INVESTIGATING ADSORPTION OF ARSENATE AND CHROMATE ONTO SHEWANELLA PUTREFACIENS

We have measured adsorption of dissolved arsenate and chromate onto the bacterium Shewanella putrefaciens (200R) as a function of pH (3-11), ionic strength (NaCl), and relative concentrations of the respective oxy-ligands (2 - 250 µm ligand per g wet bacteria) at room temperature (25 0C) in the laboratory. Experiments were carried out in batch series by equilibrating dissolved arsenate or chromate in the presence of bacteria for 4 hours, filtering the supernatants, and measurement of remaining dissolved ligand via ICP-OES.

Experimental results show that chromate is strongly adsorbed by the bacteria under circumneutral pH conditions, with little uptake at 5 < pH > 9. In contrast, little to no arsenate uptake was observed under similar experimental conditions. Chromate uptake is not strongly dependent on ionic strength. Minimal chromate uptake at low pH may be due to abiotic reduction of Cr(VI) to Cr(III), as previously reported for other microbial cells. However, the divergent behavior of chromate versus arsenate at near-neutral pH indicates that ligand-specific chemical bonding is important. Our results demonstrate that ligand uptake by bacteria under non-growth conditions can vary significantly based on ligand identity, with some oxy-ligands displaying more stable surface complexation than others. Surface complexation modeling calculations will be presented that seek to constrain surface species potentially responsible for reversible chromate adsorption onto S. putrefaciens cell membranes.