XAFS determination of the bacterial cell wall functional groups responsible for complexation of Cd and U as a function of pH

Bacteria, which are ubiquitous in near-surface geologic systems, can affect the distribution and fate of metals in these systems through adsorption re actions between the metals and bacterial cell walls. Recently, Fein et al. (1997) developed a chemical equilibrium approach to quantify metal adsorpti on onto cell walls, treating the sorption as a surface complexation phenome non. However, such models are based on circumstantial bulk adsorption evide nce only, and the nature and mechanism of metal binding to cell walls for e ach metal system have not been determined spectroscopically. The results of XAFS measurements at the Cd K-edge and U L3-edge on Bacillus subtilis expo sed to these elements show that, at low pH, U binds to phosphoryl groups wh ile Cd binds to carboxyl functional groups.