Biofilm Accelerates As(III) Oxidation on Reactive MnOx Coated Filter Sand in Groundwater Filters

Removal of carcinogenic arsenic (As) from groundwater is essential for providing 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 oxide (MnO _x ) coated filter sand, since it is unknown which pathway dominates in filters. The MnO _x 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 3 weeks, a shift from surface-catalytic to biological As-(III) oxidation was observed. Initially, surface-catalytic As-(III) oxidation (k _CHEM = 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 Polaromonas as the dominant genus, and Microscillaceae were found post experiments. Except for these changes, the microbial community on the sand grains stayed relatively similar prior to and post experiments. No significant changes in the physical-chemical properties of the MnO _x coating were found post experiments. A first-order biological As-(III) oxidation rate constant k _BIO of 4.64 min^-1 was found, yielding a half-life of 9 s. This represents a 14-fold acceleration compared with 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 MnO _x , offering a potential new pathway for the development of effective As removal systems.