Applicability of Supported Liquid Membranes for Extraction of Zn2+ and Ni2+ in Water Using Cyclohexylamine as a Carrier

Abstract

Zn²⁺ and Ni²⁺ are frequently reported in wastewater effluents with concentrations much greater than the permitted limits and thus pose a huge threat to living beings. Therefore, researchers are gaining much focus on removing these metals from water bodies. This study performed two sets of experiments to study the comparative extraction efficiencies of Zn²⁺ and Ni²⁺ from water samples using a supported liquid membrane (SLM), carrier, and solvent. For this purpose, the chloride salts of both Zn²⁺ and Ni²⁺ were used to study the movement of both ions from the acidic feed phase to the basic strip phase through SLM equilibrated with cyclohexylamine, which acts as a carrier for the transportation of Zn²⁺ and Ni²⁺. The conditions were optimized while studying four different parameters to maximize the transport of ions from the feed to the stripping phase. However, the acid concentration was kept constant at 1 M for all the experiments. Separate experiments were carried out to study the movement of Zn²⁺ and Ni²⁺ ions. The distribution coefficient, permeability coefficient, flux, and extraction efficiencies of both ions were calculated against different parameters. A comparative analysis found that Zn²⁺ ions have a greater tendency to move to the strip phase than the Ni²⁺ ions under a similar set of parameters. Extraction time reveals that approximately 87% of the Zn²⁺ ions moved to the stripping phase from the feed phase at the end of the 120^th min of the experiment. In contrast, only 72% of the Ni²⁺ ions could move to the stripping phase under similar conditions. Current studies indicate that the extraction methodologies based on SLM can effectively remove Zn²⁺ and Ni²⁺ from water samples.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The increasing population of mankind has raised the demand for almost all usable items, including buildings for shelter, offices, furniture, edibles, clothing, footwear, machinery, tools, and vehicles to make life at normal pace [1]. Many of these items include metals as a constituent [2, 3]. More and more industries are being added to the market to fulfill the increasing requirements. Although these industries are fulfilling the increasing demands, they add toxic heavy metals to the environment [4]. Heavy metals frequently reported include Zn, As, Cr, Hg, and Cd [5, 6]. The waste from the Zn industry is obtained through the hydrometallurgical Zn-winning process, and it comprises mainly Cu (approx. 28%) and Zn (approx. 22%), as well as Co, Cd, and Ni [7]. Zn concentrations in the plating industry rinse solutions range from 6 to 535 mg/L. Zn (II) is typically considered harmless, with a recommended discharge limit of roughly 5 mg/L [8–10]. Similarly, Ni²⁺ is found in tiny amounts in seawater, petroleum, and coal and minor concentrations in animals and plants. Heavy metal-bearing wastewater is being disposed of safely and efficiently [11]. Ni is poisonous and is utilized in various industries, including stainless steel, corrosion-resistant alloys, batteries, metal plating, automobile makers, and coins. The metal ions Ni²⁺ found in these companies’ wastewater cause substantial environmental contamination. Ni (II) concentrations range from 3.4 to 900 mg/L in a typical industrial effluent. According to the World Health Organization, the maximum amount of Ni in drinking water is 0.02 mg/L. Before releasing industrial effluent into water, it is critical to remove these heavy metals from the industrial effluents [12].

Unlike natural products, most synthetic industrial products are non-degradable. Such materials are becoming more prevalent in the ecosystem, polluting the air, water, and soil [13]. As a result, they accumulate in living bodies, including plants, animals, and humans, which are hazardous and may cause severe lethal diseases [14]. The harmful effects of these heavy metals are well known nowadays [15]. It is, therefore, desirable to remove hazardous and toxic metals from industrial and commercial waste before they become part of the natural environment. Various physical and chemical methods are practiced at industrial and laboratory scales to remove these toxic metals from the waste. Some of the widely used methods being employed for the said purpose include chemical precipitation [16], reverse osmosis [17], coagulation [18], adsorption, ion exchange [19, 20], solvent extraction, adsorption [21, 22], and electrolysis [23, 24]. However, all of these methods have their own limitations and drawbacks, such as secondary sludge formation [25] and its disposal, critical process conditions [26], and higher operational costs and lower efficiencies [27, 28].

Many investigations are being conducted to find a highly effective method that requires little or no critical process conditions and is cost-effective too [29, 30]. One such method is a liquid membrane (LM) system that can be used to remove the desired metal ions from the waste material and thus seems to be a promising method for future applications [31, 32]. Besides, LMs are advantageous over other technologies in cost, efficiency, energy, and solvent requirements [33–35]. The LM is a thin layer of organic/aqueous liquid, immobilized or freely floating between two phases, thus facilitating selective mass transfer of target species based on chemical affinity or complexation mechanisms. The separation process depends on different factors, including the charge density of the species under study, pore size, ion size, and charge. The extraction reagent is diluted with the suitable solvent, and the membrane is soaked into this carrier/solvent liquid, where the pore spaces in the membranes are allowed to fill in. This carrier then transports the metal ions from one membrane side to the other. Three main types of LMs are bulk LMs [36], emulsion LMs [37], and supported LMs [38, 39]. The bulk and emulsion LMs do not contain any support. The hollow fiber and the flat sheet fall into the supported LM category. Because of its many benefits over other separation methods, LMs have become the focus of extensive scientific research due to their widespread use in various industries, laboratories, and analytical fields for separation, pre-concentration, and wastewater treatment. In this study, cyclohexylamine was employed as a carrier in the SLMs for the first time to remove Zn²⁺ and Ni²⁺ ions from the feed phase. Some studies using Aliquat 336 and triethanolamine as carrier phase signify some limitations, highlighting decreased flux and selectivity loss over time [40, 41]. Cyclohexylamine readily makes complexes with the Zn²⁺ and Ni²⁺ ions. This formation of a complex is a reversible reaction. Hence, the carrier regenerates itself after transporting ions from the feed to the strip phase, making it superior to other extractants. The removal efficiency of the SLM system was compared to that of Zn²⁺ and Ni²⁺ ions under identical conditions. A greater removal efficiency of the SLM system was noticed for Zn²⁺ ions from an acidic feed to a basic strip phase.

2. Materials and Methods

2.1. Reagent and Solutions

Chemicals used in the experiments for the separation of the Zn²⁺ ions across a solid–LM include cyclohexylamine (Merck, Germany), cyclohexanone (Merck, Germany), hydrochloric acid (35.4%), zinc chloride (98%), and Celgard 2400 membrane. The membrane used is a microporous polypropylene film. The thickness of the membrane was 25 μm, the pore diameter was 0.043 μm, and the porosity was 40%. Metallic chlorides of Zn (II) and Ni (II) were used in feed phases. Zinc chloride and nickel chloride stock solutions were prepared by dissolving zinc chloride and nickel chloride in 1.0 M HCl. These two solutions were standardized by EDTA-Na titration and using xylenol as an indicator. Different concentrations of Zn used in these experiments were 0.5 × 10⁻⁴, 1.5 × 10⁻⁴, 2.5 × 10⁻⁴, 3.5 × 10⁻⁴, and 4.5 × 10⁻⁴ M, whereas the range of Ni ion concentrations included 2.5 × 10⁻⁴, 3.5 × 10⁻⁴, 4.5 × 10⁻⁴, 5.5 × 10⁻⁴, and 6.5 × 10⁻⁴ M solutions with respect to Ni²⁺. Deionized (DI) water was obtained from HAT Enterprises Islamabad with an 18-MΩ·cm resistivity and was used to make all the solutions.

2.2. Optimization of Parameters

The parameters selected for the comparative study of the extraction of Zn²⁺ and Ni²⁺ ions included the effect of carrier, that is, cyclohexylamine, the effect of membrane soaking time, the effect of initial concentration of feed phase, and the effect of the initial concentration of strip phase on the extraction of given ions. To study the parameters 1, 2, and 4, 1 M NaOH solution was used as a stripping phase. However, the concentrations of stripping phases with respect to NaOH were varied to 0.25, 0.50, 0.75, 1.00, and 1.25 M for the study of parameter 3. Cyclohexylamine was used as a carrier to study the transport behavior of both metal ions across the membrane. For the study of the Zn²⁺ ion transport, the carrier concentrations were 0.06, 0.08, 0.2, 0.4, and 0.8 M. To examine the movement of Ni²⁺ across the membrane, the used carrier concentrations for the purpose of optimization were 0.05, 0.08, 0.1, 0.3, and 0.5 M. Zn²⁺ and Ni²⁺ exhibit different affinities toward the cyclohexylamine carrier, depending on their individual solubility limits, membrane stability, and the distinct complexation behavior. This led to varying carrier concentration ranges for optimal transport in each case. Cyclohexanone was used as a diluent to prepare different concentrations of cyclohexylamine. The present study selected feed phase concentrations within a lower-to-mild range of industrial wastewater levels to ensure precise control over variables for systematically evaluating the transport mechanism. In contrast, carrier concentrations were chosen to avoid operational constraints like membrane saturation. Ensuring proper membrane design, practical scalability, optimizing carrier concentration, or ensuring multi-extraction steps, we expect similar performance at higher concentrations of metal ions.

2.3. Experimentation

The experiments were conducted in a cell at a 25 ± 1°C temperature. It was a simple cell in which different batches were run to obtain readings. The cell’s body was made up of acrylic material and two chambers. The membrane was inserted between two compartments with the clamps. The capacity of each compartment was 180 cm³. The interfacial area of the solid–LM was 12.56 cm². The stirring of the solution was carried out by using electric motors in both compartments of the system. Both the compartments contained holes for sampling the stripping and feeding phases. The speed (rpm) of the motor was uniform, that is, 1000 rpm, throughout all the experiments.

The membrane was immersed in cyclohexylamine for 24 h to equilibrate the membrane with cyclohexylamine [42]. The carrier (cyclohexylamine) filled up the pores due to capillary action. After that, the membrane was removed and washed to remove the excess carrier. After the saturation equilibration of the membrane, it was fixed in between the two chambers of the system cell. Each chamber can contain a maximum of 180 cm³ of the solution. An electric motor at 1000 rpm was used to restrict the concentration polarization at both sides of the membrane interphase. Four parameters were optimized in this study: carrier concentration, soaking time for membrane, feeding concentration, and stripping concentration. In all the experiments, 10-cm³ samples were withdrawn from the system with the help of a pipette after constant and regular intervals of time for all the experiments to optimize four different parameters. The removed samples were analyzed with atomic absorption spectrophotometer to determine the concentration of metals of interest. For these experiments, the temperature was kept at 25 ± 1°C, and the pH was maintained at 4.0. Due to protonation–deprotonation equilibria governing the metal complex formation, most of the extraction involving amine requires highly acidic media. However, in the present study, the pH value has been adjusted to moderate acidic conditions to assess the potential practical applications of cyclohexylamine carrier where extreme acidity may not be desirable. The distribution coefficient, permeability coefficient, flux, and extraction efficiency were calculated for all the optimized parameters. Distribution coefficient, sometimes also called the partition coefficient, is the ratio of the concentration of the compound in a mixture of two solvents. In these experiments, the distribution coefficient is the distribution of metal ions between the solution and the membrane. Equation (1) is used to calculate the distribution coefficient [43]$$\begin{array}{}\text{(1)}& K_D=\frac{K_D^f}{K_D^s},\end{array}$$where $K_D^f$ and $K_D^s$ represent the distribution coefficient of the feed phase and stripping phase, respectively. The rate law or first-order reaction can be used to comprehend the variation of metal ions in the SLM system with the elapsed time [44]$$\begin{array}{}\text{(2)}& -\frac{{dC}_f}{dt}={kC}_O,\end{array}$$where $C_f$, $C_O$, $t$, and $k$ represent the current (or remaining) concentration of the feed phase in mol dm⁻³, the initial feed phase concentration in mol dm⁻³, the elapsed time, and the permeability coefficient, respectively. Integrating the above equation, we get the following equation (3) [45], which can be used to determine the rate law:$$\begin{array}{}\text{(3)}& \ln \frac{C_f}{C_o}=kt.\end{array}$$

The rate constant ($k$) can be determined from the straight line drawn between $\ln C_f/C_o$ and elapsed time, $t$. Different parameters were studied using the permeability coefficient (P) or rate constant, as described by Danesi in the mass-transfer model [46]. Equation (3) is equivalent to the following equation:$$\begin{array}{}\text{(4)}& \ln \frac{C_t}{C_o}=-\epsilon \frac{S}{V}Pt,\end{array}$$where $C_o$, $C_t$, $\epsilon$, $S$, $V$, $P$, and $t$ represent the initial concentration of feed solution, the concentration of feed solution at the elapse time $t$, the porosity of the membrane, membrane area, permeability coefficient, and elapsed time, respectively.

2.4. Flux (J) Calculations

The transport of the metal ions was quantified in terms of their fluxes. To determine the flux of given ions, plots were drawn between the concentration of the feed phase versus time, that is, ${dC}_{f/s}/dt$. This value was then inserted in Equation (5), which is given below [47, 48]. The value of $V_{f/s}$ was 180 cm³, the interfacial membrane area was 12.56 cm², and the porosity of membrane was 40%$$\begin{array}{}\text{(5)}& J=\frac{{dC}_f}{dt}\frac{V_f}{S\epsilon },\end{array}$$where $V_f$, $C_f$, $\epsilon$, $S$, and $t$ represent the volume of the feed phase (in cm³), the concentration of the feed phase at $t$ elapsed time, the porosity of the membrane, membrane area, and elapsed time, respectively.

2.5. Extraction Efficiency %

The recovery (%) of metal ions can be calculated using the general formula percentage$$\begin{array}{}\text{(6)}& \text{Extraction\hspace{0.17em}efficiency\hspace{0.17em}}\%=\frac{{C_o-C}_t}{C_o}\times 100.\end{array}$$

To optimize different parameters, a series of experiments were performed to study the transport of metal ions across the solid–liquid membrane (Celgard 2400). Feed phase concentration, strip phase concentration, membrane’s soaking time in the carrier (cyclohexylamine) phase, and NaOH concentration in the strip phase were investigated in the present study. The pH of the feed phase was kept constant at 4 ± 0.2 by using an acetate buffer solution. In the first series of experiments, 0.06, 0.08, 0.2, 0.4, and 0.8 M of cyclohexylamine were used to select the most appropriate concentration of the carrier at which the Zn²⁺ ion transportation was found to be maximum. Likewise, 0.05, 0.08, 0.1, 0.3, and 0.5 M were the employed concentrations of the carrier to find the maximum transport of Ni²⁺ ions across the SLM. The feed phase and strip phase concentrations were kept constant for this series of experiments. The feed phase was 3.5 × 10⁻⁴ M with respect to Zn²⁺, whereas the strip phase was 0.1 M NaOH solution. Likewise, the concentration of Ni²⁺ was kept at 5.5 × 10⁻⁴ M, and the stripping phase was a 1.0 M solution with respect to NaOH, which was practically kept the same for all the experiments.

3. Results and Discussion

The plots in Figures 1(a) and 1(b) show that the Zn²⁺ and Ni²⁺ ion concentrations decreased in the feed phase with the passage of time. Conversely, the concentrations of Zn²⁺ and Ni²⁺ ions increased across the membrane in the strip phase, as reflected in the plots in Figure 1S (supporting data). The transport of the Zn²⁺ ion across SLM exhibits a direct relationship with the carrier concentration until 0.4 M. However, an increased concentration of Zn²⁺ in the feed phase was observed when the carrier concentration increased beyond 0.4 M. Zn²⁺ ions form a complex with cyclohexylamine at the membrane-feed phase interface. This complex releases the captured Zn²⁺ ions in the strip phase on the other side of the membrane due to the basic medium. When the concentration of the carrier was increased beyond 0.4 M, the viscosity of the cyclohexylamine also increased, which obstructed the transport of Zn²⁺ ions from feed to the strip phase. The exact mechanism was observed for the transport of Ni²⁺ ions. It is apparent from the graphs that the optimum concentration of carrier, that is, cyclohexylamine, was 0.4 M for Zn²⁺ ion movement and 0.3 M for Ni²⁺ ion movement.

[figure(s) omitted; refer to PDF]

Equation (1) was used to calculate the distribution coefficient of the Zn²⁺ and Ni²⁺ ions between the solution and membrane. Zn²⁺ ions distribute themselves in three phases, that is, the feed phase, membrane phase, and the strip phase. Total Zn²⁺ or Ni²⁺ concentration can be determined by the following equations (7) and (8):$$\begin{array}{}\text{(7)}& {\text{Zn}}^{2+}\text{total}={\text{Zn}}^{2+}\text{feed}+{\text{Zn}}^{2+}\text{strip}+{\text{Zn}}^{2+}\text{membrane},\text{(8)}& {\text{Zn}}^{2+}\text{membrane}={{\text{Zn}}^{2+}\text{total}-\text{Zn}}^{2+}\text{feed}-{\text{Zn}}^{2+}\text{strip},\end{array}$$where ${\text{Zn}}^{2+}\text{total}$, ${\text{Zn}}^{2+}\text{feed}$, ${\text{Zn}}^{2+}\text{strip}$, and ${\text{Zn}}^{2+}\text{membrane}$ represent the initial Zn²⁺ and Zn²⁺ concentrations in the feed, strip, and membrane, respectively. The same method was used to determine the Ni²⁺ ion concentration in the feed, strip, and membrane phases.

Figures 2(a) and 2(b) show that the maximum movement of Zn²⁺ and Ni²⁺ ions from the feed to the strip phase is at 0.4 and 0.3 M of carrier, respectively. Other parameters, including the distribution coefficient, permeability coefficient, flux, and extraction efficiency, were also calculated while considering these optimized values. The distribution coefficients were calculated for both the feed phase ($K_D^f$) and the strip phase ($K_D^s$) at 0.4 mole of the carrier concentration at all intervals; the sample was taken out to determine the concentration of Zn²⁺ ions. The calculated $K_D$ values were 0.329, 0.274, 0.200, 0.154, and 0.154 for the feed phase. These values indicate that Zn²⁺ ions were more concentrated in the membrane at the start of the experiment and decreased with the passage of time during the later part of the experiment. The same tendency was observed for the Ni²⁺ ions, that is, the distribution coefficient value for the feed phase before and after equilibrium was determined to be 0.28 and 0.12, respectively, at the optimum carrier concentration, which implies that Ni²⁺ ions were substantially greater in the membrane in the beginning of the experiment than near the end of the experiment.

[figure(s) omitted; refer to PDF]

Likewise, distribution coefficients were calculated for the stripping phase. At the end of the experiments, the difference in values of the distribution coefficient of the feed and strip phases was 2.57 times for Ni²⁺ and 2.1 times for Zn²⁺. The substantial difference is evidence of comparatively higher affinity of the metallic ions with the membrane in the feed phase. The affinity difference between the feed and stripping phases drives the metallic ions from the feed–membrane interphase toward the strip phase. It is worth noting that Zn²⁺ has more affinity toward the membrane than Ni²⁺ due to the comparatively more stable complex of Zn²⁺ with cyclohexylamine in the membrane. This conclusion supports our findings that Zn²⁺ and Ni²⁺ metal ions interact with cyclohexylamine differently, as seen by the differences in permeability trends we obtained for these metal ions (Tables 1 and 2). Similar observations have been reported in the literature indicating that metals with higher stability complexes tend to extract faster, whereas metals with moderate stability show more balanced transport kinetics [49, 50]. Flux was calculated using Equation (5) for different carrier (cyclohexylamine) concentrations. 10-cm³ samples were withdrawn at regular intervals, that is, at 30, 60, 90, 120, and 150 min of the experiment. The factor $dC/dt$ was used to calculate the flux. In all experiments performed at various carrier concentrations, it was observed that flux values were high at the beginning of the experiment, that is, at 30^th min. Then, the fluxes decreased gradually with time. The decreased values of fluxes with time indicate that the transportation of ions was reduced near the completion of the experiment. The comparison in Figure 3 demonstrates that the highest transportation of Zn²⁺ ions was observed at 0.4 M of the carrier concentration. In contrast, the maximum transportation of Ni²⁺ ions was found at 0.3 M of the carrier concentration.

Table 1

Permeability coefficient, $P$, values of Zn²⁺ at optimum extractant concentrations.

Sr. No.	Time (min)	Carrier concentration (mole/L)	$C_t$ (10−4 M)	$C_t/C_0$	x = ln $C_t/C_0$	$\text{Slope}=x/t$	$P=\text{slope\hspace{0.17em}}V/S_{\in }$
1	0	0.4	3.5	0	0	0	0
2	30	0.4	1.52	0.434286	−0.834	−0.028	1.00
3	60	0.4	1.2	0.342857	−0.874	−0.015	0.52
4	90	0.4	1.1	0.314286	−0.916	−0.010	0.36
5	120	0.4	1.02	0.291429	−0.990	−0.008	0.30
6	150	0.4	1.02	0.291429	−0.990	−0.007	0.24

Table 2

Permeability coefficient, $P$, values of Ni²⁺ at 0.3 mol cyclohexylamine concentration.

Sr. No	Time (min)	Carrier concentration (mole/L)	$C_t$ (10−4 M)	ln $C_t/C_0$	P
1	0	0.3	5.50	0	0
2	30	0.3	1.80	−1.12	1.33
3	60	0.3	1.56	−1.26	0.75
4	90	0.3	1.50	−1.30	0.52
5	120	0.3	1.49	−1.31	0.39
6	150	0.3	1.49	−1.31	0.31

[figure(s) omitted; refer to PDF]

Equation (4) was used to calculate the values of the permeability coefficient. The permeability coefficient was maximum at the initial stage, that is, at 30 min of the experiment, for both the metal ions because of maximum binding sites. Tables 1 and 2 show the calculated P values of both metal ions at optimum extractant concentrations. The slope of the straight-line graph also indicates that the maximum transport of metallic ions was obtained at the initial stage of the experiment.

Extraction efficiency was calculated at various carrier concentrations using Equation (6). It was observed that there was a direct relation between carrier concentration and extraction of up to a 0.4 M concentration of cyclohexylamine for Zn²⁺ and up to a 0.3 M concentration of cyclohexylamine for Ni²⁺. Beyond these concentrations of the carrier, the extraction of the given metal ions decreased (Figure 4). This decrease in the extraction of the metal ions is due to a higher viscosity of the concentrated carrier, which obstructs the transport of ions across the membrane [51]. From the data given in Tables 1 and 2, a slight variation in permeability coefficient is observed over time for both Zn²⁺ and Ni²⁺. This variation can be attributed to the membrane saturation or fouling effect over time [52]. In addition, it is also evident that Ni²⁺ exhibits a more gradual decline in permeability (Table 2) than Zn²⁺ (Table 1), signifying sustained transport due to weaker binding resulting in slow depletion of available carrier sites.

[figure(s) omitted; refer to PDF]

The effect of membrane soaking time was determined by soaking the membrane for 8, 16, 24, and 32 h. Figures 5 and S2 depict the effect of the soaking of membrane graphically. The transport of the Zn²⁺ increased up to 24 h. Beyond 24 h, the soaking of the membrane in diluent–carrier solution causes higher concentration of carrier molecule due to evaporation of diluent in the membrane. This gives rise to an increase in the concentration of the carrier, and eventually the Zn²⁺ ion movement across the membrane is hindered. Similar trends were observed for Ni²⁺ ions.

[figure(s) omitted; refer to PDF]

The distribution coefficient for the feed and stripping phases at an optimum 0.4 M carrier concentration in the latter half of the experiment was 0.157 and 0.083, respectively. The ratio represents a difference of 1.8 times, reflecting that Zn²⁺ ions have 1.8 times more affinity with the membrane in the feed phase than in the strip phase. Thus, the membrane soaking helps transport Zn²⁺ ions in SLM. Tables 3 and 4 show the values of the distribution coefficient ($K_D$) of Zn²⁺ and Ni²⁺ ions, respectively, at different experiment stages where membranes were soaked in the carrier for an optimized time duration of 24 h.

Table 3

Distribution coefficient ($K_D$) of Zn²⁺ at an optimum membrane soaking time of 24 h.

Sr. No	Time (min)	Feed phase distribution coefficient ($K_D^f$)	Strip phase distribution coefficient ($K_D^s$)	$K_D=K_D^f/K_D^s$
1	0	0.000	0.000	0.000
2	30	0.298	0.340	0.876
3	60	0.267	0.190	1.405
4	90	0.205	0.123	1.664
5	120	0.157	0.083	1.887
6	150	0.157	0.083	1.887

Table 4

Distribution coefficient ($K_D$) of Ni at an optimum membrane soaking time of 24 h.

Sr. No	Time (min)	Feed phase distribution coefficient ($K_D^f$)	Strip phase distribution coefficient ($K_D^s$)	$K_D=K_D^f/K_D^s$
1	0	0.000	0.000	0.000
2	30	0.100	0.162	0.619
3	60	0.091	0.092	0.985
4	90	0.077	0.061	1.264
5	120	0.066	0.043	1.516
6	150	0.066	0.043	1.516

Likewise in the case of nickel, before attaining equilibrium (at 30^th min) and after attaining equilibrium (at 120^th min), the distribution coefficient (Table 4) for the feed phase and stripping phase was calculated as 0.006 and 0.043, respectively, which shows that the membrane soaking period aids nickel metal ion transport in SLM.

Figures 5 and 2S (supporting data) show that the maximum transportation of Zn²⁺ ions from feed to the strip phase occurs with the membranes, which were soaked in the carrier for 24 h. The increase in the metal ion concentration continued up to 24 h. However, the transportation of metal ions of interest declined after increasing the soaking time beyond 24 h. Such a decline in the transportation of metal ions is likely due to enhanced hindrance to the movement of ions because of the metal–carrier complex in the membrane.

Fluxes (Figure 3S (supporting data)) and extraction efficiencies (Figure 6) of Zn²⁺ and Ni²⁺ ions were also calculated by using Equations (5) and (6), respectively. The permeability coefficient was observed to be maximum at 30 min of the experiment, while the extraction efficiency increased with the increase of the membrane soaking time till 24 h. As shown in Figure 3S (supporting data) and Figure 6, a significant decrease in the fluxes and, hence, in the extractions of Zn²⁺ and Ni²⁺ ions were observed through the membranes soaked for 32 h in the carrier solution. This decrease is likely due to the overcrowding of the metal ion–carrier complex meritoriously in the membrane beyond 24 h of soaking time, and protonation and deprotonation at the feed and strip interface slow down. This could be attributed to the high saturation of the carrier in the membrane by capillary action that hinders the metal ions through the SLM system.

[figure(s) omitted; refer to PDF]

3.1. Effect of Initial Concentration of Feed Phase for the Extraction of Ions

The effect of concentration of the feed phase solution on the transport of Zn²⁺ ions for the values 0.5, 1.5, 2.5, 3.5, and 4.5 [× 10⁻⁴] M and the effect of concentration of the feed phase solution on the transport of Ni²⁺ ions for the values 2.5, 3.5, 4.5, 5.5, and 6.5 [× 10⁻⁴] M were studied. Compared to the stripped phase solution, the ionic strength of the feed solution was maintained by 1.0 M HCl. The feed phase constitutes 50% HCl and 50% zinc chloride solution to study the transport of Zn²⁺ ions. Figures 7 and 4S (supporting data) demonstrate the effect of the feed phase solution on the transport of Zn²⁺ and Ni²⁺ ions across SLM. Graphs show that when the stripping phase NaOH concentration was increased, Zn²⁺ and Ni²⁺ ion concentrations in the feed solution dropped with time. It is also clear that the transport of Zn²⁺ improves with an increased feed solution concentration. The percentage recoveries/extraction efficiency gradually increases to 3.5 × 10⁻⁴ M of Zn²⁺ and then decreases. Likewise, the percentage recoveries of Ni²⁺ increase to 5.5 × 10⁻⁴ M of its concentration. Because the metal–cyclohexylamine complexation rate was highest at the feed–membrane interface, the highest extraction of Zn²⁺ ions was observed at 3.5 × 10⁻⁴ M and 5.5 × 10⁻⁴ M concentrations of Ni²⁺ for Ni²⁺ ions. The extraction of Zn²⁺ ions was reduced above 3.5 × 10⁻⁴ M concentration, whereas the Ni²⁺ ion extraction became reduced beyond a concentration of 5.5 × 10⁻⁴ M in comparison. The congestion of the metal–carrier in the membrane caused a reduction in metal ion transfer. Furthermore, greater metal ion concentrations may get adsorbed in membrane pores, preventing the metal ion transit to the stripping phase [53].

[figure(s) omitted; refer to PDF]

The distribution coefficients in the feed phase before (at 30 min) and after equilibrium (at 120 min) were 0.403 and 0.200, respectively. After reaching equilibrium, the distribution coefficient in the stripping phase was computed and found to be 0.094. The distribution coefficient data reveal much movement of Zn²⁺ ions into the stripping phase before the experiment reached equilibrium. This reflects the optimal feed solution concentration for successful Zn²⁺ extraction in SLM. In the case of Ni²⁺, the distribution coefficients were 0.286 and 0.147 in the feed phase before and after reaching the equilibrium (at 30 and 120 min of the experiment, respectively). After attaining equilibrium, the stripping phase distribution coefficient was calculated to be 0.054. Before the experiment reached equilibrium, the distribution coefficient data showed much transit of Ni²⁺ ions toward the stripping phase. This is the ideal feed solution concentration for Ni²⁺ extraction through SLM.

The distribution coefficient data reveal a lot of movement of Zn²⁺ ions into the stripping phase before the experiment reached equilibrium. The optimum feed solution concentration at which Zn²⁺ ions can be extracted effectively in SLM was 3.5 × 10⁻⁴ M, whereas 5.5 × 10⁻⁴ M was the optimum concentration for the effective extraction of Ni²⁺ ions. In optimization of feed phase, the cyclohexylamine (carrier) concentration was 0.4 × 10⁻⁴ M with 24 h of membrane’s soaking time and the feed phase consisted of 0.5, 1.5, 2.5, 3.5, and 4.5 10⁻⁴ M. The strip phase was a 1 M solution of NaOH.

Figure 5S (supporting data) shows graphical representations of fluxes of metal ions for the optimization of the concentration of stripping solutions. The greatest flux of Zn²⁺ ions was found during the experiment before reaching the equilibrium state. This behavior explains why the pH gradient controlled a couple of counter-transports. The stripping phase was highly basic, while the feed phase solution was kept at a pH of 4 by using a buffer solution. The maximum mass transport took place at the initial stage of the experiment. After reaching equilibrium, the pH gradient factor ended, and mass transmission was seen to be nil. For the stripping phase optimization, extraction efficiency was determined and is shown in Figure 8. All the statistical data from both the metal ion removal experiments demonstrated that the highest % extraction occurred at a stripping phase concentration of 1 M. This component also confirms that the pH gradient is critical for the transfer of Zn²⁺ and Ni²⁺ ions from feed to the stripping phase through the membrane.

[figure(s) omitted; refer to PDF]

The extraction efficiency of Zn²⁺ ion at various concentrations of the strip phase solution is given in Figure 8. The maximum extraction efficiency of the feed phase is obtained at 1.0 mole of NaOH in the stripping phase, which was thus the optimum value for the maximum extraction of both metal ions. This component also confirms that in the feed and stripping phases, the pH gradient is critical for the simultaneous transit of Zn²⁺ and Ni²⁺ extraction and de-extraction. The extraction mechanism of the Zn²⁺ ion through LM is by compound formation, ion pair association, or solvation. The mechanism of extraction involves the ion pair association. The extractant makes an ion complex, which is neutral and permeated through the membrane. The reaction can be represented as given below in Equations (9) and (10), where CHA stands for cyclohexylamine. The reaction occurring in the feed phase is shown in Equation (11), the reactions taking place in the membrane phase are given in Equations (12) and (13), whereas the reaction occurring in the strip phase is provided in Equation (14)$$\begin{array}{}\text{(9)}& \text{CHA}+{\mathrm{H}}^{+}⟶\text{\hspace{0.17em}CHA}{\mathrm{H}}^{+},\text{(10)}& \text{CHA}+\mathrm{n}{\mathrm{H}}^{+}⟶{\text{CHA}{\mathrm{H}}_{\mathrm{n}}}^{\mathrm{n}+},\text{(11)}& {\text{ZnCl}}_2+\text{HCl\hspace{0.17em}}⟶{{\text{ZnCl}}_3}^{-}+{\mathrm{H}}^{+},\text{(12)}& {\mathrm{H}}^{+}+{\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}⟶{\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}{\mathrm{H}}^{+},\text{(13)}& {{\text{ZnCl}}_3}^{-}+{\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}{\mathrm{H}}^{+}⟶{{\text{ZnCl}}_3}^{-}.{\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}{\mathrm{H}}^{+},\text{(14)}& {{\text{ZnCl}}_3}^{-}.{\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}{\mathrm{H}}^{+}⟶{\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}+{{\text{ZnCl}}_3}^{-}+{\mathrm{H}}_2\mathrm{O}+\text{NaCl}.\end{array}$$

The membrane stability was thoroughly evaluated in one of our previously reported studies on extracting Cr3+ ions using cyclohexylamine as a carrier in a supported LM system [53]. The results demonstrated that cyclohexylamine exhibited minimal leaching and consistently performed over multiple extraction cycles. These findings indicate cyclohexylamine-based membranes’ potential reusability and stability, suggesting their applicability for extracting metal ions such as Zn²⁺ and Ni²⁺.

3.2. Mechanism

Extraction of Zn²⁺ ion in SLM follows a coupled counter-transport system in which metal ions present in the feed phase make a complex with carrier molecule (extraction) diffuse across the SLM phase and release metal ions in the strip phase (de-extraction). Meanwhile, another component in the stripping phase makes a complex with a carrier molecule, which is then released in the feed phase by crossing the SLM. The membrane phase contains cyclohexylamine in cyclohexanone (diluent), which acts as a barrier. Cyclohexylamine extracts Zn²⁺ ion species at the feed–membrane interface. The Zn²⁺ ion–carrier complex, ${{\text{ZnCl}}_3}^{-}.{\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}{\mathrm{H}}^{+}$, formation occurs in this region, is called the extraction of metal and shifts the equilibrium toward the strip region. The complex ${{\text{ZnCl}}_3}^{-}.{\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}{\mathrm{H}}^{+}$ moves through the membrane phase by diffusion and releases the Zn²⁺ on the strip side at the strip–membrane interphase due to the reaction of a proton with OH⁻ of the strip solution. This movement of Zn²⁺ to strip is called the de-extraction of metal. The Zn–CHA complex is unstable in the basic media as H⁺ of the complex spontaneously reacts with OH⁻, destroying the charge balance (equilibrium of the complex), because of which the CHA is released for further transport of Zn²⁺ ions (Figure 9)$$\begin{array}{}\text{(15)}& {{\text{ZnCl}}_3}^{-}.{\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}{\mathrm{H}}^{+}+\text{NaOH}⟶{\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}+{{\text{ZnCl}}_3}^{-}+{\mathrm{H}}_2\mathrm{O}+\mathrm{N}{\mathrm{a}}^{+}.\end{array}$$

[figure(s) omitted; refer to PDF]

The overall reaction is given in Equation (16)$$\begin{array}{}\text{(16)}& \text{CHA}{{\mathrm{H}}_{\mathrm{n}}}^{\mathrm{n}+}+{\mathrm{Z}{\text{nCl}}_2+\mathrm{n}}^{\mathrm{n}-}⟶\text{CHA}{\mathrm{H}}_{\mathrm{n}}.{\text{ZnCl}}_2+\mathrm{n}.\end{array}$$

The above equation can also be written in a simple form as in Equation (17)$$\begin{array}{}\text{(17)}& {{\text{ZnCl}}_2+\mathrm{n}}^{\mathrm{n}-}+\text{CHA}+\mathrm{n}{\mathrm{H}}^{+}\leftrightarrows {\text{CHAH}}_{\mathrm{n}}\mathrm{·}{\text{ZnCl}}_2+\mathrm{n}.\end{array}$$

The equilibrium constant for the reaction can be written as$$\begin{array}{}\text{(18)}& {\mathrm{K}}_{{\mathrm{Z}}_{\mathrm{n}}}=\frac{\text{CHA}{\mathrm{H}}_{\mathrm{n}}\text{Zn}{\text{Cl}}_{2+\mathrm{n}}}{{\text{Zn}{\text{Cl}}_{2+\mathrm{n}}}^{-\mathrm{n}}\text{CHA}{{\mathrm{H}}^{+}}^{\mathrm{n}}}.\end{array}$$

It is evident from the equation that proton gradient between the feed and strip phase is the core driving force due to which the metal ions are extracted from the feed and de-extracted to the strip phase.

To objectively evaluate cyclohexylamine’s applicability, a comparative analysis of its performance against conventional carriers is given in Table 5. It summarizes key parameters, including selectivity, extraction efficiency, stripping requirements, cost, and environmental impact of various extractants reported in the literature. The comparative data in Table 5 position cyclohexylamine as a sustainable alternative for niche applications. However, its moderate toxicity necessitates careful process design to mitigate volatilization risks, as noted in prior studies [54].

Table 5

A comparative analysis of cyclohexylamine versus commercial extractants.

Carrier	Target metals	Selectivity	Extraction efficiency	Stripping ease	Cost (USD/kg)	Toxicity	References
Cyclohexylamine (CHA)	Zn2+, Ni2+, Cu2+	Zn2+ > Ni2+ > Cu2+	High (Zn2+: ≥ 90%)	Mild acid (H2SO4)	5–10	Moderate	[54, 55]
D2EHPA (P204)	Zn2+, Cd2+, Mn2+	Zn2+ > Cd2+ > Mn2+	Very high (≥ 95%)	Strong acid (HCl)	10–20	Low	[56, 57]
TOPO	UO22+, Fe3+	UO22+ > Fe3+	Moderate (70%–85%)	Na2CO3	20–30	Low	[58]
Aliquat 336	Co2+, Pd2+	Pd2+ > Co2+	High (anions)	Thiourea	20–40	Moderate	[59]
Cyanex 272	Co2+, Ni2+	Co2+ ≫ Ni2+	Excellent (Co2+: ∼95%)	Dilute H2SO4	30–50	Low	[60]
TBP	UO22+, RE3+	UO22+ > RE3+	Moderate (60%–80%)	HNO3	15–25	Low	[61]

4. Conclusion

In the present study, a systematic investigation on the transport behavior of Zn²⁺ and Ni²⁺ ions using cyclohexylamine-based supported LM as a carrier employing cyclohexanone as the diluent. The findings of the current study suggest that extraction and stripping efficiencies demonstrated strong pH of both feed and strip phases, with optimal performance being achieved at feed phase concentrations of 3.5 × 10⁻⁴ M for Zn²⁺ and 5.5 × 10⁻⁴ M for Ni²⁺, respectively. It is worth mentioning that this extraction efficiency was achieved using a 0.4 × 10⁻⁴ M carrier concentration, employing 24 h of membrane soaking time. Additionally, a comparative analysis on the transport kinetics of two metal ions revealed significantly faster transport kinetics for Zn²⁺, achieving an 87% extraction efficiency within 120 min compared to 72% for Ni²⁺ under identical conditions. This preferential transport of Zn²⁺ can be attributed to its greater affinity for cyclohexylamine, forming more stable complexes facilitating efficient partitioning into a 1 M NaOH strip phase. The findings highlight the membrane system’s selectivity for Zn²⁺ recovery, suggesting potential applications in hydrometallurgical processes for zinc-containing waste streams. Further optimization should address long-term membrane stability and evaluate the system’s performance in continuous operation modes to assess industrial viability. The demonstrated pH-dependent transport mechanism and concentration thresholds provide a foundation for scaling up this separation approach while maintaining selectivity between these transition metals.

Funding

The authors received no specific funding for this work.

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