NSF-BSF: Selective Transport of Divalent Cations through Polymeric Membranes Using Host-Guest Chemistry

利用海水、微咸水和城市污水等非常规水源生产饮用水，对于缓解全球水资源短缺至关重要。聚合物膜，如薄膜复合反渗透（RO）膜，自20世纪80年代初问世以来，一直处于水净化和海水淡化工艺的前沿。虽然反渗透系统是节能的，并且消耗的能量仅比脱盐的实际最小理论能量多~25%，但反渗透膜容易受到硫酸盐和二价钙、镁或钡等结垢离子引起的无机结垢。具有高浓度这些离子和/或需要高回收率的水源水，例如在内陆海水淡化中，在膜上产生无机垢的倾向特别高。众所周知，无机结垢会大大降低膜的水通量，限制膜的寿命，增加处理成本，并增加膜过程的能耗。膜结垢的经济和环境影响导致了各种缓解方法，包括调整溶液pH和添加聚合物抗结垢剂来阻止晶体生长位点。然而，现有技术的主要限制是它们需要添加化学品，例如聚合物或强酸/碱，这些化学品对环境不友好且成本高昂。与Ben-Gurion大学的研究人员合作，该项目将解决迫在眉睫的需求，一种替代的、无化学物质的方法来选择性地去除形成结垢的离子，以减轻反渗透膜表面的结垢，并提高海水淡化过程的经济性。该研究的总体目标是将生物通道的选择性机制转化为聚合物膜，以便在连续电渗析过程中选择性地去除结垢物质。研究人员假设，分子结合位点可以选择性地从离子中去除水壳（如在某些生物通道中所见），从而实现高度特异性的吸附和通过膜的运输。为了验证这一假设，将通过使用含有对目标离子具有高化学亲和力的垂坠基团的聚合物修饰传统膜的表面来创建选择性膜（任务1）。这些官能团有望产生前所未有的选择性，因为它们提供了有利的主客体配合物来选择性地去除目标离子的水壳（任务2）。然后，这个功能原型将用于阐明具有主客体化学的膜的选择性机制（任务3），以及评估这些膜的结构特性与其选择性运输之间的关系（任务4）。最后，从任务1-4中获得的见解将用于开发一种均质膜（任务5），然后将在电渗析中进行测试，以选择性去除结垢离子（任务6）。该项目的具体目标包括：(1)研究离子对化学定制聚合物的亲和力在实现选择性运输中的作用；(2)评估固有膜结构特性如何影响溶质运输和选择性；(3)利用电渗析制造坚固的膜以减少结垢电位。该项目的成果将是一种新的膜技术，能够在海水淡化之前去除形成结垢的离子，以减轻反渗透膜上的结垢。这项技术将是第一个连续分离二价离子和单价离子的方法，不需要定期使用化学品，克服了现有方法的局限性。本研究还将通过应用过渡态理论从熵和焓的角度来描述溶质输运现象，从而促进对选择性输运过程的基本理解。这些见解，以及从本研究中建立的设计原则，也将与其他溶质的分离有关，以后可能会在回收有价值的资源或从水中去除污染物方面得到应用。本研究由美国国家科学基金会和美以两国科学基金会通过特殊提交机会NSF 20-094共同资助。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Producing drinking water from unconventional water sources, such as seawater, brackish water, and municipal wastewater effluent, is crucial for alleviating global water scarcity. Polymeric membranes, such as thin-film composite reverse osmosis (RO) membranes, have been at the forefront of water purification and desalination processes since their advent in the early 1980s. While RO systems are energy efficient and consume only ~25% more than the practical minimum theoretical energy of desalination, RO membranes are susceptible to inorganic scaling caused by scale-forming ions such as sulfate and divalent calcium, magnesium, or barium. Source waters that have high concentrations of these ions and/or require high recovery rates, such as in inland desalination, have an exceptionally high propensity to produce inorganic scale on membranes. Inorganic scaling is known to drastically lower membrane water flux, limit membrane lifetime, increase treatment costs, and increase the energy consumption of membrane processes. Economic and environmental effects of membrane scaling have led to various mitigation approaches, including adjusting solution pH and adding polymeric antiscalants to block crystal growth sites. However, the primary limitation of established techniques is they require the addition of chemicals, such as polymers or strong acids/bases, that are environmentally unfriendly and costly. In collaboration with researchers at Ben-Gurion University, this project will address the imminent need for an alternative, chemical-free method to selectively remove scale-forming ions to mitigate scaling on RO membrane surfaces and improve the economics of desalination processes.The overall goal of the research is to translate selectivity mechanisms of biological channels into polymeric membranes for the purpose of selectively removing scale-forming species in a continuous electrodialysis process. The investigators hypothesize that molecular binding sites that can selectively remove water shells from ions (as seen in some biological channels) will enable highly specific adsorption and transport through membranes. To test this hypothesis, a selective membrane will be created by modifying the surface of conventional membranes with polymers comprising pendant groups with a high chemical affinity for target ions (Task 1). These functional groups are expected to yield unprecedented selectivity because they provide favorable host-guest complexes to selectively remove water shells of target ions (Task 2). This functional prototype will then be used to elucidate selectivity mechanisms for membranes with host-guest chemistry (Task 3), as well as to assess the relationship between the structural properties of those membranes and their selective transport (Task 4). Finally, the insights from Tasks 1-4 will be used to develop a homogenous membrane (Task 5), which will then be tested in electrodialysis for selective removal of scale-forming ions (Task 6). The specific objectives of the project include: (i) investigating the role of ion affinity to chemically tailored polymers in achieving selective transport, (ii) assessing how intrinsic membrane structural properties affect solute transport and selectivity, and (iii) fabricating robust membranes to reduce scaling potential using electrodialysis. The outcome of the project will be a new membrane technology capable of removing scale-forming ions prior to desalination to mitigate scaling on RO membranes. This technology would be the first continuous approach for separating divalent ions from monovalent ions without requiring periodic use of chemicals, overcoming the limitations of existing approaches. This study will also advance the fundamental understanding of selective transport processes by applying transition-state theory to describe solute transport phenomena in terms of entropy and enthalpy. These insights, along with design principles established from this study, will be relevant for separations of other solutes as well, which may later find application in reclaiming valuable resources or removing contaminants of concern from water. This research is jointly funded by NSF and The US-Israel Binational Science foundation through the special submission opportunity NSF 20-094.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.