Electrochemical adsorption of cadmium and arsenic by natural Fe-Mn nodules.

Fe-Mn nodules are widely distributed and regarded as excellent adsorbents for heavy metals. Their adsorption-desorption reactions with heavy metal ions are usually accompanied by redox processes. Herein, Fe-Mn nodules were used as adsorbents for Cd(II) and As(III,V) at a constant cell voltage under electrochemically controlled reduction and oxidation, respectively. The results showed that the adsorption performance for Cd(II) and As(III,V) was enhanced respectively due to the decrease and increase of Mn average oxidation state (Mn AOS) in Fe-Mn nodules. High birnessite content and Mn average oxidation state (Mn AOS) improved the adsorption of Cd(II) and As(III,V). The adsorption capacity for Cd(II) and total As increased with increasing voltage. With increasing pH, the adsorption capacity for Cd(II) increased first and then reached equilibrium, and that of total As decreased and then increased. The Cd(II) electrochemical adsorption capacity (129.9 mg g) and the removal efficiency for total As at 1.2 V (83.6 %) in As-containing wastewater at an initial concentration of 4.068 mg L were remarkably higher than the corresponding inorganic adsorption performance (9.46 mg g and 70.5 %, respectively). This work may further promote the application of natural Fe-Mn nodules in the adsorption of heavy metals from wastewaters.