Birnessite minerals (layer-type MnO2) participate in important biogeochemical processes, particularly trace metal sorption. When produced by bacteria,birnessite particles are typically deposited in a biofilm matrix. The impact of the biofilm on the sorption behavior of biogenic birnessite is poorly understood. In one study, Ni adsorbed preferentially to the biomass when the mineral fraction was small relative to the biomass fraction (<4% MnO2)1. Other researchers using a biomass-mineral assemblage containing 10-20% MnO2 by mass found that birnessite enhanced Zn and Ni sorption relative to the biomass alone, with trace metals adsorbing preferentially to birnessite cation vacancy sites2,3. Based on these studies, we hypothesize that changing the mineral:biomass ratios may cause organic-mineral interactions that modify the mineral structure and/or the total number of metal binding sites. We evaluated the impact of the biofilm on the reactivity of biogenic birnessite at varying MnO2:biomass ratios. The sorbents were produced using Pseudomonas puitda strain GB-1 cultures incubated with Mn concentrations ranging from 0.05�2.0 mM Mn to yield sorbents with 1�15 % MnO2on a per mass basis. The sorbents were used to conduct adsorption isotherm experiments for Ni at pH 6, as well as Ni K-edge extended X-ray absorption fine structure (EXAFS) measurements. Our results indicate a difference in the reactivity of the composite biomass-birnessite assemblage depending on the mineral to organic ratio, with the fraction of Ni partitioning to the biomass decreases as the amount of birnessite in the assemblage increases, as determined from linear combination fits of the EXAFS spectra. Our results suggest that the biomass fraction of the sorbent plays an important role in governing metal removal from solution.1Zhu, M., et al, 2010, Environmental Science&Technology 44, 4472-4478.2Pe�a, J., et al, 2011, Environmental Science&Technology 45, 7338-7344.3 Pe�a, J., et al, 2010, Geochimica.Cosmochim.Acta 74, 3076-3089.

Impact of microbial surfaces and exudates on biogenic birnessite reactivity

247th National Meeting (2014)

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