A Novel Conjugate Mechanism for Enhancing the Adsorption Capacity of Amine-Functionalized Activated Rice Husk Ash for Simultaneous Removal of Organics and Anions in Wastewater: Experimental and Theoretical Explanations

Abstract

In this study, amine-functionalized rice husk ash (TRI-ARHA) was prepared and successfully applied as a multipurpose adsorbent for simultaneous removal of organics (i.e., methyl orange, MO), nitrate, and phosphate in wastewater with adsorption capacities of around 16.6 mgMO/g, 32.9 mgNO3-N/g, and 13.4 mgPO43-P/g. These capacities were superior to those of the other commercially available materials such as activated carbon and ion-exchange resins in both individual and multipollutant adsorption experiments. In addition, the simultaneous adsorption of three components gives a higher adsorption capacity than individual adsorption for each anion (e.g., 1.18 times for nitrate and 1.11 times for phosphate). A conjugated adsorption mechanism may occur on the surface of the TRI-ARHA material, which can be ascribed to the amine groups in the MO molecule that are further activated by H+ (released from the surface amine groups) or Na+ ions (present in the solution), to form new adsorption centers for nitrate and phosphate. The integration process of H+ and Na+ to the nitrogen positions of MO was also studied by computational chemistry with the basis set of B3LYP/6-32G∗ calculated by Gaussian 16 software. The application in the treatment of real wastewaters proved that the TRI-ARHA material was more advantageous for multipollutant removal than other materials, thanks to its conjugated adsorption mechanism.