Compositions and methods for selective anion removal

Methods for removing a target anion entity, such as a phosphate ion, from fluids by treating the fluid with a substrate containing an immobilized rare earth, the substrate being either a first loaded substrate including a first immobilized cationic rare earth, the first loaded substrate being formed by precipitating a rare earth in a clay such that the rare earth is fixed inside a porous structure of the clay and/or fixed on the surface of the clay; or a second loaded substrate comprising a second immobilized cationic rare earth that is bonded to the second loaded substrate via a chelating ligand.