Oxidation of arsenite by self-regenerative bioactive birnessite in a continuous flow column reactor

Naturally occurring manganese (Mn) oxide, biogenic birnessite ((Na, Ca, K)_0.5 Mn^{III, IV}₂O₄·1.5 H₂O), is involved in the geochemical cycling of variety of metals including arsenic (As). This natural reaction was exploited in this study to develop a sustainable oxidation treatment process of As(III) to the less soluble (and less toxic) As(V). It is known that the birnessite surface becomes passivated during As(III) oxidation, which quickly decreases its reactivity. The cycle batch test and the following XANES (X-ray absorption near-edge structure) analysis in this study confirmed that combining chemical As(III) oxidation by birnessite with simultaneous birnessite regeneration by Mn-oxidizing microorganisms (Pseudomonas sp. SK3) can avoid passivation of Mn^III-precipitates and enables continuous As(III) oxidation while increasing the AOS (average oxidation state) of birnessite. This chemical/microbiological synergism was observed for the As(III) concentration range of 0.2–0.5 mM with 0.1% birnessite, wherein no net Mn loss from birnessite was noticed for complete As(III) oxidation. The continuous column test was run for 40 days at a HRT (hydraulic retention time) of 3 h by feeding a 0.2 mM As(III) solution. The As(III) oxidation efficiency of >98% was consistently achieved while strictly controlling the Mn²⁺ dissolution throughout the test period. This study concluded that by taking advantage of a robust microbial Mn-oxidizing activity, the use of “bioactive” birnessite realizes self-sustainable oxidation of As(III), without necessitating additional feed of oxidant birnessite, Mn²⁺ ions or organics.