Mechanochemically synthesized Fe-Mn binary oxides for efficient As(III) removal: Insight into the origin of synergy action from mutual Fe and Mn doping.

Iron manganese oxide resources are widely derived from the geological structure, and their combinations play an important role in the migration and transformation of arsenic. Iron oxide and manganese oxide exist generally in a mixed state in Fe-Mn oxides synthesized via the well studied co-precipitation methods using potassium permanganate and manganese/iron sulfates. Herein, a newly designed Fe-Mn-O compositing oxide with Fe-MnO, Mn-FeO, (FeMn)OOH solid solution and FeOOH as the main components, simply through solvent-free mechanical ball milling pyrolusite (MnO) and ferrihydrite (FeOOH) together has been reported. Atomic-scale integrations by doping Fe and Mn with each other were detected and an adsorption-oxidation bifunctionality was achieved, where Fe-doped MnO served as oxidizer for As(III) and amorphous/ground FeOOH acted as adsorbent first for As(III) and then As(V) from the oxidization. The maximal adsorption for As(III) could reach 44.99 mg/g and over 82.5% of As(III) was converted to As(V). More importantly, high removal ability of arsenic worked in a wide pH range of 2-10.5%, and 87.2% of its initial adsorption-oxidation capacity could be kept even after 5-cycles reuse for treating 20 mg/L As(III) with a dosage at 1 g/L. Together with the enhanced adsorption capacity by the milled FeOOH, surface electron transfer efficiency of the developed Fe-MnO surrounded with Mn-FeO has been studied for the first time to understand the oxidization effect to As(V). Besides the environment-friendliness of ball milling method, the prepared sample is quite stable without noticeable metal release into solution. Mechanism studies of arsenic removal by the as-prepared Fe-Mn-O oxide provide a new direction for improving the oxidation efficiency of MnO to As(III) based on the widely available cheap Mn and Fe oxides, contributing to the development of advanced oxidization process in the treatment of waste water.