The main objective of this study is to examine how the charge densities of four monovalent anions—fluoride (F^–), chloride (Cl^–), bromide (Br^–), and nitrate (NO₃^–)—influence their Donnan (charge) exclusion by a charged nanofiltration (NF) membrane. We systematically studied the rejection behavior of ternary ion solutions containing sodium cation (Na⁺) and two of the monovalent anions as a function of the pH with a polyamide NF membrane. In the solutions containing F^– and Cl^– or F^– and Br^–, F^– rejection was higher than Cl^– or Br^– rejection only when the solution pH was higher than 5.5, suggesting that F^– (which has a higher charge density) was repelled more strongly by the negatively charged membrane. The order of change in the activation energy for the transport of the four anions through the polyamide membrane as a response to the increase of the membrane negative charge was the following: F^– > Cl^– > NO₃^– > Br^–. This order corroborates our main hypothesis that an anion with a smaller ionic radius, and hence a higher charge density, is more affected by the Donnan (charge)-exclusion mechanism in NF. We conclude with a proposed mechanism for the role of ionic charge density in the rejection of monovalent anions in NF.

中文翻译：

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纳滤对离子电荷密度在唐南单价阴离子排斥中的作用

本研究的主要目的是研究如何四个单价阴离子氟化物（F电荷密度^- ），氯离子（CL ^- ），溴（溴^- ），和硝酸盐（NO ₃ ^-）-influence其唐南（充电）通过带电的纳滤（NF）膜排除。我们通过聚酰胺NF膜系统地研究了含钠阳离子（Na ⁺）和两个单价阴离子的三元离子溶液的排斥行为与pH的关系。在包含F ^–和Cl ^–或F ^–和Br ^–的溶液中，F ^–排斥比Cl ^–或Br ^–高。仅当溶液的pH值高于5.5时，离子才被排斥，这表明带负电的膜会更强烈地排斥F ^–（具有更高的电荷密度）。响应于膜负电荷增加，四种阴离子通过聚酰胺膜传输的活化能的变化顺序如下：F ^– > Cl ^– > NO ₃ ^– > Br ^–。该顺序证实了我们的主要假设，即具有较小离子半径并因此具有较高电荷密度的阴离子受NF中Donnan（电荷）排斥机制的影响更大。我们以离子电荷密度在NF中一价阴离子的排斥中的作用机理提出了一个结论。