Cadmium complexation by bacteriogenic iron oxides from a subterranean environment

This study quantifies the metal sorption characteristics of subterranean bacteriogenic iron oxides (BIOS) and their organic phases (intermixed intact and fragmented bacteria). A Cd2+ ion-selective electrode was used to generate high-resolution metal sorption data as a function of increasing pH. A multisite Langmuir model, along with a linear programming regression method (LPM), was applied to fit experimental data. This approach found two discrete Cd2+ binding sites for the BIOS with average -log10 equilibrium constants (pK(S,j)) of 1.06 +/- 0.19 and 2.24 +/- 0.28. Three discrete sites were obtained for the bacterial fraction, with pK(S,j) values of -0.05 +/- 0.12, 1.18 +/- 0.02, and 3.81 +/- 0.16. This indicated that the BIOS surface had a lower affinity for Cd2+ than that of the bacteria. pK(S,j) values for the BIOS were similar to those reported for pure iron oxide phases, while the organic fraction pK(S,j) spectrum was consistent with previous spectra for intact bacteria. Individual binding site densities of 0.04 +/- 0.01 and 0.05 +/- 0.02 and 0.29 +/- 0.05, 0.11 +/- 0.01, and 0.09 +/- 0.02 micromol/mg of BIOS corresponded to the iron oxide mixture and bacteria fraction, respectively. These values indicated high concentrations of strong affinity Cd2+ complexing groups on the bacterial surface. Comparison of total site densities of 0.08 +/- 0.02 and 0.48 +/- 0.06 micromol/mg of BIOS for the mixture and the bacterial phase, respectively, suggested a nonadditive character for the BIOS surface reactivity. This was emphasized by a higher affinity for Cd2+, as well as an increase in total site concentration observed for the bacterial phase. LPM was able to distinguish between the BIOS mixture and its organic fraction Cd2+ complexation characteristics. This approach is therefore a useful tool for the study of natural sorbent materials controlling metal partitioning in contaminated and pristine environments.