Calcium-based adsorbents produced by biomass material is technically feasible, costly effective and advantageous to a certain extent in capturing CO2. Since the waste of a cockle shell is found to contain around 98% calcium, it can be a good source for CaCO3 and yield CaO through the high temperature calcination process. The sorption equilibria of CO2 on synthesized CaO with different amounts of water were measured experimentally by using a volumetric method at a temperature of 2 degrees C. Under dry conditions, synthesized CaO has a low CO2 uptake, as the adsorption rate of CO2 in calcium-based sorbent is high at higher temperatures. While under wet conditions, the large increment of CO2 adsorption was observed due to formation of CO2 hydrates which were indicated by inflection of isotherms at a definite pressure of the S-shape. At the inflection point, the highest sorption capacity (5.07 mmol/g) was obtained at a water ratio of 0.55, which is about 30 times higher than dry samples. The trend of FTIR spectra were observed to be similar for raw cockle shells and wet CaO after CO2 adsorption, in contrast with spectra of CaO before adsorption. The formation of CO2 hydrate can be further verified by the changes of surface morphology of CaO, before and after adsorption. It can be concluded that calcium-based sorbent with the presence of water has the ability to be further utilized for CO2 separation.