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Nanofiltration membranes with sandwich-like mixed charge layers for high-efficiency Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+}separation
用于高效 Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+} 分离的具有类蜂窝状混合电荷层的纳滤膜

Yunhao Li a a  ^("a "){ }^{\text {a }}, Wu Kuang b b  ^("b "){ }^{\text {b }}, Haijun Yu a Yu a  Yu^("a ")\mathrm{Yu}^{\text {a }}, Dandan Liu a a  ^("a "){ }^{\text {a }}, Yanfang Liu a a  ^("a "){ }^{\text {a }}, Guodong Kang a, , ∗ , a,  , ∗ ,  ^("a, ",^(**)", "){ }^{\text {a, },{ }^{*} \text {, }} Xinmiao Liang a a  ^("a "){ }^{\text {a }}, Yiming Cao a,*** a,***  ^("a,*** "){ }^{\text {a,*** }}
李云浩 a a  ^("a "){ }^{\text {a }} 、吴匡 b b  ^("b "){ }^{\text {b }} 、海军 Yu a Yu a  Yu^("a ")\mathrm{Yu}^{\text {a }} 、刘丹丹 a a  ^("a "){ }^{\text {a }} 、刘艳芳 a a  ^("a "){ }^{\text {a }} 、康国栋 a, , ∗ , a,  , ∗ ,  ^("a, ",^(**)", "){ }^{\text {a, },{ }^{*} \text {, }} 、梁欣淼 a a  ^("a "){ }^{\text {a }} 、曹一鸣 a,*** a,***  ^("a,*** "){ }^{\text {a,*** }} a a  ^("a "){ }^{\text {a }} Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China
a a  ^("a "){ }^{\text {a }} 中国科学院大连化学物理研究所大连清洁能源国家实验室，大连，116023 b b ^(b){ }^{\mathrm{b}} Beijing OriginWater Technology Co., Ltd., Beijing, 102206, China
b b ^(b){ }^{\mathrm{b}} 北京碧水源科技有限公司有限公司、中国北京，102206

ARTICLE INFO  产品信息

Keywords:  保留字：

Nanofiltration membrane  纳滤膜
Interfacial polymerization
界面聚合
Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+}separation   Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+} 分离
Mixed charge  混合电荷
Sandwich-like mixed charge configuration
三明治式混合装药结构

Abstract  摘要

Efficient separation of monovalent/multivalent cations and anions is essential for optimizing lithium extraction from Salt-Lake to simplify production processes and reduces costs. Nevertheless, the conventional nanofiltration membranes with specific charges can only separate either cations or anions. To overcome this limitation, a novel strategy is proposed for fabricating nanofiltration membranes with “negative-positive-negative” sandwich-like mixed charge configuration. Herein, two aqueous monomers (phenylbiguanide and polyethyleneimine) with significantly different diffusion rates are employed to react with trimesoyl chloride to form a nascent polyamide layer via interfacial polymerization. Subsequently, phenylbiguanide that preferentially diffuses to top surface of nascent membrane and unreacted amine groups from polyethyleneimine are used to react with m-phenylenedisulfonyl chloride to form a polysulfonamide/polyamide functional layer. The unreacted sulfonyl chloride groups and trimesoyl chloride near aqueous phase could hydrolyzes more easily, producing negatively charged sulfonic acid and carboxyl groups on the top and bottom. Meanwhile, the main part of polyamide layer is positively charged attributed to numerous unreacted amine groups. The “negative-positive-negative” sandwichlike configuration was proven using TOF-SIMS. The fabricated membrane exhibited a selectivity of 57.22 for MgCl 2 / LiCl MgCl 2 / LiCl MgCl_(2)//LiCl\mathrm{MgCl}_{2} / \mathrm{LiCl} and 30.61 for Na 2 SO 4 / NaCl Na 2 SO 4 / NaCl Na_(2)SO_(4)//NaCl\mathrm{Na}_{2} \mathrm{SO}_{4} / \mathrm{NaCl}. Furthermore, Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+}selectivity of 57.34 was achived for the mixed salt solution ( Mg 2 + / Li + = 45 ) Mg 2 + / Li + = 45 (Mg^(2+)//Li^(+)=45)\left(\mathrm{Mg}^{2+} / \mathrm{Li}^{+}=45\right), showing good application potential in lithium extraction.
一价/多价阳离子和阴离子的有效分离对于优化盐湖锂提取以简化生产工艺并降低成本至关重要。然而，具有特定电荷的常规纳滤膜只能分离阳离子或阴离子。为了克服这一局限性，提出了一种新的策略，用于制造具有“负-正-负”类混合电荷构型的纳滤膜。本文中，采用具有显著不同的扩散速率的两种水性单体（苯基双胍和聚乙烯亚胺）与均苯三甲酰氯反应以通过界面聚合形成新生聚酰胺层。随后，优先扩散到新生膜的顶表面的苯基双胍和来自聚乙烯亚胺的未反应的胺基用于与间苯二磺酰氯反应以形成聚磺酰胺/聚酰胺功能层。 水相附近未反应的磺酰氯基团和均苯三甲酰氯更容易水解，在顶部和底部产生带负电荷的磺酸和羧基。同时，由于大量未反应的胺基，聚酰胺层的主要部分带正电荷。利用飞行时间二次离子质谱（TOF-SIMS）验证了“负-正-负”的类螺旋构型。制备的膜对 MgCl 2 / LiCl MgCl 2 / LiCl MgCl_(2)//LiCl\mathrm{MgCl}_{2} / \mathrm{LiCl} 和 Na 2 SO 4 / NaCl Na 2 SO 4 / NaCl Na_(2)SO_(4)//NaCl\mathrm{Na}_{2} \mathrm{SO}_{4} / \mathrm{NaCl} 的选择性分别为 57.22 和 30.61。此外，混合盐溶液 ( Mg 2 + / Li + = 45 ) Mg 2 + / Li + = 45 (Mg^(2+)//Li^(+)=45)\left(\mathrm{Mg}^{2+} / \mathrm{Li}^{+}=45\right) 的 Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+} 选择性达到 57.34，显示出良好的应用潜力。

1. Introduction  1.介绍

Lithium has become a strategic resource of paramount importance [1], driven by its critical role in renewable energy storage and electric vehicle batteries. It is estimated that by 2025, global lithium consumption (measured in lithium carbonate equivalent) will reach 900, 000 tons, which is 3.4 times of 2015 levels ( 265,000 tons), leading to a projected shortfall in lithium supply [2,3]. Extracting lithium from Salt-Lake brine represents a promising solution to address the growing demand for lithium resources [4,5]. Nevertheless, brine extraction presents considerable challenges, primarily due to the high concentration of bivalent Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+} ions and the significant Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+}ratio. The comparable hydration radius and chemical properties of Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+} and Li + Li + Li^(+)\mathrm{Li}^{+}further complicate the selective extraction of lithium with high purity [6], necessitating the development of innovative separation technologies.
锂已成为一种至关重要的战略资源[1]，这得益于其在可再生能源储存和电动汽车电池中的关键作用。据估计，到2025年，全球锂消费量（以碳酸锂当量衡量）将达到90万吨，是2015年水平（26.5万吨）的3.4倍，导致预计锂供应短缺[2，3]。从盐湖卤水中提取锂是解决锂资源日益增长的需求的一种有前途的解决方案[4，5]。然而，盐水提取提出了相当大的挑战，主要是由于高浓度的二价 Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+} 离子和显著的 Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+} 比。 Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+} 和 Li + Li + Li^(+)\mathrm{Li}^{+} 的可比水合半径和化学性质进一步使高纯度锂的选择性提取复杂化[6]，需要开发创新的分离技术。
Nanofiltration membranes have gained attention as a promising
纳滤膜作为一种有前途的
solution for lithium extraction from Salt-Lake brine. These membranes, characterized by charged surfaces and sub-nanometer pores, leverage Donnan effects and size sieving principles to achieve ion separation [7, 8]. Classical nanofiltration membranes are polyamide (PA) membranes produced via the interfacial polymerization of trimesoyl chloride (TMC) and polymerization (PIP) [9-11]. Nevertheless, their inherent negative charge results in poor rejection of Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+}, leading to unsatisfactory Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+}selectivity [12,13]. Researchers have developed positively charged nanofiltration membranes to enhance Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+}selectivity through strategies such as introducing [6,14,15] or grafting [12,16,17] electropositive monomers during interfacial polymerization, and constructing positively charged intermediate layers. Notably, polyethylenimine (PEI)-based PA membranes, prepared using PEI and TMC, have attracted considerable attention in the field of Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+}separation from Salt-Lake brine due to their high separation efficiency, as well as the presence of a large number of positively charged amine groups [2,
盐湖卤水提锂溶液这些膜的特征在于带电表面和亚纳米孔，利用 Donnan 效应和尺寸筛分原理来实现离子分离[7，8]。经典的纳滤膜是通过均苯三甲酰氯（TMC）的界面聚合和聚合（PIP）生产的聚酰胺（PA）膜[9-11]。然而，它们固有的负电荷导致 Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+} 的差的排斥，导致不令人满意的 Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+} 选择性[12，13]。研究人员已经开发出带正电荷的纳滤膜，通过在界面聚合过程中引入[6，14，15]或接枝[12，16，17]正电性单体以及构建带正电荷的中间层等策略来提高 Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+} 选择性。 值得注意的是，使用 PEI 和 TMC 制备的基于聚乙烯亚胺（PEI）的 PA 膜由于其高分离效率以及大量带正电荷的胺基的存在而在从盐湖卤水中分离 Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+} 的领域中引起了相当大的关注[2，

16,18,19].  16、18、19]。

Conventional nanofiltration membranes, possessing either a specific positive or negative charge, are typically restricted to separating only monovalent/multivalent cations or anions [20]. For instance, PEI-based PA membranes can effectively reject multivalent cations, while poor rejection of multivalent negative [9]. In addition to cations like Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+} and Li + Li + Li^(+)\mathrm{Li}^{+}in Salt-Lake brine, there are high concentrations of anions such as SO 4 2 − SO 4 2 − SO_(4)^(2-)\mathrm{SO}_{4}{ }^{2-} and Cl − Cl − Cl^(-)\mathrm{Cl}^{-}, along with negatively charged natural small molecules. To meet production requirements, traditional lithium extraction processes often couple membrane methods with precipitation, adsorption, or electrochemical techniques, complicating the process and significantly increasing costs [21].
具有特定正电荷或负电荷的常规纳滤膜通常仅限于分离单价/多价阳离子或阴离子[20]。例如，基于 PEI 的 PA 膜可以有效地排斥多价阳离子，而对多价阴离子的排斥较差[9]。盐湖卤水中除含有 Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+} 、 Li + Li + Li^(+)\mathrm{Li}^{+} 等阳离子外，还含有高浓度的 SO 4 2 − SO 4 2 − SO_(4)^(2-)\mathrm{SO}_{4}{ }^{2-} 、 Cl − Cl − Cl^(-)\mathrm{Cl}^{-} 等阴离子，沿着有带负电荷的天然小分子。为了满足生产要求，传统的锂提取工艺通常将膜法与沉淀、吸附或电化学技术相结合，使工艺复杂化并显著增加成本[21]。
Constructing vertically distributed composite charge layers and asymmetric network structures is a promising strategy to address these challenges [22,23]. This study proposes a novel nanofiltration membrane design featuring a vertically distributed composite charge layer with a “negative-positive-negative” sandwich-like structure. This innovative configuration simultaneously separates monovalent/multivalent cations and anions. The negatively charged top layer repels high-valence anions such as SO 4 2 − SO 4 2 − SO_(4)^(2-)\mathrm{SO}_{4}{ }^{2-}, while the positively charged intermediate layer enhances Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+}selectivity. Additionally, the negatively charged bottom layer facilitates the rapid passage of monovalent Li + Li + Li^(+)\mathrm{Li}^{+}ions in this layer (rather than through the entire functional layer).
构建垂直分布的复合电荷层和非对称网络结构是解决这些挑战的有前途的策略[22，23]。本研究提出一种新颖的纳滤膜设计，具有垂直分布的复合电荷层与“负-正-负”的类膜结构。这种创新的配置同时分离单价/多价阳离子和阴离子。带负电荷的顶层排斥高价阴离子如 SO 4 2 − SO 4 2 − SO_(4)^(2-)\mathrm{SO}_{4}{ }^{2-} ，而带正电荷的中间层增强 Mg 2 + / Li + Mg 2 + / Li + Mg^(2+)//Li^(+)\mathrm{Mg}^{2+} / \mathrm{Li}^{+} 选择性。另外，带负电荷的底层促进单价 Li + Li + Li^(+)\mathrm{Li}^{+} 离子在该层中快速通过（而不是通过整个功能层）。
Herein, two aqueous monomers with significantly different diffusion rates in the organic phase, e.g. phenylbiguanide (PBG) and PEI, were selected to fabricate a nascent PA layer through interfacial polymerization with TMC. Due to PBG (117 Da) has smaller molecular size and hydrophobic benzene ring, it exhibited a higher diffusion rate in organic phase compared to PEI with larger molecular weight (70,000 Da) and highly branched, allowing PBG to more easily reach the PA surface. Subsequently, m-phenylenedisulfonyl chloride (IC) reacted with the residual amine groups (free PBG and partially embedded PEI) on PA surface to form a PSA layer, thereby creating a PSA/PA fcomposite unctional layer. The PSA surface contained many unreacted sulfonyl chloride groups, which hydrolyzed to generate negatively charged sulfonic acid groups. Additionally, partial TMC near the aqueous phase was prone to hydrolysis, resulting in negatively charged carboxyl groups at the bottom of PA layer. Compared to the acyl chloride of TMC and the sulfonyl chloride of IC, PEI and PBG contained an excess of positively charged amine groups, leading to a positively charged core within the functional layer. This created a “negative-positive-negative” sandwichlike structure within the mixed-charge functional layer. Time of flight secondary ion mass spectrometry (TOF-SIMS) was employed to thoroughly analyze and confirm the chemical composition of this functional layer with “negative-positive-negative” sandwich-like structure. This strategy offers a novel method for the development of high-performance nanofiltration membranes and their application in lithium extraction from Salt-Lake brine.
本文中，选择在有机相中具有显著不同扩散速率的两种水性单体，例如苯基双胍（PBG）和 PEI，通过与 TMC 的界面聚合来制造新生 PA 层。由于 PBG（117 Da）具有较小的分子尺寸和疏水苯环，因此与具有较大分子量（70，000 Da）和高度支化的 PEI 相比，其在有机相中表现出更高的扩散速率，使得 PBG 更容易到达 PA 表面。随后，间苯二磺酰氯（IC）与 PA 表面的残余胺基（游离 PBG 和部分嵌入的 PEI）反应形成 PSA 层，从而产生 PSA/PA 复合功能层。PSA 表面含有许多未反应的磺酰氯基团，其水解产生带负电荷的磺酸基团。此外，靠近水相的部分 TMC 易于水解，导致 PA 层底部带负电荷的羧基。 与 TMC 的酰氯和 IC 的磺酰氯相比，PEI 和 PBG 含有过量的带正电荷的胺基，导致功能层内的带正电荷的核。这在混合电荷功能层内产生了“负-正-负”三明治状结构。利用飞行时间二次离子质谱（TOF-SIMS）对该功能层的化学组成进行了分析和确认，该功能层具有“负-正-负”的类金属结构。该方法为高性能纳滤膜的开发及其在盐湖卤水提锂中的应用提供了一种新的方法。

2. Experiment  2.实验

2.1. Materials and chemicals
2.1.材料和化学品

The polysulfone-based support membrane for ultrafiltration was obtained from Jozzon Membrane Technology Co., located in Dongying, China. For the preparation, materials such as TMC (98 %), PEI (70,000 g / mol g / mol g//mol\mathrm{g} / \mathrm{mol} and 50 wt % 50 wt % 50wt%50 \mathrm{wt} \% in water), IC ( 98 % 98 % 98%98 \% ), PBG ( 98 % 98 % 98%98 \% ), and ethanol ( 99.5 %) were sourced from Macklin Biochemical Co. Ltd., Shanghai, China. Additional chemicals such as magnesium chloride ( MgCl 2 , 99 % ) MgCl 2 , 99 % (MgCl_(2),99%)\left(\mathrm{MgCl}_{2}, 99 \%\right), lithium chloride ( LiCl , AR LiCl , AR LiCl,AR\mathrm{LiCl}, \mathrm{AR} ), sodium triphosphate ( Na 3 PO 4 , 96 % Na 3 PO 4 , 96 % (Na_(3)PO_(4),96%:}\left(\mathrm{Na}_{3} \mathrm{PO}_{4}, 96 \%\right. ), sucrose ( 99 % 99 % 99%99 \% AR), glycerol ( 99 % 99 % 99%99 \% AR), and polyethylene glycol (PEG800 , with a molecular weight of 800 g / mol , 99 % 800 g / mol , 99 % 800g//mol,99%800 \mathrm{~g} / \mathrm{mol}, 99 \% ) were acquired from Aladdin Biochemical Technology Co., Shanghai, China. Deionized water (conductivity < 2 μ S cm − 1 < 2 μ S cm − 1 < 2muScm^(-1)<2 \mu \mathrm{~S} \mathrm{~cm}^{-1} ) was prepared using a laboratory-grade purification system.
用于超滤的聚砜基支撑膜得自 Jozzon Membrane Technology Co.，位于中国东营。对于制备，材料如 TMC（98%）、PEI（70，000 g / mol g / mol g//mol\mathrm{g} / \mathrm{mol} 和 50 wt % 50 wt % 50wt%50 \mathrm{wt} \% 水溶液）、IC（ 98 % 98 % 98%98 \% ）、PBG（ 98 % 98 % 98%98 \% ）和乙醇（99.5%）来自 Macklin Biochemical Co.Ltd，中国上海另外的化学品如氯化镁 ( MgCl 2 , 99 % ) MgCl 2 , 99 % (MgCl_(2),99%)\left(\mathrm{MgCl}_{2}, 99 \%\right) 、氯化锂（ LiCl , AR LiCl , AR LiCl,AR\mathrm{LiCl}, \mathrm{AR} ）、三磷酸钠（ ( Na 3 PO 4 , 96 % Na 3 PO 4 , 96 % (Na_(3)PO_(4),96%:}\left(\mathrm{Na}_{3} \mathrm{PO}_{4}, 96 \%\right. ）、蔗糖（ 99 % 99 % 99%99 \% AR）、甘油（ 99 % 99 % 99%99 \% AR）和聚乙二醇（PEG 800，分子量为 800 g / mol , 99 % 800 g / mol , 99 % 800g//mol,99%800 \mathrm{~g} / \mathrm{mol}, 99 \% ）购自 Aladdin Biochemical Technology Co.，中国上海使用实验室级纯化系统制备去离子水（电导率 < 2 μ S cm − 1 < 2 μ S cm − 1 < 2muScm^(-1)<2 \mu \mathrm{~S} \mathrm{~cm}^{-1} ）。

2.2. Membrane preparation
2.2.膜制备

Nanofiltration membranes were fabricated via interfacial polymerization using PEI and TMC as aqueous monomers. The PEI solution ( 0.20 % w / v % w / v %w//v\% \mathrm{w} / \mathrm{v} ) was prepared in water containing 1.00 % w / v Na 3 PO 4 1.00 % w / v Na 3 PO 4 1.00%w//vNa3PO_(4)1.00 \% \mathrm{w} / \mathrm{v} \mathrm{Na} 3 \mathrm{PO}_{4} and 0.40 % w / v % w / v %w//v\% \mathrm{w} / \mathrm{v} PBG, and HCl to adjust pH to 10 . TMC was dissolved at 0.15 % w / 0.15 % w / 0.15%w//0.15 \% \mathrm{w} / v in hexane. For membrane preparation, PSf ultrafiltration membranes were first immersed in the PEI solution for 5 min . Afterward, excess solution was removed using a nitrogen gun until visibly dry. Then, the TMC hexane solution was poured onto the PEI-immersed PSf membrane and left for 30 s , forming the PA layer. Subsequently, the IC hexane solution ( 0.35 % 0.35 % 0.35%0.35 \% ) or pure hexane were poured on the nascent PA membrane and cured at 50 ∘ C 50 ∘ C 50^(@)C50^{\circ} \mathrm{C} for 10 min to prepare PBG-IC and PBG0 membranes, respectively. Finally, all nanofiltration membranes were stored in deionized water until testing.
以 PEI 和 TMC 为水相单体，通过界面聚合法制备了纳滤膜。在含有 1.00 % w / v Na 3 PO 4 1.00 % w / v Na 3 PO 4 1.00%w//vNa3PO_(4)1.00 \% \mathrm{w} / \mathrm{v} \mathrm{Na} 3 \mathrm{PO}_{4} 和 0.40 % w / v % w / v %w//v\% \mathrm{w} / \mathrm{v} PBG 的水中制备 PEI 溶液（0.20 % w / v % w / v %w//v\% \mathrm{w} / \mathrm{v} ），并用 HCl 调节 pH 至 10。将 TMC 在 0.15 % w / 0.15 % w / 0.15%w//0.15 \% \mathrm{w} / v 下溶解于己烷中。对于膜制备，首先将 PSf 超滤膜浸入 PEI 溶液中 5 分钟。之后，使用氮气枪除去过量的溶液，直到明显干燥。然后，将 TMC 己烷溶液倒在浸入 PEI 的 PSf 膜上并放置 30 秒，形成 PA 层。随后，将 IC 己烷溶液（ 0.35 % 0.35 % 0.35%0.35 \% ）或纯己烷倾倒在新生 PA 膜上，并在 50 ∘ C 50 ∘ C 50^(@)C50^{\circ} \mathrm{C} 下固化 10 分钟以分别制备 PBG-IC 和 PBG 0 膜。最后，将所有纳滤膜储存在去离子水中直至测试。

2.3. Characterizations of nanofiltration membranes
2.3.纳滤膜的表征

Prior to characterization, PBG-0 and PBG-IC membranes were vacuum-dried at 25 ∘ C 25 ∘ C 25^(@)C25{ }^{\circ} \mathrm{C} for 24 h . The chemical structure compositions of PA layer were analyzed both at the top surface and ibottom part, as well as in the vertical profile, using Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). The X-ray Photoelectron Spectroscopy (XPS, Thermo ESCALAB 250XI, USA) and Fourier Transform Infrared Spectroscopy (IR, Thermo Scientific Nicolet IS5, USA) were used to characterize the elemental compositions and chemical structure of nanofiltration membranes. The thickness, surface morphology, roughness of samples was assessed using an Atomic force microscope (AFM, Dimension Icon, Bruker, Germany). To measure the actual thickness of the functional layer, polysulfone support layer was dissolved in dimethyl formamide, and transfer functional layer to the surface of a smooth silicon wafer. The surface streaming potential ( ζ ) ( ζ ) (zeta)(\zeta) of PBG-0 and PBG-IC membranes were investigated by electrokinetic analyzer (SurPASS, Anton Paar, Austria) using 1 mM KCl aqueous solution. For the test of bottom surface, the surface of membranes were attached to the doublesided tape, then membranes were immersed into dimethyl formamide to dissolve the PSf support, with their bottom surfaces facing up. The surface hydrophilicity was determined by measuring the water contact angle (WCA) using a JC2000C contact angle instrument (Powereach, China).
在表征之前，将 PBG-0 和 PBG-IC 膜在 25 ∘ C 25 ∘ C 25^(@)C25{ }^{\circ} \mathrm{C} 下真空干燥 24 小时。利用飞行时间二次离子质谱（TOF-SIMS）分析了 PA 层的顶面、底面以及垂直剖面的化学结构组成。使用 X 射线光电子能谱（XPS，Thermo ESCALAB 250 XI，USA）和傅里叶变换红外光谱（IR，Thermo Scientific 尼科莱 IS 5，USA）表征纳滤膜的元素组成和化学结构。使用原子力显微镜（AFM，Dimension Icon，Bruker，德国）评估样品的厚度、表面形态、粗糙度。为了测量功能层的实际厚度，将聚砜支撑层溶解在二甲基甲酰胺中，并将功能层转移到光滑硅晶片的表面。 通过电动分析仪（SurPASS，Anton 帕尔，Austria）使用 1 mM KCl 水溶液研究 PBG-0 和 PBG-IC 膜的表面流动电位 ( ζ ) ( ζ ) (zeta)(\zeta) 。对于底表面的测试，将膜的表面附着到双面胶带上，然后将膜浸入二甲基甲酰胺中以溶解 PSf 载体，使其底表面朝上。通过使用 JC 2000 C 接触角仪（Powereach，中国）测量水接触角（WCA）来确定表面亲水性。

2.4. Separation performance
2.4.分离性能

The ions selectivity and water permeability of PBG-0 and PBG-IC membranes was evaluated by the custom cross-flow filtration apparatus, operated at room temperature. Filtration tests were carried out at 20 ∘ C 20 ∘ C 20^(@)C20^{\circ} \mathrm{C} with an applied pressure of 6 bar and a cross-flow velocity of 3 L min − 1 min − 1 min^(-1)\mathrm{min}^{-1}. Prior to testing, nanofiltration membranes were stabilized for 30 min.
PBG-0 和 PBG-IC 膜的离子选择性和水渗透性通过在室温下操作的定制错流过滤装置来评估。过滤试验在 20 ∘ C 20 ∘ C 20^(@)C20^{\circ} \mathrm{C} 下进行，施加的压力为 6 巴，错流速度为 3L min − 1 min − 1 min^(-1)\mathrm{min}^{-1} 。在测试之前，将纳滤膜稳定 30 分钟。
Pure water flux ( P w , L ⋅ m − 2 h − 1 ) P w , L ⋅ m − 2 h − 1 (P_(w),(L)*m^(-2)h^(-1))\left(P_{w}, \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1}\right) was calculated according to Equation (1).
根据公式（1）计算纯水通量 ( P w , L ⋅ m − 2 h − 1 ) P w , L ⋅ m − 2 h − 1 (P_(w),(L)*m^(-2)h^(-1))\left(P_{w}, \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1}\right) 。
P w = V A rea × Δ t P w = V A rea  × Δ t P_(w)=(V)/(A_("rea ")xx Delta t)P_{w}=\frac{V}{A_{\text {rea }} \times \Delta t}
Where, Δ t ( h ) Δ t ( h ) Delta t(h)\Delta t(\mathrm{~h}) denotes the duration of permeation, V ( L ) V ( L ) V(L)V(\mathrm{~L}) indicates the volume of permeate collected in Δ t Δ t Delta t\Delta t, and A rea ( m 2 ) A rea  m 2 A_("rea ")(m^(2))A_{\text {rea }}\left(\mathrm{m}^{2}\right) signifies the effective test area ( 19.6 cm 2 19.6 cm 2 19.6cm^(2)19.6 \mathrm{~cm}^{2} ). Using an inorganic salt solution instead of pure water, the water flux ( J w , L ⋅ m − 2 h − 1 ) J w , L ⋅ m − 2 h − 1 (J_(w),(L)*m^(-2)h^(-1))\left(J_{w}, \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1}\right) of nanofiltration membranes were measured according to Equation (1). The water permeability constant ( A , L ⋅ m − 2 h − 1 bar − 1 A , L ⋅ m − 2 h − 1 bar − 1 A,L*m^(-2)h^(-1)bar^(-1)A, \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1} ) was then determined through Equation (2).
其中， Δ t ( h ) Δ t ( h ) Delta t(h)\Delta t(\mathrm{~h}) 表示渗透持续时间， V ( L ) V ( L ) V(L)V(\mathrm{~L}) 表示在 Δ t Δ t Delta t\Delta t 中收集的渗透物体积， A rea ( m 2 ) A rea  m 2 A_("rea ")(m^(2))A_{\text {rea }}\left(\mathrm{m}^{2}\right) 表示有效测试面积（ 19.6 cm 2 19.6 cm 2 19.6cm^(2)19.6 \mathrm{~cm}^{2} ）。使用无机盐溶液代替纯水，根据公式（1）测量纳滤膜的水通量 ( J w , L ⋅ m − 2 h − 1 ) J w , L ⋅ m − 2 h − 1 (J_(w),(L)*m^(-2)h^(-1))\left(J_{w}, \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1}\right) 。然后通过等式（2）确定透水性常数（ A , L ⋅ m − 2 h − 1 bar − 1 A , L ⋅ m − 2 h − 1 bar − 1 A,L*m^(-2)h^(-1)bar^(-1)A, \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1} ）。
A = J w Δ P − Δ π A = J w Δ P − Δ π A=(J_(w))/(Delta P-Delta pi)A=\frac{J_{w}}{\Delta P-\Delta \pi}
Here, Δ P Δ P Delta P\Delta P (bar) and Δ π Δ π Delta pi\Delta \pi (bar) represents the operating pressure and osmotic pressure. The rejection ( R , % R , % R,%R, \% ) of single-salt (containing LiCl ,
这里， Δ P Δ P Delta P\Delta P （bar）和 Δ π Δ π Delta pi\Delta \pi （bar）表示操作压力和渗透压。单盐（含 LiCl，
MgCl 2 , NaCl MgCl 2 , NaCl MgCl_(2),NaCl\mathrm{MgCl}_{2}, \mathrm{NaCl}, andNa 2 SO 4 2 SO 4 2SO_(4)2 \mathrm{SO}_{4} with 1 g L − 1 1 g L − 1 1gL^(-1)1 \mathrm{~g} \mathrm{~L}^{-1} ) for both PBG-0 and PBG-IC membranes, were calculated by applying Equation (3).
对于 PBG-0 和 PBG-IC 膜两者，通过应用等式（3）来计算 Na MgCl 2 , NaCl MgCl 2 , NaCl MgCl_(2),NaCl\mathrm{MgCl}_{2}, \mathrm{NaCl} 和 Na 2 SO 4 2 SO 4 2SO_(4)2 \mathrm{SO}_{4} 与 1 g L − 1 1 g L − 1 1gL^(-1)1 \mathrm{~g} \mathrm{~L}^{-1} ）。
R = ( 1 − C p C f ) × 100 % R = 1 − C p C f × 100 % R=(1-(C_(p))/(C_(f)))xx100%R=\left(1-\frac{C_{p}}{C_{f}}\right) \times 100 \%
Where, C f ( g ⋅ L − 1 ) C f g ⋅ L − 1 C_(f)((g)*L^(-1))C_{f}\left(\mathrm{~g} \cdot \mathrm{~L}^{-1}\right) indicates the concentration of feed solution, whereas C p ( g ⋅ L − 1 ) C p g ⋅ L − 1 C_(p)((g)*L^(-1))C_{p}\left(\mathrm{~g} \cdot \mathrm{~L}^{-1}\right) represents the filtrate solution. The salt permeability constant ( B ; m ⋅ s − 1 B ; m ⋅ s − 1 B;m*s^(-1)B ; \mathrm{m} \cdot \mathrm{s}^{-1} ) was calculated by following Equation (4).
其中， C f ( g ⋅ L − 1 ) C f g ⋅ L − 1 C_(f)((g)*L^(-1))C_{f}\left(\mathrm{~g} \cdot \mathrm{~L}^{-1}\right) 表示进料溶液的浓度，而 C p ( g ⋅ L − 1 ) C p g ⋅ L − 1 C_(p)((g)*L^(-1))C_{p}\left(\mathrm{~g} \cdot \mathrm{~L}^{-1}\right) 表示滤液溶液。通过以下等式（4）计算盐渗透性常数（ B ; m ⋅ s − 1 B ; m ⋅ s − 1 B;m*s^(-1)B ; \mathrm{m} \cdot \mathrm{s}^{-1} ）。
B = J s ( C f − C p ) B = J s C f − C p B=(J_(s))/((C_(f)-C_(p)))B=\frac{J_{s}}{\left(C_{f}-C_{p}\right)}
Here, J S ( g ⋅ m − 2 s − 1 ) J S g ⋅ m − 2 s − 1 J_(S)((g)*m^(-2)s^(-1))J_{S}\left(\mathrm{~g} \cdot \mathrm{~m}^{-2} \mathrm{~s}^{-1}\right) denotes the salt flux. The salt and ions selectivity (such as S L i C l / M g C L 2 S L i C l / M g C L 2 S_(LiCl//MgCL2)S_{L i C l / M g C L 2} and S L i , M g S L i , M g S_(Li,Mg)S_{L i, M g} ) of all membranes was calculated based on Equations (5) and (6), respectively.
这里， J S ( g ⋅ m − 2 s − 1 ) J S g ⋅ m − 2 s − 1 J_(S)((g)*m^(-2)s^(-1))J_{S}\left(\mathrm{~g} \cdot \mathrm{~m}^{-2} \mathrm{~s}^{-1}\right) 表示盐通量。所有膜的盐和离子选择性（例如 S L i C l / M g C L 2 S L i C l / M g C L 2 S_(LiCl//MgCL2)S_{L i C l / M g C L 2} 和 S L i , M g S L i , M g S_(Li,Mg)S_{L i, M g} ）分别基于等式（5）和（6）计算。

S L i , M g = 1 − R L i + 1 − R M g 2 + = C p , L i + × C f , M g 2 + C f , L i + × C p , M g 2 + S L i , M g = 1 − R L i + 1 − R M g 2 + = C p , L i + × C f , M g 2 + C f , L i + × C p , M g 2 + S_(Li,Mg)=(1-R_(Li^(+)))/(1-R_(Mg^(2+)))=(C_(p,Li^(+))xxC_(f,Mg^(2+)))/(C_(f,Li^(+))xxC_(p,Mg^(2+)))S_{L i, M g}=\frac{1-R_{L i^{+}}}{1-R_{M g^{2+}}}=\frac{C_{p, L i^{+}} \times C_{f, M g^{2+}}}{C_{f, L i^{+}} \times C_{p, M g^{2+}}}
Inductively Coupled Plasma Optical Emission Spectroscopy (ICPOES, Optima 7300DV, USA) was utilized to determine the concentrations of Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+} and Li + Li + Li^(+)\mathrm{Li}^{+}ions in the mixed saline solution, which had a total concentration of 2 g L − 1 2 g L − 1 2gL^(-1)2 \mathrm{~g} \mathrm{~L}^{-1} and a Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+} to Li + Li + Li^(+)\mathrm{Li}^{+}mass ratio of 45 : 1 45 : 1 45:145: 1. The Stokes radii of neutral solutes, such as ethanol, glycerol, sucrose, and PEG-800, were calculated using Equation (7).
利用电感耦合等离子体发射光谱法（ICPOES，Optima 7300 DV，USA）测定混合盐溶液中 Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+} 和 Li + Li + Li^(+)\mathrm{Li}^{+} 离子的浓度，其总浓度为 2 g L − 1 2 g L − 1 2gL^(-1)2 \mathrm{~g} \mathrm{~L}^{-1} ， Mg 2 + Mg 2 + Mg^(2+)\mathrm{Mg}^{2+} 与 Li + Li + Li^(+)\mathrm{Li}^{+} 的质量比为 45 : 1 45 : 1 45:145: 1 。中性溶质（如乙醇、甘油、蔗糖和 PEG-800）的斯托克斯半径使用公式（7）计算。
r = 16.73 × 10 − 12 × M 0.557 r = 16.73 × 10 − 12 × M 0.557 r=16.73 xx10^(-12)xxM^(0.557)r=16.73 \times 10^{-12} \times M^{0.557}
Where, M ( g ⋅ mol − 1 ) M g ⋅ mol − 1 M((g)*mol^(-1))M\left(\mathrm{~g} \cdot \mathrm{~mol}^{-1}\right) represents the molar mass of neutral organic solute. The pore radius distribution of nanofiltration membranes were represented as a probability density function and calculated by applying
式中， M ( g ⋅ mol − 1 ) M g ⋅ mol − 1 M((g)*mol^(-1))M\left(\mathrm{~g} \cdot \mathrm{~mol}^{-1}\right) 表示中性有机溶质的摩尔质量。用概率密度函数表示纳滤膜的孔径分布，并应用
Equation (8).  等式（8）。
d R ( r p ) d r p = 1 2 π r p ln ⁡ σ p exp ⁡ [ − ( ln ⁡ r p − ln ⁡ μ p ) 2 2 ( ln ⁡ σ p ) 2 ] d R r p d r p = 1 2 π r p ln ⁡ σ p exp ⁡ − ln ⁡ r p − ln ⁡ μ p 2 2 ln ⁡ σ p 2 (dR(r_(p)))/(dr_(p))=(1)/(sqrt(2pi)r_(p)ln sigma_(p))exp[-((ln r_(p)-ln mu_(p))^(2))/(2(ln sigma_(p))^(2))]\frac{d R\left(r_{p}\right)}{d r_{p}}=\frac{1}{\sqrt{2 \pi} r_{p} \ln \sigma_{p}} \exp \left[-\frac{\left(\ln r_{p}-\ln \mu_{p}\right)^{2}}{2\left(\ln \sigma_{p}\right)^{2}}\right]
The geometric mean radius ( μ p , nm μ p , nm mu_(p),nm\mu_{\mathrm{p}}, \mathrm{nm} ) were assessed by the Stokes radius of neutral organic solute at a 50 % 50 % 50%50 \% rejection, while effective pore radius ( r p , n m r p , n m r_(p),nmr_{p}, n m ) represents the Stokes radius of neutral organic solute with a measured rejection of 90 % 90 % 90%90 \%. Additionally, the distribution of the membrane pore size is represented by the geometric standard deviation of the PDF curve (б), defined as the ratio of the Stokes radius corresponding to 84.13 % 84.13 % 84.13%84.13 \% rejection to that at 50 % 50 % 50%50 \% rejection. The concentration of neutral organic solute ( 0.2 g L − 1 0.2 g L − 1 0.2gL^(-1)0.2 \mathrm{~g} \mathrm{~L}^{-1} in the feed solution) was investigated by the total organic carbon analyzer (TOC-LCPH, Shimadzu, Japan).
几何平均半径（ μ p , nm μ p , nm mu_(p),nm\mu_{\mathrm{p}}, \mathrm{nm} ）通过在 50 % 50 % 50%50 \% 截留率下中性有机溶质的斯托克斯半径来评估，而有效孔半径（ r p , n m r p , n m r_(p),nmr_{p}, n m ）表示具有测量的 90 % 90 % 90%90 \% 截留率的中性有机溶质的斯托克斯半径。另外，膜孔径的分布由 PDF 曲线的几何标准偏差（Δ F）表示，其定义为对应于 84.13 % 84.13 % 84.13%84.13 \% 排斥的斯托克斯半径与对应于 50 % 50 % 50%50 \% 排斥的斯托克斯半径的比率。通过总有机碳分析仪（TOC-LCPH，Shimadzu，Japan）研究中性有机溶质（进料溶液中的 0.2 g L − 1 0.2 g L − 1 0.2gL^(-1)0.2 \mathrm{~g} \mathrm{~L}^{-1} ）的浓度。

3. Results and discussion
3.结果和讨论

3.1. Membrane design and structural composition
3.1.膜设计和结构组成

The nascent PA layer were fabricated via interfacial polymerization between ogranic amine (PBG and PEI) and TMC on the polysulfone support membrane via interfacial polymerization. Subsequently, the surface amine groups of the nascent PA layer reacted with IC (dissolved in hexane) to form a PSA/PA functional layer in this work (Fig. 1a). After heat treatment, the modified nanofiltration membrane (PBG-IC) was obtained. Fourier-transform infrared (FTIR) spectroscopy of the functional layer obtained from the PBG-IC membrane by dissolving the polysulfone support layer, revealed the characteristic peaks at 914 cm − 1 914 cm − 1 914cm^(-1)914 \mathrm{~cm}^{-1} [24] and 1636 cm − 1 1636 cm − 1 1636cm^(-1)1636 \mathrm{~cm}^{-1} [14] corresponding to sulfonamide and amide groups, confirming the successful synthesis of the PSA/PA layer (Fig. 1b). Moreover, another membrane (PBG-0) was fabricated by
采用界面聚合法，在聚砜支撑膜上通过有机胺（PBG 和 PEI）与 TMC 的界面聚合制备了初生 PA 层。随后，新生 PA 层的表面胺基与 IC（溶解在己烷中）反应以在该工作中形成 PSA/PA 功能层（图 1a）。经热处理后，得到改性纳滤膜（PBG-IC）。通过溶解聚砜支撑层从 PBG-IC 膜获得的功能层的傅里叶变换红外（FTIR）光谱显示了对应于磺酰胺和酰胺基团的 914 cm − 1 914 cm − 1 914cm^(-1)914 \mathrm{~cm}^{-1} [24]和 1636 cm − 1 1636 cm − 1 1636cm^(-1)1636 \mathrm{~cm}^{-1} [14]处的特征峰，证实了 PSA/PA 层的成功合成（图 1b）。此外，另一种膜（PBG-0）通过以下方式制备：
Fig. 1. Membrane design and structural composition. (a) Schematical illustration of the interfacial polymerization process. PBG and PEI react with TMC at water/ hexane interface to form a loose PA layer. Then, the surface of nascent membrane contains free PBG monomers and residual amine groups from partial reaction of PEI, both of which will react with the subsequent IC. (b) The infrared spectroscopy of functional layer from PBG-IC and PBG-0 membranes with dissolved polysulfone support layer. © The ultraviolet spectroscopy of PBG and PEI from aqueous phase to organic phase. (d-f) TOF-SIMS spectrum about the distribution of sulfamide (de, − SO 2 NH − − SO 2 NH − -SO_(2)NH--\mathrm{SO}_{2} \mathrm{NH}- ) and amide (f, − CO 2 NH − CO 2 NH -CO_(2)NH-\mathrm{CO}_{2} \mathrm{NH}-) in the top and bottom of PBG-IC functional layer (left: top surface, right: bottom parts).
Fig. 1.膜设计和结构组成。(a)界面聚合过程的示意图。PBG 和 PEI 与 TMC 在水/己烷界面处反应以形成疏松的 PA 层。然后，新生膜表面含有游离的 PBG 单体和来自 PEI 部分反应的残余胺基，这两者将与后续的 IC 反应。(b)具有溶解的聚砜支撑层的 PBG-IC 和 PBG-0 膜的功能层的红外光谱。© PBG 和 PEI 从水相到有机相的紫外光谱。(d-f)关于 PBG-IC 功能层的顶部和底部中磺酰胺（de， − SO 2 NH − − SO 2 NH − -SO_(2)NH--\mathrm{SO}_{2} \mathrm{NH}- ）和酰胺（f， − CO 2 NH − CO 2 NH -CO_(2)NH-\mathrm{CO}_{2} \mathrm{NH} -）的分布的 TOF-SIMS 光谱（左：顶部表面，右：底部部分）。