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

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 seri

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 aflux 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 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 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.8−10In

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

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, 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,13we 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

www.acsami.org

mode In all cases, permeate water purity was >99% at high

waterflux >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

puri fication 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 Di fferent compositions of

membranes were prepared by changing the total polymer

concen-tration 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 Agr and genipin cross-linked aerogel as

CS-Agr-G To obtain di fferent 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 min 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

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

cross-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, cross-linked 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) Thermogravi-metric 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 N2atmosphere 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 2 different kind of emulsions, namely, crude biodiesel/water and oil spill wastewater were tested No dye or arti ficial 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

=

−

J V

A t(0 t n) (1) where J is flux (L·m −2 ·h −1 ), V is volume of permeate, A is effective area

of the membrane and at zero time, t 0 , and at interval n, t n , respectively Considering water as rich phase in permeate, infrared (IR) spectroscopy has been used to quantify the amount of oil di ffused 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 (R 2 = 0.99325) and a standard error of 0.2143 mg/mL was obtained Therefore, percent rejection of oil was calculated using

⎛

⎝

⎠

⎟⎟

R C C C

where Cfand Cpare the concentration of feed and permeate solutions, respectively.

■ RESULTS AND DISCUSSIONS The preparation procedure for superhydrophilic agarose inner wall coated CS aerogel membrane is shown schematically in Figure 1 Chitosan is one of the abundant natural resource extracted from the shells of shrimp, lobster, and crabs CS is alsofibrous in nature that can be used in different forms upon chemical and physical modification, and can be chemically cross-linked using−NH2functionality 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

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.

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

H-bonding during lyophilization Proposed structure inFigure 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-anhydrogalactose linkage), 1160, and 1076 cm−1.19,20Chitosan

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−1representing 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−1upon blending with

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 (seeFigure 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 cross-linking 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 −NH2of 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 (seeFigure S1) This independent swelling behavior for different feed condition is an encouraging sign as it

Figure 2 (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 di fferent magnifications of genipin cross-linked

chitosan −agarose aerogel membrane, and (e) schematics of proposed

porous aerogel membrane formation mechanism.

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 itfiltration setup.Figure 3a 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 3b1,

b2 represent biodiesel emulsion before and after oil−water

separation at the rate of 213 L m−2h−1 Similarly,Figure 3c1, c2

shows the separation pattern of oil-spill, interestingly with

much faster rate of 284 L m−2h−1 Quality of permeate water

was analyzed for separation efficiency using FTIR

measure-ments 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−1for 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%

Further, large scale continuous flow test is essential to evaluate membrane stability and practical application Cross-flow 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

tested CS−aerogel membranes vigorously under crossflow module for 8 h collecting permeate samples in regular time interval for analysis of both emulsions (crude biodiesel and oil-spill).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−2h−1bar−1fluxes yielded∼99% pure waterFigure 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 deformationFigure 5a Extent of membrane deformation under crossflow pump pressure is evident fromFigure 5b before and after test run SEM images inFigure 5c, d also reveals the extent

of deformation clearly Under feed flow pressure large pores were seem completely collapsed

However, membranes regains significant physical character-istics after surface regeneration process by washing in DI water

regenerated membrane retained substantialflux 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 consid-erably in 25 days’ time Further, after 35 days of observation membrane lost 60−70% original mass For complete green

Figure 3 (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.

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.

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 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 biobased membranes is an economical and sustainable

solution

■ ASSOCIATED CONTENT

*S Supporting Information

The Supporting Information is available free of charge on the

ACS Publications websiteat DOI:10.1021/acsami.5b08705

Optimization of swelling, TG and DTG curves, membrane surface regeneration, and possible interaction

of agarose with genipin (PDF)

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 competingfinancial interest

S.K.N gratefully acknowledges the DST, Government of India for the DST-INSPIRE Fellowship and Research Grant (IFA12-CH-84) R.M., J.P.C., and N.V gratefully acknowledge DST (SB/EMEQ-052/2013) and CSIR, New Delhi, Government of India forfinancial support (CSC0130)

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