Ferrocyanide removal from solution by aluminum-based drinking water treatment residue

This study proposes the use of an aluminum-based drinking water treatment residue (DWTR) to adsorb ferrocyanide. The batch tests and chemical characterization results showed that ferrocyanide adsorption increased as the pH, ion strength, and the solid and solution ratio decreased, and as the initial ferrocyanide concentration increased. The pseudo-first (R² = 0.906) and pseudo-second-order (R² = 0.966) kinetic models well described the adsorption kinetics, and the adsorption isotherm was also well fittted by Langmuir (R² = 0.989) and Freundlich (R² = 0.989) models. The calculated initial ferrocyanide adsorption rate by the pseudo-second-order kinetic model was 0.0190 mg-CN g^-1 h^-1, and the estimated maximum adsorption capacity determined by the Langmuir model was 20.9 mg-CN g^-1. The main structure and elemental distributions showed nearly no change in DWTR after adsorption. Adsorption involved electrostatic interactions and ligand exchanges with Al in DWTR, as evidenced by the 1.40 eV increase in the Al binding energy after adsorption. Furthermore, ferrocyanide adsorption had a dual effect on the DWTR porosity (including both increase and decrease effect), resulting in a slight increase in the specific surface area and total pore volume of DWTR after adsorption. This dual effect was likely related to Fe present in ferrocyanide, which introduced new vacant sites on DWTR. Overall, recycled DWTR is a promising potential adsorbent for ferrocyanide.