1. Introduction
Mining is an essential driver of economic growth and prosperity for many countries. However, mining also poses serious environmental challenges, especially in the form of heavy metal contamination from acid mine drainage (AMD). AMD is the acidic and metal-rich water that results from the oxidation of sulfide-containing minerals exposed during mining activities [
1,
2]. AMD can pollute nearby water resources, endangering the ecosystem and human health [
3,
4]. Heavy metals are persistent in the environment and can accumulate in soft tissues, disrupting the normal functioning of vital organs and systems [
5,
6]. While mineral exploration drives economic advancement, addressing the potential environmental implications is essential to ensure sustainable development.
Treatment of AMD is a costly exercise due to the complex and heterogeneous nature of its composition, typical of its origin [
7]. Researchers have explored various techniques to treat AMD, such as chemical precipitation [
8], membrane filtration [
9], electrochemical treatment [
10], and biological remediation [
11]. However, these methods have drawbacks, such as low efficiency, high requirements of chemical inputs, and secondary pollution [
12]. Therefore, there is a need for novel strategies that can improve AMD treatment and remove heavy metals more effectively. Adsorption and ion exchange are promising heavy metal remediation techniques [
12,
13,
14]. Adsorption is a simple, versatile, and cost-effective technique that involves the attachment of contaminants onto the surface of an adsorbent. On the other hand, ion exchange is a technique that uses engineered polymers with fixed charges and mobile counter-ions to exchange ions with the solution based on their affinities [
15]. Both techniques provide reusability without significant performance loss, making them attractive options for heavy metals removal in aquatic media [
16,
17,
18].
Various organic and inorganic adsorbents such as activated carbon, zerovalent iron, metal-organic frameworks, clay minerals, zeolites, layered double hydroxides, and modified biochar have been proposed [
19,
20,
21]. These adsorbents can remove heavy metals from wastewater using different mechanisms, such as adsorption, reduction, precipitation, and complexation. However, these adsorbents also have drawbacks regarding their selectivity, capacity, and stability, limiting their applicability and efficiency for heavy metal remediation. For instance, metal-organic frameworks are expensive and sensitive to pH changes and water stability [
22]. Clay minerals have a low selectivity for metal ions and can be affected by competing anions [
23]. Zeolites have a narrow pH range for optimal performance and can be saturated quickly [
24].
Improving the properties and performance of these adsorbents or developing new materials that can overcome these limitations for AMD remediation when necessary; hence, the growing interest in novel adsorbents that can enhance the adsorption processes. Researchers have explored using metal oxide nanoparticles as efficient adsorbents for heavy metal removal [
25]. Metal oxide nanoparticles have a high surface area and reactivity, which enhance their adsorption capacity and selectivity [
26,
27,
28]. However, the sole usage of nanoparticles presents practical challenges, such as aggregation and instability [
29,
30]. On the other hand, ion-exchange resins utilize the Donnan membrane effect, where the fixed charge in the ion-exchange resin attracts the corresponding opposite-charged ions to the pores of the resin [
31,
32]. However, ion-exchange resins possess limited selectivity and capacity for particular ions, limiting their adsorption efficiency [
33]. The reduced adsorption capacity usually occurs due to competing ions, resulting from the simultaneous adsorption of many different ions.
To overcome these challenges, researchers have incorporated nanoparticles into polymeric matrices, such as ion-exchange resins, to create hybrid materials that combine the advantages of both components [
34,
35]. The performance of ion exchangers modified with hydrous ferric oxide (HFO) nanoparticles for phosphate removal from wastewater has been studied by various researchers [
36,
37,
38]. Gifford et al. [
39] delved into the performance of HFO or titanium dioxide nanoparticles precipitated within anion-exchange resins, targeting the removal of chromium and arsenic. Nguyen et al. [
40] prepared a composite of cation-exchange resin-supported iron and magnesium oxides/hydroxides as a potential adsorbent for nitrate ions from water. Maltseva et al. [
41] demonstrated that the sorption of arsenate ions was enhanced via ion exchangers modified with hydrated oxides of zirconium and iron.
One of the main challenges of treating AMD is its high concentration of sulfate ions, which can interfere with the removal of Cr(VI). With its divalent negative charge, sulfate challenges Cr(VI) in competing for anion-exchange resin sites. Previous studies, such as Acelas et al. [
42], have highlighted the significant negative effect of SO
42− on phosphate adsorption using a hybrid anion exchanger with embedded iron oxide. Other researchers, such as Hua et al. [
43], Kolodynska et al. [
44], and Kowalczyk et al. [
45], have investigated the delicate balance of sulfates and other anions in the adsorption of Cr(VI) using anion exchangers with embedded HFO. However, these studies used low SO
42− concentrations (40–50 mg·L
−1). In comparison, the actual sulfate concentration in AMD from the Witwatersrand Mining Basin in South Africa ranges from 1500 mg·L
−1 to 3000 mg·L
−1 [
46]. This significant difference sets the basis for this study, which aimed to simulate the high sulfate levels in AMD.
This study investigated the potential of ion-exchange resins and hydrous ferric oxide (HFO) as an adsorbent that combines two adsorption sites to remove heavy metals. HFO is a natural mineral that can form precipitates or coatings on various substrates and has a high affinity for metal ions [
47,
48]. Two types of hybrid resins were synthesized: anion-exchange resin (HAIX-HFO) for chromium (Cr(VI)) removal and cation-exchange resin (HCIX-HFO) for cadmium (Cd(II)) and lead (Pb(II)) removal in acidic and high sulfate conditions. The physicochemical properties of these hybrid resins were characterized, and their adsorption performance was evaluated under different experimental conditions. Furthermore, the feasibility study of regenerating and reusing these hybrid resins after saturation was investigated. The synthesized adsorbents have the potential for the remediation of heavy metals in aqueous environments.
2. Materials and Methods
2.1. Materials and Standards
Unless otherwise stated, all chemical reagents used in this study were of analytical grade and obtained from Sigma Aldrich. The hybrid ion-exchange resins embedded with hydrous ferric oxides (HFOs) were synthesized using FeCl
3·6H
2O, 32% HCL, 65% HNO
3, NaOH, and NaCl. Before the analysis, inductively coupled plasma optical emission spectroscopy (ICP-OES) was calibrated using a multi-elemental standard. The host resins, Amberlite IRA400 Cl form (HAIX) and IMAC HP 1110 (HCIX), were obtained from Sigma Aldrich (Merck, Johannesburg, South Africa) and Lenntech (Delfgauw, The Netherlands), respectively. As shown in
Table 1, Amberlite IRA400 Cl form is a gel-type, strong basic anion-exchange resin, while IMAC HP1110 is a strong acid cation resin. For the adsorption studies, K
2Cr
2O
7, CdSO
4·H
2O, and PbSO
4 were utilized, and the respective sulfate solutions were prepared using Na
2SO
4 (Rochelle chemicals).
2.2. Analytical Techniques for Characterization
To investigate the surface morphology and elemental composition of the dispersed nanoparticles in the hybrid ion-exchange resins, a field emission scanning electron microscope (FESEM) (model JEOL JSM-7800F), coupled with energy-dispersive X-ray spectroscopy (EDS) (Thermo Scientific Ultradry) detector was used. The FESEM was operated at an accelerating voltage of 5 kV, a magnification of 900–9500×, a resolution of 1–10 µm, and a scan time of 10 s. The EDS spectra were collected using an acquisition time of 60 s and a dead time of less than 25%.
Raman spectroscopy (Witec, Alpha 300, TS 150 Raman spectrometer), with a laser power source of 532 mW and 784.898 mW as an excitation source, was used to identify the form of the iron oxide in the hybrid ion-exchange resins. The Raman spectra were recorded in the range of 100–2000 cm−1, with a resolution of 4 cm−1, and an integration time of 10 s.
The Agilent Technologies 700 series ICP-OES was used to analyze the qualitative and quantitative levels of metals in aqueous samples. The following conditions were used to operate the ICP-OES: an RF power of 1500 W, a plasma gas maintained at a flow rate of 15 L·min−1, an auxiliary gas flow rate kept at 1.5 L·min−1, a nebulizer gas flow rate of 0.8 L·min−1, a sample uptake rate of 1 mL·min−1, and an integration time of 10 s per replicate.
2.3. Synthesis of the Hybrid Ion-Exchange Resins Embedded with Hydrous Ferric Oxide Nanoparticles
Anionic and cationic hybrid ion-exchange resins embedded with HFO nanoparticles were synthesized using a method prescribed by Pan and colleagues [
49]. The synthesis steps were conducted using a thermostatic shaker that maintained a shaking speed of 200 rpm and 25–26 °C temperature range. The synthesis of respective hybrid exchange resins (anionic or cationic) embedded with HFO nanoparticles entailed washing 100 g of Amberlite IRA 400 Cl form or IMAC HP 1110 with 500 mL of deionized water through shaking for two hours to remove fine particles and any residual organic material.
To lodge HFO into HAIX resins, the ferric chloride anionic complex was prepared by dissolving 135.42 g of FeCl3·6H2O in 500 mL of 1 M HCl to produce Fe3+ on the quaternary nitrogen sites of the Amberlite IRA 400 Cl form. The hydrolysis reaction formed [Fe(H2O)6]3+ was subsequently converted to HFO nanoparticles embedded in the resin matrix. To increase the yield, the prepared solution and the rinsed Amberlite IRA 400 Cl form were agitated in a thermostatic shaker for 24 h.
To embed HFO nanoparticles into HCIX resin, 135.42 g of FeCl3·6H2O was dissolved in 500 mL of deionized water: the Fe3+ exchanges with the Na+ counter ion of the cation-exchange resin. The FeCl3 solution was added to the rinsed IMAC HP 1110 resin and shook it in a thermostatic shaker for 24 h. Respective FeCl4− and FeCl3 solutions were decanted, followed by the addition of 300 mL 1 M NaOH aliquots and shaking while measuring the pH every 10 min using a pH meter. The solution was decanted at pH 12.
Subsequently, both resins were placed in an oven at 40 °C for 24 h. This step was followed by cooling the resins to room temperature and then rinsing with 300 mL aliquots of 1 M NaCl until the pH meter showed pH 7. After decanting the supernatant, the resins were air-dried to form HAIX-HFO and HCIX-HFO.
2.4. Adsorption Studies
This study investigated the batch adsorption of Cr(VI), Cd(II), and Pb(II) on two hybrid ion-exchange resins: HCIX-HFO and HAIX-HFO. The effects of various parameters, such as pH, sulfate concentration, contact time, and adsorbent dosage, on the removal efficiency of the three metals were examined. The adsorption data were fitted to kinetic and isotherm models to elucidate the adsorption mechanisms. Additionally, this study explored the impact of sulfate on the adsorption of metal ions and the competition between Cd(II) and Pb(II) in binary solution for the available adsorption sites. The performance of HCIX-HFO in treating actual AMD samples was also evaluated. Moreover, batch desorption studies were conducted to assess the feasibility of regenerating the heavy metal-loaded HCIX-HFO.
2.4.1. Effect of pH, Sulfate Concentration, Contact Time, and Resin Dosage
The impacts of varying pH levels on adsorption were examined. Solutions containing Cr(VI), Cd(II)), and lead (Pb(II)) at a concentration of 1 mg·L−1 were adjusted to pH values ranging from 2 to 5 using either HCl or NaOH. Subsequently, 0.005 g of HAIX-HFO was added to the Cr(VI) solutions, while 0.005 g of HCIX-HFO was added to the Cd(II) and Pb(II) solutions.
The effects of sulfate concentrations, mimicking typical levels found in AMDs, were explored. Solutions containing 1 mg·L
−1 of Cr(VI) were prepared with varying sulfate concentrations ranging from 0 to 3000 mg·L
−1 [
46,
50,
51]. The pH of the Cr(VI) solution was adjusted to 4 before being added to HAIX-HFO resin. This step and the effects of pH involved 24 h of 200 rpm agitation.
The influence of contact time on adsorption was further investigated. A Cr(VI) solution at a concentration of 1 mg·L−1 and a pH of 4 was prepared and agitated with the HAIX-HFO resin for various time intervals ranging from 5 to 360 min.
Lastly, the effect of varying the dosage of the HAIX-HFO resin on the adsorption of Cr(VI) was examined. Solutions containing Cr(VI) at a concentration of 1 mg·L−1 and a pH of 4 were prepared and agitated with different amounts of the HAIX-HFO resin ranging from 0.001 g to 0.01 g for 360 min. All steps involved maintaining the temperature at 25–26 °C, filtering the mixtures, and analyzing the remaining metal concentrations using ICP-OES.
A similar approach for studying the effects of sulfate concentration, contact time, and hybrid resin dosage on Cd(II) and Pb(II) adsorption was employed. However, these studies used the HCIX-HFO resin instead of the HAIX-HFO resin. Also, the competitive adsorption values of Cd(II) and Pb(II) were determined. A binary stock solution that contained 1 mg·L−1 Cd(II), 1 mg·L−1 Pb(II), and 3000 mg·L−1 sulfate was prepared and adjusted to pH 4. Then, 25 mL aliquots of the binary solution were added to 0.005 g portions of the HCIX-HFO resin and placed in a shaker for 24 h at 25–26 °C. The mixtures were filtered, and the remaining Cd(II) and Pb(II) concentrations in the supernatants were determined via ICP-OES.
2.4.2. AMD Remediation
An actual AMD sample from the Western Witwatersrand Mining Basin (Johannesburg, South Africa) was used in this study. The pH of the sample was 2.62, and its metal composition was determined via ICP-OES (
Table 2). Evaluation of the adsorption of heavy metals with the HCIX-HFO resin was performed by adding 25 mL of the AMD sample to 0.005 g of the HCIX-HFO resin and shaking it at 200 rpm for 360 min at 25–26 °C. Filtered solutions were measured via ICP-OES to determine the concentration of metal species.
2.4.3. Resin Regeneration
Regeneration of Pb(II) or Cd(II) metal-laden HCIX-HFO was evaluated using NaCl and NaOH. The saturated mixture of the HCIX-HFO resin with either 5% NaCl or 1 M NaOH solution was agitated for 24 h at 25–26 °C. The mixtures were filtered, and the supernatants were analyzed for Pb(II) and Cd(II) concentrations using ICP-OES.
4. Conclusions
This study entailed developing hybrid ion-exchange resins by integrating HFO nanoparticles into anionic and cationic resin matrices. The anionic hybrid resin, HAIX-HFO, showed remarkable performance in removing Cr(VI) from acidic water, achieving almost a 99.1% adsorption efficiency at pH 4 and a slightly lower one at pH 5. Despite the high sulfate concentration, which reduced Cr(VI) adsorption, HAIX-HFO exhibited selectivity for Cr(VI). The adsorption kinetics of Cr(VI) were explained via the pseudo-second-order and intraparticle diffusion models, the applicability of which varied depending on sulfate presence or absence. The R2 values for the pseudo-second-order values were 0.9929 and 0.9856 for Cr(VI) with and without sulfate, respectively. The intraparticle diffusion model revealed that sulfate on the HAIX-HFO surface lowered the Cr(VI) diffusion rate. Furthermore, the Langmuir and Temkin models were the best fit for describing the adsorption equilibrium of Cr(VI) without and with sulfate, respectively. Similarly, the cationic hybrid resin, HCIX-HFO, demonstrated optimal adsorption efficiencies of 98.2% and 97.1% at pH 4 for Cd(II) and Pb(II), respectively. The pseudo-second-order model described their adsorption kinetics in sulfate. While Cd(II) adsorption equilibrium followed the Freundlich and Langmuir models, the Freundlich model indicated that HCIX-HFO may not be a suitable Cd(II) adsorbent. In the binary solution, HCIX-HFO preferred Pb(II) over Cd(II). Furthermore, HCIX-HFO displayed its effectiveness in removing Cu(II), Ni(II), and Pb(II) from an actual acid mine drainage sample. Moreover, the use of NaCl demonstrated exceptional capability in regenerating heavy metal-laden HCIX-HFO. These findings underscore the potential of hybrid ion-exchange resins incorporating HFO nanoparticles in addressing the challenges associated with acid mine drainage treatment.