Influence of the pH-value on the photocatalytic disinfection of bacteria with TiO2 – Explanation by DLVO and XDLVO theory

A photocatalytic disinfection with P25 TiO2 was conducted at three pH-values (4, 7 and 10) in NaCl solution (c = 10 mmol L−1). Neither the osmotic stress nor solely the pH-value had a significant impact on the survival of the Escherichia coli cells. The cells were more damaged at pH 4 than at pH 7 and 10 in irradiation experiments with TiO2. The smallest disinfection rates occurred at pH 10. SEM images revealed that the surface of the cells was covered with TiO2 to a greater extent at pH 4 compared to pH 10. At pH 10, the disinfection rates increased when increasing the ionic strength and changing the NaCl solution to CaCl2. The results indicate that the disinfection rates depend on the electrostatic interaction between cells and TiO2. The differences could be qualitatively better explained by the XDLVO theory in comparison to the DLVO theory.

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► Disinfection rate is dependant on electrostatic forces.
► Close contact between photocatalyst and microorganism enhances disinfection.
► Disinfection rate can be explained by XDLVO theory.