Cadmium complexation by bacteriogenic iron oxides from a subterranean environment

Abstract

This study quantifies the metal sorption characteristics of subterranean bacteriogenic iron oxides (BIOS) and their organic phases (intermixed intact and fragmented bacteria). A Cd 2+ 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 Cd 2+ binding sites for the BIOS with average −log 10 equilibrium constants (p K S, j ) of 1.06±0.19 and 2.24±0.28. Three discrete sites were obtained for the bacterial fraction, with p K 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 Cd 2+ than that of the bacteria. p K S, j values for the BIOS were similar to those reported for pure iron oxide phases, while the organic fraction p K 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 μmol/mg of BIOS corresponded to the iron oxide mixture and bacteria fraction, respectively. These values indicated high concentrations of strong affinity Cd 2+ complexing groups on the bacterial surface. Comparison of total site densities of 0.08±0.02 and 0.48±0.06 μmol/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 Cd 2+, 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 Cd 2+ complexation characteristics. This approach is therefore a useful tool for the study of natural sorbent materials controlling metal partitioning in contaminated and pristine environments.