Adsorption behaviour of Fe(II) and Cr(VI) on activated carbon: Surface chemistry, isotherm, kinetic and thermodynamic studies

- Activated carbon was prepared from carbonized carbon of Combretum quadrangulare Kurz as a new feedstock.
- Activated carbon prepared by physical activation was presented the negative surface charge resulting in high Fe2+ adsorption ability.
- Fe-modified activated carbon exhibited good efficiency on Cr2O72− adsorption.
- Adsorptions of Fe2+ and Cr2O72− using prepared adsorbents were fitted for Langmuir isotherm and pseudo-second-order kinetic models.
- Adsorption processes of Fe2+ and Cr2O72− were spontaneous and endothermic nature.

Studied in this research is the preparation of activated carbon (AC) from the carbonized carbon (CB) of Combretum quadrangulare Kurz. Pyrolysis was used to activate the CB under (400 and 500) °C for (1-5) h. Chemical activation by 45%w/w phosphoric acid (H3PO4) was also used as an activating agent with a ratio of acid to carbon of 2.3 by weight under the same temperature and time as that of pyrolysis process. It was found that the AC prepared at 500 °C for 1 h (501) showed the best adsorption capacity. Batch absorption experiments were performed as a function of absorbent dose and pH for removal of 100 mg L−1 of Fe2+ followed by 4 mg L−1 of Cr2O72−. The 501 and commercial activated carbon (ACC) could remove (21.33 and 5.42) mg L−1 of Fe2+, respectively, (0.45 and 1.32) mg L−1 of Cr2O72−, respectively. The ACC was also found to be favoured for adsorption of non-polar I2 molecules. Physicochemical properties of prepared adsorbents such as surface area, total functional groups and pHpzc were also studied. Moreover, adsorption isotherm was found to be well fitted with Langmuir model. The adsorption kinetic followed the pseudo-second-order kinetic model. Thermodynamic study indicated that the adsorption of Fe2+ and Cr2O72− onto absorbent surface was endothermic. Desorption characteristic of Fe2+ and Cr2O72− on activated carbon was applied by HNO3. This research was proven that the activated carbon could remove Fe2+ followed by Cr2O72− effectively in water.