on Negatively Charged Membrane Surface

Proton arrangement on the membrane surface due to electrical gradient between cation and negatively charged functional groups of the membrane ---p. Conceptual delineation of the interference of carbonic acid series on the membrane surface considering the viscosity B coefficient ---p.

Membrane]

Mass Transport Principle on Membrane for Water Treatment

Where J is the flux of water [L3/L2·T], D is the diffusion constant [L2/T] and δ is the thickness of the boundary layer [L], Cm is the concentration at the membrane surface, Cp is the concentration of permeation, and Cb is the concentration of bulk solution. In other words, in Equation 5, convective mass transport (mass movement both by bulk movement and by diffusion of individual particles) is the primary cause of solute flux.

Figure 1. The conceptual diagram of concentration gradient across a membrane

Donnan Exclusion Equilibrium

Mauro (1962) recommended referring to the particle-particle Coulomb interaction instead of the Poisson equation. 12] In fact, however, the restriction to apply the equation does not come from the Poisson equation, since the thermal term is not derived from the Poisson equation but from the Maxwell-Boltzmann distribution.

Figure 2. The conceptual representation of the Donnan potential distribution on membrane surface:

Double Layer Theory

The assumption is that “if the thermal term is much larger than the electrostatic term (𝐹𝜓0−≪ 𝑅𝑇)”, it is the same thing that Mauro (1962) mentioned. In the same way that Poisson's equation was used, equation 18 is derived and the constant here is directly related to the Debye length (1 κ⁄. The result shows that the surface potential (𝜓0−) is a natural constant (e) times greater than the potential at the Debye length distance (𝜓1 κ−⁄.

The position of the shear plane is controversial, but is generally accepted to be closer to the charged surface than the Debye length and the measuring point of the zeta potential (ζ). 22] Moreover, hydration is not the intrinsic characteristic, but a function of the mutual interaction of solutes, which is the result of the dynamics of the composition to attract water molecules. Therefore, the size of the hydrated cation varies depending on the charge density of the cation and the attractiveness of the anion in a solution.

Figure 3. The electric potential decrease at the Debye length from the negatively charged membrane

Thesis Scope & Outline]

Research Objectives

As can be seen from the above, a membrane water treatment system is governed by three basic driving forces; concentration gradient, pressure gradient and electrical gradient. In particular, the nanoporous NF and RO membranes used for ion removal are strongly affected by the electrical gradient, rather than the mesoporous MF and UF membranes. An interesting point is that the cation rejection ratio on the negatively charged NF and RO membrane is proportional to a certain order.

It is concluded that the unexplored barrier hinders the diffusion of cations near the membrane surface. This study was conducted to investigate the cation rejection mechanism from the perspective of electrical dynamics in the aqueous system.

Structure of the dissertation

Method & Materials]

Surface Area Determination of RO membrane

The term membrane TFN was first introduced by Hoek and coworkers (Jeong et al., 2007). They verified the improved performance of the TFN membrane in terms of its water permeability and negative charge density compared to the existing thin film composite (TFC) membrane. Zeolite nanoparticles contribute greatly to the increased negative charge density through their negative charge.

29-32] To study the electrical interaction between membrane and cation, the TFN membrane is selected, which has electrical-apparent aptitude. 34-36] During measurement trials, the fact was verified that a waterproofing agent (a waterproofing agent) coated on a virgin membrane reacts with nitrogen gas in the high-pressure condition. To compensate for the defect, virgin membrane was soaked in Milli-Q for 30 hours to eliminate the humectant on its surface and then it was freeze-dried for a day due to the fact that the membrane can be deformed by high pretreatment temperature at BET analysis.

Membrane Module Test

33] Thus, we tried to measure with alternative methods, which are computational procedures named as BJH (Barrett-Joyner-Halenda) and D-H (Dollimore and Heal) desorption models. 35] Since the freeze-drying process evaporated the moisture in the membrane, the pretreatment temperature was set at 50℃. The retentate flow rate was set to maintain 1 LPM because it can critically affect the rejection capability of the membrane.

The amount of diffusion caused by solute shear increases with increasing Reynolds number. However, after filtering it with 0.45 μm filters (), the final concentration data of the food solution from IC () showed variation from the target results due to solubility. The solution was gradually poured into the reservoir containing the Milli-Q for compression while the holding current was being wasted.

Figure 6. Process flow diagram (PFD) of the laboratory scale module for membrane performance test

Potentiometric Titration (Acidimetry)

Potentiometric titrations were performed with an automatic titrator (702 SM Titrino, Metrohm, Herizau, Switzerland) connected to a thermal printer (LK-D10, Sewoo, Korea). Electrolyte solutions for blank titrations were sealed and purged with nitrogen gas during sample titrations to minimize interference from other ions such as the carbonic acid array. The concept of the above equation is used to measure the surface charge density of a material.

All carbonic acid can be exchanged for bicarbonate and carbonate ions after it is dissolved, because the initial pH level of the acidimetry was approximately 10.0. The acid dissociation constant of the carbonic acid series varies with the salinity state of the solution [40]. With the same method, potentiometric titrations were repeated with two types of electrolytes containing carbonic acids; sodium chloride (trace metal base, 99.999%, Aldrich) and barium chloride (trace metal base, 99.9%, Aldrich).

Figure 8. Snapshots of potentiometric titration; Note that, plastic flask containing an electrolyte solution should be sealed with Para film and purged with high purity nitrogen gas during the

Result and Discussion]

Surface Area of RO membrane

The RO membrane has a specific surface area much larger than the apparent surface area due to the nanometer pores. Therefore, a measurement of the specific surface should be preceded in order not to overestimate the results of the surface analysis. A multipoint BET estimate shows a large difference of over 40 percent from the BJH and D-H methods while there is little gap between the BJH and D-H desorption estimates below 2%.

It is reasonable to estimate the specific surface area (𝐴𝑠) as approximately 24 m2/g as multipoint BET evaluation figures have been reported to be far from theoretical approaches and design membrane fabrication values.

Cation Selectivity Priority

Rejection priority of cation on the negatively charged RO membrane (TFN); the amount of sodium permeation (rejection ratio) is clearly greater (smaller) than that of lithium permeation while a. The ranked priority shows two certain orders that it is proportional to the valence of cation and inversely proportional to ionic size. As mentioned earlier, the hydrated radii are inappropriate to account for the permeability difference, as this is an extrinsic property that varies under the state of electrolytes and pH of solutions.

23,24] Charge density, meaning valence per unit size, has a reliable proxy as one of the intrinsic properties of cations. Some researches claimed that, instead of monovalent cations, divalent cations can be brought close to the membrane surface by electrostatic attraction with the negative charge, based on an indirect sign - zeta potential variations. 41] However, if so, divalent cations entering adjacent to the membrane surface should easily cross the membrane as long as diffusion is the main mechanism of ion permeation in the NF as well as RO membrane.

Figure 9. Rejection priority of cation on the negatively charged RO membrane (TFN); the amount of sodium permeation (rejection ratio) is clearly larger (smaller) than that of lithium permeation while a

All electrolytes have a similar tendency, along with the increase in the amount of protons in the bulk solution (i.e. 10−𝑝𝐻). Consequently, the proton closest to the membrane surface can reach the activation energy faster in the case of the calcium ion, even if the proton concentration of the bulk solution (pH) is relatively insufficient compared to the others. The presence of the clustered protons on a negatively charged membrane surface is also verified from several other studies.

It is one of the important proofs that the salt rejection can be improved due to the clustered protons, the amount of which basically depends on the amount of protons in the bulk solution (10−𝑝𝐻). The only difference from the nitrogen purge cases is the presence of the dissolved carbonic acid series. Conceptual delineation of the interference of carbonic acid series on the membrane surface considering the viscosity B coefficient.

Figure 10. Different propensity of potentiometric titration due to water-proofing agent on the virgin membrane (TFN RO membrane in 0.06 M NaCl electrolyte solution)

Conclusion]

Conclusion

This study shows that the amount of protons on the negatively charged membrane surface determines the rejection ratio of cation, and the proton amount depends on the charge density of cation. Looking at the theory development process, it is assumed that the electric potential decrease, expressed numerically along the distance, may be related to the arrangement of the clustered protons. All the aforementioned results imply that the transport mechanism of cations across the diffusion dominant membranes (i.e. NF and RO) is influenced by the protons clustered near the membrane surface, as well as show possibilities to develop a new membrane filtration system.

First, certain anions can be an important factor to consider and control when designing a membrane process. The anionic composition of the feed water can affect cation rejection, as certain anions can affect the collected protons on the membrane surface. Based on the same principle, the newly developed TFN membrane synthesized with highly charged zeolite nanoparticles is expected to have higher salt rejection (both cations and anions) than the TFC membrane due to the increased charge density.

The importance of the hydrated beam and hydration shells on ion permeability during nanofiltration in dead-end and cross-flow modes. Importance of thermodynamic and physical characteristics in ion permeation during membrane separation: Hydrated radius, hydration free energy and viscous effects. Ions from the Hofmeister series and osmolytes: effects on proteins in solution and on the crystallization process.

A review of polyamide thin-film nanocomposite (TFN) membranes: history, applications, challenges and approaches, water research. Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes. Contamination of the reverse osmosis membrane by protein (BSA): Effects of pH, calcium, magnesium, ionic strength and temperature.

Acknowledgement