Enhanced properties of a positive-charged nanofiltration membrane containing quaternarized chitosan through second interfacial polymerization for the removal of salts and pharmaceuticals Open Access

Nowadays, pharmaceuticals and personal care products (PPCPs), which are bioactive trace pollutants in different environmental compartments, such as surface water, drinking water, groundwater, and sediments, are progressive issues due to the increasing urbanization (Ellis 2006; Liu et al. 2023). As PPCPs are abundant in the environment and possess significant ecological risks (endocrine disruption, bioaccumulation, inducement of bacterial antibiotic resistance, clinical toxicity, etc.), dealing with PPCPs effectively has become a serious problem (Liu et al. 2020; Oluwole et al. 2020). The major route for pharmaceutical residues to reach the aquatic environment is the direct emissions of drugs, and another source is most probably excretion from patients undergoing pharma treatments (Montforts 2006; Fatta-Kassinos et al. 2011). Kosma et al. (2010) investigated the residues of 11 PPCPs in the municipal and hospital wastewater treatment plants of Ioannina City in Western Greece. The results showed the occurrence of all target compounds in the wastewater samples and the achievement of the highest removal rate during biological treatment, which were satisfactory except for diclofenac and carbamazepine (CBZ). These could produce reproductive, neurological, and other toxicological or carcinogenicity effects on humans and other organisms through drinking water and food chains (Dai et al. 2014; Kosma et al. 2014). Besides, on account of the slight PPCP concentration and their intricate chemical structure, technologies used frequently in sewage treatment plants might not be efficient enough to remove them. So, it is imminent to find other especially impactful approaches to remove PPCPs. Owing to higher permeate flux and less rejection of monovalent salts, nanofiltration (NF) membranes exhibit more advantages in preventing PPCPs from entering aquatic environments (Bellona & Drewes 2007; Radjenović et al. 2008; Zhao et al. 2017; Liu et al. 2018a).

NF membranes have a pore size typically of 1 nm with properties in between the ultrafiltration (UF) membrane and the reverse osmosis (RO) membrane, which are all pressure-driven separation processes (Oatley-Radcliffe et al. 2017). Due to the moderate operating pressure and a high retention rate of small molecules, NF membranes have been widely utilized in desalination, wastewater treatment, and liquid concentration (Guo et al. 2021). Currently, NF membranes have attracted considerable attention for improving PPCP removal for water reclamation (Lin et al. 2018). Košutić et al. (2007) investigated the removal of antibiotics by RO/NF from model wastewater of a manufacturing plant producing veterinary pharmaceuticals. The rejection of the examined antibiotics by the selected RO and the tight NF membranes was acceptably high. The dominant rejection mechanism of the examined ionizable antibiotics by all the membranes was the size exclusion effect. Mansourpanah et al. (2011) applied trimesoyl chloride (TMC) and piperazine as reagents for the preparation of poly(piperazine amide) on a polyethersulfone UF support. The rejection of tetracycline antibiotics and sulfonamides by the NF/RO membrane exceeded 90%. Donnan exclusion and mobility of the ions across the membrane are two important factors affecting the rejection capability of ions for charged membranes. Therefore, the rejection mechanisms of trace organic micropollutants by NF membranes mainly include steric hindrance and electrostatic repulsion (Semião & Schäfer 2013; Semião et al. 2013). Typical NF membranes with thin film composite (TFC) structures have been fabricated by the interfacial polymerization (IP) of the diamine and acyl chloride (Shen et al. 2019). Lin et al. modified NF90 with 3-sulfopropyl methacrylate potassium salt and 2-hydroxyethyl methacrylate applying for the removal of CBZ, ibuprofen (IBU), sulfadiazine, sulfamethoxazole, sulfamethazine, and triclosan (Lin et al. 2019).

Nowadays, IP is one of the most common methods to develop composite NF membranes, which consist of a dense polyamide functional barrier layer and a porous support membrane (Bi et al. 2018; Chiao et al. 2020). They can both dominantly influence the NF performance including the water flux and salt rejection (Zhang et al. 2004). Compared to other techniques, the method of IP has several advantages such as easy purification of products, reproducible results, and good polymerization yield. Furthermore, IP is also applied to produce high-quality polyaniline nanofibers with control over their size, morphology, and nanofiber diameter (Li et al. 2015). Tang et al. 2008 fabricated a novel positive-charged NF hollow fiber membrane for lithium and magnesium separation through the IP of decentralized application and TMC on the polyacrylonitrile support membrane. The research demonstrated the effects of the reaction time of polymerization and the concentration of reactive monomers on the performance of the composite membrane. The permeability of a single salt followed LiCl > NaCl > MgSO₄ > MgCl₂ at pH = 9.5 and the negative repulsion of Li⁺ ions was found in the mixed solution of LiCl and MgCl₂. Tsuru et al. 2013 employed triethanolamine and TMC via IP to fabricate the polyester Kenner membrane with good acid resistance, which studied the effects of polymerization time, monomer concentration, and pH value of the aqueous solution. The tertiary amine groups of the film surface layer becoming quaternary ammonium groups contribute to advancing the flux of the membrane at low pH. In the research of Tsuru et al. (Wang et al. 2002), the permeability of the obtained NF membrane greatly increased through two-step IPs using isophthaloyl chloride and trimesoyl chloride. Recently, many efforts have been made to improve the separation performance by introducing polyethyleneimine (PEI) into the aqueous phase during the IP process.

PEI, as a cationic polyelectrolyte, contains abundant amine groups and the proportion of primary amine, secondary amine, and tertiary amine is 1:2:1, which has been widely verified for preparing high positively charged and excellent hydrophilic TFC NF membranes via the IP process thanks to its high charged density, easy protonation, and easy preparation (Lee et al. 2004; Xu et al. 2015; Shen et al. 2020; Bridge et al. 2022). Tao et al. 2022 developed a composite NF membrane-grafted PEI on the polyamide layer, in which the removal rate for heavy metals reached 90% and the water flux reached 74 L·m⁻²·h⁻¹·MPa⁻¹. Xu et al. (2019a) designed a double quaternary ammonium salt to modify the PEI polyamide NF membrane. The permeability of the modified PEI membrane was 212 L·m⁻²·h⁻¹·MPa⁻¹ and was 4.4 times that of the original PEI film. At the same time, the MgCl₂ retention rate remained at 95%. El-Gendi et al. 2018 fabricated a positively charged NF membrane with abundant –NH₃⁺ and –NH₂⁺ groups for the separation of Mg²⁺ and Li⁺ from salt lake brine, which had a high Mg²⁺/Li⁺ mass ratio. The NF membrane was prepared by IP between PEI and TMC on the support of polyethersulfone (PES) UF membrane, and the salt rejection followed the order: MgCl₂ (94.8%) > MgSO₄ (84.1%) > Na₂SO₄ (81.4%) > NaCl (36.9%) > LiCl (30.6%). Huang et al. 2015 produced new PEI asymmetric membranes to get selective polymeric membranes with high flux fitting for the separation of organic molecules from aqueous mixtures by NF.

Herein, we aimed to construct a positively charged NF membrane with comprehensive rejection to PPCPs of different physicochemical properties using a new positively charged compound quaternate chitosan (HTCC) by nucleophilic substitution reaction with glycidyl trimethyl ammonium chloride (GTMAC), PEI, and TMC via second IP (SIP). The original loose PES membrane was capable of immobilizing the positively charged polyamide layer formed by IP of PEI, TMC, and HTCC. The multilayer morphology, hydrophilicity, and surface charge were analyzed by SEM, atomic force microscope (AFM), water contact angle, and zeta potential, respectively. Meanwhile, we investigated the concentration and soaking time of PEI and HTCC on the membrane performance, respectively. In addition, the water permeability and separation capability of diluted salts and PPCPs of the modified membrane were evaluated.

Chitosan (CTS, weight-averaged molecular weight Mw ≈ 150 kDa, Sinopharm Chemical Reagent Co., Ltd) was dissolved in the solvent acetic acid (CH₃COOH ≥ 99.5%, Sinopharm Chemical Reagent Co., Ltd). The PES UF membrane (MWCO = 10 kDa) was provided by Ande Membrane Separation Technology Engineering Co., Ltd (Beijing, China). Polyethyleneimine was obtained from Maclean Biotechnology Co., Ltd. (Shanghai, China). GTMAC with a purity of ≥95% was purchased from Shanghai DB Biotechnology Co., Ltd (China). Polyethylene glycol with molecular weights of 400, 600, 800, 1,000, and 1,500 g/mol purchased from Chengdu Kelong Chemical Reagent Co., Ltd was used as the model neutral solutes to estimate the MWCO of the obtained membrane. Trimethylbenzene chloride was provided by Saan Chemical Technology Co., Ltd (Shanghai, China). Hexane was provided by Titan Technology Co., Ltd (Shanghai, China). Atenolol (ATE), CBZ, amlodipine (AML) and IBU were kindly provided by the China Institute of Food and Drug Control City, China. Other chemicals, including inorganic salts, ethanol, and sodium hydroxide, were procured from Sinopharm Chemical Reagent Co., Ltd (China). All aqueous solutions were prepared using purified water with a resistance of 18.2 MΩ·cm.

The specific preparation method of HTCC was previously described in published literature (Afonso et al. 2001). Two grams of CTS powder was dissolved in 100 mL of acetic acid with a concentration of 2 wt%. Then, a 10 wt% NaOH solution was added to adjust the pH to 10 and to precipitate the CTS. The reaction proceeded under stirring for 4 h to allow the CTS to be fully alkalized. Then, the solution was centrifuged under 3,000 r/min and turned into a flask containing 30 mL isopropanol as medium with continuously stirring for 3 h at 70 °C. Then, a certain number of mole GTMAC (n_CTS:n_GTMAC = 1:4) was added to the solution at 80 °C for 8 h. After cooling, the reaction solution was washed thrice with a 250 mL acetone solution. The precipitated solids were put into the oven at 80 °C until fully dried. Finally, the obtained material was ground into powder.

The chemical structure of the membrane surface was observed by the Fourier transform infrared spectrometer (FTIR/ATR Nicolt iS5). The sample with 1 cm² was placed on the loading stage, and then the detection device was pressed on the center of the sample module to detect, to get the FTIR spectrum in the range of 3,500–500 cm⁻¹. A scanning electron microscope (JEOLJSM-6380LV) was used to scan the surface and cross-section micromorphology of the PES substrate and the modified membrane. The morphologies of membranes were observed under appropriate magnifications of 5 and 100 k. The sample, needed to be dehydrated and golded before the surface morphology, was observed. An AFM (Dimension 310) was utilized to observe the surface structure and roughness of the modified membrane. The static contact angle was employed by a DSA30 contact angle system to discuss the change in its hydrophilicity. Additionally, the streaming potential method was adopted to detect charging Zeta potential on the surface of a dually charged membrane using the electrokinetic analyzer (SurPASS^™ 3, Austria) with 1 mM KCl solution as an electrolyte solution, to determine the isoelectric point on the surface of the membrane and analyze the influence of external pH on the surface charge.

The MWCO of the modified membrane was measured by rejection experiments of neutral organic solutes (1 g/L) with molecular weights ranging from 100 to 1,000 Da at 6 bar. The concentration of organic matter in the material liquid and the osmotic liquid was detected by the total organic carbon analyzer. According to the literature, it was known that the molecular weight with 90% retention was defined as MWCO (Liu et al. 2018b).

As shown in Figure 2(c), the rejection for MgCl₂ increased to 92% ascribed to the electrostatic effect when the HTCC concentration was 0.5%, so 0.5% was the best concentration of HTCC. This was due to the reaction of acid chloride groups that were left on the PEI/TMC membrane with HTCC, and the introduction of quaternary ammonium groups in the HTCC directly modified the surface electrical properties of the membrane from negative to positive, so that the rejection of MgCl₂ was higher than that of Na₂SO₄ (Xiang et al. 2022). As the concentration of HTCC increased, the density of HTCC molecules near the outer surface of the polyamide layer increased, and the consumed acid chloride groups also increased forming a tighter membrane surface structure, which showed a certain degree of increase in rejection and decline in flux. When the concentration of HTCC exceeded 0.5%, the rejection of Na₂SO₄ was significantly reduced due to the increase of the positive charge on the surface of the membrane and the rejection of MgCl₂ was only slightly increased. At the same time, the viscosity of the HTCC solution was too large to block the pores of the membrane, resulting in a continuous decrease in flux.

As exhibited in Figure 2(d), when the soaking time reached 10 min, the MgCl₂ rejection rate was higher than the rejection rate of Na₂SO₄ and the rejection of the two salts tended to be stable when the reaction time between 15 and 30 min, indicating that the residual acid chloride groups had been largely consumed when the reaction time was 15 min. When the reaction time exceeded 15 min, the rejection of Na₂SO₄ decreased slightly while the rejection of MgCl₂ a little increased, and the flux continued to decrease. This was because a small amount of HTCC molecules were still diffusing toward the interface and gathered on the surface of the membrane with a positive charge and high viscosity resulting in the pores of the membrane being blocked. Taken together, the reaction time was 15 min for the optimal condition.

The characteristic AFM 3D images and the average roughness of different membrane samples are shown in Figure 3(a3)–3(c3). It could be seen that M0 had an obvious ridge-and-valley structure with an averaged RMS roughness of 31.6 nm. After IP of PEI and TMC on the PES membrane, it could be observed that the surface roughness of the modified membrane had been increased relatively with more ridge-and-valley structure, attributing to the formation of the cross-linked network structure of the selective layer by PEI (Kong et al. 2016). In addition, the RMS value of the PEI/TMC membrane increased to 52.1 nm, which proves that PEI and TMC strongly reacted, forming polyamide attached to the surface of the PES membrane, causing a sharp increase in roughness. Compared with the PEI/TMC membrane, the surface roughness of the PEI/TMC/HTCC membrane was significantly increased to 26.0 nm owing to a relatively slow reaction process between HTCC and the remaining TMC, encapsulating the polyamide layer of the PEI/TMC membrane.

As shown in Figure 3(d), the PES membrane illustrated a normal water peak at 3,388 cm⁻¹, while the PEI/TMC membrane showed a significant increase in the absorption peak volume at 3,179 cm⁻¹ and a redshift. On the one hand, this was due to –COCl which originated from incompletely reacted TMC. It underwent hydrolysis to form –COOH, resulting in an increase in the absorption peak area. On the other hand, PEI and TMC were polymerized to form –CONH–, and the –OH peak overlapped with the –NH peak, causing the absorption peak to redshift (Ouyang et al. 2019). The absorption peak of the PEI/TMC/HTCC membrane at 3,179 cm⁻¹ showed a blueshift to 3,369 cm⁻¹, which is ascribed to the introduction of –OH by the load of HTCC. The PEI/TMC composite NF membrane exhibited a typical absorption peak of C = O on –CONH₂ at 1,655 cm⁻¹ (C = O stretching, amide I), which indicated IP (Semião & Schäfer 2013). The absorption peak at 1,655 cm⁻¹ on the PEI/TMC/HTCC membrane was smaller because the composite structure introduced by the second step had no amide bond production and covered the original amide structure.

The visual diagram of the contact angle of the PES membrane, PES/(PEI), PES/(PEI/TMC), and PES/(PEI/TMC/HTCC) membrane is shown in Figure 3(e). By introducing a layer of PEI, the water contact angle of the PES/(PEI) membrane was a little smaller than that of the PES membrane, illustrating that the amino group in PEI improved the hydrophilicity of the PES membrane. When the interface polymerization between PEI and TMC occurred, the contact angle of the PEI/TMC membrane dropped to 13.77° because the acid chloride group on the remaining TMC was hydrolyzed to form excellent hydrophilic carboxyl groups (Li et al. 2022). After SIP, the contact angle of the PES/(PEI/TMC/HTCC) membrane quickly rose to 51.70°, thanks to the massive consumption of –NH on the HTCC and the reduction of hydrolysis of acid chloride groups.

We have successfully prepared a positively charged composite NF membrane that has been modified with HTCC containing quaternarized chitosan. It is prepared from natural chitosan for the removal of PPCPs through SIP. The concentration of PEI and its soaking time and the concentration of HTCC and its soaking time were determined as 1.5%, 20 min and 0.5%, 15 min to achieve the optimal membrane, respectively. The surface microstructure of the optimal membrane was studied by SEM and AFM. The amphoteric membrane showed a MWCO of 481 Da. Our experimental results indicated that the PEI/TMC/HTCC membrane exhibited excellent rejection performance to various inorganic salts and PPCPs by Donnan exclusion and steric hindrance. At the same time, the anti-fouling performance and stability of the PEI/TMC/HTCC membrane were improved. Therefore, this study provides a new strategy for the preparation of NF membranes in the removal of PPCPs.

Our work was financially supported by the National Natural Science Foundation of China (Grant No. 51208259). Facility support was acknowledged to the Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, Nanjing University of Science and Technology (Grant No. 30920140122008).

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

The authors declare there is no conflict.