Arsenic detoxification by iron-manganese nodules under electrochemically controlled redox: Mechanism and application

Iron-manganese binary oxides are characterized by high oxidation and adsorption capability and widely applied to arsenic (As) detoxification in contaminated waters. Despite of their lower preparation cost relative to synthesized iron-manganese binary oxides, the low adsorption capacity of natural iron-manganese oxides largely hinders their application. Here, electrochemically controlled redox was employed to improve the As(III,V) removal performance of iron-manganese nodules in a symmetric electrode system, and the removal mechanism and electrode reusability were also examined. Experimental results showed that both the electrochemical reduction and oxidation of birnessite in iron-manganese nodules contributed much to As(III,V) removal. Higher cell voltage facilitated a higher removal efficiency of total As within 0-1.2 V, which reached 94.7% at 1.2 V for actual As-containing wastewater (4068 μg L^-1). The efficiency was obviously higher than that at open circuit (81.4%). Under electrode polarity reversal, the alternating reduction dissolution and oxidation recrystallization of birnessite in iron-manganese nodules promoted their contact with As, enhancing the total As removal efficiency from 75.6% to 91.8% after five times of repeated adsorption. This research clarifies the effect of electrochemical redox on As(III,V) detoxification by iron-manganese oxides, and expands the application of natural iron-manganese nodules in the treatment of As-contaminated wastewaters.