# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 4 Characterization Techniques for Fouling Analysis
### Zeta Potential  
Fouling can change the membrane surface charge and by measuring the zeta potential of new and fouled membranes one can evaluate the possibility of foulant(s) to adhere or not to the membrane surface. It is a measure of the magnitude of electrostatic repulsion or attraction between particles. In general, zeta potential is a key indicator of the stability of colloidal particles and it is a good method to describe double-layer properties of colloidal dispersions [15; 103]. Various factors affect the zeta potential such as the concentration, composition, temperature and pH of the solution as well as the  
Figure 6: Ultrasonic signal responses of the clean NF membrane (blue line) and differential signals (red line) obtained at 300 min of fouling operation with the feed solution of 1000 mg/L silica and 1000 mg/L NaCl under a constant pressure operation. Adapted with permission from [116], Copyright Elsevier, 2015.  
membrane surface properties (i.e., charge, roughness, chemical heterogeneity, etc.) [122]. Therefore, it is a useful technique to determine whether a membrane is modified in order to improve the considered process performance. The determination of the zeta potential can be carried out using several methods such as the streaming potential, vibrational potential, electrophoresis, electro-osmosis, etc. For fouling characterizations streaming potential and electrophoresis are the most considered methods [123].  
Electrophoresis is the movement of charged particles or polyelectrolytes in a liquid under the influence of an external electric field. The electrophoretic mobility of a charged particle is determined by the balance of electrical and viscous forces that could be used to determine the zeta potential. The streaming potential is measured when the electrolyte solution is forced with a known pressure through a channel [124]. A resulting voltage is measured between electrode probes on either side of the channel and compared with the voltage at zero applied pressure. The streaming potential can be related to the zeta potential by factors that include the electrical conductivity, fluid viscosity and the structure of the channel [123]. More experimental details of zeta potential can be found in [122].  
[PERSON], et al. [103] measured the streaming zeta potential of PP and PTFE membranes after their use in MD for the treatment of different pretreated model manure feed solutions. It was stated that new PP and PTFE exhibited a small negative zeta potential (above \(-30\) mV, at pH \(\approx 9\)). The fouled PTFE membrane exhibited the more negative zeta potential (above \(-40\) mV, at pH \(\approx 9\)) indicating the higher electrostatic repulsion between model manure solution and the membrane surface. However, for the PP after adsorptive fouling with model manure solution the zeta potential was above \(-15\) mV (at pH \(\approx 9\)) (see Figure 7) suggesting an increased fouling potential due to decreased electrostatic repulsion between the membrane surface and the model manure solution.  
[PERSON] et al. [29, 30] characterized both commercial and prepared MD membranes by means of zeta potential at different pH values and claimed that the negatively charged MD membranes were resistant to dye adsorption and subsequent fouling.  
Figure 7: Zeta potential of polytetrafluoroethylene (PTFE) and PP membranes as a function of pH after adsorptive fouling. Reprinted with permission from [103], Copyright Elsevier, 2015.