研析多孔導體介質驅動過電流下離子分離機制與脫鹽績效

Abstract

在於全球水資源短缺與資源匱乏危機下，發展循環技術以從廢水中回收有價值資源，已成為備受關注議題；電力驅動分離技術相較於傳統分離技術，具潛力以較低能耗直接從水中去除離子和微量污染物，同時可減少化學品使用和二次廢棄物產生，且不僅可淨化水質，亦可從廢水中回收寶貴資源，符合循環水經濟之概念。其中，可操作於過電流（Overlimiting Current，簡稱OLC）條件下之電動力分離技術，例如：震波電透析（Shock Wave Electrodialysis，簡稱SWED）及電去離子（Electrodeionization，簡稱EDI）等，因具可提高程序之能源效率或降低成本等優勢於近年受矚目。有鑑於此，本論文旨在研析多孔導體介質於OLC條件下，對於離子分離機制與脫鹽績效之影響，研究目的包括：(1) 建立結合理論機制利用COMSOL Multiphysics數值模擬，研析多孔導體材料驅動之過電流離子分離之機制，例如震波形成、離子傳輸行為與過電流分離效能之關鍵機制；(2) 建立並操作可模組化之 SWED 系統，包含進行實驗室規模過電流驅動離子分離模組設計與關鍵孔洞帶電材料製備，以探討不同操作條件與幾何設計對於實驗回收表現的影響與分離關鍵績效指標之關係，並鑑別關鍵操作參數；(3) 建立並驗證離子交換樹脂複合材料產生過電流之機制性物理模型，量化不同傳輸相（例如：陽離子樹脂、陰離子樹脂、溶液）對整體導電度之貢獻，提供設計電透析與電去離子（EDI）材料之依據。具體研究結果包括： 1. 建立震波電透析技術之質量傳輸模型 本研究使用 COMSOL Multiphysics 建構二維多單元之震波電透析模型，模擬震波形成、解析離子傳輸機制，並在不同操作條件下評估其分離效能。該模型整合 Nernst-Planck 模式、Darcy 定律與一階電滲流（Electroosmosis）理論，以描述 SWED 單元內部的離子濃度分佈、離子通量、電流分佈與流體速度等參數。藉由濃度、速度與電位分佈的等高線圖，清楚描繪各種傳輸現象，有助於掌握最佳操作條件，並提供可擴展系統設計之依據。模擬結果分析顯示，表面電荷密度、施加電壓及流速為影響震波發展與分離效率的關鍵參數；此外，進一步提出兩項關鍵指標， 「震波高度」與「震波長度」，以描述脫鹽區域之發展狀態，作為評估震波電透析系統效能之定量依據，並可用為預測脫鹽去除率。 2. 開發可模組化之震波電透析技術及其脫鹽效能評估 本研究提出首個具有多單元的可擴展模組化 SWED 裝置，該系統擁有 4000 mm² 之有效膜面積及 2.5 mL/min 之流速，處理能力為既有文獻中裝置之十倍以上。透過本研究實驗驗證，證實在過電流操作區間能顯著提升脫鹽效率，尤其在降低流道中孔洞帶電（矽酸鹽玻璃）間隔層厚度時效果更為顯著。當矽酸鹽間隔層厚度由 1.0 cm 減少至 0.4 cm 時，以兩組厚度為 4 mm 的多孔矽酸鹽玻璃濾材作為通道配置，結果顯示可連續操作於脫鹽率維持在 90%以上，能耗約為 45 kWh/m³，對應電流效率約為 75%。本研究證實 SWED 系統可於實際較大規模水處理及資源回收領域之可行性與能源效率，為未來推動 SWED 技術示範之重要參考依據。 3. 建立離子導體材料於電透析技術之離子傳輸模型及機制 本研究提出一基於體積比例之離子導體材料（Ionically Conductive Materials，簡稱ICMs）離子傳輸物理模型，描述離子於離子交換樹脂（Ion Exchange Resin，簡稱IER）複合材料中之傳輸行為。此模型架構於體積比例加權，可成功預測陽離子、陰離子與溶液相在空間中分佈下之比電導，結果顯示：此模型成功預測不同材料組成下之比電導行為（Specific Electrical Conductance），並透過固定型樹脂片（IER Wafer）與傳統鬆散型樹脂（Loose Resin）系統之實驗數據進行驗證，顯示高度符合性與統計可靠性，模型迴歸判定係數（R²）均高於 0.95，本研究證實ICMs材料提升離子傳輸效率之機制，未來可適用於電去離子（EDI）相關系統之模型預測與優化。此外，本研究亦將此 ICMs整合至 SWED 系統，實驗結果顯示在施加 0.12 A、0.25 A 與 0.50 A 之電流下，稀釋側的去除效率穩定維持在約 30%，各電流條件下變化不大；然而，濃縮側的濃度提升則隨電流增加而明顯上升，由 ~15% 增加至超過 50%。該結果顯示，雖然導電性複合多孔材料可引發 OLC 現象，但無法形成典型衝擊波。 綜合以上，本研究深化對於 OLC 機制之理解，同時提供SWED技術設計與放大高效分離系統的實務建議。未來研究方向建議聚焦於導電多孔材料之開發、系統可擴展性評估，以及擴大應用（例如有機溶液分離）技術之探討。綜合以上，本研究促進對 OLC 分離技術機制的認識，並提供具體物理意義解釋，以有助於未來廢水資源化與可擴展電化學分離系統設計之研發與應用突破。

Access to clean water is essential for achieving the United Nations Sustainable Development Goal 6, yet global water scarcity remains a critical challenge. Electrically-driven electrokinetic separation technologies offer energy-efficient, low-carbon alternatives to conventional methods by directly removing ions and trace pollutants while minimizing chemicals use and secondary waste. Embracing circular water economy principles, these technologies enable not only water purification but also the recovery of valuable resources from wastewater, contributing to sustainable water, food, and energy systems. Recent advances, such as shock wave electrodialysis (SWED) and electrodeionization (EDI), have demonstrated potential for enhancing ion transport under overlimiting current (OLC) conditions. The OLC condition is a regime associated with improved energy efficiency, enhanced membrane performance, and greater applicability to complex wastewater streams. The objectives of this research include to (1) construct and simulate two-dimensional Multiphysics models to investigate shock-wave formation, ion transport dynamics, and separation performance under OLC conditions; (2) experimentally validate the model by fabricating and operating multicell SWED systems under varying operational and geometric parameters; and (3) develop and validate a mechanistic conductivity model that describes ion transport within IER-based composite materials, providing predictive insight for material design in electrodialysis and EDI applications. To achieve these goals, this study investigates the development, modeling, and experimental validation of OLC separation technology, with a focus on SWED systems and ionically conductive materials (ICMs). 1. Modeling the Mass Transfer in SWED A two-dimensional multicell SWED model was developed using COMSOL Multiphysics to simulate shock wave formation, analyze ion transport mechanisms, and assess separation performance under varying operational conditions. The model incorporates the Nernst-Planck equation, Darcy’s law, and first-order electroosmosis to capture the local concentration profiles, ionic fluxes, current distribution, and fluid velocity within SWED cells. Contour plots of concentration, velocity, and electric potential are presented to visually illustrate transport phenomena and assist in identifying optimal operating conditions for scalable system design. Key parameters such as surface charge density, applied voltage, and flow velocity were shown to significantly influence shock wave development and, consequently, separation efficiency. Furthermore, two defining characteristics, i.e., shock wave height and length, were proposed as metrics for evaluating performance in cross-flow deionization systems. 2. Evaluating Experimental Performance of Salt Separation by Scalable SWED To complement the simulation results, we experimentally verified the occurrence of extreme salt depletion in the multi-cell SWED system, attributed to OLC and the formation of a deionization shock. We presented the first scalable SWED device with multiple cell pairs, designed for desalination. The system features an active membrane area of 4000 mm² and a flow rate of 2.5 mL/min, offering over ten times the capacity of existing devices. Experimental validation with this scalable SWED stacks confirmed that operating in the OLC regime enhances desalination efficiency, particularly when reducing the thickness of silicate glass spacers. It highlights that a reduction in silica spacer thickness from 1.0 cm to 0.4 cm significantly improves performance, and channel setups can benefit from these effects. For the configuration with 2 cells of 4 mm thickness porous silicate glass frit media, it shows continuous salt rejection exceeding 90% with energy consumption is about 45 kWh/m³, corresponding current efficiency was around 75%. This work represents a significant step toward realizing SWED as a practical, energy-efficient solution for large-scale water treatment and resource recovery. 3. Constructing a Model to Elucidate Ion Transport Mechanisms in ICMs A mechanistic conductivity model was developed to describe ion transport within the ICM composites. The model, based on a volume-fraction-weighted framework, successfully predicted specific conductance by accounting for the spatial distribution of cationic, anionic, and solution phases. Validation against experimental data showed strong agreement with all coefficients of determination (R2) > 0.95, supporting its applicability in electrodialysis and EDI contexts. Furthermore, IER wafers (one type of ICMs) were integrated into the SWED system. The experimental result shows that at applied currents of 0.12 A, 0.25 A, and 0.50 A, the removal efficiency in the dilute stream remains relatively constant at around 30%, with only minor variation across different current conditions, while concentration enrichment increases significantly with current, rising from approximately 15% to over 50% at 0.5 A. It demonstrates that although OLC behavior could also be triggered by composite conductive media, the formation of a shock wave could not be formed. Finally, this study not only deepens the mechanistic understanding of OLC phenomena but also offers practical insights for the design and scaling of advanced separation systems. Future research directions are proposed in the areas of conductive porous media development, system scalability, and application to organic solution separations. In conclusion, this study deepens the mechanistic understanding of OLC separation and provides practical insights for designing scalable, energy-efficient systems for wastewater resource recovery.