Magnetic Zr-MOFs nanocomposites for rapid removal of heavy metal ions and dyes from water

Highlights

• Amino-decorated Zr-MFCs were prepared by a facile and efficient strategy.

• The Zr-MFCs can effectively removal metal ions/organic dyes from aqueous solution.

• Anionic and cationic dyes could be selectively separated and removed by Zr-MFCs.

• These Zr-MFCs materials can be easily regenerated for reuse.

Abstract

Amino-decorated Zr-based magnetic Metal-Organic Frameworks composites (Zr-MFCs) were prepared by a facile and efficient strategy. The nano-sized Fe₃O₄@SiO₂ core (about 15 nm) was coated with a shell of Zr-MOFs (about 5 nm) by means of in-situ growth. And, Fe₃O₄@SiO₂@UiO-66 and its amino derivatives (Fe₃O₄@SiO₂@UiO-66-NH₂ and Fe₃O₄@SiO₂@UiO-66-Urea) were successfully prepared by using different precursors. The obtained Zr-MFCs were demonstrated to be efficient adsorbents for metal ions/organic dyes removal from aqueous solution, with high adsorption capacity and fast adsorption kinetics. It was found that the amine-decorated MFCs were highly efficient for metal ions/dyes removal compared to raw MFC-O. Among them, MFC-N exhibited the highest capacity for Pb²⁺ (102 mg g⁻¹) and methylene blue (128 mg g⁻¹), while MFC-O exhibited the highest capacity for methyl orange (219 mg g⁻¹). Moreover, anionic and cationic dyes could be selectively separated and removed from the mixed solution just by adjusting the solution pH with Zr-MFCs as the adsorbents. And these Zr-MFCs materials can be easily regenerated by desorbing metal ions/organic dyes from the sorbents with appropriate eluents, and the adsorption capacity can be remained unchanged after 6 recycles. The obtained results demonstrated the great application potential of the prepared MFCs as fascinating adsorbents for water treatment.

Introduction

With the rapid industrial development, accidental and purposive dumping of industrial waste such as heavy metal ions and dyes has contributed to the serious problem of water pollution, which is dangerous for the health of living organisms (Wang et al., 2014, Sun et al., 2011). Consequently, a variety of techniques have been adopted for the uptake of ions or dyes from aqueous solution, including chemical precipitation, adsorption, membrane systems and ion exchange (Hashim et al., 2011, Gupta et al., 2009, Kumar and Guliants, 2010, Dabrowski et al., 2004). Among them, adsorption featuring with simple and fast separation, efficient and low-cost has been extensively used for water treatment (Gupta and Suhas, 2009, Ali and Gupta, 2007). The choice of adsorbents largely influence the adsorption capacity and selectivity in the adsorption process to a great extent, and the low adsorption capacities and poor selective sorption have limited the application of common adsorbents (e.g. activated carbon (Mohan et al., 2008), zeolites (Meshko et al., 2001) and natural fibers (Zein et al., 2010). Therefore, development of novel adsorbents with high efficiency and selectivity for target pollutants has provoked great interest and become the main orientation in adsorption field presently. In this regard, some newly appeared materials (carbon nanotubes (Liu et al., 2013, Upadhyayula et al., 2009), graphene (Han et al., 2014, Cong et al., 2012), layered double hydroxides (Ma et al., 2014a, Ma et al., 2014b), and other materials (Taskin et al., 2014, Li et al., 2014) have been investigated as the adsorbents for heavy metals trapping, and they have exhibited great potential as the adsorbents for water treatment. While tedious high-speed centrifugation or filtration separation after adsorption is required, hindering the extensive application of such adsorbents.

Recently, widespread application of magnetic substrates to address this issue facilitates the isolation of trapped species and separation from sample solutions for reuse (Kaur et al., 2014). The combination of magnetic nanoparticles (MNPs) and desirable building blocks or components could contribute to cooperatively enhanced performance during adsorption.

Metal-Organic Frameworks (MOFs) and/or coordination polymers with permanent porosity, which were constructed through connecting inorganic metal nodes and organic building blocks via coordination bonds, have received wide attention in catalysis, gas storage and drug delivery, etc. Their excellent chemical and thermal stability, well-order porosity, high surface area and easy functionalization of their pores or outer-surface are highly favourable to adsorption (Hasan and Jhung, 2015, Burtch et al., 2014) and separation (Voorde, V. B., 2014). MOFs have been widely utilized to remove hazardous pollutants (Hasan and Jhung, 2015, Ayati et al., 2016, Roushani et al., 2016a), such as dyes or heavy metal ions in water, demonstrating a great application potential in water treatment. Magnetic MOFs composites (MFCs), which integrate the advantages of fast separation of magnetic materials and the superior properties of MOFs, have been widely used in different technological fields (Ricco et al., 2013), and offer an attractive alternative platform for dyes/heavy metal ions capture. However, a large proportion of the MFCs are functionalized with HKUST-1 currently (Zhao et al., 2015a, Zhao et al., 2015b, Xiong et al., 2015, Bagheri et al., 2012, Ke et al., 2012, Wang et al., 2013), and HKUST-1 is unstable under acid solution and would release toxic Cu²⁺ (Flemming and Trevors, 1989, Huang et al., 2015). Comparatively, MOFs containing zirconium metal nodes with strong Zr(IV)-O bonds exhibit better stability in water across a broad pH range (Cavka et al., 2008, Kandiah et al., 2010a, Kalidindi et al., 2015, Piscopo et al., 2015). In addition, Zr-MOFs inherit remarkable thermal and mechanical stability (Cavka et al., 2008, Wu et al., 2013). Due to the nodes with abundant Zr-bound hydroxides, Zr-MOFs exhibited excellent performance for removal of selenium species from drinking water (Howarth et al., 2015). However, the preparation/application of magnetic Zr-MOFs in water treatment is still scarce (Zhao et al., 2014, Zhang et al., 2015). Even more distressing is that the present approaches usually require specific surface modification for the in-situ growth of Zr-MOFs on the MNPs surface and the prepared MFCs are in micro-size (>200 nm). On the other hand, it has been proven that amino modification can be an effective strategy to increase the uptake of metal ions or dyes from aqueous solution (Ricco et al., 2015, Haque et al., 2010, Haque et al., 2014, Roushani et al., 2016b). So it is quite attractive to develop more simple and efficient methods to prepare amino-functionalized nano-sized Zr-MFCs.

Herein, a serious of nano-sized Zr-MFCs, including Fe₃O₄@SiO₂@UiO-66 (denoted as MFC-O), Fe₃O₄@SiO₂@UiO-66-NH₂ (denoted as MFC-N) and Fe₃O₄@SiO₂@UiO-66-Urea (denoted as MFC-U), have been prepared successfully through a facile one-pot hydrothermal approach (Scheme 1). All the prepared MFCs showed core-shell structure, and the shell thickness is adjustable through adjusting the quantity of predecessor. These nano-sized Zr-MFCs were investigated for the extraction of both dyes and metal ions. They exhibited excellent adsorption property both for heavy metal ions and cationic/anionic dyes, and relatively high thermal and chemical stability. Besides, fast separation and good recovery of MFCs from aqueous solution can be realized just by applying external magnetic fields, demonstrating a very good application prospect for dyes/heavy metal ions removal from environmental water.