Transport of Heavy Metals Pb(II), Zn(II), and Cd(II) Ions across CTA Polymer Membranes Containing Alkyl-Triazole as Ions Carrier

The polymer membranes of cellulose triacetate -o-NPPE-1-alkyl-triazole (alkyl= hexyl, octyl, decyl) were characterized by non-contact atomic force microscopy (AFM). The influence of membrane morphology on transport process was discussed. 1-Alkyl-triazole derivatives are new cheap compounds that have the ability to bind metal ions in an acidic medium. These membranes were used for the investigation of the facilitated transport of Zn(II), Cd(II), and Pb(II) ions from an aqueous nitrate feed phase. The initial flux values of metal ions transport depend on the type of carrier used. The maximum value of the initial flux for Zn(II) ions was equal to 12.34 × 10−6 molm−2s−1 (for PIMs with 1-decyltriazole). In the case of Zn(II) and Cd(II) ions as the hydrophobicity of the carrier increases, the separation coefficients SZn(II)/Cd(II) slightly increase from 1.8 to 2.4, while for Zn(II) and Pb(II) ions separation coefficients SZn(II)/Pb(II) decrease. The highest recovery factors (RF) were found for Zn(II) ions (c.a. 90%). The RF values of Cd(II) ions increase from 58% to 67%. The highest RF value for Pb(II) is 30%. The rate-limiting step in the transport of Zn(II), Cd(II) and Pb(II) ions across PIMs with 1-alkyltriazole may be the diffusion coefficient of the carrier-cation complex. The AFM images show that the distribution of the carrier in the tested membranes is homogeneous over the entire surface. The roughness values determined for PIMs with alkyltriazole are slightly higher than the roughness of PIM with the commercial carrier, for example D2EHPA.


Introduction
The demand for metals has been increasing for many years. Heavy metals are widely used in metallurgy, electroplating, chemical, tannery, petrochemicals and paper manufacturing industries, etc.: The use of heavy metals for industrial production is the main reason why these metals are present in wastewater, sewage and waste [1]. Without proper waste water treatment, heavy metals will be released into water bodies and cause severe damage to the environment and ecosystem [2][3][4][5].
Among heavy metals, cadmium, lead, and zinc are considered the most toxic and hazardous to the environment [6][7][8]. They are currently implemented in a significant number of industries such as the production of cables, batteries, pigments, paints, steels and alloys, as well as the metal, glass, and plastic industries. The discharge of these industries causes the contamination of the aquatic environment by these heavy metals [9]. Metals removed from the sewage can be successfully reused in industry, therefore sewage and metal-bearing waste have become a secondary source of metals. Conventional methods of wastewater treatment, such as precipitation [10], ion exchange [11], flotation [12] or Membranes 2022, 12, 1068 2 of 14 coagulation, require the use of large volumes of reagents, resulting in the formation of large amounts of waste containing heavy metals. For example, industrial wastewaters containing 0.1 g/dm 3 of copper, cadmium, or mercury ions give 10-, 9-, and 5-times larger amounts of sediments, respectively [13]. Also, liquid-liquid extraction is widely used in the separation of heavy metals. However, due to the toxicity of the water-insoluble solvents used, these methods are very risky, especially when used on an industrial scale [14]. Therefore, it is necessary to search for alternative, cheap and ecological methods of purifying sewage and wastewater from various types of pollutants, especially from heavy metal ions.
Over the decades, it has been proven that liquid membranes, in particular polymer inclusion membranes (PIM), are a better alternative than solvent extraction methods for the separation and recovery of various metal ions [14][15][16][17][18]. The advantage of using PIMs is the possibility of simultaneous extraction and re-extraction and their greater durability and stability compared to liquid membranes [19]. The PIMs are formed by evaporating a solution containing a polymer matrix, a plasticizer, and a carrier [20]. In polymer inclusion membranes, the carrier is not washed out of the matrix, and in addition, these membranes do not contain any solvent. These membranes are characterized by a much longer life, better mechanical properties and chemical resistance compared to liquid membranes (LM) [21,22]. These types of membranes are increasingly used to remove a number of heavy metals from wastewater, including Cu, Zn, Pb, Ni, Co, Cd [1,2,14,23].
The present article deals with a competitive transport of zinc(II), lead(II), and cadmium (II) ions from a dilute aqueous solutions using PIM with 1-alkyltriazoles. The values of three parameters: initial flux (J 0 ), selectivity coefficient (S M(1) / M(2) ), and recovery factor of a given metal after 24 h (RF) were selected for the comparative analysis of the transport process. The aim of the study was also to investigate the influence of the physical properties of PIMs on the efficiency of separation of the studied ions.
Cadmium, lead, and zinc are considered the most toxic and hazardous to the environment [23][24][25]. They are currently implemented in a significant number of industries such as cables, batteries, pigments, paints, steels and alloys, metal, glass, and plastic industries. The discharge of these industries causes the contamination of the aquatic environment by these heavy metals [26].

Reagents
Analytical grade chemical reagents: zinc(II), cadmium(II), and lead(II) nitrates, sulphuric acid, and tetramethylammonium hydroxide were purchased from (POCh, Gliwice, Poland Physical properties of the 1-alkyltriazoles are listed in the Table 1. (1) Physical properties of the 1-alkyltriazoles are listed in the Table 1.

Equipment
Metal ions concentrations in aqueous phases were analyzed with AAS 240FS Spectrometer, Agilent, Santa Clara, CA, USA (AAS-atomic absorption spectroscopy). Measurements were made for the following emission lines of the analyzed elements: 213.9 nm, 228.8 nm, and 247.6 nm for Zn, Cd, and Pb respectively.
The thickness of the PIM was measured using a digital micrometer (Panametrics ® Magna-Mike ® 8500 (San Diego, CA, USA)) with an accuracy of 0.1 µ m.
A surface characterization study of the PIMs was performed by atomic force microscopy (AFM) using Atomic-force MultiMode Scanning Probe Microscope IIIa (Digital Instruments Vecco Metrology Group, Santa Barbara, CA, USA).
Membrane spectrophotometric studies were performed using a Bruker Invenio R Infrared Spectrophotometer (Ettlingen, Germany) equipped with a broadband BeamSplitter and the ATR Quest attachment by Specac (Orpington, UK). The research was carried out in the wavenumber range from 3750 to 300 cm -1 .

Polymer Inclusion Membrane Preparation
PIMs were prepared as reported in the earlier paper [27][28][29][30][31]. The feed phase was a 3component aqueous solution of Pb(II), Zn(II), and Cd(II) ions with a concentration of C0,M =0.001 M of each ion. The feed phase pH was kept constant (pH = 4.0) by adding tetramethylammonium hydroxide and controlled by pH meter. The receiving phase was 0.01 M HCl. At the receiving phase, metal ions concentrations were measured. The membrane film (at surface area of 4.9·10 −4 m 2 ) was tightly clamped between two cell compartments. Both, i.e., the source and receiving aqueous phases (45 cm 3 each) were mechanically stirred at 600 rpm. Metal concentration was determined by taking small samples (0.1 cm 3 each) of the aqueous receiving phase at different times.

Parameters Characterizing the Transport Process
The process of transport across PIMs is characterized by the initial flux (Jo), selectivity coefficient (S), and recovery factor (RF) [33,34].  Physical properties of the 1-alkyltriazoles are listed in the Table 1.

Equipment
Metal ions concentrations in aqueous phases were analyzed with AAS 240FS Spectrometer, Agilent, Santa Clara, CA, USA (AAS-atomic absorption spectroscopy). Measurements were made for the following emission lines of the analyzed elements: 213.9 nm, 228.8 nm, and 247.6 nm for Zn, Cd, and Pb respectively.
The thickness of the PIM was measured using a digital micrometer (Panametrics ® Magna-Mike ® 8500 (San Diego, CA, USA)) with an accuracy of 0.1 µm.
A surface characterization study of the PIMs was performed by atomic force microscopy (AFM) using Atomic-force MultiMode Scanning Probe Microscope IIIa (Digital Instruments Vecco Metrology Group, Santa Barbara, CA, USA).
Membrane spectrophotometric studies were performed using a Bruker Invenio R Infrared Spectrophotometer (Ettlingen, Germany) equipped with a broadband BeamSplitter and the ATR Quest attachment by Specac (Orpington, UK). The research was carried out in the wavenumber range from 3750 to 300 cm -1 .

Polymer Inclusion Membrane Preparation
PIMs were prepared as reported in the earlier paper [27][28][29][30][31]. The feed phase was a 3-component aqueous solution of Pb(II), Zn(II), and Cd(II) ions with a concentration of C 0 , M =0.001 M of each ion. The feed phase pH was kept constant (pH = 4.0) by adding tetramethylammonium hydroxide and controlled by pH meter. The receiving phase was 0.01 M HCl. At the receiving phase, metal ions concentrations were measured. The membrane film (at surface area of 4.9 × 10 −4 m 2 ) was tightly clamped between two cell compartments. Both, i.e., the source and receiving aqueous phases (45 cm 3 each) were mechanically stirred at 600 rpm. Metal concentration was determined by taking small samples (0.1 cm 3 each) of the aqueous receiving phase at different times.

Parameters Characterizing the Transport Process
The process of transport across PIMs is characterized by the initial flux (J o ), selectivity coefficient (S), and recovery factor (RF) [33,34].
The kinetics of metal ions transport across membranes was described by equation: where c t and c o are the metal ions concentration (M) in the feed phase at a given time, and the initial metal ions concentration, respectively; k is the rate constants (s −1 ), t is the time of transport (s). The permeability coefficient (P) was calculated from: where V is the volume of the aqueous feed phase, and A is an effective area of the membrane. The initial flux (J 0 ) is defined as: The selectivity coefficient (S) was defined as the ratio of initial fluxes for M1 and M2 metal ions, respectively: To describe the efficiency of metal removal from the feed phase, the recovery factor (RF) was calculated:

Membrane Characteristics
The thickness of membranes before and after transport was found to be the same and were 30, 31, 33 µm for 1, 2, 3, respectively.
The analysis of surface pore characteristics of the polymer membrane was made using the NanoScope v.5.12 AFM image processing program, which enabled the calculation of PIM's roughness (R q ). Figure 1 shows the AFM images of PIMs with 1-alkyltriazole (1-3) as the carrier in three-dimensional form with format of 5.0 × 5.0 µm. The distribution of the carrier in the investigated membrane after evaporation of the dichloromethane is homogeneous on the entire surface.
The roughness (R q ) parameter is the standard deviation of the z values within the box cursor and is calculated as: The average roughness (R q ) of the membrane was calculated using atomic force microscopy (AFM) for n different sites, and they are shown in Table 2. As seen in Table 2 the roughness of the membranes increases with increasing length of the alkyl substituent in the carrier molecule higher than the commercial carrier, D2EHPA (4.7 nm) that was used by Salazar-Alvarez [35]. However, the roughness values for CTAo-NPPE-alkyltriazole membranes are comparable to the roughness found for PIM with alkylimidazoles (3.7-7.2 nm) [23] and with an imidazole derivative of azothiacrown ethers (3.3-5.3 nm) [36].
The roughness (Rq) parameter is the standard deviation of the z values within the box cursor and is calculated as: The average roughness (Rq) of the membrane was calculated using atomic force microscopy (AFM) for n different sites, and they are shown in Table 2.

Thermal Stability of PIM with 1-Alkyltriazole
The CTA-o-NPPE membrane with 1-hexyltriazole was also tested for their thermal stability. About 20 mg of membrane were heated at 10 • C/min under nitrogen from 25 • C to 800 • C. Figure 2 shows thermograms of CTA-o-NPPE membranes with and without 1-hexyltriazole (1) before and after ion transport. The CTA-o-NPPE membrane with 1-hexyltriazole was also tested for their thermal stability. About 20 mg of membrane were heated at 10°C/min under nitrogen from 25 °C to 800 °C. Figure 2 shows thermograms of CTA-o-NPPE membranes with and without 1hexyltriazole (1) before and after ion transport. As seen from Figure 2 the degradation of CTA-o-NPPE membrane with 1-hexyltriazole proceeds in two steps. The ranges of decomposition temperatures are summarized in Table 3.  As seen from Figure 2 the degradation of CTA-o-NPPE membrane with 1-hexyltriazole proceeds in two steps. The ranges of decomposition temperatures are summarized in Table 3.

FT-IR Analysis of the PIM with 1-Hexyltriazole
The membrane with 1-hexyltriazole was analyzed using FT-IR spectroscopy. The spectra are presented in Figure 3.
The interpretation of IR spectra was made using IRPal 2.0 program. Table 4 shows indicated bonds of signals recorded in Figure 3.
As reported in the literature, the degradation of a CTA membrane occurs in two steps; the first one over a range of 292-320 °C (main step) and the other one over a range of 450-476 °C (the charring of products) [37][38][39].

FT-IR Analysis of the PIM with 1-Hexyltriazole
The membrane with 1-hexyltriazole was analyzed using FT-IR spectroscopy. The spectra are presented in Figure 3. The interpretation of IR spectra was made using IRPal 2.0 program. Table 4 shows indicated bonds of signals recorded in Figure 3.

Separation of Zn(II) from Zn(II)-Cd(II)-Pb(II) Mixture
In the first series of experiments, competitive transport of Zn(II), Cd(II), and Pb(II) ions from aqueous nitrate solutions containing the metal species at concentration 0.001 M across CTA polymer membrane with alkyltriazole as the ionic carrier and o-NPPE as the plasticizer into 0.01 M HCl was investigated.
In the absence of an ion carrier in the PIM, i.e., when the membrane contained only CTA and o-NPPE, no significant flow of metal ions was detected.
The relationship between ln(Ct/Co) and time for the transport of Zn(II), Cd(II), and Pb(II) ions across PIMs containing 1-3 triazole derivatives is shown in Figure 4.

Separation of Zn(II) from Zn(II)-Cd(II)-Pb(II) Mixture
In the first series of experiments, competitive transport of Zn(II), Cd(II), and Pb(II) ions from aqueous nitrate solutions containing the metal species at concentration 0.001 M across CTA polymer membrane with alkyltriazole as the ionic carrier and o-NPPE as the plasticizer into 0.01 M HCl was investigated.
In the absence of an ion carrier in the PIM, i.e., when the membrane contained only CTA and o-NPPE, no significant flow of metal ions was detected.
The relationship between ln(C t /C o ) and time for the transport of Zn(II), Cd(II), and Pb(II) ions across PIMs containing 1-3 triazole derivatives is shown in Figure 4. The relationship ln(C t /C o ) = f(t) was linear (Figure 4), which was confirmed by high values of determination coefficients (R 2 ) (above 0.98). Standard deviation of rate constant was determined below 5%.
As shown in Figure 4, transport can be described by first-order kinetics in relation to the concentration of metal ions. It is in agreement with the mathematical model proposed by Danesi [40].

Carrier
Metal Ions J 0 , µmolm −2 ·s −1 As can be seen from Table 5, for all carriers, the initial zinc ion flux has the highest value, which means that Zn (II) ions were transported at the highest rate. Lead(II) ions were transported with the lowest.

Selectivity Orders and Selectivity Coefficients S Zn(II)/M(II) S Pd(II)/M(II)
The initial flux values of metal ions transport also depend on the type of carrier used and increases in the order 1 < 2 < 3. The observed trend increases with an increase in the hydrophobicity of the carrier (with an increase in the length of the -R groups in the carrier molecule). The maximum value of the initial flux for Zn(II) ions was equal to 12.34 × 10 −6 molm −2 s −1 (for 3).
Comparing the values of the initial fluxes (Table 5) with the roughness values (Table 2) of the membranes, it can be concluded that the increase in roughness increases the transport of each metal ion.
In the case of zinc and cadmium ions as the hydrophobicity of the carrier increases, the values of separation coefficients S Zn(II)/Cd(II) slightly increase from 1.8 for carrier 1 to 2.4 for 3 while for zinc and lead ions the values of separation coefficients S Zn(II)/Pb(II) decrease. The highest value of the separation coefficients S Zn(II)/Pb(II) equal to 16.2 was obtained for carrier 1.
The Figure 5 presents the proposed mechanism of the metal ions transport across PIMs with alkyltriazole.
At the interface of the feed phase and the PIM surface, metal ions bind to the carrier molecules (alkyl triazoles) present in the membrane to form complex compounds. The complexes diffuse to the opposite surface of the membrane where the metal ions are released into the receiving phase.
The Pb(II), Zn(II), and Cd(II) ions with alkyltriazole (L) form 6-coordinate octahedral complexes [ML 6 ] 2+ . In the case of Zn(II) and Cd(II) ions, an additional phenomenon is the ease of changing the coordination number from 6 to 4, and thus the geometry of the coordination sphere from octahedron to tetrahedron [29,41,42]. This is illustrated by the Equation (8).
The transport mechanism presented in Figure 5 can be described as follows: 1. Transport: at the interface feed phase/ membrane: The differences in the transport speed of the examined ions across the PIM can be explained by comparing the influence of their diameter on the values of the initial fluxes ( Figure 6).
The diameters of the Zn(II), Cd(II), and Pb(II) are 1.48Å, 1.90Å, and 2.36Å, respectively [43]. As the cation diameter increases, the value of the initial ion fluxes decreases ( Figure 6).
The transport of metal ions is influenced by both the size of the cation and the size of the carrier molecule.
at the interface receiving phase/membrane: L(membrane) + H + (aq/HCl) ↔ HL(membrane) at the interface membrane/feed phase: HL(membrane) ↔ L(membrane) + H + (aq) The differences in the transport speed of the examined ions across the PIM can be explained by comparing the influence of their diameter on the values of the initial fluxes ( Figure 6). The diameters of the Zn(II), Cd(II), and Pb(II) are 1.48Å , 1.90Å , and 2.36Å , respectively [43]. As the cation diameter increases, the value of the initial ion fluxes decreases ( Figure 6).
The transport of metal ions is influenced by both the size of the cation and the size of the carrier molecule. Figure 7 shows the graphs of the correlation (Co-Ct) as a function of time for the transport of Zn(II), Cd(II), and Pb(II) ions across PIM with 1-alkyltriazole 1-3.   The diameters of the Zn(II), Cd(II), and Pb(II) are 1.48Å , 1.90Å , and 2.36Å , re tively [43]. As the cation diameter increases, the value of the initial ion fluxes decr ( Figure 6).

Membrane Diffusion Coefficients of Zn(II), Cd(II), and Pb(II) Ions across PIMs with 1-Alkyltriazole
The transport of metal ions is influenced by both the size of the cation and the s the carrier molecule. Figure 7 shows the graphs of the correlation (Co-Ct) as a function of time fo transport of Zn(II), Cd(II), and Pb(II) ions across PIM with 1-alkyltriazole 1-3.  According to Tor et al. [44], the diffusion coefficients of metal ions (D o ) can be calculated from the equation:

Membrane Diffusion Coefficients of Zn(II), Cd(II), and Pb(II) Ions across PIMs with 1-Alkyltriazole
where: d o -the thickness of the membrane and ∆o could be evaluated by plotting (C o -C t ) vs. time.
Obtained values of diffusion coefficients are presented in Table 6.
Values of diffusion coefficient determined in this study are comparable with these presented in literature data for different membranes. They are in the range from 10 −12 to 10 −6 cm 2 /s and show that the limiting step of the process is the transfer of metal complex across membrane barrier [35,44]. The value of the diffusion coefficient of M(II)-carrier species of 3.12 × 10 −11 -2.38 × 10 −7 cm 2 /s is comparable to PIMs with 1-alkylimidazole for which the values of diffusion coefficients range from 10 −12 to 10 −8 cm 2 /s [23].

Recovery of Metal
In order to describe the efficiency of metal removal from the feed phase, the recovery factor (RF) was calculated from Equation (5). Figure 8 shows the values of the recovery factor Zn(II), Cd(II), and Pb(II) ions from the feed phase during the 24-hrs transport across PIMs with 1-alkyl-triazoles. Values of diffusion coefficient determined in this study are comparable with these presented in literature data for different membranes. They are in the range from 10 −12 to 10 −6 cm 2 /s and show that the limiting step of the process is the transfer of metal complex across membrane barrier [35,44]. The value of the diffusion coefficient of M(II)-carrier species of 3.12•10 −11 -2.38•10 −7 cm 2 /s is comparable to PIMs with 1-alkylimidazole for which the values of diffusion coefficients range from 10 −12 to 10 −8 cm 2 /s [23].

Recovery of Metal
In order to describe the efficiency of metal removal from the feed phase, the recovery factor (RF) was calculated from equation (5). Figure 8 shows the values of the recovery factor Zn(II), Cd(II), and Pb(II) ions from the feed phase during the 24-hrs transport across PIMs with 1-alkyl-triazoles. As shown in Figure 8, the values of recovery factors (RF) depend on the carrier used in the membrane. The RF values for Zn(II) and Cd(II) ions increase with increasing hydro- As shown in Figure 8, the values of recovery factors (RF) depend on the carrier used in the membrane. The RF values for Zn(II) and Cd(II) ions increase with increasing hydrophobicity of the carrier molecule, hence they are the highest for 1-decyltriazole (3) in contrast to the RF value for Pb(II) ions, which is the lowest for this carrier. The highest recovery factors (RF) were found for Zn(II) ions (c.a. 90%). The RF values of Cd(II) ions are 58%, 63%, and 67% for 1, 2, and 3, respectively.
The lowest RF values were obtained for Pb(II) ions, which are the slowest transported by this type of membrane. The highest RF value for Pb(II) is 28% for carrier 1.

Conclusions
1-Alkyl triazole derivatives are new cheap compounds that have the ability to bind metal ions in an acidic medium. They can be used as carriers in PIMs.
The 1-alkyltriazoles under study can be used for separation of the Zn(II) ions from an equimolar mixture of the Zn(II), Cd(II) and Pb(II) ions during transport across PIMs. The transport of Zn(II), Cd(II) and Pb(II) ions can be described by first-order kinetics in relation to the concentration of metal ions.
The initial flux values of metal ions transport depend on the type of carrier used and increases with the increase in the hydrophobicity of the carrier molecule in the order 1 < 2 < 3. The maximum value of the initial flux for Zn(II) ions was equal to 12.34 × 10 −6 molm −2 s −1 (for 3).
In the case of zinc and cadmium ions as the hydrophobicity of the carrier increases, the values of separation coefficients S Zn(II)/Cd(II) slightly increase from 1.8 for carrier 1 to 2.4 for 3 while for zinc and lead ions the values of separation coefficients S Zn(II)/Pb(II) decrease.
The highest value of the separation coefficients S Zn(II)/Pb(II) equal to 16.2 was obtained for carrier 1. The highest recovery factors (RF) were found for Zn(II) ions (c.a. 90%). The RF values of Cd(II) ions are 58%, 63%, and 67% for 1, 2, and 3, respectively. The highest RF value for Pb(II) is 30% for carrier 2.
The rate-limiting step in the transport of Zn(II), Cd(II) and Pb(II) ions across PIMs with 1-alkyltriazole may be the diffusion coefficient of the carrier-cation complex.
The values of the initial fluxes of all metal ions transported are low, which means that membranes of this type can be used in industry in the next step of wastewater treatment when the concentrations of toxic metals (Zn, Cd and Pb) are low.