Response Surface Optimization of Chromium (IV) Removal with Teff Straw-Based Activated Carbon

Abstract

This study examined the use of activated carbon made from teff (Eragrostis tef) straw to adsorptively remove hexavalent chromium (Cr (VI)) from aqueous solution. Activated carbon (TSAC) from teff straw was synthesized using phosphoric acid (H₃PO₄) followed by calcination at high temperature. Then, it was characterized using FTIR, SEM, XRD, BET, and pHpzc analysis. The BET surface area of TSDAC was obtained as 823.63 m²/g. The adsorption parameters used in this study were an activation temperature of 400–600 °C, an impregnation ratio of 1:4–1:6, and an H₃PO₄ concentration of 1–3 M. The adsorption parameters were then optimized using the Response Surface Methodology (RSM) of Central Composite Design (CCD). Based on the CCD, the quadratic regression model was obtained with an R² of 0.9819. Furthermore, this study examined the relationship between the removal effectiveness and the initial concentration of Cr (VI), contact time, adsorbent dose, and solution of pH. As a result, the optimal parameters of the study were an H₃PO₄ concentration of 3 M, activation temperatures of 502.87 °C, and impregnation ratios of 6 g of H₃PO₄ per 1 g of TS, and the result was 94.21 % of chromium (VI) ion removal. This result was shown as an efficient adsorption of Chromium (VI) ion. Different models such as Langmuir, Freundlich, Redlich-Peterson, and isotherm models were shown to be appropriate with the adsorption data and suggested that both the monolayer and heterogeneous surface adsorption. The dynamics of the adsorption study were best represented by the pseudo-second-order kinetic model. The thermodynamic studies verified that the adsorption process is endothermic and spontaneous. Based on the findings in this study, the TSDAC adsorbent synthesis from a raw material of teff straw is promising.