Rare earth elements (REEs) are critical materials that are widely used in various industries and innovative technologies including clean energy. The US relies heavily on international supplies of REEs and may be at high risk of supply disruption due to potential geopolitical conflicts. Thus, it is critical for the US to seek new alternative and sustainable technologies for domestic REE production. Electronic and industrial wastes contain reasonably high REE content and may offer unique and new opportunities for REE recovery. Our research aims to design and investigate molecular mesoporous materials for effective and selective REE extraction/separation from mild acidic extract of electronic and industrial wastes. Here we reported phosphonate-functionalized magnetic mesoporous silica (MMS) for REE separation from citric acid simulant and real municipal solid waste incineration (MSWI) ash extracts.
MMS material was synthesized using cetyltrimethylammonium bromide template in the presence of magnetite nanoparticles. The MMS material was functionalized with phosphonate ligand (MMS-PP) using a post-synthesis method. Both MMS and MMS-PP materials were characterized by various analytical methods. The MMS materials were evaluated for four highest REEs in MSWIA, lanthanum (La), cerium (Ce), neodymium (Nd), yttrium (Y) recovery, from citrate extraction simulants. In 50 mM citric acid (pH ~2.1), the La recovery rates of MMS decreased to nearly zero, but the La recovery rate of MMS-PP remained at 91%. With different La loadings in 50 mM citric simulant, the La adsorption capacity was 13.3 mg/g, compared to ~3 mg/g for MMS, with >60% recovery rate at the La loading of ~200 ppm. The MMS-PP material exhibited excellent selectivity for La separation against competing Na⁺ and Ca²⁺, but its selectivity against Al³⁺ and Fe³⁺ was moderate. In addition, the adsorption reaction of La onto MMS-PP was fast, completing within 10 minutes, and the MMS-PP materials remained effective at La separation after four adsorption-desorption cycles. For the real MSWI ash extraction solution (50 mM sodium citrate, 0.01 M HCl, pH ~2.0), the MMS-PP exhibited comparative extraction efficiency of ~10 for Y (recovery rate 55.6% and ~6 for Dy (30.4%) relative to those for Ce (3.8%), La (7.0%), Nd (5.1%) and Pr (4.1%). In addition, a polyethyleneimine (PEI) functionalized MMS material shows 40-50% recovery rates for all these REEs from the above real ash extraction solution.

Lei Hu

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