Disclosed is a composition, comprising: an insoluble rare earth oxyanion compound comprising a rare earth cation and an oxyanion having an element with an atomic number selected from the group consisting of 13, 22, 31, 32, 42, 44, 45, 49, 50, 52, 72, 75, 77, 78, 83, and 92; and a rare earth compound, wherein the insoluble rare earth oxyanion compound is derived from an aqueous phase reaction of the rare earth compound with the oxyanion.;Also disclosed is a process for removing oxyanions of an element having an atomic number selected from the group consisting of 5, 13, 14, 22 to 25, 31, 32, 40 to 42, 44, 45, 49 to 52, 72 to 75, 77, 78, 82, 83 and 92 from an aqueous feed containing one or more of said oxyanions, which process comprises contacting said aqueous feed with an aqueous solution of one or more soluble rare earth compounds to precipitate one or more of said oxyanions from said feed and thereby produce an aqueous fluid having a reduced concentration of said oxyanions as compared to said feed.
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