Cross-Linked Carbon Nanotube Adsorbents for Water Treatment

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Surfaces, Interfaces, and Applications

Cross-linked carbon nanotube adsorbents for water treatment: tuning the sorption capacity through chemical functionalization Myriam Barrejón, Zois Syrgiannis, Max Burian, Susanna Bosi, Tiziano Montini, Paolo Fornasiero, Heinz Amenitsch, and Maurizio Prato ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20557 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

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ACS Applied Materials & Interfaces

Cross-linked carbon nanotube adsorbents for water treatment: tuning the sorption capacity through chemical functionalization Myriam Barrejón*a, Zois Syrgiannisa, Max Burianb, Susanna Bosia, Tiziano Montinia,c, Paolo Fornasieroa,c, Heinz Amenitschb, Maurizio Prato*a,d,e aDepartment

of Chemical and Pharmaceutical Sciences, Università degli Studi di Trieste, Via

Licio Giorgieri 1, Trieste 34127, Italy bInstitute

of Inorganic Chemistry, Graz University of Technology, Stremayrgasse 9/V, 8010

Graz, Austria cINSTM

consortium and ICCOM-CNR Trieste Research Units, Università degli Studi di

Trieste, Via Licio Giorgieri 1, Trieste 34127, Italy dCarbon

Bionanotechnology Group CICbiomaGUNE, Paseo Miramón 182, 20014 Guipúzcoa,

Spain eBasque

Foundation for Science, Ikerbasque, Bilbao 48013, Spain

KEYWORDS: carbon nanotube-based adsorbents, cross-linking, water treatment, organic pollutants, adsorption capacity.

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ABSTRACT

The development of carbon-based membrane adsorbent materials for water treatment has become a hot topic in recent years. Among them, carbon nanotubes (CNTs) are promising materials because of its large surface area, high aspect ratio, great chemical reactivity and low cost. In this work, free-standing CNT adsorbents are fabricated from chemically cross-linked single-walled carbon nanotubes (SWCNTs). We have demonstrated that by controlling the degree of cross-linking, the nanostructure, porous features and specific surface area (SSA) of the resulting materials can be tuned, in turn allowing the control of the adsorption capacities and the improvement of the adsorption performance. The cross-linked CNT adsorbents exhibit a notably selective sorption ability and good recyclability for removal of organics and oils from contaminated water.

Introduction Industrial activity produces a continuous increase of waste and pollution in our aquatic systems. In order to mitigate these damages, a variety of techniques have been developed, however, most of these methods imply high cost and energy consumption. In the last decades, membrane separation technologies have gained much attention as an alternative to conventional separation methods,1,2 and concretely their use for water purification has been one of the most successful applications. Membranes for oil-water separation,3 adsorption of organic contaminants4,5 or desalination processes6,7 have been widely described in the literature. Polymeric membranes are the most widely used materials in wastewater treatment applications,7,8 however, these membranes show important limitations. The most significant limitations are their easy degradation in harsh solvents and their limited antifouling and 2 ACS Paragon Plus Environment

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mechanical properties, which result in lack of stability in long-term applications and shortens their working life span.9 In general, the ideal sorbent material for membrane technologies must present high porosity, large specific surface area, high selectivity and excellent recyclability.10 Therefore, an intelligent design of new adsorbents would imply the optimization of these features, while the tuning of these parameters is relevant for optimizing the membrane performance in terms of permeability and selectivity. In recent years, carbon nanotubes (CNTs), owing to their high surface area, low density, super-hydrophobicity and enhanced mechanical properties, have been on the top among the effective materials available for the development of adsorbents for composite membranes, appearing as an interesting alternative to current state-of-the-art adsorbents.11–13 Furthermore, their tubular form is a key point to provide porosity in membrane structures affording outstanding water-transport properties. Adsorbent materials consisting of pure CNTs can be prepared; however, the solubility issues make difficult to obtain a homogeneous distribution of such CNTs within the adsorbent. Additionally, mechanical strength and stability of pristine CNT-based adsorbents are still limited as they are thermodynamically stabilized by weak van der Waals forces.14 To overcome these drawbacks, extensive investigations on chemical modifications have been conducted in the last decades, where active organic groups are generated/grafted on the surface of carbon nanotubes. In most of the cases, the functionalization approaches are based on the application of acid treatment before further functionalization of the CNT surface.15 The subsequent chemical modification of oxidized CNTs with polymers for their application in wastewater treatment technologies has been also extensively studied. 6,16–18 Other studies instead focus on the direct attachment of polymers to the sidewalls of CNTs, either through in situ polymerization or long chain polymer attachment.19,20 However, although chemical 3 ACS Paragon Plus Environment


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functionalization of CNTs could substantially increase the sorption capacity towards various water pollutants, it decreases their ability for binding hydrophobic organic water pollutants such as polyaromatic compounds that interact strongly with the sidewall of CNTs through π-π interactions. Therefore, great attention needs to be taken during the functionalization of CNTs, so that one can preserve their hydrophobic nature for multi-pollutants water treatment. In this direction, the chemical modification of CNT-based adsorbents through approaches that preserve the hydrophobic structure has been also addressed, usually based on the interaction of CNTs with hydrophobic polymers such as polystyrene or polytetrafluoroethylene.21–24 However, modification of CNTs through these methods still yields adsorbents with limited long-term stability and sometimes reduced mechanical strength due to the wall rupture.25 Chemical cross-linking of CNTs appears as a promising strategy for enhancing the mechanical properties of the resulting materials, providing adsorbent materials with higher resistance, stability, and long-term performance.26 Hence, the generation of stronger CNT-CNT interactions via cross-linking processes could be used to overcome the most common weaknesses of membranes fabricated for water filtration processes and it is also a promising approach to develop adsorbent materials with tunable pore size and rougher surfaces, favoring the creation of interstitial spaces between tubes, acting as major trap for small pollutants and increasing the adsorption capacity.27 Few examples have been described in the literature about the preparation of CNT free standing adsorbent materials for water treatment based on the cross-linking of CNTs, and most of the existing studies are based on the use of polymers as cross-linkers.28,29 In a recent study Park et al.30 have demonstrated that the mechanical properties of cross-linked CNT-based materials strongly depend on the degree of cross-linking, length of cross-linkers, and diameter of the employed CNTs, concluding that the incorporation of aromatic cross-linkers between carbon nanotube fibers, enhances the mechanical strength of the resulting composite materials. 4 ACS Paragon Plus Environment

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Based on these findings, we present here the preparation of highly flexible and robust singlewalled carbon nanotube (SWCNT) adsorbent materials with tunable structure through crosslinking with bis-aromatic-amine containing compounds for their application in membrane technologies. The new materials are fully characterized and the capacity of adsorption of the resulting CNT adsorbents for the removal of different aromatic water pollutants is studied. The performance of the adsorbent materials for oil/water separation and the removal of organic solvents is also investigated.

Results and discussion 1.

Synthesis, preparation and characterization of the CNT adsorbents NanoIntegris's HiPCO SWCNTs were used to fabricate the CNT adsorbents through cross-

linking processes, and aryl diazonium salt chemistry was employed to build up the desired networks. For this purpose, bisdiazonium compounds consisting of benzidine derivatives, which have been previously described as effective cross-linking agents for CNTs and graphene,31,32 acted as bridges between the SWCNTs. As shown in the scheme in Figure 1, the synthetic procedure comprised two different steps (see supporting information for further details). During the first step SWCNTs were allowed to react with benzidine in the presence of isoamyl nitrite for 24 hours, yielding SWCNTs with low cross-linking degree and called from here on LCD-SWCNT. In a second step, LCD-SWCNT was employed as the starting material and step one was repeated yielding SWCNTs with substantially greater degree of cross-linking and called from here on HCD-SWCNT. This second round of cross-linking was intended to result in stronger and more stable CNT-based adsorbent materials. The final products were purified through the common washing/filtration/sonication/work-up procedures. After the cross-linking and purification processes, the adsorbent materials were fabricated through the very well-known vacuum filtration method33 using a typical glass microfiltration setup (Figure 5 ACS Paragon Plus Environment


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1, bottom right side). Briefly, a homogeneous dispersion of cross-linked SWCNTs (LCD- and HCD-SWCNTs) in tetrahydrofuran (THF) is poured onto a PTFE membrane; then, after proper drying, the free-standing film of CNTs that can be manipulated by tweezers is easily obtained by etching (Figure 1, bottom left side). The film thickness was controlled by modifying the amount of material employed for the filtration process. The resulting crosslinked CNT adsorbents can be then used as filtering material for the separation of different water pollutants. As observed in Figure 1, the obtained SWCNT free-standing films were extremely flexible and exhibited remarkable mechanical strength and stability. The flexibility was tested by rolling the fabricated CNT films into cylinders, which returned to their original form after unrolling without suffering from any cracks or defects.34 This is a key feature for the application of the cross-linked CNTs films as adsorbents in long-term water treatment applications.


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Figure 1. Top part: Schematic representation of the cross-linking procedure through several rounds of functionalization with bis-aromatic linkers. Bottom part: CNT film fabrication setup and SWCNT adsorbent material obtained through vacuum filtration. The successful cross-linking of SWCNTs was initially confirmed by Raman spectroscopy. As observed, the D-band, which was hardly observed for the pristine SWCNTs (p-SWCNT) became more distinguishable after reaction with the bis-aromatic compound (Figure 2 left and inset). The ratio of ID/IG exhibited a general increasing trend when going from p-SWCNT to LCD-SWCNT, and more so for HCD-SWCNT. This fact confirmed the presence of higher degree of functionalization in HCD-SWCNT and consequently the successful achievement of a higher number of bridges between the nanotubes (higher cross-linking degree). These results were in perfect agreement with those of the thermogravimetric analyses, which developed a group coverage of one group every 234 carbon atoms (536 μmol/g) and one group every 98 carbon atoms (944 μmol/g) for LCD-SWCNT and HCD-SWCNT, respectively (Figure 2 right). 100

1,0 0,4

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Weight loss %

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80

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1000 1500 2000 Raman Shift cm-1

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Figure 2. Black line: p-SWCNTs; Red line: LCD-SWCNTs and blue line: HCD-SWCNTs. Left: Raman studies of the cross-linked materials in comparison with the p-SWCNTs. Right: TGA curves of the cross-linked materials in comparison with the p-SWCNTs.



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Apart from the evident new structural features of the cross-linked CNTs that show buckypaper-like appearance, the morphological features at the nanoscale level of the CNT adsorbents were studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) (Fig. 3 and S2). TEM images of the new CNT-based adsorbents (LCD- and HCD-SWCNTs) in comparison with the pristine SWCNTs, revealed the presence of highly entangled structures consisting of randomly networked CNTs, that was even more pronounced after the second round of functionalization (Figure 3 top part). This new feature was a key point for the preparation of the desired adsorbent materials, facilitating the fabrication of stable and free-standing materials. By scanning electron microcopy (SEM), morphological changes were also evident when increasing the degree of functionalization (Figure 3 bottom part and S2). After the first round of functionalization, a rough and more compact surface was observed. Small pores were still observable on the surface, however, much smaller than those of the starting material. A more notable change was observed after the second round of functionalization, showing a large carpet of CNTs with imperceptible porosity.


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Figure 3. TEM images of a) p-SWCNT, b) LCD-SWCNTs and c) HCD-SWCNTs; SEM images of d) p-SWCNTs, e) LDC-SWCNTs and f) HDC-SWCNTs. Statistical image analysis of the SEM images using ImageJ software35 yielded information on the pore size distribution (Fig. S3). To this end, the images were processed first by thresholding

(the thresholded features

are

displayed

in red

and

background

is

displayed in grayscale in Fig. S3). The software then identifies the pores and quantifies the area and Feret’s diameter, that is the longest distance between two tangents of the pore boundary. The pore size distribution was then calculated from the Feret’s diameter data with an average pore diameter of 15.94 nm for LCD-SWCNT and 9.73 nm for HCD-SWCNT. It is worth mentioning here that this method allows a fair approximation of the pore diameter, since it will measure the longest distance. However, it allows us to confirm the existence of bigger pores in LCD-SWCNTs in comparison to HCD-SWCNTs. These findings suggest that the presence of more bridges between the CNTs results in the formation of smaller cavities and empty spaces in the adsorbent material. Small angle X-ray scattering (SAXS) yielded information on the nanostructure of the CNTs before and after the functionalization. The reference measurement of untreated p-SWCNTs (Figure 4a) revealed the formation of a hierarchical super-structure as depicted in Figure 4b: pSWCNTs formed hexagonally packed strands (with diameter of approx. 5 nm), which in turn aligned to form multi-strand fibers (with diameter of approx. 10 nm) [see supporting Information - SAXS model fitting for a detailed model description]. As the hexagonal packing of the CNTs within the hexagonal strands was very dense (no sub-nanometer pores accessible), the random alignment of multiple strands to a single fiber left nano-defects. The Functionalization with benzidine (yielding LCD-SWCNT and HCD-SWCNT) affected neither the hexagonal-strands (constant hexagonal lattice parameter) nor the hierarchical 9 ACS Paragon Plus Environment


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network formation (steady aggregate Porod contribution) [see supporting information Table S1]. However, in these cases, the formation of nanoparticles with an approx. diameter of 3.0 nm was witnessed, likely connected to the aggregation of benzidine molecules on the strand and fiber surface. Consistently, this scattering contribution became significantly stronger and the approx. diameter increased to 3.6 nm in the second functionalization step. Additionally, an increase in hexagonally packed strand dimeter from 5 to 6 nm and the multi-strand fiber diameter from 10 to 13 nm was observed with functionalization, respectively. These results were in accordance with those from the AFM studies that revealed diameters ranging from 7 to 12 nm for p-SWCNT that slightly increased in both cross-linked materials, LCD-SWCNT and HCD-SWCNT (see supporting information Figure S1). From this, it can be concluded that benzidine functionalization occurs at two structural sites: i) between the hexagonally packed strands, so filling the nano-defects within the multi-strand fibers, and ii) on the surface of the multi-strand fibers, where molecular aggregation forms organic nanoparticles and causes further alignment of single strands with pre-existing fibers.

Figure 4. SAXS investigation of the reference and functionalized compounds. a) SAXS

GFF

patterns of p-SWCNT (black), LCD-SWCNT (Red) and HCD-SWCNT (blue) together with 10 ACS Paragon Plus Environment

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the respective full-pattern refinement fits (see Supporting Information of model details). b) Hierarchical model derived from the SAXS data of the p-SWCNT reference pattern: single CNT form hexagonally packed strands of approx. 5 nm, which further align to multi-strand fibers of approx. 10 nm. To gain deeper insight into the nanostructure and the porous features of the CNT adsorbent materials, nitrogen adsorption–desorption measurements were performed. As shown in Figure 5 the investigated samples presented type IV isotherms, typical of mesoporous materials.36 The textural properties of the different materials are summarized in Table 1.

Figure 5. N2 physisorption isotherms at the liquid nitrogen temperature. Black line: pSWCNTs; Red line: LCD-SWCNTs and blue line: HCD-SWCNTs Table1. Results from textural analysis of the materials. DM (nm) c Sample

p-SWCNT

SSA (m2/g) a

604

CPV (cm³/g) b

1.176

Adsorption

Desorption

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LCD-SWCNT

210

0.975

49

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HCD-SWCNT

427

1.059

52

3.7 / 33

a

Specific Surface Area calculated applying the BET method.

b

Cumulative Pore Volume.

c

Relative maxima of the pore distribution calculated applying the Barrett-Joyner-Halenda

(BJH) method.37 All the samples showed high surface areas, although the derivatized products (LCDSWCNT and HCD-SWCNT) showed lower surface area with respect to the p-SWCNTs. As observed, the surface area was calculated to be 210 m2 g-1 for LCD-SWCNT; however, an increase of the cross-linking degree led to an increase of the specific surface area (427 m2 g-1 for HCD-SWCNT). This could be attributed to the formation of additional nanoparticles resulting from the aggregation of benzidine molecules on the CNT strands, as previously described in the SAXS studies. However, also the presence of smaller and new pores resulting from the creation of new bridges may play an important role in the increase of the specific surface area. Nevertheless, all the samples showed high and comparable cumulative pore volumes. p-SWCNT showed also a significant amount of micropores, accounting for a microporous surface area of 180 m2/g and a microporous volume of 0.150 cm3/g. In the other cases, the contribution of micropores was negligible. The size distribution analysis revealed that, for all the samples, mesopores had a bottle-neck shape, with the inner part larger than the entrance. From these results we can predict that thanks to the high mesopore volume and surface area the CNT-based adsorbents could be ideal candidates for the removal of contaminants from water. On the other hand, adsorption capacity is directly proportional to the specific surface area as the number of active sites available for adsorption is higher.38–40 Thus, 12 ACS Paragon Plus Environment

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HCD-SWCNT is expected to have highest adsorption capacity in comparison to LCDSWCNT when this adsorbent is interfaced with organic pollutants and oils. 2. Application of the CNT adsorbents to water treatment In this study, an investigation of the adsorption properties of the different SWCNT adsorbent materials is presented, and the adsorption capacity of different water pollutants is studied as a function of the cross-linking density and the specific surface area. It is worth mentioning here that even though p-SWCNTs presented the higher specific surface area, this material did not show the free-standing paper-like structure required for the preparation of the desired adsorbents, probably due to weak van der Waals interactions that are not sufﬁcient to make the pristine CNT films mechanically strong, and therefore only LCD-SWCNT and HCDSWCNT were tested for the water treatment studies. For the study, the free-standing CNT cross-linked films acted as filtering materials by placing them inside the cell of the vacuum filtration setup. 2.1

Adsorption of aromatic compounds

Contamination of water by aromatic compounds is a common problem due to the disposal of industrial and agricultural waste that has increased in the last decades. These pollutants have adverse impacts on ecosystems and human health due to their ubiquitous and toxic nature.41,42 In the last years, various methods have been developed for the removal of aromatic pollutants from water,2 being one of the most common employed methods focused on the use of adsorbent materials with high binding affinity to remove even trace amounts of aromatic pollutants. Among them, graphitic materials such as CNTs are promising as adsorbents due to the strong non-covalent interactions with aromatic organic compounds.42,43 Indeed, recent studies about the adsorption of aromatic compounds onto CNTs and graphene, have demonstrated higher adsorption capacities than activated carbon, which is currently the most widely used material for water purification.44–46 However, improved adsorption performance may be achieved by 13 ACS Paragon Plus Environment


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employing chemically modified CNT-based membranes with enhanced structural properties and adsorbent behavior. Methylene blue (MB), diquat dibromide (DqDb) and 1-pyrenebutyric acid (PBA) were chosen for the study of the sorption capacity of our cross-linked CNT adsorbent materials. MB is a water pollutant produced from textile, plastic and dye industries. Though MB is not strongly hazardous, acute exposure can cause some harmful effects, such as increased heart rate, vomiting, shock, cyanosis, jaundice, and quadriplegia.47 DqDb is one of the most widely used pesticides in plant growth and causes serious health and environmental damage due to its toxic nature.48 PBA was chosen just as a model of polycyclic aromatic compounds. The adsorption capacity of the cross-linked CNT adsorbents was then evaluated using solutions of the aforementioned polyaromatic compounds at a concentration of 1.5 x 10-2 mg/ml (Fig. 6 and Table 2). For the study, the solution was poured into the cell of a vacuum filtration system at 2 mbar, using the cross-linked CNTs as adsorbent material. As observed in Table 2, after the filtration process through LCD-SWCNT adsorbents with an average thickness of approximately 25 μm, MB showed a concentration decrease of 40%, while an increase of the adsorbent thickness up to 50 μm, showed higher adsorption capacity (around 67% concentration decrease). On the other hand, when using the HCD-SWCNT adsorbents with 50 μm thickness, the adsorption was even higher than that observed in the previous case, reaching a retention capacity of up to 83%. Similar results were observed during the filtration of water containing DqDb and PBA, where, for the same thickness, PBA and DqDb showed higher degree of retention when the solutions where filtered through the adsorbent with higher degree of cross-linking (HCD-SWCNT). Table 2. Retention percentages after filtration through the cross-linked CNT adsorbents with different thicknesses.


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Aromatic pollutants

Methylene

Diquat dibromide

1-pyrenebutyric

blue (MB)

(DqDb)

acid (PBA)

LCD-SWCNT (25 μm)

40 %

-

-

LCD-SWCNT (50 μm)

67 %

20 %

28 %

HCD-SWCNT (50 μm)

83 %

28 %

50 %

Adsorbent

Figure 6. UV-Vis spectra of 1) MB solution in H2O, 2) DqDb solution in H2O and 3) PBA solution in H2O. Solid line: starting solution; dotted line: LCD-SWCNT (25 μm); dashed line: LCD-SWCNT (50 μm); dashed and dotted line: HDC-SWCNTs (50 μm).



In order to ensure more reliable and accurate results on the filtration of aromatics through the cross-linked CNT adsorbents, we conducted four filtration experiments for each aromatic pollutant through LCD- and HCD- SWCNT adsorbents of 50 μm-thick, whose results are collected in a bar chart in Figure 7 (left side). In agreement with the results shown in the table, the reduction of the concentration was higher during the filtration experiments through HCDSWCNT for all the cases. The error bars indicated the standard deviation of four separate filtration experiments for each CNT adsorbent. Finally, in an attempt to get extra information of the adsorption capacity of our CNTadsorbents towards aromatic compounds the penetration curves were determined by assessing the amount of aromatic pollutant adsorbed onto HCD-SWCNTs pieces per unit area (Figure 7 right side). For the studies, solutions of the polyaromatic compounds at a concentration of 1.5 x 102 mg/ml were again prepared. The amount of pollutant adsorbed onto the surface of the CNT adsorbents was determined following the procedure described in the supporting information (see section “sorption of aromatic compounds”). The results showed almost a linear increase of adsorbed pollutant at the beginning followed by a saturation region after 90 minutes, which represents filling of the total number of sites that are exposed for binding pollutant on the material surface. Similar behaviour was observed previously in the literature for the adsorption of aromatic compounds onto CNTs/graphene hybrid materials.12 1,8

100

Methylene blue Diquat dibromide Pyrenebutyric acid

1,6 LCD-SWCNT HCD-SWCNT

80 60 40

g of pollutant/m2

Concentration decrease %

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Figure 7. Right: Bar graphs of the mean values of concentration decrease determined from four different filtration experiments with different aromatic water pollutants for LCD- and HCD- SWCNT adsorbents of 50 μm-thick. Left: Plots of pollutant uptake per unit area as a function of time for HCD-SWCNT adsorbents. Something important to remark here is the lower level of retention observed for DqDb with respect to the retention of MB and PBA. As carbon-based adsorbents are prone to adsorb by π–π interactions between adsorbent and adsorbate respective unsaturated bonds, this suggests that the lower aromatic character of DqDb, results in a decrease of the interactions with the CNT sidewalls and therefore in a lower removal percentage of this water pollutant. The findings arising from these studies confirm the existence of higher level of retention observed in the presence of HCD-SWCNT, that could be attributed to its higher cross-linking degree and larger specific surface area, as previously deduced from the N2 physisorption and SAXS experiments: a higher specific surface area is available for adsorption of pollutant molecules, improving performance of the adsorbent material. Most importantly, the level of retention of aromatic compounds can be tuned by modifying the thickness of the adsorbent and the degree of cross-linking between the CNTs. This latter approach could be useful in the future development of CNT-based adsorbents to increase their adsorption capacity and improve their performance. 2.2

Separation of oils and water-in-oil emulsions

The frequent oil spills released during industrial accidents can cause damage in ecosystems, including plants and animals, and contaminate water for drinking and other purposes. In many cases, wastewater contains emulsified oil/water mixtures which is always difficult and challenging to separate. Therefore, the development of functional materials for efficient treatment of oil-contaminated water is crucial. 17 ACS Paragon Plus Environment


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Due to the outstanding properties mentioned in the previous sections, CNTs have demonstrated to be promising for their application in the field of oil–water separation, offering a large number of advantages.22,49,50 Sun et al.28 reported p-phenylenediamine modified CNTs synthesized via diazotization salt reaction, where the functionalization induced an increase of the surface roughness and lowered their surface free energy by the introduction of the organic molecules.23,25,51 The resulting materials exhibited a notably absorbing ability for removal of organics and oils from water. Shi et al.52 reported that SWCNT networks could be used for the separation of emulsified oil/water mixtures, which is even more challenging due to presence of droplets of very small sizes. Based on these results, the capacity of our cross-linked CNT adsorbents for the removal of organic and oil contaminants from water was evaluated. Before studying the separation capacity of the new CNT adsorbents for water/oil mixtures, the wetting behavior of water and oil droplets on the cross-linked CNT adsorbents was determined. For this purpose, a rough estimation of the water and oil contact angles was performed taking zoomed pictures of water and oil droplets placed onto the surface of HCD-SWCNT and determining the contact angle using the computer program ImageJ35 (Figure 8). As observed in Figure 8, when a water droplet (3 µL) was placed onto the HCD-SWCNT adsorbent, it remained there without spreading (Figure 8a), showing a contact angle equal to 105º which is usually attributed to hydrophobic surfaces with poor wettability. In contrast, when a droplet of oil was placed onto the HCD-SWCNT adsorbent (Figure 8b) it spread quickly and a nearly zero contact angle was reached showing super-oleophilic properties. This behavior could be attributed to the increase of surface roughness, as explained before, because of the presence of linkers among the nanotube strands that lowers the surface free energy after functionalization.51 This specific wetting properties clearly suggest their potential use for separation of oil/water mixtures, given the high oil affinity that exhibit the as-prepared CNT adsorbents.


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Figure 8. HCD-SWCNT adsorbent images where is evidenced the wetting behavior of water (left) and oil (right) droplets. Inset: Zoom with contact angle determined with ImageJ.35 Figure 9 shows the process of sorption of a drop of toluene in water when it comes in contact with the CNT adsorbents (see supporting information for further details). Toluene colored with oil red was dropped onto the surface of water, and then, a piece of HCD-SWCNT was brought in contact with the drop of toluene floating on the surface. The drop of toluene was fully absorbed leaving a clean and almost transparent phase of water.



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Figure 9. Removing a toluene droplet (labelled with oil Red) from the surface of water using the HCD-SWCNT adsorbent. These results confirmed the potential use of the cross-linked CNTs as selective adsorbent to remove oils and organic solvents from water. On the other hand, oil/water emulsions can be completely separated by CNT films when the pore size is far less than the droplet size of these emulsions.52 Thus, the combination of nanometer-sized porous structure with high specific surface area observed for the cross-linked CNT adsorbents led us to study the capacity of separation of emulsified oil/water mixtures. For the sake of comparison, a surfactant-free water in cyclohexane emulsion was permeated through LCD-SWCNT and HCD-SWCNT adsorbents (50 μm) (Figure 10). As shown in Figure 10a, cyclohexane immediately permeated through the CNT adsorbents while the wateroil emulsion was retained on the top part. The permeation flux (at a pressure of 2 mbar) was calculated as a function of the permeate volume per unit adsorbent area, being (2.1 ± 0.2) x 102 L m-2 h-1 and (1.6 ± 0.1) x 102 L m-2 h-1 for LCD-SWCNT and HCD-SWCNT, respectively.


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Figure 10. Top part: a) Photograph of the setup for the separation of water-in-cyclohexane emulsion where cyclohexane selectively permeates through HCD-SWCNT; b) photographs of the water-in-cyclohexane emulsions before and after filtration; Bottom part: optical microscopy images of c) feed solution, d) filtrate through LCD-SWCNT (50 μm) and e) filtrate through HCD-SWCNT (50 μm). As observed in Figure 10b, the collected filtrate was transparent compared with the original milky white feed emulsion (after the filtration through both CNT-adsorbent materials), indicating that the emulsion droplets de-emulsified once touching the CNT-adsorbents. This fact confirmed the excellent separating properties of the cross-linked SWCNT adsorbents for emulsified water/oil mixtures. In order to get deeper insight into the different capacities of adsorption of both adsorbents when these were faced to water/oil emulsions, optical microscopy was used to assess the separation effectiveness. As observed in the Figure 10c, the feed presented numerous water 21 ACS Paragon Plus Environment


Page 22 of 32

droplets with an average size of about 20-25 μm. After filtration through LCD-SWCNT, we still observed in the filtrate small water droplets, with an average size 3-6 μm. However, filtration through HCD-SWCNT was much more effective and the water droplets observed in the

filtrate

showed

diameters

below

1

μm.

These

results

suggest

that

the

hydrophobic/superoleophilic nature of the cross-linked CNT adsorbents allows the permeation of oils while repelling water. The differences in pore size between LCD- and HCD-SWCNTs seems to govern the degree of separation of the water/oil emulsions in these studies, where HCD-SWCNT allows only the permeation of nanoscale water droplets. These findings confirm again the possibility of controlling the CNT-based adsorbents performance through chemical modification via cross-linking approaches, as well as the importance of developing methods to control the porosity and specific surface area of carbon-based adsorbent materials. 2.3. Performance of the cross-linked CNT-adsorbents To further demonstrate the potential of the CNT adsorbents for the sorption of organic solvents and oil from water, we investigated the weight gain ratio (α) of LCD-SWCNT and HCD-SWCNT for several kinds of organic solvents and exhausted oil. The weight gain ratio of a membrane is defined as α = (mabsorbed - mdry)/mdry x 100 w%, where mdry and mabsorbed are the weight of CNT adsorbents before and after adsorption of organics or oils.28 For the study, pieces of the cross-linked CNT adsorbents were submerged for 30 minutes inside the different organic solvents and exhausted oil, and the weight gain ratio (α) was calculated (see supporting information for further details). The cross-linked CNTs adsorbents showed very high sorption capacities for all the tested organic solvents (Figure 11). As observed, better performance was again evident for the adsorbent with higher specific surface area (HCDSWCNT) showing a capacity of adsorption of up to 22 times its own weight. These findings supported again the theory that the adsorption capacity could be tuned, and promising adsorption rates could be reached, by increasing the degree of cross-linking between CNTs. 22 ACS Paragon Plus Environment

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The variation in adsorption observed for the different solvents predominantly depends on the density, resulting in higher adsorption when increasing the solvent density.

Figure 11. Sorption capacity of LCD-SWCNT and HCD-SWCNT for oil and organic solvents. The recyclability of adsorbent materials for the removal of organics or oils from water is an important factor to evaluate the adsorbents performance. In order to evaluate if the CNT adsorbents could be useful for practical applications, the recyclability of HCD-SWCNT was evaluated. In this particular case, the recyclability was achieved by a washing procedure consisting of several cycles of acetone followed by drying at 100 ºC for 3 hours. In Figure 12 is observed the relationship between the weight gain of the adsorbents and the recycle numbers tested for the adsorption of different organic solvents by HCD-SWCNT, that was the adsorbent with higher adsorption capacity. As observed, the sorption capacity was only slightly decreased after 9 cycles, which confirmed the good recyclability of our cross-linked SWCNT adsorbents.



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Figure 12. Adsorption recyclability of HCD-SWCNT for different organic solvents.

Conclusion A facile and versatile approach for the preparation of hydrophobic adsorbent materials based on the chemical cross-linking of carbon nanotubes has been presented in this work. We have developed a novel methodology for the improvement of the CNT adsorbent’s performance consisting in the modification of the degree of cross-linking. Two different factors, pore size and specific surface area, are influenced via chemical cross-linking, allowing the tuning of the adsorption capacity of CNT-based adsorbents. The proposed approach, that simultaneously results in the enhancement of the mechanical properties and stability of the adsorbents, could be exploited in future membrane technologies based on CNTs to design CNT-based materials with great adsorption properties, just by increasing the degree of cross-linking. The resulting cross-linked CNT adsorbents allow the effective removal of different types of organic pollutants. Furthermore, the efficient separation of water/oil emulsions is demonstrated, that is normally achieved through the use of super-hydrophobic adsorbent materials consisting of hybrids based on CNTs and polymers. The cross-linked CNT adsorbents show high selectivity, 24 ACS Paragon Plus Environment

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long-term stability and recyclability, and represent a step forward towards the development of high-performance and multifunctional CNT-based adsorbents, promising for their application in cost-effective water purification technologies. ASSOCIATED CONTENT Figure S1. AFM images of p-SWCNTs and both cross-linked materials; Figure S2. SAXS experimental scattering pattern and illustrations; Figure S3. Average pore size distribution; synthesis of the cross-linked materials; sorption of aromatic compounds; procedure for the study of the sorption capacity; preparation and procedure for the separation of water/oil emulsions; permeation flux calculation method; SAXS model fitting. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the University of Trieste, Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Ministero dell’Università e della Ricerca (MIUR) (FIRB prot. RBAP11ETKA and Cofin. Prot. 2010N3T9M4). The authors acknowledge the CERIC-ERIC Consortium for the access to experimental facilities (Austrian SAXS beamline).

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Cross-Linked Carbon Nanotube Adsorbents for Water Treatment

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