Novel Adsorbent Based on Banana Peel Waste for Removal of Heavy Metal Ions from Synthetic Solutions

Due to its valuable compounds, food waste has been gaining attention in different applications, such as life quality and environment. Combined with circular economy requirements, a valorization method for waste, especially banana waste, was to convert them into adsorbents with advanced properties. The banana waste, after thermal treatment, was used with high removal performances (100%) for the removal of heavy metals, such as Cr, Cu, Pb, and Zn, but their small particle size makes them very hard to recover and reuse. For this reason, a biopolymeric matrix was used to incorporate the banana waste. The matrix was chosen for its remarkable properties, such as low cost, biodegradability, low carbon footprint, and reduced environmental impact. In this research, different types of materials (simple banana peel ash BPA and combined with biopolymeric matrix, ALG–BPA, CS–BPA) were prepared, characterized, and tested. The materials were characterized by means of attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), optical microscopy (OM), scanning electron microscopy (SEM), and tested for the removal of metal ions from synthetic solutions using atomic absorption spectroscopy (AAS). The ALG–BPA material proved to be the most efficient in the removal of heavy metal ions from synthetic solution, reaching even 100% metal removal for Cr, Fe, Pb, and Zn, while the CS-based materials were the least efficient, presenting the best values for Cr and Fe ions with a removal efficiency of 34.14% and 28.38%, respectively. By adding BPA to CS, the adsorption properties of the material were slightly improved, but also only for Cr and Fe ions, to 37.09% and 57.78%.


Introduction
Some of the biggest problems the world is facing today are gravitating around waste and pollution. Resources are extracted, processed, used, and ultimately stored as waste. At the end of the life cycle, the waste is usually incinerated (or heat processed) or stored in the field. Circular economy tries to combat both these problems by applying the "3 Rs": Reduction (demand and/or consumption of resources, materials, and products), Reuse, and Recycling (return of materials to another life cycle) [1]. In this case, waste is always considered a value-added substance. The conventional methods to deal with food waste include thermal and chemical treatment that generate smaller molecules (CO, CO 2 , CH 4 , H 2 O, NH 3 , etc.), solids (compost, slag, and ash), but the environmental risk of air pollution does not disappear, contributing to the increase in greenhouse gases and the amount of waste and wastewater [2]. By comparison, the recovery of waste through emerging technologies does not lead to the destruction of nutrients or other useful components, and

Preparation of the Banana Peel Ash (BPA)
A modified method proposed by Yang et al. [41] was used to obtain BPB, where the banana peels were charred and then activated using a 30% NaOH solution instead of KOH. To obtain the BPB, banana peels were dried in an oven at 140 • C for 4 h. The resulting banana peels were then subjected to two-stage pyrolysis in a calcination furnace. In the first stage of pyrolysis at a temperature of 700 • C, for 1 h, and a heating rate of 3 • C/min, amorphous coal was obtained. To remove partially charred intermediate compounds, the product was treated with NaOH at a weight ratio of 1:3, followed by a heating process at 800 • C for 1 h. After these steps, the BPA was washed with distilled water and dried until constant mass.

Preparation of the Alginate Microbeads
To prepare the ALG and ALG-BPA (1:1) microbeads, first, a 1% solution was prepared by dissolving the desired amount of sodium alginate in hot water and under vigorous stirring. The formed viscous solution was put in an ultrasound bath to eliminate the air bubbles. Half of the solution was put aside to later prepare the ALG microbeads, and the other half was mixed with the BPA to form the ALG-BPA (1:1) microbeads. The ALG and ALG-BPA blend were dripped in a 2% CaCl 2 bath for reticulation and microbead formation, as can be seen in Figure 1. containing heavy metal ions as the first matrix for future experiments regarding real industrial waters.

Preparation of the Banana Peel Ash (BPA)
A modified method proposed by Yang et al. [41] was used to obtain BPB, where the banana peels were charred and then activated using a 30% NaOH solution instead of KOH. To obtain the BPB, banana peels were dried in an oven at 140 °C for 4 h. The resulting banana peels were then subjected to two-stage pyrolysis in a calcination furnace. In the first stage of pyrolysis at a temperature of 700 °C, for 1 h, and a heating rate of 3 °C/min, amorphous coal was obtained. To remove partially charred intermediate compounds, the product was treated with NaOH at a weight ratio of 1:3, followed by a heating process at 800 °C for 1 h. After these steps, the BPA was washed with distilled water and dried until constant mass.

Preparation of the Alginate Microbeads
To prepare the ALG and ALG-BPA (1:1) microbeads, first, a 1% solution was prepared by dissolving the desired amount of sodium alginate in hot water and under vigorous stirring. The formed viscous solution was put in an ultrasound bath to eliminate the air bubbles. Half of the solution was put aside to later prepare the ALG microbeads, and the other half was mixed with the BPA to form the ALG-BPA (1:1) microbeads. The ALG and ALG-BPA blend were dripped in a 2% CaCl2 bath for reticulation and microbead formation, as can be seen in Figure 1. The beads were left overnight for reticulation in the CaCl2 solution and then washed with distilled water until the total removal of Cl − ions (testing with AgNO3). After washing, the beads were left to dry overnight, and they are presented in Figure 2. The beads were left overnight for reticulation in the CaCl 2 solution and then washed with distilled water until the total removal of Cl − ions (testing with AgNO 3 ). After washing, the beads were left to dry overnight, and they are presented in Figure 2.

Preparation of the Chitosan Microbeads
To prepare the CS and CS-BPA (1:1) microbeads, first, a 1% solution was obtained by dissolving the desired amount of chitosan in 1% acetic acid and under vigorous stirring. The formed viscous solution was put in an ultrasound bath to eliminate the air bubbles. Half of the solution was put aside to later prepare the CS microbeads, and the other half was mixed with the BPA to form the CS-BPA (1:1) microbeads. The CS solution and the CS-BPA blend were crosslinked with glutaraldehyde for reticulation and the blends started to separate as the microbeads were forming. Then, the materials were washed abundantly with distilled water using a Büchner funnel. After washing, the materials were left to dry overnight and are presented in Figure 3.

Characterization of the Microbeads
Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (Interspec 200-X Spectrophotometer, Interspectrum, Tõravere, Estonia) was used to characterize the samples before and after the addition of BPA.
To determine the structure and morphology of the eco-materials, Optical Microscopy (OM) (OLYMPUS BX51 M microscope, Tokyo, Japan) and Scanning electron microscopy (SEM) coupled with energy-dispersive spectra EDS (QUANTA 450 FEG microscope, Eindhoven, The Netherlands), equipped with a field emission gun and a 1.2 nm resolution X-ray energy dispersive spectrometer, with a resolution of 133 eV) were used.

Removal Efficiency Testing of the Materials
Batch adsorption experiments analysis of a multielement aqueous solution were performed by using a ContrAA ® 800D Atomic Absorption Spectrometry (AAS) system from Analytik, Jena, Germany, with acetylene flame with specific wavelengths between 185 and 900 nm. In total, 0.5 g of each material was put in contact with 100 mL of a multielement solution of 0.5 mg/L concentration. The water solutions containing the metal ions were prepared by diluting the standard solution with purified water. These solutions were in the pH range 4-5. The materials were left in contact with the solution for one hour, and

Preparation of the Chitosan Microbeads
To prepare the CS and CS-BPA (1:1) microbeads, first, a 1% solution was obtained by dissolving the desired amount of chitosan in 1% acetic acid and under vigorous stirring. The formed viscous solution was put in an ultrasound bath to eliminate the air bubbles. Half of the solution was put aside to later prepare the CS microbeads, and the other half was mixed with the BPA to form the CS-BPA (1:1) microbeads. The CS solution and the CS-BPA blend were crosslinked with glutaraldehyde for reticulation and the blends started to separate as the microbeads were forming. Then, the materials were washed abundantly with distilled water using a Büchner funnel. After washing, the materials were left to dry overnight and are presented in Figure 3.

Preparation of the Chitosan Microbeads
To prepare the CS and CS-BPA (1:1) microbeads, first, a 1% solution was obtained by dissolving the desired amount of chitosan in 1% acetic acid and under vigorous stirring. The formed viscous solution was put in an ultrasound bath to eliminate the air bubbles. Half of the solution was put aside to later prepare the CS microbeads, and the other half was mixed with the BPA to form the CS-BPA (1:1) microbeads. The CS solution and the CS-BPA blend were crosslinked with glutaraldehyde for reticulation and the blends started to separate as the microbeads were forming. Then, the materials were washed abundantly with distilled water using a Büchner funnel. After washing, the materials were left to dry overnight and are presented in Figure 3.

Characterization of the Microbeads
Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (Interspec 200-X Spectrophotometer, Interspectrum, Tõravere, Estonia) was used to characterize the samples before and after the addition of BPA.
To determine the structure and morphology of the eco-materials, Optical Microscopy (OM) (OLYMPUS BX51 M microscope, Tokyo, Japan) and Scanning electron microscopy (SEM) coupled with energy-dispersive spectra EDS (QUANTA 450 FEG microscope, Eindhoven, The Netherlands), equipped with a field emission gun and a 1.2 nm resolution X-ray energy dispersive spectrometer, with a resolution of 133 eV) were used.

Removal Efficiency Testing of the Materials
Batch adsorption experiments analysis of a multielement aqueous solution were performed by using a ContrAA ® 800D Atomic Absorption Spectrometry (AAS) system from Analytik, Jena, Germany, with acetylene flame with specific wavelengths between 185 and 900 nm. In total, 0.5 g of each material was put in contact with 100 mL of a multielement solution of 0.5 mg/L concentration. The water solutions containing the metal ions were prepared by diluting the standard solution with purified water. These solutions were in the pH range 4-5. The materials were left in contact with the solution for one hour, and

Characterization of the Microbeads
Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (Interspec 200-X Spectrophotometer, Interspectrum, Tõravere, Estonia) was used to characterize the samples before and after the addition of BPA.
To determine the structure and morphology of the eco-materials, Optical Microscopy (OM) (OLYMPUS BX51 M microscope, Tokyo, Japan) and Scanning electron microscopy (SEM) coupled with energy-dispersive spectra EDS (QUANTA 450 FEG microscope, Eindhoven, The Netherlands), equipped with a field emission gun and a 1.2 nm resolution X-ray energy dispersive spectrometer, with a resolution of 133 eV) were used.

Removal Efficiency Testing of the Materials
Batch adsorption experiments analysis of a multielement aqueous solution were performed by using a ContrAA ® 800D Atomic Absorption Spectrometry (AAS) system from Analytik, Jena, Germany, with acetylene flame with specific wavelengths between 185 and 900 nm. In total, 0.5 g of each material was put in contact with 100 mL of a multielement solution of 0.5 mg/L concentration. The water solutions containing the metal ions were prepared by diluting the standard solution with purified water. These solutions were in the pH range 4-5. The materials were left in contact with the solution for one hour, and every 15 min, samples were taken from the solution to test the remaining concentration of metal ions selected in the study (Cd, Co, Cr, Cu, Fe, Mn, Pb, and Zn). From the absorbance given by the AAS, the concentrations were calculated, and the removal efficiency was calculated using the following formula: where C i was the concentration of the water solution before contact with adsorbante material (at 0 min), and C f was the concentration of the solution after a certain amount of time (f = 15, 30, 45, 60 min).

Results and Discussion
The spectrum for the BPA sample presented in Figure 4 shows a peak located at 3358 cm −1 , which is attributed to the stretching vibration of the O-H bonds, that suggests that the material contains amorphous silicate or possible aluminum silicate and the peak at 1639 cm −1 is attributed to bending vibrations of H 2 O molecules [42]. The peaks at 1419 cm −1 and 1018 cm −1 are specific for stretching vibration of Si-O-Si and Al-O-Si bonds [43].
Materials 2021, 14, x FOR PEER REVIEW 5 of 16 every 15 min, samples were taken from the solution to test the remaining concentration of metal ions selected in the study (Cd, Co, Cr, Cu, Fe, Mn, Pb, and Zn). From the absorbance given by the AAS, the concentrations were calculated, and the removal efficiency was calculated using the following formula: where C was the concentration of the water solution before contact with adsorbante material (at 0 min), and C was the concentration of the solution after a certain amount of time (f = 15, 30, 45, 60 min).

ATR-FTIR Analysis
The spectrum for the BPA sample presented in Figure 4 shows a peak located at 3358 cm −1 , which is attributed to the stretching vibration of the O-H bonds, that suggests that the material contains amorphous silicate or possible aluminum silicate and the peak at 1639 cm −1 is attributed to bending vibrations of H2O molecules [42]. The peaks at 1419 cm −1 and 1018 cm −1 are specific for stretching vibration of Si-O-Si and Al-O-Si bonds [43].

ALG and ALG-BPA Microbeads
The ATR-FTIR analyses for the alginate-based microbeads are presented in Figure 5, and the peak data are presented in Table 1. The spectrum for the ALG sample presents peaks from stretching and bending vibration given by the basic functional groups present in the calcium alginate chemical structure. Stretching vibrations of O-H bonds of alginate appear in the range of 3000-3600 cm −1 , and the stretching vibrations of aliphatic C-H were observed at 2920-2850 cm −1 . The bands at 1650 and 1400 cm −1 were attributed to asymmetric and symmetric vibrations of carboxylate salt ions and 1029 cm −1 (C-O stretch, primary hydroxyl group) [27,31,44]. The ALG-BPA spectrum has a lower intensity for the OH band, indicating the encapsulation of the BPA in the ALG.

ALG and ALG-BPA Microbeads
The ATR-FTIR analyses for the alginate-based microbeads are presented in Figure 5, and the peak data are presented in Table 1. The spectrum for the ALG sample presents peaks from stretching and bending vibration given by the basic functional groups present in the calcium alginate chemical structure. Stretching vibrations of O-H bonds of alginate appear in the range of 3000-3600 cm −1 , and the stretching vibrations of aliphatic C-H were observed at 2920-2850 cm −1 . The bands at 1650 and 1400 cm −1 were attributed to asymmetric and symmetric vibrations of carboxylate salt ions and 1029 cm −1 (C-O stretch, primary hydroxyl group) [27,31,44]. The ALG-BPA spectrum has a lower intensity for the OH band, indicating the encapsulation of the BPA in the ALG.  The ATR-FTIR analyses for the chitosan-based microbeads are presented in Figure 6, and the peak data are presented in Table 2. The basic characteristic peaks of the chitosan were shown at 3308 cm −1 (O-H stretch and N-H stretch, overlapped), 2929 and 2865 cm −1 (C-H stretch), 1641 cm −1 (NH2 deformation), 1556 cm −1 (N-H bend), 1029 cm −1 (C-O stretch, primary hydroxyl group), as reported in the literature [27,45]. When compared to the CS spectrum, the one for CS-BPA shows that the peak for O-H stretch and N-H stretch has a lower intensity, meaning that the BPA is bounded to the CS chemical structure.

CS and CS-BPA Microbeads
The ATR-FTIR analyses for the chitosan-based microbeads are presented in Figure 6, and the peak data are presented in Table 2. The basic characteristic peaks of the chitosan were shown at 3308 cm −1 (O-H stretch and N-H stretch, overlapped), 2929 and 2865 cm −1 (C-H stretch), 1641 cm −1 (NH 2 deformation), 1556 cm −1 (N-H bend), 1029 cm −1 (C-O stretch, primary hydroxyl group), as reported in the literature [27,45]. When compared to the CS spectrum, the one for CS-BPA shows that the peak for O-H stretch and N-H stretch has a lower intensity, meaning that the BPA is bounded to the CS chemical structure.     Figure 7 for the simple ALG sample and Figure 8 for the ALG-BPA 1:1 sample.      It can be observed that for the ALG sample, the size is located in the range of 900-1200 µm, these being characteristic of the particles after dehydration. They maintain their spherical shape, as can be seen in Figure 7. The incorporation of BPA in the ALG mass can be seen in Figure 8, the white particles of BPA being evenly distributed in the mass of ALG, having dimensions in the range of 20-50 µm. It can be observed that for the ALG sample, the size is located in the range of 900-1200 µm, these being characteristic of the particles after dehydration. They maintain their spherical shape, as can be seen in Figure 7. The incorporation of BPA in the ALG mass can be seen in Figure 8, the white particles of BPA being evenly distributed in the mass of ALG, having dimensions in the range of 20-50 µm.

CS and CS-BPA (1:1) Micro Beads
CS and CS-BPA microbeads were analyzed by optical microscopy (OM) under a polarized light optical microscope. The images obtained at magnifications of 50, 100 and 200×, respectively, are shown in Figure 9 for the simple CS sample and Figure 10  It can be observed in Figure 8 that the size of crosslinked CS particles is located in the range of 300-900 µm, but also the glassy aspect of the surface, these being characteristic  Figure 9 for the simple CS sample and Figure 10  It can be observed that for the ALG sample, the size is located in the range of 900-1200 µm, these being characteristic of the particles after dehydration. They maintain their spherical shape, as can be seen in Figure 7. The incorporation of BPA in the ALG mass can be seen in Figure 8, the white particles of BPA being evenly distributed in the mass of ALG, having dimensions in the range of 20-50 µm.  Figure 9 for the simple CS sample and Figure 10  It can be observed in Figure 8 that the size of crosslinked CS particles is located in the range of 300-900 µm, but also the glassy aspect of the surface, these being characteristic  It can be observed that for the ALG sample, the size is located in the range of 900-1200 µm, these being characteristic of the particles after dehydration. They maintain their spherical shape, as can be seen in Figure 7. The incorporation of BPA in the ALG mass can be seen in Figure 8, the white particles of BPA being evenly distributed in the mass of ALG, having dimensions in the range of 20-50 µm.  Figure 9 for the simple CS sample and Figure 10  It can be observed in Figure 8 that the size of crosslinked CS particles is located in the range of 300-900 µm, but also the glassy aspect of the surface, these being characteristic It can be observed in Figure 8 that the size of crosslinked CS particles is located in the range of 300-900 µm, but also the glassy aspect of the surface, these being characteristic of CS particles, after dehydration. The incorporation of BPA in the CS mass can be observed in Figure 9 by the white particles in the dehydrated CS mass, and respecting the same magnification order, the size of the BPA aggregates in the range 20-150 µm.

BPA
In order to study the morphology of the BPA surface obtained from banana peel waste, scanning electron microscopy (SEM) analyses, coupled with energy dispersive spectroscopy (EDS), were performed and also to identify the elements present in the sample. As can be seen in Figure 11a, the surfaces show homogeneity and porous appearance of the ash particles. The EDS pattern for the BPA sample ( Figure 11b) exhibit a carbon content of 6.68%, oxygen 53.44%, magnesium 1.31%, aluminum 1.02%, silicon 35.97%, potassium 0.47%, and calcium 1.12%. served in Figure 9 by the white particles in the dehydrated CS mass, and respecting the same magnification order, the size of the BPA aggregates in the range 20-150 µm.

BPA
In order to study the morphology of the BPA surface obtained from banana peel waste, scanning electron microscopy (SEM) analyses, coupled with energy dispersive spectroscopy (EDS), were performed and also to identify the elements present in the sample. As can be seen in Figure 11a, the surfaces show homogeneity and porous appearance of the ash particles. The EDS pattern for the BPA sample ( Figure 11b) exhibit a carbon content of 6.68%, oxygen 53.44%, magnesium 1.31%, aluminum 1.02%, silicon 35.97%, potassium 0.47%, and calcium 1.12%.
(a) (b) Figure 11. SEM images for BPA sample at 4000× (a) and the EDS pattern (b).

ALG and ALG-BPA Microbeads
The SEM-EDS analysis was used to determine the surface morphologies of ALG and ALG-BPA (1:1) microbeads, and it is presented in Figures 12 and 13. For the ALG sample (Figure 12a), the surface revealed nonaggregate microspheres with irregular surfaces, high roughness, and cracks caused by collapsing of the polymer layers during dehydration. Similar results were reported by Lagoa et al. [22]. The energy dispersive X-ray (EDS) patterns of ALG (Figure 12b For the ALG-BPA (1:1) sample (Figure 13a), the surface shows small beads deposited in the cracks and all over the surface, indicating the encapsulation of the BPA in the ALG matrix. The energy dispersive X-ray (EDS) patterns of ALG-BPA (1:1) (Figure 13b) exhibit

ALG and ALG-BPA Microbeads
The SEM-EDS analysis was used to determine the surface morphologies of ALG and ALG-BPA (1:1) microbeads, and it is presented in Figures 12 and 13. For the ALG sample (Figure 12a), the surface revealed nonaggregate microspheres with irregular surfaces, high roughness, and cracks caused by collapsing of the polymer layers during dehydration. Similar results were reported by Lagoa et al. [22]. The energy dispersive X-ray (EDS) patterns of ALG (Figure 12b Figure 9 by the white particles in the dehydrated CS mass, and respecting the same magnification order, the size of the BPA aggregates in the range 20-150 µm.

BPA
In order to study the morphology of the BPA surface obtained from banana peel waste, scanning electron microscopy (SEM) analyses, coupled with energy dispersive spectroscopy (EDS), were performed and also to identify the elements present in the sample. As can be seen in Figure 11a, the surfaces show homogeneity and porous appearance of the ash particles. The EDS pattern for the BPA sample ( Figure 11b) exhibit a carbon content of 6.68%, oxygen 53.44%, magnesium 1.31%, aluminum 1.02%, silicon 35.97%, potassium 0.47%, and calcium 1.12%.
(a) (b) Figure 11. SEM images for BPA sample at 4000× (a) and the EDS pattern (b).

ALG and ALG-BPA Microbeads
The SEM-EDS analysis was used to determine the surface morphologies of ALG and ALG-BPA (1:1) microbeads, and it is presented in Figures 12 and 13. For the ALG sample (Figure 12a), the surface revealed nonaggregate microspheres with irregular surfaces, high roughness, and cracks caused by collapsing of the polymer layers during dehydration. Similar results were reported by Lagoa et al. [22]. The energy dispersive X-ray (EDS) patterns of ALG (Figure 12b For the ALG-BPA (1:1) sample (Figure 13a), the surface shows small beads deposited in the cracks and all over the surface, indicating the encapsulation of the BPA in the ALG matrix. The energy dispersive X-ray (EDS) patterns of ALG-BPA (1:1) (Figure 13b) exhibit

CS and CS-BPA Microbeads
The morphology and structure of the CS and CS-BPA microbeads are presented in Figures 14 and 15. The CS sample presents a nonporous, smooth membranous phase consisting of dome-shaped orifices, microfibrils, and crystallites. The energy dispersive X-ray (EDS) patterns of CS microbeads (Figure 14b In Figure 15a, we can see that the surface of the CS-BPA (1:1) sample is irregular, with different aggregates formed on the surface, indicating the encapsulation of BPA in the chitosan chemical structure. The energy dispersive X-ray (EDS) patterns of the sample (Figure 15b) exhibit carbon content of 64.74 wt.%, oxygen of 33.28 wt.%, silicon of 1.73 wt.%, and traces of aluminum. The Al appears due to the support on which the sample is analyzed in the SEM equipment.

CS and CS-BPA Microbeads
The morphology and structure of the CS and CS-BPA microbeads are presented in Figures 14 and 15. The CS sample presents a nonporous, smooth membranous phase consisting of dome-shaped orifices, microfibrils, and crystallites. The energy dispersive X-ray (EDS) patterns of CS microbeads (Figure 14b In Figure 15a, we can see that the surface of the CS-BPA (1:1) sample is irregular, with different aggregates formed on the surface, indicating the encapsulation of BPA in the chitosan chemical structure. The energy dispersive X-ray (EDS) patterns of the sample (Figure 15b) exhibit carbon content of 64.74 wt.%, oxygen of 33.28 wt.%, silicon of 1.73 wt.%, and traces of aluminum. The Al appears due to the support on which the sample is analyzed in the SEM equipment.

Testing of the BPA
Samples were taken for analysis every 15 min, up to 60 min, with three repetitions per experiment. The averages of the three absorbance values read for each element are represented graphically in Figure 16a. In Figure 15a, we can see that the surface of the CS-BPA (1:1) sample is irregular, with different aggregates formed on the surface, indicating the encapsulation of BPA in the chitosan chemical structure. The energy dispersive X-ray (EDS) patterns of the sample (Figure 15b) exhibit carbon content of 64.74 wt.%, oxygen of 33.28 wt.%, silicon of 1.73 wt.%, and traces of aluminum. The Al appears due to the support on which the sample is analyzed in the SEM equipment.  It can be observed from Figure 16b that the removal efficiency is high for all the metals studied, reaching 100% for Zn after only 30 min of contact and for Cd and Fe after 60 min of contact. For all metals studied, the removal efficiency was over 97% after 30 min of contact. Similar results for different metal ions were obtained by other researchers when using wood ash. In a study by Mosoarca et al. [37], ash obtained from wood-burning was used to retain Mn 2+ ions from wastewater. They found that the material presents an adsorption efficiency of over 90% but can be increased to over 98% by parameter control, with best values obtained for pH~6, a material dose of 10 g/L. Wood ash was also studied by Pehlivan et al. [38] for the removal of Cr 4+ ions. They observed that the adsorption of Cr 4+ ions was higher at a pH between 2 and 2.5. Borlodoi et al. studied how ash obtained from banana peels can remove iron ions from groundwater from a concentration of 20 mg/L to 0.3 mg/L [39]. Das et al. [40] obtained ashes from bamboo, banana leaf, banana rind, banana pseudo-stem, and rice husk, tested them for the removal of Fe 2+ ions from water samples and found the most efficient to be the banana pseudo-stem ash.
Due to the small particle size that provides a high surface area and thus a maximum efficiency in removing metals, the regeneration process is difficult. Thus, after testing in multielement solution, for 60 min, the BPA material was dried and weighed, and it was observed that the recovery yield of the BPA material is about 25%; the losses thus influ- It can be observed from Figure 16b that the removal efficiency is high for all the metals studied, reaching 100% for Zn after only 30 min of contact and for Cd and Fe after 60 min of contact. For all metals studied, the removal efficiency was over 97% after 30 min of contact. Similar results for different metal ions were obtained by other researchers when using wood ash. In a study by Mosoarca et al. [37], ash obtained from wood-burning was used to retain Mn 2+ ions from wastewater. They found that the material presents an adsorption efficiency of over 90% but can be increased to over 98% by parameter control, with best values obtained for pH~6, a material dose of 10 g/L. Wood ash was also studied by Pehlivan et al. [38] for the removal of Cr 4+ ions. They observed that the adsorption of Cr 4+ ions was higher at a pH between 2 and 2.5. Borlodoi et al. studied how ash obtained from banana peels can remove iron ions from groundwater from a concentration of 20 mg/L to 0.3 mg/L [39]. Das et al. [40] obtained ashes from bamboo, banana leaf, banana rind, banana pseudo-stem, and rice husk, tested them for the removal of Fe 2+ ions from water samples and found the most efficient to be the banana pseudo-stem ash.
Due to the small particle size that provides a high surface area and thus a maximum efficiency in removing metals, the regeneration process is difficult. Thus, after testing in multielement solution, for 60 min, the BPA material was dried and weighed, and it was observed that the recovery yield of the BPA material is about 25%; the losses thus influence the high efficiency of the material. Based on these findings, the possibility of incorporating BPA material in biopolymeric ecological matrices, such as chitosan and alginate, was studied.

Testing the Polysaccharide-BPA Materials
The materials were agitated with a magnetic stirrer to enhance the metal adsorption, and samples were taken every 15 min to observe the decrease in metal ions concentration. The metals studied were Cd, Co, Cr, Cu, Fe, Mn, Pb, and Zn. It can be observed from Figure 17a,b, that, when using ALG and ALG-BPA materials, in a period of 1 h, the absorbance for most metal ions decreases significantly, indicating a decrease in metal ion concentration in the water sample. Similar results for ALG-based materials were observed by researchers. In a study by Rapa et al. [33], alginate was combined in different proportions with starch and nano clays to form some new composites for the removal of Cu 2+ ions from waters. They observed that the optimal ratio between components alginate/starch/n-clay was 1/2/3 (dried components) with a maximum removal efficiency of 95% for Cu 2+ ions after 25 h of contact. The removal of Cu 2+ , Mg 2+ , Fe 2+ , and Pb 2+ by using alginate combined with different nanocellulose biosorbents was studied by Abou-Zeid et al. [32]. They found that tri-carboxylate cellulose nanofibers (TPC-CNF) combined with alginate presented high removal efficiency for the metal ions studied with the best values obtained for Pb (95%) and Cu (92%), but a lower removal efficiency for Mg (54%) and Fe (43%). was studied.

Testing the Polysaccharide-BPA Materials
The materials were agitated with a magnetic stirrer to enhance the metal adsorption, and samples were taken every 15 min to observe the decrease in metal ions concentration. The metals studied were Cd, Co, Cr, Cu, Fe, Mn, Pb, and Zn. It can be observed from Figure 17a,b, that, when using ALG and ALG-BPA materials, in a period of 1 h, the absorbance for most metal ions decreases significantly, indicating a decrease in metal ion concentration in the water sample. Similar results for ALG-based materials were observed by researchers. In a study by Rapa et al. [33], alginate was combined in different proportions with starch and nano clays to form some new composites for the removal of Cu 2+ ions from waters. They observed that the optimal ratio between components alginate/starch/n-clay was 1/2/3 (dried components) with a maximum removal efficiency of 95% for Cu 2+ ions after 25 h of contact. The removal of Cu 2+ , Mg 2+ , Fe 2+ , and Pb 2+ by using alginate combined with different nanocellulose biosorbents was studied by Abou-Zeid et al. [32]. They found that tri-carboxylate cellulose nanofibers (TPC-CNF) combined with alginate presented high removal efficiency for the metal ions studied with the best values obtained for Pb (95%) and Cu (92%), but a lower removal efficiency for Mg (54%) and Fe (43%). For the CS and CS-BPA (1:1) microbeads, it can be observed in Figure 18a,b that the impact over the removal efficiency of heavy metal ions is less noticed in the case of the CS sample, while the CS-BPA sample proved to be efficient in partial removal of some metal ions. Ngah and Fatinathan [34] studied the removal of Cu(II) ions from an aqueous solution of chitosan beads, chitosan-GLA 1:1, 2:1 ratio beads, and chitosan-alginate beads, and based on their results, all materials are efficient for Cu(II) removal from water solutions. Modified chitosan was studied by Zhao et al. for the removal of Cd(II) and Pb (II) ions [35]. In a study by Matei et al. [36], magnetite nanoparticles were incorporated in a chitosan matrix and tested for the removal of Cr 6+ ions from aqueous solutions. The biocompatible composite proved to be suitable for monolayer adsorption of Cr 6+ ions on the porous surface of the material and presented a removal efficiency of 91% for a concentration of 0.5 mg/L Cr 6+ ions. For the CS and CS-BPA (1:1) microbeads, it can be observed in Figure 18a,b that the impact over the removal efficiency of heavy metal ions is less noticed in the case of the CS sample, while the CS-BPA sample proved to be efficient in partial removal of some metal ions. Ngah and Fatinathan [34] studied the removal of Cu(II) ions from an aqueous solution of chitosan beads, chitosan-GLA 1:1, 2:1 ratio beads, and chitosan-alginate beads, and based on their results, all materials are efficient for Cu(II) removal from water solutions. Modified chitosan was studied by Zhao et al. for the removal of Cd(II) and Pb (II) ions [35]. In a study by Matei et al. [36], magnetite nanoparticles were incorporated in a chitosan matrix and tested for the removal of Cr 6+ ions from aqueous solutions. The biocompatible composite proved to be suitable for monolayer adsorption of Cr 6+ ions on the porous surface of the material and presented a removal efficiency of 91% for a concentration of 0.5 mg/L Cr 6+ ions.
The removal efficiencies for heavy metal ions from water solution for the four types of microbeads are presented in Figure 19a-h. It can be observed that the most efficient material for the removal of most metals studied is the ALG-BPA (1:1) microbeads. They present over 95% removal efficiency for the metals studied and 100% removal efficiency for Cr, Fe, Pb, and Zn. The ALG microbeads also presented good removal efficiencies of metal ions, with values over 90%. The least efficient material was the CS microbeads. The best values were obtained for Cr and Fe ions with a removal efficiency of 34.14% and 28.38%, respectively. By adding BPA to CS, the adsorption properties of the material were slightly improved, but also only for Cr and Fe ions, to 37.09% and 57.78%. The removal efficiencies for heavy metal ions from water solution for the four types of microbeads are presented in Figure 19a-h. It can be observed that the most efficient material for the removal of most metals studied is the ALG-BPA (1:1) microbeads. They present over 95% removal efficiency for the metals studied and 100% removal efficiency for Cr, Fe, Pb, and Zn. The ALG microbeads also presented good removal efficiencies of metal ions, with values over 90%. The least efficient material was the CS microbeads. The best values were obtained for Cr and Fe ions with a removal efficiency of 34.14% and 28.38%, respectively. By adding BPA to CS, the adsorption properties of the material were slightly improved, but also only for Cr and Fe ions, to 37.09% and 57.78%.  The removal efficiencies for heavy metal ions from water solution for the four types of microbeads are presented in Figure 19a-h. It can be observed that the most efficient material for the removal of most metals studied is the ALG-BPA (1:1) microbeads. They present over 95% removal efficiency for the metals studied and 100% removal efficiency for Cr, Fe, Pb, and Zn. The ALG microbeads also presented good removal efficiencies of metal ions, with values over 90%. The least efficient material was the CS microbeads. The best values were obtained for Cr and Fe ions with a removal efficiency of 34.14% and 28.38%, respectively. By adding BPA to CS, the adsorption properties of the material were slightly improved, but also only for Cr and Fe ions, to 37.09% and 57.78%.

Conclusions
Banana waste was thermally treated to prepare a new eco-material with a high removal efficiency of heavy metal ions from synthetic solutions. The BPA obtained, although very efficient (over 97% removal efficiency for all metals investigated), presented a big problem with recovery and reuse, so the BPA was incorporated in biopolymeric matrixes (ALG and CS). These materials were tested in aqueous solutions containing heavy metal ions as the first matrix for future experiments regarding real industrial waters. The materials were characterized in terms of structural and morphological properties using ATR-FTIR, OM, SEM-EDS, and batch adsorption tests using AAS. From FTIR analysis, it was observed a lowering in the concentration of hydroxyl groups, indicating the presence of BPA in the structure of ALG and CS-based microbeads. From the OM and SEM-EDS analysis, the morphology and surface structure of the materials were studied, and it was observed that the materials with BPA presented an irregular surface and irregular aggregates. The most relevant results were obtained for ALG-BPA microbeads, which presented 100% removal efficiency for Cr, Fe, Pb, and Zn and above 90% for the other metal ions. The least adsorbent material was the one with CS, presenting results for Cr and Fe ions with a removal efficiency of 34.14% and 28.38%, respectively. By adding BPA to CS, the adsorption properties of the material were slightly improved but also only for Cr and Fe ions to 37.09 % and 57.78%. Although adding BPA to CS improved the adsorption properties of the material, it was still less efficient than the ALG-based microbeads in the removal of heavy metals from water solutions.

Conclusions
Banana waste was thermally treated to prepare a new eco-material with a high removal efficiency of heavy metal ions from synthetic solutions. The BPA obtained, although very efficient (over 97% removal efficiency for all metals investigated), presented a big problem with recovery and reuse, so the BPA was incorporated in biopolymeric matrixes (ALG and CS). These materials were tested in aqueous solutions containing heavy metal ions as the first matrix for future experiments regarding real industrial waters. The materials were characterized in terms of structural and morphological properties using ATR-FTIR, OM, SEM-EDS, and batch adsorption tests using AAS. From FTIR analysis, it was observed a lowering in the concentration of hydroxyl groups, indicating the presence of BPA in the structure of ALG and CS-based microbeads. From the OM and SEM-EDS analysis, the morphology and surface structure of the materials were studied, and it was observed that the materials with BPA presented an irregular surface and irregular aggregates. The most relevant results were obtained for ALG-BPA microbeads, which presented 100% removal efficiency for Cr, Fe, Pb, and Zn and above 90% for the other metal ions. The least adsorbent material was the one with CS, presenting results for Cr and Fe ions with a removal efficiency of 34.14% and 28.38%, respectively. By adding BPA to CS, the adsorption properties of the material were slightly improved but also only for Cr and Fe ions to 37.09% and 57.78%. Although adding BPA to CS improved the adsorption properties of the material, it was still less efficient than the ALG-based microbeads in the removal of heavy metals from water solutions.