*Review* **Magnetic Nanohydrometallurgy Applied to Lanthanide Separation**

#### **Fernando M. de Melo, Sabrina N. Almeida and Henrique E. Toma \***

Instituto de Química, Universidade de São Paulo, São Paulo 05508-000, Brazil; fernando.menegatti.melo@usp.br (F.M.d.M.); sabrina\_nobrega@hotmail.com (S.N.A.) **\***

 Correspondence: henetoma@iq.usp.br

Received: 24 April 2020; Accepted: 8 June 2020; Published: 11 June 2020

**Abstract:** Lanthanides play an important role in modern technology because of their outstanding optical, electronic, and magnetic properties. Their current hydrometallurgical processing involves lixiviation, leading to concentrates of elements whose separation requires exhaustive procedures because of their similar chemical properties. In this sense, a new nanotechnological approach is here discussed, involving the use of iron oxide nanoparticles functionalized with complexing agents, such as diethylenetriaminepentaacetic acid (DTPA), for carrying out the magnetic extraction and separation of the lanthanide ions in aqueous solution. This strategy, also known as magnetic nanohydrometallurgy (MNHM), was first introduced in 2011 for dealing with transition metal recovery in the laboratory, and has been recently extended to the lanthanide series. This technology is based on lanthanide complexation and depends on the chemical equilibrium involved. It has been better described in terms of Langmuir isotherms, considering a uniform distribution of the metal ions over the nanoparticles surface, as evidenced by high angle annular dark field microscopy. The observed affinity parameters correlate with the lanthanide ion contraction series, and the process dynamics have been studied by monitoring the nanoparticles migration under an applied magnetic field (magnetophoresis). The elements can be reversibly captured and released from the magnetically confined nanoparticles, allowing their separation by a simple acid-base treatment. It can operate in a circular scheme, facilitated by the easy magnetic recovery of the extracting agents, without using organic solvents and ionic exchange columns. MNHM has been successfully tested for the separation of the lanthanide elements from monazite mineral, and seems a promising green nanotechnology, particularly suitable for urban mining.

**Keywords:** lanthanide separation; magnetic nanohydrometallurgy; complexing nanoparticles; magnetic nanoparticles; magnetophoresis; monazite processing; urban mining
