DLVO Approach to the Flocculability of a Photosynthetic H<sub>2</sub>-Producing Bacterium, <i>Rhodopseudomonas acidophila</i>

The DLVO theory was used to explore the flocculation characteristics of a H<sub>2</sub>-producing photosynthetic bacterium, <i>Rhodopseudomonas acidophila.</i> The relationship between the surface characteristics of this strain and its flocculability was evaluated. Its flocculability was governed by both electrolyte concentration and pH, and the appropriate electrolyte concentration and pH were found to be 0.1 M NaCl solution and pH 7.0, respectively. In addition, the extracellular polymeric substances produced by <i>R. acidophila</i> were observed to have a significant effect on its flocculation. The effective Hamaker constant between <i>R. acidophila</i> and water was only 2.27 × 10<sup>-23</sup> J, suggesting that the contribution of van der Waals interaction energy to the total interaction energy could be neglected. As a result, the bacterial particles could not overcome the total energy barrier to flocculate effectively. Otherwise, because the repulsive total interfacial free energy between the bacterial cells and water was positive, the cell particles of <i>R. acidophila</i> repelled each other, resulting in a great stability of the cell suspensions.