1. Introduction
The availability of precious metals, such as gold and palladium, is becoming very scarce and natural resources are depleting. This is due to their demand in numerous applications ranging from jewellery to automotive industries [
1,
2,
3,
4,
5,
6]. Due to their broad application, limited stock and the high price of precious metals make it vital to recycle and recover them from industrial waste [
7]. Gold (Au) and palladium (Pd) metals are economically important as investments and as currency. Recovering and recycling precious metals from waste have gained much attention in research industries [
8,
9]. Mohamed et al. [
10], reported in their work that, there is about 300 g per ton of gold in computer motherboards and approximately 200 g in mobile phones [
10]. Out of the top six PGMs, Pd plays a key role in manufacturing processes in industries such as fossil fuel refinery, automobile, electronics and pharmaceutical [
11,
12,
13]. Several researchers argue that to preserve Au(III) and Pd(II) resources, as well meeting the future market demands, there is a need to develop cheap and eco-friendly processes for the recovery of PGMs from secondary sources.
The most widely used analytical techniques to quantify Pd and Au in industrial waste and natural ore include graphite furnace atomic absorption spectrometry [
14,
15], X-ray fluorescence spectrometry [
16], ultraviolet-visible spectrometry [
17], atomic absorption spectroscopy [
16,
18,
19] and inductively coupled plasma techniques [
20,
21]. Spectrometric methods have attracted attention due to their low cost and simplicity. Among these methods, inductively coupled plasma-atomic emission spectroscopy (ICP-OES) is favourable because it is robust, can handle wide ranges of the sample matrix and has the ability to analyse multi-elements [
22,
23]. However, due to the low concentration, difficulties in extraction and digestion of precious metals in waste materials, there is a need for the application of sample pre-treatment before ICP-OES determination.
For this reason, several micro-extraction methods have been employed for the effective adsorption of precious metals from waste. These include dispersive liquid-liquid microextraction [
24,
25,
26], dispersive solid-phase (micro)extraction (DSPME) [
27] and magnetic solid-phase extraction (MSPE) [
2,
28,
29]. Among miniaturized SPE techniques, MSPE technology offers remarkable features such as rapid separation of the adsorbent from the sample matrix, effective interaction between the adsorbent and the analytes and elimination of centrifugation step [
30,
31]. However, in MSPE the sensitivity, affinity and efficiency of an adsorbent depend on the nature of adsorbent material used [
31]. For instance, magnetic nanoparticles can provide large surface modification and therefore, an enhancement of adsorption capacity, which can make it to interact with various types of analytes. For instance, in a study reported by Jalilian et al. [
18], they investigated the extraction and determination of Au (III), Pd(II) and Ag(I) using a magnetic adsorbent. They used thiourea-HCl to elute metal ions from the adsorbent and quantified those using flame atomic absorption spectroscopy (FAAS). The maximum capacities were ranging between 49–50 mg g
−1. They successfully applied the method to extract analytes in wastewater. However, it is critical and significant to search for new materials with high extraction efficiency and adsorption capacity.
Our previous study, magnetic Fe
3O
4/Mg-Al-layered double hydroxide (LDH) nanocomposite has proven to be effective in the recovery of gold(III) and iridium(IV) from mine soil samples [
32]. Promising results that maximum adsorption capacities and recoveries ranging from 115–124 mg g
−1 and 80%–109% were obtained. The high adsorption capacity and acceptable percentage recoveries were attributed to the incorporation of LDH material. Layered double hydroxide has been extensively studied as a potential adsorbent for the sorption removal of various pollutants [
33,
34]. This is due to their structural characteristics such as ion exchange and memory effect which makes LDH to be favourable for extraction and adsorption of pollutants (especially anions) in acidic and basic media. However, LDH, on its own, lacks practical applicability due to the difficulties in separation and recovery of the adsorbents from sample solutions media.
Therefore, the aim of this work was to investigate the potential application of core-shell nanostructured magnetic (Fe3O4@SiO2) nanocomposite functionalized with layered double hydroxide (Mg-Al-LDH) nanoflakes for the extraction and recovery of gold(III) and palladium(II) from aqueous leachate solutions of PGM ore and concentrate. The objectives of this article are: (1) to prepare and characterise layered double hydroxide-coated with magnetic nanoparticles (Fe3O4@SiO2@Mg-Al-LDH) nanocomposite, and (2) to apply the Fe3O4@SiO2@Mg-Al-LDH nanocomposite as the adsorbent in the recovery of Au(III) and Pd(II) from aqueous leachate solutions of PGM ore and concentrate prior to ICP-OES quantification. The effect of sample pH, mass of adsorbent, extraction time, eluent type and concentration on the extraction and recovery were investigated using response surface methodology.
2. Materials and Methods
2.1. Materials and Instrumentation
The reagents employed in this study were all of analytical grade unless otherwise specified in the methods and they were used as obtained. Double distilled water (Ultra-pure (type 1) supplied by Milli-Q water system (Merk, Darmstadt, Germany) was used for the entire study. Ammonium hydroxide solution (25% v/v) was obtained from Associated Chemical Enterprises, (Pty) Ltd., Johannesburg, South Africa) and was used in the synthesis of the adsorbent and pH adjustments. Iron (II) chloride tetrahydrate (FeCl2·4H2O), aluminium chloride (AlCl3), iron (III) chloride hexahydrate (FeCl3·6H2O), absolute ethanol, tetraethylorthosilicate (TEOS) and magnesium chloride hexahydrate (MgCl2·6H2O) used for the synthesis of magnetic Mg-Al-LDH composite were acquired from Sigma-Aldrich (St. Louis, MO, USA). Gold and palladium singe element standard (1000 mg L−1) purchased from HG-LGC (Teddington, UK) were used to prepare synthetic samples for adsorption and the calibration of the instrument. South African Reference materials, that is, SARM 186 platinum group metal (PGM) concentrate and SARM 107 PGM ore were obtained from Mintek (Analytical Services Division, Johannesburg, South Africa). The quantification of Au(III) and Pd(II) recovered from sample solutions was determined using an ICP-OES (iCAP 6500 Duo, Thermo Scientific, Loughborough, UK) equipped with a charge injection device (CID) detector. The operating conditions were applied as suggested by the manufacturer. The pH of the solutions was adjusted using H1 9811–5, pH meter ((HANNA Instruments, Smithfield, RI, USA). A Scientech ultrasonic cleaner (Labotec, Midrand, South Africa) with a volume of 5.7 L (internal dimensions: 300 mm × 153 mm × 150 mm) was used to facilitate the adsorption, extraction, and recovery (elution) process.
2.2. Preparation of Fe3O4@SiO2@Mg–Al LDH Nanocomposite
The synthesis of Fe
3O
4@SiO
2@Mg–Al LDH was obtained via sol-gel method. Magnetic nanoparticles were synthesized by following a previously reported method by Munonde et al. with minor changes [
35]. The Fe
3O
4@SiO
2 microsphere was obtained through a sol-gel method which was previously reported by [
36]. Briefly, 850 mg magnetic spheres (Fe
3O
4) previously synthesised were dispersed into a round-bottom flask charged with ethanol, water at a ratio of 4:1 (
v/
v) and 3.60 mL concentrated ammonia solution 25%–30% NH
3 basis). The suspension was sonicated in an ultrasonic bath for 30 min to enable uniform dispersion. This was followed by dropwise addition of 2.5 mL of tetraethyl orthosilicate (TEOS), and the mixture was stirred continuously for 8h at 70 °C. The resultant Fe
3O
4@SiO
2 microspheres were collected using an external magnetic field, washed with ethanol five times, and dried at 50 °C for 2 h.
The deposition of nanoflakes of Mg–Al-LDH onto Fe
3O
4@SiO
2 microspheres via ultrasonication to form Fe
3O
4@SiO
2@Mg–Al-LDH were carried out as reported by Zhao et al. [
36]. Briefly, 0.10 g Fe
3O
4@SiO
2 nanocomposite was dispersed in 50 mL deionized water-methanol mixture at pH 10. via ultrasonication. The pH of the deionised water-methanol mixture (1:1) was adjusted to 10 addition 0.53 g Na
2CO
3 and 0.4 g NaOH solids. 20 mL 1:1 methanol:water containing 1.13 g AlCl
3, 1.476 g MgCl
2·6H
2O was added dropwise to the Fe
3O
4@SiO
2 suspension at a constant rate using a burette under vigorous stirring. It should be noted that the pH of the solution was maintained at pH 10. When the reaction was completed, the mixture was further agitated via ultrasonication for 1 h. The product was collected via an external magnet and redispersed in 70 mL of deionized. The product was further exfoliated under ultrasound dispersion for another 1 h. The Fe
3O
4@SiO
2@Mg–Al-LDH nanocomposite was separated from the supernatant using an external magnet, washed with ethanol, and dried at 50 °C under vacuum for 2 h.
2.3. Characterization
The crystalline structure of Fe3O4 Mg–Al-LDH, Fe3O4@SiO2, and Fe3O4@SiO2@Mg–Al-LDH nanocomposite were investigated using a PANalyticalX’pert Pro X-ray diffraction crystal analyser (XRD, PANalytical, Almelo, The Netherlands) with nickel-filtered CuKα radiation. The transmission electron microscopy (TEM, JEM-2100, JEOL, Tokyo, Japan) was used to record the TEM images of the adsorbents. The morphological studies of the prepared adsorbent were also assessed using scanning electron microscopy (SEM, TESCAN VEGA 3 XMU, LMH instrument coupled with energy dispersive X-ray spectroscopy (EDS) to confirm the elemental composition of the composite. The nitrogen adsorption–desorption (Brunauer–Emmett–Teller (BET)) analysis (ASAP2020 V3. 00H, Micrometrics Instrument Corporation, Norcross, GA, USA) was used to determine the surface properties of the adsorbent. The zeta potential of the nanocomposite was measured by analysing suspension of 0.1 g adsorbent at different pH (1–12) using a Nano ZS Zetasizer (Malvern Instruments Limited, Malvern, Worces, UK).
2.4. Extraction and Recovery of Au(III) and Pd(II) from Synthetic Samples
2.4.1. Selection of Eluent Type
The elution or desorption of Pd(II) and Au(III) was studied using the analyte-loaded adsorbent and different elution solvents. Briefly, 0.15 g Fe3O4@SiO2@Mg–Al-LDH nanocomposite was placed in sample bottles containing 50 mL of 500 µg L−1 adjusted to pH 3.5 with 0.01 mol L−1 HCl or NaOH and sonicated for 20 min. The supernatant solution was carefully discarded with aid of an external magnet. 5.0 mL of 3.0 mol L−1 HNO3, HCl, and aqua regia was replaced in the sample bottle containing an analyte-loaded adsorbent. The elution or desorption process was carried by allowing the adsorbent to interact with the eluent solvent under ultrasonication for 10 min. The supernatant and the adsorbent were separated using magnetic separation and the supernatant was filtered and analysed using ICP-OES.
2.4.2. Optimization Using Central Composite Design (CCD)
The extraction and recovery of Pd(II) and Au(III) was carried by placing different masses of the adsorbent (50–200 mg) in as sample bottles containing 50 mL of 500 µg L
−1 of Pd(II) and Au(III) solution (pH 2–9 adjusted with 0.01 mol L
−1 HCl or NaOH). The adsorption process was achieved by ultrasonication for 5–30 min. Aqueous solutions and the adsorbent were separated via magnetic decantation. The adsorbed analytes were recovered using 3.0 mol L
−1 HCl acid after sonication for 10 min, magnetically separated, filtered and analysed with ICP-OES. The optimization of the extraction and recovery process was achieved using central composite design (CCD). The most influential variables were mass of adsorbent (MA) sample pH and extraction time (ET), their minimum (−), central (0) and higher (+) levels are presented in
Table 1.
2.5. Determination of Adsorption Capacity
The adsorption equilibrium experiments were carried out under optimized conditions. Briefly, 112 mg of adsorbents were added into 100.0 mL model solution (pH 4.0) containing Pd(II) and Au(III) at initial concentrations ranging from 2–10 mg L
−1 (prepared using a single stock solution of each analyte) for 17.5 min. Four commonly used isotherms, i.e., Freundlich, Langmuir, Toth and Sips models were applied to interpret the experimental data obtained for ICP-OES analyses. The concentration of Au(III) and Pd(II) in the procedure blanks and model sample solution (before adsorption) was determined using ICP-OES. The analytical data from the ICP-OES was used to calculate adsorption capacity (
qe, mg g
−1) (Equation (1)).
where
C0 and
Ce are initial and equilibrium concentrations (mg L
−1) of Au(III) and Pd(II),
V is the volume of the sample (L), and
m is the mass of the adsorbent (g).
2.6. Regeneration and Reusability Studies
The desorption of adsorbed Au(III) and Pd(II) was carried out by 3.0 mol L−1 hydrochloric acid. Briefly, 10 mL of desorption solution was added to 50 mL reagent bottles containing spent adsorbent loaded with analytes of interest. The samples were sonicated for 20 min to ensure that all the analytes have been desorbed. The desorbed analytes from the spent adsorbent were analysed using ICP-OES. After the desorption process the adsorbent was washed with deionised water followed by ethanol and dried at 50 °C in a vacuum oven for 2 h.
2.7. Effect of Interfering Ions
Gold and palladium may coexist with other metals and therefore, the effect of interfering ions was investigated under optimum conditions. Briefly, 50 mL of synthetic samples (pH 4.0 adjusted with 0.01 mol L−1 HCl or NaOH) containing analytes of interest was fixed at 1.0 mg L−1 and major and trace metals (As, Zn(II), Ni(II), Mn(II), Cr(III), Na(I), Ca(II), Ba, and Cr(III)−), at 10 mg L−1 were added to sample bottles containing 112 mg of the adsorbent. The mixed-ion sample solution was sonicated for 17.5 min and the analytes were eluted using 3.0 mol L−1 HCl. The eluent was analysed using ICP-OES.
2.8. Application to Real Samples
Briefly, 1000 mg of SARM 186 PGM concentrate and SARM 107 PGM ore were accurately weighed and placed in 50 mL of polypropylene tube. Then, 10 mL of aqua regia (6 mL of HNO3 and 4 mL and HCl) and 5-mL of the concentrated H2O2 was added. The sample was then digested for 60 min at 60 °C using a Digi block. The residue was diluted to 50 mL using deionised water. The extraction and recovery of Pd(II) and Au(III) from leachate samples were carried out under optimum conditions. Briefly, 50 mL leachate solutions at pH 4.0 (adjusted with 0.01 mol L−1 HCl or NaOH) was placed in a sample bottle containing 112 mg of the adsorbent. The extraction process was achieved via ultrasonication for 17.5 min. Supernatant and residual adsorbent were separated via an external magnet. The adsorbed analytes were recovered using 3.0 mol L−1 HCl acid and the elution process was assisted by ultrasonic power 10 min. The supernatant containing the recovered analytes were separated via magnetic decantation and the liquid was filtered and analysed using ICP-OES.