Experimental study of the effect of EDTA on Cd adsorption by Bacillus subtilis: a test of the chemical equilibrium approach

The interactions between aqueous metals, organic acids, and bacteria can significantly affect mass transport in many geologic settings. This study examines the Cd–EDTA–Bacillus subtilis system to investigate these interactions, and to test the applicability of a chemical equilibrium approach to quantify aqueous and surface complexation reactions that occur in the system. The experiments indicate that fully-protonated EDTA adsorbs onto the bacterial surface through hydrophobic interaction, and that adsorption of the deprotonated EDTA molecule is negligible. Aqueous EDTA can strongly compete with the bacterial surface for aqueous Cd, and the presence of aqueous EDTA significantly diminishes Cd adsorption onto B. subtilis. Independent of the experimental measurements, we use chemical equilibrium modeling to estimate the extent of Cd adsorption in the Cd–EDTA–bacteria systems. The observed adsorption behavior is in excellent agreement with the estimations, suggesting that chemical equilibrium modeling can successfully account for the distribution of mass in a system in which both aqueous and surface metal–organic complexation occurs.