Quantifying metal adsorption onto bacteria mixtures: A test and application of the surface complexation model

In this study, we measure Cd, Co, Sr, and Zn adsorption onto mixtures of Gram-positive and Gram-negative bacteria, containing the bacterial species Streptococcus faecalis, Staphyloccocs aureus, Sporosarcina ureae, Bacillus subtilis, Bacillus licheniformis, Escherichia coli, Pseudomonas aeruginosa, Enterobacter aerogenes, Proteus vulgaris, and Serratia marscecens. Metal adsorption experiments were conducted with mixtures of Gram-positive bacteria, Gram-negative bacteria, and mixtures consisting of both bacterial types. The objective is to compare the metal adsorption behavior of Gram-positive and Gram-negative bacteria, and determine if the surface complexation model can be applied to describe metal adsorption onto complex assemblages of different bacterial types. The experimental results indicate that the metal adsorption behaviors of Gram-positive mixtures, Gram-negative mixtures, and mixtures of both Gram-positive and Gram-negative bacteria are nearly identical. We describe the metal adsorption data with a site-specific chemical equilibrium model, which can successfully describe the pH and bacteria.-metal ratio effects on metal adsorption. The metal binding constants determined with the bacteria mixtures in this study are similar to the binding constants that have previously been measured for B. subtilis, suggesting that adsorption constants can be estimated using thermodynamic parameters determined for a limited number of bacterial species. We then apply the metal adsorption constants determined for bacteria mixtures to a hypothetical bacteria-water-rock system to illustrate the utility of the surface complexation approach.