Manganese-coated filter sand, pre- and post- arsenic exposure
Removal of carcinogenic arsenic (As) from groundwater is essential for safe drinking water. Arsenate (As(V)) is more effectively removed in groundwater filters than arsenite (As(III)), making the oxidation of As(III) to As(V) a key step in the treatment process. This study distinguishes between surface catalytic and biological As(III) oxidation on natural manganese oxides (MnOx) coated filter sand, since it is unknown which pathway dominates in filters. The MnOx coated sand was collected from a full-scale groundwater filter and consisted of a mixture of different abiotically and biologically formed Mn oxides, such as Birnessite and Todorokite. A lab-scale filter setup was operated with As(III)-containing water. Within three weeks, a shift from surface catalytic to biological As(III) oxidation was observed. Initially, surface catalytic As(III) oxidation (kCHEM= 0.318 min-1) was coupled to Mn(II) release at a ratio of 0.96, approximating the stoichiometric ratio of 1. This coupling disappeared over time, indicating the biological nature of the reaction as confirmed by microbial inhibition. An increase in relative abundance of the known As-oxidizing families Comamonadaceae, with the genus Polaromonas as the dominant species, and Microscillaceae were found post experiments. Except for these changes, the microbial community on the sand grains stayed relatively similar prior and post experiments. No significant changes in the physical-chemical properties of the MnOx coating were found post experiments. A first-order biological As(III) oxidation rate constant kBIO of 4.64 min-1 was found, yielding a half-life of 9 seconds. This represents a 14-fold acceleration compared to surface catalytic oxidation, revealing that kinetic limitations rather than surface passivation can be attributed to the loss of surface-catalytic oxidation. Our study demonstrates that biological oxidation of As(III) can outpace the acknowledged oxidizing power of MnOx, offering a potential new pathway for the development of effective As removal systems.