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Figure 1.2 Schematic of the different types of remediation techniques for ARD neutralization or prevention.

      Similarly, passive treatments, although being the least expensive approach, also make use of neutralizing agents. This technology is effective in mine waste with low acidic content and minimal water flow rate fluctuation. The major drawback of passive technology is that it requires a lengthier operative time for effective remediation, compromising its usage in modern mining activities [48]. Although investigations and pilot analysis have indicated that active and passive approaches can positively handle AMD [49], the continuous use of chemicals to neutralize AMD has raised some environmental concerns, making this procedure unsustainable and costly. In avoiding the use of excess chemicals, Tabelin and coworkers introduced two passivation techniques known as microencapsulation and galvanic microencapsulation techniques that could potentially prevent pyrite oxidation at the point source. The microencapsulation technique uses a redox-reactive organic carrier that is highly sensitive to pyrite in mine tailings to coat pyrite surfaces and thus prevent AMD formation. Meanwhile, galvanic encapsulation is based on galvanic interactions between pyrite and metals with lower rest potentials to suppress the oxidation of sulfide minerals [50]. However, these methods are not sufficient as they only suppress sulfide oxidation on a temporary basis, besides which sulfide oxidation is a natural process that is slow and difficult to prevent. A recent study has shown that active and passive biological remediation techniques offer great advantages, permanently removing sulphate and metals from mine drainage and having a high ability to recover the valuable metals [51]. However, the shortcomings of these methods is the high cost involved and the difficulty in setting up the processes, calling for a more sustainable and cost-effective technique for the remediation of AMD from mining MSWs.

1.3 Dendrimer as Extraction Agent of Rare Earth Element in AMD

Polymeric materials such as dendrimer nanoparticles have been proven to effectively recover metals from wastewater [52, 53]. Dendrimers are three-dimensional, nanosized polymeric material with great surface chemistry consisting of numerous end groups that can be manipulated or functionalized as modules for a thriving nanotechnology industry [54]. Intensive research studies have been conducted to evaluate the feasibility of retrieving REE from unconventional sources using different techniques (Table 1.4). However, these techniques also have their shortcomings involving cost and, therefore, developing an appropriate cost-effective method to extract them from AMD could make it a perfect source of REE. The use of dendrimers as an absorbent for the removal of metals in solution is a growing trend, and structurally, it has a tree-like branched nature consisting of a central core, an interior and exterior branched cell with the potential of forming more branches through a repetitive synthesis of monomers known as generation growth [55]. The higher generation growth of dendrimers is believed to carry more functional groups on their molecular surfaces. This allows them to interact with solid surfaces, acting as an adsorbent or ligands soluble in water to remove solvated toxic metals [56–58]. The properties of dendrimers are dependent mainly on these functional groups, such as amine, carboxyl, or hydroxyl groups, attached to dendrimers, which are used primarily for adsorption reactions of different targets [55, 59]. For instance, amine-terminated dendrimers, such as polyamido-amine dendrimers (PAMAMs), exhibit a high binding affinity for metal ions to their surface via coordination with the amine or acid functionality [60].

Table 1.4 Recovery of REE using different techniques.

      1.3.1 Poly(amidoamine) (PAMAM) Dendrimers

Poly(amido-amine) (PAMAM) was first reported in the 1980s and became the first dendrimer family to be produced and marketed [61]. It is a class of monodisperse, hyperbranched polymer with rich terminal amino functional groups that can be precisely controlled and functionalized with hydroxyl (OH), the carboxylic acid (COOH) or conjugated to hydrocarbon chains [62]. Full generational growth of PAMAM dendrimer can be synthesized with the aid of a microwave by first separately dissolving EDA and multi-ester in methanol, then allowing both solutions to cool down before gradually adding the multi-ester to EDA solution warmed at ambient temperature for several days [63]. PAMAM has gained a lot of research interest as the most widely studied dendrimer due to its unique characteristics such as good biocompatibilities, a high specific surface area, good chemical stabilities, high-capacity chelating agents for metal ions, and many tertiary and primary amine groups in its inner and surface structure [64, 65]. As such, research interest has increased over the years for using PAMAM dendrimers for the adsorptive removal of heavy metals from soil and aqueous solution [66] (Table 1.5). Adsorption is commonly employed due to its simplicity, low cost, and high efficiency.

      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