Metal Adsorption onto Gram-Negative Bacteria from an Oil-contaminated Site

This paper reports thermodynamic parameters for metal adsorption onto a gram-negative bacterium, which would allow bacterial effects to be included in models of metal transport in the subsurface. Potentiometric titrations were used to determine the different types of sites present on bacterial cell walls. Stability constants for adsorption of a variety of metals to each site was determined from batch adsorption experiments at varying pH with constant metal concentration, and the number of sites involved in metal uptake was confirmed by sorption isotherm experiments at constant pH. Titrations revealed three distinct acidic surface sites on the bacterial, with pK values of 4.3±0.1, 6.9±0.5 and 8.9±0.5, corresponding to carboxyl, phosphate and hydroxyl/amine groups, with surface densities of 5.0±0.7×l0^-4, 2.2±0.6×l0^-4 and 5.5±2.2 × lO_-4 mol/g of dry bacteria. Only carboxyl and phosphate sites are involved in metal uptake, with surface affinities of the order Cu>Pb>Zn based on the following stability constants: Log K_cmboxyl: Zn²⁺ = 3.3±0.1, Pb²⁺ = 4.3±0.2, and Cu²⁺ = 4.4±0.2, Log K_Phosphate: Zn²⁺ = 5.1±0.1 and Cu²⁺ = 6.0±0.5. Some of these stability constants are similar to those for common organic compounds found in natural soil solutions, suggesting that bacteria may play an important role in metal attenuation in the subsurface.

• 1. INTRODUCTION

• 2. THEORY

• 3. EXPERIMENTAL METHODS

• 4. RESULTS AND DISCUSSION

• 5. SUMMARY

• 6. ACKNOWLEDGEMENTS

• 7. REFERENCES