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

利用海水、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。自20世纪80年代初问世以来，聚合物膜，如薄膜复合反渗透(RO)膜，一直处于水净化和海水淡化过程的前沿。虽然反渗透系统是节能的，并且只比实际的海水淡化最低理论能量高出约25%，但反渗透膜容易受到硫酸盐和二价钙、镁或钡等结垢离子造成的无机结垢的影响。具有高浓度这些离子和/或需要高回收率的源水，例如内陆海水淡化，极有可能在膜上产生无机垢。众所周知，无机结垢会显著降低膜的水通量，限制膜的寿命，增加处理成本，并增加膜过程的能源消耗。膜结垢的经济和环境效应导致了各种缓解方法，包括调节溶液pH和添加聚合物阻垢剂来阻止晶体生长位置。然而，现有技术的主要局限性是它们需要添加不环保且成本高昂的化学品，如聚合物或强酸/碱。与本-古里安大学的研究人员合作，该项目将解决对选择性去除结垢离子的迫切需求，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理转化为聚合物膜，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，能够选择性地从离子中去除水壳的分子结合部位(如在一些生物通道中看到的)将使高度特异的吸附和通过膜的运输成为可能。为了验证这一假设，将通过用含有对目标离子具有高化学亲和力的侧基的聚合物来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络合物来选择性地去除目标离子的水壳(任务2)。这一功能原型随后将用于阐明具有主客体化学的膜的选择性机理(任务3)，以及评估这些膜的结构特性与其选择性传输之间的关系(任务4)。最后，任务1-4的见解将用于开发均质膜(任务5)，然后将在电渗析中进行测试，以选择性地去除结垢离子(任务6)。该项目的具体目标包括：(I)研究离子与化学定制聚合物的亲和力在实现选择性传输中的作用，(Ii)评估膜的固有结构属性如何影响溶质传输和选择性，以及(Iii)利用电渗析制造坚固的膜以降低结垢电位。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结垢离子，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，而不需要定期使用化学品，克服了现有方法的局限性。通过应用过渡态理论从熵和热焓的角度描述溶质迁移现象，也有助于加深对选择性迁移过程的基本认识。这些见解，以及从这项研究建立的设计原则，也将与其他溶质的分离相关，这可能会在以后应用于回收宝贵的资源或从水中去除令人担忧的污染物。这项研究由NSF和美国-以色列两国科学基金会通过特别提交机会NSF 20-094共同资助。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.