Arsenite simultaneous sorption and oxidation by natural ferruginous manganese ores with various ratios of Mn/Fe

Abstract

Arsenite [As(III)], is more toxic and difficult to remove from aqueous systems compared to arsenate [As(V)], and as such poses a significant health risk to humans and the environment. In order to make it more amenable to efficient removal, an oxidation pre-treatment of As(III) is particularly important. Compared to synthetic adsorbents, the low cost and abundance of natural ferruginous manganese ore (NFMO), make it a potentially attractive adsorbent for application in large-scale treatments. Here, we investigated the simultaneous oxidation and adsorption behaviour of As(III) with three NFMOs, each consisting of different Mn/Fe molar ratios. Results demonstrated that the NFMO with a high Mn:Fe ratio had a stronger oxidation capacity for As(III), while As(III) adsorption efficiency of NFMOs increased with increasing Fe content. The sorbent dosage was an important factor for As(III) oxidation with NFMOs having higher Mn contents, but not for ores with low contents. An pH increasing from 6 to 7.9 enhanced the As(III) oxidation by NFMOs. Furthermore, the As(III) oxidation rate of NFMO increased by 68% with the addition of pyrophosphate (PP), and increasing PP concentrations led to higher oxidation rates. During the process, PP formed complexes with Mn(III), thus accelerating the conversion of Mn(IV) to Mn(III). Later, the dissolution of these complexes led to a generation of additional oxidation adsorption sites, favouring As(III) oxidation. FTIR and XPS analyses further confirmed that reduction of Mn(IV) and Mn(III) species played a vital role in the oxidation of As(III) to As(V). As(III) removal by NFMO was attributed to the joint effect of sorption and oxidation processes, where Manganese oxide was responsible for As(III) oxidation, while Fe oxide played a primary role in the arsenic sorption.