『Abstract
Data must be collected over a wide pH range to accurately model
the adsorption of protons and metal onto bacterial surfaces; however,
alterations in the structural and chemical properties of bacterial
surfaces resulting from exposure to acidic solutions may affect
the mechanisms of cation binding. Binding properties of bacteria
may also be affected by nutrient and oxygen levels present during
their growth. We measured Cd, Co, and Pb adsorption onto bacteria
by using (1) bacteria washed with acidic solutions (pH≒1.5), (2)
non-acid-washed bacteria exposed to acidic parent solutions, and
(3) non-acid-washed bacteria exposed to neutral parent solutions.
The purpose was to determine the effect of acidic conditions on
the adsorptive properties of Pseudomonas mendocina, Pseudomonas
aeruginosa, Bacillus subtilis, and Bacillus cereus.
We also measured Co adsorption onto bacteria (Pseudomonas fluorescens)
grown under nutrient-rich and nutrient-limited conditions and
onto the facultative bacterium Shewanella oneidensis MR-1
grown under aerobic and anaerobic conditions. Bacteria exposed
to acidic solutions adsorbed more metals than bacteria not exposed
to such solutions. We attribute the increase in adsorption to
the irreversible displacement of structurally bound Mg and Ca
by protons. After displacement, the protonated sites can participate
in reversible metal adsorption reactions. Thermodynamic modeling
suggests that concentrations of functional group sites on bacterial
surfaces increase by as much as five times in response to acid
washing, assuming that stability constants for the bacterial surface
complexes remain the same. Although the sizes of the bacteria
changed markedly in response to nutrient limits and oxygen content
during growth, the mass-normalized extent of Co adsorption onto
both P. fluorescens and S. oneidensis MR-1 was independent
of growth conditions. We conclude that adsorption constants derived
from experiments in which the bacteria are never exposed to acidic
conditions probably provide the most accurate estimates of the
extent of bacteria-metal adsorption in natural settings.
Keywords: Metals; Sorption; Acid; Bacteria; Anaerobic; Surface
complexation modeling
1. Introduction
2. Materials and methods
2.1. Bacterial growth and harvest
2.2. Metal adsorption experiments
2.3. Acidification and dissolved organic carbon experiments
3. Results
4. Discussion
4.1. Effect of acid on the adsorptive properties of bacterial
surfaces
4.2. Effect of growth conditions on binding properties of bacterial
surfaces
4.3. Modeling of bacteria-metal adsorption reactions
4.4. Implications for describing bacteria-metal adsorption in
nature
5. Summary and conclusions
Acknowledgements
References