壓力驅動的奈米過濾系統：以電輔助薄膜及溫度梯度下的聚電解質多層薄膜進行離子分離的研究

Abstract

奈米過濾技術近幾十年來被廣泛應用於水處理和溶液分離等多個領域。是一種壓力驅動的薄膜分離技術，其孔徑介於反滲透和超濾之間，有效地從水中分離離子、有機分子和粒子。 聚電解質多層膜是一種高度定制化的薄膜製備技術，利用正負電荷的多層聚電解質來控制薄膜的選擇性和滲透性。這些薄膜通常用於分離具有大顆粒或分子以及多價離子的溶液。在第一章中，我們考慮了各種因素例如孔徑、進料濃度和薄膜兩側溫差的影響，研究聚電解質多層膜對於溶液中各離子的脫鹽率、鈉離子和鎂離子之間的選擇性，圓柱狀奈米孔道的體積通量。我們發現小孔徑能夠提高選擇性。然而，在固定的施加壓降下增加雙層數量（即聚電解質多層膜的厚度）卻不能顯著提高選擇性，因為此時奈米孔道的體積通量會降低。除此之外，稀薄的溶液使奈米孔道具有更厚的電雙層，能夠產生較好的選擇性和體積通量。另外，為了降低能耗，我們也在薄膜兩側施加溫度梯度提供熱驅動力，從而減少所需的施加壓降，其中溫度梯度可以顯著提高選擇性和體積通量，但具體效果取決於梯度的方向。 傳統奈米過濾薄膜要在不降低通透率的情況下實現高離子脫鹽率存在一定的困難。在第二章中，我們嘗試通過電輔助的方法增強奈米孔道的過濾性能，其中導電材料在可極化薄膜中充當虛擬陰極。我們通過增加奈米孔道的表面電荷密度，強化離子與奈米孔壁之間的電荷交互作用來克服以往必須在增加脫鹽率和體積通量之間作取捨的困境。然而，與薄膜官能基團中解離出的極化電荷相比，電輔助的方法表現出一些反效果，例如當施加壓力降不夠大但足夠高的額外偏壓時，電極之間因電位差而產生的電場會影響到施加偏壓的驅動力，導致奈米孔道的體積通量略微降低。在本研究中，陰極的位置、陽極到奈米孔壁的距離以及薄膜的介電常數等皆是決定奈米過濾效率的主要因素。此外，我們也證明了此系統能在相對濃度較高的氯化鈉溶液中保持一定水平的脫鹽率和體積通量，同時不需要施加過大的壓力降和額外偏壓作為驅動力，顯示了奈米過濾技術在海水淡化和水處理方面仍具有很大的發展潛力。

Nanofiltration (NF) has been widely utilized in the past few decades in versatile applications such as water treatment and solution separation. It is a pressure-driven membrane separation technology having a pore size between reverse osmosis (RO) and ultrafiltration, effectively separating ions, organic molecules, and particles from water. Polyelectrolyte multilayer membranes (PEMMs) are highly customized membrane technologies that utilize layers of positively and negatively charged polyelectrolytes to control membrane selectivity and permeability. These membranes are commonly used to separate components of solutions with large size particles/molecules and multivalent ions. In Chapter 1, the individual ion rejection rate, the selectivity between Na^+ and Mg^(2+) (S(Na^+/Mg^(2+) )) and the volumetric flux (vf) of PEMMs, which is considered by an effective cylindrical nanopore, is investigated theoretically with consideration of the effects from various factors, e.g., pore size, feed concentration, and a temperature difference applied across the membrane. Due to electrostatic interactions between ions and nanopore surface, the results show that small pore radius leads to greater S(Na^+/Mg^(2+) ). Increasing the number of bilayers (and PEMM thickness) under fixed ΔP cannot significantly improve S(Na^+/Mg^(2+) ) owing to the decline in the vf. Dilute possesses thick electric double layer, leading to high S(Na^+/Mg^(2+) ) and high vf. To lower energy consumption, a temperature gradient can be applied across the membrane to provide thermal driving force and thus reduce the required ΔP. Temperature gradients can appreciably raise S(Na^+/Mg^(2+) ) or vf, depending on the direction of the gradient. In addition, achieving a high ion rejection rate in conventional NF membranes without compromising permeability poses significant challenges. In Chapter 2, we investigate theoretical enhancements in the NF performance of a cylindrical nanopore through electrically assisted method, where conductive materials within a polarizable membrane act as the virtual cathode. The approach focuses on increasing the charge interaction between ions and nanopore walls by augmenting surface charge density, thereby utilizing high charge density to mitigate the trade-off between rejection rate and volumetric flux (U_P). Unfortunately, compared to polarizable charge dissociated from membrane functional groups, the electrical assistance system exhibits some counterproductive effects. For instance, when pressure drop ΔP is not sufficiently large but high external voltage ΔV is applied, there are interactions between electric body force generated by the potential difference from the electrodes and pressure-driven force, leading to a slightly lower U_P. Factors such as the position of anode, radial distance from cathode to nanopores, and membrane dielectric constant significantly enhance NF efficiency in this study. Moreover, the system demonstrates its ability to maintain a certain level of rejection rate and U_P for relatively concentrated NaCl solutions while simultaneously increasing ΔP and ΔV without excessively large values. These findings suggest promising applications in desalination and water treatment using NF technology.