A treatment method for coal fly ash, and in particular sodic fly ash, comprises 1) contacting the coal fly ash with anhydrite, and 2) contacting the coal fly ash in the presence of water with at least one additive. The material obtained from the contacting steps (1) and (2) may be dried. The steps (1) and (2) may be carried simultaneously or sequentially. The additive may comprise at least one component selected from the group consisting of strontium-containing compounds, barium-containing compounds, dolomite, a dolomite derivative such as calcined or hydrated dolomite, water-soluble sources of silicate such as sodium or potassium silicate, iron-containing compounds, and any combinations thereof. A particularly preferred additive comprises sodium silicate. The method may be effective in reducing the sodium content in the fly ash (Na.sub.2O), reducing the alkalinity of the fly ash, and/or stabilizing at least one heavy metal such as selenium and/or arsenic to reduce their leachability.

Provided is a carbon dioxide adsorbent with which large quantities of carbon dioxide can be adsorbed and removed even under conditions having low carbon dioxide concentrations such as when under subatmospheric pressure or when under an environment having a carbon dioxide partial pressure of less than atmospheric pressure, said carbon dioxide adsorbent exhibiting excellent adsorption activity. A carbon dioxide adsorbent including at least a ZSM-5 zeolite including barium (Ba) or strontium (Sr) is characterized in that the ZSM-5 zeolite includes M-O-M bonds (M being Ba or Sr, and O being oxygen). The M-O-M bonds interact strongly with carbon dioxide, and thus carbon dioxide can be adsorbed effectively and in large volumes even under conditions having low carbon dioxide concentrations.