Adsorptive removal of Cr (VI) by Fe-modified activated carbon prepared from Trapa natans husk

Fe-modified activated carbon (THAC-Fe) was developed from a low-cost aquatic plant residue, Trapa natans husk, and tested for its ability to remove Cr (VI) from aqueous solutions. The surface characteristics of the carbons before and after modification were measured. The results showed that impregnation with Fe increased the carbon surface area, introduced more acidic functional groups, and improved the adsorption ability by nearly three times as compared to the original activated carbon. The effects of solution pH, adsorbent dose, contact time and initial Cr (VI) concentration on the adsorption of Cr (VI) by THAC-Fe were investigated. The adsorption capacity decreased sharply with the increase of solution pH. The kinetics data followed the pseudo-second-order model and the rate of chromium (VI) uptake was found to be controlled by external mass transfer and intra-particle diffusion throughout the entire adsorption period. Boyd plot confirmed that film diffusion was the rate-limiting step in the adsorption process. Equilibrium data fit well to the Tempkin and Freundlich models. The maximum adsorption capacity was 11.83 mg/g at room temperature as calculated by Langmuir equation. According to the calculated thermodynamic parameters, the adsorption was a spontaneous endothermic process. Since Cr (VI) cannot be easily desorbed using distilled water or NaOH solutions, chemisorption is the likely mode of adsorption. According to these results, THAC-Fe is a promising adsorbent for the removal of Cr (VI) from wastewater.