Apparatus, process and article for treating a gas containing one or more of a chemical and/or biological contaminant. The process includes contacting the gas with an aggregate composition comprising an insoluble rare earth-containing compound to form a gas depleted of chemical and active biological contaminants. The insoluble rare earth-containing compound can include one or more of cerium, lanthanum, or praseodymium. The composition comprises no more than two elements selected from the group consisting of yttrium, scandium, and europium when the aggregate has been sintered. A suitable insoluble cerium-containing compound can be derived from cerium carbonate. In one embodiment, the aggregate composition consists essentially of one or more cerium oxides, and optionally, a binder. Although intended for a variety of fluid treatment applications, such applications specifically include the treatment of breathing gases such as air that may contain chemical and/or biological contaminants.