Disclosed herein are embodiments of a polymer-functionalized particle for using in isolating and extracting solutes, such as rare earth metals, lithium, and the like. The polymer-functionalized particles exhibit strong resistance to agglomeration and degradation even in high ionic strength and/or temperature environments. A post-particle synthesis method for making the polymer-functionalized particle is disclosed, along with a magnetic separation device and that can be used in system embodiments to facilitate use and regeneration of the polymer-functionalized particles in solute extraction.