group on the surfaces of dendrimer producing anions in solution, and since lanthanides are hard Lewis acids and prefer binding to hard donor atoms, such as oxygen [74], will eventually bind with the REE ions on the surfaces of PAMAM-COOH@MNPs via electrostatic force of attraction (Eq. 1.2).
1.4.2.3 Reactor 3 – Separation Tank
From the mixing tank, the solution is then pumped into reactor 3, which is designed with an external magnetic field. This is where the separation process will take place with the deployment of an external magnetic field. Since the dendrimer has the potential to respond to an external magnetic field, it will be separated from the rest of the solution and drained into tank 4 for the final recovery step. The residual effluent will be pumped into tank 5 for further treatment before it can be decanted into the environment or used for consumption and agricultural purposes. It should be noted that the residual effluent is neutral and cannot pose any further harm to the environment. However, it should be treated to ensure that it is safe for consumption.
1.4.2.4 Reactor 4 – Recovery of REEs Metals
Since the surfaces of the dendrimer nanoparticles have a high density of active sites, it will enhance the absorptivity of REEs in the acidic effluent, and at reactor 4, only these materials will be present. However, the REEs can be recovered from the dendrimer nanoparticles by slight acidification to initiate the protonation of these metal ions in the solution (Eq. 1.3). This property makes PAMAM dendrimers promising recyclable chelating agents for the metal ion separation [63]. The individual lanthanides can be extracted using an ion-exchange method where the ionic solution of the lanthanides is flush through a column containing resins and each ion is bonded to the resin with various strengths based on their ion size. After recovery of the lanthanides, the pH of the solution is then adjusted, and the formation of PAMAM-COOH@MNPs is achieved and can be reused.
(1.3)
1.5 Challenges and Opportunities for the Future of Metal Mining
Mining activities provide the many raw materials needed for modern technology, and our future is deeply dependent on them despite their serious environmental and social impact. The use of REEs has skyrocketed, and the future demand is uncertain, which makes it difficult to control and manage their waste as the rush for these minerals hasten mining operators to dig the ore, sell the metals, and, once the deposit is exhausted, walk away and start another mine elsewhere without prior planning to clean up their waste [75]. This attitude has created a lot of unattended landfills around the world for decades now, polluting the environment. Therefore, current mining practices need to change. Moreover, all stakeholders involved need to engage more responsibly by implementing cleaner technologies to minimize their environmental threat. However, mining companies lack these technological innovations to improve the sustainability of their mining operations and require more research endeavours to assist them. However, research has proven that the voluminous tailings dams or landfills after mining activities have become a viable source for some of these precious metals, which can be more economical to recover or recycle than mine ore deposits. The formation of AMD is a viable source for REEs, and despite its environmental consequences, it has presented another option for obtaining these scarce commodities to meet their rising demand. Therefore, the technological ideas to recover REEs from AMD presented in this chapter have opened up future opportunities for mining operators to easily incorporate these cost-effective methodologies in their extraction design to clean up their waste and extract valuable metals to boost their yield and generate more income. The most important step for this technological idea is to engineer an alkaline treatment technology to optimize a high concentration of REEs in the acidic waste before using magnetic dendrimer nanoparticles to aid their recovery. The many functional groups on the surfaces of dendrimers with binding affinity to multiple metal ions are the most intriguing aspect of this technological innovation. However, the difficulty inherent with this procedure will be separating the individual REEs from each other to yield pure single elements, and this will require optimization of factors such as pH to control their recovery.
1.6 Conclusion
The formation of AMD from sulfidic ores during mining activities is an inevitable process that can be enhanced as a secondary source for REEs using an effective alkaline treatment technology engineered to optimize a high concentration of REEs in the waste. Despite the occurrence of metals in rocks and soil as a natural constituent of the Earth’s crust, the leaching of AMD products into the environment will enhance their presence at an elevated concentration significantly higher than the acceptable levels prescribed by the World Health Organization (WHO). Therefore, it is imperative to treat AMD to a useable standard as defined in the water quality guidelines. On the contrary, AMD is a valuable source of REEs which can be recovered to compensate for their shortage in supply needed for advancement of technological developments. Although the recovery of rare earth metals from AMD remains a great challenge, the use of nano-absorbent such as dendrimer has enhanced their recovery, making AMD a future viable source for these precious metals. Dendrimers are an exceptional cost-effective material with surfaces that are easily designed to suit any intended application. The use of magnetic PAMAM dendrimer nanoparticles surface-functionalized with succinic acid presented in this chapter is a pragmatic approach with dual functionality. The succinic functional group attached on the surfaces of the dendrimer will protonate in the acidic effluent to anionic groups, which will increase the binding ability of the polymer with REEs in solution via electrostatic attraction, and its magnetic property will be used to separate the bound metal ions from the solution. This integrated approach will recover REEs from the acidic effluent using a low-cost material that itself is environmentally friendly to generate income for mining operators, thus stopping the pollution cycle.
Acknowledgment
A. J. L. thanks Prof. Elvis Fosso-Kankeu and Prof. Martin Mkandawire for their excellent mentorship and continued support. The authors also acknowledge the support from Prof. Bhekie B. Mamba, the University of South Africa (UNISA), North-West University and Cape Breton University.
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