Comparative study of sorption kinetics and equilibrium of chromium (VI) on charcoals prepared from different low-cost materials

In the present study three types of agricultural waste materials, peanut shell, lapsi seeds and energy grass were carbonized by the same carbonization methods using mineral acids at elevated temperature. The resulted charcoals having approximately 60% carbon content were applied as low-cost adsorbents for the removal of Cr(VI) from aqueous solutions. The sorption kinetics at pH = 1 and the sorption isotherms at different pHs were studied. Chromium concentration was measured by flame atomic absorption spectrometry (F-AAS) in the residual solution by an indirect way and was additionally checked by direct measurement using an XRF spectrometer. The adsorption isotherms were modeled by Langmuir and Redlich–Peterson equation. In order to achieve the maximal adsorption pH = 1 was found to be optimal at every investigated charcoals. The maximal adsorption capacity varied between 205 and 230; 85 and 138; 34 and 40; and 30 and 33 mg g− 1 at pH = 1; 2; 3; and 4 respectively at the different charcoals. The obtained pseudo-second-order rate constant was k2 = 4.0 × 10− 4 g mg− 1 min− 1 at each type of activated carbons. The bond of Cr(VI) to the functional groups of charcoal seems to be irreversible in the case of dried Cr(VI) saturated charcoals. Both the model studies and the dissolution experiments provided evidence for this result. It was supposed on the basis of our results, that Cr(VI) bonds chemically to the activated carbon consequently the Cr(VI) saturated charcoals had no more contaminating effect in the environment.

► A new chemical treatment method was developed to prepare high-capacity charcoal from agricultural wastes.
► Low-cost adsorbents were applied for removing Cr(VI) from aqueous solution.
► Maximum chromate sorption capacity was found to be exceeding 200 mg g− 1.
► The sorption was modeled by pseudo-second order kinetics and by Langmuir isotherm.
► FTIR_ATR result revealed that carboxylic and phenolic groups are responsible for chromate adsorption.