Mechanism of Synergistic Adsorption of Arsenic and Cadmium by Aluminium-substituted Ferrihydrites

Abstract

The synergistic process and mechanism of aluminum (Al)-substituted ferrihydrites on arsenic[As(Ⅴ)] and cadmium[Cd(Ⅱ)] were studied under laboratory conditions. The results showed that synergistic adsorption and coprecipitation of As and Cd by Al-substituted ferrihydrites was clearly affected by both the pH of solution and the order in which heavy metals were added. The solution in which As co-existed with Cd for 72 hours, at a pH of 6.0 to 6.5, the As and Cd adsorption capacity of Al-substituted ferrihydrites containing 20% Al (AF20) reached 60.9 mg·g-1 and 17.1 mg·g-1, respectively. The removal rates of As and Cd were 96.0% and 73.0%, respectively. Arsenic and Cd were synergistically adsorbed into the internal pores of AF20 particles, and the synergistic adsorption effect of AF20 on As and Cd was clear. Adding Cd to the solution containing As, for 72 hours, and with a pH of 6.1 to 6.5, the As and Cd adsorption capacity of AF20 was 58.1 mg·g-1 and 12.4 mg·g-1, respectively. The removal rates of As and Cd were 96.0% and 48.3%, respectively. Adsorption of As limited the fixation of Cd by AF20. When adding As to the solution containing Cd, for 72 hours, with a pH of 9.5 to 9.8, fixed amounts of As and Cd on AF20 were 20.9 mg·g-1 and 24.4 mg·g-1, respectively. The removal rates of As and Cd were 38.8% and 98.9%, respectively. The coprecipitation of As and Cd by AF20 was clear. The resulting insoluble As and Cd compounds distributed the Cd distribution in a sparse strip and impeded the further adsorption of As. The results show that Al-substituted ferrihydrites can synergistically adsorb and coprecipitate As and Cd in contaminated environmental media.