Ultra-Selective Polyamide Membrane from Metal Organic Framework Assembly Regulated Interfacial Polymerization for Desalination and Water Reuse

Yue Wen Tongji University Ruobin Dai Tongji University Xuesong Li Tongji University Xingran Zhang Tongji University Xingzhong Cao Institute of High Energy Physics https://orcid.org/0000-0001-5011-5912 Zhichao Wu Tongji University Shihong Lin Vanderbilt University https://orcid.org/0000-0001-9832-9127 Chuyang Tang University of Hong Kong Zhiwei Wang (  zwwang@tongji.edu.cn ) Tongji University https://orcid.org/0000-0001-6729-2237


INTRODUCTION
The escalating global water shortage motivates sustainable water resource management for supplying clean, safe, and adequate water 1,2 . To date, reverse osmosis (RO) has become a leading technology to address the severe shortages in fresh water supply through desalination and wastewater reuse 3,4 . Although existing polyamide (PA) RO membranes exhibit high water-salt selectivity with salt rejection beyond 99% (for seawater desalination), they remain inadequate in removing certain harmful and regulated constituents from feedwater 5,6 .
For example, N-nitrosodimethylamine (NDMA) is a carcinogenic disinfection byproduct commonly found in the RO permeate of wastewater reuse plants 7 . Historically, high concentrations of NDMA (400,000 ppt on site and 20,000 ppt offsite) had been found in aquifers in southern California where RO-recycled wastewater was used for groundwater replenishment, which prompts California to implement a stringent action level for NDMA as low as 10 ppt. 8 Unfortunately, commercial RO membranes show unsatisfactory rejection of NDMA (30-80% for lab-scale tests [9][10][11] and merely 5-60% at pilot-to full-scale levels 12 ), which necessitates highly energy-intensive advanced oxidation processes (e.g., ultraviolet coupled with peroxide) to destroy NDMA in RO permeates 13,14 . The poor rejection of NDMA is attributed to its small molecular weight (74 g mol -1 ), uncharged state at pH 6-8, and strong dipole moment (6.3 Debye) 13,15 . Another challenge faced by commercial RO membranes in seawater desalination is the inadequate removal of boron that is neutral (as B(OH)3, molecular weight: 61.8 g mol -1 ) in seawater. Due to the low boron rejection (approximately 40-80%) by commercial RO membranes 16,17 , the desalinated permeate often requires an additional second-pass RO treatment, which increases the cost, energy consumption, and footprint of seawater desalination plants. The poor rejection of these important species by commercial RO membranes calls for the development of ultraselective RO membranes for more cost-effective seawater desalination and wastewater reuse.
Recent studies have shown that steric hindrance plays a critical role in determining transport through RO membranes 10,11 . Efforts have been dedicated to decrease the free volume of PA layers, involving heat treatment 5,18 and surface modification (e.g., surface coating or 'plugging') 9,19 . However, the improvement of solute rejection via these approaches is at the expense of water permeability 5,9,20 . While other efforts to fabricate more selective membranes have been attempted recently, such as incorporating porous nanomaterials 21,22 , stacking two-dimensional (2D) nanoflakes 23,24 , and integrating biomimetic nanochannels 1,25,26 , no study has thus far reported the production of defectfree membrane that effectively rejects small neutral solutes such as NDMA and boron.
Additionally, the scalable fabrication of membranes based on novel materials still face significant challenges.
The performance of the thin-film composite (TFC) RO membrane is primarily dependent on the PA active layer 27 , which is fabricated via an interfacial polymerization (IP) reaction between an amine monomer, e.g., m-phenylenediamine (MPD), and an acyl chloride monomer, e.g., trimesoyl chloride (TMC) 28 . As the IP process involves extremely fast and uncontrolled reaction at the water/oil interface 22,29 , the formation of the PA active layer with both fast water transport and outstanding solute rejection remains as a substantial technical challenge.
Amphiphilic nanoflakes, which possess highly asymmetric surface wettability and distinct physicochemical properties 30,31 , have garnered considerable attention in drug delivery 32 , catalysis 33 , and oil recovery 34 . Inspired by their great potential in interfacial properties regulation 35 , herein, we develop an ultra-selective polyamide RO membrane by enhancing the IP process at the interface to create a crumpled, free-standing, and ultrathin nanofilm with an intrinsic thickness of ~5 nm and a high cross-linking degree of ~97%.
The amphiphilic metal-organic framework (MOF) nanoflakes self-assemble at the water/hexane interface and regulate the heat dissipation and the transport of MPD monomers during IP ( Figure 1). This process, namely MOF assembly regulated IP (MARIP), results in an ultrathin PA nanofilm with unprecedentedly high rejections of NDMA (90.3±0.4%) and boron (90.1±3.4%). The mechanisms of amphiphilic nanoflakes in enhancing IP were further investigated using doppler broadening energy spectroscopy (DBES), isothermal titration calorimetry and molecular simulations.

RESULTS
Formation of free-standing polyamide nanofilm via MARIP. CuBDC MOF were used as the amphiphilic nanoflakes for regulating the IP process due to their large porosity, high aspect ratio, and asymmetric wettability (Supplementary Figure 2). The large difference in surface energy between the hydrophilic segments (i.e., -COOH groups from BDC on the edges of nanoflakes) and the hydrophobic segments (i.e., aromatic and aliphatic sites from BDC organic linkers exposed in the lateral dimensions) led to strong adsorption of CuBDC at the interface, resulting in an interfacial self-assembly of the MOF nanoflakes. 36 The formation of interfacial MOF layer was corroborated by the exceptional ability of MOF nanoflakes to stabilize oil/water mixture: when MOF nanoflakes were added in an oil/water mixture, stable oil-in-water emulsions formed spontaneously and almost immediately  To confirm the role of MOF nanoflake assembly in MARIP, we further fabricated Mix-MOF PA membrane by adding TMC/hexane solution to the system immediately (within 5 s) after MOF nanoflakes were added to water (without forming an interfacial selfassembly). Without horizontal alignment at the interface, the MOF nanoflakes were incorporated into the nanofilm during the IP process. In the absence of interfacial  Table 1), and more importantly, better solute rejection 45 . The denser PA nanofilm from MARIP was further confirmed by Doppler broadening energy spectroscopy (DBES). As a lower S parameter represents a smaller free volume (or sub-nanometer pores), 46 the consistently lower range of the S parameter of MARIP-0.1 as compared to that of PA films formed in FIP or conventional IP with support layer indicates that MARIP-0.1 has a smaller free volume ( Figure 2h).

Separation performance of polyamide nanofilms from MARIP. The separation
performance of the PA membranes was evaluated using crossflow filtration. The FIP membrane had a relatively low water permeance and low salt rejection, even lower than that of TFC-PA membrane prepared via conventional IP with a support layer. In contrast, the water permeance of the MARIP-0.1 and MARIP-0.2 membranes was 262% and 277% of that of the FIP membrane, respectively (Figure 3a). The dramatic permeance enhancement for PA membranes formed from MARIP was attributed to: (i) the larger effective filtration area for water transport and (ii) the lower intrinsic thickness of the active layer that reduces the water transport distance. As shown in Figure 3a

DISCUSSION
Interfacial polymerization (IP) is a complex and non-equilibrium process, where two monomers polymerize rapidly and irreversibly near the interface between two immiscible phases 49 . In MARIP, the amphiphilic MOF nanoflakes were used to enhance IP process to obtain an ultra-selective PA layer with simultaneous improvement of water permeance and selectivity. We hypothesize that the ultra-selectivity of PA membranes from MARIP is attributable to the combined effects of (i) accelerated IP reaction resulting from increased interfacial temperature due to hindered heat dissipation, and (ii) facilitated trans-interface transport of MPD caused by the reduced energy barrier (Figure 4) and enhanced electrostatic attraction at the interface. The heat release rate is closely related to PA nanofilm formation, and the intensive heat release accelerates the whole reaction process and enhances the cross-linking degree of the polymer matrix 52 . The local temperature rise could lead to "interfacial boiling", facilitating degassing of the solution to generate bubbles that led to the formation of nanovoids structure in the resulting PA film 28 . The heat release rate during IP process, measured by isothermal titration calorimetry (ITC), is shown in Figure 4a and Supplementary Figure 14.
With MOF nanoflakes at the interface, the heat release rate was remarkably higher than that of the control sample (Supplementary Figure 14). The addition of MOF nanoflakes nearly tripled the average heat release rate compared to that of the control FIP reaction ( Figure 4a). Compared with other porous nanomaterials with high thermal conductivity (e.g., carbon nanotubes 53 , graphene oxide 54 , and metal foam 55 ), MOF nanoflakes have a significantly lower thermal conductivity of 0.2231 W m -1 K -1 and thus impose a substantially higher resistance against heat dissipation towards aqueous phase during IP process. The heat retained by MOF nanoflakes accelerated the IP process which in turn released more heat, creating a positive feedback loop to promote a highly vigorous IP process. The accelerated IP process substantially improved the cross-linking degree (and thus the selectivity) of the resulting PA nanofilms. Moreover, with the 2D interfacial MOF self-assembly, the generated heat and nanobubbles were trapped between PA film and MOF nanoflakes during MARIP. The confined heat and nanobubbles drove the MPD monomers to the less dense area of the nascent nanofilm where the MPD further reacted with TMC to eliminate defects (i.e., spontaneous self-healing) 56 and promote the formation of a defect-free PA film that is highly crosslinked throughout.
During the IP process, MPD monomers diffuse from aqueous phase to the organic phase to form the PA nanofilm in organic phase near the interface 29 . The diffusivity of amine monomers in the reaction zone is of critical importance, i.e., a higher diffusivity benefits faster and more complete IP, leading to the formation of a denser PA matrix 57  We further performed molecular dynamics (MD) simulation of heat dissipation across the water/hexane interface in the presence of MOF. After 4 ns of the heat input, the atoms near the MOF/hexane system had higher temperature with a uniform distribution ( Figure  4c). The equilibrium temperature at the interface in the water/MOF/hexane system was significantly higher than that of the water/hexane system (Figure 4d), which was mainly due to the larger interfacial thermal resistance in the presence MOF (Supplementary Movie 2). For water/hexane system, the transfer of heat produced in IP was mainly resisted by the water/hexane interface with a thermal resistance of 4.38×10 -9 m K W -1 (Supplementary Table 2). In the water/MOF/hexane system, however, the heat transfer was resisted by the MOF/hexane interface, MOF, and MOF/water interface with a total thermal resistance of 1.39×10 -8 m K W -1 (Supplementary Table 3). The simulation confirmed that the presence of interfacial MOF hinders the heat dissipation, which benefits the formation of denser polyamide nanofilm.
Molecular dynamic simulation was also performed to evaluate the free energy barrier for MPD transport through the water/hexane and water/MOF/hexane interfaces. The simulation shows that MPD molecule has the lowest free energy at the water/hexane interface (Figure 4e), which can be explained by the fact that the hydrophilic amino groups of MPD could form strong hydrogen bonds with water while the hydrophobic benzene rings have a higher affinity to hexene. As a result, the MPD molecules tend to assume a 'α ' configuration at the interface that leads to a lower free energy. Once the MPD molecules transport across the interface into the hexane phase, they assume a ' β ' configuration with which one amino group enters the hexane phase with another remaining in water. When the MPD molecule completely entered the hexane phase, it changed to 'γ' configuration. Based on the position and configuration (which depends on position), we can define three states, i.e., α, β, and γ states for the MPD molecules.
The results from molecular dynamic simulation suggest that the γ state is unstable, i.e., the spontaneous transport of a single MPD molecule across the interface (without concentration gradient sustained by reaction in the hexane phase) is energetically unfavorable. The MPD molecule is most energetically favorable at the interface with a significant global energy minimum. The transport of an MPD molecule from the interface to the hexane phase involves an energy penalty of 9.30 kT at 25ºC (Figure 4e). The addition of the MOF nanoflakes changes the energy landscape for interfacial MDP diffusion. While the overall energy penalty for MPD transport to the hexane phase from the aqueous phase or from the interface is not reduced, the presence of the MOF nanoflakes at the water/hexane interface creates many local energy minima in the free energy curve that now becomes highly "zigzagging" (Figure 4f). These local energy minima result in metastable states with relatively small energy penalties in-between, thereby facilitating the transinterface transport of MPD.
As the pore size of MOF is similar to the molecular size of MPD (Supplementary Figure   19a and 19b), the zigzagging of the free energy curve is likely attributable to the periodic distribution of hydrophilic Cu-O groups and hydrophobic benzene ring groups of MOFs along the transport direction. While the strong molecular attraction between amino groups of MPD and Cu-O groups of MOF nanoflakes reduced the free energy, the repulsion between amino groups and benzene ring groups increased the free energy. In the presence of multiple local minima, the transport of MPD across the water/hexane interface only needs to overcome the largest of the energy penalties between two adjacent minima, which is considerably (~48%) lower than that in an MOF-free system. In other words, the reduced maximum energy penalty due to the presence of interfacial MOF assembly facilitates transport of MPD across the interface.
Interestingly, the simulation also shows that MPD molecules transport through the MOF In summary, we have developed a MOF assembly regulated IP process to fabricate highly crosslinked and ultra-selective PA membranes with performance transcending the upper bound of perm-selectivity tradeoff. Not only these membranes have a substantially higher water permeance compared with that of commercial TFC-PA membranes (~125% enhancement), they also achieved unprecedentedly high rejections (>90%) of small neutral molecules (e.g., boron and NDMA) that are regulated but challenging for existing RO membranes to remove. These PA membranes from MARIP can produce water meeting the stringent standard for potable water quality with a single-pass RO, leading to significant potential of cost-saving for seawater desalination and wastewater reuse. The mechanisms for forming these high-performance membranes in the presence of amphiphilic MOF nanoflakes include the confinements of trans-interface heat transfer and gas nanobubbles formed in-situ, and the enhancement of the trans-interface transport of diamine monomers.
These factors together promote the formation of a highly cross-linked, ultra-thin, and corrugated PA active layer that is both highly water permeable and exceptional in rejecting salts and micropollutants. To monitor the diffusion process of MPD monomers from aqueous phase with/without nanoflakes to organic phase through interface, UV-vis spectra-photometer (TU1810, PERSEE, China) was modified, with the height of UV-light 1 mm above the water/hexane interface 62 . The change in absorbance at the monitoring point was recorded every 30 s once the hexane solution was added into the system. For the MOF addition, 0.10 wt.% MOF/hexane solution was added to fully cover the surface of aqueous phase and evaporated thoroughly before the addition of hexane solution.

METHODS
Separation performance tests. Membrane separation performance was tested using a laboratory-scale crossflow filtration setup at 16.0 bar and 24 ± 0.1°C as described in our previous study 63 . A membrane coupon (6.25 cm 2 ) was pre-compacted with DI water at a crossflow rate of 22.4 cm s -1 for 6 h before the measurement of water flux and salt rejection.
Water flux (Eq. (1)) was calculated by weighing the mass of the permeate over time.
where W (L·m -2 ·h -1 ) is water flux, (g) is the mass of the permeate collected over a designated time interval ( , h), Am is the membrane surface area (m 2 ), and (kg·L -1 ) is the density of water. The membrane water permeance (A) was determined as the ratio of water flux over the applied pressure.
The salt rejection was tested using a feed solution containing 2000 mg L -1 NaCl. The salt rejection (R) was determined according to Eq. (2): where the conductivity of the feed Cf (mg L -1 ) and permeate Cp (mg L -1 ) were determined using an electrical conductivity meter (Cole-Parmer, USA).
The salt permeability coefficient (B) was determined using Eq. (3): where the term exp (− W ) is used to correct the concentration polarization effect, and is the mass transfer coefficient in the crossflow cell calculated according to She et al. 64 To assess the membrane separation performance, we further measured the rejection of the inorganic micropollutant (boron, Mw=61.8 g mol -1 ) and the very small neutral micropollutant (NDMA, Mw=74.1 g mol -1 ). The concentration of boron (5 mg L -1 , at pH

Computation simulation.
To explore the effect of the MOF nanoflakes on the dissipation of reaction heat from the MOF/hexane interface to the water and hexane phases, we simulated the heat diffusion process in both water/MOF/hexane and water/hexane systems for comparison. A symmetrical setup of the thermal diffusion system was used in the simulation. Heat was continuously added to the two slabs at a specific rate by changing the velocity of the atoms, with two cold slabs at constant temperatures located in the water and hexane phases, respectively (Supplementary Figure 16). Both the water/hexane system and water/MOF/hexane system contain 4,172 water molecules, 936 hexane molecules, and 100 MPD molecules and 36 TMC molecules in the water and hexane phases, respectively.
For water/MOF/hexane system, the MOF nanoflake has a size of 4×6×6 unit cells with periodic conditions in the y-and z-directions, which corresponds to a size of about 21×39×43 Å 3 . The slabs for adding heat were in the hexane phase close to the MOF/hexane interface. The releasing rate of heat from the interfacial polymerization between MPD and TMC with an interfacial area of 1,677 Å 2 was estimated to be 7.0×10 -7 kcal mol -1 ps -1 according to the bond energy between MPD and TMC. The release rate of heat was determined by quantum calculations in combination with the measured diffusion rate of MPD across the MOF/hexane interface. Considering the sizes of the simulated system and MOF were several orders smaller than that in the experiments, the rate of adding heat to the slabs, Q1, was set to 0.70 kcal mol -1 ps -1 to simulate the heat from the exothermic interfacial polymerization reaction. The cold slabs in the water and hexane phases were maintained at a temperature of 298 K with the Langevin thermostat, and heat would be removed from these two slabs since a microcanonical (NVE) ensemble was applied to the whole system. The damping factor of the Langevin thermostat was set to 0.1 ps according to the work of Li et al. 67 The amounts of heat removed from these two slabs in every unit time were accumulatively collected. The molecular dynamics simulation was conducted with the Lammps package. All the slabs have a thickness of 5 Å. The simulations were run for 4,000,000 steps with a timestep of 1 fs. The heat was added to the system every 10 steps, and the removed heat from the cold slabs were collected every 100 steps. Before the simulation of heat diffusion, the system had been fully equilibrated in an NPT ensemble at a temperature of 298 K and a pressure of 1 atm. Since the system was symmetric in the xdirection, we averaged the temperature profile of the system in the x-direction.
To get the insight of trans-interface resistance of MPD from water to hexane with and without MOF, the combination of Umbrella Sampling 68 and weighted histogram analysis method (WHAM) 69,70 was used to calculate the necessary free energy of MPD that transported through water/hexane interface. The systems were simulated with NVE thermodynamic ensemble at 298 K temperature. Two simulated systems were constructed, i.e., one is water/hexane system, and the other is water/MOF/hexane system. Both