Low-cost activated carbon (AC) samples obtained from waste coffee grounds were used for CO 2 and CH 4 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 m 2 g − 1) and high adsorption capacities of 2.6 mmol g − 1 and 1.1 mmol g − 1 at 25 ◦ C for CO 2 and CH 4, respectively. Adsorbent samples were prepared by loading of Fe 3+ metal ions onto ACs and their adsorption capacities were compared with those of nonloaded ACs. As expected, the loading of Fe 3+ metal ions increased the adsorption capacities at all temperatures and the adsorption capacity of Fe 3+ -loaded AC carbonized at 800 ◦ C was 3.1 mmol g − 1 for CO 2 and 1.2 mmol g − 1 for CH 4 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 CO 2 and CH 4, respectively. According to our findings, bio-based ACs can be used as an effective and alternative adsorbent for capturing different gas molecules.