Constant growth of industry demand for rare earth elements (REE) makes it urgent to develop more efficient approaches to their production not only from mineral ores, but also from secondary sources. Thus, promising alternative sources of REE are their aqueous solutions, such as, for example, wastewater from hydrometallurgy, leaching solutions of metallurgical slags, acidic effluents of mine drainage, acidic solutions formed during recovery of valuable elements from microelectronic boards and magnets. Membrane methods of separation of liquid media allow both concentration of the sum of REEs and, most importantly, isolation of individual REEs. This review focuses on a variety of membrane methods applicable to REE recovery. It includes a brief description of the principle of each type of membrane separation and summarizes information on the use of each specific method for solving REE extraction problems. In addition, the review considers some methods of modeling of REE membrane separation, which is important due to the presence of dozens of parameters on which the final efficiency of the developed membranes depends.

Complex [Cu(H₂L)₂(Н₂О)₂] has been obtained by the reaction of copper(II) with 2-hydroxy-5-ethylphenylphosphonic acid (H₃L), and its structure was studied by X-ray diffraction. Protonation constants of acid H₃L and stability constants of its complexes with Cu²⁺ in water have been determined by potentiometric titration. Complex [Cu(H₂L)₂(Н₂О)₂] has been found to show low toxicity and high analgesic activity.

The known methods of preparation of cyclen-containing phosphonic acids, promising components of radiopharmaceuticals, have been improved, and new approaches have been developed. Application of 31Р and

2-Hydroxy-5-methoxyphenylphosphonic acid (H ₃ L ¹ ) and the complex [Cu(H ₂ L ¹ ) ₂ (H ₂ O) ₂ ] were synthesized and characterized by IR spectroscopy, thermogravimetry, and X-ray diffraction analysis. The polyhedron of the copper atom is an axially elongated square bipyramid with oxygen atoms of phenolic and of monodeprotonated phosphonic groups at the base and oxygen atoms of water molecules at the vertices. The protonation constants of the H ₃ L ¹ acid and the stability constants of its Cu ²⁺ complexes in water were determined by potentiometric titration. The protonation constants of the acid in water are significantly influenced by the intramolecular hydrogen bond and the methoxy group. The H ₃ L ¹ acid forms complexes CuL ^‒ and CuL ₂ ^4‒ with Cu ²⁺ in water.