# Effects of Nano-TiO\({}_{2}\) Mediated Photocatalysis on _Microystis aeruginosa_ Cells
## 3 Results and Discussion
### Adsorption Heat of Cations on the M. aeruginosa Cells  
Figure 6 shows the zeta potential as a function of the addition of cations (Li\({}^{+}\), Na\({}^{+}\), K\({}^{+}\), Mg\({}^{2+}\), Ca\({}^{2+}\), Sr\({}^{2+}\), Ba\({}^{2+}\), Al\({}^{3+}\), and Fe\({}^{3+}\)). Both the charge density and the zeta potential decreased significantly upon the addition of cations. The surface of _M. aeruginosa_ cells carried negative charges and adsorbed cations as a result of electrostatic attraction. Among the cations of the first main group, the ability to neutralize negative charges ranked as K\({}^{+}\) > Na\({}^{+}\) > Li\({}^{+}\). Similarly, the cations of the second main group ranked as Ba\({}^{2+}\) > Sr\({}^{2+}\) > Ca\({}^{2+}\) > Mg\({}^{2+}\). Within each group, the metal ion with higher charge density had stronger ability to neutralize negative charges. Cations of the second main group are divalent, and thus showed stronger ability to neutralize negative charges than those of the first main group. Furthermore, Al\({}^{3+}\) and Fe\({}^{3+}\) had the strongest ability to neutralize negative charges and changed the zeta potential the most dramatically. The results suggested that in neutralizing negative charges, the valence of the metal ion was the main driver, and the charge density of the metal ion also affected the strength of the impact. Among all tested metal ions, Al\({}^{3+}\)and Fe\({}^{3+}\) showed the strongest ability in changing the surface charges of the _M. aeruginosa_ cells.  
Table 2 shows the heat released when the cations bound to the cell surface. An exothermal process was observed for all tested cations. Theoretically, the binding reaction is accompanied with the formation of a chemical bond that is weaker than the covalent bond and similar to the hydrogen bond. During binding, the energy of the cation-cell system was reduced and heat was released. This heat depended on the valence and the charge density of the added cations, and ranked as K\({}^{+}\) > Na\({}^{+}\) > Li\({}^{+}\), Ba\({}^{2+}\) > Sr\({}^{2+}\) > Ca\({}^{2+}\) > Mg\({}^{2+}\), and Al\({}^{3+}\) > Ba\({}^{2+}\) > K\({}^{+}\). This order is reasonable because the valence and the charge density of the cation affected the bond strength and bond energy [25], which in turn determined the amount of heat released. The exothermal process indicated a negative enthalpy change. Besides, during electrostatic interaction, the cations migrated from the solution and were bound to the cell surface. The degree of order increased and the entropy of the system decreased, which in fact impeded the adsorption process. Nevertheless, the negative enthalpy change was strong enough to overcome the entropy factor and realize the binding process. Therefore, the electrostatic adsorption on the cell surface was \"enthalpy-driven\".  
Figure 6: Zeta potential of _M. aeruginosa_ cells after addition of different cations.  
The adsorption heat of cations was lower for the treated cells than the native cells. As outlined before, the amine (-NH\({}_{2}\)) groups were modified by free radicals during photocatalysis. Consequently, the charge density on the cell surface decreased, which led to weaker binding between cations and the cells.