Tuning polyamide membrane chemistry for enhanced desalination using Boc-protected ethylenediamine and its in situ Boc-deprotection

The global scarcity of freshwater resources, exacerbated by population growth and industrialization, necessitates the development of advanced desalination technologies. Polyamide membranes, developed four decades ago, have revolutionized the desalination industry, particularly thin-film composite (TFC) membranes created via interfacial polymerization (IP). This method offers greater control over layer chemistry and structure compared to older cellulose-based membranes, leading to optimized flux and salt rejection. Recent research focuses on semi-aromatic membranes, offering further tunability. Despite advancements, challenges remain, including balancing flux and rejection, improving chlorine resistance, antifouling properties, lifespan, and biofouling resistance. Two main strategies exist for enhancing performance: synthesizing new monomers for IP or modifying existing membranes chemically or physically. The first is less explored due to synthesis complexities, while the second, involving post-polymerization modifications, is more common. Studies have shown that adding hydrophilic groups (like sulfonic acid or aliphatic amines) enhances flux, sometimes without significantly affecting salt rejection. This work aims to improve polyamide membrane performance through a novel in situ chemical modification using Boc-protected ethylenediamine (EDA) during IP, followed by controlled deprotection to introduce hydrophilic ammonium groups that enhance the membrane's characteristics.

Previous research has explored various approaches to improve the performance of polyamide desalination membranes. Yong et al. demonstrated enhanced permeate flux by adding *m*-phenylenediamine-5-sulfonic acid post-polymerization, attributing the improvement to hydrophilic sulfonic acid groups. Similarly, Prera et al. regulated the aqueous phase composition by adding 1,3-diamino-2-hydroxypropane (DAHP), leading to increased flux. Aliphatic amino acids, such as L-arginine (Chen et al.) and L-lysine (Xu et al.), have also been investigated as aqueous additives, resulting in improvements in both flux and salt rejection. These studies highlight the potential of modifying the aqueous phase composition during IP to tailor membrane properties. The current study builds upon this knowledge by investigating the impact of ethylenediamine-monoboc (EDA-Boc), a molecule that allows for in situ tuning of the membrane's active layer via Boc-deprotection, thus providing a controlled introduction of hydrophilic groups.

This study focused on incorporating Boc-protected ethylenediamine (EDA-Boc) into the polyamide active layer during interfacial polymerization (IP) with m-phenylenediamine (MPD) and trimesoyl chloride (TMC). One amino group in EDA was protected with a Boc group, allowing the other to react with TMC. Three membranes were fabricated: MPD-TMC (control), MPD-TMC-EDA-Boc (Boc-protected EDA incorporated), and MPD-TMC-EDA-Deboc (Boc-deprotected). Free-standing active layers were also synthesized for characterization purposes. Characterization techniques included ATR-FTIR spectroscopy (to identify functional groups), solid-state ¹³C NMR (to confirm the chemical structure), elemental analysis (CHNS/O, to determine elemental composition and crosslinking degree), scanning electron microscopy (SEM, to analyze surface and cross-sectional morphology), energy-dispersive X-ray spectroscopy (EDX, for elemental mapping), atomic force microscopy (AFM, to measure surface roughness), and water contact angle (WCA) measurements (to assess surface hydrophilicity). Desalination performance was evaluated using DI water and various salt solutions (NaCl, MgCl2, Na2SO4, MgSO4) as feeds under different pressures, determining permeate flux and salt rejection. Antifouling properties were investigated using bovine serum albumin (BSA) as a fouling agent, and flux recovery was assessed after backwashing. The synthesis of EDA-Boc involved reacting ethylenediamine with di-tert-butyl dicarbonate in dichloromethane, followed by purification. The IP involved preparing aqueous amine solutions (MPD and MPD/EDA-Boc) and an organic TMC/n-hexane solution. The polyamide layer was synthesized on polysulfone/polyethylene terephthalate (PSf/PET) supports via a non-solvent-induced phase separation (NIPS) method. Boc-deprotection was achieved using a 20% HCl aqueous solution. Calculations for crosslinking degree used standard equations based on elemental analysis. Permeate flux and salt rejection calculations employed standard equations based on volume, time, membrane area, and conductivity measurements.

ATR-FTIR, ¹³C NMR, and elemental analysis confirmed the successful incorporation of EDA-Boc and subsequent Boc-deprotection. SEM revealed variations in surface morphology across the three membranes, with the MPD-TMC-EDA-Deboc membrane showing a denser and more uniform active layer. The thickness of the active layer changed from 145 nm (MPD-TMC), 244 nm (MPD-TMC-EDA-Boc) and 187 nm (MPD-TMC-EDA-Deboc). Elemental analysis indicated an increased carbon percentage and decreased oxygen percentage in the MPD-TMC-EDA-Deboc membrane due to Boc-deprotection. The degree of crosslinking showed a slight increase from MPD-TMC to MPD-TMC-EDA-Deboc. AFM measurements showed that MPD-TMC-EDA-Boc membrane was smoother (6.3 nm) than MPD-TMC (7.3 nm) and MPD-TMC-EDA-Deboc (7.9 nm) membrane. WCA measurements indicated increased hydrophilicity in the MPD-TMC-EDA-Deboc membrane (55°) compared to MPD-TMC (70°). Desalination tests with DI water showed a significant increase in permeate flux for both MPD-TMC-EDA-Boc and MPD-TMC-EDA-Deboc membranes compared to the control at various pressures. The MPD-TMC-EDA-Deboc membrane exhibited superior salt rejection (98 ± 0.5% for NaCl) compared to both the MPD-TMC and MPD-TMC-EDA-Boc membranes. Seawater nanofiltration experiments showed high salt rejection (97%) and flux (23 L m⁻² h⁻¹) for the MPD-TMC-EDA-Deboc membrane. The antifouling tests showed that although the MPD-TMC-EDA-Deboc membrane fouled more than the other two membranes, it also exhibited the highest flux recovery (95 ± 0.5%) after backwashing. The increased hydrophilicity of MPD-TMC-EDA-Deboc resulting from the ammonium groups positively impacted permeate flux. The enhanced salt rejection was attributed to the repulsive interaction between the positively charged ammonium ions and cations in the feed.

The results demonstrate the effectiveness of the Boc-protection/deprotection strategy for enhancing the performance of polyamide desalination membranes. The in situ modification allows for controlled introduction of hydrophilic ammonium groups, improving hydrophilicity and flux. The enhanced salt rejection is attributed to both increased crosslinking and the electrostatic repulsion between the positively charged surface and cations in the feed. The higher flux recovery after backwashing suggests improved antifouling properties. The MPD-TMC-EDA-Deboc membrane outperforms the control and other membranes reported in the literature. The method avoids the potential drawbacks of post-polymerization modifications or the use of inorganic fillers, which can cause membrane defects. This approach provides a facile and effective method for tuning membrane properties.

This study successfully demonstrated the use of Boc-protected ethylenediamine and in situ Boc-deprotection to enhance the desalination performance of polyamide membranes. The resulting MPD-TMC-EDA-Deboc membrane exhibited superior salt rejection, permeate flux, and antifouling properties compared to the control. The method offers a promising strategy for tuning the active layer chemistry of polyamide membranes to optimize their performance for desalination applications. Future research could explore other Boc-protected amines or different deprotection methods to further enhance membrane properties. Investigating the long-term stability and chlorine resistance of these modified membranes would also be valuable.

The study used a specific fouling agent (BSA) to simulate membrane fouling. The results may not be entirely representative of all types of fouling encountered in real-world desalination applications. The long-term stability and durability of the modified membranes under various operating conditions (e.g., high pressure, high salinity, presence of other foulants) require further investigation. The study's scale was laboratory-based, and further research is necessary to assess the scalability and economic feasibility of the approach for large-scale desalination applications.