Increasing demand for fossil fuels is associated with massive atmospheric CO2 levels. Considering that numerous studies have been published with CO2 capturing techniques, utilizing techniques are yet in early stage with financial or technical issues. As a part of chemical conversion in CO2 utilization, this paper investigated the performance of a CO2 and H2O mixture (CHM) onto activated carbon fibers (ACF) for surface modification. CHM-treated ACF samples were prepared at a pressure of 20 bar with 100 °C of water vapor and 750 μL of CO2 for 1 h through the gas-phase, and labeled as C-ACF850. For the control sample, N-ACF850 was also prepared by the impregnation of nitric acid. Physiochemical analyses revealed that the overall characteristics of C-ACF850 lay between ACF850 and N-ACF850. C-ACF850 experienced minimized surface area decrement (21.92% better than N-ACF850), but increased surface functional groups (50.47% better than ACF850). C-ACF850 also showed preferable adsorption efficiency on selected metals, in which case both physical and chemical properties of adsorbent affect the overall adsorption efficiency. In this regard, a novel applicability of CHM may present an appealing alternative to traditionally used strong acids.