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

Membranes fabrication and characterizationThe Fig. 1 shows the possible reactions leading to MPD-TMC and MPD-TMC-EDA-Boc active layers decorated on the polymeric support. Once the EDA-Boc is incorporated in the membrane active layer, it can be readily deprotected leading to the generation of hydrophilic functional groups on the membrane. The possible reaction of Boc-deprotection and generation of ammonium ions in the membrane active layer is given in the Fig. 1. Hence, the current study was aimed at decorating the membrane active layer with such a molecule that can be in situ tuned resulting in enhancing the performance of the membranes in terms of permeate flux and antifouling potential while keeping the rejection considerably high.Fig. 1The possible reactions, active layers of MPD/TMC and MPD-TMC-EDA-Boc active layers, and Boc-deprotection in the case of MPD-TMC-EDA-Deboc active layers.MPD-TMC, TMC-EDA-Boc, and MPD-TMC-EDA-Deboc membranes were characterized by several techniques. ATR-FTIR analysis is shown in Fig. 2. Owing to the thin active layer and limited detection capability of the FTIR technique, the FTIR analysis was carried out for the free-standing active layers which were synthesized under the same conditions that were used for the fabrication of the membranes. In the case of the MPD-TMC active layer, several functional groups were observed which include N-H stretching vibrations located in the range of 3400 cm−1 followed by the peaks located at around 3000 cm−1 which are attributed to the aromatic -C-H stretching of the benzene rings of MPD and TMC19. It is important to note that upon Boc-deprotection, a broad peak of the N-H functional groups was observed in the case of the MPD-TMC-EDA-Deboc active layer. This observation confirmed the fact that the Boc-deprotection has resulted in a change in the amine functions in the active layer which can enhance the hydrophilicity of the membrane. Similarly, other peaks related to carbonyl groups of the amide linkage (-CONH) can be found in the active layer located at around 1600 cm−1.Fig. 2FTIR spectra of MPD-TMC, MPD-TMC-EDA-Boc, and MPD-TMC-EDA-Deboc free-standing active layers.To further establish the structure of the membranes, we carried out solid-state 13C NMR measurements of the free-standing active layers of the MPD-TMC-EDA-Boc (Fig. 3a) and MPD-TMC-EDA-Deboc (Fig. 3b) active layers of the membranes. The MPD-TMC-EDA-Boc membrane possessed small peaks located at 19.7 ppm, 32.1 ppm, and 38.1 ppm corresponding to the carbon atoms of the Boc and EDA. The peaks spanning from 106.6 ppm to 138.6 ppm can be attributed to the aromatic carbon atoms of the polyamides due to MPD/TMC while the peak at 155.3 ppm can be attributed to the carbonyl ( > C = O) group of the amide linkage of the polyamide network. After Boc-deprotection, the peak due to Boc is completely absent in the NMR spectrum of the MPD-TMC-EDA-Deboc membrane. The peaks located at the aliphatic region of 19 to 38 ppm shifted and merged with the peaks of the aromatic region, which can be attributed to the protonation of the amino groups yielding ammonium ions. Owing to the positive charge on N atoms of the ammonium ions, the carbon atoms of the EDA are more deshielded, and hence the peaks are shifted and merged with aromatic peaks. However, the other peaks such as the peaks previously identified in the MPD-TMC-EDA-Boc membrane remained intact including the carbonyl peak. These findings suggested that the Boc groups were successfully removed from the active layer yielding quaternary ammonium ions in the membrane active layer.Fig. 3: NMR analysis of the membrane active layers.Solid state 13C-NMR spectra of (a) MPD-TMC-EDA-Boc and (b) MPD-TMC-EDA-Deboc membranes.The variation in the chemical compositions of the active layers of the membranes is obvious and is a representation of the chemical changes brought about due to the incorporation of EDA-Boc and subsequent Boc-deprotection. The percentage of each element changed across all the membranes where the carbon percentage increased from MPD-TMC to MPD-TMC-EDA-Boc to MPD-TMC-EDA-Deboc. This increase in the percentage of C can be attributed to the presence of EDA-Boc which contains more C than oxygen and nitrogen. This fact can also be observed in decreased concentrations of O and N in the MPD-TMC-EDA-Boc and MPD-TMC-EDA-Deboc membranes. The highest variation in the composition of membranes was observed for the MPD-TMC-EDA-Deboc membrane where the C was 68 ± 0.5%, and O was reduced to 15 ± 0.5%. The decrease in the percentage of O can be attributed to the loss of tert-butoxy groups owing to Boc-deprotection. The percentage of N was observed to be 13.7% in the case of MPD-TMC-EDA-Deboc membrane which might be attributed to two reasons which are Boc-deprotection causing readjustment of elemental composition whereas the second reason is the addition of chloride ions (Cl−) as counter ions of the ammonium ions. Additional evidence for ammonium ions was also obtained by CHNS analysis of the active layers of MPD-TMC-EDA-Boc and MPD-TMC-EDA-Deboc membranes. The CHNS analysis revealed that the percentage of hydrogen (H) was increased from 5.1% to 5.4% for MPD-TMC-EDA-Boc and MPD-TMC-EDA-Deboc membranes, respectively. These slight variations of elemental composition agree with the composition of the reaction mixture because the concentration of EDA-Boc was low during IP.Since the polyamide active layer is grown as a crosslinked network, the degree of crosslinking was also calculated for all the fabricated membranes as given in Table 1. A minor variation in the degree of crosslinking was observed from MPD-TMC to MPD-TMC-EDA-Boc and MPD-TMC-EDA-Deboc membranes. To avoid interference from the membrane supporting layers, the degree of crosslinking was calculated by using free-standing active layers of all membranes that were fabricated under similar conditions that were used for membrane fabrication. The degree of crosslinking of the membrane was calculated by elemental analysis by using the set of equations from 1 to 318 which are generally used for MPD/TMC reactions.$${\rm{Crosslinking\; Degree}}( \% )=\frac{X}{X+Y}\times 100$$
(1)
$${\rm{X}}+{\rm{Y}}=1$$
(2)
$${\rm{O}}/{\rm{N}}=\frac{3X+4Y}{3X+2Y}$$
(3)
Where “X” represents the crosslinked parts of the polyamide while “Y” represents the linear fractions of the polyamide active layer of the membrane, and ‘O/N’ is the Oxygen/Nitrogen ratio found by elemental analysis.Table 1 The elemental composition of the active layers of all three membranes and the degree of crosslinkingFollowing the determination of the structure and variations in the chemistry of the active layers, we proceeded further to investigate the other features of the membranes which included surface morphology analysis, cross-sectional studies, elemental composition, surface roughness, and wettability of the membrane surface. For surface SEM analysis, an appropriate area of each membrane was subjected to SEM analysis as shown in Fig. 4. The surface morphology analysis of the MPD-TMC membrane revealed the existence of a polyamide active layer where the polyamide chains were arranged in the form of a network with valleys in between the ridges (Fig. 4a–c). A variation in the surface morphology was observed in the case of the MPD-TMC-EDA-Boc membrane where the SEM micrographs (Fig. 4d–f) showed the existence of a relatively smoother surface having the presence of a fine polyamide active layer. The altered surface morphology can be attributed to the reaction of the EDA-Boc amine with some of the acid chloride groups in TMC which results in a different growth of the polyamide active layer than typical MPD/TMC morphology. These observations are obvious as a minor variation in the chemistry of the participating monomers induces major changes in the structure of the membrane which eventually results in changes in the performance of the membrane. On moving further to the MPD-TMC-EDA-Deboc membrane, a completely different surface morphology was observed. The surface SEM micrographs of the MPD-TMC-EDA-Deboc membrane revealed the reappearance of a more refined ridge and valley network of the polyamide (Fig. 4g–i). The Boc-deprotection of the MPD-TMC-EDA-Boc membrane led to the generation of a fine, dense, and uniform network. Compared to the MPD-TMC control membrane, the ridges are sharper with relatively deeper valleys for the MPD-TMC-EDA-Deboc membrane. This can be attributed to the fact that the treatment of the MPD-TMC-EDA-Boc membrane with 20% HCl led to the removal of the bulky Boc groups along with all oligomers generated during the IP process. This led to the generation of a rougher membrane surface. Hence, the comparison of the SEM micrographs of the membranes confirmed the expected variation in the structural features of the membranes which has resulted in a variation in the performance of the membranes during desalination experiments.Fig. 4: Surface SEM analyses of the fabricated membranes.SEM micrographs showing the surface morphology of MPD-TMC (a–c); MPD-TMC-EDA-Boc (d–f); and MPD-TMC-EDA-Deboc (g–i) membranes at different magnifications.Cross-sectional SEM analysis of the membranes was also carried out. Different magnifications of the cross-sectional SEM micrographs of all the fabricated membranes MPD-TMC (Fig. 5a–c), MPD-TMC-EDA-Boc (Fig. 5d–f), and MPD-TMC-EDA-Deboc (Fig. 5g–i) are presented. An intact asymmetric membrane structure was observed for all three membranes where the lowest layer represents the nonwoven fabric (PET), the middle layer is PSF UF support, and the top layer is the polyamide active layer of the membranes. This asymmetric structure is an essential structural feature of the membranes which is essential for desalinating the salts present in the feed and provides free passageways to the permeating water molecules through the membranes. The active layer is a highly dense skin layer having extremely fine pores that separate the salts from the permeating water owing to size exclusion in addition to other mechanisms involved in desalting the water. The thickness of the active layers of the membranes was measured by ImageJ software where the thickness was found to be varied from one membrane to another. The thickness of the polyamide active layers for MPD-TMC, MPD-TMC-EDA-Boc, and MPD-TMC-EDA-Deboc was found to be 145 nm, 244 nm, and 187 nm, respectively. Therefore, on Boc-deprotection, the thickness of the membrane was decreased which could result in reduced mass transfer resistance during filtration and passage of the feed through the membrane. The PSF UF support possesses several finger-like projections which can be attributed to the non-solvent-induced phase inversion/separation (NIPS) where the solvent DMAc is rapidly de-mixed from the polymer matrix and dissolves into water. These finger-like projections offer free channels for the unhindered transport of permeating water molecules through the membrane.Fig. 5: Cross-sectional SEM analyses of the fabricated membranes.Cross-sectional view of membranes MPD-TMC (a–c); MPD-TMC-EDA-Boc (d–f); and MPD-TMC-EDA-Deboc (g–i) at varying magnifications.The elemental composition of the membranes was determined by EDX analysis of the membranes (Fig. 6). A similar composition to that of the free-standing active layers having C, N, and O was observed for all the membranes structure.Fig. 6: EDX analyses of the fabricated membranes.EDX spectra of MPD-TMC (a); MPD-TMC-EDA-Boc (b); and MPD-TMC-EDA-Deboc (c) membranes.The mapping analysis of the membrane also confirmed the uniform distribution of all the elements that were found in the membranes (Fig. 7). The intensity of the dots in each map is related to the concentration of that element in the membrane structure where carbon being the most abundant has the highest intensity of the C atoms in the map followed by O and N. The concentration of C and N is almost the same whereas O is slightly more concentrated compared to N. Normally, a N:O of 1 is expected for the completely crosslinked polyamide network confirming that each N is part of the amide bond (-CONH) and linked to the O atom of the >C = O group of the amide bond. Hence, in an ideal situation, the concentration of N should be equal to that of O but in the polyamide active layer certain acid chloride groups are also hydrolyzed during IP upon exposure to the aqueous medium and hence the concentration of O is slightly more than that of N. This fact has also been observed in the elemental composition of the free-standing active layers of the membranes. Moreover, the membrane’s bulk porosity was also calculated using the given Eq. (4)20. The membrane’s bulk porosity was determined to be 9.6, 5.7, and 6.6% for MPD-TMC, MPD-TMC-EDA-Boc and MPD-TMC-EDA-Deboc membranes, respectively.$$\varepsilon\, \left( \% \right)=\frac{{W}_{{wet}}-{W}_{{dry}}}{V{\rho }_{w}}\times 100$$
(4)
Where Wwet, and Wdry are the wet and dry weights of the membrane in kg, V is the volume of the dry membrane in m3 and \({\rho }_{w}\) is the density of the water in kg m−3.Fig. 7: Mapping analyses of the fabricated membranes.Elemental mapping images of MPD-TMC (a); MPD-TMC-EDA-Boc (b); and MPD-TMC-EDA-Deboc (c) membranes showing the distribution of surface elements.Another extremely important feature of the membranes is the surface roughness which contributes to several factors such as surface wettability and membrane surface fouling during filtration experiments. The surface roughness of the membranes was determined by atomic force microscopy (AFM) analysis of the membranes. The 2D and 3D AFM images of the membranes are given in Fig. 8. As depicted by the SEM surface micrographs of the membranes, the MPD-TMC-EDA-Boc (Fig. 8b) membrane appeared smoother compared to the MPD-TMC (Fig. 8a) and MPD-TMC-EDA-Deboc (Fig. 8c) membranes. An average surface roughness (Ra) of 6.3 nm was recorded for MPD-TMC-EDA-Boc membranes compared to 7.3 nm for MPD-TMC and 7.9 nm for MPD-TMC-EDA-Deboc membranes. Moreover, the reaction of the MPD and TMC can also be possibly impacted to a certain extent as some of the sites of the TMC are covered by EDA-Boc which eventually impacts the way of growth of the polyamide active layer during IP. However, upon Boc-deprotection, the tert-butoxy groups are removed from the polyamide active layer coupled with the removal of certain oligomers yielding a rougher membrane surface as has been observed in the case of MPD-TMC-EDA-Deboc membrane. These surface roughness values of the membranes agree with the surface morphologies observed for the membranes during SEM analysis.Fig. 8: Surface roughness analyses of the membranes by atomic force microscopy.2D and 3D AFM microscopic images of (a) MPD-TMC, (b) MPD-TMC-EDA-Boc and (c) MPD-TMC-EDA-Deboc illustrate the surface roughness of prepared membranes.Another closely related feature is the membrane’s surface wettability which affects the performance of the membranes such as the antifouling potential and permeability. The wettability of a membrane depends on several factors such as chemistry, the active layer, and the surface roughness of the membrane. Generally, the membrane surface wettability is determined by the water contact angle (WCA) of the membranes where the lowering of WCA value is associated with increasing the hydrophilicity of the membrane and vice versa. Generally, Young’s model considers the fact that the increase in membrane surface roughness causes an increase in its surface wettability. On the other hand, the Wenzel model considers the fact that a decrease in membrane surface roughness causes a decrease in the water contact angle (WCA) of the membrane which has also been observed in literature21,22. In the current scenario, this relationship between surface roughness and hydrophilicity is true only when we move from the MPD-TMC membrane to MPD-TMC-EDA-Boc as the surface roughness is decreased from 7.3 nm to 6.3 nm. Hence, the WCAs for MPD-TMC and MPD-TMC-EDA-Boc membranes were measured to be 70.0° and 68.0° (Fig. 9). However, in the case of the MPD-TMC-EDA-Deboc membrane, the surface hydrophilicity increases (WCA = 55.0o) with an increase in the membrane surface roughness (Ra = 7.87 nm). This observation highlights the fact that the surface roughness of the membrane is not the only contributing factor in controlling the wettability of the membrane, the chemistry of the membrane plays a significant role in determining the surface wettability of the membranes. The MPD-TMC-EDA-Deboc membrane has an altered membrane chemistry owing to the generation of hydrophilic ammonium ions on the surface of the membrane. The additional hydrophilic functionality on the membrane surface results in the lowering of the WCA of the membrane resulting in increased surface hydrophilicity.Fig. 9Contact angle measurements of prepared membranes.Desalination performance of the membranesFollowing characterization and establishing the structural features of the membranes, the performance of the membranes was evaluated. Initially, the effect of feed pressure on the permeate flux was measured by using DI water as feed while the membranes were installed in parallel on a crossflow filtration system. An obvious increase in permeate flux was observed with an increase in feed pressure (Fig. 10). The control MPD/TMC membrane was measured to have the lowest flux compared to the two other membranes. The highest pressure applied in the current study was 30 bar where a greater permeate flux was observed for the MPD-TMC-EDA-Boc (33.0 ± 0.5 L m−2 h−1) and MPD-TMC-EDA-Deboc (37.0 ± 0.2 L m−2 h−1) membranes compared to the MPD-TMC membrane (26.0 ± 0.2 L m−2 h−1) (Fig. 10). This increase in permeate flux from MPD-TMC to MPD-TMC-EDA-Deboc membrane is anticipated from the structure of the membranes. The MPD-TMC control membrane possesses a normal polyamide network having only amide linkage (>CONH) as the hydrophilic functional group of the membrane whereas the other component is a hydrophobic benzene ring. On the other hand, the MPD-TMC-EDA-Boc membrane has not only additional amide linkages but also a linear aliphatic component (EDA-Boc) which enhances the hydrophilicity and causes a slight increase in the flexibility of the polyamide active layer of the membrane. Hence, the MPD-TMC-EDA-Boc membrane demonstrated a higher permeate flux. In the case of the MPD-TMC-EDA-Deboc membrane, the generation of ammonium ions on the membrane develops a strong hydration layer owing to interaction with the water molecules which in turn causes an increase in the permeate flux of the membrane. Furthermore, the WCA measurements of the membranes also revealed that the MPD-TMC-EDA-Deboc membrane has the lowest WCA (55o) among all the fabricated membranes. This lowering of WCA reflects an increase in hydrophilicity which eventually results in increased permeate flux. Hence, the membrane surface chemistry is crucial in contributing to the flux enhancement of the membrane. Despite a slight increase in the degree of crosslinking from MPD-TMC to MPD-TMC-EDA-Deboc membrane, the flux has increased which further confirms the fact that the permeate flux is governed by multiple factors including membrane surface wettability, chemical structure, and crosslinked physical network.Fig. 10Effect of feed pressure on the permeate flux of the fabricated MPD-TMC, MPD-TMC-EDA-Boc, and MPD-TMC-EDA-Deboc membranes using deionized water as feed where the membranes are installed in parallel on a crossflow filtration system. The error bars show the standard deviation (n = 3).Following measurements of permeate flux, another important parameter is the measurement of the rejection of the salts by the fabricated membranes. The rejection of both monovalent (NaCl) and divalent salts (MgCl2, Na2SO4, and MgSO4) was measured by using the fabricated membranes at a pressure of 20 bar (Fig. 11). The rejection of NaCl stayed >93% for all the fabricated membranes including the control MPD-TMC membrane. In comparison to the control MPD-TMC membrane, the MPD-TMC-EDA-Boc and MPD-TMC-EDA-Deboc membranes showed higher NaCl rejection. A NaCl rejection of 95 ± 0.5% was measured for the MPD-TMC-EDA-Boc membrane while it was further increased to 98 ± 0.5% for the MPD-TMC-EDA-Deboc membrane. The rejection of divalent salts was measured to be >95% for all the membranes whereas again the MPD-TMC-EDA-Deboc membrane showed the highest salt rejection (>98%) among all the fabricated membranes. Among all the salts studied in the current work, MgCl2 was the most rejected salt, having a rejection of 99% (Fig. 11). Like flux, the rejection of salts by the membrane involves several mechanisms which include size exclusion, the crosslinked polyamide network, surface charge/charge exclusion, solution diffusion, and advective transport of salts through the membrane23,24. As far as the size exclusion is concerned, the hydrated monovalent ions being smaller in size can readily pass through the membrane compared to bigger salt ions. Therefore, the difference between the rejection of monovalent and divalent salts can be described by simply considering the size exclusion principle. Now as we consider the divalent salts, MgCl2 was measured to have the highest rejection among all the tested salts which can be explained by considering several factors such as the size, charge, and number of ions per molecule. Magnesium has a charge of +2 and a bigger hydration radius compared to Na+ (0.86 nm cf. 0.72 nm). Similarly, SO4-2– has a greater hydration radius than Cl– (0.76 nm cf. 0.66 nm)25,26. Hence, considering the hydration radii of the ions, salts such as MgSO4 are expected to have the highest rejection. A rejection of 98.5 ± 0.5% was measured for MgSO4 which is comparable to the rejection of MgCl2 (99.0 ± 0.5%). The slightly higher rejection of MgCl2 can be attributed to not only the hydration radius but also to the number of ions being rejected. In the case of MgCl2, three ions per molecule are rejected by the membrane under filtration conditions. The rejection of Na2SO4 (98.0 ± 0.5%) is comparable to that of NaCl (98%) which highlights a significant observation that the rejection of the salts depends more on the nature of the ions involved in addition to the charge on the ions27. The higher salt rejection performance of the MPD-TMC-EDA-Deboc membrane can be attributed to the presence of positively charged ammonium ions on the surface of the membrane due to the Deboc of the EDA groups. A repulsive force exists between the ammonium ions on the surface of the membranes and cations such as Na+ and Mg2+ present in the feed leading to higher salt rejection. The increasing degree of crosslinking (Table 1) from MPD-TMC to MPD-TMC-EDA-Boc and MPD-TMC-EDA-Deboc membranes is also a contributing factor to increasing the rejection of salts by the membrane. In addition, the decrease in bulk porosity of the MPD-TMC-EDA-Deboc membrane can also result in higher rejection rates since less number and narrower pathways are available for the penetration of the salts. This is attributed to the increased resistance provided by the narrow channels of the polyamide framework of the membranes. Therefore, altering the active layer chemistry impacts the chemical and physical structure of the membrane which eventually results in varied desalination performance of the membrane. For the sake of comparison, the seawater NF permeate at a TDS level of 33,700 ppm was collected from the permeate of the NF membrane, and the desalination potential of the fabricated membranes was explored. The MPD-TMC-EDA-Deboc membranes showed a rejection rate of 97%, while the MPD-TMC-EDA-Boc membrane had a rejection rate of 94%, and the MPD-TMC membrane rejected 92% of the TDS.Fig. 11The rejection performance of the mono, divalent ions, and the seawater NF permeate by the three fabricated membranes; MPD-TMC, MPD-TMC-EDA-Boc, and MPD-TMC-EDA-Deboc at 20 bar. The error bars show the standard deviation (n = 3).Following the salt rejection, we also studied the pure water permeate flux of different salt-containing feeds at a constant transmembrane pressure of 20 bar. In comparison to DI water used as feed, the permeate flux of the salt-containing feeds was lower (Fig. 12). In the case of DI water feed, the permeate flux was recorded to be 22 ± 0.5 L m−2 h−1, 23 ± 0.5 L m−2 h−1, and 27 ± 0.5 L m−2h−1 for MPD-TMC, MPD-TMC-EDA-Boc and MPD-TMC-EDA-Deboc membranes, respectively. However, with NaCl as feed, the permeate flux was recorded to be 25 ± 0.5 L m−2 h−1 for the MPD-TMC-EDA-Deboc membrane. A further decrease in permeate flux was recorded in the case of divalent salts such as MgCl2 and MgSO2. Hence, the nature of the salt impacts not only the salt diffusion but also the permeate flux. If we closely observe the permeate flux profiles of the different salt-containing feeds, it can be observed that the permeate flux in the case of NaCl is more (25 ± 0.5 L m−2 h−1) than that of Na2SO4 (23 ± 0.5 L m−2 h−1). Similarly, the permeate flux of MgCl2 is slightly more (23.5 ± 0.5 L m−2 h−1) than that of MgSO4 (23 ± 0.5 L m−2 h−1) for the MPD-TMC-EDA-Deboc membrane. This observation suggests that the nature of the anions present in the feed affects the permeate flux of the MPD-TMC-EDA-Deboc27. These results can be explained because the MPD-TMC-EDA-Deboc membrane possesses positively charged ammonium ions decorated on the surface of the membrane which develops strong repulsion for the cations while anions are attracted towards the membrane surface. The attachment of the anions on the membrane surface results in a lowering of the permeate which is also reflected by the lower NaCl rejection. The attachment of the anions on the membrane surface provides a shielding effect and hence the membrane does not remain repulsive to Na+ which owing to its smaller hydration radius passes through the membrane under higher feed pressure. In the case of seawater NF permeate, the permeate flux showed a slight decline which is well anticipated owing to concentration polarization (CP). The MPD-TMC-EDA-Deboc membranes showed a permeate flux of 23 L m−2 h−1, MPD-TMC-EDA-Boc membranes possessed a permeate flux of 20 L m−2 h−1 and the MPD-TMC membrane had a permeate flux of 18.5 L m−2 h−1 for the seawater NF permeate.Fig. 12Comparative filtration data of three membranes at fixed solute concentration and 20 bar pressure. The error bars show the standard deviation (n = 3).The study was further extended by including another important parameter related to the performance of the membrane which is the antifouling potential of the membrane. The desalination membranes are fed with a feed after several pretreatment steps which minimize the evidence of membrane fouling. However, it has been observed that membrane fouling is an inevitable process whether it is biofouling, inorganic, or organic fouling on the membrane surface. In the current study, 200 ppm BSA solution was used as feed to simulate the proteinaceous organic foulants found in seawater. The BSA feed was run for 6 continuous hours in a crossflow mode at 20.0 bar which resulted in a decrease in the permeate flux of the membranes. This decrease in permeate flux was greater initially, then gradually stabilized and remained plateaued. The rapid decrease in permeate flux of the membrane during the initial stages of the fouling experiments is obvious due to the abundant availability of the attachment sites for foulants at the surface of the membrane. Hence, the foulants are rapidly attached to the membrane surface while with time the attachment sites are not available for further attachment leading to a plateaued performance of the membrane during later stages of the fouling experiments. The flux decreased from 21.0 ± 0.5 L m−2 h−1 to 19.0 ± 0.5 L m−2 h−1 for the MPD-TMC membrane and a decrease of 22.0 ± 0.5 L m−2 h−1 to 19.0 ± 0.5 L m−2 h−1 was observed for MPD-TMC-EDA-Boc membrane (Fig. 13). In the case of MPD-TMC-EDA-Deboc membrane, a larger decrease in permeate flux was observed where the permeate flux of the membrane decreased from 27.0 ± 0.5 L m−2 h−1 to 24 ± 0.5 L m−2 h−1. Although the MPD-TMC-EDA-Deboc membrane possesses a relatively more hydrophilic surface, foulants like BSA also have several charged groups that develop interactions with the membrane’s active layer. BSA is a protein that is composed of amino acid chains crosslinked through several disulfide bonds and hence BSA has both hydrogen bond donors and acceptor groups such as >C = O, -NH, -CONH, NH2, -COOH, and -SH groups which could develop interactions with the membrane.Fig. 13Variation of permeate flux with time for the membranes for representative experiments in the presence of fouling agent (bovine serum albumin), at 20 bar.After studying the fouling behavior of the membrane, our next objective was to use a cleaning protocol for making the membranes reusable. For this sake, we backwashed the membrane using DI water at 20 bar for 1 h and the results are demonstrated in Fig. 14. As we know all the membranes underwent minor fouling during the 6 h of fouling test with BSA and the flux decline of 10%, 11%, and 12% was observed for MPD-TMC, MPD-TMC-EDA-Boc, and MPD-TMC-EDA-Deboc, respectively. Although the MPD-TMC-EDA-Deboc membrane has more fouling compared to that of the other two membranes, it also demonstrated the largest flux recovery of 95.0 ± 0.5% of its original flux. Hence, the hydrophilic surface of the MPD-TMC-EDA-Deboc membrane favors the removal of loosely held foulants owing to the stronger sheer force of the water molecules resulting in cleaning the membrane surface.Fig. 14Variation in permeate flux observed after backwashing of fouled membranes, at 20 bar.The desalination performance of the MPD-TMC-EDA-Deboc membrane was compared with similar membranes. It was found to be among the most promising in terms of permeate flux and salt rejection. This method is notably advantageous as it relies on the covalent bonding of EDA-Boc in the membrane’s active layer, thereby avoiding issues such as inhomogeneity and defects caused by inorganic fillers like TiO2 in the organic polyamide layer. Table 2 provides comparative data showing the performance of the MPD-TMC-EDA-Deboc membrane alongside other RO membranes reported in the literature.Table 2 Comparison of MPD-TMC-EDA-Deboc membrane with other membranes reported in the literatureInterestingly, tuning the active layer chemistry of the polyamide membranes has demonstrated enhancement in the desalination potential of the polyamide thin film composite membranes. The in-situ Boc-deprotection of the EDA-Boc by using 20% HCl solution led to the generation of ammonium ions in the membrane active layer which was reflected by the lowering of WCA from 70o for the MPD-TMC membrane to 55o for MPD-TMC-EDA-Deboc membrane. This increase in membrane surface hydrophilicity correlated with an enhancement of the permeate flux of the membrane from 26 L m−2 h−1 for MPD-TMC membrane to 37 L m−2 h−1 MPD-TMC-EDA-Deboc membrane at 30 bar. Moreover, the salt rejection showed an increase from 94% NaCl to 98% NaCl rejection. The fouling test by using BSA feed for six continuous hours showed a minor decrease ≈ of 10% in permeate flux which was recovered up to 95% by backwashing the membranes using DI water. Hence, this work demonstrates the potential of this approach for tuning the chemistry of the membrane’s active layer to enhance the desalination potential of the membranes.