Chaudhery 2015 chitosan based aerogel membrane for robust oil in water emulsion separation

Research Article www.acsami.org Chitosan-Based Aerogel Membrane for Robust Oil-in-Water Emulsion Separation Jai Prakash Chaudhary,†,§ Nilesh Vadodariya,†,§ Sanna Kotrappanavar Nataraj,*,‡,§ and Ramavatar Meena*,†,§ † Process Design and Engineering Division, CSIR-Central Salt & Marine Chemicals Research Institute, G B Marg, Bhavnagar-364002, Gujarat India ‡ RO Membrane Division, CSIR-Central Salt & Marine Chemicals Research Institute, G B Marg, Bhavnagar-364002, Gujarat India § AcSIR-Central Salt and Marine Chemicals Research Institute, G B Marg, Bhavnagar-364002, Gujarat India S Supporting Information * 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 cross-linking 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 was recovered with a 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 KEYWORDS: biopolymer, emulsion, wastewater, aerogel membrane, oil−water separation ■ 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 are generally classified into 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 methods 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, advanced materials have emerged in different forms 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 leads to preferable selection of oil over water These new class of © 2015 American Chemical Society materials have shown excellent separation properties because of their large surface area, high porosity and can be easily custommade to fit the final application.18 However, owing to their distinctive features like sustainability & biodegradability in addition to superhydrophilicity and high surface area, bio-based aerogels are a better choice for oil−water emulsion separation Therefore, present study explores the 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 is used as pore forming 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 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 biodiesel/water emulsion, crude vegetable oil/water emulsion and highly contaminated oil spill wastewater For sustainable applications, membranes were tested under crossflow filtration Received: September 15, 2015 Accepted: October 20, 2015 Published: October 20, 2015 24957 DOI: 10.1021/acsami.5b08705 ACS Appl Mater Interfaces 2015, 7, 24957−24962 Research Article ACS Applied Materials & Interfaces linking After that, each gel was cut into 0.4 mm slices and lyophilized to obtain 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 Methods Agarose solutions were prepared by Autoclaving it using Autoclave ES-315 (TOMY SEIKO Co., Ltd., Japan) Further, crosslinked gel were subjected to Lyophilization using VirTis Benchtop, Freeze-dryer, United States for getting final aerogel membranes FTIR spectra were recorded on a PerkinElmer design instrument (Spectrum GX, USA) The aerogels membrane were analyzed 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 mg of each sample under N2 atmosphere with a heating rate of 10°/min The solid state UV−vis spectra were measured using Shimadzu UV-3101PC spectrophotometer (JAPAN) Membrane Testing The aerogel membranes were tested for their separation performances using simple funnel with membrane sitting in neck to selective passage of water Initially different kind of emulsions, namely, crude biodiesel/water and oil spill wastewater were tested No dye or artificial color was used to distinguish feed samples (either to water or to oil) As emulsions were stable, the initial time was recorded as soon as the oil/water emulsion was poured into the container funnel Subsequently, both permeation rate and purity were 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 Emulsion 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 flux for aerogel membrane were calculated as 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) All other chemicals were used as received without further purification Crude biodiesel oil was procured from our institute pilot plant, oil-spill collected from ship breaking yard, Alang, Gujarat and used without any further purification Further, emulsions were prepared in 20:80 v/v ratio of crude biodiesel:water by vigorous stirring 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 cross-linked aerogel as CSAgr-G To obtain different membranes, 450−900 mg of agarose was taken in separate beakers having 75 mL of distilled water and solubilized by autoclaving it at 120 °C for 15 In another set of beakers, 50−100 mg of chitosan was dissolved in 25 mL of 0.05 M acetic acid (Figure 1) Then chitosan solution was added to the viscous J= V A(t0 − tn) (1) −2 −1 where J is flux (L·m ·h ), V is volume of permeate, A is effective area of the membrane and at zero time, t0, and at interval n, tn, 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 1−100 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, percent rejection of oil was calculated using ⎛ Cf − C p ⎞ ⎟⎟ × 100 %R = ⎜⎜ ⎝ Cp ⎠ (2) where Cf and Cp are the concentration of feed and permeate solutions, respectively ■ Figure 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 RESULTS AND DISCUSSIONS The preparation procedure for superhydrophilic agarose inner wall coated CS aerogel membrane is shown schematically in Figure 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 cross-linking To make large pore size agarose solution under vigorous stirring conditions followed by addition of genipin (10−40 mg in 0.5 mL methanol) with continuous stirring at 80 °C and gradually cooled to room temperature to form hydrogel After 5−10 min, the color of whole solution starts changing from transparent to blue due to the cross-linking Resulting hydrogel was left for 2−3 days at room (25 °C) temperature for complete cross24958 DOI: 10.1021/acsami.5b08705 ACS Appl Mater Interfaces 2015, 7, 24957−24962 Research Article ACS Applied Materials & Interfaces CS (∼3435 cm−1) to ∼3400 cm−1 in CS-Agr This remained unchanged upon genipin cross-linking 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 Cross-linking of CS was further confirmed using solid UV spectroscopy as shown in Figure 2b 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 cross-linking in CS matrix which induces dark green color to aerogel membrane.13,22 Figure 2c, 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 lyophilization 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 analysis used to determine nature of membrane transformation and their thermal stability TGA results (see Figure S2a−d) of the blend (Agr−CS), as well as genipin cross-linked blend (Agr−CS−G) 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 cross-linked blend (Agr−CS−G) retained as high as 41.08% residual mass at 599.5 °C 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 indicating network stability which may be due to the hydrogen bonding between Agr−OH and lone pair of −NH2 of CS The cross-linked blend shows the lowest mass loss and greatest thermal stability which may be the result of the formation of strong covalent and hydrogen bonding during this process On the basis of 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 2e First, when CS and Agr were mixed together, limited interaction of CS with Agr forms island clusters where mass of agarose covered with CS polymer boundaries gel Further, when cross-linking agent genipin was added to CS−Agr mixture inter and intrachain cross-linking of CS initiates forming dense column-like walls holding Agr gel mass inside When gel undergoes lyophilization water trapped in agarose crystallizes pushing agarose to CS walls So, it leads to confirmation that super hydrophilic CS with macropore in which Agr thin-layer surrounded the stable CS walls with interwall cross-linking was used for selective separation of water from oil−water emulsions Aerogel membranes were 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 % cross-linking agent (genipin) showed moderate swelling (∼32%) in both pure water and emulsion (see Figure S1) This independent swelling behavior for different feed condition is an encouraging sign as it aerogel membrane, agarose was used as a gelling agent which also helped in creating highly porous aerogel membrane Interestingly, agarose also played a role in enhancing hydrophilic property by interacting with chitosan through Hbonding during lyophilization Proposed structure in Figure 1c shows genipin readily cross-links chitosan at 80 °C, while agarose undergoes H-bonding interaction with −OH of chitosan making it stable gel at room temperature To confirm CS cross-linking and CS−Agr interactions in aerogel membrane FTIR and solid-UV measurements on both control CS−Agr and genipin cross-linked CS−Agr were analyzed In all cases, IR spectrum of individual constitutes CS, Agr, and genipin were also recorded to the highlight changes Figure 2a shows FTIR analysis of all constituents where agarose exhibited characteristics peaks at 932 (due to 3,6- Figure (a) FTIR spectra of agarose−chitosan (Agr−CS−G) aerogel cross-linked 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 of genipin cross-linked chitosan−agarose aerogel membrane, and (e) schematics of proposed porous aerogel membrane formation mechanism 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 and 1100 cm−1 were attributed to C−O and C−N stretching vibrations Genipin exhibited characteristics band at 1443 cm−1 which is 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 cross-linking indicated presence of ring stretching of heterocyclic amine The shoulder at 1641 cm−1 representing CO stretch also appeared in the product.21 Presence of additional characteristics agarose and chitosan peaks in FTIR spectrum of cross-linked CS−Agr-−G further confirms that the 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 24959 DOI: 10.1021/acsami.5b08705 ACS Appl Mater Interfaces 2015, 7, 24957−24962 Research Article ACS Applied Materials & Interfaces Further, large scale continuous flow test is essential to evaluate membrane stability and practical application Crossflow membrane testing unit with feed chamber and specially made membrane/permeate chamber are shown schematically in Figure 4a−c, respectively Unlike many reported articles, we indicates the affinity of membrane surface for selective absorption and subsequent permeation of water Prewetted membrane of 2.0 cm diameter was fixed in the neck of a funnel to make it filtration setup Figure 3a shows the Figure (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 tested CS−aerogel membranes vigorously under crossflow module for h collecting permeate samples in regular time interval for analysis of both emulsions (crude biodiesel and oilspill) Figure 4d 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 ∼99% pure water Figure 4e 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 5a Extent of membrane deformation under crossflow pump pressure is evident from Figure 5b before and after test run SEM images in Figure 5c, d also reveals the extent of deformation clearly Under feed flow pressure large pores were seem completely collapsed However, membranes regains significant physical characteristics after surface regeneration process by washing in DI water (Figure S4a), which was further used repeatedly Surface regenerated membrane retained substantial flux of water from contaminated emulsion (Figure S4b) For being sustainable and green, material after use should undergo biodegradation We tested CS-based aerogel membranes for biodegradation after several cycles of use by keeping used membrane in soil for natural degradation Figure 5e gives photographs of aerogel membrane undergoing biodegradation at different time interval Under normal soil conditions membrane biodegraded considerably in 25 days’ time Further, after 35 days of observation membrane lost 60−70% original mass For complete green Figure (a) Photograph of coin size aerogel membrane used to separate (b) biodiesel/water emulsion (c) oil-spill wastewater emulsion collected from ship breaking yard (b1 and b2) biodiesel/ water emulsion before and after separation and (c1 and c2) oil-spill wastewater emulsion before and after separation (d, e) FTIR analysis of feed emulsion and pure oil samples (and received oil spill) and permeates water characterized for their purity 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 3b1, b2 represent biodiesel emulsion before and after oil−water separation at the rate of 213 L m−2 h−1 Similarly, Figure 3c1, c2 shows the separation pattern of oil-spill, interestingly with much faster rate of 284 L m−2 h−1 Quality of permeate water was analyzed for separation efficiency using FTIR measurements Figure 3d, e shows characterization of feed (stable emulsion) and permeate and pure water along with control pure oil for both biodiesel and oil-spill, respectively Emulsion and pure oil samples have characteristic peaks at 1745 and 2930 cm−1 for CO of esters and C−H stretching of oils, respectively Whereas, in both permeates all significant peaks disappeared The % rejection of oil was calculated using standard plot for different concentration of oil-in-water emulsion, which for both case noted to be >99% 24960 DOI: 10.1021/acsami.5b08705 ACS Appl Mater Interfaces 2015, 7, 24957−24962 Research Article ACS Applied Materials & Interfaces ■ Optimization of swelling, TG and DTG curves, membrane surface regeneration, and possible interaction of agarose with genipin (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: sknata@gmail.com; sknataraj@csmcri.org *E-mail: rmeena@csmcri.org; ramavatar73@yahoo.com Fax: +91-278-2567562 Tel: +91-278-2567760 Notes CSIR-CSMCRI Communication No 159/2015 The authors declare no competing financial interest ■ ACKNOWLEDGMENTS S.K.N gratefully acknowledges the DST, Government of India for the DST-INSPIRE Fellowship and Research Grant (IFA12CH-84) R.M., J.P.C., and N.V gratefully acknowledge DST (SB/EMEQ-052/2013) and CSIR, New Delhi, Government of India for financial support (CSC0130) ■ Figure (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 process it is significant that using bio-origin to biodegradation a material completes the life cycle ■ CONCLUSIONS In summary, present study demonstrates that macroporous 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 stability 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 macroporous 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 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Water-Soluble Fluorogenic alginic Acid Derivatives Carbohydr Res 2011, 346, 527− 533 24962 DOI: 10.1021/acsami.5b08705 ACS Appl Mater Interfaces 2015, 7, 24957−24962 ... 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... with macropore in which Agr thin-layer surrounded the stable CS walls with interwall cross-linking was used for selective separation of water from oil water emulsions Aerogel membranes were further... gelling agent which also helped in creating highly porous aerogel membrane Interestingly, agarose also played a role in enhancing hydrophilic property by interacting with chitosan through Hbonding