Elsevier

Journal of Membrane Science

Volume 642, 15 February 2022, 119930
Journal of Membrane Science

Surfactants attached thin film composite (TFC) nanofiltration (NF) membrane via intermolecular interaction for heavy metals removal

https://doi.org/10.1016/j.memsci.2021.119930Get rights and content

Highlights

  • Positively charged NF membrane was developed through surfactant post-treatment.

  • IP built dense PA layer was optimized by monomers concentration adjustment.

  • CTAB and SDS endowed superior positively and negatively charged surfaces.

  • The surfactant layer offers a superior NF performance.

  • CTAB attached TFC membrane owns excellent heavy metals removal ability and stability.

Abstract

Heavy metals are well-known pollutant in the water body, which can threaten our health in various ways. Since size exclusion and electrostatic repulsion play a significant role in heavy metals removal for membrane technology, hereon, a positively charged nanofiltration membrane with a dense polyamide (PA) layer and a surfactant functionalized layer (SFL) was successfully developed. The optimization of the PA layer was achieved by adjusting the concentration of piperazine (PIP) and trimesoyl chloride (TMC) during interfacial polymerization and the thin film composite (TFC) membrane formed by 0.4% (w/v) PIP and 0.01% (w/v) TMC was selected for further study due to its gratifying performance. Then, the membrane was developed by attaching surfactants in which cetyl trimethyl ammonium bromide (CTAB) endowed a superior positively charged surface whereas sodium dodecyl sulfate (SDS) intensified negative charge compared to the original membranes. Furthermore, the SFL attached membrane was successfully obtained by attaching CTAB with various concentration and grafting time. The feed concentration of all metal ions and salts adopted for evaluating NF performances were 500 mg‧L−1. The best performance showed a high water permeability of 21.5 ± 0.6 L‧m−2‧h−1‧bar−1 and 96.6 ± 1.4% rejection for MgCl2. Overall, the as-developed SFL attached membrane has found its potential application in heavy metal removal in which the separation efficiency was over 98% against Zn2+, Ni2+, 96%–98% against Cu2+, >94% against Ca2+ and >85% against Pb2+ with a high efficiency of water reclamation simultaneously.

Introduction

With the increasing crisis of water scarcity, wastewater reclamation has become an effective way to improve the utilization of water resource and reduce the environmental pollution [[1], [2], [3], [4]]. Recently, heavy metals in water environment have become a major problem in the continuous progress of industrialization [5,6]. Although heavy metals (like zinc, copper, nickel, lead, etc.) are generally present at low concentrations in the water body, they can cause a serious threat to both human health and ecosystem, especially for its toxicity, accumulation, non-biodegradable and durability [[7], [8], [9]]. Therefore, removing heavy metals from wastewater is of critical importance prior to water reuse.

Currently, the promising membrane technologies are preferred in the removing heavy metal ions from contaminated water resources among common methods for its high efficiency, environmental friendly and modularity [[10], [11], [12]]. Nanofiltration (NF) plays a vital role in wastewater purification and desalination, while it is stating between ultrafiltration (UF) and reverse osmosis (RO) [13]. Moreover, NF owns the particular merit for retaining small molecules (>200 Da) and multivalent ions, while let water and monovalent ions pass through rapidly. It has great potentials in mixed salts separation, salts/organics separation, heavy metal salts removal and dyes purification [[14], [15], [16]]. Commercial NF270, NF90 (DOW), AFC40 and AFC80 (PCI) membranes have already been developed for removing of heavy metals [[17], [18], [19]].

Thin-film composite (TFC) polyamide (PA) membrane is a predominant structure in designing high-performance NF membranes. It consists of a porous sub-layer and a dense PA layer, the latter dominate the separation process in terms of exclusion effect and electrostatic interaction. Currently, the positively charged nanofiltration membrane are popular in the field of heavy metals removal [[20], [21], [22]]. Li et al. have studied the influence of the surface charge of NF membrane on the Mg2+/Li+ separation, which demonstrated the PC membrane was prefer to retain Mg2+ both by experiment and simulation [23]. In order to enhance the membrane surface electropositivity, monomers with abundant active amino groups like poly (amidoamine) and chitosan were adopted. Besides, polyethylenimine (PEI) was also a monomer used to fabricate positively charged membranes with excellent stability [[24], [25], [26]]. However, the affinity between those monomers and the substrates surface resulted in the poor adherence which may further impede the formation of the PA layer. Among recent studies, nanoparticles, surfactants or acid acceptance agent can be utilized to solve the issue, but the preparation process and the cost can be further improved [[27], [28], [29], [30]].

Comparatively, these micro-molecular amine monomers, such as piperazine (PIP), are easier to form PA layer with trimesoyl chloride (TMC) and have been widely used in fabricating TFC NF membranes [31,32]. In general, most part of the acyl chloride groups in organic phase TMC are bonded with amine molecules to form PA layer, while small part of them transformed into carboxyl groups after contacting with aqueous solution during molecular diffusion. Additionally, the carboxyl groups will be reproduced unceasingly after IP process, as the residual acyl chloride groups in the TFC membrane was posed in plenty water. Thus, the TFC membrane carries negative charge on its surface when prepared with micro-molecule amine monomers, and therefore is adverse to heavy metals removal. Hereto, in this work, we would proposed a facile strategy to post-treatment the PA layer in order to adjust the charge property of the membrane surface [33].

The effect of surfactants on TFC NF membranes properties have been studied [34,35]. The cationic surfactants cetyl trimethyl ammonium bromide (CTAB) have been verified as an approach to strengthen the charge of NF membrane [36]. The surfactants always adhere to the membrane by IP through doped in aqueous solution, which limit the increase of membrane surface charge, and thus post treatment have been considered. Lin et al. post-treated the PEI/TMC surface with quaternization to fine tune its surface properties and obtained 94.4% MgCl2 rejection with pure water permeability of 9.3 L‧m−2‧h−1‧bar−1 [37]. As the size exclusion also plays an important role in separating process. Decreasing the membrane pore size is an advisable route for improving metal ions removal. Tian et al. have proposed a polypiperazinamide surface with positive rejection for Ni2+, Zn2+, Cu2+ and Cd2+ only by reducing its pore size via decreasing TMC concentration [38].

In this work, a polypiperazinamide surface was prepared and further optimized during IP. Effects of low monomers concentration were investigated including 0.2–1.0% (w/v) of PIP and 0.005–0.150% (w/v) of TMC. On this basis, positively and negatively charged surfaces were acquired by surfactants solution attachment in terms of electrostatic adsorption and hydrophobic interaction (Fig. 1). NF performances and the heavy metals removal ability of the resultant surfactant functionalized layers (SFLs) were comprehensively evaluated and the detailed mechanism was studied. Additionally, the peculiarities of the SFLs were also characterized including morphology, composition, pore size and the surface properties. This study may offer a simple and effective method for post-treating TFC membranes.

Section snippets

Materials and chemicals

Polyethersulfone (PES, 50,000 Da) used as the support was obtained from Hangzhou Water Treatment Technology Development Center Co., Ltd.. TMC (98%) and PIP (GR) were obtained from Sigma-Aldrich. CTAB (AR) and sodium dodecyl sulfate (SDS, AR) were supplied by Sinopharm Chemical Reagent Co., Ltd. (China). Inorganic salts including sodium sulfate (Na2SO4, 99.0%), magnesium sulfate (MgSO4, 98.0%), magnesium chloride hexahydrate (MgCl2·6H2O, 98.0%), sodium chloride (NaCl, 99.9%) and heavy metals

Characteristics of as-fabricated surfaces without post-treatment

Five raw samples were first prepared by fixing PIP solution at 0.6% (w/v) and varying the TMC concentration within the range of 0.005–0.150% (w/v). In order to study the effect of PIP concentration on properties, 0.2–1.0% (w/v) PIP solutions were fixed with 0.010% (w/v) TMC. Results including chemical composition, SEM images, pore size, WCA and zeta potentials are shown in supporting information (SI), respectively. Those parameters are listed in Table 2. The characteristic peak Cdouble bondO at 1661 cm−1

Conclusions

In this study, surfactants (including CTAB and SDS) attached NF membranes were successfully developed by post-treatment technology, and the CTAB attached membrane was used for heavy metals removal. Firstly, various TFC NF membranes were fabricated via IP, the concentration of PIP aqueous solution (0.2–1.0%, w/v) was investigated for the effects on membrane properties, as well as TMC concentration (0.005–0.150%, w/v). The P4 NF membrane formed by 0.4% (w/v) PIP and 0.01% (w/v) TMC was selected

CRediT authorship contribution statement

Sun-Jie Xu: Conceptualization, Supervision, Formal analysis, Funding acquisition. Qian Shen: Data curation, Writing – original draft. Li-Han Luo: Investigation. Yi-Hao Tong: Investigation. Yu-Zhe Wu: Software. Zhen-Liang Xu: Supervision, Funding acquisition. Hai-Zhen Zhang: Validation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors gratefully acknowledge for the financial support received from Project funded by China Postdoctoral Science Foundation (2021M691013) and the research funding provided by National Natural Science Foundation of China (22078092 and 21978082).

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    Sun-Jie Xu and Qian Shen contributed equally to this work.

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