Quantifying the Effects of Bacillus subtilis Cell Walls on the Precipitation of Copper Hydroxide from Aqueous Solution

This paper describes a series of experiments that determine the effects that nonmetabolizing bacterial cells have on the precipitation of Cu(OH)_2(s). Copper adsorption and precipitation onto the gram-positive soil bacterium Bacillus subtilis was measured using batch experiments in 0.1 M NaClO₄ as a function of pH, and aqueous Cu activity at 25° C. To differentiate between adsorption and precipitation of Cu onto the bacterial cell walls, we utilized equilibrium solubility measurements in control and bacteria-bearing systems along with surface complexation modeling. Because we explicitly account for Cu adsorption, and because the control experiments clearly define the saturation concentrations, these experiments constrain the conditons at which mineral precipitation occurs both in bacteria-bearing and bacteria-free systems. The bacteria-bearing systems exhibit enhanced Cu-removal from solution relative to the bacteria-free control experiments at pH values less than those at which Cu precipitates in control experiments. However, the enhanced adsorption can be best explained by accounting for Cu adsorption onto the bacterial cell wall; because the precipitation curve observed in the bacteria-bearing systems is coincident with that observed in the control experiments the results of these experiments demonstrate that nonmetabolizing bacteria cell walls do not induce Cu(OH)_2(s) precipitation at otherwise undersaturated conditions.