Recyclable nanoscale zerovalent iron (nZVI)-immobilized electrospun nanofiber composites with improved mechanical strength for groundwater remediation
Graphical abstract
Introduction
Groundwater remediation is facing big challenges in increasingly emerging pollutants. Nanoscale zero-valent iron (nZVI), with its high reducibility towards heavy metals and halogenated organics, have been a promising material/technology widely studied for in-situ groundwater remediation [1]. However, individual nZVI particles are prone to agglomeration, sedimentation and oxidization [2,3]. Traditional methods were to add surfactants [4], suspending agents [5] and stabilizers [2] to ensure the activity and uniformity of nZVI particles amidst groundwater. Nevertheless, these auxiliary additives, adsorbed contaminants or even nZVI itself may render a secondary pollution as long as they remained in groundwater without recycle [6].
To solve these problems, a method using membrane as a carrier to load nZVI was put forward and recognized [7,8]. Basically, membrane-immobilized nZVI not only can avoid nZVI agglomeration and sedimentation in the groundwater, but also can recycle the used nZVI particles and the adsorbed contaminants to avoid potential secondary pollution [9]. Particularly, electrospun polymeric nanofiber membrane with its high recyclability [10], high surface-area-to-volume ratio [11] and easy-to-functionalized properties [[12], [13], [14], [15]] is a suitable technology for nZVI immobilization.
In terms of the material for nZVI immobilization, polymers with carboxylic groups (–COOH), are mainly selected to fabricate membranes to bond with nZVI [16]. Polyacrylic acid (PAA) with abundant –COOH group is one of the main polymers for nZVI immobilization [9,17]. Besides, PAA is also the most commonly-used stabilizer against nZVI aggregation. However, PAA does not have enough mechanical strength because of its intrinsic non-homogeneous structure as well as the lack of effective energy dissipation mechanism [18]. Meanwhile, lacking mechanical strength is one of the critical problems for the use of membrane-supported nZVI [19,20]. Therefore, to apply for groundwater filtration process, the mechanical strength of a PAA-based membrane should be enhanced.
According to previous studies, the use of polymer with high molecular weight is a good way to improve polymer mechanical strength [21], because the long-chain structure of high molecular weight polymers can effectively increase the possibility of a polymer entanglement, thereby increasing interconnection among polymers [22]. Besides, crosslinking between the main chain polymer and cross-linkers is another way to improve the mechanical strength of the polymer by enhancing intermolecular forces among polymer molecules [23]. Gong et al. [24] induced a double–network structure with various hydrophilic polymers and successfully increased PAA polymer mechanical strength. Subsequently, Lin et al. [18] further developed a simplified dual-crosslinking method using acrylamide and Fe(III) to crosslink acrylic acid monomer by covalent bond and ionic bond, respectively. The novel dual-crosslinked acrylic acid hydrogel showed adequate toughness, high mechanical strength, and good self-recovery. However, there is no study reported regarding the use of high molecular weight PAA and dual-crosslinking method to increase the mechanical strength of PAA electrospun membrane, and employ it for nZVI immobilization.
Thus, this study investigated the influence of PAA molecular weight and the feasibility of the dual-crosslinking method on the mechanical strength of electrospun membranes. The conceptual mechanism of membrane strength enhancement in this study is presented in Fig. 1. Our strategy was to compare the effect of PAA molecular weight on the mechanical strength, thus we used PAA with two different molecular weights: 2000 (M2k, Fig. 1a) and 450000 (M450k, Fig. 1b) to conduct the first crosslinking with polyvinyl alcohol (PVA). Subsequently, PAA-PVA membranes (M2k and M450k) were dual-crosslinked by adding Fe(II) (Fig. 1c) or Fe(III) (Fig. 1d) as the ionic cross-linker respectively to investigate its effect on ionic crosslinking among polymers.
Meanwhile, interestingly, in terms of nZVI immobilization, Fe(II) and Fe(III) ions are also the substrates for nZVI reduction, which means the Fe(II) or Fe(III) can be either the ionic cross-linkers for membrane construction or the electron acceptors for nZVI generation onto the PAA-PVA membrane. Therefore, we tried to figure out the optimum balance between the increase of membrane mechanical strength and the increase of nZVI immobilization onto the membrane. Regarding the selection of Fe(II) and Fe(III), the PAA-PVA membrane could complex more Fe(II) ions, because each Fe(II) will just occupy two –COOH groups from PAA (Fig. 1c) while each Fe(III) will take three –COOH groups (Fig. 1d). On the other hand, Fe(III) with three coordination bonds (COO–Fe) may have better crosslinking capability for membrane strengthening than Fe (II) with only two coordination bonds [25].
Therefore, this study has two main objectives. One is to develop a high mechanical strength membrane for nZVI immobilization. The other is to investigate the optimum strategy to make the developed membrane a better nZVI immobilization and filtration performance.
Section snippets
Materials
PAA (Mw = 2000 and Mw = 450,000), PVA (Mw = 85,000–124,000, 87–89% hydrolyzed), Fe2 (SO4)3, (FeSO4·7H2O), NaBH4, Cd (NO3)2 and ethanol were ordered from Sigma-Aldrich. The deionized water was prepared with Millipore Milli-Q water system and purged by N2 for 20 min before use to remove the dissolved oxygen.
Polymer solution preparation
A long-chain PAA (Mw = 450,000) was used to compare with the short-chain PAA (Mw = 2000) in this study. For a better nZVI capture, all the mixed PAA/PVA polymer solutions were prepared with a
Effect of molecular weight on membrane fabrication
Fig. 4 shows the SEM images and fiber size distributions of pristine (as-spun) and thermally-crosslinked PAA/PVA nanofiber membranes with two molecular weights of 2 k and 450 k. In terms of as-spun membranes (with prefix A in membrane code), both A-M2k (Fig. 4a) and A-M450k (Fig. 4b) showed beadless and defect-free membranes with cylindrical nanofibrous structure. The average fiber sizes of the two as-pun membranes were 620 ± 60 nm and 422 ± 212 nm, respectively. A-M450k had a lower average
Conclusions
In this study, we successfully improved the mechanical strength of the PAA-PVA membrane by increasing PAA molecular weight and introducing the Fe(II)/Fe(III)-complexed dual-crosslinking structure. The developed high molecular weight composites could also immobilize more nZVI particles due to higher surface areas and more available –COOH groups. The nZVI membranes reduced from Fe(II) and Fe(III) had different performances in mechanical strength, Cd(II) removals, nZVI immobilization and
Acknowledgement
The authors would like to thank the funding support from the Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (Project number: 4.1.18-13/14). The grants from Smart Civil Infrastructure Research Program under the Korean Ministry of Land, Infrastructure and Transport (18SCIP-B145909-01), as well as from the Technology Innovation Program (10080342, Development of Concrete Photocatalytic Finishing Plate for De-NOx) funded by the MOTIE, Korea are also
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