Enhancing chlorine resistance in polyamide membranes with surface & structure modification strategies Open Access

Although more than half of our planet's surface comprises water, economic and social development has critically been flustered by the shortage of freshwater globally (Shannon et al. 2008; Werber et al. 2016). Water scarcity is a pioneer problem for its consumers, either industries or agriculture (Jacangelo et al. 1997; Bodzek et al. 2012; Eliasson 2015). To overcome this situation, different methods are adopted in which wastewater recycling and desalination are the most striking (Shao & Chen 2013). Among the membrane-based water treatment technologies, reverse osmosis plays a pivotal role in desalination and wastewater reclamation to produce superior quality water for industrial and household use (Cadotte et al. 1988; Li & Wang 2010; Werber et al. 2016). The reverse osmosis (RO) RO process has broadly been used for the desalination of water, considering its higher operability and separation efficiency. However, a trade-off subsists in water permeability and salt rejection of membranes used commercially. The RO process's broad utilization depends on polyamide thin-film composite membranes (Greenlee et al. 2009; Wang et al. 2012). In the early days, the reverse osmosis membranes were made of cellulose diacetate (CDA), cellulose triacetate (CTA), or their blend. However, nowadays, the RO process is frequently based on polyamide thin film composite (PA-TFC) membranes with the advantage of efficient salt rejection and water permeation (Li et al. 2016; Ferjani et al. 2000; Shaban et al. 2020; Zhang et al. 2020). The major drawback of PA-TFC membranes is fouling, leading to increased permeation resistance and decreased water flux (Yu et al. 2018; Yang et al. 2019), but enhanced polyamide membranes are important for the desalination and treatment of saline wastewater from various industries. Simultaneously, some foulants can be accumulated on membranes surfaces immersed in groundwater, wastewater, or seawater.

The foulants are of four types: organic, inorganic, colloidal, and biological (Dong Kang & Ming Cao 2012; Idrees 2020). These foulants (Figure 1) form a cake layer, leading to decreased flux and increased differential pressure and operational cost. There are several fouling control strategies based on foulant's type (Abbasi-Garravand et al. 2017; Guo et al. 2020). Colloidal and inorganic fouling could be effortlessly controlled by physical and chemical methods like coagulation/flocculation, media filtration, scale inhibitor/acid dosing (Hafsi et al. 2004; Jiang et al. 2017). On the other hand, organic and biofouling are burdensome to control because these two kinds of fouling grow in cooperation with each other (Jeong et al. 2016). To mitigate the accumulation of some foulants, chloramines, chlorine dioxide, ozone, free chlorine, ultraviolet (UV), as well as potassium permanganate, the chlorine is most abundantly used for sterilization and disinfection of membrane elements as it is found very useful to deactivate a large variety of microorganisms (Buch et al. 2008; Zhang et al. 2015). Generally, chlorination of membrane mainly involves N-Chenlorination by hydrogen substitution on amide nitrogen, tracked by the ring chlorination that is an irreversible process. However, continuous exposure to chlorine ions in the solution will demolish the cross-linked polymeric membrane structure, causing lowering salt rejection and shortening service life (Yu et al. 2011; Do et al. 2012a). Therefore, it is a distinct requirement to modify the dual functioned polyamide-based reverse osmosis membranes compatible with chlorine resistance and antifouling properties for long term desired durability.

In order to lift the inclusive performance of RO membranes, various modifications have been investigated, including grafting (Guo et al. 2019), surface coating (Xia et al. 2020), surface bio-adhesion (Habimana et al. 2014), in-situ surface modification (Wang et al. 2019), and fusion of nanomaterials into membranes. However, they isolate the polyamide layer from free influence with the feedwater containing chlorine contents proven effective in reducing chlorine degradation and foulants deposition. Injection of hydrophilic nanoparticles as graphene oxides (GO) (Peyki et al. 2015), carbon nanotubes (CNTs) (Zhao et al. 2014), and graphitic carbon nitride (g-C₃N₄) (Liu & Xu 2016) into polyamide membranes have been given more consideration because of excellent modification and processing simplicity (Saenz De Jubera et al. 2013; Ali et al. 2016). Particularly in modern years, using two-dimensional nanomaterials in RO membranes has been distinctively studied due to their abundant active sites and higher particular surface area, supporting more favorable nanochannels for water to pass, maintaining salt repulsion. Antioxidant properties of graphene oxide were found to be very helpful in chlorine resistance for polyamide membranes (Chae et al. 2015). Kim et al. (2015) observed that graphene oxide could enhance chlorine resistance more productively than carbon nanotubes. Chae et al. (2015) found a modified polyamide membrane with better chlorine resistance by embedding graphene oxide into the membrane layers (Mi 2014; Idrees 2020). Besides, GO nanosheets have excellent filmmaking properties in many ways, such as spin-coating, dip-coating, vacuum filtration, and layer by layer self-assembly (Mi 2014). Kim et al. (2014) reported layer by layer graphene oxide and NH₂-GO self-assembly by charge adsorption on the polyamide membrane.

Graphene oxide quantum dots (GOQDs) were a compelling additive substance to increase water flux (Fathizadeh et al. 2019). GOQDs, having 3–20 nm diameter dimensions, have been used in anti-bacterial research because of their unique characteristics, such as ultrasmall lateral size (Nurunnabi et al. 2013), morphology (Cong et al. 2014), and cytotoxicity (Wu et al. 2013), etc. Therefore, graphene oxide quantum dots are the good nominee to replace graphene oxide TFN polyamide membranes. GOQDs are employed to manufacture functionalized GOQD-PVDF membrane and establish better antifouling, anti-bacterial, and hydrophilic performance. Water flux increased with GOQD grafting of PVDF from 500 L·m⁻²·h⁻¹·bar⁻¹ to >3800 L·m⁻²·h⁻¹·bar⁻¹ with improved anti-bacterial properties. It is observed that hydrophilicity improved significantly and a dramatic decrease in water contact angle from 118.5 to 34.3° by coating GOQDF on PVDF (Zhang et al. 2014a).

Citrazinic acid (C₆H₈O₇·H₂O), N-hydroxysuccinimide (NHS), M-phenylenediamine (MPD) (flakes, 99%), dichloromethane (≥99.8%) ethylenediamine (EDA), trimesoyl chloride (TMC) (98%), n-hexane (≥95%), sodium hydroxide (NaOH, 10 wt%), sodium hypochlorite (NaOCl, 10 wt%), sodium chloride (NaCl, >99%), citric acid (99%), 1-(3-dimethyl aminopropyl)-3-ethyl carbodiimide hydrochloride (EDC), 2-(N-morpholino) ethane sulfonic acid (MES) and ammonia (28.0–30.0% NH₃ solution) were used without further purification. Toray reverse osmosis membrane (ROMEMBRA for brackish water) (USA) was used to support the polyamide N-GOQD membrane.

Nanoparticles of N-GOQDs were prepared using hydrothermal treatment by citric acid carbonization with ammonia (Ho et al. 2016; Fathizadeh et al. 2019). Shortly, an aqueous solution of citric acid (100 mg/mL) and aqueous ammonia solution with a volume ratio of 4:1 was set-up into an autoclave (Teflon-Lined) at a temperature of 180 °C for 12 hours. The resulting solution (pale yellow) was dialyzed using a dialysis tube soaked in deionized water to remove excess ammonia and impurities for 4 hours. After dialysis, to remove the impurities cluster, aqueous dispersion with a dialysis bag was centrifuged three times at 10,000 rpm. The resilient was gathered for antecedent membrane preparation.

Adjusted pH 5.5–6.0 by HCl and an aqueous solution of MES was prepared. Membrane surface grafting by EDA was organized in three steps:

• (1)

  Surface activation of the membrane by EDC/NHS: to obtain EDC/NHS solution, 0.1 g EDC was mixed to 200 mL MES buffer solution. Polyamide membrane was PLUNGED for 10 min in the MES-EDC solution, and for 30 min, 0.05 g N-hydroxy succinimide (NHS) was mixed. Using EDC/NHS, –COOH was activated on the surface of the membrane.

• (2)

  EDA grafting onto the membrane surface: 0.1wt % of ethylenediamine (EDA) was mixed in the prepared solution. After 16 hours, the activated membrane was hatched in the solution, and washed thoroughly with water.

• (3)

  N-GOQDs grafting on EDA grafted membrane: similar to the first step, N-GOQDs was mobilized with EDC/NHS. EDA grafted membrane was immersed in the activated solution of N-GOQD for 16 hours in a dark environment. Finally, the membrane with grafting of N-GOQDs was attained and washed with deionized water.

Chlorine solution with 1000 ppm NaOCl with adjusted pH 7.0 using HCl was used as a standard chlorine solution. Salt rejection and initial water flux were tested before membrane soaking in the chlorine solution. Later, the membrane was periodically washed with deionized water after soaking in NaOCl solution for a particularized time and analyzed for salt removal performance, including salt rejection and water flux. Salt rejection volume debilitation is a significant depiction of chlorine resistance membranes with good chlorine resistance to exhibit less attenuation of salt rejection and water flux.

Synthesized N-GOQD particles were fully dispersed without superficial agglomeration and had almost consistent size division between 3 and 8 nm (Fathizadeh et al. 2019). N-GOQD particles are unreactive in aqueous solution at room temperature and it is expected that some particles have nitrogen functioned graphene oxide layers (Zhang et al. 2014a; Ho et al. 2016). The oxidation degree of GO and N-GOQD is 47.1 and 51.5%, respectively, proving that GO has a lower degree of oxidation than N-GOQD. Also, GO (1.82%) has a lower N to C atoms ratio than N-GOQD (13.25%). It means NGOQD has more hydrophilic and surface polar groups (such as N—H, C—O, and C=O) than GO (Wei et al. 2010a).

Grafting of chemicals on the surface of membranes is a valuable modification method. High activity in the hydrogen substitution reaction of carboxylic acid proved that aromatic polyamide chain grafting sites were best possible to appear on these groups (De Gooijer et al. 2004a; Liu & Sun 2008). After a high temperature or high reaction time, grafting sites do not exist on the end group of polyamide chains (De Gooijer et al. 2004b; Xu et al. 2007). Homoplastically, EDC/NHS can activate—COOH on N-GOQD, amines are used to achieve a covalent bond.

N-GOQD with particular grafting times and concentrations were utilized to attain increased water flux and modified desalination performance of the membrane. Changes with distinct N-GOQD concentration on desalination performance of membrane with fixed 12 h grafting times are shown in Figure 2. No apparent result on membrane separation performance was observed with N-GOQDs concentrations. Best desalination values, NaCl rejection rate 96.2% and water flux 40.02 L·m⁻²·h⁻¹ compared to 94%, and 36.4 L·m⁻²·h⁻¹ of the pristine membrane concentration of 0.002 mg/mL with grafted membrane have to be achieved. After N-GOQDs grafting, hydrophilicity on the membrane surface improved, resulting in elevated water flux.

Another modification factor in controlling surface properties and quantity of quantum dots is grafting time. Results with different grafting times are observed and analyzed in Figure 3. At grafting reaction time 12 h, both salt rejection and water flux enhanced as liken to the pristine membrane and observed a dominant increase in water flux from 36.0 to 40.02 L·m⁻²·h⁻¹, which is 11.3%. Thus, all grafted membranes show improved salt rejection than pristine membranes after 160 h of chlorine resistance test. The modified membrane showed better resistance for chlorine after 12 h grafting time. It means that this is an optimum reaction time for chlorine resistance and desalination performance. The minimum value of water flux at 16 h reaction time can be observed in Figure 3, which deduced that at 16 h reaction time, chlorine resistance and grafting amount is maximum. However, as per Figure 4, salt rejection at 12 hours is maximum. It means that an increase in reaction time above 12 h carboxyl and amino groups of nitrogen graphene oxide quantum dots leads to a self-polymerization reaction; the number of quantum dots increases on the membrane surface. That is why the chlorine resistance of the membrane is not acceptable, and water flux decreases at 16 h.

For a particular time, pristine membranes soaked in NaOCl solution (1000 ppm). A salt rejection test for each soaking time was performed and variations are shown in Figure 2. Choline soaking increased water flux (Figure 2) and decreased salt rejection (Figure 3). With N-GOQDs grafted concentration of 0.002 g/mL, salt rejection is 80.1% at 280 h of reaction time, which is more excellent than 60.3% of the pristine membrane. These results show that chlorine resistance improved in N-GOQDs grafted membranes.

A new two-dimensional series of transition metal carbides and nitrides MXenes Ti₃C₂T_x have materialized since 2011 (Naguib et al. 2011). Ti₃C₂T_x is abundantly used in super capacitance, electromagnetic shielding, and electrocatalysis (Lukatskaya et al. 2013; Peng et al. 2019). Due to its stability and extraordinary penetration performance, it is used in water purification and for the removal of heavy metal ions from water. Remanan et al. (2020) manufactured a GO/ Ti₃C₂T_x nanofiltration membrane, providing magnificent removal of methylene blue solution, maintaining a high-water flux due to the hydrophilicity of Ti₃C₂T_x. Wang et al. (2020) tried to develop a strategy for the first time to boost the performance of RO membranes by the inclusion of Ti₃C₂T_x into the PA layer and studied its effects on permeability, antifouling, and chlorine resistance.

Camphorsulfonic acid (CSA, 99%), m-phenylenediamine (MPDA, 99.5%), bovine serum albumin (BSA, 96%), trimesoyl chloride (TMC, 98%), sodium dodecylsulphate (SDS, 98%) , triethylamine (TEA, 99%), sodium hypochlorite (NaOCl, 6–14%), titanium aluminum carbide (Ti3AlC2>99%, 400 mesh), lithium fluoride (LiF, 99%), n-hexane (laboratory reagent, 97%), hydrochloric acid (HCl) and sodium chloride (NaCl, >99%) were used without further purification. Toray reverse osmosis membrane (ROMEMBRA) (USA) was utilized as the support of poly amide and mexene embedded PA- Ti₃C₂T_x membranes. Demineralized water was employed in all the experiments.

First of all, a mixture of 0.5 g lithium fluoride (LiF) and 12 ml HCL (5 mol/L) was prepared, and 0.5 g Ti₃AlC₂ was added for 72 hours at 60 °C. The centrifugation was applied to the obtained suspension under 6000 rpm for 4 minutes. Then, the resulting precipitates were washed by demineralized water until a pH value of above 7.5. Lastly, the precipitates were vacuum dried at 60 °C for 30 hours to obtain the Ti₃C₂T_x powder.

The interface polyamide membrane was synthesized on the polysulfone(PS) membrane (Morgan & Kwolek 1996; Amin et al. n.d.). The preparation of PA-Ti₃C₂T_x membrane was arranged into the following three steps:

• (1)

  NaNO₂ Removal: The PS membrane was first washed with demineralized water five times and then submerged in demineralized water for 28 hours to remove NaNO₂ contamination.

• (2)

  Ti₃C₂T_x Application: MPD, SDS, and TEA solution were prepared, and Ti₃C₂T_x was mixed with it. The obtained mixture's pH was maintained to 11 by CSA, and the membrane was immersed in it for 4 minutes, after that in 1.5 wt% TMC and n-hexane solution for 1 minute.

• (3)

  Heat Treatment: Heat was applied to the membrane at 55 °C for 8 min. Finally, the PA-Ti₃C₂T_x membrane was attained and stored in demineralized water for studies.

NaOCl was used as the primary source of chlorine to test the chlorine resistance of the membrane. Salt rejection and water flux were tested before the membrane's exposure to chlorine solution. The synthesized membrane was submerged in a 3000 ppm NaOCl solution by applying a method of static chlorination. The membrane was frequently washed by demineralized water for a specific time after each exposure and the salt rejection and water flux were analyzed. Membrane exhibits chlorine resistance with a slight decrease in salt rejection and an increase in water flux.

The salt rejection and water permeance of the PA and PA-Ti₃C₂T_x membranes were estimated, as shown in Figure 5. The PA-Ti₃C₂T_x showed that the salt rejection (97.5–98.3%) was almost the same as that of the PA membrane (98.5%) and excellent water permeability (2.4–2.6 L·m⁻²·h⁻¹·bar⁻¹) higher than that of the PA membrane (1.8 L·m⁻²·h⁻¹·bar⁻¹), indicating the advantageous characteristics of Ti₃C₂T_x to impede the trade-off impact (Geise et al. 2011; Werber et al. 2016). This can be accredited to the –OH groups produced by the Ti₃C₂T_x, which creates the additional cross-linking of the PA- Ti₃C₂T_x selective layer. Figures 5 and 6 also compare the results of this study's separation performance, membranes modified by nanomaterials in literature, and commercial RO membranes.

Commercial RO membrane (Park et al. 2019) had a low water permeability (1.3 L·m⁻²·h⁻¹·bar⁻¹) but a higher salt rejection (99.3%). GO membrane (Wei et al. 2010b; Yin et al. 2016) showed a trade-off between salt rejection and water permeability (96.4% rejection at 0.9 L·m⁻²·h⁻¹·bar⁻¹ or 93.8% rejection at 2.8 L·m⁻²·h⁻¹·bar⁻¹). In contrast, the PA-Ti₃C₂T_x membrane obtained in this study exhibits extraordinary performance, suggesting the addition of Ti₃C₂T_x as a solution of the RO membrane's trade-off effect (Geise et al. 2011; Werber et al. 2016).

The PA-membrane was immersed in a 3000 ppm NaOCl solution for 5 hours in intervals of 1 hour to study the membrane's chlorine resistance. There is a considerable increase in the water flux of the PA membrane (3.6 L·m⁻²·h⁻¹·bar⁻¹) (Figure 6), while there is a decrease in the salt rejection (93%) (Figure 7) after exposure to NaOCl. The structure of the PA membrane was significantly damaged due to the active chlorine solution, and the infiltration of both ions and water molecules is caused by the pores between the membrane's functional layers.

On the other hand, the chlorine resistance of the PA-Ti₃C₂T_x membrane was determined in the same conditions. The flux of PA-Ti₃C₂T_x increased after 5 hours of chlorine exposure (Figure 8), and the salt rejection was maintained above 97% (superior to the 93% of PA) (Figures 6 and 7). These results show that the embodiment of Ti₃C₂T_x reduces the effect of chlorine on the cross-linked bonds of PA and provides stability to the structure of PA, presenting the membrane as chlorine resistant. Figure 9 shows the comparison of PA-Ti₃C₂T_x and PA-GO (Wei et al. 2010b), PA-Silica (Peyki et al. 2015), PA-Alumina (Saleh & Gupta 2012), PA-TiO₂ (Kim et al. 2003), and PA-Zeolite (Jeong et al. 2007).

Another approach to prevent chlorine damage of the TFC membrane is the introduction of reducing agents into the PA layer as they will consume the chlorine due to its oxidizing nature, playing the role of chlorine scavengers (Spear 1992). Lee & US (2010) and Wilson et al. (1977) and first attempted to produce the chlorine resistant TFC membranes by reducing agents containing carbon-carbon double bonds. Thioether units have a superior reducing ability (Kaczorowska et al. 2005; Zhang et al. 2014b) and can quickly introduce into the PA chains (Zhang et al. 2012; Javadi et al. 2013). Cao (2018) introduced the thioether units into the PA Layer of TFC membranes for the first time to serve as chlorine captors and studied the effect on performance, morphology, chemical structure, and specifically examined the chlorine resistance of the membrane.

M-phenylenediamine (MPD, ≥99%), demineralized water, thiodibenzoyl chloride(TDC, ≥98%,), triethylamine (TEA, ≥99%), sulfonic acid (SA, ≥98%), hydrochloric acid (HCl, 36.5%), sodium hypochlorite (NaOCl, 14.5%), trimesoyl chloride (TMC, 98%), thiobenzoic acid (TDA, 90%), sodium chloride (NaCl, ≥99.5%) thionyl chloride (SOCl₂, 97%) N-hexane(anhydrous, ≥99%) and sodium hydroxide (NaOH, ≥99.5%) were used without further purification. Toray reverse osmosis membrane (ROMEMBRA) (USA) was used to support the polyamide TFC membrane.

Synthesis of thio-dibenzoyl chloride (TDC) 90 mL thionyl chloride (SOCl₂) and 14.8 g of TDA were mixed, and 0.7 mL of dry pyridine was added to it. The mixture was stirred in a flask at ambient temperature under nitrogen (N₂) atmosphere in the absence of light for 4 hours, and then the mixture was refluxed for 24 h. After that, the mixture was distilled and dried under reduced pressure. The obtained residue was separated at 80 °C with solvent petroleum ether (70–85 °C). Finally, the solvent was evaporated at reduced pressure, and the product was dried under a vacuum at 60 °C.

At the start, a piece of PA-membrane was washed with demineralized water and set between the stands of polytetrafluoroethylene (PTFE). After that, 4.5 weight % SA and three weight % TEA was mixed in 100 mL water, and 60 mL of 3% MPD (w/v) was added to this solution and spilt into the frame, keeping in contact with the PS membrane for at least 10 min. The solution was then removed, and 60 mL of 0.5% TMC poured into the frame for a contact time of 1 min; afterward, the membrane was washed with n-hexane three times to remove any residual reagent. During interfacial polymerization, the thioether units were added to the PA layer by substituting trimesoyl chloride (TMC) with thio-dibenzoyl chloride (TDC) partly. 0%-TDC-TFC and 30%-TDC-TFC membranes were prepared and dried in air for 10 min and stored in demineralized water at 5 °C for subsequent studies.

Water flux and salt rejection of membranes were measured before chlorination. The soaking method under two different sets of conditions was used to test the membrane's chlorine resistance. For the first set, membranes were soaked to 100, 200, 300, 500, 1000, 2000, and 3000 ppm NaOCl solution for 1 hour at pH=4 and for the second set the NaOCl concentration was kept constant at 2000 ppm and exposure time was varied as 1, 5, 15 and 24 hours at pH=11. After exposure to chlorine, the membranes were flushed with demineralized water for 5 min, and water flux and salt rejection were analyzed to test the chlorine resistance.

Due to the degradation of the PA layer, the performance of the TFC-RO membrane generally declined. On chlorine attack in the acidic pH range, reversible N-chlorination first destroys the amide N-H bond. PA symmetry and intermolecular hydrogen bond face a severe disorder leading towards a tighter structure and a hydrophobic surface resulting in a considerable decrease in water flux. On the other hand, hydrolysis promoted by chlorination drives the N-chlorination reaction at a high pH range, directing towards polymer rift and inflammation. As a result, the salt rejection decreases (Do et al. 2012b, 2012c). The effect of chlorine exposure was examined in Figure 10 at 0 and 30%-TDC-TFC membranes for water flux and salt rejection.

• (a)

  Chlorination at pH 4: The variations in salt rejection and normalized water flux of 0%-TDC-TFC and 30%-TDC-TFC membranes exposed with 100, 200, 300, 500, 1000, 2000, and 3000 ppm NaOCl solution for 1 h at pH=4 is shown in Figures 10 and 11, respectively. There is no considerable effect on the salt rejection of both membranes, but the flux shows apparent changes. The flux of 0%-TDC-TFC membrane increased at 100 ppm exposure and then declined abruptly to 50% of the pristine membrane at an exposure of 200 ppm. The other 30%-TDC-TFC membrane presented very different behavior. The normalized flux was raised to 1.67 after exposure to 100 ppm than 1.08 for the 0%-TDC-TFC membrane. On 200 ppm exposure, the flux was 0.97, much higher than the 0%-TDC-TFC membrane, which was only 0.63.

• (b)

  Chlorination at pH 11: The salt rejection and normalized water flux of 0%-TDC-TFC and 30%-TDC-TFC membranes exposed with 3000, 9000, 33,000, and 49,000 ppm NaOCl solution 1 h at pH=11 are shown in Figures 12 and 13, respectively. Both membranes show the same trend of increase in normalized flux as the exposure increased. At low concentration, the normalized salt rejection abruptly increased and then slowly decreased as the exposure increased. On 3000 ppm exposure, the normalized salt rejection of 30%-TDC-TFC membrane was below 1 and two times more than 0%-TDC-TFC membrane. These results show that the introduction of thioether units improved the chlorine resistance of the membrane.

Polyamide composite membranes revised with graphene oxide (GO), oxidized graphitic carbon nitride (OGCN), or graphitic carbon nitride (g-C₃N₄) are made by interfacial polymerization. Graphitic carbon nitride with a permeable coated structure is typically practiced in organic water contamination (Chen et al. 2016; Zhao et al. 2016; Shen et al. 2017). Oxidized graphitic carbon nitride can provide bountiful oxygen-containing functional groups and great aqueous dispersity because of superficial protonation. OGCN involves aromatic tri-s-triazine units that can offer a steady medium for water (Zhang et al. 2009; Paquin et al. 2015).

Graphene oxide and carbon nanotubes can enhance the chlorine resistance of PA membranes (Park et al. 2010; Kim et al. 2015); however, graphene oxide's arrangement process is complicated, making graphene oxide membranes costly. Graphitic carbon nitride (g-C₃N₄) has a graphene-like structure, and preparation is straightforward (Gao et al. 2017). Graphitic carbon nitride can be oxidized with large oxygenic groups to oxidized graphitic carbon nitride, which has more chlorine resistance like graphene oxides. Polyamide membranes are doped with oxidized graphitic carbon nitride; thus, salt rejection and water flux increased compared to the pristine membrane (Tian Yi Ma et al. 2014). It is observed that chlorine resistance and separation performance improved at a concentration of 0.01 g/L. Salt rejection of oxidized graphitic carbon nitride (OGCN) polyamide membrane decreased by 15% after a treatment time of 40 hours, 30% of graphitic carbon nitride (g-C₃N₄), 22% of graphene oxide (GO) polyamide membrane, and 27% of the pristine membrane (Wang et al. 2016).

Benzene-1,3,5-tricarbonyl chloride (trimesoyl chloride), M-phenylenediamine (MPDA), graphene oxide powder, sulfuric acid (H₂SO₄), nitric acid (HNO₃), hydrochloric acid (HCl), sodium hydroxide (NaOH), sodium chloride (NaCl), sodium hypochlorite (NaOCl), n-hexane and urea. Toray reverse osmosis membrane (ROMEMBRA) (USA) was used to support the polyamide TFC membrane.

One stage thermal polymerization technique was used to prepare g-C₃N₄. A muffle furnace was used to heat urea from 20 to 550 °C at a heating rate of 5 °C/min for 2 hours. At room temperature, the heated specimen was cooled, grounded into powder, and collected. Oxidizing g-C₃N₄ to synthesize oxidized graphene carbon nitride. HNO₃ and H₂SO₄ solution with a volume ratio of 2:1 was used with g-C₃N₄ at room temperature and stirred for 20 hours. Deionized water was used to wash the obtained yellow-white product. The product is dried at room temperature, which is oxidized graphene carbon nitride.

Interfacial polymerization of trimesoyl chloride and M-phenylenediamine was used to synthesize the polyamide membrane. The membrane surface was washed several times with deionized water and dried thoroughly. M-phenylenediamine solution with 2 wt% specification was used to soak the membrane for 5 minutes and the membrane surface was dried thoroughly. To build an interfacial polymerization membrane was soaked in a 0.1 wt% trimesoyl chloride solution. The membrane was dried entirely using an oven at 80 °C for 5 min. The membrane as washed with deionized water and stored. The same method was used to prepare membranes with OGCN, g-C₃N_4, and GO using an MPDA solution.

Resistance to chloride ions in the polyamide membrane is distinguished by variation in membrane separation performance. Strong chlorine resistance will be examined when there is less variation in salt rejection and water flux. Firstly, membrane separation performance was tested, adopting a cross-flow configuration with a 2000 ppm NaCl solution. Chlorine solution with 1000 ppm NaOCl with adjusted pH 7.0 using HCl was used as a standard chlorine solution. Salt rejection and initial water flux were tested before membrane soaking in the chlorine solution. Later, the membrane was periodically washed with deionized water after soaking in NaOCl solution for a particularized time and analyzed for salt removal performance, including salt rejection and water flux. Salt rejection volume debilitation is a significant depiction of chlorine resistance. Membranes with good resistance of chlorine exhibit less attenuation of salt rejection and water flux.

Prepared chemicals with different concentrations of PA-GO, PA g-C₃N₄, and PA OGCN were doped on membranes and analyzed. Carbon to nitrogen bond of pristine and polyamide OGCN membranes decreases 7.6 and 1.2% respectively after chlorine treatment, signifying that CGNO has chlorine capture ability. Modified membranes have better water flux than that of the pristine membrane. Higher water flux has been observed with higher doping concentrations, as shown in Figure 14. Water flux with MPD solution at 0.01 g/L concentration of GO, g-C₃N₄, OGCN is 6.03, 5.31, and 4.89 L·m⁻²·h^−1, respectively. The hydrophilic properties of these nanosheets promote water molecules adsorption.

It is observed that salt rejection variation is the opposite of water flux, which is much higher at a concentration of 0.01 g/L but decreases gradually with an increase in nanosheets concentration from 0.01 g/L (Figure 15). This is due to the collection of excessive nanotubes in membranes, preventing interfacial polymerization and leading to defects in membranes. These defects increase with higher penetration of nanosheets in membranes, and they can also increase salt rejection with moderate distribution (Jadav & Singh 2009; Kim et al. 2013). Salt rejection is maximum at a moderate concentration of 0.01 g/L. The analysis shows that g-C₃N₄ doped membranes exhibit higher water flux and OGCN doped membranes exhibit higher salt rejection.

After oxidation reactions, OGCN membranes contain many nitrogen and hydrogen bonds that provide more active sites to attack chlorine active radicles. Nitrogen and hydrogen bonds may go through oxidation reactions, which leads to the improvement of salt rejection. Analyzing these two factors, oxidized graphitic carbon nitride membranes exhibit better chlorine resistance than GO membranes. On the other hand, GO membranes show better water flux ability than OGCN membranes, as shown in Figure 16.

Oxidized graphene carbon nitride has smaller nanolayers and is dispersed quickly, so they cause less damage to polyamide membranes than g-C₃N₄. After 14 hours of soaking time in NaOCl solution, salt rejection of the OGCN doped membrane decreased to 83%, while the pristine membrane has a 71.7% decrease in salt rejection, as shown in Figure 17. The experiment shows that g-C₃N₄ does not have a particular impact on chlorine resistance but oxidized graphitic carbon nitride positively impacts chlorine resistance.

In this work, four different membranes, PA-NGOQDs, PA-Ti₃C₂T_x, PA-Thioether, and PA-OGCN, have been developed by surface and structural modifications in membranes study chlorine resistance. This was done by analyzing the salt rejection and water flux of the membranes along the chlorine exposure time with respect to the pristine membrane.

Figure 18 shows the salt rejection of all four membranes with respect to time comparing them with that of pristine membrane. It can be seen that the salt rejection of the PA-N-GOQDs membrane is 83.8%, PA-Ti₃C₂T_x membrane is 97.7%, PA-Thioether membrane is 98.8%, PA-OGCN membrane is 86%, and pristine membrane is 94% after 5 hours of chlorine exposure.

Figure 19 exhibits the normalized flux of membranes with respect to time comparing them with that of pristine membrane. The normalized flux of PA-N-GOQDs membrane is 87.9%, PA-Ti₃C₂T_x membrane is 87.4%, PA-Thioether membrane is 89.6%, PA-OGCN membrane is 81.5%, and pristine membrane is 86.5% after 5 h of chlorine exposure. These results represent that the salt rejection and normalized flux of membrane with thioether units are less affected by chlorine exposure and higher than other membranes studied, evincing that it is more chlorine resistant.

It is observed that the thioether membrane shows better water flux and salt rejection properties than other membranes. On the other hand, the preparation cost of PA-NGOQDs, PA-Ti₃C₂T_x, and PA-Thioether are high as compared to PA-OGCN. Oxidized graphitic carbon nitride membrane requires a soaking time of 40 hours, which is very high and did not show satisfactory results of water flux and salt rejection compared to other membranes.

The authors gratefully acknowledge the writing encouragement of Engr. Muhammad Jazib Zafar and Zunnurain Hussain.

All relevant data are included in the paper or its Supplementary Information.