II. Chapter 2. Background and literature review
2.3 Polyamide (PA) membrane
2.3.3 Characterizations of polyamide membrane
2.3.3.5 Zeta potential (ζ)
Polymeric filtration membranes (e.g., polyamide membrane) or ion-exchange membranes naturally are ionizable characteristics at the membrane surface. Consequently, the sign (negative/positive) and amount of charge on membrane surface depends on condition of aqueous feed solutions such as ionic concentration and solution’s pH. Feed solutions are normally complex mixtures which contains charge carrying substances such as surfactants, polyelectrolytes, ions, and macromolecules. The membrane’s surface charge can be changed by complexation/interaction between charge carrying chemicals and the membrane surface. Because the complexation/interaction between membrane surfaces and these species in aqueous phase determines the membrane’s surface charge, measurement of the zeta potential of the membrane is important parameter to explain membrane performances or fouling phenomenon.
The structure, charged species which near to the surface of membrane, is known as the electrical double layer (EDL) [56]. Charge spreading of the EDL is most concentrated at closest to the surface of membrane, and its potential decreases in accordance with increasing distance from the surface of membrane as shown in Fig. 2.15. Ions near to the surface of membrane are existed in the immobile Stern layer. This layer can be subdivided into the inner Helmholtz layer: (1) the layer containing partly dehydrated ions attached to the membrane surface electrostatically between the surface of membrane and the inner Helmholtz plane (IHP in Fig. 2.15), and (2) the outer Helmholtz layer containing fully hydrated ions which are counter-ions to the inner Helmholtz ions between the inner and outer Helmholtz planes (OHP in Fig. 2.15). In the immobile Stern layer, the charge spreading and electrical potential are measured by interactions between ions/dipoles and the surface of membrane. Ions can freely move by thermal driven motion where beyond the immobile Stern layer which is called by the diffuse Gouy- Chapman layer.
There are various methods to measure electrokinetic properties such as streaming potential, electro-osmosis, electrophoresis, and sedimentation potential. Because this work was conducted by electrophoresis method, only electrophoresis method is briefly summarized as follows: The ζ potential values of the membrane surface is measured by electrophoresis method using a quartz cell. When electric field exists in the electrophoresis chamber, an asymmetric electroosmotic flow happens through the membrane surface. Because of the amassment of the cations from the electrolyte solution, this electroosmotic flow allows to move the monitoring particles on the membrane surface. Then, the ζ potential values on the membrane surface can be calculated using the Smoluchowski equation (2-4) which uses electrophoretic mobility of the monitoring particles [70].
28 ζ =4πηU
εrε0 (2 − 4)
where ζ is the ζ potential measured by electrophoresis method (mV), η is the viscosity of liquid medium (0.89× 10−3 Pa s), U is the monitoring particle’s electrophoretic mobility (cm2 V−1 s−1), εr and ε0 are permittivity of the liquid medium (78.38) and vacuum (8.854 × 10−12 s m−1), respectively.
Figure. 2.15 Schematic illustration for charge spreading of the ionic species which near to the membrane surface (the electrical double layer) and the resultant zeta potential with increasing distance from the membrane surface [56].
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Surface charge of membrane affects not only rejection of the charged organic/inorganic matters by the membrane but also different fouling tendency in the pressure-driven process.
Firstly, electrostatic interactions between charged organic solutes and a surface charge of membrane have often been published to be a rejection mechanism significantly [71]. Hu et al. studied that charged organics had higher rejections than neutral organics by negative charge of membrane surface because of electrostatic repulsion between charged of solute molecules and membrane surface [72]. The membrane surface of RO, NF and UF membranes frequently has a negative charge to increase the rejection of negatively charged natural organic matters by donnan exclusion effect [73]. Negative surface charge of the membranes are increased by increasing the pH of feed solutions due to a dissociation of carboxylic and sulfonic functional groups on membrane surface [44, 74], resulting increase rejection of negatively charged organic/inorganic matters.
Secondly, the electrostatic charge of membrane surface is a mainly important consideration for the decrease of membrane fouling when feed solution contains charged foulants. When surface of the membrane and the foulant have same charge, electrostatic repulsion forces between surface of the membrane and the foulants prevent the foulant deposition on the membrane surface thereby reducing the fouling [75]. For example, humic materials in aquatic environments are took into account to be the major element of natural organic matter (NOM), are refractory anionic macromolecules with low/moderate molecular weight [75]. Humic material contains both aromatic and aliphatic components with primarily carboxylic/phenolic functional groups [75]. Consequently, humic materials mostly are negatively charged in the pH range of natural waters [76]. Mika et al. showed that at pH 4–5 the membrane (NTR-7450 from Nitto Denko and Desal-5 from Osmonics/Desal company) has slightly negative charge and humic acid are almost uncharged thereby promoted fouling at pH 4-5 condition [77]. To solve this issue, polyvinyl alcohol (PVA) coated the membrane with highly negatively charged may show stable flux because of the strong electrostatic repulsion between membrane surface and negatively charged NOM [50]. Therefore, measurement of zeta potential is important to investigate effect of surface charge on membrane performance and fouling phenomenon.
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