Title: Membrane characterization by dynamic hysteresis: measurements; mechanisms; and implications for membrane fouling

Journal: Journal of Membrane Science

Authors: Sangyoup Lee1, Eunsu Lee1, Menachem Elimelech2, and Seungkwan Hong1*

Institute: 

1School of Civil, Environmental & Architectural Engineering, Korea University, 1, 5-ka, Anam-Dong, Sungbuk-Gu, Seoul, 136-713, Republic of Korea
2Department of Chemical Engineering, Environmental Engineering Program, Yale University,
New Haven, Connecticut 06520-8286, USA

The original and creativity of paper: This paper presented a novel method of membrane surface characterization using dynamic hysteresis. Moreover, mechanisms governing and factors affecting dynamic hysteresis, as well as featuring the chemical aspects of dynamic hysteresis were investigated. In this case, they point out that dynamic hysteresis can be applied to indicate membrane fouling. 

Summary

There is a new method to evaluate fouling propensity using dynamic hysteresis. Dynamic hysteresis, which represent the difference between the forces applied to a membrane surface when it is advanced into and withdrawn from a liquid, is the tool for measuring the distribution of surface charge on membrane surface. The results obtained from this method can be indicated fouling propensity. 

Dynamic hysteresis and chemical surface heterogeneity

The zeta potential and dynamic hysteresis of the four membranes were determined. Fig. 1 shows that the SW-30HR membrane reveals a negative charge throughout the whole pH range, whereas the other three membranes have an isoelectric point between pH 4 and 5. 



FIGURE 1. Membrane zeta potential as a function of solution pH. Experiments were carried out with 10 mM KCl as a background electrolyte and temperature of 25 °C.

Fig. 2 shows that dynamic hysteresis of RO membranes varies with variations in solution pH. This is due to the different of heterogeneity of the functional groups on the membrane surface. The dynamic hysteresis of SW-30HR is relatively low and uniform because its surface is largely composed of acidic functional groups. Additionally, the dynamic hysteresis values decrease with increasing solution pH. 



FIGURE 2. Variation of the dynamic hysteresis of RO membranes with respect to solution pH: (a) SW-30HR, (b) RE-8040, (c) SWC-5, and (d) TM-820.

Fig. 3 illustrates the distribution of charge on membrane surface with respect to variation of solution pH. They found that the number of negatively charged functional groups across the membrane surface was proportional to solution pH. At low pH values, the membrane surface is partially uncharged. However, when the pH increases the uncharged acidic functional groups deprotonate and become negatively charged resulting in uniform of membrane surface.




FIGURE 3. Conceptual description of the changes in charge distribution of a homogeneous surface with respect to solution pH. This description corresponds to the SW-30HR membrane, which exhibits no isoelectric point as shown in Fig. 1.

In case of the other membranes, they displayed different behavior. Their dynamic hysteresis values were fluctuated near the isoelectric point due to the chemical properties of the membranes. Therefore, positively charged functional groups deprotonate when the isoelectric point of the membrane is approached, and then imparting to the membrane surface a net neutral charge (Fig. 4). 



FIGURE 4. Conceptual description of the changes in charge distribution of a heterogeneous surface with respect to solution pH. This description corresponds to the RE-8040, SWC-5, and TM-820 membranes, which exhibit isoelectric points as shown in Fig. 1.

Relating dynamic hysteresis to membrane fouling

Fouling experiments with alginate as were performed to examine a potential of using dynamic hysteresis for fouling propensity prediction. The results were presented in Fig. 5, the SWC-5 membrane showed the highest flux reduction followed by the TM-820, RE 8040, and SW-30HR membranes. This indicates that the initial flux decline is directly related to the dynamic hysteresis. 



FIGURE 5. Flux reduction of RO membranes determined from bench-scale NOM fouling runs. The percent flux reduction was measured 30 min after the initiation of the fouling run with 500 mg/L alginate. Other experimental conditions were: initial flux of 15 m/s, crossflow velocity of 8.5 cm/s, background electrolyte of 10 mM NaCl, and temperature of 20 ºC.

Furthermore, the relation between the flux reduction and dynamic hysteresis, as well as the relation between the flux reduction and zeta potential showed Fig. 6. It can be seen that the dynamic hysteresis has a stronger correlation with the flux decline than the membrane zeta potential. Since the zeta potential only shows the average of the membrane surface charge, not the distribution, a membrane with high zeta potential but heterogeneous charge will be more easily fouled compared to a membrane with a uniform charge distribution. 




FIGURE 6. Relating flux reduction to (a) dynamic hysteresis and (b) zeta potential of RO membranes. Regression coefficient (R2) is included.

According to this study, it is clearly confirmed that dynamic hysteresis can be an optional tool for characterizing membrane surfaces and assessing membrane fouling propensity.



Application & further study: An application of dynamic hysteresis is very useful for membrane surface characterizations as well as fouling propensity evaluation. Therefore, this method can be an option tool for membrane surface study.

By Monruedee Moonkhum
Email: moon@gist.ac.kr