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

Bacterial cell membranes display a strong affinity for a wide variety of aqueous metal cations and have the potential to affect the mass transport of these cations through adsorption reactions in water-rock systems. Prior studies have focused on determining the thermodynamic stability constants of heavy metals and radionuclides; however, the constants for Mn-bacterial surface complexes formed on manganese oxidizing bacteria remain unmeasured. We measured the rate, extent, and reversibility of Mn(II) adsorption onto a bacterial species capable of Mn-oxidization (Pseudomonas putida), and onto a bacterial species that does not promote Mn-oxidization (Bacillus subtilis). The extent of adsorption was measured as a function of both pH and metal loading in order to determine the thermodynamic stability constants of the Mn-bacterial surface complexes that form on the bacteria and to test whether Mn oxidizers exhibit unusual Mn adsorption behavior relative to species that do not oxidize Mn. Furthermore, we determined the effect of bacterial extracellular polymeric substances (EPS) on Mn(II) adsorption by conducting experiments with and without EPS removal from the biomass. The experimental results indicated that Mn(II) adsorption onto B. subtilis and P. putida was rapid and reversible. The extent of Mn(II) adsorption onto both bacterial species increased with increasing pH, but P. putida adsorbed significantly more Mn(II) than did B. subtilis across most of the pH range studied. Both the adsorption measurements and the calculated stability constants indicate that P. putida, a Mn oxidizing bacterial species, exhibited significantly enhanced Mn adsorption relative to that observed for B. subtilis. The enhanced Mn(II) adsorption exhibited by P. putida suggests that bacteria may adapt the metal binding environments within their cell envelopes in order to optimize bioavailability of metals that are beneficial to their metabolism. Experiments conducted at low metal-loading conditions yielded stability constants for the Mn(II)-bacterial surface complexes that were less than or similar to those calculated for the high metal-loading conditions, suggesting that Mn(II)-sulfhydryl binding does not dominate under low metal loading conditions as it does for other metals. Removal of EPS molecules from P. putida led to significantly reduced extents of Mn(II) adsorption, suggesting that Mn(II)-EPS binding plays at least some role in the overall adsorption of Mn(II) onto P. putida biomass.