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 si gnificantly affect mass transport in many geologic settings. This study exa mines the Cd-EDTA-Bacillus subtilis system to investigate these interaction s, and to test the applicability of a chemical equilibrium approach to quan tify aqueous and surface complexation reactions that occur in the system. T he experiments indicate that fully-protonated EDTA adsorbs onto the bacteri al surface through hydrophobic interaction, and that adsorption of the depr otonated EDTA molecule is negligible. Aqueous EDTA can strongly compete wit h the bacterial surface for aqueous Cd, and the presence of aqueous EDTA si gnificantly diminishes Cd adsorption onto B. subtilis. Independent of the e xperimental measurements, we use chemical equilibrium modeling to estimate the extent of Cd adsorption in the Cd-EDTA-bacteria systems. The observed a dsorption behavior is in excellent agreement with the estimations, suggesti ng that chemical equilibrium modeling can successfully account for the dist ribution of mass in a system in which both aqueous and surface metal-organi c complexation occurs. (C) 1999 Elsevier Science B.V. All rights reserved.