1. Introduction
Stringent environmental regulations and depletion of the world’s mineral resources have urged for the removal and recovery of heavy metals from the metallurgical production and hydrometallurgical processing waste and secondary sources in complex leach solutions. Despite their toxicity, Cd and Cu are used in industries, such as metal refining, mining, electroplating, and manufacturing of alloys. Cd is often found in industrial waste by-products, such as Cd-rich dust, Cu-Cd slag, and hydrometallurgical leachates, along with other heavy metals, e.g., Cu, Ni, Zn, etc. [
1]. Selective separation and recovery of Cd from wastewater containing various metallic constituents can be achieved by chemical precipitation, adsorption, ion exchange, solvent extraction, electrolysis, etc. [
2]. Different metal species with almost identical valence configurations in the same mixture allows co-transport, and it makes selective extraction a tough challenge. Selective extraction of Cd(II) is feasible in the presence of Zn(II), Ni(II), Co(II), Mn(II), Fe(II), Ca(II), and Mg(II), but Cu(II) and Pb(II) were found to be co-extracted with Cd(II) [
3]. The presence of Cd in the same mixture was found to interrupt the physiological balance of other metals [
4].
Liquid-liquid extraction (LLE) is one of the most versatile techniques used for the selective separation, recovery, and purification of aqueous media containing metal ions [
5]. LLE is a simple and quick method with low operational cost [
6]. LLE utilizes the principle of analyte (metal cation) distribution ratios between two immiscible liquids (generally consist of one organic and one aqueous phase) in contact with each other to achieve separation. The separation of metal within a multi-element mixture is known as selectivity. To justify the efficiency of LLE in selective separation of one metal over the other metal, the distribution ratios and separation factors of metals using selected extractant are used by most researchers [
7,
8]. Several studies have been reported in the literature on different combinations of organic extractants have been intensively investigated for the extraction of Cd(II) from synthetic solutions, industrial wastewaters, and complex leach solutions. Organophosphorus-based extractants, such as di-(2-ethylhexyl) phosphoric acid (D2EHPA), 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC88A), and di-2,4,4-trimethylpentyl phosphinic acid (Cyanex 272), were used to extract of Cd(II) from synthetic sulfate solutions by cation exchange mechanism [
9]. Free fatty acid-rich oil derived from palm kernel distillate had been introduced for Cu(II) extraction, as green and renewable extractants without the need of diluent for its low melting point, low density, low water solubility, and moderate viscosity [
10]. However, the new fatty acid has not been tested with the separation of other metals, and its selectivity for other metals is not specified. Ionic liquids (IL) are known as task-specific extractants and have been highlighted in various scientific publications for their improved and adjustable physicochemical properties, such as thermal stability, high polarity, negligible vapor pressure, non-flammability, and wide range of miscibility with other organic solvents [
11,
12]. Quaternary ammonium-based IL, such as tri-octyl methylammonium chloride (Aliquat 336), and phosphonium-based IL, e.g., trihexyl(tetradecyl)phosphonium chloride (Cyphos IL101), were used to extract Cd(II) [
13]. Rapid extraction equilibria were achieved, where 99% of Cd(II) ions were extracted using 0.2 M Aliquat 336 and 0.04 M Cyphos IL101 in kerosene, respectively. Trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl) phosphinate (Cyphos IL 104) was used for separation of Cd(II) over Cu(II), Co(II) and Ni(II) and the separation factors were found in order of S
Cd/Cu < S
Cd/Co < S
Cd/Ni [
14].
Neutral extracting agents like tributyl phosphate (TBP) and trioctylphosphine oxide (TOPO) have high extraction coefficients for many metals and organic solutions, but low selectivity when used as sole extractant. TBP has been discovered to control third-phase formation, as a phase modifier. D2EHPA-TBP has been proven for the improved extraction of several metal cations, such as Cu(II) [
15,
16], Co(II) [
17,
18], Fe(III) [
19], Ni(II) [
20], Zn(II) [
21]. There is no literature traceable regarding the extraction of Cd(II) using the combination of Aliquat 336 and TBP. Application of masking agent enhances the selective separation of metal ions by means to suppress the interference of unwanted constituents in a system without forming elaborate separation. Masking agents are also metal-complexing agents, introduced to improve the separation factor in the extraction procedures. Separation factor of Cd(II) over Zn(II) was increased by more than 500 times using D2EHPA as extractant and an aqueous hexadentate ligand with nitrogen donors,
N,
N,
N′,
N′-tetrakis(2-pyridylmethyl)ethylene diamine (TPEN) as a masking agent for Zn(II) [
22]. Ethylenediaminetetraacetic acid (EDTA) has been proved as the most useful masking agent, by forming anionic complexes with several metal ions. EDTA acted as a masking agent in the source phase for Fe(III), Cu(II), Ni(II), and Zn(II) during the selective permeation of uranium using sodium carbonate as the receiving phase [
23]. 0.1 M EDTA was added into ammonia buffer at the stripping phase at a ratio of 4 to 1 to separate Cd(II) from Zn(II) and Ni(II) [
13].
The investigation of the capability of Aliquat 336-TBP in extracting Cd(II) and the efficiency of EDTA in suppressing the co-extraction of Cu(II) has not been reported in the literature. The current work aims to find out the selective extraction of Cd(II) over Cu(II) ions by using Aliquat 336 (extractant) with TBP (phase modifier) and EDTA as a masking agent. Various process variables affecting the extraction were studied and optimized using response surface methodology (RSM). RSM has been proven to be an effective statistical tool to evaluate the interaction between variables [
24,
25]. In this study, the two-level fractional factorial design was used for screening experiments, whereas central composite design was used for the optimization of significant parameters. Analysis of the second-order model was conducted to achieve the optimum response. Qualitative Fourier transform infrared (FTIR) measurement was recorded on the extractant and their combination to compare the change of bonds in the organic phase after extraction.
2. Materials and Methods
2.1. Chemicals and Reagents
Copper sulfate pentahydrate (CuSO4·5H2O) (≥99.6% purity) and cadmium sulfate hydrate (CdSO4·H2O) (≥98% purity) were obtained from Merck. The organic extractants used to extract the metal ion were TBP (≥99% purity), and Aliquat 336 (≥95% purity) from Sigma-Aldrich. The dilution of organic extractant was performed using commercial toluene purchased from Sigma-Aldrich.
Nitric acid (HNO3) (≥65%), sulfuric acid (H2SO4) (≥98%), ethylenediaminetetraacetic acid (EDTA) (≥98%), sodium hydroxide (NaOH) (≥99%) and sodium sulfate (Na2SO4) (≥99%) were purchased from Merck. Glassware was cleansed with Decon 90 and washed before soaking in 5% HNO3. Deionized water was used for final washing of glassware and to prepare all the aqueous mixtures.
2.2. Equipment
Aqueous and organic phases were mixed using a digital overhead stirrer (IKA, Microstar 7.5 control). Initial and final equilibrium pH (pH
eq) readings of aqueous phase were measured by pH meter (Sension+ pH3, Hach, Loveland CO, USA). The Cd(II) and Cu(II) ions concentration before and after extraction studies were analyzed separately using a flame atomic absorption spectrophotometer (FAAS) (AA-7000, Shimadzu, Tokyo, Japan) after appropriate filtration and dilution. Air-acetylene (Air-C
2H
2) flame of 2300 °C was used for atomization of all samples. Analyses of Cd(II) and Cu(II) were conducted, based on
Table 1.
FTIR spectrometer (Frontier™, Perkin Elmer, MA, USA) with universal attenuated total reflectance polarization (FTIR-ATR) was operated in the mid-infrared region with wavenumbers spanning from about 600 cm−1 to 4000 cm−1 for investigation of infrared spectra of the organic phase before and after extraction.
2.3. Preparation of Aqueous and Organic Mixtures
The aqueous mixture was prepared from CdSO4·H2O and CuSO4·5H2O with 200 mM Na2SO4 as inert salt in deionized water. Various concentrations of EDTA (10 mM to 100 mM) were added to the aqueous solution, as a masking agent. The organic mixture was prepared with Aliquat 336 as extractant, TBP or TOPO as phase modifier, and toluene as diluent. Different concentrations of Aliquat 336 (50–200 mM) and phase modifier (0–100 mM) were tested. TBP and TOPO were used as phase modifier to reduce the formation of emulsion and enhancement of phase separation during LLE. Karl-Fischer determination for fresh Aliquat 336 had been identified to have 1.7 wt% water, and toluene had 0.03 wt% water. Due to a small amount of Aliquat 336 added into toluene (as diluent) utilized in this experiment, the water content in the mixture of Aliquat 336 in toluene was not significant.
2.4. Selective Liquid-Liquid Extraction of Cd(II) and Cu(II) Ions
Extraction studies were conducted by combining 20 mL of the organic mixture (Aliquat 336 and TBP) with the aqueous mixture (100 mg/L Cd(II) and Cu(II) ions) at a ratio of 1:1. The mixture was stirred with an overhead stirrer at 150 rpm for 10 min and left to separate for 5 min. The pHeq of the aqueous mixture was recorded before adjusting to preferable pH by adding in small drops of H2SO4 or NaOH. The mixture was mixed again and left to separate for 5 min until the desired pHeq was obtained. To collect the separated aqueous sample, an aqueous phase containing extracted metals was obtained after phase disengagement using a separating funnel. The separated aqueous samples were analyzed to determine the concentrations of metal contents, Maq of Cu(II) and Cd(II), respectively, with FAAS after appropriate dilution.
Standard error was found to be less than 1% after triplicate runs of the experiment. The percentage of extraction (E%) of metal ions was calculated using Equation (1).
where
Mi (mg/L) is the initial metal ion concentration in aqueous phase, and
Maq (mg/L) is the final metal ion concentration after LLE studies.
2.5. Screening and Optimization of Operating Parameters
In this study, response surface methodology (RSM) was used for analytical optimization of variables that govern the selective extraction of Cd(II) over Cu(II) ions significantly. All experiments were run in triplicate, and the relative standard deviation between replicated samples was less than 2%. Minitab software (Release 17, Minitab Inc., State College, PA, USA) was used to analyze the data involved in RSM. A 2
5–1 fractional factorial design was applied to screen five important parameters within their specific ranges, namelym equilibrium pH (pH 2–5), Aliquat 336 concentration Aliquat 336 (50–100 mM), TBP concentration TBP (50–100 mM), concentration of EDTA (10–50 mM), and organic to aqueous ratio (O:A) (1:1 to 2:1) for their significance on the response (E%). Other factors, such as mixing time (10 min), operating temperature (28 ± 1 °C), mixing speed (150 rpm), concentration of inert salt Na
2SO
4 (200 mM), initial concentration of Cd(II) and Cu(II) ions
Mi (100 mg/L), diluent type (toluene) were fixed at specific values, based on earlier findings. Sixteen experimental runs were conducted for each metal whereby the range for each parameter was determined based on the earlier findings [
26,
27].
The optimization experiment was studied using central composite design (CCD) on selective extraction of Cd(II) over Cu(II) ions obtained from the screening experiments. Level 0 represents the center value of each parameter, the low (−1) and high (+1) values, the extreme low (−α) values, and the extreme high (α) values for each parameter studied [
28]. Parameters that did not have a significant impact on the selective extraction of Cd(II) over Cu(II), were fixed at low values. Other parameters were fixed as described in screening experiments. The optimum operating conditions for maximum Cd(II) separation from Cu(II) using Aliquat 336-TBP in toluene with EDTA were evaluated by Minitab software. Composite desirability (
D) was used to evaluate the optimum value of significant parameters by identifying the degree of satisfaction.
A second-order response function in Equation (2) was selected to best fit the response data for multiple regression analysis:
where
y is the dependent variable,
,
,
and
are the regression coefficients of intercept, linear, quadratic, and interaction variables, respectively,
xi and
xj are the independent variables, and ε is the error for the effects of excluded parameters. By using Minitab software, the coefficients gathered from optimization experiments, were determined by the least square’s method [
28] to best fit the regression model.
4. Conclusions
Investigation on the addition of phase modifier at different pH clearly revealed that TBP enhanced the separability of Cd(II) over Cu(II) at pH 4.5 to pH 5.5. The obtained experimental result had proven the possibility of using Aliquat 336 with TBP to selectively separate Cd(II) with high extractability up to 86.37% over Cu(II) (0.59%) after adding EDTA (50 mM) as a masking agent. Screening of five parameters using a 25−1 fractional factorial design, consisted of equilibrium pH (pHeq), Aliquat 336, TBP, EDTA, and organic to aqueous ratio (O:A) revealed that only Aliquat 336 and EDTA have significantly influenced the selective extraction of Cd(II) over Cu(II), followed by optimization with central composite design. A second-order quadratic model was evaluated, and its R2 value (0.998) had proven that the model was highly significant and suitable to predict the extraction of Cd(II) in the range of variables studied with a deviation error of 2.76%. The optimum operating parameters for maximum separation of Cd(II) over Cu(II) were determined as follows: Aliquat 336 of 99.64 mM, TBP of 50 mM, EDTA of 48.86 mM, O:A of 1, pHeq of 5, mixing speed of 150 rpm, the operating temperature of 28 °C and mixing time of 10 min. FTIR results have proven that the presence of the interaction between Cd(II) with Aliquat 336. The reusability of Aliquat 336 and its role as the main extractant for extraction of Cd(II) has been proven.