Candidate: Haile A. Mengistu
Degree of: Doctor of Philosophy
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 25º C and 1 bar. The following stability constants are generated:
log K1int = 19.0, log K2int = 14.3 ± 0.17, log K3int = 8.32 ± 0.27.
Adsorption of arsenate on Shewanella putrefaciens is weak having a thermodynamic stability below the prediction limits of our approach. However, we generated new sets of thermodynamic constants for bacterial adsorption of chromate that fits all the chromate concentration range (0.5 to 5 mmol chromate/ g bacteria) and the entire pH range (3 – 11.5) reasonably well with the following values:
log K1int = 2.0, log K2int = 7.7, log K3int = 8.9, log K4int = 8.7, and log K5int = 1.3
Our recommended values provide a close fit to the experimental data over a wide range of conditions, and should be adopted in exercises modeling of the chemical speciation of phosphate and chromate in natural systems containing HFO and bacteria. .
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