Self-Cleaning Nanofiltration Membranes: Mineralized Interlayer Engineering

Crosslinking mineralized interlayer engineering of in situ self-cleaning
nanoltration membranes based on polyethylene substrates
Yunhuan Chen , Xinyue Liu , Weier Wang , Xiaoxiao Duan
*
, Yongsheng Ren
*
State Key Laboratory of High-efciency Utilization of Coal and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Faculty of Frontier Science
and Technology, Ningxia University, Yinchuan 750021, PR China
ARTICLE INFO
Keywords:
Nanoltration
Hydrophilic modication
Catalytic self-cleaning
Interfacial polymerization
ABSTRACT
Conventional nanoltration (NF) membranes are limited by their greater thickness and less stable ultraltration
substrates, as well as the signicant challenges posed by membrane fouling. In this work, we developed com-
posite NF membranes with a multifunctional mineralized interlayer through metal polyphenol network (MPN)
precursor-mediated cross-linking mineralization on hydrophobic polyethylene (PE) substrates. The incorporation
of a mineralized layer enhanced the wettability of PE and optimized the structure of polyamide (PA) via an
interlayer modulation strategy, resulting in PA-Fe
3
O
4
-PE membranes exhibiting water permeance up to 21.9
LMH bar
−1
and selectivity up to 68.8 for Cl
-
/SO
4
2-
. Furthermore, PA-Fe
3
O
4
-PE displayed a highly polarized
surface that signicantly improved its antifouling properties. The conned space within PA-Fe
3
O
4
-PE enabled
efcient regeneration through in situ self-cleaning, achieving ux recovery above 95% during all three fouling-
regeneration cycles while maintaining high stability and recoverability under extreme real-world wastewater
conditions. This study provides novel insights into the preparation of multifunctional composite NF membranes
and their sustainable application in water treatment.
1. Introduction
Reversed osmosis (RO) and nanoltration (NF) technologies have
experienced a surge in popularity following the launch of polyamide
(PA) thin lm composite (TFC) membranes [1,2]. These membrane
technologies have rapidly emerged as the dominant players in water
treatment, particularly desalination, due to their numerous advantages
including high energy efciency, treatment efcacy, and small footprint
[3]. In comparison to RO membranes, NF membranes exhibit a slight
compromise in solute rejection capacity while achieving higher water
permeability. However, the combined mechanism of size exclusion and
electrostatic effects endows NF membranes with the capability to
selectively separate charged solutes [4,5]. Despite the promising appli-
cation prospects of NF membranes in material concentration, selective
solute separation, and water purication, their permeability-selectivity
trade-off and inevitable membrane fouling issue pose signicant chal-
lenges to further advancements [6–8]. Consequently, the development
of NF membranes with enhanced permselectivity and antifouling/self-
cleaning properties is crucial for their ideal utilization in water
treatment.
To date, the majority of industrially useful NF membranes have been
obtained by preparing PA layers on porous supports by interfacial
polymerization (IP) [9,10]. Common substrates for preparing TFC
membranes using IP include ultraltration (UF) membranes such as
polysulfone (PSF), polyacrylonitrile (PAN), and polyethersulfone (PES),
which are chosen due to their suitable hydrophilicity and porosity [11].
However, the higher thickness of these UF membranes results in longer
water transport paths and higher transmembrane resistance for the
prepared NF membranes, leading to undesired permeability outcomes
[12–14]. In this case, the utilization of a thin and tough porous substrate
such as commercially available polyethylene (PE) membranes as an
alternative to UF substrates is anticipated to enhance water permeability
[15]. However, despite addressing the issue of inadequate water
permeability caused by thickness and porosity, hydrophobic PE mem-
branes encounter challenges in establishing a continuous water layer on
their surface during IP, thereby rendering the preparation of PA layers
based on PE substrates nearly unattainable [16,17]. Consequently,
enhancing the hydrophilicity of the PE substrate becomes a fundamental
prerequisite for conducting the IP reaction. Hydrophilic treatment of
hydrophobic substrates has been extensively documented, including
plasma-induced grafting of polyethylene glycol (PEG) onto poly(vinyl-
idene uoride) (PVDF) and polypropylene (PP), as well as oxidization of
* Corresponding authors at: No.539, Helanshan West Road, Xixia District, Yinchuan, Ningxia Hui Autonomous Region, 750021, PR China.
E-mail addresses: [email protected] (X. Duan), [email protected], [email protected] (Y. Ren).
Contents lists available at ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
https://doi.org/10.1016/j.cej.2024.158926
Received 4 October 2024; Received in revised form 20 December 2024; Accepted 22 December 2024
Chemical Engineering Journal 504 (2025) 158926
Available online 24 December 2024
1385-8947/© 2024 Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

PP using a chromic acid solution [12,18]. However, these methods are
typically complex and require high energy consumption, potentially
leading to damage to the pore structure of the membrane. Therefore, it is
imperative to explore a gentle and easily controllable method of modi-
cation that is suitable for large-scale applications, thereby enabling its
implementation in industrial manufacturing.
When seeking strategies to overcome this challenge, mussel
chemistry-directed biomimetic coatings offer a glimpse of the solution.
Natural polyphenols possess high interfacial activity and can form
continuous coatings on inert substrates, which have been demonstrated
to gently modify hydrophobic substrates [12,19]. Moreover, metal-
polyphenol networks (MPNs) show great promise in constructing
multifunctional interfacial coatings due to the strong coordination be-
tween metal ions and polyphenols [20–22]. The combination of
different compounds with excellent electronic properties results in new
composite materials, which have attracted great technological interest
in recent years [23]. For instance, MPN can facilitate the mineralization
of metal ions (such as complex iron oxides) to achieve fenton-like cat-
alytic oxidation function [24–27]. Additionally, the mineralized inter-
layer mediated by MPN enhances aqueous-phase monomer storage
capacity and retards monomer diffusion, thereby regulating the IP
process [28]. Distinguished by its deviation from conventional catalyst
preparation methods, this mild mineralization process obviates the need
for intricate procedures, especially for iron spinel compounds [29–31].
In addition, the potential of spinel is conrmed by its biomedical ap-
plications [32]. Therefore, guided by mussel chemistry, MPN deposition
and mineralization may realize our vision of gently modifying PE sub-
strates to construct a multifunctional mineralized coating that includes
hydrophilic modication of PE membranes, interlayer modulation
strategies for optimizing IP reactions, as well as achieving in situ cata-
lytic self-cleaning.
To demonstrate the aforementioned concept, a multifunctional
mineralized coating was fabricated on a hydrophobic PE substrate
through MPN-mediated crosslinking mineralization (Fig. 1a), which
serves three purposes: (i) hydrophilicization of the PE substrate to be
appropriate for the IP reaction; (ii) mineralized interlayer to regulate
the diffusion of monomers in the IP reaction; and (iii) efcient in situ
catalytic self-cleaning via conned spaces constructed by composite
membranes. The resulting NF membranes exhibit exceptional water
permeability, solute separation selectivity, and high surface polarity,
effectively breaking the permeability-selectivity trade-off while
demonstrating excellent antifouling properties. Moreover, due to their
conned space, these composite NF membranes can be regenerated
through rapid in-situ catalytic self-cleaning even under extreme real-
world wastewater conditions while maintaining remarkable stability
and recyclability.
2. Materials and methods
2.1. Materials and chemicals
Commercial PE microporous membranes with a thickness of ~ 20
μ m
were obtained from SK geo centric Ltd. (Korea). Polysulfone (PSF) ul-
traltration (UF) membranes (MWCO: ~20 kDa) provided by Xiamen
Xuwu Membrane Technology Co. Tris-HCl buffer (1 M, pH =8.8) was
purchased from Shanghai Yuanye Biotechnology Co. Tannic acid (TA),
FeCl
2
⋅4H
2
O, FeCl
3
⋅6H
2
O, LiCl, NaCl, MgCl
2
, Na
2
SO
4
and MgSO
4
are all
analytically pure (AR) provided by Sinopharm Chemical Reagent Co.
Piperazine (PIP, 99.5 %), hexane (AR), ethylene glycol (AR), poly-
ethylene glycol (PEG, WM: 200, 300, 400, and 600 Da, chemically pure
(CP) and sodium alginate (SA, CP) were obtained from Shanghai Aladdin
Biochemical Sci & Tech Co. Trimesoyl chloride (TMC, >98 %) are
purchased from Shanghai McLean Biochemical Technology Co.
2.2. Membrane preparation
The preparation of composite nanoltration (NF) membranes is
divided into two distinct stages: the mineralization process of the
polyethylene (PE) substrate and the subsequent formation of a poly-
amide (PA) layer, as elaborated in Supplementary Information.
Fig. 1. MPN-mediated in situ cross-linking mineralization. a) Schematic illustration of MPN coating fabrication and in situ cross-linking mineralization inspired by
mussel biochemistry. b-e) Surface SEM images of PE, MPN-PE, and Fe
3
O
4
-PE substrates, and f) cross-section and EDS mapping images of Fe
3
O
4
-PE substrate,
indicative the elemental distribution of C, O, and Fe.
Y. Chen et al.
Chemical Engineering Journal 504 (2025) 158926
2

2.3. Characterization methods
The membrane surface wettability and surface free energy (SFE)
were characterized by measuring the water contact angles (WCAs) and
ethylene glycol contact angles (ECAs) on the membrane surface. Raman
spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray
photoelectron spectroscopy (XPS), and energy-dispersive X-ray spec-
troscopy (EDS) were employed to analyze the chemical composition of
the membrane. Scanning electron microscopy (SEM) and atomic force
microscopy (AFM) were utilized for observing the morphology of the
membrane. A comprehensive introduction to membrane characteriza-
tion is provided in Supplementary Information.
2.4. Membrane separation assay
The membrane separation and fouling/regeneration experiments
were conducted using a at plate cross-ow device with a membrane
cell effective area of 24 cm
2
in this study. Supplementary Information
provides detailed information on the experimental process and meth-
odology related to membrane separation, fouling, and regeneration.
2.5. Computational simulation
The Gaussian 09 software was employed for structure optimization
and binding energy calculations, based on Density Functional Theory
(DFT) with the B3LYP exchange–correlation functional [33]. For main
group elements, the 6-311G** basis set was utilized, while for the
transition metal element iron (Fe), the SDD pseudopotential basis set
was employed to enhance calculation accuracy. Additionally, Multiwfn
software was used to perform electrostatic potentials and differential
charge densities analysis [34].
3. Results and discussion
3.1. Mpn-mediated cross-linking mineralization
The process of mineralization/hydrophilic modication of the
polyethylene substrate is depicted in Fig. 1a, and the entire minerali-
zation process was conducted under mild room temperature conditions.
Importantly, conventional metal oxide preparation involves oxidative
processes where excess and deciency of oxygen can increase or
decrease the degree of oxidation of the 3d-metal [35]. In contrast, the
MPN-mediated mineralization process has no complex reaction steps,
and the entire mineralization process relies on a single control variable,
time, and is therefore easily manageable in an industrial manufacturing
process. According to the proposed scheme, MPN is deposited onto the
polyethylene substrate as the initial mineralization precursor, thereby
providing a plethora of reaction sites for subsequent crosslinking
mineralization reactions. A brous structure was observed for the
nascent PE substrate (Fig. 1b), while EDS mapping of its surface and
cross-section only revealed the distribution of elemental C (elemental H
could not be detected by EDS). The loading of MPN resulted in the
emergence of randomly dispersed nano-dots on the surface of MPN-PE
(Fig. 1c), accompanied by the presence of elemental O and Fe, which
were absent in the PE substrate, as observed through both surface and
cross-section mapping (Fig. S2b). Upon completion of crosslinking
mineralization, uniform and dense nanoparticles formed on the surface
of Fe
3
O
4
-PE (Fig. 1d–e), leading to a certain degree increase in surface
roughness (Fig. S3). It is well known that the electronic properties for
the complex oxides strongly depend on the average crystallite size and
crystallite size distribution [36]. Thus, the dense nanoparticles on
Fe
3
O
4
-PE potentially allow for superior electronic properties. According
to Wenzel’s model [37]. the nanoscale surface protrusions can signi-
cantly augment the hydrophilicity of the membrane surface, thereby
rendering the Fe
3
O
4
-PE substrate amenable for implementation in the IP
process. Moreover, uniform distribution of O and Fe elements was
observed on both the surface and cross-section of Fe
3
O
4
-PE membranes
through membrane surface and cross-section morphology analysis and
EDS mapping (Fig. 1f and Fig. S2). This nding suggests that MPN-
mediated cross-linking mineralization not only occurs on the mem-
brane surface but also within its pores. Due to the abundance of hydroxyl
groups in MPN, the MPN-mediated cross-linking mineralization process
not only yields compact nanoparticles but also introduces a signicant
number of hydrophilic functional groups onto the PE substrate, thereby
substantially enhancing its water wettability. In accordance with the
dissolution-diffusion model, hydrophilic membranes exhibit a steeper
concentration gradient for water during operation, facilitating efcient
water transport [3].
To gain further insights into the mineralization process of cross-
linking, Raman spectra were acquired for the three substrates (Fig. 2a).
In the case of MPN-PE and Fe
3
O
4
-PE, evidence of catechol coordination
with Fe
3+
(Fe-O resonance) was observed within the range of approxi-
mately 530–640 cm
−1
, while novel peaks arising from C-OH vibrations
and C-O-Fe
3+
coordination were detected at 1342 and 1472 cm
−1
[19,38]. These ndings provide conrmation regarding the incorpora-
tion of TA and MPN networks onto PE substrates. Additionally, a weak
peak at 680 cm
−1
is observed for Fe
3
O
4
-PE, which can be attributed to
the formation of Fe
3
O
4
minerals [39]. Normally, it would be necessary to
determine the atomic positions of the cations in the initial Fe
3
O
4
singlet,
or whether other phases and composites would form [40]. However, in
this paper the source of the iron in Fe
3
O
4
can be clearly identied as Fe
2+
and Fe
3+
, thus discharging the possibility of forming other composites.
As depicted in Fig. 2b, with respect to atomic composition, MPN-PE
exhibits a lower Fe content (0.22 %) on its surface, while Fe
3
O
4
-PE
demonstrates higher loading of Fe (8.86 %) and O (32.21 %), attributed
to the continuous crosslinking mineralization process. The O 1 s and Fe
2p XPS spectra of MPN-PE and Fe
3
O
4
-PE were analyzed to investigate
the chemical compositional shifts during the mineralization process
(Fig. 2c). Deconvolution of the O 1 s spectrum revealed the presence of
–OH, C =O, and C-O peaks in MPN-PE, while the Fe 2p spectrum
exhibited a characteristic peak corresponding to Fe (III) [41]. For Fe
3
O
4
-
PE (Fig. 2d), the signal of Fe-O from metal oxides was additionally
detected in the deconvolution of its O1s, as well as its Fe 2p was shown
to be a mixed valence of Fe (II) and Fe(III) [42]. By combining different
types of polymer with oxides and carbon-based materials the new
composites with increased and attractive electronic properties could be
fabricated[43]. It is noteworthy that the high abundance of Fe
2+
in-
dicates the presence of signicant oxygen vacancies in the Fe
3
O
4
coating, which effectively facilitates the activation of H
2
O
2
and the
generation of reactive oxygen species (ROS) components [44]. More-
over, oxygen vacancies effect on exchange interactions [45]. This in situ
degradation/mineralization process of pollutants based on the Fenton
reaction is exactly the mechanism of action of catalytic self-cleaning in
this work.
The thickness of the Fe
3
O
4
coating can be estimated by determining
the average pore diameter before and after mineralization. As depicted
in Fig. 2e, the virgin PE membrane had an average pore diameter of
121.5 nm, whereas that of Fe
3
O
4
-PE was reduced to 108.4 nm, indi-
cating a mineralized coating thickness of approximately 6.6 nm
(Fig. S4). Although a reduction in pore size may reduce the water
transport channels within the membrane[46], this change in ux due to
the change in pore size is negligible since the pristine PE substrate has no
water ux at low pressures (2 bar) (Fig. 2f). The pristine PE substrate
exhibited a water contact angle (WCA) of 104.1◦, indicating complete
hydrophobicity and resulting in a high liquid breakthrough pressure
(LEP) of 19.3 bar at 2 bar[47]. In contrast, the Fe
3
O
4
-PE membrane with
inside-out hydrophilic modication showed a reduced WCA of 50.5◦and
LEP decreased to 0.7 bar, leading to a dry membrane ux of 19.7 LMH
and an impressive wet membrane ux as high as 102.9 LMH under an
operating pressure of 2 bar. In conclusion, the afore mentioned char-
acterization results conrm that MPN-mediated crosslinking minerali-
zation enables the formation of a Fe
3
O
4
mineralized layer on
Y. Chen et al.
Chemical Engineering Journal 504 (2025) 158926
3

Fig. 2. Physicochemical properties of the original and modied substrates. a) Raman spectroscopy and b) XPS analysis of PE, MPN-PE and Fe
3
O
4
-PE substrates. XPS
O1s and Fe 2p spectra of c) MPN-PE and d) Fe
3
O
4
-PE. e) Pore size distribution and f) WCA, LEP, and water ux (2 bar) of PE and Fe
3
O
4
-PE (where the wet membrane
was obtained by pre-soaking for 1 h in DI).
Fig. 3. Interaction mechanism of MPN precursors. a1-a2) Optimized structure of TA molecule (substituted with a gallate branch) and its spatial arrangement of
electron cloud. b1, c1) Equilibrium conguration of Fe
3+
interacting with the ester group /phenolic hydroxyl group (b/c), b2, c2) Corresponding electron cloud
arrangement, b3, c3) The 3D isosurfaces depicting electron density difference and b4, c4) 2D contour maps near the ester group /phenolic hydroxyl group. The blue
and yellow surfaces in the 3D isosurfaces represent values equivalent to 0.001 and −0.001, respectively. The dashed blue and solid red lines in the 2D contour maps
correspond to regions where there is a decrease and an increase in electron density, respectively. (For interpretation of the references to colour in this gure legend,
the reader is referred to the web version of this article.)
Y. Chen et al.
Chemical Engineering Journal 504 (2025) 158926
4

hydrophobic PE substrate. Simultaneously, this process effectively en-
hances the hydrophilicity of the PE substrate, thereby facilitating the
preparation of composite NF membranes via IP.
3.2. Mechanism of MPN-mediated cross-linking mineralization
As proposed, MPN precursors serve as both modiers of membrane
surfaces and crucial agents in the formation and stabilization of Fe
3
O
4
coatings. The MPN with Fe
3+
as a cross-linking center provides reaction
sites and nucleation sites necessary for achieving the mineralization
transformation of Fe
3
O
4
[24]. Furthermore, it is imperative to consider
that the strength of the interaction between Fe
3+
and TA exerts a sub-
stantial inuence on the process of mineralization [39,48]. Therefore, it
is imperative to analyze the molecular interactions between Ta and
Fe
3+
. The interaction between TA molecules and Fe
3+
in aqueous solu-
tion was assessed using density-functional theory (DFT). To streamline
the calculations, gallic acid branches were employed as a simplied
representation of TA. The optimised molecular structure of TA and its
electron cloud arrangement are depicted in Fig. 3a1-a2, where the ester
group and phenolic hydroxyl group are predicted to interact strongly
with Fe
3+
. Specically, the O-Fe bond formed by the ester group exhibits
a binding energy of −3.79 eV at a distance of 2.02 Å from Fe
3+
(Fig. 3b1). On the other hand, the phenolic hydroxyl group forms an O-
Fe bond with a relatively longer distance (2.44 Å) from Fe
3+
, resulting in
a binding energy of −3.03 eV (Fig. 3c1). In comparison, the binding
energy of Fe
3+
to the phenolic hydroxyl group was slightly lower than
that of the ester group, probably due to the more distant spatial location.
However, it should be noted that chelating coordination among neigh-
boring hydroxyl groups may lead to a more signicant interaction than
what is calculated [25]. The electron cloud distributions before and after
TA binding to Fe
3+
(Fig. 3a2, b2, and c2) exhibit a pronounced
augmentation in the electron density surrounding Fe
3+
, thereby indi-
cating the occurrence of electron transfer from the ester group/phenolic
hydroxyl group to the adjacent Fe
3+
ion, which facilitates the binding of
Fe
3+
. To gain further insights into the variations in electron density
during the action of MPN precursors, the different charge densities of
ester groups and phenolic hydroxyl groups before and after their inter-
action with Fe3 +were investigated. As shown in Fig. 3b3 and c3
(representing ±0.001 isosurfaces; other scenarios are illustrated in
Fig. S5 and S6), the interaction between TA and Fe
3+
results in a sphe-
roidal electron-rich (blue) structure encompassing the iron nucleus,
irrespective of the Fe
3+
site of action. From the 2D contour maps
(Fig. 3b4 and c4), the center of this electron-rich structure surrounding
the Fe nucleus exhibits an electron-decient blue dashed line, indicating
an electron-integration interaction between TA and Fe
3+
. Furthermore,
the O-Fe interaction leads to enhanced electron density around Fe
3+
,
Fig. 4. Morphological characteristics, compositional analysis, and inherent properties of polyethylene-based composite NF membranes. a) Surface SEM, b) Surface
AFM and c) Separation layer thickness characterization of PA- Fe
3
O
4
-PE membrane. d) Surface XPS spectra of PE substrate and PA-Fe
3
O
4
-PE membrane. e) WCA and
SFE of several substrates and PA- Fe
3
O
4
-PE membrane. f) Repulsion curves of PA- Fe
3
O
4
-PE towards uncharged solute (PEG) and its pore diameter distribution. g)
Comparison of pore size ranges (All apertures with a probability density greater than 0.005 in the PDF curve were counted) of conventional PA-PSF and PA- Fe
3
O
4
-PE
membranes prepared by IP on PSF and Fe
3
O
4
-PE substrates. h) WP and rejection of prepared PA- Fe
3
O
4
-PE membranes for treatment of inorganic salt solutions (2000
ppm, 7 bar). i) Comparative analysis of permeability and separation selectivity (Cl
-
/SO
4
2-
) between PA- Fe
3
O
4
-PE membranes and NF membranes on
different substrates.
Y. Chen et al.
Chemical Engineering Journal 504 (2025) 158926
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