Water Purification and Microplastics Removal Using Magnetic Polyoxometalate‐Supported Ionic Liquid Phases (magPOM‐SILPs)

Abstract Filtration is an established water‐purification technology. However, due to low flow rates, the filtration of large volumes of water is often not practical. Herein, we report an alternative purification approach in which a magnetic nanoparticle composite is used to remove organic, inorganic, microbial, and microplastics pollutants from water. The composite is based on a polyoxometalate ionic liquid (POM‐IL) adsorbed onto magnetic microporous core–shell Fe2O3/SiO2 particles, giving a magnetic POM‐supported ionic liquid phase (magPOM‐SILP). Efficient, often quantitative removal of several typical surface water pollutants is reported together with facile removal of the particles using a permanent magnet. Tuning of the composite components could lead to new materials for centralized and decentralized water purification systems.

Abstract: Filtration is an established water-purification technology.H owever,d ue to lowf low rates,t he filtration of large volumes of water is often not practical. Herein, we report an alternative purification approach in whichamagnetic nanoparticle composite is used to remove organic, inorganic, microbial, and microplastics pollutants from water.T he composite is based on ap olyoxometalate ionic liquid (POM-IL) adsorbed onto magnetic microporous core-shell Fe 2 O 3 / SiO 2 particles,g iving am agnetic POM-supported ionic liquid phase (magPOM-SILP). Efficient, often quantitative removal of several typical surface water pollutants is reported together with facile removal of the particles using apermanent magnet. Tuning of the composite components could lead to new materials for centralized and decentralized water purification systems.
Access to clean water is still amajor challenge in large parts of the world, and many water resources in developing countries carry high concentrations of organic pollutants, heavy metals or microbial contaminants. [1][2][3] In addition, microplastic particles have recently been identified as contaminants of emerging concern (CEC) which can enter the food chain upon uptake by marine organisms. [4,5] Micro-plastics can bind and concentrate persistent organic pollutants (POPs), which amplifies their public health impact. [6] Often, water purification relies on as eries of operations including chemical coagulation, flocculation, sedimentation, filtration and disinfection which produce safe drinking water from contaminated surface or ground water. [7,8] Tr aditionally,d ifferent filters which target specific pollutants are connected in line to enable stepwise water purification. Filter materials typically include porous adsorbents, such as zeolites,m inerals,o ra ctive carbon. [9] However, treatment of large volumes of water or the deployment in remote areas require alternative methods which combine ease of use with minimum technological requirements and the ability to simultaneously remove multiple contaminants. Composites are promising materials to this end, as their target properties can be tuned by independent modification of each component. Recently,s ome of us have explored the removal of water contaminants by developing so-called polyoxometalate-supported ionic liquid phases (POM-SILPs). [10] This composite is based on commercial porous silica particles which are surface-functionalizedw ith waterimmiscible polyoxometalate ionic-liquids (POM-ILs) capable of binding organic and inorganic contaminants. [11] ThePOM-ILs [12] combined lacunary Keggin tungstate anions featuring heavy-metal binding sites [13,14] with long-chain quaternary organo-ammonium cations [15,16] which act as antimicrobials. Integration of the POM-SILP composite in filter cartridges allowed the simultaneous removal of organic, inorganic,a nd microbial contamination from water. However,t he system requires filtration processing which is limited to small water volumes,a nd overcoming this challenge is either energyintense (using pressurized systems) or materials-intense (using more filter materials).
Herein, we propose water purification by magnetic particles as ap romising alternative to filtration which could be employed in various water treatment scenarios.Incontrast to filtration, magnetic water purification could facilitate the treatment of large volumes of water, and can in principle be used without further infrastructure if particle removal is possible using simple permanent magnets. [17] Pioneering studies have explored the removal of aqueous pollutants using magnetic particles,i ncluding heavy-metal cation removal by amino acid-modified iron oxide, [18] organic pollutant removal by graphene oxide-functionalized magnetic particles, [19] and separation of freshwater algae using silicacoated magnetic particles. [20] Recently,ground-breaking studies reported the use of light-driven magnetic microswimmers for the collection and removal of microplastics from water. [21] Herein, we target multi-pollutant removal based on coreshell particles composed of as uperparamagnetic iron oxide (Fe 2 O 3 ,h ematite) core encased in ap orous silica shell. [22,23] We hypothesized that this architecture enables magnetic removal and at the same time stable POM-IL surfaceanchoring.W ed emonstrate that the resulting magnetic POM-SILP (magPOM-SILP) composite effectively binds organic, inorganic,m icrobial, and microplastic pollutants from water, and can easily be recovered using ap ermanent magnet. Tr eatment of large water volumes therefore becomes possible.T oour knowledge,this is the first report of magnetic POM-SILPs (MagPOM-SILPs), therefore their synthesis is described briefly:t he magnetic iron oxide/silica core-shell precursor particles are synthesized by an adapted reverse water-in-oil microemulsion method at elevated temperature with cyclohexane as the organic phase. [22] Reaction of the surfactant Brij 56 with aqueous Fe III solution and subsequently with Si(OEt) 4 leads to spherical Fe 2 O 3 @SiO 2 coreshell nanoparticles which were washed, dried, and calcined (420 8 8C) to give the microporous Fe 2 O 3 @SiO 2 composite 1. Thea verage particle size of 1 was 16 nm, the BET specific surface area was approximately 270 m 2 g À1 and the BJH pore volume was 0.97 cm 3 g À1 ;f or further characterization details, see Supporting Information. Composite 1 is used as nonmodified reference compound throughout this study.
TheP OM-IL is synthesized as described in the literature [10,12,24,25] by combination of the lacunary-Keggin cluster anion ([a-SiW 11 O 39 ] 8À ) [26] and the antimicrobial tetra-n-heptyl ammonium cation (Q 7 (= (n-C 7 H 15 ) 4 N + ) [27] (Figure 1). The magPOM-SILPs were prepared by dispersing SiO 2 @Fe 3 O 4 particles (4.0 g) in aP OM-IL solution in acetone (50 mL, [POM-IL] = 3.36 mm, m(POM-IL = 1.0 g) and subsequent vacuum drying, giving the composite magPOM-SILP 2 with aP OM-IL loading of 20 wt %, see Supporting Information for details.A saresult of the partial filling of the pores in 2 with the POM-IL, the BET specific surface area of 2 was reduced to around 100 m 2 g À1 and the BJH pore volume was 0.70 cm 3 g À1 ;for further characterization details,see Support-ing Information. Compound 2 was obtained as dry and freeflowing powder which can be easily handled, while the POM-IL precursor is ah ighly viscous liquid which would make deployment for water purification difficult.
We performed aseries of water-purification tests using the magPOM-SILPs for removal of pollutants often found in water samples.A queous samples (5 mL) of the respective pollutant at health-relevant concentrations were prepared, and the magPOM-SILP 2 or the reference particles 1 (50 mg) were dispersed in the polluted sample and magnetically stirred. After stirring for 24 h, the magnetic particles were removed using apermanent magnet (see Supporting Video). Note that no leaching of any components of 2 into the aqueous phase was observed after stirring for 24 hu sing inductively coupled plasma atomic emission spectroscopy (ICP-AES) and C,H,N,elemental analysis.
We explored heavy-metal removal from water using the metal-ion pollutant models [2] Pb 2+ ,N i 2+ ,C o 2+ ,a nd MnO 4 À according to the standard procedure described above (Figure 1). Them etal-ion concentrations are set to levels significantly above the WHO guideline levels to test removal efficiency and simulate acute pollution scenarios (Table 1). [28] ICP-AES analyses of the solutions after particle removal show metal removal efficiencies between 75-99 mol %for the magPOM-SILP 2,while the non-modified reference 1 showed significantly lower removal efficiencies in the approximately 35-50 mol %r ange ( Table 1, entries 1-5).
We then explored the removal of organic pollutants from water using the triphenylmethane (trityl) dye Patent Blue V (PBV, Figure 2) as amodel for textile dye pollutants. [29] To this end, aqueous solutions of PBV were stirred with magPOM-SILP 2 using the standard experimental procedure described above.D ye removal was quantified by UV/Vis spectroscopy ( Figure 2) and it was observed that the magPOM-SILP 2 removes more than 99 %o ft he dye while the non-modified reference 1 showed only 6% removal (Table 1, entry 6). The increased dye removal by 2 is assigned to the high affinity of the POM-IL to interact with organic species,due to the large, hydrophobic Q 7 cations. [11] We hypothesized that the viscous POM-IL coating on the magnetic nanoparticle surface could be well suited for

Angewandte Chemie
Communications attaching the magPOM-SILP particles to microplastics,a nd thereby enable their magnetic recovery from water. To this end, we used commercial colloidal solutions of spherical polystyrene (PS) beads (diameter 1 mma nd 10 mm, PS bead concentration:0 .1 wt %( = 1gL À1 )) as models of environmentally persistent microplastics.P Sp article removal was quantified using dynamic light scattering (DLS,s ee Supporting Information for details). Ther emoval experiments were carried as described above (V solution = 5mL, t binding = 24 h). Our experiments demonstrated quantitative removal of both the 1 mmand 10 mmPSbeads using magPOM-SILP 2.Incontrast, the reference 1 showed no microplastics removal, see Table 1, entries 7a nd 8.
To gain some insights into the kinetics of the binding of 2 to the microplastic particles,w ep erformed the removal experiment described above (using both PS bead sizes) at ar educed binding time of 6h,w hich also resulted in quantitative PS bead removal. In addition, we demonstrated that three consecutive recycling runs using the same batch of 2 are possible,a ll of which show quantitative removal of both the 1 mma nd the 10 mmP Sb eads (see Supporting Information). Next, we examined the microplastics removal capacity of 2 by performing the above experiment, but using PS bead solution volumes of 20 mL and 50 mL. Forboth solutions we note PS bead removal efficiencies over 90 %b ased on DLS analyses,s ee Supporting Information. This emphasizes that the magPOM-SILPs are capable of removing microplastic model compounds from large volumes of water.
To gain insights into the interactions between magPOM-SILP 2 and the PS beads,w ep erformed scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/ EDX) of the magnetically recovered, dried samples.A s shown in Figure 3, the significantly smaller particles of 2 cover large parts of the surface of the PS beads and thus render them susceptible for magnetic removal. We suggest that this surface attachment of 2 to the PS beads is due to hydrophobic interactions between the POM-IL coating and the PS surface, see Supporting Information for proposed scheme.Inaddition, PS bead aggregation is observed which could be induced by 2, and further aggregation studies are currently underway to understand the surface attachment in more detail. In sum, PS microplastics removal by magnetic particle attachment could provide ameans of treating larger volumes of water which are not amenable to classical filtration. [21] Some of our previous research has confirmed the bactericidal properties of POM-ILs, [10,12,25] therefore we hypothesized that magPOM-SILP 2 would be able to purify water heavily contaminated with bacteria. Thea ntibacterial water purification properties of the magPOM-SILPs were tested against gram-negative E. coli and gram-positive B. subtilis. [12] Briefly,aqueous solutions of the microporous magPOM-SILP 2 were inoculated with 10 6 CFU mL À1 of E. coli or B. subtilis and incubated at 37 8 8Cf or 1hbefore removing the particles with am agnet and quantifying the bacteria present in the supernatant solutions.AtamagPOM-SILP 2 concentration of 1mgmL À1 ,t he bacterial removal was 58 %f or E. coli and 100 %f or B. subtilis,w hile at ac oncentration of 10 mg mL À1 the bacterial removal efficiency was 100 %for both bacterial strains.T he antibacterial effect was confirmed and characterized using electron microscopies (SEM and TEM). Ther eusability of the magPOM-SLIP nanoparticles was tested over three cycles of inoculation with E. coli or B. subtilis,p article separation, and particle washing with water and subsequent reuse with af resh inoculum of the bacteria. Theb actericidal effect of 2 against B. subtilis remained unaltered after three cycles,w hile the effect of the particles on E. coli was reduced after the second cycle ( Figure 4A), which is commensurate with other studies using magnetic nanoparticles for water purification. [30,31] In conclusion, we report the first example of magnetic polyoxometalate-supported ionic liquid phases (magPOM- Figure 2. Removal of the water-soluble aromatic model pollutant Patent Blue V(PBV) from water.L eft:U V/Vis spectra before purification (blue), after purification using reference 1 (red) and after purificationu sing magPOM-SILP 2 (green).
[PBV] 0 = 32 mm.Adsorption time:2 4h.Details see Table 1entry 6. Inset:molecular structure of PBV.Right:photographs of the PBV solutions before and after purification( with 2). SILPs) and their use in water purification. Them agPOM-SILP composite is capable of removing organic, inorganic, microbial, and microplastic pollutants from water using ar ange of target-specific removal modes.H igh removal efficiencies are reported together with initial insights into an ew mode of microplastics removal by surface-binding of magnetic particles.I nf uture,w ew ill explore how optimization of the individual components can be used to improve the capacity of the systems and investigate their coupling to electromagnetic recovery systems for use under more realistic operating conditions.