Investigation of Oxy-Anion Legand Adsorption onto Surfaces of Iron Oxide and Bacteria in Groundwater Aquifers

Author

Haile Mengistu, Western Michigan University

Date of Award

12-2006

Degree Name

Doctor of Philosophy

Department

Geosciences

First Advisor

Dr. Johnson Haas

Second Advisor

Dr. Carla Koretsky

Third Advisor

Dr. Alan Kehew

Fourth Advisor

Dr. Jeremy Fein

Abstract

We have measured adsorption of dissolved phosphate onto synthetic hydrous ferric oxide (HFO), dissolved arsenate and chromate onto the cell membrane of a bacterium species, S. putrefaciens as a function of pH, ionic strength, and relative concentrations of the respective oxy-ligands. Our experimental data were used to constrain optimal values for surface complexation reactions involving dissolved oxi-ligands (phosphate, and chromate) and the HFO surface and the cell membrane of S. putrefaciens respectively according to the diffuse layer model.

Our results provide a more accurate fit to experimental measurements over a broader range of pH (3 - 12), ionic strength and total relative phosphate concentration (12.5 - 500 μmol phosphate/g HFO) with a consistent set of model equilibrium constant (log K) values at 25° C and 1 bar. The following stability constants are generated:

log K₁^int = 19.0, log K₂^int = 14.3 ± 0.17, log K₃^int = 8.32 ± 0.27

Modeling of bacterial arsenate uptake data was not conducted because of weak adsorption below the prediction limits of our approach.

Multiple modeling of bacterial adsorption of chromate was performed to fit the chromate concentration data (0.5 to 5 μmol chromate/ g bacteria) in the entire pH range (3 - 11.5) and the following complexes are suggested as reasonable adsorption reactions to fit part of the experimental data:

>200Ra-POH₂-CrO₄^2- and >200-COO-CrO₄³: Log K^int = 8.1 and 1.5 respectively

>200Ra-COO-Ca-HCrO₄ : Log K^int = 10.5 and

>200Ra-NH₂-HCrO₄^-: Log K^int = 15.0

Non-metabolic reduction of chromate is suggested as a possible explanation to diminished adsorption at low pH-regions (pH < 4 ). Unless the model interpretation is backed by additional direct evidence, it is not possible to be certain that the three surface reactions "correctly" represent chromate adsorption on bacterial cell walls. It is therefore essential that additional bacterial surface characterization and possible complexation sites should be made available by making use of direct spectroscopic techniques (FTIR) or liquid chromatography of cell membrane extracts.

Access Setting

Dissertation-Open Access

Recommended Citation

Mengistu, Haile, "Investigation of Oxy-Anion Legand Adsorption onto Surfaces of Iron Oxide and Bacteria in Groundwater Aquifers" (2006). Dissertations. 3506.
https://scholarworks.wmich.edu/dissertations/3506