Nanostructured membranes for enzyme catalysis and green synthesis of nanoparticles

Article abstract of DOI:10.1002/cssc.201100211

Macroporous membranes functionalized with ionizable macromolecules provide promising applications in high capacity toxic metal capture, nanoparticle syntheses, and catalysis. Our low-pressure membrane approach has good reaction and separation selectivities, which are tunable by varying pH, ionic strength, or pressure. The sustainable green chemistry approach under ambient conditions and the evaluation of a reactive poly(acrylic acid) (PAA)-modified polyvinylidene fluoride (PVDF) membrane is described. Two distinct membrane types were obtained through different methods: 1) a stacked membrane through layer-by-layer assembly for the incorporation of enzymes (catalase and glucose oxidase), providing tunable product yields and 2) Fe/Pd nanoparticles for degradation of pollutants, obtained through an in situ green synthesis. Bioreactor-nanodomain interactions and mixed matrix nanocomposite membranes provide remarkable versatility compared to conventional membranes.

Full text of DOI:10.1002/cssc.201100211

DOI: 10.1002/cssc.201100211
Nanostructured Membranes for Enzyme Catalysis and
Green Synthesis of Nanoparticles
Vasile Smuleac,[a] Rajender Varma,[b] Babita Baruwati,[b] Subhas Sikdar,[b] and
Dibakar Bhattacharyya*[a]
Macroporous membranes functionalized with ionizable macro-
molecules provide promising applications in high capacity
toxic metal capture, nanoparticle syntheses, and catalysis. Our
low-pressure membrane approach has good reaction and sep-
aration selectivities, which are tunable by varying pH, ionic
strength, or pressure. The sustainable green chemistry ap-
proach under ambient conditions and the evaluation of a reac-
tive poly(acrylic acid) (PAA)-modified polyvinylidene fluoride
(PVDF) membrane is described. Two distinct membrane types
were obtained through different methods: 1) a stacked mem-
brane through layer-by-layer assembly for the incorporation of
enzymes (catalase and glucose oxidase), providing tunable
product yields and 2) Fe/Pd nanoparticles for degradation of
pollutants, obtained through an in situ green synthesis.
Bioreactor–nanodomain interactions and mixed matrix nano-
composite membranes provide remarkable versatility com-
pared to conventional membranes.
Membrane-based separations and reactions have wide applica-
tions ranging from clean water production to selective separa-
tions, chemical synthesis, and biotechnology.[1–3] The availability
of high-capacity membranes for efficient, selective catalysis
with facile in situ regeneration is much needed for economic
and sustainable exploitation of a variety of applications, such
as green synthesis of chemicals, toxic metals removal, or toxic
organics destruction in polluted water. Despite tremendous ad-
vances in separation science, membranes with superior selec-
tivity are still desirable. An example of remarkable selectivity
found in nature is in biological membranes containing aqua-
porins, which transport water at high rates with complete re-
moval of protons.[4,5] The advances in self-assembled[6,7]
carbon[8] nanotubes and alumina nanochannels[9,10] and nano-
composite membranes[11,12] have provided elegant techniques
with high permeability and selectivity.
Figure 1. Multifunctional membranes for metal capture and catalytic reac-
tions. a) Single charged polymer layer for toxic metal capture; b) Supported
metallic NPs for catalysis; c) Electrostatic enzyme immobilization in LbL
assembly for catalysis.
tors and 2) in situ synthesis of metal nanoparticles using a
“green” reducing agent (vitamin C) for environmentally impor-
tant reactions. All functionalization steps, including enzyme in-
corporation and nanoparticle formation, were carried out
using water-based chemistry.
Functionalization of microfiltration-type membranes with
polymers and biomolecules[13,14] to create nanostructured do-
mains in pores offers high transmembrane flux and exceptional
versatility, extending their application from separations to toxic
metal capture, nanoparticle synthesis, and catalytic reactions.
If the selected macromolecule is a polyelectrolyte, the incorpo-
ration in the membrane pores provides both highly charged
field and high toxic metal sorption capacity (Figure 1a).[15]
The metal-sorbed membrane can be easily converted to cata-
lytic membranes (Figure 1b) through reduction to metallic
nanoparticles. The electrostatic assembly based on layer-by-
layer (LbL) deposition of polyelectrolytes can be used for the
incorporation of active enzymes (Figure 1c) without covalent
attachment.
Results and Discussion
The LbL technique is simple and versatile; it most commonly
involves the alternate deposition of oppositely charged poly-
mer layers and is suitable for the assembly of complex supra-
[a] Dr. V. Smuleac, Prof. D. Bhattacharyya
Dept. of Chemical and Materials Engineering
University of Kentucky
Lexington, KY 40506 (USA)
Fax:(+1)859 323 1929
E-mail: db@engr.uky.edu
[b] Dr. R. Varma, Dr. B. Baruwati, Dr. S. Sikdar
Sustainable Technology Division
Reactive, catalytic membranes have been studied and a
common platform, poly(acrylic acid) (PAA)-functionalized poly-
vinylidene fluoride (PVDF) membrane (650 nm pore size), was
used as a support for 1) enzyme incorporation using the LbL
technique for the formation of multilayer membrane bioreac-
National Risk Management Research Lab/USEPA
Cincinnati, OH 45268 (USA)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100211.
ChemSusChem 2011, 4, 1773 – 1777
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D. Bhattacharyya et al.
molecular structures[16,17] and the incorporation of enzymes
without covalent attachment. Other techniques, such as site-di-
rected immobilization,[18] usually employ complex and expen-
sive molecular biology methodologies. We aim to use a simple
method for enzyme immobilization that ensures high catalytic
activity and stability as well as the versatility required for the
electrostatic immobilization of various enzymes.
in the Experimental Section; subsequent deposition occurred
through multiple electrostatic interactions between the ad-
sorbing polyelectrolyte and the oppositely charged layer al-
ready on the membrane. Poly(allylamine) hydrochloride (PAH)
comprised the a second layer to form a PAA–PAH-functional-
ized PVDF membrane with an overall positive charge.
Enzyme immobilization on the polyelectrolyte-assembled
membranes occurs through electrostatic interactions. Catalase
(isoelectric point, pI=5.7) was immobilized on positively
charged membranes at pH 7 to give catalase an overall nega-
tive charge for easy incorporation into a positively charged
membrane [PAA–PAH, pKa (PAH)ꢀ8.8]. The membrane coating,
assembly formation, and catalase incorporation sequence is
shown in Scheme 1. The second enzyme, glucose oxidase (pI=
Stacked membranes for enzymatic catalysis
The chemical or pharmaceutical industries often use several re-
actions (e.g., parallel, sequential, etc.) for syntheses; many sep-
aration and purification steps are needed to obtain a pure
product with high yield, which increases energy consumption
and cost. Bifunctional catalytic membranes can, however, be
synthesized easily and applied to reactions to obtain high se-
lectivity and product yield with minimal separation steps. This
is illustrated in Figure 2, in which two model enzymes are
Scheme 1. PVDF membrane modification inside pores with charged
polymers and catalase immobilization.
4.2) was immobilized in a three-layer assembly consisting of
PAA–PAH–PSS, which carries an overall negative charge (PSS=
polystyrene sulfonate). This was conducted at pH 7, at which
Figure 2. Bifunctional enzyme-catalyzed reactions in nanostructured
membranes.
used, that is glucose oxidase immobilized on the upper mem-
brane and catalase on the lower. In this particular system, the
hydrogen peroxide reaction product in the first membrane is
the reactant in the second, as detailed in Equation (1). Gluconic
acid, a high value chemical commonly used in calcium or
iron(II) gluconate nutrition supplements, is obtained without
additional separation steps.
glucose oxidase has an overall positive charge, opposite to
that of the membrane. This enzyme catalyzes the well-known
reaction 2H2O2!2H2O+O2.
Catalase activities in the free and immobilized forms were
evaluated by using the Michaelis–Menten model, the rate data
for which was obtained under a pressure-driven convective
flow, ensuring accessibility to all active sites. Figure 3a shows
the hydrogen peroxide degradation rates at four substrate
concentrations. At saturation conditions the membrane-immo-
bilized catalase activity is very similar to that of homogeneous
phase catalysis (90%), which is superior to many immobiliza-
tion techniques. It is well known that other common enzyme
immobilization techniques,[18] such as covalent attachment,
result in significant loss of activity. In particular, glucose ox-
idase has demonstrated a sevenfold reduction in activity upon
Glucose oxidase
Glucose
Gluconic acidþH2O2
ƒƒƒƒƒƒƒ!
ð1Þ
Catalase
O2þH2OþGluconic acid
ƒƒƒ!
Glucose oxidase, immobilized through LbL assembly on vari-
ous membranes, have been reported previously.[19,20] We use
an immobilized catalase enzyme in a polyelectrolyte multilayer
assembly formed within the membrane pore domain under
convective conditions. The PVDF membrane was functionalized
with PAA by in situ polymerization of acrylic acid, as described
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Nanostructured Membranes for Enzyme Catalysis
JV the permeation flux (cm3 cm2 sꢁ1) through volume V. In addi-
tion, V=eAL, where e is the porosity (70% average, from man-
ufacturer’s data) and L the membrane thickness (125 mm). The
flux (and hence t) can be modulated by changing the applied
pressure; varying the pressure between 30 and 140 kPa caused
a flux change from 3–12ꢁ10ꢁ4 cm3 cm2 sꢁ1. Low pressure oper-
ations reduce energy consumption significantly. The relation-
ship between the residence time and reactant conversion at
steady state is shown in Figure 3b. Variation of operating pres-
sure can therefore modulate product yield as well as flux (i.e.,
throughput rates).
In addition to the development of individual membranes,
the in situ hydrogen peroxide generation was investigated
using the membrane reactors in series described above
(Figure 2). The upper membrane contained glucose oxidase
and the lower catalase, which were incorporated at pH 7 into a
PAA–PAH–PSS and PAA–PAH layer assembly, respectively. To
prove the in situ hydrogen peroxide formation, the upper
membrane was mounted in a filtration cell and an aerated
pH 7 feed solution containing 0.2 L of 50 mm glucose was
passed convectively through the membrane. After a residence
time of 0.7 s (flux of 12.6ꢁ10ꢁ4 cm3 cm2 s at a pressure of
140 kPa), permeate solutions were found to contain 0.9 mm of
H2O2. Two membranes containing glucose oxidase and cata-
lase, respectively, were then mounted in the filtration cell
using the aforementioned procedure. The concentration of hy-
drogen peroxide in the permeate was found to be zero (i.e.,
100% conversion of H2O2 to oxygen by catalase). Even with an
eightfold (ca. 5.3 s) increase in residence time, the H2O2 conver-
sion was still 100%. This is owed to the excess of catalase
(21300 vs 1550 units of glucose oxidase; for glucose oxidase,
one unit oxidizes 1.0 mmol of b-d-glucose to d-gluconolactone
and H2O2 per min at pH 5.1 and 35 8C; whereas for catalase,
one unit decomposes 1.0 mmol of H2O2 per min at pH 7.0 and
25 8C) in the LbL assembly, however, these loadings are easily
adjustable for specific conversion requirements.
Green synthesis of bimetallic nanoparticles in membranes
The same PAA-functionalized (for metal ion capture) mem-
brane platform was extended to synthesize metal nanoparti-
cles (NPs) in membrane pores using a greener method. The
properties of nanoscale metal particles often differ from those
of bulk materials, enabling applications in areas such as het-
erogeneous catalysis,[23] magnetism,[24] or molecular biology.[25]
For water purification applications, particularly those involving
detoxification of chlorinated organics, the Fe and Fe/Pd bimet-
allic NP systems are commonly used,[26] although other systems
such as Fe/Ni and Fe/Cu have also been reported.[27,28] Owing
to their magnetic properties, Fe (or Fe/Pd bimetallic), NPs tend
to agglomerate rapidly in water to form micron-size or larger
aggregates, thus losing reactivity.[29] Various techniques have
been employed to control the particle size by using additives
or coating with a protective layer.[26,29] Another approach to
control the particle size and prevent agglomeration is to syn-
thesize supported particles; we have used the PAA-coated
PVDF membrane extensively as a platform for this purpose.
Figure 3. a) Rate of hydrogen peroxide decomposition as a function of sub-
strate concentration at pH 6.92, t=228C, [H2O2]initial =2.9 mm, using 0.17 mg
of immobilized catalase (&) or 0.31 mg of catalase homogeneous phase
(*); b) Hydrogen peroxide conversion as a function of residence time in the
membrane pore with catalase enzyme in an LbL assembly at pH 6.92,
t=228C, [H2O2]initial =2.9 mm, using 0.17 mg of immobilized catalase, and
showing the immobilized catalase activity to be 90% that of homogenous
phase catalysis (mmol minꢁ1 mgenzymeꢁ1).
H2O2
covalent immobilization on a porous alumina support com-
pared with the activity in the homogeneous (bulk) phase.[21]
Further to reports on catalase immobilization on various
supports,[22] our approach allows for the tuning of product
yield through variation of residence time (t) in membrane
pores. The residence time was calculated as t=V/(AJV), where
V is the membrane volume, A the external area (33.2 cm2), and
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D. Bhattacharyya et al.
In contrast to other work that has reported the use of toxic
borohydride as a reducing agent,[30] we show that highly reac-
tive NPs (Fe, Pd, or Fe/Pd) can be synthesized easily in mem-
branes using “green” reducing agents. Nontoxic, biodegrad-
able materials, such as ascorbic acid (vitamin C),[31–33] grape
pomace,[34] or polymers,[35] have been recently applied to ho-
mogeneous phase Fe/Pd NP synthesis. We report the synthesis
of supported Fe/Pd NPs in membrane pores using ascorbic
acid as a reducing agent.
A “green” synthesis of bimetallic Fe/Pd NPs in PAA-function-
alized PVDF membranes is shown in Scheme 2. The first step,
PAA coating on PVDF membrane, is described in the Experi-
Figure 4. a) Fe and Pd reduction with ascorbic acid; b) SEM of the Fe/Pd NP
immobilized on the membrane; and c) FTIR spectrum before (ascorbic acid)
and after (dehydroxyascorbic acid) reduction.
agent is shown in Figure 4b.The NPs are uniform, with an aver-
age size of approximately 30 nm. The bimetallic NP composi-
tion was 3.1 wt.% Pd relative to Fe; an energy dispersive X-ray
spectrum and elemental analysis is shown in the Supporting
Information, Figure S2.
Scheme 2. PVDF membrane modification inside pores with charged poly-
mers and subsequent Fe/Pd NP synthesis, where AA is acrylic acid, EG is eth-
ylene glycol (cross-linking agent), and KPS is potassium persulfate (initiator).
The presence of Fe [(110), (200)] and Pd [(111), (200)] peaks
in the NP EDX pattern showed iron to be in metallic a-Fe
form. In the simultaneous reduction of Fe and Pd, ascorbic
acid is oxidized to dehydroascorbic acid. The FTIR spectrum in
Figure 4c, “Rxn” line, shows a new peak in the 1750–
1680 cmꢁ1 region, corresponding to the C=O stretching fre-
quency of the ketone bond formed in the reaction.
mental Section. Prior to ion exchange, PAA-functionalized
PVDF membranes were immersed in an NaCl (5–10 wt.%) solu-
tion at pH 10 for at least 3 h to convert ꢁCOOH to the
ꢁCOONa form. In the next step, the membrane was washed
with deionized ultrafiltered water until the pH of the washing
solution became neutral. The membrane was then immersed
in PdCl2 solution at pH 4.7 for 3 h. Feed solution volume and
Conclusions
concentration were typically 50 mL and 20 mgPd2+ L
ꢁ1, respec-
We have developed a simple and versatile method of enzyme
catalysis and nanoparticle synthesis using a common platform,
poly(acrylic acid)-coated polyvinylidene fluoride membrane,
and “greener” methods. The enzymes were incorporated in
polymer multilayer-assembled membranes through electrostat-
ic interactions and maintained a high activity upon immobiliza-
tion comparable to that of the homogeneous phase catalysis.
Membranes containing different enzymes can be stacked and
used as reactors in series and the reaction yields can be modu-
lated by adjusting parameters such as the enzyme loading on
each membrane and the residence time, which is in turn relat-
ed to permeate flux. We have demonstrated the direct and
“green” synthesis of bimetallic Fe/Pd particles in a membrane
domain. Compared to the established method, which uses
toxic sodium borohydride, this approach uses biodegradable
ascorbic acid and thus has a minimized environmental impact.
tively. Nitrogen gas was bubbled through the feed solution to
minimize oxidation. Reduction with ascorbic acid (50 mL, 0.1m)
in the presence of FeCl3 (180 mgFe Lꢁ1) ensured Fe/Pd bimetallic
NP formation.
Bimetallic Fe/Pd particles were also formed in diffusion
mode; the experimental setup is shown in the Supporting In-
formation, Figure S1. The PVDF–PAA membrane was mounted
between two chambers, one containing K2PdCl4 and the other
a mixture of FeCl3 and ascorbic acid; each diffused inside the
membrane pores. As a consequence, the Fe/Pd NPs were
formed primarily in the membrane pores rather than on the
surface layer; this is the major difference between the diffusive
and soaking modes. In the latter approach the NPs are formed
both on the surface layer and within the membrane pores.
The reduction potential (E0 =0.06 V) of ascorbic acid is suffi-
cient to reduce Pd2+ (E0 =0.951 V), but not Fe2+ (E0 =ꢁ0.44 V).
Ascorbic acid can, however, simultaneously reduce the Fe and
Pd, as shown in Figure 4 a. The overall potential for the three
reactions is positive (negative free energy), enabling the reduc-
tion of Pd2+ and Fe2+ to Pd0 and Fe0, respectively. An SEM
image of the Fe/Pd NPs after using ascorbic acid as reducing
Experimental Section
PVDF membranes with a thickness of 125 mm and an average pore
size of 650 nm were purchased from Millipore. Glucose oxidase
from Aspergillus niger and catalase from bovine liver were pur-
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Nanostructured Membranes for Enzyme Catalysis
[2] M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Marinas,
A. M. Mayes, Nature 2008, 452, 301–310.
[3] N. N. Li, A. G. Fane, W. S. W. Ho, T. Matsuura, Advanced Membrane Tech-
nology and Applications, Wiley & Sons, New York, 2008.
[4] D. Fu, A. Libson, L. J. W. Miercke, C. Weltzman, P. Nollert, J. Krucinski,
R. M. Stroud, Science 2000, 290, 481–486.
[5] E. Tajkhorshid, P. Nollert, M. O. Jensen, L. J. W. Miercke, J. O’Connell,
R. M. Stroud, K. Schulten, Science 2002, 296, 525–530.
[6] E. N. Savariar, K. Krishnamoorthy, S. Thayumanavan, Nat. Nanotechnol.
2008, 3, 112–117.
chased from Aldrich; acrylic acid, ascorbic acid, dehydroascorbic
acid, palladium chloride, potassium tetrachloropalladate(II), titani-
um(IV) oxysulfate, and PAH and PSS (average MWꢀ70000 gmolꢁ1
)
were supplied by Aldrich. The protein assay kit was obtained from
BioRad Laboratories. Trichloroethylene, ferric chloride hexahydrate,
hydrogen peroxide, sodium hydroxide, and deionized ultrafiltered
water were purchased from Fisher Scientific. Ethylene glycol and
potassium persulfate were purchased from Mallinckrodt and EM
Science, respectively.
[7] P. Kohli, C. C. Harrell, Z. Cao, R. Gasparac, W. Tan, C. R. Martin, Science
2004, 305, 984–986.
[8] B. J. Hinds, N. Chopra, T. Rantell, R. Andrews, V. Gavalas, L. G. Bachas,
Science 2004, 303, 62–65.
[9] T. C. Merkel, B. D. Freeman, R. J. Spontak, Z. He, I. Pinnau, P. Meakin,
A. J. Hill, Science 2002, 296, 519–522.
[10] W. Yave, K. V. Peinemann, S. Shishatskiy, V. Khotimskiy, M. Chirkova,
S. Matson, E. Litvinova, N. Lecerf, Macromolecules 2007, 40, 8991–8998.
[11] W. Chen, Z. Q. Wu, X. H. Xia, J. J. Xu, H. Y. Chen, Angew. Chem. 2010,
122, 8115–8119; Angew. Chem. Int. Ed. 2010, 49, 7943–7947.
[12] S. J. Li, J. Li, K. Wang, C. Wang, J. J. Xu, H. Y. Chen, X. H. Xia, Q. Huo, ACS
Nano 2010, 4, 6417–6424.
PVDF membranes were functionalized with poly(acrylic acid)
through in situ polymerization of acrylic acid. The polymerization
reaction was carried out in aqueous solution, comprising 30%
acrylic acid, the cross-linking agent ethylene glycol (molar ratio
ethylene glycol/acrylic acid=1:6.5) and the initiator potassium per-
sulfate (1 wt.%). The PVDF membrane[36] was dipped in the poly-
merization solution for 2 min, sandwiched between two Teflon
plates and placed in an oven at 908C for 4 h. Nitrogen gas was
supplied continuously to flush out oxygen, which acts as an inhibi-
tor for the polymerization reaction. To ensure thorough wetting,
hydrophilic PVDF membranes were used.
[13] M. Ulbricht, Polymer 2006, 47, 2217–2262.
[14] M. L. Bruening, D. M. Dotzauer, P. Jain, L. Ouyang, G. L. Baker, Langmuir
2008, 24, 7663–7673.
[15] S. M. C. Ritchie, L. G. Bachas, T. Olin, S. K. Sikdar, D. Bhattacharyya, Lang-
muir 1999, 15, 6346–6357.
The amount of enzyme in the feed and permeate solutions was
determined using the Bradford protein assay procedure.[37] A cali-
bration curve was obtained from a bovine serum albumin standard
solution. An acidic dye was then added to the protein solution,
and the absorbance of the dye–protein complex was measured at
595 nm, with a standard deviation for three measurements of less
than 0.1%. The amount of enzyme immobilized on the membrane
could be estimated by material balance analysis.
[16] G. Decher, Science 1997, 277, 1232–1237.
[17] O. Ikkala, G. Brinke, Science 2002, 295, 2407–2409.
[18] D. A. Butterfield, D. Bhattacharyya, S. Daunert, L. G. Bachas, J. Membr.
Sci. 2001, 181, 29–37.
[19] V. Smuleac, D. A. Butterfield, D. Bhattacharyya, Langmuir 2006, 22,
10118–10124.
Hydrogen peroxide degradation was analyzed by using a well es-
tablished colorimetric method.[38] An acidified titanium(IV) oxysul-
fate solution was added, and the absorbance of the yellow-colored
titanium–peroxo complex was measured at 407 nm. Over the con-
centration range 2–100 mgLꢁ1, the standard deviation was less
than 0.5%.
[20] S. Datta, C. Cecil, D. Bhattacharyya, Ind. Eng. Chem. Res. 2008, 47, 4586–
4597.
[21] S. J. Li, C. Wang, Z. Q. Wu, J. J. Xu, X. H. Xia, H. Y. Chen, Chem. Eur. J.
2010, 16, 10186–10194.
[22] Y. Wang, F. Caruso, Adv. Funct. Mater. 2004, 14, 1012–1018.
[23] F. Tao, M. E. Grass, Y. Zhang, D. R. Butcher, J. R. Renzas, Z. Li, J. Y. Chung,
B. S. Mun, M. Salmeron, G. A. Somorjai, Science 2008, 322, 932–934.
[24] S. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser, Science 2000, 287,
1989–1992.
[25] N. L. Rosi, D. A. Giljohann, C. S. Thaxton, A. K. R. Lytton-Jean, M. S. Han,
C. A. Mirkin, Science 2006, 312, 1027–1129.
[26] F. He, D. Zhao, Appl. Catal. B 2008, 84, 533–540.
[27] B. Schrick, J. L. Blough, A. D. Jones, T. E. Mallouk, Chem. Mater. 2002, 14,
5140–5147.
[28] N. Zhu, H. Luan, S. Yuan, J. Chen, X. Wu, L. Wang, J. Hazard. Mater.
2010, 176, 1101–1105.
[29] A. H. Lu, E. L. Salabas, F. Schuth, Angew. Chem. 2007, 119, 1242–1266;
Angew. Chem. Int. Ed. 2007, 46, 1222–1244.
The amount of Pd captured during ion exchange and Fe reduced
in the presence of ascorbic acid was determined by material bal-
ance analysis. The concentrations for Fe and Pd in the feed and
permeate solutions were quantified by using a Varian SpectrAA
220 fast sequential atomic absorption spectrometer equipped with
a Fisher Scientific hollow cathode lamp. For measurements involv-
ing Fe, the lamp was operated at a wavelength of 386.0 nm and
the calibration plot created by using four different concentrations
of Fe ranging from 25 to 200 mgLꢁ1; R2 =0.9993 and the average
analytical error was 2%. For Pd measurements, the lamp was oper-
ated at a wavelength of 246.6 nm and the linear calibration range
was between 0.2 and 28 mgLꢁ1 Pd, with an analysis error of less
than 2%.
[30] J. Xu, D. Bhattacharyya, J. Phys. Chem. C 2008, 112, 9133–9144.
[31] M. N. Nadagouda, R. S. Varma, Cryst. Growth Des. 2007, 7, 2582–2587.
[32] F. He, J. Liu, C. B. Roberts, D. Zhao, Ind. Eng. Chem. Res. 2009, 48, 6550–
6557.
[33] J. Liu, F. He, T. M. Gunn, D. Zhao, C. B. Roberts, Langmuir 2009, 25,
7116–7128.
[34] B. Baruwati, R. S. Varma, ChemSusChem 2009, 2, 1041–1044.
[35] J. Virkutyte, R. S. Varma, Chem. Sci. 2011, 2, 837–846.
[36] V. Smuleac, L. G. Bachas, D. Bhattacharyya, J. Membr. Sci. 2010, 346,
310–317.
[37] M. M. Bradford, Anal. Biochem. 1976, 72, 248–254.
[38] P. A. Clapp, D. F. Evans, T. S. S. Sheriff, Anal. Chim. Acta 1989, 218, 331–
334.
Acknowledgements
This study was supported by the NIEHS-SRP program and by
DOE-KRCEE.
Keywords: electrostatic
interactions
·
enzymes
·
immobilization · membranes · nanoparticles
Received: April 25, 2011
[1] E. Drioli, L. Giorno, Comprehensive Membrane Science and Engineering,
Elsevier, Amsterdam, 2010.
Revised: July 6, 2011
Published online on November 15, 2011
ChemSusChem 2011, 4, 1773 – 1777
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
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