To enhance the capture of low-level indoor CO₂, a commercial zeolite (13X) was modified with alkali and alkaline earth metals using an ion-exchange method. Although the calcium-impregnated sorbent (zeo-Ca) showed the largest adsorption capacity, with a strong binding force for carbon dioxide, its regeneration by heat treatment was very difficult. Moisture in the gas flow caused significant decreases in CO₂ adsorption capability as well as in the lifetime of the adsorbents. As for the regeneration gas, the test showed that nitrogen would hinder the CO₂ adsorption more significantly than helium gas. Water vapour and nitrogen gas molecules are apt to competitively occupy the available sites of the adsorbent over the CO₂ molecules.

The CO₂ adsorption selectivity of plain activated carbon nanofibers (ANF) is generally low. For enhancement, nitrogen functionalities favorable for CO₂ adsorption are usually tethered to the ANF. In the current study, we adopted chemical impregnation using 0.5 wt% tetraethylenepentamine (TEPA) solution as an impregnant. To enhance the impregnation of TEPA further, preliminary oxidation of the nanofibers with 70% HNO₃ was conducted. The effects of HNO₃ and TEPA treatments on the modified ANFs were investigated for physical (using N₂ monosorb, thermogravimetric analyzer, scanning electron microscopy) and chemical (X-ray photoelectron spectrometer) changes. From the results, we found that although TEPA impregnation reduced the specific surface area and pore volume of the ANFs (from 673.7 and 15.61 to 278.8 m²/g and 0.284 cm³/g, respectively), whereas the HNO₃ pre-oxidation increased the number of carboxylic groups on the ANF. Upon TEPA loading, pyridinic nitrogen was tethered and further enhanced by pre-oxidation. The surface treatment cumulatively increased the amine content from 5.81% to 13.31%. Consequently, the final adsorption capacity for low (0.3%) and pure CO₂ levels were enhanced from 0.20 and 1.89 to 0.33 and 2.96 mmol/g, respectively. Hence, the two-step pre-oxidation and TEPA treatments were efficient for improved CO₂ affinity.