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Anti-fouling Cellulose Hybrid Bio-membrane for Effective Oil/Water Separation Ravichandran Honnavally Kollarigowda, Sinoj Abraham, and Carlo Montemagno ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09087 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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ACS Applied Materials & Interfaces

Anti-fouling Cellulose Hybrid Bio-membrane for Effective Oil/Water Separation Ravichandran H. Kollarigowda, ab Sinoj Abraham ab* and Carlo D. Montemagno ab* a

Ingenuity Lab, 11421 Saskatchewan Drive NW, Edmonton, Alberta, Canada - T6G2M9

b

Department of Chemical and Materials Engineering University of Alberta, Edmonton, Alberta,

Canada -T6G2V4. KEYWORDS. Cellulose Bio-membrane, Anti-fouling Polymeric Membrane, Oil/Water Separation, Oil Absorption, Hydrophobic Block Co-polymer.

ABSTRACT. Oil/water separation has been of great interest worldwide due to the increasingly serious environmental pollution caused by the abundant discharge of industrial wastewater, oil spill accidents and odors. Here, we describe simple and economical super-hydrophobic hybrid membranes for effective oil/water separation. Eco-friendly, anti-fouling membranes were fabricated for oil/water separation, waste particle filtration, the blocking of thiol-based odor materials, etc., by using cellulose membrane filter (CM). CM was modified from their original super hydrophilic nature into a super-hydrophobic surface via a reversible addition– fragmentation chain transfer (RAFT) technique. The block co-polymer of Poly[(3(trimethoxysilyl)propyl acrylate)-block-myrcene] was synthesized using a “grafting-from”

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approach on the CM. The surface contact angle that we obtained was over 160O, and absorption tests of several organic contaminants (oils and solvents) exhibited superior levels of extractive activity and excellent reusability. These properties rendered this membrane a promising surface for oil/water separation. Interestingly, myrcene blocks thiol (through “-ene-” chemistry) contaminants, thereby bestowing a pleasant odor to polluted water by acting as an anti-fouling material. We exploited the structural properties of cellulose networks and simple chemical manipulations to fabricate an original material that proved effective in separating water from organic and nano/micro particulate contaminants. These characteristics allowed our material to effectively separate water from oily/particulate phases as well as embed anti-fouling materials for water purification, thus making it an appropriate absorber for chemical processes and environmental protection.

Introduction

Oil pollution is a worldwide problem based on increasing oily wastewater produced by the petrochemical, textile, and food industries, as well as frequent oil spill accidents.1-2 Oil/water separation has been of great global interest as a result of increasingly serious environmental pollution caused by abundant discharge of industrial wastewater, oil spill accidents, odors, etc.3-5 Oil-polluted water usually contains harmful chemicals, which cause harm to human health and damage to the ecosystem.6 Therefore, advanced materials or techniques are urgently required to effectively separate various oil/water mixtures. Conventional techniques, such as gravity separation, skimming, and flotation, are useful for the separation of free oil/water mixtures, but are not applicable to oil/water emulsions.7-8 They suffer from limitations of low efficiency and high operational costs. As an alternative, researchers have recently emphasized the effectiveness


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of utilizing a particular wettability to design novel materials for oil/water separation.9-11 Superwetting materials with different affinities to water and oil are believed to hold much promise in realizing higher efficiency oil/water separation as water and oils are intrinsically immiscible.11 For example, superhydrophobic-superoleophilic materials, such as PTFE-coated mesh,12 silicone-modified polyester textile,13 and polysiloxane-based gel,14 have been developed to separate free oil/water mixtures by either membrane filtration or affinity absorption of oil from water.15-18 The separation of water-in-oil emulsions also becomes available if the pore size of a hydrophobic-oleophilic membrane is rationally designed to be smaller than the emulsified water droplets.19-21 More recently, graphene-based materials have been successfully employed as sorbents, exhibiting superior uptake capabilities.22-24 However, there are disadvantages that lie in high costs and scarce availability for practical applications. Natural sorbents, such as wool fibers,25 zeolites,26 activated carbons,27 and collegen,28 have had better results due to their microporosity, low cost, and biodegradability. Cellulose is the main raw material of paper and cotton fabrics; it is not only abundant in nature, but is also light in weight, stable, and is easily disposed of. More importantly, cellulose-based materials (e.g., filter paper, membrane) can be chemically modified for various applications.29-31 Typically, a superhydrophobic and superoleophilic surface can be achieved by dipping, spincoating, or spraying filter paper with a solution, suspension, or emulsion of hydrophobic nanoparticles type.30-32 They are mostly stable and reusable, but have undesirable properties such as high costs, disposal difficulties, complex fabrication procedures and lack of long-term stability of these spin coated or sprayed polymers could highly reduce their range of applications. For this reason, the use of a covalently grafted hydrophobic polymer on a cellulose membrane, described


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in this work, might be a real step forward in the realization of stable platforms for the effective oil/water separation. In this paper, we present a facile and cost-effective process to obtain a hydrophobic and antifouling surface on cellulose membrane (CM) and establish its application for efficient oil/water separation and thiol filtration. The biodegradability factor is a significant characteristic of our CM, rendering it an eco-friendly method compared to conventional materials and techniques. Specifically, CM was modified from being superhydrophilic in nature with block-copolymers of silane and myrcene monomers via RAFT polymerization. Interestingly, this material demonstrated a robust oil absorption capacity and effectively entrapped particles in complex water-oil microparticle systems through a straightforward and inexpensive chemical modification. This membrane also facilitated thiol filtration by blocking thiols on the surface through thiol-ene conjugation. Moreover, the material is reusable, completely biodegradable, and can be employed as a reliable agent for wastewater treatment. Experimental Methods All chemicals and solvents were purchased from Sigma-Aldrich at the highest purity available and used without further purification. All the chemical synthesis was carried out under dry nitrogen using schlenk standard technique. The cellulose of whatman filter membrane thimbles (CM) (33mm x 94mm, CAT no-2800-339) was purchased from Buckinghamshire UK. Experimental Section Synthesis of Poly (3-(trimethoxysilyl)propyl acrylate) on CM. 1.3 g (5.2 mmol) 3(trimethoxysilyl)propyl acrylate of monomer was dissolved in 30 ml of toluene in a 250 ml schlenk flask and degassed for 30 minutes under argon atmosphere. The monomer solution was degassed by three freeze-thaw cycles for the complete removal of dissolved oxygen from the


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system. Meanwhile, the 2 mg (0.012 mmol) of AIBN initiator solution was degassed by bubbling nitrogen for 30 minutes. After that, the monomer and initiator solutions were transferred into the schlenk flask, which contained RAFT-immobilized CM, under argon atmosphere. The flask was sealed with Teflon tape and heated at 110 °C for 24 hours. The reaction was stopped by exposure to air. The substrate was then washed with toluene by sonication for 20 minutes at room temperature, the washing process was then repeated three times to remove unimmobilized polymer on the CM substrate. The substrate was then dried in a vacuum oven at 40 °C for 24 hours. Synthesis of Poly [(3-(trimethoxysilyl)propyl acrylate)-block- Myrcene] on CM. 0.7 g ( 5.2 mmol) of myrcene monomer was dissolved in 50 ml of toluene in a 250 ml shlenk flask and degassed for 30 minutes under argon atmosphere. The monomer solution was degassed by three freeze-thaw cycles for the complete removal of dissolved oxygen from the system. Meanwhile, 2 mg (0.0121 mmol) of AIBN initiator solution was degassed by bubbling nitrogen for 30 minutes. The monomer and initiator solutions were then transferred into the shlenk flask which contained silane polymerized CM, under argon atmosphere. The flask was sealed with Teflon tape and heated at 110 °C for 24 hours. The reaction was stopped by exposure to air. The substrate was then washed with toluene by sonication for 20 minutes at room temperature, and the washing process was repeated three times to remove unimmobilized polymer on the CM substrate. The substrate was then dried in a vacuum oven at 30 °C for 24 hours. For more experimental methods, differential scanning calorimetry (DSC) curve and videos can be found in the Supporting Information. Results and discussion


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Cellulose filter membrane (CM) consists of degradable and renewable biopolymers with superb mechanical properties. However, its application in many technologically significant fields is restricted based on the lack of reactive functional sites that provide desirable surface properties.

Scheme 1. Chemical structure of the super hydrophobic block copolymer on cellulose membrane via RAFT technique. Reactions and Conditions. a) 0.0246 mmole of CDI, 0.0109


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mmole of RAFT agent, 40 ml DMF at RT for 24 hours. b and c) 5.2 mmole of silane/myrcene monomer, Initiator, 0.0121 mmol, 60 ml toluene at 110OC for 24 hours. In the schematic representation of the modified CM, blue lines represent the cellulose backbone, orange red segments stand for silane polymer while the green slice corresponds to myrcene. The use of cellulose-based materials could thus be extended to new areas by altering and tailoring its chemical and physical properties. Here, we modified CM from that have a hydrophilic nature into featuring super hydrophobicity with a antifouling surface for water/oil separation. The cellulose hydroxyl group was activated by immersing it into a DMF and water system for additional functionalization. Activated CM was functionalized with 2(Dodecylthiocarbonothioylthio)-2-methylpropionic acid (RAFT agent, chain transfer agent; CTA) by acid/hydroxyl esterification. The hydrophobic nature of the silane-based monomer of 3(trimethoxysilyl) propyl acrylate was polymerized onto CM with the RAFT technique utilizing 2,2′-Azobis(2-methylpropionitrile) as an initiator. RAFT is one of the key methods for controlling radical polymerization to yield a narrow polydispersity index and a multi-functional group for further modification.33 We also introduced another hydrophobic monomer, myrcene, that was grafted onto the surface as a polymer block, providing a fouling removal agent. Silanegrafted CM serves as a macro initiator and myrcene was polymerized with it, in agreement with the theoretical predictions of loading monomer units, confirming the efficiency of our polymerization strategy. The chemical structure and a graphical representation of the modification via the CM are presented in Scheme 1. Wetting Performances and Morphology performance Water contact angle (WCA) measurement permitted the determination of the change in hydrophilicity through the introduction of the block co-polymer onto the CM, see Figure 1. The


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contact angle of the solo CM was not established due to the superhydrophilicity of the hydroxyl groups (Figure 1a). By grafting the RAFT agent onto the CM, the contact angle slightly increased to 20O±6, the long alkyl chains of these RAFT agents were considered the reason for this change.

Figure 1. WCA of measurements of a) cellulose membrane (super hydrophilic) and b) block copolymer of 3-(trimethoxysilyl)propyl acrylate –block- myrcene grafted on CM. Photographs of water droplets on the surface of CM and modified CM (d) and (dyed with rhodamine B, methyl blue and CuSO4, respectively).

When introducing silane onto the surface, the contact angle rose from 20O±6 to 140O±5. Further grafting of myrcene onto the surface resulted in an increase in the contact angle to 160O±4 (see Figure 1b). This suggested that the block co-polymer was successfully polymerized onto the cellulose surface.


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Scanning electronic microscopy (SEM) examination showed that the cellulose fibers with silane polymerization formed small micro-hair-like structures on the fibers, as portrayed in Figure S2. After polymerizing with myrcene, more particles were aggregated and the cellulose fiber surface was completely covered (Figure 2d). As silane and myrcene polymers are hydrophobic in nature, the block co-polymers were aggregated on the whole fiber surface, as observed in Figures 2c and Figure 2d.

Figure 2. Effect of a Block-copolymer treatment on the microstructural features of cellulose modified membrane. (a and b) SEM image of dehydrated dermis of CM and (c-d) morphology of block copolymer of 3-(trimethoxysilyl)propyl acrylate –block- myrcene and inserted micrograms were high magnification resolution. Chemical Composition


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The chemical characterization of the modified CM was carried out with two powerful spectroscopic techniques, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XPS verified the elemental composition of the modified membrane. Survey scan and highresolution XPS elemental scans of C 1s, O 1s, Si 2p and N 1s were reported in Figure 3b. Recorded signals were in line with the elemental composition of the modified CM and were fit employing different model Gaussian/Lorentzian curves corresponding to the types of bonds present in the membrane.

Figure 3. Elemental composition was identified by XPS and Raman spectra of modified cellulose membrane. a) survey spectrum of XPS and b) C 1s, O 1s, Si 2p and N 1s of the modified membrane. c) Raman shift spectra for 3-(trimethoxysilyl)propyl acrylate –block-


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myrcene. Functional groups derived from the incorporation of silane and myrcene fibers are patent by bands at 1100 cm-1. d) Raman mapping of the vibrational mode at 1100 cm-1 of block co-polymer. In the high-resolution C 1s scans, the carbon atom of the carboxyl group was clearly distinguished at 288.5 eV relative to the aliphatic carbon atoms. The silane peak, present at 532 eV can be attributed to the SiOx/C-O linkage. The binding energies at 286 and 285 eV confirmed C-CO- /C-N and C-C/CH bonds of the RAFT copolymer, respectively. The N 1s peak indicated a main component at 400.0 eV, which was attributed to the amide linkage, see Figure 3b. We then performed Raman spectroscopy analysis to confirm elemental composition. Grafted myrcene and silane polymer membrane was characterized by contact angle measurements to validate the block co-polymerization process. This allowed us to identify the chemical structure of the synthesized sample and to map the distribution of a specific species over the examined surface area. From the acquired spectra, we confirmed the presence of Si–O–Si stretching at 1020 cm−1 (Figure 3c) along with pol(myrcene). Figure 3d features the spatial distribution of the band centered at 1025 cm-1, which is relatively uniform and explains the silane and myrcene moiety over the mapped area (see, Figure 3d). Absorption of oil and organic solvents Functionalized strips of CM selectively collected vegetable oil from an oil/water mixture (Figure 4a), whereas untreated CM absorbed water (Figure S4a). To verify the selectivity from the absorption water phase, we placed a modified CM sample in an aqueous phase for 10 min (figure 4b). In this case, we did not observe any staining of the samples (see Supporting Information, Figure S4b, S4c and SV1). The absorptive capacity of the modified CM was evaluated by measuring the weight increase after immersion in water/oil mixtures of different commercially


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available oils and solvents. In general, a favorable level of absorption was observed for all samples (Figure 4a). In particular, maximum absorption was achieved in the case of crude oil (20 ±5 G) and silicone oil (16 ±3 G), whereas minimum absorption was recorded for hexane and PETether (5± 3 G). To improve their absorptive potency, we cut the CM strips into small pieces (∼2−4 mm x 2 mm) and exposed them to the water/oil mixture (see Supporting Information, Figure S5).


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Figure 4. Absorption capacity of the modified CM and schematic view of the reutilization cycle for various organic oils. a) Absorption capacity for six different organic oils, for selective organic solvent absorption. b) Vegetable olive oil stained with Oil Red was used for this graphical description. Water/oil mixture is prepared (A) and a modified CM strip is placed in the mixture. After complete oil absorption, the modified CM is immersed in hexane in which a solid−liquid extraction occurred (B). Hexane is allowed to evaporate and dry and the modified CM can be used again for selective oil absorption. c) photograph of extracted oil, absorption/extraction cycles of crude oil (dark color) and red dye oil (red color). d) The recycled absorption of the modified CM for n-hexane. The recycled immersion of the modified CM with crude oil (black line) and red dye oil (red line). The cut-up modified CM proved to absorb a greater volume of oil, and this was the case for all mixtures tested. We also addressed modified CM reusability by performing a series of oil absorption/extraction cycles. Figures 4b depict the scheme for modified CM regeneration. In brief, CM absorbed the contaminant agent in the form of red-stained vegetable oil (Figure 4b). The oil was then extracted with hexane. Performing three sequential extractions of 5 min each was sufficient to remove the absorbed oil phase from the CM. The absorption/extraction cycles were repeated several times (5 to 10). This procedure was followed in the same manner with two oil systems - crude oil, which was collected from the Alberta oil reservoir, and another red dye oil system. The absorption/extraction cycles were repeated several times (~5), and the absorption capabilities of the modified CM were not diminished by the hexane extraction. Through evaluating the contact angle and the Raman spectrum, we were able to confirm that the block copolymers were grafted onto the surface and that the contact angle was ~160O, which was the same as the original. In fact, the proportion of absorption of crude oil was higher compared to the


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red dye oil - more than 20 ±5 G of weight gain was observed with the crude oil, compared to the up to 15 ±3 G weight gain observed with the red dye oil. (see Figure 4). Many inspiring research work on nanoporous bio based membrane have been published about the separation oil from an oil/water mixture.34-37 Superhydrophobic surface material adsorbs oil and penetrate greatly via pores on the nanomaterial whereas water phase is respelled, this is the phenomenon for designing the nanomaterial for oil extraction. Using this phenomenon many materials were modified on surface or coating on bio surface.30-32 Particularly, coating or sprayed surface materials lack in long-term stability and reusability of the membrane, but covalently grafted hydrophobic polymer on a cellulose membrane is more desired for this application. Additionally, compared to the reported adsorbents made from nanocellulose fibers exhibited low density and also absorption capacity for various oils and organic solvents, arising from the good dispersion and stable suspension of nanocellulose (in mg scale).37-39 In the present work, modified CM has good oil absorption, particularly high crude oil absorption capacity and more importantly it is reusable for multiple times without losing the chemical stability of the membrane. Therefore, we foreseen that this nanomaterial would be promising green material for oil/water extraction field. Filtration of crude oil/water mixtures and their separation Polymer functionalized CM was cut into circles of ~2 cm diameter and utilized in the custommade filtration flask as portrayed in Figure 5. Crude oil, obtained from the Alberta oil reservoir (contains 15 ml of oil, 7 ml of water and 600 mg of nano/micro particles), was directly filtered using these membranes to remove both the particles and oil from the water.


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This demonstrated that the modified CM can be used not only for oil/water extractions, but also for oil purification. When a crude oil was poured onto the membrane, oil selectively permeated through the mesh quickly and flowed into the flask below, while the particles were inhibited on the membrane surface. The separation process was driven solely by gravity, as shown in Figure 5a (see Supporting Videos SV3). Almost no water can be observed in the collected oil, showing efficient separation behavior. The flux rate of solo hexane was much faster compare to crude oil (Figure 5c), but when it diluted with hexane the flow rate was much better (see Supporting Video SV2).

Figure 5. a) Photograph of crude oil filtration using modified CM. Crude oil/water mixture containg inorganic/organic particles was filtered through a gravity based filtration set up. b) the


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snaps of water mixture particles (dark yellow) and filtered oil in hexane (light yellow), respectively. c) Infuse flux for various mixtures of modified CM. d) and e) after separation modified CM stripes display the before (white) after (dark brown) filtration of crude oil and their particles. Filtration through the filters yields purified oil. Inset: WCA of modified CM before and after separation. We then re-analyzed the water contact angle to confirm the stability of the grafted block copolymers, which were shown to be well-attached and not cleaved from the surface. In addition, the separation process of modified CM can be recycled stably. After each cycle, the surface WCA no changes were observed, demonstrating that modified CM possess excellent reusability. The filtration setup and contact angle before and after filtration is presented in Figure 5 and Supporting Video SV2 and SV3. Antifouling properties Performance evaluation with respect to myrcene reactivity was carried out by passing through the CM strong odorous chemicals to facilitate thiol chemistry with myrcene. As thiol is one of the most odorous materials, it is a real challenge to filter thiols within an open atmosphere. Here, we prepared a 0.3 mmol solution of mercaptoethanol in ethanol, was filtered through the modified CM. We found that, following filtration, the classical thiol odor was significantly reduced, see figure S5 (Raman spectra). This could be because the thiols were cleaved by air and generated radicals through the double bond of myrcene.40 Interestingly, these materials also bestowed a pleasant odor upon the water. The fibrillar nature of cellulose is not disrupted by filtration and the particles were heavily retained, even at the reticular dermis level with a distinct scent. Furthermore, these modified CMs were completely biodegradable and therefore offer


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sustainability in terms of waste water cleaning. In this case, the membranes were rinsed in acetone for extracting oil, dye, crude oil, etc. Interestingly, the presence of solubilized salts in water did not affect purification efficiency, which could enable effective filtration of sea water, and this process may be supplemented with oils and nano/micro particles. Conclusions In this work, we developed a new material for selective use in the separation of oil/water emulsions. It is produced using a simple and low-cost chemical procedure with substrates in the form of cellulose filter membrane (CM) that is very common across most laboratories. The attractive properties of CM include natural abundance, little to no toxicity, stability under different testing conditions, easy processing, and ease of disposal. Biodegradability is also a significant characteristic of CM, which makes it an eco-friendly separation medium versus conventional materials and methods. The chemical modification involved the block copolymerization of 3-(trimethoxysilyl)propyl acrylate –block- myrcene via the RAFT approach. Absorption testing with several organic contaminants (oils and solvents) exhibited high levels of extractive activity as well as promising reusability. Additionally, the material naturally possesses a fibrillar structure that fosters simultaneous capture of oils and particulate solids. Furthermore, the fragrance of myrcene was capable of blocking the foul odor of thiol molecules, leaving the water with a pleasant scent. This compendium of properties makes this substrate a very promising, versatile, and efficient alternative to current absorbers for capturing organic waste, anti-fouling purposes, oil separation and microparticulate contaminant purification systems. ASSOCIATED CONTENT The Supporting Information is available free of charge http://pubs.acs.org/.


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The characterization WCA, DSC, Oil absorption and Antifouling characterization. Movie SV1 shows the water repels with modified CM. Movie SV2 shows the oil permeability through modified CM and water prohibition. Movie of SV3 shows the water/oil mixture separation.

AUTHOR INFORMATION Corresponding Author * [email protected] and [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the Province of Alberta and Alberta Ingenuity Technology Futures (AITF). ACKNOWLEDGMENT The authors would like to acknowledge the National Institute for Nanotechnology and Nanofab of the University of Alberta for equipment and instrumentation. This work was supported by the Province of Alberta, Alberta Innovates Technology Futures (AITF) and the National Institute for Nanotechnology. The authors would like to acknowledge the National Institute for Nanotechnology and Nanofab of the University of Alberta for the equipment and instruments.

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TOC graphic.


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