CHEMICAL EQUILIBRIUM MODELING TECHNIQUES FOR THE ANALYSIS OF HIGH RESOLUTION BACTERIAL METAL SORPTION DATA

In this study, an ion selective electrode was used to monitor binding of Cd²⁺ on two bacteria, Bacillus subtilis (Gram+) and Escherichia coli (Gram-), as a function of increasing pH. A competitive Langmuir sorption isotherm was used along with a linear programming pK_S spectrum method (LPM) or FITEQL 3.1 to fit experimental data. Results obtained with simulated data showed that LPM is less sensitive than FITEQL to variations in sorption data. Application of the LPM to experimental data found three discrete metal binding sites on B. subtilis and E. coli with –log equilibrium constant (pK_S) values of –0.80±0.20, 0.63±0.09, and 2.35±0.10, and -0.60±0.10, 0.25±0.19 and 1.93±0.17 respectively, at a constant ionic strength, I=0.1 M (KNO₃). The corresponding site densities were 0.09±0.01, 0.07±0.01 and 0.07±0.01, and 0.01 ± 0.00, 0.02±0.01 and 0.04±0.01 ìmoles of Cd²⁺/mg of B. subtilis or E. coli. From FITEQL, pK_S values of –1.18±0.15, 0.40±0.11 and 2.31±0.32 for B. subtilis and -1.46±0.34, 0.20±0.12 and 1.87±0.12 for E. coli were recovered with site densities of 0.10±0.07, 0.07±0.06 and 0.06±0.02, and 0.02±0.00, 0.02±0.00 and 0.04±0.04 ìmoles of Cd²⁺/mg of B. subtilis or E. coli respectively. Both LPM and FITEQL produced feasible results, but LPM was less sensitive to error and did not require an a priori assumption of the number of binding sites. Recognition of variations in reactive site densities and binding constant values by means of LPM, or non-linear least squares methods like FITEQL, is important for constraining the parameterization of geochemical speciation models to predict the binding of metals by bacteria in aqueous environments. This combined approach would help assess, the role of bacterial cell surfaces in metal transport and reactivity, in particular, when compared to other geochemically relevant solids such as hydrous ferric oxides.