聚合物反渗透膜中污染物的截留机理
Rejection mechanisms for contaminants in polymeric reverse osmosis membranes
摘要：
尽管反渗透（RO）水净化的成功，污染物排斥的分子水平的物理化学过程没有很好地理解。在这里，我们进行NEMD模拟模型聚酰胺反渗透膜，以了解在水中的离子和中性污染物的传输和拒绝的机制。我们观察到，拒绝的变化与离子的大小非单调。特别地，尿素的截留率（2.4 A半径）高于乙醇（2.6 A半径），并且有机溶质的截留率（2.2-2.8 A半径）低于Na+（1.4 A半径）或Cl-（2.3 A半径）。我们表明，这可以解释在膜中的溶质可及的分子间体积和溶质-水对相互作用能。如果膜的分子结构中最小的开放空间都大于水合溶质，那么溶质-水对相互作用能无关紧要。然而，当聚合物结构中的开放空间使得溶质必须脱落至少一个水分子以穿过膜分子结构的一部分时，对相互作用能控制溶质排斥。水分子在离子的溶剂化壳层中的高对相互作用能使得水分子难以脱落，从而增强了对离子的排斥。另一方面，有机溶质-水相互作用能由与溶质氢键合的水分子控制。尿素分子比醇分子具有更多的氢键位点，导致尿素的截留率高于乙醇，乙醇是一种大小相似但氢键位点较少的分子。这些研究结果强调了溶质的溶剂化壳和溶质-水-膜化学在反渗透的背景下的重要性，从而提供了新的见解，在RO膜中的溶质传输和排斥。
英文摘要：
Despite the success of reverse osmosis (RO) for water purification, the molecular-level physico-chemical processes of contaminant rejection are not well understood. Here we carry out NEMD simulations on a model polyamide RO membrane to understand the mechanisms of transport and rejection of both ionic and neutral contaminants in water. We observe that the rejection changes non-monotonously with ion sizes. In particular, the rejection of urea, 2.4 A radius, is higher than ethanol, 2.6 A radius, and the rejections for organic solutes, 2.2-2.8 A radius, are lower than Na+, 1.4 A radius, or Cl-, 2.3 A radius. We show that this can be explained in terms of the solute accessible intermolecular volume in the membrane and the solute-water pair interaction energy. If the smallest open spaces in the membrane's molecular structure are all larger than the hydrated solute, then the solute-water pair interaction energy does not matter. However, when the open spaces in the polymeric structure are such that solutes have to shed at least one water molecule to pass through a portion of the membrane molecular structure, the pair interaction energy governs solute rejection. The high pair interaction energy for water molecules in the solvation shell for ions makes the water molecules difficult to shed, thus enhancing the rejection of ions. On the other hand, the organic solute-water interaction energies are governed by the water molecules that are hydrogen bonded to the solute. Urea molecules have more hydrogen-bonding sites than alcohol molecules, leading to a higher rejection of urea than occurs for ethanol, a molecule of similar size but with fewer hydrogen bonding sites. These findings underline the importance of the solute's solvation shell and solute-water-membrane chemistry in the context of reverse osmosis, thus providing new insights into solute transport and rejection in RO membranes.
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