A reversible adsorbent for carbon dioxide, which can be used as a decontaminant of closed environments, has been devised. It consists of a novel material formed by a surface modified silica gel with active sites for the chemisorption of CO.sub.2 at room temperature combined with a clay which acts as an agglomerant in a preferred weight ratio of 80/20. The carbon dioxide is liberated under heating at 110.degree. C. The surface area of the material ranges between about 120 and about 240 m.sup.2 /g with a total pore volume between about 0.4 and about 0.8 cc/g where about 80% of the total pore volume contains pores of a diameter between about 60 .ANG. and about 180 .ANG.. The material is capable of adsorbing between about 7.5 and 11.7 cc of dry CO.sub.2 per gram and can be regenerated between about 70.degree. and about 150.degree. C. without losing any adsorptive capacity. Dry carbon dioxide is chemisorbed as a surface ammonium carbamate which becomes a surface bicarbonate in the presence of water.