As3+ removal by Ca-Mn-Fe3O4 with and without H2O2: Effects of calcium oxide in Ca-Mn-Fe3O4

Abstract

As3+ removal by Ca-Mn-Fe3O4 composites, which contained various wt% of Ca, are investigated. Immobilization of Ca (i.e. as crystalline forms including CaO2) and Mn (i.e. as an amorphous hydrous manganese oxide) on Fe3O4 were identified, and it was revealed that the co-immobilization of Ca and Mn (i.e. especially the wt% ratio of Ca:Mn:Fe = 2:3:1) provided higher Ca wt% with greater surface area. The increasing Ca wt% (i.e. 6, 14, 17, and 19%) gradually increased the reactivity of H2O2 to oxidize As3+ to Ass. Moreover, it is suggested that superoxide anion produced from the catalytic decomposition of H2O2 reduces Mn4+ to Mn2+, which is further released into solution. On the other hand, As3+ adsorption was decreased with the highest Ca wt% in Ca-Mri-Fe3O4. It was concluded that the increasing Ca wt% positively affected As3+ oxidation but an excess Ca wt% negatively affected As3+ adsorption. The higher As3+ adsorption was observed when Ca wt% was 17 (i.e. the wt% ratios of Ca:Mn:Fe = 2:3:1). Without H2O2, As3+ was adsorbed and oxidized by Ca-Mn-Fe3O4 itself. It is suspected that As3+ oxidation is due to H2O2 produced from CaO2. Mechanisms for As3+ removal by Ca-Mn-Fe3O4 with and without H2O2 are proposed. (C) 2014 Elsevier B.V. All rights reserved.