Potassium hydroxide and ball-mill modified rice husk biochar for the elimination of Zn2+ and Hg2+ from aqueous solution

Abstract

As a result of rampant industrial and agricultural activities worldwide, heavy metal pollution of aquatic bodies has become a common occurrence, resulting in significant damage to the ecological environment and human health. There is an urgent need to develop sustainable and efficient technologies to remove metals commonly found in aquatic bodies. Rice husk is a highly productive agricultural byproduct, and improper biomass disposal can cause environmental issues such as air pollution. Therefore, this study investigated the efficacy of rice husk biochar and its modified derivatives in eliminating Zn2+ and Hg2+ from aqueous solutions. The rice husk underwent pyrolysis at a temperature of 500°C and was subsequently modified through ball milling. Subsequently, the pristine biochar and ball-milled biochar were immersed in a potassium hydroxide solution for further modification. The adsorption performance of biochar on metal ions was studied using diverse pH, dosage, kinetics, and isotherm experiments. Among all the adsorbents, potassium hydroxide modified ball-milled biochar exhibited the maximum adsorption capacity for Zn2+ (24.96 mg/g) and Hg2+ (330.38 mg/g). The adsorption capacity increased by 182% and 23%, respectively, compared to the pristine biochar. Langmuir model described the adsorption of Zn2+ (R2 = 0.95 - 0.99) well, whereas the Freundlich model was found to be most suitable for the adsorption of Hg2+ (R2 = 0.96 - 0.99). All the kinetics experimental datasets were accurately described by the pseudo second order model (R2 = 0.99). The main mechanisms for the Zn2+ and Hg2+ adsorption include pore-filling, electrostatic interaction, ion exchange, and complexation. It was concluded that potassium hydroxide modified ballmilled biochar could effectively remove Zn2+ and Hg2+ from contaminated water. Future studies could determine the most effective modification techniques of rice husk biochar to co-adsorb multiple metal ions to explore the competing adsorption mechanisms.