples depends on the concentration of the BBS solution used for the functionalization: in these experiments the maximum loading has been reached by A-BBS0.4 with 7.2% of organic matter immobilized.

Zeta Potential
The zeta potential measurements have been used in order to evaluate the surface charge of the support and its modification after BBS functionalization. This is a very important indication, dealing with adsorption, in order to forecast the type of substrate that can be subjected to efficient interaction with the materials. Figure 4 shows the trend of zeta potential in the pH range 3-11. From the measured trends it is possible to conclude that reference A material possesses a positive surface charge in the range 4-7.9 and this makes easy an interaction of the solid with negatively charged substrates. At neutral pH, in particular, BBS molecules bring a negative charge given essentially by dissociated COOH and phenolic Ph-OH groups, therefore the interaction with the alumina positive solid surface occurs very easily. Consequently, the isoelectric point (IEP) of hybrid materials shifts to lower pH increasing the BBS loading.

Effect of CV Concentration
The chemical structure of CV is reported in Scheme 1. Different concentrations of dye from 10 to 100 ppm have been used for adsorption experiment keeping constant the amount of hybrid absorbents. Results reported in Figure 5 indicate that an increase of the dye concentration leads to a decrease in the amount of dye adsorbed from 43% to 17% for A-BBS0.1, from 80% to 37% for A-BBS0.2, and from 91% to 67% for A-BBS0.4. Although the removal decreases increasing CV concentration because of the adsorbing materials saturation, the amount of dye removed by A-BBS0.4 remains at the value of about 70%, indicating a very good performance. On this material the adsorption of atenolol and carbamazepine was performed.

Effect of CV Concentration
The chemical structure of CV is reported in Scheme 1.

Effect of CV Concentration
The chemical structure of CV is reported in Scheme 1.

Scheme 1. Structure of Crystal Violet.
Different concentrations of dye from 10 to 100 ppm have been used for adsorption experiment keeping constant the amount of hybrid absorbents. Results reported in Figure 5 indicate that an increase of the dye concentration leads to a decrease in the amount of dye adsorbed from 43% to 17% for A-BBS0.1, from 80% to 37% for A-BBS0.2, and from 91% to 67% for A-BBS0.4. Although the removal decreases increasing CV concentration because of the adsorbing materials saturation, the amount of dye removed by A-BBS0.4 remains at the value of about 70%, indicating a very good performance. On this material the adsorption of atenolol and carbamazepine was performed. Different concentrations of dye from 10 to 100 ppm have been used for adsorption experiment keeping constant the amount of hybrid absorbents. Results reported in Figure 5 indicate that an increase of the dye concentration leads to a decrease in the amount of dye adsorbed from 43% to 17% for A-BBS0.1, from 80% to 37% for A-BBS0.2, and from 91% to 67% for A-BBS0.4. Although the removal decreases increasing CV concentration because of the adsorbing materials saturation, the amount of dye removed by A-BBS0.4 remains at the value of about 70%, indicating a very good performance. On this material the adsorption of atenolol and carbamazepine was performed.

Adsorption Isotherms and Model Application
In Figure 6 the removal of CV by the hybrid adsorbents was reported in form of adsorption isotherms. Clearly A-BBS0.1 reached the saturation in our experimental conditions, whereas A-BBS0.2 and, even more, A-BBS0.4 are still far from the saturation.
Langmuir and Freundlich models were applied to the experimental data and the most significant results are reported in Table 4. A very good fitting was obtained applying Langmuir model, as indicated by very high values of r 2 . qm obtained by this model defines the amount of CV saturating the samples (monolayer amount): the value obtained for the best adsorbent A-BBS0.4 corresponds to a maximum adsorption of about 35 mg/g. The RL values (always 0 < RL < 1) obtained for all the samples indicate a favored adsorption, as also suggested by Freundlich n values always higher than 2.

Adsorption Isotherms and Model Application
In Figure 6 the removal of CV by the hybrid adsorbents was reported in form of adsorption isotherms. Clearly A-BBS0.1 reached the saturation in our experimental conditions, whereas A-BBS0. 2 and, even more, A-BBS0.4 are still far from the saturation.

Adsorption Isotherms and Model Application
In Figure 6 the removal of CV by the hybrid adsorbents was reported in form of adsorption isotherms. Clearly A-BBS0.1 reached the saturation in our experimental conditions, whereas A-BBS0.2 and, even more, A-BBS0.4 are still far from the saturation.
Langmuir and Freundlich models were applied to the experimental data and the most significant results are reported in Table 4   Langmuir and Freundlich models were applied to the experimental data and the most significant results are reported in Table 4. A very good fitting was obtained applying Langmuir model, as indicated by very high values of r 2 . q m obtained by this model defines the amount of CV saturating the samples (monolayer amount): the value obtained for the best adsorbent A-BBS0.4 corresponds to a maximum adsorption of about 35 mg/g. The R L values (always 0 < R L < 1) obtained for all the samples indicate a favored adsorption, as also suggested by Freundlich n values always higher than 2.  Figure 7 reports the kinetic of adsorption relative to two different CECs, Carbamazepine and Atenolol, whose structures are shown in Scheme 2. These CECs were chosen on the basis of their chemical structure: both molecules show positively charged structure at pH 6.5 [58], but while Atenolol possessed a branched aliphatic chain, Carbamazepine is a polyaromatic and more compact molecule.     Figure 7 reports the kinetic of adsorption relative to two different CECs, Carbamazepine and Atenolol, whose structures are shown in Scheme 2. These CECs were chosen on the basis of their chemical structure: both molecules show positively charged structure at pH 6.5 [58], but while Atenolol possessed a branched aliphatic chain, Carbamazepine is a polyaromatic and more compact molecule. Scheme 2. Structure of Carbamazepine [59] (left side) and Atenolol (right side) and related pKa [60,61].

Removal of CECs
The efficiency of the material remained acceptable with the larger polar molecule atenolol which is more easily adsorbed with 51% of removal in 30 min thanks to its polar structure and consequently stronger interaction with adsorbent, whereas in the case of carbamazepine only 32% of removal was reached in 30 min suggesting a worse affinity between the material and the polyaromatic substrate.
Langmuir and Freundlich models were also applied to CEC adsorption and the most significant results are reported in Table 5. Good fittings were observed applying both Langmuir and Freundlich models as indicated by very good values of r 2 . qm obtained by these models defines the amount of Carbamazepine and Atenolol saturating the samples (monolayer amount): the values obtained for the Carbamazepine and Atenolol correspond to maximum adsorption of about 3.06 and 3.26 mg/g respectively. A comparison study of capacity of adsorption for different adsorbents was performed Scheme 2. Structure of Carbamazepine [59] (left side) and Atenolol (right side) and related pKa [60,61].
The efficiency of the material remained acceptable with the larger polar molecule atenolol which is more easily adsorbed with 51% of removal in 30 min thanks to its polar structure and consequently stronger interaction with adsorbent, whereas in the case of carbamazepine only 32% of removal was reached in 30 min suggesting a worse affinity between the material and the polyaromatic substrate.
Langmuir and Freundlich models were also applied to CEC adsorption and the most significant results are reported in Table 5. Good fittings were observed applying both Langmuir and Freundlich models as indicated by very good values of r 2 . q m obtained by these models defines the amount of Carbamazepine and Atenolol saturating the samples (monolayer amount): the values obtained for the Carbamazepine and Atenolol correspond to maximum adsorption of about 3.06 and 3.26 mg/g respectively. A comparison study of capacity of adsorption for different adsorbents was performed and the results are reported in Table 6. Considering the literature, we have reached promising results in particular in the adsorption of Carbamazepine.

Conclusions
New hybrid absorbents were prepared following a very easy procedure and characterized via TGA, XRD, Zeta potential, FTIR spectroscopy, and nitrogen adsorption then, they were tested towards the removal of selected pollutants. BBS can functionalize the alumina surface given the electrostatic attraction of opposite charges carried by the two components. In this way, cationic pollutants can be captured by the negatively charged surface of the adsorbent. The electrostatic interaction is, therefore, the major driving force for the adsorption process, but the polarity of the pollutant molecules affects both kinetics and adsorbed amounts as less polar molecules interact at lower extent and in longer time.
The strength of these new hybrid materials is not so much in photocatalytic efficiency, (the comparison in CV removal is reported in Figure 8), as in the simplicity of preparation and in its definitely "green" way. In fact, not only the efficiency of the materials should be taken in account but also other aspects deserve to be considered, i.e., the easiness and the green aspects related to the materials preparation procedure and the energy and economic savings. In fact, several materials with very good performances reported in Table 1 are prepared following complex procedures and use of non-green processes. In order to perform this comparison, we considered only the materials with better performance in CV adsorption with respect to the A-BBS hybrid system and we classified them on the basis of the following aspects: energy consumption (calcination and/or thermal treatment at temperature higher than 200 • C), multistep/complex synthesis, use of non-green compounds (acids, bases, solvents and so on). In the classification we assigned a color code from green to orange to red considering the absence of these negative aspects or their simultaneous presence during the preparation. As evidenced in Figure 8, several materials possessing much better adsorption capacity with respect to the A-BBS system show an orange or red color, whereas only few, as A-BBS0.4, show the green color.
From this perspective, the A-BBS materials can be developed further and their performance optimized in view of their use in in-field applications. Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials green to orange to red considering the absence of these negative aspects or their simultaneous presence during the preparation. As evidenced in Figure 8, several materials possessing much better adsorption capacity with respect to the A-BBS system show an orange or red color, whereas only few, as A-BBS0.4, show the green color. From this perspective, the A-BBS materials can be developed further and their performance optimized in view of their use in in-field applications.  Table 1). The colors have been assigned on the base of aspects related to possible upscaling of the processes involving the materials: energy consumption, multistep/complex synthesis, use of non-green compounds, as described in the text.  Table 1). The colors have been assigned on the base of aspects related to possible upscaling of the processes involving the materials: energy consumption, multistep/complex synthesis, use of non-green compounds, as described in the text.
Author Contributions: All the authors contributed the same way to the preparation of this paper.
Funding: This project has received funding from the European Union's Horizon 2020 research and innovative programme under the Marie Sklodowska-Curie grant agreement No 645551.

Conflicts of Interest:
The authors declare no conflict of interest.