Ultra-fast adsorption of the industrial cationic dye pollutant using nitric acid-activated rice straw biochar: insights into adsorption mechanisms

In this study, a novel inexpensive and environmentally friendly, ultra-fast acid-activated rice straw biochar for the adsorption of the cationic industrial dye methylene blue has been synthesized. The physicochemical properties of the non-activated and acid-activated biochar were assessed with FESEM, surface area analyzer, XRD, FTIR, Raman, XPS, CHNS, and thermogravimetric analysis. The results showed a 4-fivefold increase in the adsorption capacity of the biochar upon activation with nitric acid. The acid-activated biochar exhibited a maximum adsorption capacity of 78.24 mg.g-1 at alkaline pH. The acid-activated and non-activated biochar reached their maximum adsorption capacity in less than 5 min. The acid-activated biochar showed functional groups enriched in -COOH, -C = O, -NH2, and -OH as compared to the non-activated biochar. The acid-activated biochar also had higher graphitization and an increase in surface area than the non-activated biochar. The Gibbs free energy and activation energy studies clearly revealed the adsorption of MB on biochar to be physiosorption in nature. The main adsorption mechanism of the ultra-fast removal of MB by the acid-activated biochar involves electrostatic interactions, hydrogen bonding, n-pi, and pi-pi interactions which may be due to the presence of the oxygen-containing functional groups and pi-electron-rich domains. Overall, the acid-activated biochar with higher adsorption capacity is a promising low-cost and ultra-fast bioadsorbent for the removal of cationic dyes from alkaline aqueous environments.