During the past few decades, there has been a continuous increase in the applications of Rare Earth Elements (REE) and their alloys, making them critical elements for development of modern industries ^[1]. The main target markets using REEs include magnets, metallurgy, catalysts, polishing powders, batteries, mobile phones and other high-technology gadgets. With their growing demand and continuous supply risk, urban mining of REEs from different end-of-life products and industrial waste has gained increasing attention ^[2]. As such, permanent magnets, which have various applications in the development of new technological devices as well as for green energy production, are one of the most promising secondary sources of REEs that can be recycled and reused ^[3]. Common Rare Earth-based magnets include Neodymium–iron–boron (FeNdB) and Samarium–cobalt (SmCo) magnets ^[4]. One of the key challenges in the recycling of magnet materials lies in the need to separate REE from Late Transition Metals (LTM), which form their constituents or major materials of their casings ^[5]. The most common technology for REE separation is acidic leaching with different leaching agents such as hydrochloric and sulfuric acids ^[6][7]. Iron is the major component in FeNdB and in the casing of magnets in electronics. It needs to be removed in the step previous to the separation of all other components, for both economic and technical reasons. The well-established approaches for separation of iron from leachates include either precipitation of iron hydroxide during controlled elevation of pH ^[8] or the precipitation of all other components apart from iron by addition of oxalic acid and organic base ^[9]. The precipitate can then be calcined and re-dissolved in acid. Subsequent separation of the components can be achieved by solvent extraction, ion exchange, or adsorption ^[10]. Most of the established industrial methods require, however, repeated steps to obtain the desired purity, thus generating large amounts of hazardous and, in the case of primary ore treatment, even radioactive waste ^[11][12]. As an alternative to the traditionally used methods, the application of solid-phase extraction (SPE) using nanosized functional adsorbents has been proven to be an effective and more environmentally friendly method for REE recovery ^[13]. Various types of nanosorbents have been synthesized for SPE recovery of REEs ^{[14][15][16][17][18][19][20][21]}. Several reviews have already been devoted to the discussion of the advantages and challenges in the application of functional solid adsorbents for REE separation ^[22][23]. Recent works have shown that organic–inorganic functionalized silica nanoparticles possess great adsorption capacity and selectivity towards many metal ions, including heavy metals and REEs ^[24]. Functional groups such as iminodiacetic acid (IDA), diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) and triethylenetetraminehexaacetic acid (TTHA) were grafted on dense SiO₂ nanoparticles as well as SiO₂ core–shell magnetic nanoparticles and tested for different REEs adsorption and separation. High adsorption capacities of up to 300 mg of RE³⁺/g were reached, and distinct selectivity trends towards different REEs depending on the complexonate ^[25][26][27].

The majority of earlier successfully applied ligands belonged to the classes of either complexons, i.e., amino carboxylic acids ^[28], or crown ethers/cyclenes. These types of ligands revealed strong affinity to both REE and LTM. In this paper, the new ligands (N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane L1 and N-(2-Aminoethyl)-3-AminoIsobutylmethyl Dimethoxysilane L4—a derivative of ethylene diamine, known for high affinity to Ni²⁺ and Cu²⁺ and, to a lesser extent, for Co^{2+ [29][30][31]}; 2-(2-Pyridilethyl) Thiopropyltrimethoxysilane L2—a derivative of pyridine with potentially good affinity to LTM and also a sulfur bridge ^[32][33]; and Triethoxy(3-isothiocyanatopropyl)silane L3 and Triethoxy(3-thiocyanatopropyl)silane L5—derived from isothiocyanate with potential affinity to Ni²⁺ and Co^2+ [34], were selected and grafted on SiO₂ nanoparticles to achieve selectivity for separating LTM from REEs in mixed solutions (Figure 1).