Production of Nanoparticles from Acid Mine Drainage

9.8.1 Introduction

Nanoparticles represent an active area of research and a techno-economic sector with full expansion for applications (Jeevanandam et al., 2018). Nanoparticles are solid particles ranging in size from 1 to 100 nm (Yah et al., 2012; Ansari et al., 2019). Of particular interest in this section of the chapter are metallic nanoparticles which are regarded as nano-sized metals with dimensions (length, width, thickness) within the size range of 1 to 100 nm (Kumar et al., 2018). In general, nanoparticles have gained prominence in technological advancements due to their tunable physicochemical characteristics such as melting point, wettability, electrical and thermal conductivity, catalytic activity, light absorption and scattering resulting in enhanced performance over their bulk counterparts (Jeevanandam et al., 2018). The subsequent section gives a number of examples of nano-metallic particles that have been generated from AMD.

9.8.2 Selected Typical Studies of Production

of Nanoparticles from Acid Mine Drainage

Nanoscale zerovalent iron (nZVI) particles were investigated by Crane and Sapsford (2017) for the extent at which the particles could be used for the selective formation of Cu bearing nanoparticles from AMD. A methodology by Wang and Zhang (1997) was used to synthesise pure nZVI by dissolving 7.65 g of FeS04-7H20 into 50 mL of Milli-Q water (>18.2 Mft cm) and the pH was adjusted to 6.8 using 4-M NaOH. The NaOH was added drop-wise to avoid the formation of hydroxo-carbonyl complexes. The salts were reduced to nZVI by the addition of 3.0 g of NaBH4. The nanoparticle products were separated by centrifugation, rinsed with absolute ethanol and then centrifuged again. This step was repeated three times. Thereafter, the nanoparticles were dried in a vacuum desiccator for 72 h and then stored in an argon filled (BOC, 99.998%) MBraun glovebox until required for use. Prior to conducting any nZVI-AMD experiments, the AMD that was collected from a disused open cast Cu-Pb-Zn mine in Wales (UK) was removed from the refrigerator and allowed to equilibrate at the ambient laboratory temperature of 20.0 ± 1°C for 24 h. All batch experiments comprised adding 200 mL of the AMD into 250-mL glass jars, and after the addition of nZVI, the batch experimental systems were sonicated for 120 s using an ultrasonic bath. Basically, batch experiments were conducted containing unbuffered (pH 2.67 at t = 0) and pH buffered (pH < 3.1) AMD which were exposed to nZVI of about 0.1-2.0 g/L. Each system was then sealed and placed on the benchtop. About 5 mL of aque- ous-nZVI suspensions were periodically taken out using an auto-pipette. The extracted suspensions were centrifuged at 4000 rpm for 240 s after which the supernatant became clear (i.e., all of the nZVI were centrifuged to the bottom of the vial). The solid nZVI at the base of each centrifuge vial was collected and analysed using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and high resolution transmission electron microscopy (HRTEM). The results of the study by Crane and Sapsford (2017) demonstrated that nZVI was selective for Cu, Cd, and A1 removal (> 99.9% removal of all metals within 1 h when nZVI > 1.0 g/L) from unbuffered AMD despite the co-existence of numerous other metals in the AMD such as Na, Ca, Mg, K, Mn, and Zn. Basically, the addition of nZVI concentrations of >1 g/L to the AMD leads to rapid and near total selective removal of Cu, Al, and Cd from the solution through a combination of mechanisms including cementation (for Cu), precipitation and sorption to corrosion products (for Al and Cd). The selectivity of nZVI for Cu can be further enhanced by the application of an acidic pH buffer due to the restriction of Zn, Cd sorption onto nZVI corrosion products along with the concurrent prevention of hydrolysis and precipitation of Al as Al(OH)3. For example, an acidic pH buffer enabled a similar high Cu removal, but maximum removal of only < 1.5% and < 0.5% Cd and Al, respectively. The HRTEM-EDS confirmed the formation of discrete spherical nanoparticles comprised of up to 68% weight Cu, with a relatively narrow size distribution (typically 20-100-nm diameter). On the other hand, the XPS confirmed such nanoparticles as containing Cu°, with the Cu removal mechanism, therefore, likely to have been via cementation with Fe°. Overall the results demonstrate that nZVI is effective for the selective formation of Cu°-bearing nanoparticles from acidic wastewater. Therefore, this technique has proven to be a highly useful mechanism for the valorisation of Cu-bearing AMD, thereby unlocking a new economic incentive for AMD treatment. It is also noted that the Cu°-bearing nanoparticles from acidic wastewater such as AMD have a wide range of applications including catalysis, optics, electronics, and as antifungal/antibacterial agents.

Wei and Viadero (2007b) studied the recovery of ferric iron from AMD via an oxidation selective precipitation process and used it as a feed stock to synthesise magnetite nanoparticles by co-precipitation of ferric and ferrous iron at the pH of 9.5 under an inert atmosphere. Raw AMD pumped from abandoned underground coal mines in North Central West Virginia was collected and sealed in high-density polyethylene bottles. At the laboratory, the solids and debris in the water samples were removed by settling and the remaining suspended solids were removed by filtration through a 0.45-gm membrane. Thereafter, the samples were stored at 4°C prior to metal recovery experiments. Iron recovery from AMD was achieved by an oxidation selective precipitation process as described by Wei et al. (2005). Initially, raw AMD was oxidised with hydrogen peroxide. Thereafter, the pH of the solution was raised to 3.5-4.0 with the addition of 4-N sodium hydroxide solution, where iron was precipitated as ferric hydroxide/oxyhydroxide which was separated from AMD by centrifugation. The ferric hydroxide/ oxyhydroxide solids recovered were resolubilised with sulphuric acid to achieve a clear solution, which was used as feed stock to synthesise magnetite nanoparticles. As discussed in a study by Wei et al. (2005), the supernatant from centrifugation was further neutralised with sodium hydroxide to pH 6.0-70 and an aluminium-rich precipitate was obtained after settling.

Wei and Viadero (2007b) state that the magnetite nanoparticles were synthesised through co-precipitation at room temperature, which required the presence of both ferric and ferrous iron at a ratio of 2:1. Resolubilised ferric iron from the iron recovery process was used as the ferric iron source for the generation of magnetite nanoparticles from AMD. A typical synthesis process was as follows: a solution of [Fe3+]:[Fe2+] = 2:1 of which the concentrations of Fe3+:Fe2+ ranged from 0.02 M:0.01 M, 0.04 M:0.02 M, to 0.08 M:0.04 M was prepared and mixed for 30 min under a N2(g) atmosphere to prevent oxidation by completely removing dissolved oxygen from solution as previously discussed by Kim et al. (2001). Thereafter, a solution of 6.4-M NH4OH was added to raise the pH to 9.5. The crystals of magnetite were allowed to grow for 30 min, under vigorous mixing and N2 bubbling. The black precipitate (magnetite) was then isolated from solution using an external superconducting high gradient magnetic separation system. The magnetite nanoparticles were then washed three times with deoxygenated deionised water until the pH was near neutral (-7.5). Lastly, the nanoparticles were vacuum-dried and later characterised. The synthesis test results from the study by Wei and Viadero (2007b) demonstrated that it was feasible to prepare magnetite nanoparticles via the co-precipitation method with the recovered ferric iron from AMD. Based on X-ray diffraction analysis, the iron oxide phase in the black precipitate was magnetite. Through scanning and transmission electron microscopic studies, it was demonstrated that most of the magnetite particles ranged from 10 to 15 nm and were spheroidal or cubic in shape. The results of this study have shown that the synthesis of magnetite nanoparticles from AMD with the iron recovered from AMD was technically feasible.

Silva et al. (2017) assessed the feasibility of iron recovery by selective precipitation and the synthesis of goethite particles for use as pigment. The AMD was collected from a drainage channel near a coal tailings deposit in the state of Santa Catarina (Brazil). Iron recovery was achieved by selective precipitation of 1 L of AMD at a pH of 3.6. In this study, the method used for iron precipitation was similar to the methods used by Wei et al. (2005) and Menezes et al. (2009). The pH of the AMD was increased and maintained at 3.6 ± 0.1, with the addition of a 4-M NaOH solution to precipitate the iron as ferric hydroxide/oxyhydroxide. The ferric hydroxide/oxyhydrox- ide precipitate was separated from the remaining solution by centrifugation at 3 000 rpm. After centrifugation, the ferric hydroxide/oxyhydroxide precipitate obtained at pH 3.6 was washed with distilled water at pH of

3.6 ± 0.1, re-suspended, and centrifuged, and the cycle was repeated three times. The final precipitate was dissolved in nitric acid so as to achieve a clear acidic iron solution which subsequently formed an iron-hexa-aqua-ion complex [Fe(H20)6] . The solution of an iron-hexa-aqua-ion complex was alkalised with potassium hydroxide to a pH of 12.0 so as to form ferrihy- drite. Thereafter, the mixture was diluted with water and heated to 70°C for 60 min for goethite crystallisation. The synthesised goethite particles were prepared in two different forms: (1) as a paste - the goethite particles were centrifuged and prepared as a water suspension containing about 50% solids, and (2) as a powder - the goethite particles were dried at 60°C. The solids were further analysed for particle size (laser diffraction, in aqueous solution with 1% sodium polyacrylate dispersing agent), specific surface area (BET), crystalline compounds (X-ray diffraction) and elemental chemical composition (atomic absorption spectroscopy). Thereafter, the coloured pastes were produced with commercial white Portland cement in a water/cement ratio of 1:2.5 to which goethite powder was added at a powder/cement mass ratio of 1:10. The results showed that the study was able to recover iron from AMD using a selective precipitation process at pH 3.6, and the iron was used to synthetise goethite particles. Furthermore, the results showed that goethite particles were successfully produced. The goethite particles varied from 0.04 to 5.0 pm when produced as paste suspension and 0.04 to 25.0 pm when dried at 60°C and converted to a solid powder. The pigment was also successfully used in a 10% pigment/cement paste mixture to colour a white cement paste giving it a yellow ochre colour.

The study by Kwon et al. (2016) focused on the formation of iron oxide from AMD and used it for the adsorption of heavy metals. The AMD used in the study was collected from the abandoned mine in Korea. The iron oxide recovery system from the AMD included the flow equalisation tank, neutralisation tank, and the precipitation tank. Iron oxide was collected at the precipitation tank where the solution pH was maintained at 3.4 to reduce the precipitation of impurities (e.g., Al. Zn, Cu, etc.). The iron oxides were classified based on precipitation time of 1 h and 24 h. The collected precipitates were dried at 40°C for 24 h after centrifugation and were later characterised. The sorption capacity of the iron oxides was determined in a batch reactor for heavy metals (e.g., As, Pb, and Cu) removal. The concentration of each metal was 1 mmol/L, and the adsorbent to liquid ratio was 10 g/L. The adsorbent and heavy metal ions were mixed on an orbital shaker at 200 rpm for 4 h at room temperature. After sorption equilibrium the mixture was filtered with 0.45-pm filter and analysed for the remaining metal concentrations. The results showed that the iron oxide was successfully synthesised from the AMD solution by controlling solution pH. The synthesised iron oxides were found to be good adsorbents for heavy metals such as As, Cu, and Pb. However, the sorption characteristics were highly dependent on the type of iron oxide and solution pH. For example, iron oxide sampled after 1 h had higher sorption capacity for Pb and Cu than iron oxide sampled after 24 h. However, iron oxide obtained at 24 h had higher sorption capacity for As than iron oxide sampled after 1 h. The results also showed that the two iron oxides had similar particle distribution patterns, but iron oxide samples obtained at 24 h were of relatively bigger size.

In a study by Kefeni et al. (2015), the possibility of synthesizing magnetic nanoparticles with and without heat from pure chemicals and real AMD by co-precipitation method was explored. Analytical grade chemicals of purity >98% of FeCl3-6H20, FeS047H20 and Co(N03)2-6H20 (Merck, Darmstadt, Germany) were used to synthesise Fe304 and CoFe204 magnetic nanoparticles by the co-precipitation method. For each experimental set-up for pure chemicals, the required amount of salts (Fe3+/Fe2+ or Fe3+/Co2+ at a mole ratio of 2:1) was weighed and dissolved by deionised water in 1.5-L volumetric flask and the pH of the solution was recorded. For each set, six replicates of 200 mL of the solution were measured separately and added to 250-mL beaker. For comparison purpose, pHs of paired samples were adjusted to 8.5 and

11.5 by using 5-N NaOH (aqueous); and in between 8.5 and 9 by using 25% NH4OH (aqueous). For each set of a pair of samples, one was heated at 60°C and the other was not, whilst both samples were stirred continuously for 2 h. Thereafter, the samples were filtered and the precipitates were washed using deionised water until the pH of the filtrate was about 7. The precipitates were then dried at 105°C in the oven for 6 h to remove water and other volatile substances adhered to synthesised magnetic nanoparticles. Amongst the dried samples, four samples were selected and a small portion of each sample was heated at 500°C in muffle furnace for 3 h to examine the effect of high temperature. Finally, the synthesised magnetic nanoparticles were stored at room temperature for investigation of their capacity to remove metals from AMD.

In the same study by Kefeni et al. (2015), samples of simulated AMD were prepared from FeClv6H,0, FeS04-7H-,0, CrCl3-6HUO, Al(N03)3-9Hi0, Co(N03)2-6H20, MnS04H20, ZnS04 -6H26, and Ni(N03)2-6H20 (Merck, Darmstadt, Germany). The salts were weighed to achieve a desired 2:1 mol ratio of trivalent to bivalent ions present in the spinal structure of ferrite. For the real AMD that was collected, the types and concentrations of metals present in the AMD were measured, in order to determine the amount of AMD that is needed to be oxidised and mixed with fresh AMD so as to attain the 2:1 ratio of trivalent:divalent metals in the ferrite. However, only Fe3+:Fe2+ ratio was considered due to low concentration of other trace metals (trivalent or divalent) in the real AMD studied. For the oxidation of Fe2+ to Fe3+ about 1 L of fresh AMD was taken and added into 2.5-L plastic container, and its pH was adjusted to between pH 5 and 6 using either NaOH (aqueous) or NH4OH (aqueous) and then aerated for 2 h using compressed air.

The results of the study by Kefeni et al. (2015) showed that it is possible to synthesise Fe304 and CoFe204 from their corresponding pure chemical binary salts. When the method was applied to simulated and real AMD, it showed that formation of well-crystalline magnetic nanoparticles at lower pH and temperature from both samples were hampered due to interference and combined effect of the metal cations. For example, these observations were reflected by the formation of Fe304, y-Fe203, Mn304, Mn02, and ZnO mixtures as major components from real AMD. However, increasing the pH and temperature increased crystalline size of synthesised magnetic nanoparticles. In other words, the results showed that the higher pH and temperature are favourable conditions for the formation of magnetic nanoparticles from AMD than the binary cations from the standard chemicals. Under all conditions, higher intensity and better resolution of XRD peaks of synthesised magnetic nanoparticle were obtained when NH4OH (aqueous) was used for neutralisation than NaOH (aqueous). Generally, this study demonstrated the possibility of synthesizing magnetic nanoparticles from real AMD by optimizing pH, temperature, and string time. The results also indicated that treating AMD in the presence of magnetic nanoparticle seeds accelerated the formation of ferrite and resulted in increased magnetic moment of ferrite sludge. In general, this study demonstrated the possibility of converting environmental pollutants into commercially valuable chemicals.

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