An aggregate composition and process for making the aggregate composition. The aggregate composition includes an insoluble rare earth-containing compound and a polymer binder. The insoluble rare earth-containing compound can include one or more of cerium, lanthanum, or praseodymium. A suitable insoluble cerium-containing compound can be derived from cerium carbonate or a cerium salt. In a specific embodiment, the aggregate composition consists essentially of one or more cerium oxides, the polymer binder and optionally a flow aid. A process for making the composition includes mixing the insoluble rare earth-containing compound with a polymer binder to form a mixture, and subjecting the mixture to mechanical, chemical and/or thermal treatment to adhere the rare earth compound to the polymer binder. The aggregate composition can be used in a variety of fluid treatment applications to remove one or more chemical and biological contaminants in a fluid.