Environmentally Friendly Î²-CyclodextrinâIonic Liquid Polyurethane

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Environmentally friendly #-cyclodextrin – ionic liquid polyurethanes modified magnetic sorbent for the removal of PFOA, PFOS and Cr(VI) from water Abu Zayed Md Badruddoza, Bikash Bhattarai, and Rominder PS Suri ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02186 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017

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ACS Sustainable Chemistry & Engineering

Environmentally friendly β-cyclodextrin – ionic liquid polyurethanes modified magnetic sorbent for the removal of PFOA, PFOS and Cr(VI) from water Abu Zayed Md Badruddoza, ‡†* Bikash Bhattarai, ‡ Rominder P.S. Suri‡ ‡

NSF IUCRC Water and Environmental Technology (WET) Center,

Civil and Environmental Engineering, Temple University, 1947 North 12 th St., Philadelphia, PA 19122, USA

†

Present address: Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, E18-520, Cambridge, MA 02139, USA

*Corresponding author: [email protected]

Abstract Emerging contaminants such as perfluorinated compounds (PFCs) and heavy metals are of increasing concerns due to their detrimental effects on environment and human health. Their mixtures are often present at contaminated sites that pose a challenge in water remediation. Herein, we report a multifunctional magnetic sorbent (Fe3O4-CDI-IL MNPs) that was prepared by modifying the magnetic Fe3O4 nanoparticle with β-cyclodextrinionic liquid (β-CD-IL) polyurethanes for the removal of perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and Cr(VI) from aqueous solution. The successful grafting of β-CD-IL polymer on the magnetic nanoparticles was confirmed by FTIR, ζ potential, TGA and VSM. The sorption behaviors of Fe3O4-CDI-IL MNPs were investigated in terms of sorption kinetics, isotherms, simultaneous removal capability and reusability. The kinetic results showed that the sorption of PFOS, PFOA and Cr(VI) reached equilibrium within 4, 6 and 3 h, respectively, and the pseudo-second-order kinetic model best described the kinetic data. The solution pH had more obvious

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effects on the sorption of PFOA and Cr(VI) than that of PFOS. The coupling of ionic liquid with β-CD polymer backbone could significantly enhance the removal efficiencies of both PFOS and PFOA. The sorption isotherms indicated that the heterogeneous sorption capacities of Fe3O4-CDI-IL MNPs were 13,200 and 2,500 μg/g for PFOS and PFOA, respectively, and the monolayer sorption capacity was 2,600 μg/g for Cr(VI) ions. The Cr(VI)-PFC binary sorption experiments exhibited a decrease in sorption capacities for PFCs, but the removal of Cr(VI) was unaffected with the introduction of PFCs as cocontaminants. The hydrophobic interactions and electrostatic attraction were mainly involved in the PFC sorption process, whereas the ion-exchange and reduction was responsible for Cr(VI) sorption. In addition, Fe3O4-CDI-IL MNPs could be readily recovered with a permanent magnet, regenerated and reused at least ten times without any significant efficiency loss. This multifunctional sorbent thus shows potential in the removal of coexisting toxic contaminants from water or wastewater.

Keywords: perfluorinated compounds, emerging contaminants, heavy metals, adsorption, cyclodextrinionic liquid, multifunctional sorbent


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Introduction Environmental pollution by emerging contaminants (ECs) and heavy metals in different water sources and industrial effluents has aroused the public concern over the last decade. In particular, perfluorinated compounds (PFCs) as ECs have attracted continuously increasing attention recently because of their extremely persistent nature in the environment, notable bioaccumulation and potential toxicity to human beings and animals.1,2 The unique physical and chemical properties such as, hydrophobicity and oleophobicity as well as high chemical stability have led to extensive use of PFCs in many applications. Perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) are the most common PFCs used in many industries and detected ubiquitously up to μg L-1 level in aqueous environments.2 In an electroplating plant, PFOS is usually utilized as an inhibitor of chromium (Cr) fog in order to reduce the chromium contaminant in the atmosphere.3 Eventually the wastewater effluent discharged from electroplating plants contains a high concentration of PFOS mixed with hexavalent chromium. Hexavalent chromium is also highly toxic and hazardous.4,5 This toxic and carcinogenic metal is usually released to the environment mainly by industrial activities, such as electroplating, metal finishing, leather tanning, chromate manufacture and wood preservation. Exposure to Cr(VI) in drinking water, even at a trace level, increases the risk for human beings; it may trigger vomiting, skin irritation, lung cancer, kidney and liver damage.5 Therefore, due to their adverse impacts on environment and human health, an effective and simultaneous removal of such contaminants from various water sources is of significant importance.

Sorption process is considered as an effective and economical approach for improving water quality and can be used to remove a wide variety of trace pollutants.6–11 To date, various sorbents such as boehmite,12 alumina,13 activated carbon,14 carbon nanotubes,15 chitosan polymers (MIPs),16,17 and clay minerals18 have been reported as sorbents for the removal of PFOS and PFOA from water. Similarly, activated carbon is widely used as a sorbent for the removal of Cr(VI) due to its high surface area and abundant micropores. 19 However, further treatment is often needed to enhance its adsorption capacity and selectivity, such as


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oxidation, acidation, alkalization and impregnation of foreign materials and also to regenerate, thus increasing the cost and limiting its applications.20,21 Recently, magnetic nanoparticles (MNPs) such as iron oxide nanostructured materials have been considered as potential sorbents in applications of environmental pollution cleanup.22–24 The advantages of MNPs are attributed to their high specific surface area, easy separation under external magnetic fields, and ease of surface functionalization. Magnetic cyclodextrin-nanocomposites are of particular interest in environmental applications.25–29 β-cyclodextrin (β-CD) is a cyclic oligosaccharide composed of α-(1,4)-linked D-glucopyranoses, which has a hydrophobic inner cavity with a hydrophilic external surface.30 The introduction of cyclodextrin that is known to form host-guest inclusion complexes with various non-polar species onto the surfaces of MNPs may render them water dispersible and capable of host-guest chemistry, thereby increasing the surface areas and improving their sorption capacities. Therefore, cyclodextrin complexation has become a promising choice of procedure for decontaminating techniques. In recent years, room temperature ionic liquids (ILs) that are known as salts of an organic cation, are also of increasing interest due to their remarkable properties including excellent thermal and chemical stability, negligible vapor pressure, biocompatibility and good extractabilities for different species.31 Various types of ionic liquids have been used for the liquid-liquid extraction, liquid phase micro-extraction and separation of inorganic and organic substances from aqueous media.31–34 Recent studies on modification of cyclodextrin with several types of ILs showed enhanced molecular enantioselectivity towards chiral analytes

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and also improved

sorption capacity with organic pollutants.36

Both cyclodextrins and ILs are environmentally friendly and can improve the stability of functional materials.28,37–39 Modifying magnetic nanoparticles both with β-CD and IL allows new materials that possess assortment of multiple properties, such as inclusion properties, ion-exchange capabilities, magnetic separation convenience and high surface area. However, to the best of our knowledge, β-CD-IL polymer functionalized magnetic composites have not yet been used as sorbents. Further, no or very less work was focused on the simultaneous removal of ECs and heavy metals from water. The mixture of both


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organic and inorganic contaminants is often present at environmental sites that pose a challenge in remediating water. However, the conventional water treatment processes are unable to remove both classes of contaminants simultaneously to μg L-1 concentration level. Due to the fundamentally different physiochemical properties of these contaminants, research is of utter importance focusing on remediating organic and inorganic contaminants concurrently rather than with a single and separate treatment.

We report herein the development of a novel multi-functional magnetic sorbent based on cyclodextrin-IL polymers, namely Fe3O4-CD-IL MNPs, where β-CD-IL molecules were covalently linked to the nanoparticle surface through crosslinking. This work was aimed at evaluating its performance as a sorbent for the effective and simultaneous removal of PFOA, PFOS, and Cr(VI) at environmentally relevant concentrations. The effects of solution pH, contact time, initial concentrations of the contaminants and sorbent dosage on the sorption performance were investigated in this study. The reusability of Fe3O4-CDIL MNPs was also explored using the repeated sorption− desorption regeneration tests in a multicomponent system. Furthermore, the underlying mechanisms and the possible interactions between the sorbent and sorbates are discussed.

Experimental Materials Fe3O4 nanoparticles ( 6.5).23,29 The effect of solution pH could also be explained by the electrostatic interaction between Fe3O4-CDI-IL MNPs surface and charged sorbates. At lower pH, the sorption was caused by the electrostatic attraction between the positively charged sorbent and Cr(VI) ions. However, if the pH continued to increase, the surface would be negatively charged, greatly weakening the electrostatic attraction between them, thus reducing the removal efficiency. Moreover, there was a competition between OH- and Cr(VI) ions, especially at a high pH.

Kinetic results showed that the sorption rate of Cr(VI) ions was fast initially and reached equilibrium within 3 h (Figure S7A). The good fit (R2~ 0.99) obtained using the pseudo second-order kinetic model indicated that the sorption of Cr(VI) onto Fe3O4-CDI-IL MNPs could be best described by this model. This also suggested that the chemisorption process might be the rate-limiting step.48 The sorption of Cr(VI) onto Fe3O4-CDI-IL MNPs was also found to fit better to the Langmuir model with a correlation coefficient (R2) value of 0.988, suggesting that Cr(VI) sorption on the sorbent was of a monolayer coverage. The maximum Cr(VI) sorption capacity (qm) of Fe3O4-CDI-IL MNPs was 2600.0 μg/g (Table S2).


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Figure 3. Removal of Cr(VI) ions with Fe3O4-CDI-IL sorbent at different pH (initial concentration of Cr(VI) ions: 1000 μg/L, contact time: 24 h).

Competitive sorption The effect of Cr(VI) on the sorption of PFOS and PFOA using Fe 3O4-CDI-IL MNPs is shown in Figure 4A. It is observed that anionic chromate had more obvious effect on the sorption of PFOA than that of PFOS. This suggested that anionic chromate ions could compete with PFOA on the same (ion exchange) sorption sites. The sorption of PFOA was due to both CD inclusion in the hydrophobic cavity, and electrostatic interactions between the PFOA anions and the protonated surface of the sorbent. In contrast, the little decrease of PFOS removal further implied the significant role of hydrophobic interactions through host-guest inclusions for PFOS sorption. Similar trends were also observed when PFOS or PFOA interacted with the sorption sites on Fe3O4-CDI-IL MNPs in the presence of anionic humic acid (details are in the Supporting Information). It was also observed that at higher Cr(VI) ion concentrations (> 10 mg/L), the removal of PFOA increased. Though this phenomenon was not clearly understood, however,


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one possible reason might be that at higher Cr(VI) concentrations, more Cr(VI) ions were sorbed on the nanoparticles that underwent a reduction to Cr(III)) to a certain extent (SI-D4 in the Supporting Information), resulting in the creation of new sorption sites for PFCs sorption. The Fe3O4-CDI-IL sorbent was also tested for the simultaneous removal of PFOA, PFOS, and Cr(VI) ions in a tri-component system containing a mixture of 200 μg/L of each PFOA and PFOS and 1000 μg/L of Cr(VI) ions. The results for the removal efficiencies of the sorbent in a tri-component system are compared to those obtained for a single-component system (Figure 4B). Maximum removal of about 99% of both PFOA and PFOS and 96% of Cr(VI) ions was achieved with a dosage of 3.0 g/L in this tri-component system. The removal of PFOS and Cr(VI) was not affected by the introduction of other contaminants in the mixture. However, the removal rate of PFOA was slightly reduced at lower sorbent dosages. These results demonstrate that this multifunctional sorbent shows the potential in removing both organic contaminants and heavy metals from a water mixture present at environmentally relevant concentrations with higher removal efficiencies.

Figure 4. (A) Effect of Cr(VI) ions on the removal of PFOA and PFOS using Fe3O4-CDI-IL sorbent in a binary mixture. (Initial concentration of each PFOA and PFOS: 50 μg/L, sample volume: 50 mL, sorbent dosage: 0.2 g/L, pH: 5.5, contact time: 24 h). (B) Removal of PFOA, PFOS and Cr(VI) ions in a singleand tri-component mixture with Fe3O4-CDI-IL sorbent (initial concentration of each PFOA and PFOS: 200 μg/L and Cr(VI) ions: 1000 μg/L, pH: 3.0).


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Sorption mechanism

OH

OH O

OH

O

n OH

n

OH

O O

OH

OH

O

OH OH

O

O

OH

OH

OH O

OH

O

OH

OH

n

n

OH

O

O

OH

n

OH

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n OH O

OH

OH

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OH

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OH

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O

OH

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Cr(VI) ions (HCrO 4-, Cr2O72-) PFOA or PFOS Ionic liquid component

adsorption via hydrophobic inclusion

adsorption via electrostatic interactions and ion-exchange

Figure 5. Schematic of the sorption mechanisms of PFCs and Cr(VI) ions onto Fe3O4-CDI-IL MNPs.

Based on the above results and discussion, the possible sorption mechanisms for the simultaneous removal of PFOS, PFOA and Cr(VI) are proposed and discussed in Figure 5. It appears that each component of our sorbent played a critical role in its functioning: the modified cavities of CDs are primarily responsible for the capture of the PFOS and PFOA molecules through host-guest inclusion. On the other hand, the ionic liquid (IL) component plays roles not only for providing the sorption sites for metal ions (chemisorption) but also in contributing substantially in the sorption of both PFOS and PFOA. The imidazolium part in IL component imparts the electrostatic attractive forces for PFOS and PFOA, which might be the primary driving force to bind the anions onto the sorbent surface, thereby enhancing the sorption capacities of PFOS and PFOA. Previous studies suggest that the formation of micelles and hemi-micelles of PFOA or PFOS in the solution and their subsequent accumulation on the sorbent surface may be involved in the sorption process.49 However, the formation of micelles and hemi-micelle is


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probably unlikely in our study, since the concentrations of PFOA and PFOS used were far below their critical micelle concentrations (4573 mg/L for PFOS and 15,696 mg/L for PFOA) 50 and the hemi-micelles usually form on the sorbent surface when the PFOS or PFOA concentrations are in the range of 0.01– 0.001 of the CMC.51 Similar to the PFC sorption, the electrostatic attraction between positively charged sorbent and Cr(VI) anionic species presumably played the initial driving force for the anions to be sorbed onto the surface. Ion exchange and reduction (to some extent) mechanisms were then involved during the sorption process, which was confirmed from XPS studies (see the Supporting Information).

Reusability of Fe3O4-CDI-IL sorbent From a practical perspective, stability and reusability are most important features of an advanced sorbent. As we observed that the solution pH greatly affected the sorption of PFOA and Cr(VI) ions on Fe3O4CDI-IL; therefore, we anticipated that the use of desorption media with varying pH could help to desorb these ions from the sorbent. Firstly, Cr(VI)-loaded Fe3O4-CDI-IL MNPs was investigated to regenerate using different desorption eluents for metal desorption (Table S4 in supporting information). Among them, the desorption buffer, 0.2 M NaOH + 0.5 M NaCl solution could desorb the highest amount of Cr(VI) ions (85-90%) from the Cr(VI)-laden sorbent. It was also found that the quantitative desorption efficiencies using this medium were 83.4 and 44.0% for PFOA and PFOS, respectively. This result of PFOS desorption is in agreement with that of effects of pH on PFOS sorption. As the sorption of PFOS was not affected by the pH significantly, the use of buffers with different pH might not be helpful for complete PFOS desorption. Previous studies showed that organic solvents, such as methanol, acetone could be good candidates for the regeneration of the sorbent in case of CD hydrophobic inclusion being dominant in the sorption process.

25,44,52

However, the desorption medium (0.2 M NaOH + 0.5 M NaCl

solution) was used further in order to evaluate the regeneration capabilities of Fe3O4-CDI-IL sorbent in a tri-component system, where the concentration of PFOA, PFOS, and Cr(VI) ions were 200, 200 and 1000 μg/L, respectively and the sorbent dosage used was 3 g/L. The results for the removal efficiencies of PFOA, PFOS, and Cr(VI) ions in each cycle are shown in Figure 6. It shows that the Fe3O4-CDI-IL


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sorbent could be easily recovered from the solution with a simple magnetic field, regenerated and then reused for at least ten times without any significant loss of its removal efficiency. In addition, no leaching of iron ions from this magnetic sorbent was observed into the aqueous phase, which further implied the negligible dissolution of the magnetic core under the stated experimental conditions.

Figure 6. A regeneration study of Fe3O4-CDI-IL sorbent with PFOA, PFOS, and Cr(VI) ions in a ternary mixture. (For sorption study: initial concentration of each PFOA and PFOS 200 μg/L, Cr(VI) ions 1000 μg/L, sorbent dosage 3 g/L, sample volume 10 mL, pH 3.0, contact time 24 h; For desorption study: 10 mL of 0.2 M NaOH+ 0.5 M NaCl solution used as a desorption buffer, contact time 24 h)

Concluding remarks In summary, we developed a novel multi-functional sorbent; cyclodextrin-IL polymer functionalized magnetic nanoparticles (Fe3O4-CDI-IL MNPs) and demonstrated the sorptive removal of PFOS, PFOA and Cr(VI) at environmentally relevant concentrations from aqueous solutions. The sorption of PFOS, PFOA, and Cr(VI) on this magnetic sorbent reached equilibrium within 4, 6, and 3 h, respectively and followed the pseudo-second-order kinetic model. The Sips model could best describe the sorption of PFCs, whereas the Langmuir model could describe well the Cr(VI) sorption. Our sorbent exhibited a


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greater sorption toward PFOS than PFOA, and this could be attributed to the hydrophobicity and functional groups of PFOS. The solution pH had more significant effects on the sorption of PFOA and Cr(VI) than that of PFOS. The simultaneous removal of both organic and inorganic pollutants was ascertained because of their dual sorption sites (host-guest inclusion and ion-exchange capabilities) rendered by cyclodextrin moieties and IL component, respectively. The contaminants had negligible effects on each other in regard to the removal efficiencies within the lower concentration range. The hydrophobic interactions mainly played a dominant role in the sorption of PFCs on Fe3O4-CDI-IL MNPs. In contrast, the primary mechanisms for the removal of Cr(VI) were the electrostatic interactions and ion exchange. The magnetic iron oxide core renders this sorbent magnetically responsive, thereby facilitating their separation and regeneration from the complex aqueous solutions. In addition, the synthesized sorbent maintains the high removal efficiency after ten cycles of sorption-desorption regeneration tests. Overall, the chemical stability, the efficient sorption performance, the easy separability, and the excellent recyclability of these magnetic nanocomposites make them excellent candidates as solid phase extraction materials for water treatment of co-existing toxic pollutants.

Acknowledgments The funding for this work was provided by the National Science Foundation (NSF) IUCRC—Water and Environmental Technology (WET) Center. Supporting Information Additional details regarding thermogravimetric analysis of cyclodextrin, Fe3O4 MNPs and Fe3O4-CDI-IL MNPs, zeta potential measurement, VSM analysis, active groups quantitative analysis, and XPS analysis of Fe3O4-CDI-IL MNPs, sorption equilibrium and kinetic data, desorption data for Cr(VI) are included.

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Environmentally friendly β-cyclodextrin – ionic liquid polyurethanes modified magnetic sorbent for the removal of PFOA, PFOS and Cr(VI) from water

Synopsis An effective and sustainable magnetic sorbent based on β-cyclodextrin–ionic liquid was developed for PFOA, PFOS and Cr(VI) removal from water

TOC

OH OH O

OH

O

n OH

n

OH

O O OH

OH

OH

O

O

n

OH

O

n OH O

OH

OH

n

OH

O

OH

O OH

O

n

O

OH OH

O

O

OH

OH

OH

OH

OH

n

n

OH

O

O

OH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

Cr(VI) ions (HCrO 4-, Cr2O72-) PFOA or PFOS Ionic liquid component

adsorption via hydrophobic inclusion


adsorption via electrostatic interactions and ion-exchange

Environmentally Friendly Î²-CyclodextrinâIonic Liquid Polyurethane

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