Chitosan-Based Aerogel Membrane for Robust Oil-in-Water Emulsion

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Chitosan-Based Aerogel Membrane for Robust Oil-in-Water Emulsion Separation Jai Prakash Chaudhary, Nilesh Vadodariya, Nataraj Sanna Kotrappanavar, and Ramavatar Meena ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08705 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 20, 2015

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ACS Applied Materials & Interfaces

Chitosan-Based Aerogel Membrane for Robust Oil-in-Water Emulsion Separation Jai Prakash Chaudhary,a,c Nilesh Vadodariya,a,c Sanna Kotrappanavar Nataraj,b,c* Ramavatar Meenaa,c* a

Scale-Up & Process Engineering Unit Discipline, CSIR-Central Salt & Marine Chemicals Research Institute, G. B. Marg, Bhavnagar-364002, Gujarat, India. b RO-Division, CSIR-Central Salt & Marine Chemicals Research Institute, G. B. Marg, Bhavnagar-364002, Gujarat, India. c AcSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar-364002, Gujarat, India E-mail: [email protected]; [email protected] Fax: +91-278-2567562; Tel: +91-278-2567760 E-mail: [email protected], [email protected] KEYWORDS. Biopolymer, emulsion, wastewater, aerogel membrane, oil-water separation ABSTRACT: Here we demonstrate, direct recovery of water from stable emulsion waste using aerogel membrane. Chitosan based gel was transformed into highly porous aerogel membrane using bio-origin genipin as crosslinking agent. Aerogel membranes were characterized for their morphology using SEM, chemical composition by FTIR and solid-UV. Further, aerogel was tested for recovery of high quality water from oil spill sample collected from ship breaking yard. High quality (with >99% purity) water recovered with flux rate of >600 L.m-2.h-1.bar-1. After repeated use aerogel membranes were tested for greener disposal possibilities by biodegrading membrane in soil.

INTRODUCTION Oil-in-water or water-in-oil emulsions are stable liquid/liquid systems which cause serious environmental problem in absence of proper separation techniques. 1-3 Oil-water emulsions generally classified in to 3 categories depending on their stability namely, loose, medium and tight emulsions. Loose and medium emulsions can be easily phase separated. However, a tight emulsion cause serious problems and requires proper demulsification agent or method to break the emulsion. Large quantities of industrial emulsion wastes discharged to water bodies pose greater threat to aquatic life in particular causing rapid increase in the chemical oxygen demand (COD) and biological oxygen demand (BOD).4 Demulsifiers have been popular choice among many to safely separate oil-water emulsions. But in the recent past, there have been several attempts made to selectively separate water from oil or oil from water using different materials.5-7 Membrane based processes like reverse osmosis (RO) and ultrafiltration (UF) in combination with demulsifiers have been tested under different conditions for removal of oil from emulsion wastewater. 8-10 In the last two years, more advanced materials have emerged in different form like aerogels,11,12 foam membranes,13 polysaccharide agents,14 surface modified fabrics and inorganic meshes for successful separation of oil/water mixtures. 15-17 Unique 3D network of hydrophilic aerogels preferably select water over oil, similarly transforming surface to hydrophobic, oil was preferably separated from water. These new class of materials have shown excellent separation properties because of their large surface area, high porosity and can be easily custom made to fit the final application.18 However, bio-based aerogels owing distinctive features like sustainability and biodegradable aspects in addition to super-hydrophilicity and high surface area make them better choice for oil-water emulsion

separation. Therefore, present study explores use of highly porous polysaccharide chitosan based aerogel membrane for recovering water from oil-spill and stable emulsions. Macroporous aerogel was prepared using agarose and chitosan mixtures. Here agarose used as both pore formation agent as well as surface coating on highly cross-linked chitosan network. This unique feature helps in robust selection of water from stable emulsions. In our previous study, 13 we demonstrated gelatin as minor constituent for preparing superhydrophilic aerogel membrane, but for sustainable and large scale applications stable aerogel filter is vital. Therefore, here highly cross-linked chitosan acts as support network along with inducing hydrophilicity to aerogel. As prepared membrane was tested for selective water separation from bio-diesel/water emulsion, crude vegetable oil/water emulsion and highly contaminated oil spill wastewater. For sustainable applications, membranes were tested under crossflow filtration mode. In all cases, permeate water purity was >99 % at high water flux >600 L.m-2.h-1. EXPERIMENTAL SECTION Materials Hydrophilic polysaccharide agarose was extracted from red seaweed Gracilaria dura following the method reported in literature.19,20 Chitosan was purchased from Sigma Aldrich and Genipin was purchased from Challenge Bioproducts Co. Ltd. (Taiwan). And all other chemicals were used as received without further purification. Biodiesel and crude oil were procured from our institute pilot plant, oil-spill collected from ship breaking yard, Alang, Gujarat and used without any further purification. Further oil:water emulsions were also prepared in 20:80 v/v ratio of simply by vigorous stirring.

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Preparation of aerogel membrane Different compositions of membranes were prepared by changing the total polymer concentration ranging from 0.5 % to 2 % w/v keeping Agarose: Chitosan ratio constant (9: 1 w/w) in all formulations, and tested for their oil/water separation performances. Membranes were designated in abbreviation as follows, chitosan as CS, agarose as Agr, genipin as G, blend of chitosan-agarose as CS-Agr and genipin crosslinked aerogel as CS-Agr-G. To obtained different membranes 450 to 900 mg agarose was taken in separate beakers having 75 ml distilled water and solubilized by autoclaving it at 120 oC for 15 min. In another sets of beakers 50 to 100 mg chitosan was dissolved in 25 ml of 0.05M acetic acid (Figure 1). Then chitosan solution was added to the viscous agarose solution under vigorous stirring conditions for 10 min at 80 oC followed by addition of genipin (10-40 mg in 0.5 ml methanol) with continuous stirring at 80 oC and gradually cooled to room temperature to form hydrogel. After 5-10 min, the colour of whole solution starts changing from transparent solution to blue colour due to the cross linking and resulting hydrogel was left for 2-3 days at room (25 oC) temperature for complete crosslinking. After that each gel was cut to 0.4 mm thick slices and lyophilized to obtained aerogel samples for separation applications.13 The best result was obtained with aerogel obtained with 1 % w/v of total polymer concentration (Agr: CS = 9: 1 w/w), and was considered optimum polymer concentration in this study. Flux (J) of permeate and % rejection oil in permeate was calculated as method reported in our previous paper.13 Methods Agarose solutions were prepared by Autoclaving it using Autoclave ES-315 (TOMY SEIKO Co., Ltd, Japan). Further, cross-linked gel were subjected to Lyophilisation using VirTis Benchtop, Freeze dryer, United States for getting final aerogel membranes. FTIR spectra were recorded on a Perkin-Elmer design instrument (Spectrum GX, USA). The aerogels membrane were analysed for their surface morphology and pore characteristics for both control and cross-linked chitosan membranes using scanning electron microscopy (SEM) on a Carl-Zeiss Leo VP 1430 instrument (Oxford INCA). Thermogravimetric analysis (TGA) was carried out using Mettler Toledo Thermal Analyzer, (TGA/SDTA 851e, Switzerland). TGA was carried out using 6 mg of each sample under N2 atmosphere with a heating rate of 10o/min. The solid state UV-vis spectra were measured using Shimadzu UV3101PC spectrophotometer (JAPAN). 13C CP MASS spectra of each constituent were recorded on a Bruker Avance-II 500 (Ultra Shield, Switzerland) spectrometer under ambient condition. Membrane testing The aerogel membranes were tested for their separation performances using simple funnel with membrane sitting in neck to selectively passage water. Initially 2 different kind of emulsions namely, crude bio-diesel/water and oil spill wastewater were tested. No dye or artificial colour was used to distinguish feed samples (either to water or to oil). As emulsions were stable before pouring to container funnel and time was recorded to separate water from 20:80 (v/v) oil/water emulsion. Subsequently, both permeation rate and purity was evaluated. To evaluate compression breaking, recyclability and large scale continuous operations, membranes were tested in crossflow conditions. Membranes were fitted in crossflow membrane testing unit comprising of hollow chamber. Emul-

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sion feed was continuously circulated using booster pump (KEMFLOW) with nominal flow rate of 1.8 LPM capable of maintaining pressure between 0–10 bar. Restricting needle valves were provided in the membrane kit to control the flow rates. The permeate purity (% rejection of oil) and flux for aerogel membrane were calculated using following equations;

𝐽𝑛 = 𝐴(𝑡

𝑉

0 −𝑡𝑛 )

…………………………..(1)

Where, Jn is flux (L.m-2.h-1), V is volume of permeate, A is effective area of the membrane and to at zero time and at interval n, respectively. Considering water as rich phase in permeate, infrared (IR) spectroscopy has been used to quantify the amount of oil diffused with permeate water. Prior to permeate sample analysis, we calibrated standard curve for different concentrations of oil-in-water. Six standard solutions over the range of 1 to 100 mg.L-1 oil-in-water were prepared for stable emulsion using sonication bath. Further, these samples were subjected to FTIR analysis. Established calibration range fitted well with linearity and accuracy were observed with a correlation coefficient (R2=0.99325) and a standard error of 0.2143 mg/mL was obtained. Therefore, % rejection of oil was calculated using equation;

%𝑅 =(

∁𝑓 − ∁𝑝 ∁𝑝

) x100……..…………………(2)

Where, Cf and Cp are the concentration of feed and permeate solutions, respectively. Results and Discussions

Figure 1. Schematics of preparing chitosan-based aerogel membrane (a) control, (b) genipin cross-linked chitosan aerogel and (c) genipin-chitosan cross-linked chemical structure with inner walls of CS linked with agarose in H-bonding.

The preparation procedure for superhydrophilic agarose inner wall coated CS aerogel membrane has shown sche-


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matically in Figure 1. Chitosan is one of the abundant natural resource extracted from the shells of shrimp, lobster, and crabs. CS is also fibrous in nature that can be used in different forms upon chemical and physical modification, and can be chemically cross-linked using –NH2 functionality of CS. Further, CS can also be transformed in to a stable scaffold-like structure by controlling degree of crosslinking. To make large pore size aerogel membrane, agarose used as gelling agent which also helped in creating highly porous aerogel membrane. Interestingly, agarose also played in enhancing hydrophilic property by interacting with chitosan through Hbonding during lyophilisation. Proposed structure in Figure 1(c) shows genipin readily crosslinks chitosan at 80 oC, while agarose undergoes H-bonding interaction with –OH of chitosan making it stable gel at room temperature. To confirm CS crosslinking and CS-Agr interactions in aerogel membrane FTIR and solid-UV measurements on both control CS-Agr and genipin cross-linked CS-Agr were analysed. In all cases, IR spectrum of individual constitutes CS, Agr and genipin were also recorded to highlight changes.

Figure 2. (a) FTIR spectra of agarose-chitosan (Agr-CS-G) aerogel crosslinked using genipin, and (b) Solid UV spectra of pristine and cross-linked CS-Agarose aerogel membranes and their precursors, (c&d) SEM images of different magnifications Genipin cross-linked chitosan-agarose aerogel membrane, and (e) schematics of proposed porous aerogel membrane formation mechanism.

Figure 2(a) shows FTIR analysis of all constituents where agarose exhibited characteristics peaks at 932 cm-1 (due to 3, 6-anhydrogalactose linkage), 1160 and 1076 cm-1.19,20 Chitosan exhibited characteristic stretching vibrations at 1645 cm-1 (C=O stretching vibration), and additional peaks between 1000-1100 cm-1 were attributed to C-O and C-N stretching vibrations. Genipin exhibited characteristics band at 1443 cm−1 was assigned to a ring stretching mode in the genipin molecule. While appearance of new band at 1415 cm-1 in product (Agr-CS-G) after genipin crosslinking indicated presence of ring stretching of heterocyclic amine. The shoulder at 1641 cm-1 represents the stretching of C=O also appeared in

the product.21 Presence of additional characteristics agarose and chitosan peaks in FTIR spectrum of crosslinked CS-AgrG further confirms that the main backbone of pristine agarose and chitosan remained intact during modification. The main noticeable change appeared in the shift of broader Agr stretching peak (-OH) at ~3438 cm-1 upon blending with CS (~3435 cm-1) to ~3400 cm-1 in CS-Agr. This remained unchanged upon genipin crosslinking in CS-Agr-G. Therefore, hydroxyl (-OH) groups present in agarose make hydrogen bonding interaction with N lone pair of the amide group of chitosan resulting in columnar structure in which CS walls are coated with Agr. Crosslinking of CS was further confirmed using solid UV spectroscopy as shown in Figure 2(b). Pristine genipin exhibits sharp characteristic peak at 240 nm, whereas none of the control CS, Agr or blend CS-Agr shown any recognizable peaks. But, when genipin added to blend gel (CS-Agr-G), spectra shift sharply for characteristic genipin peak to 282 nm with much less intensity. During the process, a significant change also happens with the appearance of additional peak at 600 nm. This confirms extended conjugation of genipin crosslinking in CS matrix which induces dark green colour to aerogel membrane.13,22 Figure 2 (c&d) shows SEM morphologies of large pore size CS-based aerogel membrane at different magnifications, which clearly indicateds pore size distribution was in macroporous range of 40-50 µm. Close view of SEM images reveals that lyophilisation process induced well-ordered pattern to CS membrane. Formation of large column-like pore structure may also be attributed to uniform size CS polymer chains which trapped agarose gel mass in it. Thermogravimetric (TGA) was used to determine nature of membrane transformation and their thermal stability. TGA results (see, Figure S2(a-d)) of the blend (Agr-CS) as well as genipin crosslinked blend (Agr-CS-G) prepared in the presence of crosslinking agent showed the high thermal stability in comparison to pristine constituents. The minimum residual mass of 20.53%, 25.05% and ~31.21% was obtained for Agr, CS and CS-Agr blends, respectively. However, genipin crosslinked blend (Agr-CS-G) retained as high as 41.08% residual mass at 599.5oC. Therefore, it directly implies the rigid network as a result of genipin crosslinking in aerogel membranes. As shown mass loss was higher in control polymers and was maximum for agarose. The blend Agr-CS shows less mass loss compared to control polymers indicates network stability may be due to the hydrogel bonding between Agr– OH and lone pair –NH2 of CS. The crosslinked blend shows the lowest mass loss and greatest thermal stability may be the result of the formation of strong covalent and hydrogen bonding during this process. Based on FTIR, solid UV and SEM analytical evidence it can be assumed that CS-Agr gel formation is robust and following mechanism can be proposed for aerogel membrane formation as shown in Figure 2(e). Firstly, when CS and Agr mixed together limited interaction of CS with Agr forms island clusters where mass of agarose covered with CS polymer boundaries gel. Further, when crosslinking agent genipin added to CS-Agr mixture inter and intrachain crosslinking of CS initiates forming dense column-like walls holding Agr gel mass inside. When gel undergoes lyophilisation water trapped in agarose gets crystalizes pushing agarose to CS walls. So, it leads to confirm that super hydrophilic CS with macropore in which Agr thin-layer surrounded by stable CS walls with inter-wall crosslinking used for selective separation from oil-water emulsions.


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Aerogel membranes further characterized for their thermal stability and swelling properties both in pure water and oil/water emulsion to determine best membrane composition. Membrane with composition of 0.04 wt% crosslinking agent genipin showed moderate swelling (~60 %) in both pure water and emulsion (see Figure. S1). This independent swelling behavior for different feed condition is an encouraging property as it indicates affinity of membrane surface for selective absorption and subsequent permeation of water. Pre-wetted membrane of 2.0 cm diameter was fixed in the neck of a funnel to make it filtration set-up. Figure 3(a) shows the membrane used to filter biodiesel emulsion and highly contaminated oil-spill sample (Figure 3b&c). Time dependent selective separation of water from oil was evaluated. Figure 3(b1&b2) represent biodiesel emulsion before and after oil water separation at the rate of 213 L.m-2.h-1. Similarly, Figure 3(c1&c2) shows the separation pattern of oil-spill, interestingly much faster rate of 284 L.m-2.h-1. Quality of permeate water analyzed for separation efficiency using FTIR measurements. Figures 3(d&e) shows characterization of feed stable emulsion and permeate and pure water along with control pure oil for both biodiesel and oil-spill, respectively.

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oils, respectively. Whereas, in both permeates all significant peaks disappeared. The % rejection of oil was calculated using standard plot for different concentration oil-in-water emulsion, which for both case noted to be >99%.

Figure 4. (a&b) crossflow membrane testing unit used for Crude biodiesel-based emulsion separation where (c) schematic depicts membrane and permeate chamber. (d) FTIR analysis of emulsion feed and permeates collected at different time intervals in a long term run, and (e) give % oil rejection and flux (L.m-2.h-1.bar-1) trend for different oil/water emulsions.

Further, large scale continuous flow test is an essential to evaluate membrane stability and practical application. Crossflow membrane testing unit with feed chamber and specially made membrane/permeate chamber shown schematically in Figure 4(a,b&c), respectively. Unlike many reported articles, we tested CS-aerogel membranes vigorously under crossflow module for 8 hours collecting permeate samples in regular time interval for analysis both emulsions (crude biodiesel and oil-spill) were tested. Figure 4(d) gives FTIR analysis of feed and permeates at different time intervals, which reveals significant rejection of oil in permeate. It is evident that over 650 L.m-2.h-1.bar-1 fluxes yielded with ~99 % pure water Figure 4(e). It is also important that the flux and rejections were consistent for several hour of continuous and repeated run. One of the advantages using CS-aerogel membrane is post emulsion separation, membrane surface can be easily regenerated by simple washing. Membrane were also tested vertically to examine the fouling and extent of membrane deformation Figure 5(a). Extent of membrane deformation under crossflow pump pressure is evident from Figure 5(b) before and after test run. SEM images in Figure 5(c&d) also reveals the extent of deformation clearly. Under feed flow pressure large pores were seem completely collapsed.

Figure 3. (a) Photograph of coin size aerogel membrane used to separate (b) bio-diesel/water emulsion (c) oil-spill wastewater emulsion collected from ship breaking yard (b1 & b2) biodiesel/water emulsion before and after separation and (c1 & c2) oil-spill wastewater emulsion before and after separation. (d) & (e) gives FTIR analysis of feed emulsion and pure oil samples (& received oil spill) and permeates water characterized for their purity.

Emulsion and pure oil samples have characteristic peaks at 1745 and 2930 cm-1 for C=O of esters and C-H stretching of

However, membranes regains significant physical characteristics after surface regeneration process by washing in DI water (Figure S4(a)), which was further used repeatedly. Surface regenerated membrane retained substantial flux of water from contaminated emulsion (Figure S4(b)). For being sustainable and green, material after use should undergo biodegradation. We tested CS-based aerogel membrane after several cycle of use, keeping used membrane in soil for natural degradation. Figure 5(e) gives photographs of aerogel membrane undergoing biodegradation at different time interval. Under normal soil conditions membrane biodegraded considerably in 25


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days’ time. Further, after 35 days of observation membrane lost 60-70% original mass. For complete green process it is significant that using bio-origin to biodegradation a material completes the life cycle.

Acknowledgements CSIR-CSMCRI Communication No. 159/2015. SKN gratefully acknowledges the DST, Government of India for the DST-INSPIRE Fellowship and Research Grant (IFA12-CH-84). RM, JPC and Nilesh gratefully acknowledge DST (SB/EMEQ-052/2013) and CSIR, New Delhi, Government of India for financial support (CSC0130). Notes and references

Figure 5. (a) crude oil (Biodiesel) emulsions was tested in different crossflow filtration mode to check the extent of membrane deformation and fouling, (b) photograph give before and after filtration and washing of membrane tested in crossflow mode, (c&d) gives corresponding SEM images of membrane before and after filtration and (e) CS-aerogel membranes subjected to biodegradation in soil and recorded for its degradation process.

Conclusions In summary, present study demonstrates that microporous aerogel membranes have several advantageous properties with respect to their use in separation oil-water emulsions. The attractive properties of aerogel membranes include natural abundance, less-to-no toxicity, and stable under different testing conditions, easy to process and dispose. Biodegradability factor is a significant characteristic of the aerogel membrane which makes it eco-friendly separation medium in comparison to conventional materials and methods. Over 600 L.m-2.h-1.bar-1 with ~99 % pure water is a promising feature of our microporous membrane. Aerogel membrane also works in an advanced crossflow configuration which opens new avenue to faster water reclamation process from large industrial streams. One of the prospective focus using aerogel membrane is to reclaim water from oil or gas exploration operations. On the other hand, oil-water emulsion wastes and oil sludge are easy to process through with improved rate of dewatering process. Therefore, water recovery using continuous filtration process using bio-based membranes is an economical and sustainable solution.

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Chitosan-Based Aerogel Membrane for Robust Oil-in-Water Emulsion Separation Jai Prakash Chaudhary,a,c Nilesh Vadodariya,a,c Sanna Kotrappanavar Nataraj,b,c* Ramavatar Meenaa,c*


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Chitosan-Based Aerogel Membrane for Robust Oil-in-Water Emulsion

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