Christopher J. Daughney

A chemical equilibrium model for metal adsorption onto bacterial surfaces

Abstract This study quantifies metal adsorption onto cell wall surfaces of Bacillus subtilis by applying equilibrium thermodynamics to the specific chemical reactions that occur at the water-bacteria interface. We use acid/base titrations to determine deprotonation constants for the important surface functional groups, and we perform metal-bacteria adsorption experiments, using Cd, Cu, Pb, and Al, to yield site-specific stability constants for the important metal-bacteria surface complexes. The acid/base properties of the cell wall of B. subtilis can best be characterized by invoking three distinct types of surface organic acid functional groups, with pK a values of 4.82 ± 0.14, 6.9 ± 0.5, and 9.4 ± 0.6. These functional groups likely correspond to carboxyl, phosphate, and hydroxyl sites, respectively, that are displayed on the cell wall surface. The results of the metal adsorption experiments indicate that both the carboxyl sites and the phosphate sites contribute to metal uptake. The values of the log stability constants for metal-carboxyl surface complexes range from 3.4 for Cd, 4.2 for Pb, 4.3 for Cu, to 5.0 for Al. These results suggest that the stabilities of the metal-surface complexes are high enough for metal-bacterial interactions to affect metal mobilities in many aqueous systems, and this approach enables quantitative assessment of the effects of bacteria on metal mobilities.

A comparison of the thermodynamics of metal adsorption onto two common bacteria

The cell walls of bacteria are known to adsorb a variety of metals, and thus they may control metal mobilities in many low-temperature aqueous systems. In order to quantify metal adsorption onto bacterial surfaces, recent studies have applied equilibrium thermodynamics to the specific chemical and electrostatic interactions occurring at the solution–cell wall interface. However, to date, few studies have used this approach to compare the surface properties and metal affinities of different species of bacteria. In this study, we use acid–base titrations to determine the concentrations and deprotonation constants of specific surface functional groups on Bacillus licheniformis. The cell wall displays carboxyl, phosphate and hydroxyl surface functional groups, with pKa values and 1s errors of 5.2±0.3, 7.5±0.4 and 10.2±0.5, respectively. We perform metal–B. licheniformis adsorption experiments using Cd, Pb, Cu and Al. The average log K values for the Cd-, Pb-, Cu- and Al–carboxyl stability constants, with 1s errors, are 3.9±0.5, 4.6±0.3, 4.9±0.4 and 5.8±0.3, respectively. Finally, we compare the surface characteristics and metal affinities of B. licheniformis to those of Bacillus subtilis, as determined by Fein et al. [Fein, J.B., Daughney, C.J., Yee, N., Davis, T., 1997. A chemical equilibrium model of metal adsorption onto bacterial surfaces. Geochim. Cosmochim. Acta 61, 3319–3328]. Our investigations indicate that these two species of bacteria have different relative and absolute concentrations of surface sites and slightly different deprotonation and metal adsorption stability constants. We relate these variations in surface properties to variations in metal affinity in order to predict metal mobilities in complex, natural systems.

Aqueous complexation of cadmium, lead, and copper by 2,4,6-trichlorophenolate and pentachlorophenolate

The subsurface mobility of metals and polychlorinated phenols occurring together in contaminated groundwaters may be significantly affected by the extent of aqueous complexation between them. However, no previous experimental studies have examined these interactions. In light of this, the aqueous complexation of Cd, Pb, and Cu by both 2,4,6-trichlorophenolate (TCP−) and pentachlorophenolate (PCP−) has been studied at 25°C. Experimental data gathered by ion selective electrode potentiometry and ultraviolet spectrophotometry indicate that metal-chlorophenol complexation occurs, and we interpret the experimental data in terms of a single 1:1 complex in each system. The log stability constants for the complexes, with 2σ errors, are calculated to be: Cd(TCP)+, 2.5 ± 0.3; Pb(TCP)+, 3.0 ± 0.5; Cu(TCP)+, 4.9 ± 0.4; Cd(PCP)+, 2.9 ± 0.3; Pb(PCP)+, 2.8 ± 0.5; and Cu(PCP)+; 4.2 ± 0.4. Based on these values, a simple correlation technique has been applied to estimate stability constants involving other metals and chlorophenols. Calculations using these stability constants suggest that metal-chlorophenolate complexation can drastically alter metal and/or chlorophenol mobilities in contaminated groundwaters.

Effect of Growth Phase and Metabolic Activity on the Adhesion of Escherichia coli K-12 AB264 to Quartz and Lepidocrocite

The transport of bacteria through soils is controlled in part by their adhesion to mineral surfaces. We studied the adhesion of Escherichia coli K-12 to two representative soil minerals (quartz and lepidocrocite), as the growth phase of the population, the metabolic state of the cells, and the pH of the solution were independently varied. Acid-base titrations and electrophoretic mobility measurements were used to investigate the effects of cell and mineral surface speciation and electric charge on the adhesion process. Significant adhesion to lepidocrocite was observed, decreasing at higher pH values presumably in response to the decreasing electrostatic attraction between the cells and the mineral surface. Adhesion of inactive cells (poisoned with streptomycin) was more extensive than for non-poisoned cells, for both mineral substrates. Further research is warranted to determine if other bacterial species display similar relationships between adhesion, cell metabolic state, mineral sorbent, and solution pH.