pH and pCl Operational Parameters in Some Metallic Ions Separation with Composite Chitosan/Sulfonated Polyether Ether Ketone/Polypropylene Hollow Fibers Membranes

The development of new composite membranes is required to separate chemical species from aggressive environments without using corrective reagents. One such case is represented by the high hydrochloric acid mixture (very low pH and pCl) that contains mixed metal ions, or that of copper, cadmium, zinc and lead ions in a binary mixture (Cu–Zn and Cd–Pb) or quaternary mixture. This paper presents the obtaining of a composite membrane chitosan (Chi)–sulfonated poly (ether ether ketone) (sPEEK)–polypropylene hollow fiber (Chi/sPEEK/PPHF) and its use in the separation of binary or quaternary mixtures of copper, cadmium, zinc, and lead ions by nanofiltration and pertraction. The obtained membranes were morphologically and structurally characterized using scanning electron microscopy (SEM), high resolution SEM (HR–SEM), energy dispersive spectroscopy analysis (EDAX), Fourier Transform InfraRed (FTIR) spectroscopy, thermogravimetric analysis, and differential scanning calorimetry (TGA-DSC), but also used in preliminary separation tests. Using the ion solutions in hydrochloric acid 3 mol/L, the separation of copper and zinc or cadmium and lead ions from binary mixtures was performed. The pertraction results were superior to those obtained by nanofiltration, both in terms of extraction efficiency and because at pertraction, the separate cation was simultaneously concentrated by an order of magnitude. The mixture of the four cations was separated by nanofiltration (at 5 bars, using a membrane of a 1 m2 active area) by varying two operational parameters: pH and pCl. Cation retention could reach 95% when adequate values of operational parameters were selected. The paper makes some recommendations for the use of composite membranes, chitosan (Chi)–sulfonated poly (ether ether ketone) (sPEEK)–polypropylene hollow fiber (Chi/sPEEK/PPHF), so as to obtain the maximum possible retention of the target cation.


Introduction
Metals with atomic numbers (Z) greater than iron's (Z = 26) are known as "heavy metals", and their technical and economic importance is so great that their per capita consumption was an indicator of quality of life during the period of intensive industrial development [1,2]. Of these metals, copper, cadmium, zinc, and lead have outstanding technical applications in electronics and electrical engineering, construction, transportation,  2 , NaCl, chitosan, and glacial acetic acid (analytical grade, Sigma-Aldrich Chemie GmbH, Steinheim, Germany) were used in the studies. NaOH pellets, H 2 SO 4 (96%), HCl 35% ultrapure and NH 4 OH 25% (analytical grade) were purchased from Merck KGaA Darmstadt, Germany.

Materials and Methods
Ultrapure water was used for preparing the feed solutions.
Ultrapure water was used for preparing the feed solutions.

Materials
The characteristics of the polymeric compounds and derivatives used in the study are presented in Table 1. Organic polar solvents 1.9 Chi Acidulated water 6.5 * pKa values as reported in scientific literature [53][54][55] The hollow polypropylene fibers used as support for membranes were provided by GOST Ltd., Perugia, Italy [56−58].

Sulfonated Polyether Ether Ketone (sPEEK) Preparation
Three hundred mL H2SO4 of 96% concentration was placed in a 500 mL glass vial with a sealing cap, after which 25 g of polymer (PEEK) was gradually added, stirring PEEK 30.000 Sulfuric acid -Steinheim, Germany) were used in the studies. NaOH pellets, H2SO4 (96%), HCl 35% ultrapure and NH4OH 25% (analytical grade) were purchased from Merck KGaA Darmstadt, Germany. Ultrapure water was used for preparing the feed solutions.

Materials
The characteristics of the polymeric compounds and derivatives used in the study are presented in Table 1. Organic polar solvents 1.9 Chi Acidulated water 6.5 * pKa values as reported in scientific literature [53][54][55] The hollow polypropylene fibers used as support for membranes were provided by GOST Ltd., Perugia, Italy [56−58].

Sulfonated Polyether Ether Ketone (sPEEK) Preparation
Three hundred mL H2SO4 of 96% concentration was placed in a 500 mL glass vial with a sealing cap, after which 25 g of polymer (PEEK) was gradually added, stirring sPEEK Organic polar solvents 1.9 Membranes 2022, 12, x FOR PEER REVIEW 4 of 26 continuously, manually, to avoid agglomeration of the polymer. After about 2 hours of stirring, the bottle with th e polymer solution was kept without stirring for up to 24 hours in order to promote complete dissolution of the polymer in acid. This time range should not be exceeded because additional sulfonation of the polymer would occur. After processing the reagent mass by removing traces of sediment and filtering suspended materials (gels or other impurities), a clear light orange solution of 4.4% sulfonated poly (ether ether ketone) (sPEEK) was obtained. The gravimetric percentage composition of the solution, determined by electrochemical titration, was 4.40% sulfonated poly (ether ether ketone) (sPEEK), 90.36% H2SO4, and 5.24% water.
According to the same method, a solution of 30 g PEEK and 300 mL of 96% sulfuric acid was prepared. A solution of 5.2% sPEEK concentration was obtained, with various degrees of sulfonation depending on the storage time of the solution. The gravimetric percentage composition of the solution, determined by electrochemical titration, was 5.20% sulfonated poly (ether ether ketone) (sPEEK), 89.33% H2SO4, and 5.47% water. The colors of the solutions varied from light orange to brown, indicating the different degree of sulfonation ( Figure 1) [55,56].

Sulfonated Polyether Ether Ketone (sPEEK) Preparation
Three hundred mL H 2 SO 4 of 96% concentration was placed in a 500 mL glass vial with a sealing cap, after which 25 g of polymer (PEEK) was gradually added, stirring continuously, manually, to avoid agglomeration of the polymer. After about 2 h of stirring, the bottle with the polymer solution was kept without stirring for up to 24 h in order to promote complete dissolution of the polymer in acid. This time range should not be exceeded because additional sulfonation of the polymer would occur. After processing the reagent mass by removing traces of sediment and filtering suspended materials (gels or other impurities), a clear light orange solution of 4.4% sulfonated poly (ether ether ketone) (sPEEK) was obtained. The gravimetric percentage composition of the solution, determined by electrochemical titration, was 4.40% sulfonated poly (ether ether ketone) (sPEEK), 90.36% H 2 SO 4 , and 5.24% water.
According to the same method, a solution of 30 g PEEK and 300 mL of 96% sulfuric acid was prepared. A solution of 5.2% sPEEK concentration was obtained, with various degrees of sulfonation depending on the storage time of the solution. The gravimetric percentage composition of the solution, determined by electrochemical titration, was 5.20% sulfonated poly (ether ether ketone) (sPEEK), 89.33% H 2 SO 4 , and 5.47% water. The colors The determination of the degree of sulfonation and the acidity index were reported [55][56][57], and their presentation can be easily followed electrochemically, but also chemically.

Obtaining Composite Membranes
The solution of sulfonated poly (ether ether ketone) (sPEEK) was used as-is for impregnation of membranes used as such for impregnating hollow-fiber polypropylene membranes using the impregnation method [57,58]. sPEEK/HFPP (sulfonated poly (ether ether ketone)/polypropylene hollow-fiber) composite membranes can be conditioned either by drying or immersion in water or aqueous solutions. In order to obtain the chitosan/sulfonated poly (ether ether ketone)/polypropylene hollow-fiber Chi/sPEEK/PPHF composite membrane, the sulfonated poly (ether ether ketone)/polypropylene hollow-fiber sPEEK/PPHF composite membrane was immersed in a solution of 3% chitosan in 3% acetic acid [59]. Each type of membrane is washed with water and dried in vacuum for 24 h at 50 • C.
Drying of the obtained membranes is necessary for characterization, but for use in the separation tests this step can be avoided. However, the membrane polypropylene support is not affected by drying, being carried out at a moderate temperature and under vacuum. At the same time, the attack capacity of sulfuric acid is diminished by the presence of polyether ether ketone.
Membrane materials are characterized by the determination of porosity [60], morphology [61], thermal characteristics [62], and the ion-exchange capacity and retention of heavy metal ions [63].

Separation Techniques
The binary solution (Cu 2+ -Zn 2+ ) can be prepared from the available reagents without restrictions, but the aqueous solutions containing Cd 2+ -Pb 2+ of all four cations require precautions and therefore the corresponding nitrates were used.
In order to perform the tests, nanofiltration and pertraction experiments were performed in installations with a tubular configuration module [65], presented in Figures 2 and 3. The volume of solution subjected to the experiments was 10 L, and the ion concentration was in the range of 10 −6 -10 −4 mol/L. The chosen operational parameters were pH and pCl, the variation of which was conducted with hydrochloric acid, sodium hydroxide, or ammonia and sodium chloride.   The operating conditions of pertraction are presented for each test performed separately, and the nanofiltration was carried out in the installation with the characteristics shown in Table 2. The pertraction installation has the same geometric characteristics of the membrane module (length, diameter, and membrane surface), but operates at atmospheric pressure. The circulation of the source phase was ensured by the volumetric recirculation pumping through the exterior of the hollow-fiber composite membranes, and of the receiving phase through its interior. The flow rate of the source phase was 100-1000 mL/min, and of the receiving phase was 10-100 mL/min.

Performances Materials, Membranes, and Processes Determination Procedure
The fluxes from the source phase [55,56] were determined against the measured permeate mass within a determined time range, applying the following equation: where: M = permeate mass (g), S = effective surface of the membrane (m 2 ), t = the time necessary to collect the permeate volume (h). The extraction efficiency (EE%) in the separation process or retention (R) in the nanofiltration process of analytes was calculated using the concentration or absorbance of the solutions [57,58]: where c f is the final concentration of the solute (metallic ions), and c 0 is the initial concentration of the solute (metallic ions). The concentration of metal ions was determined spectrophotometrically for binary solutions, considering the additivity of the individual absorbances of the complexed ions (see Equation (3)). For the ternary system, the concentrations were determined through atomic absorption spectrometry (AAS) using the characteristic wavelengths: where A t is the overall absorbance, and A 1 ad A 2 are the specific absorbances of each ion at the chosen wavelength [62,64].

Equipment
The scanning microscopy studies, SEM and HR-SEM were performed on a Hitachi S4500 system (Hitachi High-Technologies Europe GmbH, Mannheim, Germany) [66].
Thermal analysis (TG-DSC) was performed with a STA 449C Jupiter apparatus from Netzsch (NETZSCH-Gerätebau GmbH, Selb, Germany). Each sample weighed approximately 10 mg. The samples were placed in an open alumina crucible and heated up to 900 • C with 10 K·min −1 rate, under flow of 50 mL·min −1 dried air. As reference, we used an empty alumina crucible. The evolved gases were analyzed with a FTIR Tensor 27 from Bruker (Bruker Co., Ettlingen, Germany), equipped with a thermostat gas cell [67].
The UV-Vis analyses of the solutions were carried out on a CamSpec M550 Spectrophotometer (Spectronic CamSpec Ltd., Leeds, UK) [68].
The electrochemical processes were followed up with a PARSTAT 2273 Potentiostat (Princeton Applied Research, AMETEK Inc., Berwyn, PA, USA). A glass cell with a threeelectrode setup was used [69].
The pH and pCl of the medium were followed up with a combined selective electrode (HI 4107, Hanna Instruments Ltd, Leighton Buzzard, UK) and a multi-parameter system (HI 5522, Hanna Instruments Ltd., Leighton Buzzard, UK) [70].

Results and Discussions
Separation of low-concentration ions from various aqueous systems of complex composition is an important goal in the field of membranes and membrane processes. Heavy metal ion-poor systems have been treated with various membrane processes including reverse osmosis, direct osmosis, nanofiltration, dialysis, electrodialysis, or liquid membranes. Both the need to use a small amount and number of the reagents and recuperative separation continue to encourage research.
This paper used synthetic solutions that properly simulate aqueous solutions from the recovery of waste from electronics and electrical engineering, especially those from Cu-Zn and Pb-Cd batteries. Such solutions contain, for example, in concentrations of 10 −6 -10 −4 mol/L, ions of copper, cadmium, zinc, and lead in the mixture.
The use of operational parameters pH and pCl is based on the equilibria (4)−(7) that were established in the hydrochloric supply solutions: These equilibria have been extensively studied [71][72][73] and chemical speciation plays a key role in the approached membrane separation processes.

Scanning Electron Microscopy Study
The morphology obtained through scanning electron microscopy provides important information both for the operation for the conditioning of the membranes in the permeation module. Figure 3 shows sections and details of the composite membranes obtained. It can be observed that polypropylene hollow-fiber membrane (PPHF) (Figure 4a (Figures 4c and 5), and the membrane surface had pores and micropores specific to ultrafiltration membranes [74]. Subsequent deposition of chitosan to obtain the composite membrane chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow fiber (Chi/sPEEK/HFPP) did not significantly increase the thickness of the membrane wall ( Figure 4e), but significantly changed the appearance of the surface composite fibers (Figure 4f).    It is interesting that energy dispersive X-ray analysis (EADX) ( Figure 6) highlights the change in surface composition. Thus, if only carbon and oxygen atoms appear in the polypropylene support (Figure 6a), in the composite membranes both the atomic ratio C:O and the composition change due to the appearance of sulfur atoms (Figure 6b,c). Although it is a strictly local analysis, energy dispersive X-ray analysis (EADX) is very useful to qualitatively follow the process of membrane formation as well as the process of separation of the complex system. When evaluating the layers of composite membrane the mode of examination by electron microscopy must be taken into account, under advanced vacuum. In such conditions, the only thickness that was maintained was that of the supporting polypropylene fiber (unaffected by the vacuum). In the normal state, the sPEEK or Chi/sPEEK layers of the composite membranes were strongly hydrated, while in vacuum they were completely dehydrated, thus affecting the illustrated results (see Figures 4c-f and 5).
It is interesting that energy dispersive X-ray analysis (EADX) ( Figure 6) highlights the change in surface composition. Thus, if only carbon and oxygen atoms appear in the polypropylene support (Figure 6a), in the composite membranes both the atomic ratio C:O and the composition change due to the appearance of sulfur atoms (Figure 6b,c). Although it is a strictly local analysis, energy dispersive X-ray analysis (EADX) is very useful to qualitatively follow the process of membrane formation as well as the process of separation of the complex system.

Fourier Transform InfraRed (FTIR) and UV-Vis Spectrometry Analysis
Structurally, the studied membranes have specific functions that could be highlighted by Fourier Transform InfraRed (FTIR) spectral analysis (Figure 7). At the same time, the UV-Vis spectrum indicated significant differences ( Figure 8).
The FTIR spectra revealed, for each constituent of the composite membrane (Chi/sPEEK/PPHF), characteristic absorption bands; for sPEEK ( Figure 7b)), one can observe the peak at 1656 cm −1 (as a small shoulder) assigned to the stretching mode of C=O, and the indicative peaks for the presence of O=S=O groups at 1223, 1078, and 1019 cm −1 . The sharp peak at 1592 cm −1 (C=C) had a reduced intensity and this feature is attributed to the crosslinking of the sPEEK chains with oxygen, as described in the literature [75][76][77][78]. The PPHF FTIR-ATR spectrum ( Figure 7a) showed peaks centered at 2955, 2920, 2866, and 2843 cm −1 , very characteristic features of stretching vibration modes of -CH 2 -, -CH-, and -CH 3 groups from polypropylene material. It is a known fact that the polypropylene polymer is susceptible to oxidation; therefore, in the PPHF spectrum one can observe a maximum at 1731 cm −1 corresponding to the formation of oxygen-containing groups. For the composite membrane (Chi/sPEEK/PPHF) (Figure 7c), a strong band centered at 3291 cm −1 corresponds to N-H and O-H stretching, as well as the intramolecular hydrogen bonds present in the chitosan structure. The absorption bands at around 2921 and 2869 cm −1 can be attributed to C-H symmetric and asymmetric stretching, respectively. The presence of residual N-acetyl groups was confirmed by the bands at around 1645 cm −1 (C=O stretching of amide I) and at 1558 cm −1 that correspond to the N-H bending of amide II. The strong band at 1026 cm −1 corresponds to C-O stretching and obscured the other characteristic bands of sPEEK and PPHF in this region.
These characteristics are useful to the experimenter that will be able to effortlessly select the fibers needed to enter the specific permeation module. UV-Vis spectrometry examination may also be an indicator of membrane aging [78].

Thermal Behaviour of Membrane Materials and Composite Membrane
Thermal analysis, TG-DSC (thermogravimetry and differential scanning calorimetry), was performed with a STA 449C F3 apparatus, from Netzsch (Selb, Germany), between 20 and 900 • C, in a dynamic (50 mL/min) air atmosphere. The evolved gases were analyzed with a FTIR Tensor 27 from Bruker (Bruker Co., Ettlingen, Germany) equipped with a thermostatic gas cell.
The diagram of the three basic materials for obtaining composite membranes is presented as a whole in Figure 9.

Fourier Transform InfraRed (FTIR) and UV-Vis Spectrometry Analysis
Structurally, the studied membranes have specific functions that could be highlighted by Fourier Transform InfraRed (FTIR) spectral analysis (Figure 7). At the same time, the UV-Vis spectrum indicated significant differences ( Figure 8).

Fourier Transform InfraRed (FTIR) and UV-Vis Spectrometry Analysis
Structurally, the studied membranes have specific functions that could be highlighted by Fourier Transform InfraRed (FTIR) spectral analysis (Figure 7). At the same time, the UV-Vis spectrum indicated significant differences ( Figure 8).  The FTIR spectra revealed, for each constituent of the composite membrane (Chi/sPEEK/PPHF), characteristic absorption bands; for sPEEK ( Figure 7b)), one can observe the peak at 1656 cm −1 (as a small shoulder) assigned to the stretching mode of C=O, and the indicative peaks for the presence of O=S=O groups at 1223, 1078, and 1019 cm −1 . The sharp peak at 1592 cm −1 (C=C) had a reduced intensity and this feature is attributed to the crosslinking of the sPEEK chains with oxygen, as described in the literature [75][76][77][78]. The PPHF FTIR-ATR spectrum (Figure 7a) showed peaks centered at 2955, 2920, 2866, and 2843 cm −1 , very characteristic features of stretching vibration modes of -CH 2 -, -CH-, and -CH 3 groups from polypropylene material. It is a known fact that the polypropylene polymer is susceptible to oxidation; therefore, in the PPHF spectrum one can observe a maximum at 1731 cm −1 corresponding to the formation of oxygen-containing groups. For the composite membrane (Chi/sPEEK/PPHF) (Figure 7c), a strong band centered at 3291 cm −1 corresponds to N-H and O-H stretching, as well as the intramolecular hydrogen bonds present in the chitosan structure. The absorption bands at around 2921 and 2869 cm −1 can be attributed to C-H symmetric and asymmetric stretching, respectively. The presence of residual N-acetyl groups was confirmed by the bands at around 1645 cm −1 (C=O stretching of amide I) and at 1558 cm −1 that correspond to the N-H bending of amide II. The strong band at 1026 cm −1 corresponds to C-O stretching and obscured the other characteristic bands of sPEEK and PPHF in this region.
These characteristics are useful to the experimenter that will be able to effortlessly select the fibers needed to enter the specific permeation module. UV-Vis spectrometry examination may also be an indicator of membrane aging [78]. These characteristics are useful to the experimenter that will be able to effortlessly select the fibers needed to enter the specific permeation module. UV-Vis spectrometry examination may also be an indicator of membrane aging [78].

Thermal Behaviour of Membrane Materials and Composite Membrane
Thermal analysis, TG-DSC (thermogravimetry and differential scanning calorimetry), was performed with a STA 449C F3 apparatus, from Netzsch (Selb, Germany), between 20 and 900 °C, in a dynamic (50 mL/min) air atmosphere. The evolved gases were analyzed with a FTIR Tensor 27 from Bruker (Bruker Co., Ettlingen, Germany) equipped with a thermostatic gas cell.
The diagram of the three basic materials for obtaining composite membranes is presented as a whole in Figure 9.

Thermal Behaviour of Membrane Materials and Composite Membrane
Thermal analysis, TG-DSC (thermogravimetry and differential scanning calorimetry), was performed with a STA 449C F3 apparatus, from Netzsch (Selb, Germany), between 20 and 900 • C, in a dynamic (50 mL/min) air atmosphere. The evolved gases were analyzed with a FTIR Tensor 27 from Bruker (Bruker Co., Ettlingen, Germany) equipped with a thermostatic gas cell.
The diagram of the three basic materials for obtaining composite membranes is presented as a whole in Figure 9. The composite membrane sample presented three mass loss steps on the TG curve ( Figure 10). The first loss of 3.32% was recorded up to 185 °C. Two small endothermic effects accompanied the process. The first one, with a minimum at 58.1 °C, can be ascribed to the loss of some residual water molecules as the evolved gases FTIR analysis indicated The composite membrane sample presented three mass loss steps on the TG curve ( Figure 10). The first loss of 3.32% was recorded up to 185 • C. Two small endothermic effects accompanied the process. The first one, with a minimum at 58.1 • C, can be ascribed to the loss of some residual water molecules as the evolved gases FTIR analysis indicated ( Figure 11). The composite membrane sample presented three mass loss steps on the TG curve ( Figure 10). The first loss of 3.32% was recorded up to 185 °C. Two small endothermic effects accompanied the process. The first one, with a minimum at 58.1 °C, can be ascribed to the loss of some residual water molecules as the evolved gases FTIR analysis indicated ( Figure 11). The melting of the PP fibers generated the second effect with the onset at 160.7 °C [68]. In the temperature interval 185-420 °C we recorded a mass loss of 45.10% in multiple, overlapped processes as indicated by the multiple peaks from DSC curve. Both PP and chitosan polymeric chains were broken in smaller fragments after 200 °C, and the oxidation of these fragments generated the overall exothermic small peak from 302.2 °C [79]. In parallel, sPEEK went through a desulfonation process. On the DSC curve the thermal effects overlapped, endothermic from decomposition and exothermic from oxidation, the resulting overall exothermic effect, indicating the prevalence of the oxidation processes. The exothermic peak from 410.8 °C is due to oxidation of the last PP fragments [70]. The last mass loss step, representing 47.14%, took place after 420 °C when slow oxidation of chitosan and sPEEK fragments was completed, coupled with the burning of the carbonaceous residual mass from 617.2 °C. The melting of the PP fibers generated the second effect with the onset at 160.7 • C [68]. In the temperature interval 185-420 • C we recorded a mass loss of 45.10% in multiple, overlapped processes as indicated by the multiple peaks from DSC curve. Both PP and chitosan polymeric chains were broken in smaller fragments after 200 • C, and the oxidation of these fragments generated the overall exothermic small peak from 302.2 • C [79]. In parallel, sPEEK went through a desulfonation process. On the DSC curve the thermal effects overlapped, endothermic from decomposition and exothermic from oxidation, the resulting overall exothermic effect, indicating the prevalence of the oxidation processes. The exothermic peak from 410.8 • C is due to oxidation of the last PP fragments [70]. The last mass loss step, representing 47.14%, took place after 420 • C when slow oxidation of chitosan and sPEEK fragments was completed, coupled with the burning of the carbonaceous residual mass from 617.2 • C.
The 3D FTIR plot (Figure 11a) presents the evolution of the FTIR spectrum vs. temperature. By projecting this map in 2D space (wavenumber vs. temperature), we can easily identify the components and temperature intervals when they are eliminated from the sample (Figure 11b). The FTIR spectra recorded for the evolved gases indicate the presence of water at low temperature (under 100 • C) and some traces of acetic acid over 150 • C. In the second mass-loss step, over 185 • C the FTIR analysis of evolved gases indicated the presence of CO 2 , H 2 O, and traces of CO, which are normal components for the oxidation reaction of organics. We also identified the presence of SO 2 from sPEEK desulfonation (the absorption band from 1381 cm −1 ), acetic acid from chitosan decomposition (the absorption bands from 1796 and 1180 cm −1 ), and other saturated and unsaturated hydrocarbons (the absorption bands around 3000 cm −1 ).
The melting of the PP fibers generated the second effect with the onset at 160.7 • C [68]. In the temperature interval 185-420 • C we recorded a mass loss of 45.10% in multiple, overlapped processes as indicated by the multiple peaks from DSC curve. Both PP and chitosan polymeric chains were broken in smaller fragments after 200 • C, and the oxidation of these fragments generated the overall exothermic small peak from 302.2 • C [79]. In parallel, sPEEK went through a desulfonation process. On the DSC curve the thermal effects overlapped, endothermic from decomposition and exothermic from oxidation, the resulting overall exothermic effect, indicating the prevalence of the oxidation processes. The exothermic peak from 410.8 • C is due to oxidation of the last PP fragments [70]. The last mass loss step, representing 47.14%, took place after 420 • C when slow oxidation of chitosan and sPEEK fragments was completed, coupled with the burning of the carbonaceous residual mass from 617.2 • C. The 3D FTIR plot (Figure 11a) presents the evolution of the FTIR spectrum vs. temperature. By projecting this map in 2D space (wavenumber vs. temperature), we can easily identify the components and temperature intervals when they are eliminated from the sample (Figure 11b). The FTIR spectra recorded for the evolved gases indicate the presence of water at low temperature (under 100 °C) and some traces of acetic acid over 150 °C. In the second mass-loss step, over 185 °C the FTIR analysis of evolved gases indicated the presence of CO2, H2O, and traces of CO, which are normal components for the oxidation reaction of organics. We also identified the presence of SO2 from sPEEK desulfonation (the absorption band from 1381 cm −1 ), acetic acid from chitosan decomposition (the absorption bands from 1796 and 1180 cm −1 ), and other saturated and unsaturated hydrocarbons (the absorption bands around 3000 cm −1 ). The 3D FTIR plot (Figure 11a) presents the evolution of the FTIR spectrum vs. temperature. By projecting this map in 2D space (wavenumber vs. temperature), we can easily identify the components and temperature intervals when they are eliminated from the sample (Figure 11b). The FTIR spectra recorded for the evolved gases indicate the presence of water at low temperature (under 100 • C) and some traces of acetic acid over 150 • C. In the second mass-loss step, over 185 • C the FTIR analysis of evolved gases indicated the presence of CO 2 , H 2 O, and traces of CO, which are normal components for the oxidation reaction of organics. We also identified the presence of SO 2 from sPEEK desulfonation (the absorption band from 1381 cm −1 ), acetic acid from chitosan decomposition (the absorption bands from 1796 and 1180 cm −1 ), and other saturated and unsaturated hydrocarbons (the absorption bands around 3000 cm −1 ).

Preliminary Ion Separation Tests with Prepared Composite Membranes
The recuperative separation of copper, cadmium, zinc, and lead ions was studied by two membrane processes: nanofiltration and pertraction (Figures 2 and 3), using binary systems: copper-zinc and lead-cadmium, as well as quaternary systems: copper, cadmium, zinc, and lead.
The nanofiltration or pertraction module had a usable membrane area of 1 m 2 , the difference in behavior of the membranes being imposed by the forced circulation under pressure (nanofiltration) or flow through the outside of the membrane of the source phase with the capture of the ions transported in the receiving phase inside the membrane (pertraction) (Figures 2 and 3).
The operation in the nanofiltration process was performed under a pressure of 4−6 bar, with a recirculation flow rate of 0.1-1 L/min for the source phase through the exterior of the composite hollow-fiber membrane (Table 2, Figure 3).
In the case of pertraction, the module played the role of the contactor. The source phase circulated through the outside of the composite hollow-fiber membrane at a flow rate of 0.1-1 L/min, and inside them the receiving phase had a flow rate of 0.10-0.100 L/min ( Figure 2).
The variable operational parameters were pH and pCl, trying to use as few reagents as possible, considering the supply with a strong acidic solution that also contains excess chloride ions (coming from both hydrochloric acid and sodium chloride).
The pH was adjusted either with sodium hydroxide solution or with ammonia, which is also formed in elution systems.

Nanofiltration Separation of Binary Systems with Composite Membranes from 3 mol/L Hydrochloric Acid Solutions
The first system studied consisted of a 3 mol/L hydrochloric acid solution containing copper and zinc ions in an equimolar mixture of 10 −4 mol/L concentration. This system can be separated with chitosan membranes (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow fiber (Chi/sPEEK/PPHF), since sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow fibers (sPEEK/PPHF) are unable to interact with [MCl 4 ] 2− anions.
Copper and zinc form with hydrochloric acid complex combinations of different stabilities depending on their concentration equilibria (4) and (5). Because in the presence of 3 M HCl zinc forms a complex [ZnCl 4 ] 2− anion, while copper remains in the form of Cu 2+ cations or less stable complexes, the two ions can be separated by passage through a module with a membrane of chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)polypropylene hollow fiber (Chi/sPEEK/PPHF). The complex anion [ZnCl 4 ] 2− will be retained on the membrane while Cu 2+ will pass through the module without being retained, the tetra-chloro cupper complex being much less stable [80].
Elution of Zn 2+ from the module was conducted in the presence of HCl 3·10 −2 M, which no longer provided conditions for the existence of complex anions and Zn 2+ cations left the membrane (Figure 12). The scheme shown in Figure 12a is specific to the nanofiltration of chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow-fiber (Chi/sPEEK/PPHF) membranes. and copper ions compared with that of cadmium and zinc ions ( Figure 13). The recuper-ative separation of zinc and cadmium ions, which form more stable tetra-chloro complexes, after 4 hours of operation, exceeded 70%, while for copper and especially lead ions it did not reach 40 %. One explanation for these results would be that the membrane, however, retains some of the lead or copper ions either in the form of anion complexes or expels them (does not allow passage through the membrane) as positive ions.
The results in a single separation stage are promising, especially since the concentration of test ions in the feed was relatively high, i.e., 10 −4 mol/L.  The second system studied consisted of a 3 mol/L hydrochloric acid solution containing lead and cadmium ions in an equimolar mixture of 10 −4 mol/L concentration.
The lead chloride was insoluble and that of cadmium was very weakly dissociated, but in strongly hydrochloric solution (3 mol/L HCl) and at a temperature of 50 • C, by nanofiltration through chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow-fiber (Chi/sPEEK/PPHF) membranes, the cadmium as an anion [CdCl 4 ] 2− was retained while the lead passed into the permeate as lead cations (Figure 12b).
The appearance of the retention-elution curves in the case of nanofiltration (5 bars and 200 mL/min feed solution flow) indicated a worse performance when separating lead and copper ions compared with that of cadmium and zinc ions ( Figure 13). The recuperative separation of zinc and cadmium ions, which form more stable tetra-chloro complexes, after 4 h of operation, exceeded 70%, while for copper and especially lead ions it did not reach 40%. One explanation for these results would be that the membrane, however, retains some of the lead or copper ions either in the form of anion complexes or expels them (does not allow passage through the membrane) as positive ions.
The results in a single separation stage are promising, especially since the concentration of test ions in the feed was relatively high, i.e., 10 −4 mol/L.  Figure 14a shows the separation of the copper-zinc system of equimolar concentration 10 −4 mol/L through membranes of chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow fiber (Chi/sPEEK/PPHF) in the pertraction module in which 1.0 mol/L ammonia receptor solution flowed through the fibers, which contributes to the fixation of zinc ions as a tetra-ammonia ion. The process control was performed by monitoring the concentration of zinc ions in the receiving phase, the end of the process being considered when reaching the degree of recovery of 90% of zinc. Unlike nanofiltration, in this process, the tetra-chloro-zincate ion passed through the membrane, and the cupric ion remained mostly in the supply.

Pertraction Separation of Binary Systems with Composite Membranes from 3 mol/L Hydrochloric Acid Solutions
On one hand, this behavior is justified by the particular mechanism of pertraction, in which the cationic groups of chitosan in the strongly acidic environment favor the transport of the more stable anion, ZnCl4 2− , but also the fact that copper ions, Cu 2+ , reaching the interface with the basic ammonia solution are retained by the membrane which, in that section, will have the free amino groups.
The lead-cadmium system behaved in the same way (Figure 14a), thus confirming the proposed transport mechanism (Figure 14b). After four hours of operation, the recovery of the two ions in the receiving solution exceeded 90%. It is noteworthy that the cadmium ion separated with noticeably better efficiency from its lead system than the zinc ion from the zinc-copper system.  Figure 14a shows the separation of the copper-zinc system of equimolar concentration 10 −4 mol/L through membranes of chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow fiber (Chi/sPEEK/PPHF) in the pertraction module in which 1.0 mol/L ammonia receptor solution flowed through the fibers, which contributes to the fixation of zinc ions as a tetra-ammonia ion. The process control was performed by monitoring the concentration of zinc ions in the receiving phase, the end of the process being considered when reaching the degree of recovery of 90% of zinc. Unlike nanofiltration, in this process, the tetra-chloro-zincate ion passed through the membrane, and the cupric ion remained mostly in the supply.

Pertraction Separation of Binary Systems with Composite Membranes from 3 mol/L Hydrochloric Acid Solutions
On one hand, this behavior is justified by the particular mechanism of pertraction, in which the cationic groups of chitosan in the strongly acidic environment favor the transport of the more stable anion, ZnCl 4 2− , but also the fact that copper ions, Cu 2+ , reaching the interface with the basic ammonia solution are retained by the membrane which, in that section, will have the free amino groups.
The lead-cadmium system behaved in the same way (Figure 14a), thus confirming the proposed transport mechanism (Figure 14b). After four hours of operation, the recovery of the two ions in the receiving solution exceeded 90%. It is noteworthy that the cadmium ion separated with noticeably better efficiency from its lead system than the zinc ion from the zinc-copper system.
The advantage of the extraction process, from the point of view of the higher extraction efficiency, is doubled by the fact that the receiving solution, being 10 times smaller in volume than the receiving solution (1L NH 3 aqueous solution 1.0 mol/L), led to the concentration of the separate chemical species. An important disadvantage of extraction is the additional consumption of reagents (ammonia).

Separation of Quaternary Systems with Composite Membranes by Nanofiltration
If the source of the aqueous solution from which the copper, zinc, cadmium, and lead ions are to be recovered has a pH that falls within the normal pH scale, as is the case for surface waters contaminated with the ions considered, the use of nanofiltration through the prepared composite membranes can have as operational parameters both the pH of the feed and the pCl (salinity induced with sodium chloride). The advantage of the extraction process, from the point of view of the higher extraction efficiency, is doubled by the fact that the receiving solution, being 10 times smaller in volume than the receiving solution (1L NH3 aqueous solution 1.0 mol/L), led to the concentration of the separate chemical species. An important disadvantage of extraction is the additional consumption of reagents (ammonia).

Separation of Quaternary Systems with Composite Membranes by Nanofiltration
If the source of the aqueous solution from which the copper, zinc, cadmium, and lead ions are to be recovered has a pH that falls within the normal pH scale, as is the case for surface waters contaminated with the ions considered, the use of nanofiltration through the prepared composite membranes can have as operational parameters both the pH of the feed and the pCl (salinity induced with sodium chloride). Table 3 presents the main parameters of copper, zinc, cadmium, and lead ions in aqueous solutions [80,81] of variable pH and pCl considered in establishing the operating parameters for the nanofiltration of solutions containing a mixture of these ions. These parameters must be correlated with the ionic charge of the functional groups of the composite membrane (Table 2), depending on the pH of the aqueous environment in which it operates, as follows: at pH up to 1.9, the functional groups were in the form -SO3H and -NH 3+ ; between 2 and 6.4 were-SO 3− and -NH 3+ ; and after pH 6.5 the groups became -SO 3− and NH2 (Figure 15).  Table 3 presents the main parameters of copper, zinc, cadmium, and lead ions in aqueous solutions [80,81] of variable pH and pCl considered in establishing the operating parameters for the nanofiltration of solutions containing a mixture of these ions. These parameters must be correlated with the ionic charge of the functional groups of the composite membrane (Table 2), depending on the pH of the aqueous environment in which it operates, as follows: at pH up to 1.9, the functional groups were in the form -SO 3 H and -NH 3+ ; between 2 and 6.4 were-SO 3− and -NH 3+ ; and after pH 6.5 the groups became -SO 3− and NH 2 (Figure 15).
At the same time, the way in which the pH variation is achieved can influence the efficiency of the separation (retention) of the four cations in the nanofiltration process of 10 L equimolar solution 10 −4 mol/L ions of copper, zinc, cadmium, and lead, at 5 bars with chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow-fiber (Chi/sPEEK/PPHF) composite membranes with active surfaces of 1 m 2 , at a recirculation rate of 0.20 L/min. The pH variation can be achieved by neutralizing the initial stock solution of zero pH (1 mol/L HCl solution) with solid sodium hydroxide or using only hydrochloric acid. In the first case, the pH varied from 0 to 8, and pCl remained identical, and in the second case both pH and pCl had identical values (Table 4).  The pH variation can be achieved by neutralizing the initial stock solution of zero pH (1 mol/L HCl solution) with solid sodium hydroxide or using only hydrochloric acid. In the first case, the pH varied from 0 to 8, and pCl remained identical, and in the second case both pH and pCl had identical values (Table 4). The obtained results showed that the ion retention depended on both pH and pCl, because in the aqueous solution there were competitive equilibria of formation of chlorides (MCl 2 ), tetra-chloro-complexes ([MCl 4 ] 2− ), hydroxides (M(OH) 2 ), or even aqua complexes and/or hydroxy complexes. The data in Table 2 are a good benchmark for justifying the retention values, but they are not enough because the interaction of each species with the membrane is complex and very different from case to case. This is also due to the fact that the tasks of the functional groups vary with the change of the pH of the supply solution.
Some recommendations regarding the separation of copper, zinc, cadmium, and lead ions from a mixture with membranes of chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow fiber (Chi/sPEEK/PPHF) can be as follows:

•
The separation of zinc and cadmium ions showed the narrowest variation range, most likely because these ions were retained to the same extent either by the cationic or anionic groups of the composite membrane.

•
Copper separation was excellent at pCl and pH values as high as possible.

•
The separation of lead in environments with sufficiently low pCl was little-influenced by the pH value. However, caution is advised when both pH and pCl are high. • At high pH values, the separation of copper, zinc, and cadmium ions was very good because they interacted with the membrane in both sulfonic and amino groups.
It should be noted that in repeated uses the membranes lose their qualities (retention decreases for all cations studied), most likely due to the detachment of the composite membrane from the support (Figure 16).
The obtained results showed that the ion retention depended on both pH and pCl, because in the aqueous solution there were competitive equilibria of formation of chlorides (MCl2), tetra-chloro-complexes ([MCl4] 2− ), hydroxides (M(OH)2), or even aqua complexes and/or hydroxy complexes. The data in Table 2 are a good benchmark for justifying the retention values, but they are not enough because the interaction of each species with the membrane is complex and very different from case to case. This is also due to the fact that the tasks of the functional groups vary with the change of the pH of the supply solution.
Some recommendations regarding the separation of copper, zinc, cadmium, and lead ions from a mixture with membranes of chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow fiber (Chi/sPEEK/PPHF) can be as follows: • The separation of zinc and cadmium ions showed the narrowest variation range, most likely because these ions were retained to the same extent either by the cationic or anionic groups of the composite membrane.

•
Copper separation was excellent at pCl and pH values as high as possible.

•
The separation of lead in environments with sufficiently low pCl was little-influenced by the pH value. However, caution is advised when both pH and pCl are high.

•
At high pH values, the separation of copper, zinc, and cadmium ions was very good because they interacted with the membrane in both sulfonic and amino groups.
It should be noted that in repeated uses the membranes lose their qualities (retention decreases for all cations studied), most likely due to the detachment of the composite membrane from the support (Figure 16). The application of nanofiltration for the separation of the quaternary system must take into account the permeate flows of the composite membrane chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow fiber (Chi/sPEEK/PPHF), which were much lower than the polypropylene support (PPHF) or the sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow-fiber (sPEEK/PPHF) membrane (Table 5). The application of nanofiltration for the separation of the quaternary system must take into account the permeate flows of the composite membrane chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow fiber (Chi/sPEEK/PPHF), which were much lower than the polypropylene support (PPHF) or the sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow-fiber (sPEEK/PPHF) membrane ( Table 5).
The support polypropylene membrane is a membrane specific to microfiltration and has relatively low water flows, being hydrophobic. Although the sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow-fiber (sPEEK/PPHF) membrane had the pores of the support covered, by increasing the pressure, specific ultrafiltration flows were obtained. However, it must be borne in mind that at the contact surface with the supply it had the hydrophilized layer of sulfonated polyether ether ketone (sPEEK). Finally, the composite membrane of chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)polypropylene hollow fiber (Chi/sPEEK/PPHF) had specific nanofiltration flows, and the operation at 5−6 bar was performed only for technical and economic reasons (obtaining an acceptable flow at medium working pressure). At this stage of the study, the increase in pressure above 6 bar was not taken into account both for energy consumption reasons but also because the composite membrane must be optimized by a possible crosslinking to avoid detachment from the support ( Figure 16).
Among the objectives to be achieved by developing the study of this type of membrane, the following must be found:

•
Tracking the influence of the molecular weight of chitosan; • Degree of sulfonation of the poly-ether-ether-ketone; • Decreasing the thickness of the polymer membrane support layer; • Optimization of flow-retention under the imposed pressure limit conditions.

Conclusions
In this study, we presented the obtaining of a composite membrane of chitosan (Chi)sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow fiber (Chi/sPEEK/PPHF), which was characterized morphologically, structurally, and from the point of view of the separation performances of copper, cadmium, zinc, and lead ions, in either binary mixture (Cu-Zn and Cd-Pb) or quaternary mixtures, in the conditions of strongly hydrochloric systems (very low pH and pCl). The separation of binary systems from solutions of hydrochloric acid 3 mol/L was performed both by nanofiltration and pertraction. The quaternary system was separated by nanofiltration under variable pH and pCl conditions. The obtained membranes were morphologically and structurally characterized by scanning electron microscopy (SEM), high-resolution SEM (HR-SEM), energy dispersive spectroscopy analysis (EDAX), Fourier Transform InfraRed (FTIR) spectroscopy, thermal gravimetric analysis, and differential scanning calorimetry (TGA).
Preliminary separation tests showed that binary systems could be efficiently separated by both nanofiltration and pertraction. Extraction would be more advantageous in terms of separation efficiency (90% is reached), but also because the chemical species that is extracted is concentrated by almost an order of magnitude. Nanofiltration has the advantage of a simpler operation and applicability to multiple systems, but the separation efficiency is strongly influenced by both pH and pCl.
Depending on the target cation and the pH and pCl conditions, retentions of over 90% (for Pb) and almost 95% (for Cu) could be obtained.
The study carried out here opens the perspective of obtaining and characterizing new chitosan (Chi)-sulfonated poly (ether ether ketone) (sPEEK)-polypropylene hollow-fiber (Chi/sPEEK/PPHF) composite membranes using different types of chitosan, but also sulfonated polymer compounds that improve not only the stability and lifetime of the membranes but also of the process performances.