Shefer Idit, Lopez Kian, Straub Anthony P, Epsztein Razi
Faculty of Civil and Environmental Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel.
Department of Civil, Environmental and Architectural Engineering, University of Colorado Boulder, Boulder, Colorado 80309-0428, United States.
Environ Sci Technol. 2022 Jun 21;56(12):7467-7483. doi: 10.1021/acs.est.2c00912. Epub 2022 May 13.
Membrane technologies using reverse osmosis (RO) and nanofiltration (NF) have been widely implemented in water purification and desalination processes. Separation between species at the molecular level is achievable in RO and NF membranes due to a complex and poorly understood combination of transport mechanisms that have attracted the attention of researchers within and beyond the membrane community for many years. Minimizing existing knowledge gaps in transport through these membranes can improve the sustainability of current water-treatment processes and expand the use of RO and NF membranes to other applications that require high selectivity between species. Since its establishment in 1949, and with growing popularity in recent years, Eyring's transition-state theory (TST) for transmembrane permeation has been applied in numerous studies to mechanistically explore molecular transport in membranes including RO and NF. In this review, we critically assess TST applied to transmembrane permeation in salt-rejecting membranes, focusing on mechanistic insights into transport under confinement that can be gained from this framework and the key limitations associated with the method. We first demonstrate and discuss the limited ability of the commonly used solution-diffusion model to mechanistically explain transport and selectivity trends observed in RO and NF membranes. Next, we review important milestones in the development of TST, introduce its underlying principles and equations, and establish the connection to transmembrane permeation with a focus on molecular-level enthalpic and entropic barriers that govern water and solute transport under confinement. We then critically review the application of TST to explore transport in RO and NF membranes, analyzing trends in measured enthalpic and entropic barriers and synthesizing new data to highlight important phenomena associated with the temperature-dependent measurement of the activation parameters. We also discuss major limitations of the experimental application of TST and propose specific solutions to minimize the uncertainties surrounding the current approach. We conclude with identifying future research needs to enhance the implementation and maximize the benefit of TST application to transmembrane permeation.
使用反渗透（RO）和纳滤（NF）的膜技术已广泛应用于水净化和海水淡化过程。由于存在复杂且尚未完全理解的传输机制组合，RO和NF膜能够在分子水平上实现不同物质的分离，多年来一直吸引着膜领域内外研究人员的关注。减少这些膜传输方面现有知识的差距，可提高当前水处理工艺的可持续性，并将RO和NF膜的应用扩展到其他需要对不同物质具有高选择性的领域。自1949年建立以来，随着近年来其日益普及，用于跨膜渗透的艾林过渡态理论（TST）已在众多研究中得到应用，以从机理上探索包括RO和NF在内的膜中的分子传输。在本综述中，我们批判性地评估了应用于拒盐膜跨膜渗透的TST，重点关注从该框架中可获得的关于受限传输的机理见解以及与该方法相关的关键局限性。我们首先展示并讨论常用的溶液扩散模型在从机理上解释RO和NF膜中观察到的传输和选择性趋势方面的有限能力。接下来，我们回顾TST发展中的重要里程碑，介绍其基本原理和方程，并建立与跨膜渗透的联系，重点关注在受限条件下控制水和溶质传输的分子水平焓垒和熵垒。然后，我们批判性地回顾TST在探索RO和NF膜传输方面的应用，分析测量的焓垒和熵垒趋势，并综合新数据以突出与活化参数的温度依赖性测量相关的重要现象。我们还讨论了TST实验应用的主要局限性，并提出了具体解决方案，以尽量减少围绕当前方法的不确定性。我们最后确定了未来的研究需求，以加强TST在跨膜渗透中的应用并最大化其益处。
