Fabrication of a novel polyvinylidene fluoride membrane via binding SiO2 nanoparticles and a copper ferrocyanide layer onto a membrane surface for selective removal of cesium
Graphical abstract
Introduction
Increasing industrial activities accelerate the release of radionuclides [1,2]. Cesium (137Cs) is the most hazardous and abundant radionuclide in the aqueous environment, due to its high energy gamma ray emission (661.9 keV) and relatively long half-life (30.1 years) [3]. In the past years, several methods have been studied for removing Cs from aqueous solution which includes precipitation, solvent extraction, adsorption/ion exchange and membrane technology [[4], [5], [6], [7]]. Membrane technology is increasing as pretreatment in desalination and used in wastewater treatment containing Cs because it offers significant advantages compared with conventional removal methods [5,8,9]. In our previous studies, reverse osmosis (RO) membrane has demonstrated good capabilities in removing of Cs, but the RO membrane is limited by membrane fouling and high energy consumption [9,10]. Low pressure membranes (LPMs) have been applied widely in the world from a cost-benefit perspective due to that their filtration pressures were just one tenth of RO membranes [11,12]. However, it is difficult to remove Cs by LPMs through mechanical filtration, and the membranes are no selective [13,14]. As a result, now LPMs filtration systems Cs has to be eliminated by including adsorbents in the solution upstream of the membrane or by utilizing an accessional processing unit [5,[15], [16], [17]].
Transition metal ferrocyanides have been reported to be one of the most promising adsorbents due to their superb high selectivity and large adsorption capacities for Cs, thereby being extensively studied in the last decade [[18], [19], [20], [21], [22]]. Among the transition metal ferrocyanides, copper ferrocyanide (CuFC) was demonstrated as an efficient and environmentally friendly Cs scavenger [19,[23], [24], [25]]. CuFC had strong affinities to Cs, which was less affected by the coexisting cations and solution pH [[26], [27], [28]]. In generally, as fine powers, the CuFC crystals are difficult to use directly in the filtration of Cs separated from aqueous solution [29]. The excellent performances of CuFC and the development of modification technology unprecedentedly promote the development of novel CuFC composite membranes with actual value. The substrate membrane prevents CuFC from leaching into the treated water and also mechanically supports the CuFC layer. By simultaneously taking advantage of the CuFC layer and membrane substrate, the composite membrane can be an effective material with great potential for use in radioactive wastewater remediation. CuFC has been directly precipitated into the pore of membrane, which had the low stability and decreased the flux, resulting in low selectivity of Cs [30,31]. Recently, the CuFC crystals have been impregnated to various materials such as silica, zeolite and magnetic nanoparticles (NPs), in order to improve the mechanical properties of the absorbent [28,32,33]. To improve the bonding strength and mitigate membrane pore blocking caused by the CuFC particles deposition, the hydrophilic silica (SiO2) layer was introduced into the CuFC composite membrane. Polyvinylidene fluoride (PVDF) with outstanding mechanical strength, good thermal stability, and high chemical resistance has been widely applied as a LPMs material for industrial process [[34], [35], [36]]. Some literatures on embedding SiO2 NPs onto PVDF membranes have indicated the membrane flux and hydrophilicity were highly improved [35,37]. On the other hand, CuFC type particles could coat onto SiO2 through chemical bonding, and present a high selectivity to Cs [28,38,39]. To the best of our knowledge, no study has developed the CuFC-coated NPs onto the membrane surface synchronously with tight binding for property improvement.
This study explores a novel and facile approach to fabricate a highly selective PVDF composite membrane for Cs via post fabrication tethering of hydrophilic SiO2 and CuFC NPs on the membrane surface. A tight chemical bond for binding SiO2 NPs (surface-tailored with amine functional groups) on the membrane surface was formed by the introduction of Hydroxyl groups onto the PVDF membrane surface by chemical processes. Afterward, the fixed SiO2 NPs were covalently bond to CuFC NPs in the presence of copper chloride and potassium ferrocyanide. The CuFC-functionalized composite membranes were sufficiently characterized, and the effect of competitive ions and organic matter on Cs removal in both synthetic and nature water were investigated.
Section snippets
Materials
Isotope of cesium nitrate (CsNO3) was obtained from Acros and used as the surrogate for 137Cs due to their similar chemical characteristics. Copper chloride (CuCl2), potassium ferrocyanide (K3[Fe(CN)6]•3H2O), hexane, and trimesoyl chloride (TMC, 98%) were supplied by Sigma-Aldrich. SiO2 NPs modified with amino groups (300 nm, w/v: 2.5%) and tetrabutylammonium fluoride (TBAF, 98%) were purchased from Aladdin industrial corporation (Shanghai, China). Humic acid (HA) was provided by Sinopharm
Characterization of the modified membranes
To form reactive groups (−COCl) and loading amino-modified SiO2 NPs onto membrane surface, the pristine PVDF membrane was pretreated in KOH solution and TMC hexane media, respectively [34]. Afterward, the CuFC was further spiked onto the SiO2 NPs of the membrane surface. The detailed modification processes are shown in Fig. 1.
The pristine PVDF membrane was conduct by KOH to promote the production of unsaturated double bonds using for nucleophilic addition process to inducing the hydroxyl group.
Conclusions
- 1
One highly selective PVDF membrane for Cs removal was firstly prepared via a facile and effective strategy binding SiO2 and CuFC onto the PVDF membrane surface synchronously. And, a covalent bond tightly bound CuFC NPs onto the SiO2 surface. The obtained functionalized membrane exhibited a relatively high permeate flux and selectivity for Cs, and it can bind up to 1440.4 mg m−2 for Cs.
- 2
The CuFC/SiO2/PVDF membrane proved to have high stability in a wide pH and physical stress, and shown the
Acknowledgements
This study was financially supported by the Major Science and Technology Program for Water Pollution Control and Treatment of China (2015ZX07406006) and the National Natural Science Foundation of China (Grant No.41603109, Grant No. 21677015and Grant No.51238006).
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