1.3.2 Principle REE Extraction Using PAMAM
Despite the extensive application of PAMAM dendrimers, no report has outlined their direct use for the recovery of REEs from AMD. This can be attributed to the low pH of the acidic effluent, which instead favors the protonation of the terminal amine functional groups, producing cationic charge on their surfaces to deactivate their adsorptive capacity to metal ions through electric repulsive forces. However, suppose the surfaces of PAMAM dendrimers are functionalized with carboxyl functional groups such as succinic acid. In that case, it will protonate in the acidic effluent to anionic groups, which will increase the binding ability of the polymer with metal ions in the acidic solution via electrostatic attraction. Considerable research interest has been focused on using magnetic nanoparticles (MNPs) as supporting cores for dendrimers to remove heavy metals from wastewater due to their intrinsic magnetic property, which allows for convenient separation through the deployment of an external magnetic field [67]. This feature improves the economic viability of the separation and re-use of dendrimers in a wide range of applications [55]. Therefore, magnetic PAMAM dendrimer nanoparticles will be synthesized, and their surfaces functionalized with succinic acid (PAMAMCOOH@MNPs) (Figure 1.3) and used as a perfect absorbent to recover REEs from AMD.
Table 1.5 Use of PAMAM dendrimer for the removal of metal ions.
| Contaminants | Mode of removal | Study outcome | Reference |
| Cu2+ and Pb2+ | A single and binary component system using modified carbon nanotubes (CNTs) with four generations of poly-amidoamine dendrimer (PAMAM, G4) for the removal of Cu2+ and Pb2+. | The study was very effective, achieving high adsorption capacities for copper and lead (3333 and 4870 mg/g respectively). | [68] |
| Mn(II) | silica-gel supported amino- and ester-terminated polyamidoamine dendrimers. The adsorption of the contaminant in solution is dependent on the terminal group. | The results revealed that amino-terminated polyamidoamine dendrimers could be potentially used as promising adsorbents for the effective removal of Mn(II) from an aqueous solution. | [69] |
| Cd(II), Pb(II) and Cu(II) | As-synthesized magnetic graphene oxide nanocomposite was grafted polyamidoamine dendrimer. | The adsorption capacities of metal ions were 435.85, 326.729 and 353.59 mg g-1 for Cd(II), Pb(II) and Cu(II), respectively, which proves excellent removal ability of the metal ion from water and wastewater. | [70] |
| Hg2+ | Carboxyl-terminated hyperbranched poly(amidoamine) dendrimers grafted superparamagnetic nanoparticles with the core-shell structure for selective removal of mercury from the aquatic sample. | As a result, the carboxyl-terminated hyperbranched poly(amidoamine) dendrimers grafted superparamagnetic nanoparticles displayed excellent properties and rebinding ability toward Hg2+ ions. | [71] |
| Ni2+, Fe2+, and Fe3+ | Metal ion remediation using polyamidoamine dendrimers as a chelating agent. | Metal ion removal rates from simulated wastewater were evaluated for these metal ions, and the complexation of Ni2+ to internal tertiary amine sites occurred more rapidly than that of Fe3+, which was more rapid than Fe2+. | [72] |
Figure 1.3 Schematic of magnetic PAMAM succinamic dendrimer nanoparticle.
1.4 Designed a Recovery System for REE from AMD
1.4.1 Process Overview
Acid mine drainage is a complex effluent characterized by low pH with multiple leached ions depending on their geological formation. The key to recovering these metal ions from waste is based on the fundamental understanding and characterization of the solution chemistry for any given mining site. The use of PAMAM-COOH@MNPs for REEs recovery is very critical as it will stop further production of sludge and adsorb multiple metal ions on their surfaces within the acidic medium. Figure 1.4 is a general representation of the procedure to recover REEs from AMD. However, this design will consider AMD with a high concentration while noting the presence of sulfate ions as the main anion load present in the solution. All the chemical species present in the AMD are mostly available in their oxidized state. The design consists of different stages called reactors, which will be described accordingly with supportive chemical equations to quantify each stage.
Figure 1.4 Systematic representation of the proposed processes for the recovery of REEs and water from ARD heavily laden with metals.
1.4.2 Components and Their Functions
1.4.2.1 Reactor 1 – Collection Tank
This is the initial stage of the reaction where the acidic effluent collected from mine drainage is being stored in a tank for filtration. The filtration process will remove all solid materials and coarse impurities such as algae and suspended rock particles.
1.4.2.2 Reactor 2 – Mixing Tank
Immediately after filtration, the acidic effluent will be pumped into the mixing tank for further treatment. At this stage, sulfate ions that are considered the main impurities will be removed to prevent them from reacting with metal ions in the effluent as such, lowering the grade of metals to be recovered. To circumvent the effects of sulphate ions, lime water (calcium hydroxides) from a different tank is pumped simultaneously with PAMAM-COOH@MNPs into the mixing tank. The pH is controlled using the lime water until it reaches the neutral point while stirring the reaction mixture at 400 rpm for 24 hours. The process of controlling the pH using calcium hydroxides is known as stage precipitation, which prevents the sulphate ions from complexing with the REEs within the acidic condition. The introduction of Ca2+ ions will react with the sulfate ligands as the rate of bonding interaction between these two ions is faster than the REE ions in solution due to the high reactivity of Ca2+ ions (Eq. 1.1). Eventually, the proportion of REE ions in the solution will subsequently increase, and its recovery will be enhanced [73]. The introduction of PAMAM-COOH@ MNPs at the initial stage under an acidic condition causes the protonation of the
