Adsorption of ferrous ions onto Bacillus subtilis cells

Bacteria are known to have reactive surfaces that can affect the cycling of dissolved elements through sorption reactions. The properties of adsorption of various metallic ions (Cd2+, Pb2+, etc.) onto various types of Gram-positive or -negative bacterial surfaces have been measured in several previous studies, but the behavior of ferrous ions has been somewhat left out, possibly because ferrous ions are easily oxidized under standard laboratory conditions. In this paper, the adsorption of ferrous ions onto Bacillus subtilis (a Gram-positive bacterium) was measured under anoxic conditions as a function of pH for various Fe(II)/bacteria ratios. Our results first indicate that Fe(II) adsorption increased with pH and that the adsorption was reversible. An equilibration time of one hour was found sufficient to reach equilibrium between the bacterial surfaces and the solution. Acid-base titrations of the bacterial suspensions were also performed. The titration data could be fitted using a simple three-site model. Neglecting the electrostatic component, the three sites had pK values of 4.45 (pK(1)), 6.74 (pK(2)), and 9.08 (pK(3)) with corresponding concentrations of 5.6, 5.3, and 6.1 (X 10(-4) mol/dry g of bacteria), respectively. The high density of sites available for proton adsorption suggests that protons can easily diffuse within the cell walls. The adsorption data for Fe(II) was fitted with a single-site model. A Langmuir adsorption isotherm suggested that the site concentration available for the adsorption of the ferrous ions was equal to 3.5 x 10(-4) mol/g which as far less than for protons. The modeling analysis of the adsorption isotherms also indicated that the adsorption took place onto the type 2 sites (pK(2)=6.74), or type 3 sites (pK(3)=9.08) and that the adsorption constant of the Fe(II) ions onto the protonated sites was equal to -log(T) = -1.22+/-0.10. A two-site model did not improve the quality of the fit, but indicated that the type 1 sites (pK(1)=4.45) could contribute only to a limited extent to the adsorption process. (C) 2004 Elsevier B.V. All rights reserved.