Bacteria play an integral role in the biogeochemical cycling of a wide range of metals. Because adsorption is the first step in metal bioavailability for many processes, it is crucial to determine the identities and thermodynamic stabilities of metal-bacteria surface complexes in order to construct rate laws for metabolic processes that involve metals. We measured the thermodynamic stabilities of Mn-bacteria complexes that form on two Mn oxidizing bacterial species in order to serve as a foundation for kinetic rate laws of biogenic Mn oxidation. Mn(II) adsorption onto the Mn oxidizing bacteria P. putida and B. subtilis was measured under anoxic conditions. A known mass of cells was added to a weighed volume of 0.1 M NaClO4 containing 4 ppm of Mn(II) to form a parent cell suspension of 20 g wet cell L-1. We conducted adsorption kinetics experiments in which the parent solution pH held constant at 6.5. For the Mn(II) adsorption experiments as a function of pH at fixed ionic strength and metal:bacteria ratio, the parent solution was divided into individual test tubes, and the pH of each solution was adjusted to cover the pH range of 3.0 to 8.0. The bacteria-Mn(II)-electrolyte suspension was centrifuged and filtered prior to Mn(II) determination by ICP-OES. The concentration of Mn(II) adsorbed onto bacteria was determined by difference between this value and the total Mn(II) in each system. The Mn(II) adsorption kinetics experiments indicated that B. subtilis and P. putida adsorbed a maximum of 81% and 92% of the total Mn(II), respectively. The maximum removal of Mn(II) by B. subtilis and P. putida was reached at 3hrs and 1hr, respectively. The extent of Mn(II) adsorption increased with increasing pH for both bacterial species. Mn(II) adsorption onto both bacterial species was modeled using a surface complexation model. The B. subtilis and P. putida datasets each required Mn binding onto 3 sites to account for the adsorption behavior as a function of pH, and the data constrain the values of the stability constants for each Mn(II)-bacterial complex. Calculated log Kads for Mn(II)-site complexes are different between the two studied bacteria, indicating that stability constants for different species can vary and that these values must be determined for each species of interest. These Kads values are used as a basis to interpret Mn(II) oxidation kinetics measurements, enabling us to calibrate rate laws based on bacterial surface speciation of Mn(II).

Thermodynamic modeling of Mn(II) adsorption onto manganese oxidizing bacteria

251st National Meeting (2016)

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