Acid mine drainage pollution remediation using hybrid chelating ion-exchange/HZrO2 nanocomposite adsorbents
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Environmental pollution due to acid mine drainage (AMD) is a worldwide concern because of its high content of toxic metals and acidity. The toxic metal species present in AMD tends to affect negatively the whole ecological system where it is discharged, and this requires an elective solution to remedy the environment. In this study, hydrated ZrO2 nanoparticles (HZO) were irreversibly dispersed within chelating ion-exchange resins using the precipitation method, resulting in HZO-260, HZO-207, HZO-214, HZO-4195 and HZO-900 organic/inorganic nanosorbents which were used for the removal of metals from AMD. The synthesized nanosorbents were characterized using SEM–EDS, FTIR and XRD. The effect of time, adsorbent dosage and pH on Al(III) adsorption was investigated using the batch technique. The SEM–EDS confirmed the incorporation of HZO within all the parent resins, while XRD showed that the hybrid materials were amorphous. The adsorption of Al(III) occurred through physisorption and was favourable only onto HZO-260 as revealed by the data modelling. Metal levels were determined using the ICP-OES technique. The HZO-260 removed 100% Al(III) in acidic conditions and was successfully regenerated for reuse using a NaCl–NaOH binary solution (pH > 12). HZO-260 removed selected metals (Al, Cr, Mn, Fe, Ni, Co, Cu, Zn, Pb and Cd) from environmental AMD. Therefore, HZO-260 has a promising potential as an adsorbent for AMD remediation.
KeywordsAdsorption Metal removal Organic/inorganic nanosorbents Zirconium oxide nanoparticle
Mining is one of the important economic sectors that form the economic backbone of most countries worldwide [3, 21, 40]. The minerals of interest usually occur in nature with metal sulphide rocks such as iron sulphide (pyrite) in their strata, and pyrites are of paramount concern to environmentalists because upon exposure to the atmosphere, they quickly react with water and air to produce acidic effluents known as acid mine drainage (AMD) [4, 16]. AMD is characterized by high concentrations of H+, Fe2+ and SO42− species. Once AMD is formed, either in the mine voids or in the stockpiles of waste rock, it eventually finds its way through percolation, seepage, leachate, run-offs or even decanting to pollute water sources, thus rendering the water acidic . When acidic water is in contact with different geologic materials such as mineral rocks, then toxic metals can be easily leached out and the solution gets laden with metal ions . Generally, AMD acts as a vehicle for toxic metal pollution in the environment and the same toxic metals are a known recalcitrant bio-accumulative systemic toxins which cause carcinogenic, mutagenic, teratogenic, fetotoxic, neurotoxic and nephrotoxic effects to humans . Moreover, the acidic metal-laden water upsets the whole ecological system as AMD-polluted water is always devoid of life . The water habitat and aesthetic values are destroyed by AMD pollution. The worst part is that once AMD is produced, it persists for centuries, thus polluting the environment endlessly .
AMD is so disastrous to the environment such that in the year 1987 the United States Environmental Protection Agency (USEPA) declared it second only to global warming and stratospheric ozone depletion in ecological risk [16, 29]. Since then, treatment technologies have been sought worldwide to remediate toxic metal pollution in the environment. Unfortunately, traditional AMD treatment technologies use chemicals that produce wastes that are hazardous to the environment . Therefore, there is a need for green technologies to be explored to save the environment from toxic metal (AMD) pollution.
One of the attractive technologies to effectively address AMD pollution to the environment involves adsorption process. These processes are an attractive technology for toxic metal removal from solutions, due to the fact that they employ adsorbents with high surface area-to-volume ratio, thus only small amounts are required, and the waste generated may not lead to secondary pollution. Moreover, the loaded adsorbents may also be amenable to regeneration for multiple reuse and recovery of the pollutants as valuable products. Thus, adsorption processes are cheap, simple and environmentally friendly . Metal oxides are renowned metal scavengers in the environment and are a good choice of adsorbents for dissolved metals [10, 14, 17]. Therefore, since metal oxides have a special affinity for dissolved metals in the environment, they can be carefully engineered for the efficient removal of the same from solutions. Their adsorptive properties are enhanced when they are in the nanoscale . Advantages of metal oxide nanoparticles (MONPs) as adsorbents include high adsorption efficiency, fast kinetics, amphoteric surfaces in addition to the fact that their synthesis involves procedures that are safe, simple and cost-effective . Unfortunately, MONPs suffer from aggregation and weak mechanical strength to withstand flow-through systems for application purposes . To overcome this limitation, researchers have dispersed MONPs into various polymeric materials for support-producing hybrid composite adsorbents with multiple advantageous properties [34, 39, 44].
Zirconium (IV) oxide is one of the polyvalent metal oxides that can remove both organic and inorganic pollutants from water. Zirconium oxide has the advantage of high chemical stability  and is rich in surface hydroxyl groups , which make it attractive for use as an adsorbent for metals from highly acidic solutions like AMD. According to the literature, the use of organic-supported hydrated Zr(IV) oxide nanocomposites has been reported for the removal of metals from wastewaters. Cho and co-workers prepared a ZrO2/chitosan beads adsorbent (HZOCBs) for the removal of F− and Pb2+ from solution . HZOCBs exhibited efficient and rapid removal of Pb(II) ions with a maximum adsorption capacity of 222.2 mg/g. The adsorption of Pb(II) on the hybrid material occurred in a single-layer fashion. In another study, Pan and others fabricated a porous anion exchanger-supported hydrous Zr oxide adsorbent (HZO-201) and used it for the removal of As from groundwater and acidic mine water . HZO-201 demonstrated a superior removal of As from the waters as compared to the Fe oxide-laden hybrid with the same host (HFO-201). The hybrid resin was amenable to regeneration for reuse using NaOH-NaCl binary solution with negligible loss in adsorption capacity. Padungthon and co-workers also synthesized a hybrid anion-exchange resin impregnated with ZrO2 nanoparticles (HAIX-Zr) for the removal of both As(III) and As(V) ions from water . The hybrid resin selectively removed both As species in the background of high concentration of competing ions. Similar to the previous study, they found that HAIX-Zr can be efficiently (> 90%) regenerated for reuse using the binary solution. They concluded that the hybrid adsorbent is very stable, it can immobilize the adsorbed As forever, and finally, the exhausted As-loaded nanosorbents can be safely disposed of in a landfill without the As leaching off into the environment.
In this study, the chemically stable Zr(IV) oxide was dispersed within various chelating resins for support-producing hybrid chelating ion-exchange resins (HLIX) for the removal of metals from the hostile AMD. Chelating resins exhibit better removal of metal ions over cation and anion-exchange resins. Thus, they are a good choice of support material for MONPs for metal pollution remediation. To the best of our knowledge, chelating resin-supported hydrated zirconium oxide nanosorbents for metal pollution remediation in acidic industrial wastewaters including AMD were for the first time produced and reported by this study. The major constituents of the Wits gold mines’ AMD in South Africa are Al(III), Fe(II) and Mn(II) [18, 24]. Of the metals, Al(III) has received less attention from researchers, yet it is as much toxic as the so-called heavy metals [22, 30, 46]. Secondly, Al(III) is the most sensitive to high conductivity and low pH conditions, which are both characteristic of AMD, where it readily leaches from geologic material to contaminate water resources . Hence in this study, Al(III) was used to investigate the adsorption efficiency of the synthesized HLIX. In humans, Al can affect both hard and soft tissues leading to detrimental health effects. Aluminium poisoning causes bone disease, osteomalacia; the Al(III) ions compete and exchange sites with the Ca2+ in the bones. When Al affects the nervous system, it causes Alzheimer’s disease and in cells it compromises the immune system . Aluminium is also carcinogenic. In aquatic life, Al(III) smothers the gills disturbing the enzymes responsible for uptake of ions leading to death . In soils Al inhibits the nutrient uptake by the plant roots  leading to dwarfism or even death of the plants. This may compromise food security and may result in deforestation. Aluminium in AMD impacted water (pH < 4.5) exists as an Al-SO4+ complex and as the most toxic free ion, Al3+. Both species increase with the decrease in pH of the polluted water .
Therefore, the objectives of this study were to impregnate hydrated Zr oxide nanoparticles into different macroporous polystyrene-based chelating resins producing hybrid HZO nanocomposites for the adsorption of metal ions from solutions, to characterize the fabricated hybrid nanocomposite materials and finally to use them for the removal of metal ions from environmental AMD.
2 Experimental procedures
In this work, hybrid ion-exchange Zr(IV) oxide adsorbents were synthesized by precipitation followed by mild thermal treatment and then characterized using a range of techniques to evaluate the success of the synthesis. Batch adsorption studies to evaluate the adsorbents’ performance and the mechanisms of the adsorption were carried out using synthetic solutions composed of the major AMD components: Al(III), Fe(II), Mn(II) and SO42−. This was followed by the desorption of the metals from the loaded resins. Finally, the adsorbents were tested on environmental AMD samples obtained from a defunct gold mine located in the western part of Gauteng Province, South Africa, for the removal of 10 selected metals, namely Al, Cr, Mn, Fe, Ni, Co, Cu, Zn, Pb and Cd.
2.2 Synthesis of hybrid Zr(IV) oxide adsorbents
All chemicals used in the experiments were of analytical grade purchased from Sigma-Aldrich (Johannesburg, South Africa). Five different macroporous polystyrene cross-linked divinyl benzene (DVB) resins were used as host materials for Zr(IV) oxide nanoparticles producing hybrid HZO adsorbents to study their efficiency on metal ions adsorption. Three of these host resins (alkylaminophosphonate, iminodiacetate and thiourea) were of the weakly acidic chelating type, while the other two were anionic (a weakly basic chelating bis-picolylamine and a strongly basic quaternary ammonium types) producing HZO-260, HZO-207, HZO-214, HZO-4195 and HZO-900 nanocomposites, respectively. The host resins were washed by shaking 50 g of each with 250 mL DI water in a prewashed 500-mL conical flask using an orbital shaker set at a speed of 150 rpm for 2 h. This procedure was carried out 3 times with fresh DI water each time. A 0.1 M solution of 99.99% zirconium (IV) sulphate hydrate was prepared using de-ionized water and was used to load the Zr(IV) ions into the L260 host resin. All the other host resins were loaded using 1 M solution of zirconium oxychloride (ZrOCl2·8H2O). Metal loading solutions for anion exchangers were prepared by dissolving the metal salt (ZrOCl2·8H2O) in 1 M dilute HCl in an ice bath . The full synthesis procedure for the hybrid nanosorbents was done in three steps. The first step was the loading of the Zr ions into the 50 g washed resin by shaking with 250 mL loading solution in an orbital shaker set at 150 rpm in room temperature for 24 h. The supernatant was decanted and discarded appropriately. The next step was the simultaneous desorption and precipitation of the zirconium ions loaded within the resin beads by shaking with 250 mL of a binary NaCl–NaOH (1 M of each) solution under the same conditions as in the previous step. The solution was decanted and discarded likewise. The resin beads were then washed with DI water more than 10 times to remove all residual binary solution. The clean resins were finally dried in an oven set at 40 °C overnight for 16 h. Hence, the hybrid chelating ion-exchange metal oxide resins (HLIX-Zr) were produced for the purpose of this study.
2.3 Characterization of synthesized resins
Physical characterization of the synthesized resins was carried out on both the host and hybrid resins. The crystallinity was studied using XRD model Rigaku Smartlab X-ray Diffractometer at room temperature using Cu-Kα radiation (λ = 0.154059 nm) operated at 45 kV and 200 mA in a 2θ range of 5°–90° and speed 2° min−1. The qualitative qualities of the dispersed nanoparticles were studied using HRSEM model JEOL JSM-7800F Field Emission Scanning Electron Microscope (FESEM) coupled with Thermo Scientific Ultradry EDS detector. Functional groups on the adsorbents’ surfaces were investigated using Fourier transform infrared model PerkinElmer FTIR spectrometer Frontier (spectrum 100 spectrometer) by ATR method in the range of 400–4000 cm−1 at a resolution of 4 cm−1.
2.4 Adsorption studies
Adsorption experiments were carried out using the batch equilibrium technique. Synthetic solution containing about 50 mg/L of each of Al(III), Fe(II) and Mn(II), the main metal constituents of AMD, was prepared in the presence of about 3000 mg/L SO42− and used to model the adsorption efficiency of the synthesized nanosorbents. The salts that were used to prepare the ternary synthetic solution were supplied as Al2(SO)4 hydrate (Mr = 242.2 g/mol), MnSO4·H2O (Mr = 98.08 g/mol) and FeSO4·7H2O (Mr = 278.01 g/mol), and the weights for the salts used were 0.6356 g, 0.3000 g and 0.4976 g in 2 L, respectively. The batch tests were carried out in 100-mL Erlenmeyer flasks using 50 mL of synthetic solution in an orbital shaker set at 200 rpm, at 25 °C. Each flask was covered with parafilm to avoid evaporation during the adsorption process. A fixed amount of adsorbent (20 mg) at different time intervals (5, 10, 20, 30, 60, 180, 360 min) was used to study the effect of contact time on Al(III) ions’ adsorption. The effect of adsorbent dosage (0–12 g/L) for the predetermined times on adsorption efficiency was conducted. The effect of initial pH (1.5–3.5 and 10.0–11.5) on adsorption of Al(III) ions was investigated at predetermined time and resin dosage. Al(III) precipitates around pH 4 and redissolves above pH 8; thus, no pH study can be conducted in this pH range for the Al(III). Kinetics and isotherm modelling was carried out using the adsorption data in the pH ranges where Al(III) is dissolved. Regeneration of the loaded hybrid resin was carried out using a NaCl–NaOH binary solution (3% w/v of each salt). The metal-loaded hybrid resin was washed with DI water to remove excess synthetic solution before 50 mL of regenerant solution was added. The flask was then shaken in the same conditions as the adsorption process. The amounts of metal ions in the solutions were estimated using Agilent Technologies 700 Series ICP-OES.
2.5 Application to environmental sample
The best performing hybrid nanosorbent was finally tested on environmental AMD, obtained from a defunct gold mine located in the western part of Gauteng Province, South Africa, for the removal of 10 selected metals (Al, Cr, Mn, Fe, Ni, Co, Cu, Zn, Pb and Cd). All experiments were carried out in triplicate, and the average result was reported. Blanks were also run together with the samples to establish the effect the glassware had on the adsorption of the metal ions.
2.6 Data analysis
3 Modelling of adsorption data
3.1 Kinetics models
3.2 Adsorption isotherm models
4 Results and discussion
4.1 Characterization of nanocomposites
4.2 Effect of contact time on Al3+ adsorption
Kinetics model parameters for Al3+ adsorption on HZO hybrids at solution pH 1.8
4.3 Effect of adsorbent dosage
Freundlich, Langmuir and Temkin isotherm constants for the adsorption of Al3+ on HZO nanocomposites
8.34 × 10−9
4.83 × 10−18
9.13 × 105
3.64 × 1031
All the other nanosorbents’ adsorption processes showed best agreement with the Langmuir model (R2 > 0.99). This result implies that the adsorption sites on each adsorbent surface have equal energy (homogeneous) and there were no interactions between adsorbed species resulting in a single layer of the adsorbed metals. However, the negative values of KL give negative values of qmsx which implies that the Langmuir model is inadequate for explaining the adsorption processes of these nanocomposites [1, 2]. This finding is reinforced by that the four nanocomposite adsorbents had the Langmuir model separation factor RL > 1, which implies that the adsorption of Al(III) on their surfaces was not favourable; adsorption is favourable in the range 0 < RL < 1. This result concurs with the findings on the effect of pH which showed low removal efficiencies for these hybrid adsorbents in acidic conditions (Fig. 5b–e). Moreover, the big Chi-square values show that the difference between the experimental and calculated capacity using the model is significant, confirming the inadequacy of the model to describe the adsorption processes regardless of being the model of best fit as per the coefficient of determination (R2 > 0.99).
4.4 Effect of pH on the pattern of adsorption
The pH is the master parameter in the adsorption of metals from solutions. Figure 5a shows that hybrid HZO-260 removed all (100%) of the Al(III) in acidic conditions and adsorption was unfavourable in alkaline conditions. The preferred solution pH for efficient and sufficient use of the HZO-260 adsorbent is 2.0–2.4 because the final solution pH (6.6–7.5) is within pH 6.5–8.5, the drinking water guideline by the United States Environmental Protection Agency (USEPA) . Thus, this nanocomposite is best suited for AMD remediation. HZO-207 (Fig. 5b) adsorbed Al(III) (85%) the best at alkaline pH (10.2) achieving a final solution pH of 8.1 which is within the drinking water guideline. This adsorbent may be suitable for the recovery of metals from alkaline solutions though this may be a very difficult process to control because at pH 11 desorption is favourable. HZO-214 adsorbed 100% Al(III) at pH 11.3 yielding a final solution pH of 7.66 that meets the drinking water guideline as shown in Fig. 5c. Similarly, the use of this adsorbent would pose a great challenge because at pH 10 metal adsorption is very unfavourable and the same is applicable at pH around 12. These two conditions would make the use of this hybrid material a very difficult process to control because of the very narrow pH operation range. Therefore, it is not cost-effective to adsorb Al(III) in alkaline conditions. It should be noted that the adsorption of Al(III) was not considered in the pH range of 4.5–9, where Al(III) precipitates as Al(OH)3. Above pH 9, the Al(OH)3 dissolves and Al(III) ions are released into the solution.
Summary of the effect of pH observations for the hybrid adsorbents under study
Adsorption efficiency (%)
Final solution pH
1.6, 2.0, 2.4, 3.4
2.5, 6.6, 7.5, 9.1
Hybrid HZO-4195 and HZO-900, as shown in Fig. 5d and e, respectively, adsorbed Al(III) (82% and 84%, respectively) best at alkaline conditions of pH 10.2, but the final solution pH (9.6 and 11.3) failed to meet the USEPA drinking water guideline. The host resins for these two hybrid materials are a weak base chelating resin (bis-picolylamine) and a strong base anion exchanger, respectively; hence, they have fixed positive charges that exert Donnan membrane effect on the negative Al(III) complex, Al(OH)4−, in the alkaline medium. In light of the estimated pHpzc (2.5 and 2.2) for the two adsorbents and the negative Al(III) complex in alkaline pH, the adsorption process must have happened through chemical ion-exchange with both the mobile anion of the host resin and the negative surface functional groups of the Zr oxide nanoparticles (ligand exchange). The slight difference in performance between these two hybrid materials may be attributed to the basic anionic strength of the host material. Nevertheless, both hybrid resins can be used to remove some metals in alkaline wastewater as part of a treatment train.
4.5 Regeneration studies
4.6 Application on environmental AMD
Mean metal levels (mg/L) in environmental AMD
Based on this study, incorporation of Zr oxide nanoparticles into the chelating resins was found very important. This is in terms of enhancing metal removal efficiency from AMD as well as improving the stability of the adsorbent in acidic media. Amongst the synthesized hybrid resins, HZO-260 was found the best in adsorption efficiency of metals at a lower pH ≤ 3.4. The other nanocomposites, HZO-207, HZO-214, HZO-4195 and HZO-900, can remove metals appreciably only in alkaline solutions, which is not cost-effective. Adsorption of Al(III) by HZO-260 from real AMD occurs through physisorption (chemical ion-exchange). Application of HZO-260 for metal removal from the environmental AMD sample revealed that removal of Mn(II) in acidic medium is possible. Furthermore, in order to apply HZO-260 in future for real AMD remediation, possible number of recycle and reuse, cost–benefit calculation and optimization of all parameters need to be addressed. Overall, from these results hybrid HZO-260 nanocomposite is a potential and promising candidate for future AMD pollution remediation.
The authors are grateful for laboratory working space from the College of Agriculture and Environmental Sciences (CAES), use of ICP-OES from the Analytical Chemistry Department, use of XRD and HRSEM from the Physics Department and funding from the NanoWS Research Unit, all from UNISA Science Campus, Johannesburg, South Africa.
Compliance with ethical standards
Conflict of interest
The author(s) declare that they have no competing interests.
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