Low-cost activated carbon (AC) samples obtained from waste coffee grounds were used for CO2 and CH4 adsorption. ACs were prepared by chemical activation and carbonized at three different temperatures. AC carbonized at 800 ◦ C showed a relatively high surface area (582.92 m2 g?1) and high adsorption capacities of 2.6 mmol g?1 and 1.1 mmol g?1 at 25 ◦ C for CO2 and CH4 , respectively. Adsorbent samples were prepared by loading of Fe3+ metal ions onto ACs and their adsorption capacities were compared with those of nonloaded ACs. As expected, the loading of Fe3+ metal ions increased the adsorption capacities at all temperatures and the adsorption capacity of Fe3+ -loaded AC carbonized at 800 ◦ C was 3.1 mmol g?1 for CO2 and 1.2 mmol g?1 for CH4 at 25 ◦ C. The isosteric heats of adsorption were calculated at 0–35 ◦ C with the range of 20–35 kJ mol?1 and 18–23 kJ mol?1 for CO2 and CH4 , respectively. According to our findings, bio-based ACs can be used as an effective and alternative adsorbent for capturing different gas molecules.