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Comparison of Adsorption and Selectivity
Characteristics for 4-Nitrophenol Imprinted
Polymers Prepared via Bulk and Suspension
Polymerization
Rıdvan Say,
1,2 ,
*Arzu Erso
¨z,
1
I
˙zzet S¸ener,
3
Ayc¸a Atılır,
1
Sibel Diltemiz,
1
and Adil Denizli
4
1
Department of Chemistry, Anadolu University, Eskis¸ehir, Turkey
2
BI
˙BAM (Plant, Drug and Scientific Researches Center), Anadolu
U
¨niversitesi, Eskis¸ehir, Turkey
3
Department of Chemistry, Pamukkale University, Denizli, Turkey
4
Department of Chemistry, Hacettepe University, Ankara, Turkey
ABSTRACT
This manuscript describes a method for the selective removal of phenolic
compounds from aqueous medium by imprinted polymers, which is the
noncovalent approach based on both hydrogen bonding and hydro-
phobic interaction. These imprinted polymers were prepared by
both bulk polymerization (IP1) and suspension polymerization (IP2) of
methacryloylantipyrine (MAAP) in the presence of azobisisobutyronitrile
(AIBN) as an initiator and cross-linking EGDMA and was imprinted with
3471
DOI: 10.1081/SS-200028939 0149-6395 (Print); 1520-5754 (Online)
Copyright #2004 by Marcel Dekker, Inc. www.dekker.com
*Correspondence: Rıdvan Say, Department of Chemistry, Anadolu U
¨niversitesi, Fen
Faku
¨ltesi, Kimya Bo
¨lu
¨mu
¨, 26470 Eskis¸ehir, Turkey; E-mail: rsay@anadolu.edu.tr.
SEPARATION SCIENCE AND TECHNOLOGY
Vol. 39, No. 15, pp. 3471–3484, 2004
4-nitrophenol. The effect of polymerization technique (bulk and suspen-
sion) on the adsorption and selectivity of phenolics was investigated. The
maximum adsorption capacities of these IP1 and IP2 polymers were 25.9
and 27.6 mg/g for 4-nitrophenol, respectively. It was observed that the
4-nitrophenol binding capacity decreased with increasing pH. The selec-
tivity experiments showed that the polymer prepared via suspension pol-
ymerization has high-binding ability for 4-nitrophenol to other phenolics
compared to the polymer prepared via bulk polymerization.
Key Words: Phenol; Adsorption of phenol; Molecularly imprinted
polymer (MIP); Bulk and suspension polymerization.
INTRODUCTION
Phenolic compounds have interested different researchers because of
their physiological and physical-chemical properties as well as their anti-
carcinogenic and high-antioxidant capacity. These compounds are polluting
substances present in the aquatic environment as by-products of the
coal and oil industry or the result of pesticide and drug decay.
[1]
The U.S.
Environmental Protection Agency (EPA) and European Union have included
phenol and various chlorophenols and nitrophenols in their lists of priority
pollutants to be monitored in the aquatic environment. The maximum level
allowed for these compounds in publicly supplied water is 0.5 mg/mL.
Because of their carcinogenic properties there is a great need in wastewater
for a new and selective technique for phenol separation.
The removal and/or detoxification of phenolic compounds can be done
by microbial degradation, and chemical oxidation using agents such as
ozone, hydrogen peroxide, or chlorine dioxide. The main limitation of these
methods is their low efficiency in the removal of trace levels of phenols.
Adsorption is a well-known removal technique for organic compounds from
water. The use of activated carbon as an adsorbent for the removal of phenolic
compounds is common practice.
[2 – 4]
The adsorption behavior of various
phenols by polymeric adsorbents and ion exchange resin,
[5 – 7]
use of clay
and organoclay as an adsorbent,
[8,9]
and molecularly imprinted adsor-
bents
[10,11]
have been studied for the adsorption of phenol from aqueous
solution.
Molecular imprinting is a method for making selective binding sites
in synthetic polymers by using molecular template. Target molecules
(i.e., phenolics) can be used as templates for imprinting cross-linked polymers.
After the removal of the template, the remaining polymer is more selective.
The selectivity of the polymer depends on various factors such as the size
Say et al.3472
and shape of the cavity and rebinding interactions. Covalent interactions,
[10,12]
noncovalent interactions such as hydrogen bonding,
[11,13,14]
p
-
p
bonding and
hydrophobic interaction,
[15]
electrostatic interactions,
[16]
and metal ion
coordination
[17 – 19]
can be exploited to organize the functional monomers
around the template. MIPs prepared by suspension and bulk polymerization
have been synthesized. Imprinted polymers are usually prepared by bulk poly-
merization using thermal or UV radiation, but polymerized particles limited
mass transfer. MIPs prepared by suspension polymerization are more suitable
products for separation strategies.
The objective of this study is to investigate the adsorption behavior of
4-nitrophenol and phenolic compounds on imprinted polymers prepared by
both bulk polymerization (IP1) and suspension polymerization (IP2). For this
purpose, methacryloylantipyrine (MAAP) monomer, which has
p
electron-
rich aromatic ring, firstly was synthesized by our group using antipyrine,
which is hydroxy radical capture and spectroscopic reagent for phenols, and
then was polymerized. The 4-nitrophenol, which has an electron-poor
aromatic ring, was chosen as a template molecule. Phenol, chlorophenol,
2-nitrophenol, and cresol were used as adsorbates because they are some of
the organic pollutants that need to be removed from wastewater. Association
constant (K
ass
), number of accessible sites (N), and binding ability were
also evaluated to get information about the specific interaction between
4-nitrophenol imprinted polymers (IP1 and IP2) and 4-nitrophenol.
EXPERIMENTAL
Materials
Both 4-aminoantipyrine and pyridine were supplied by Aldrich
and used as received. Ethyleneglycoldimethacrylate (EGDMA) was obtained
from Fluka A.G., distilled under reduced pressure in the presence of hydro-
quinone inhibitor, and stored at 48C until use. Azobisisobutyronitrile
(AIBN), phenol, and p-nitrophenol were also obtained from Fluka. The
4-nitrophenol, 2-chlorophenol were supplied from Aldrich, o-cresol, sodium
dihydrogen phosphate were supplied from Riedel-de Haen, and sodium tetra-
borate was supplied from Codex. Methacryloylchloride was obtained from
Sigma. All other chemicals were of reagent grade and were purchased from
Merck AG. All water used in the experiments was purified using a Barnstead
(Dubuque, IA) ROpure LP reverse osmosis unit with a high-flow cellulose
acetate membrane (Barnstead D2731) followed by a Barnstead D3804
NANO pure organic/colloid removal and ion exchange packed-bed system.
4-Nitrophenol IP Prepared via Bulk and Suspension Polymerization 3473
Instrumentation
Capillary electrochromatography experiments were carried out with a
Prince CEC 760 model 3D capillary electrochromatography system, equipped
with a diode-array detector. Uncoated fused-silica capillary (75 mm ID) with a
total length of 35.0 cm was used for the MECC separations. The capillary was
conditioned before use and between runs with 0.2 M NaOH and rinse buffer
for 20 min. Inlet buffer injection was performed for 0.2 sec at 100 mbar,
analyte for 0.1 sec at 20 mbar, the detection wavelength was 210 nm, and
the separation voltage was maintained at 15 kV. The concentration of phenolic
compounds was 50 ppm. Phosphate-borate buffer with pH 7.0 was selected for
optimization experiments and sodium dodecyl sulfate was used as a surfactant.
The column temperature was maintained at 258C. Automated capillary
rinsing, sample introduction, and execution of the electrophoretic runs were
controlled by a personal computer. Data processing was carried out with a
Dax.7.1 3D software. Three replicate runs were performed for all conditions.
Fourier transform infrared spectroscopy (FTIR) spectra of P(MAAP-
co-EGDMA) and microbeads were obtained through the use of a FTIR spec-
trophotometer (Jasco Corparation, made in Japan; FT/IR-300 E). A Fisher
Scientific, Accumet
w
Basic AB15 pH-meter was used to measure pH values.
Synthesis of MAAP
The following experimental precedure was applied for the synthesis of
MAAP:
[20]
4-aminoantipyrine (0.5 g; 2.463 mmol) and pyridine (0.2 mL;
2.46 mmol) were dissolved in 100 mL of dry CHCl
3
and the solution was
cooled to 08C. Then, methacryloylchloride (0.26 mL; 2.46 mmol) was
poured slowly into this solution by stirring magnetically at room temperature
for 2 h. At the end of this chemical reaction period, the solution was washed
with dilute 50 mL of HCl and 50 mL of dilute NaOH. Then, the organic
phase was evaporated in a rotary evaporator. The residue was crystallized in
petroleumbenzene-ethylacetate.
Melting point was found at 132 – 1338C, Yield % 70. FT-IR (KBr, cm
21
):
770 –710 cm
21
(monosubstituted benzene ring), 1580 – 1500 cm
21
(conju-
gation at aromatic ring, strong two or three bands), 1600 cm
21
(methacryl
double-band), 1642 cm
21
(amide carbonyl band), 1730 cm
21
(carbonyl band
at cyclis ketone position), 2975 – 2925 cm
21
(C22H band), 3260 cm
21
(N55H band). 1H-NMR (CHCl
3
): 2.05 ppm 3H singlet (22C55C22CH
3
,
vinyl methyl), 3.0 ppm 3H singlet (22C22CH
3
), 3.35 ppm 3H singlet
(22N22CH
3
), 5.5 ppm 1H singlet (22CH
a
55C22), 5.8 ppm 1H singlet
(22CH
b
55C22), 7.25 –8.80 ppm 4H multiplet (aromatic, CDCl
3
peak is also
Say et al.3474
observed at 7.3 ppm with aromatic peaks), 8.80 ppm 1H singlet (aromatic),
9.1 ppm 1H singlet (N22H).
Preparation of 4-Nitrophenol Imprinted Polymer (IP1) via
Bulk Polymerization
For the preparation of imprinted polymer (IP1), which is the noncovalent
approach based on both hydrogen bonding and hydrophobic interaction, the
template 4-nitrophenol (2.0 mmol) was dissolved in 22 mL of acetonitrile in
a flask. The functional monomer MAAP (12 mmol), the cross-linking
EGDMA (57 mmol), and the initiator AIBN (1.6 mmol) were then added to
the flask. After degassing and nitrogen purging, the flask was sealed and
allowed to polymerize at room temperature for 24 h under UV irradiation.
The obtained IP1 hard polymers were crushed, ground, and wet-sieved
using acetone to obtain regularly sized particles between 25 and 44 mm. The
polymer particles were suspended and refluxed with NaOH in methanol to
remove 4-nitrophenol template molecules. The particles were then extensively
washed with water and methanol until no more 4-nitrophenol was released.
Nonimprinted blank polymer in the absence of 4-nitrophenol was pre-
pared and treated with the same method.
Preparation of 4-Nitrophenol Imprinted Polymeric
Microbeads (IP2) via Suspension Polymerization
The IP2 beads were prepared by modified suspension polymerization
technique. A typical suspension copolymerization procedure of IP2 beads
was given as follows: The dispersion medium was prepared by dissolving
0.2 g polyvinylalcohol within 60 mL of distilled water. MAAP/4-nitrophenol
(2 mmol/2 mmol) preorganized mixture and 1.05 g methylmethacrylate
(MMA) were mixed into 8.0 mL EGDMA/12.0 mL toluene, and 0.100 g of
2,20-azobisisobutyronitrile (AIBN) was dissolved within the monomer
mixture. This solution was then transferred into the dispersion medium
and magnetically stirred (at a constant stirring rate of 600 rpm) in a glass poly-
merization reactor (100 mL), which was in a thermostatic water bath. The
reactor was flushed by bubbling nitrogen and then was sealed. The reactor
temperature was kept at 808C for 7 h. Then the polymerization was completed
at 908C in 3 h. After polymerization, the IP2 beads were separated from
the polymerization medium. The residuals (e.g., unconverted monomer,
initiator) were removed by a cleaning procedure and dried in a vacuum
oven at 708C for 48 h.
4-Nitrophenol IP Prepared via Bulk and Suspension Polymerization 3475
The polymer microbeads were suspended and refluxed with NaOH
in methanol to remove the 4-nitrophenol template molecule. The particles were
then extensively washed with water and methanol until no more 4-nitrophenol
was released.
Nonimprinted blank polymer microbeads in the absence of 4-nitrophenol
were prepared and treated with the same method.
IP2 microbeads were spherical in shape with a size range of 5– 120 mmin
diameter. The specific surface area of IP2 microbeads was found as 144.6 m
2
/g.
The equilibrium swelling ratio of the IP2 microbeads was determined in water
at 258C and found to be 20.5%.
Adsorption of Phenolic Compounds
Adsorption of phenolic compounds from aqueous solutions was investi-
gated in batch experiments. Effects of the initial phenolic compounds concen-
tration, pH of the medium on the adsorption rate, and adsorption capacity were
studied. The suspensions were brought to the desired pH by adding sodium
hydroxide and nitric acid. The pH was maintaned in a range of +0.1 units
until equlibrium was attained. In all experiments, polymer concentration
was kept constant at 25 mg/25 mL. The concentration of the phenolic com-
pounds in the aqueous phases after desired treatment periods was measured
by using a MECC. The experiments were performed in replicates of three,
and the samples were analyzed in replicates of three as well. For each set of
data present, standard statistical methods were used to determine the mean
values and standard deviations. Confidence intervals of 95% were calculated
for each set of samples in order to determine the margin of error. Adsorption
values (mg/g) were calculated as the difference in phenolic compounds
concentration of the pre- and postadsorption solutions divided by the weight
of IP1 particles or IP2 microbeads.
Association constant (K
ass
) and the number of accessible sites (N) for the
specific interaction between the template imprinted polymer (IP1 and IP2) and
the template itself were determined by Scathard’s plots using the equation
B/F¼2K
ass
BþK
ass
PN, where P is the concentration of MIP, B is rebinding
4-nitrophenol concentration, and F is free 4-nitrophenol concentration.
[21]
The binding ability based on molecular imprinting effect was evaluated in
terms of “imprinting-induced promotion of binding” (IPB). This value is
defined by
IPB ¼Aimp Anonimp
Anonimp
Say et al.3476
Here, A
imp
is the amount of the guest that was bound by imprinted
polymer under the conditions just described and A
non-imp
is the corresponding
value for the nonimprinted polymer.
[22]
RESULTS AND DISCUSSION
Comparison of 4-Nitrophenol Adsorption
Comparison of Adsorption Time for 4-Nitrophenol
The equilibrium adsorption time of 4-nitrophenol on the IP1 and IP2
polymers was investigated by changing the adsorption time between
15 min –12 h. These batch experiments were performed by using 4-
nitrophenol solutions. The initial concentrations of phenolic compounds
were kept constant at 50 mg/L. As seen in Fig. 1, adsorption amounts of
phenolic compounds increased with time, and saturation levels were
reached within 2 h for IP2 and around 10 h for IP1. As can be seen from the
figure, 4-nitrophenol was adsorbed on IP2, which has templates by surface
imprinting much faster than IP1, which has mass-transfer limitations due to
bulk polymerization.
Figure 1. The adsorption rates of 4-nitrophenol on IP1 and IP2. The initial concen-
tration of 4-nitrophenol is 50 mg/Lat258C and pH 3.0.
4-Nitrophenol IP Prepared via Bulk and Suspension Polymerization 3477
The experiments related with adsorption time have been shown a wide
range of adsorption rates. Ravi et al. used activated carbon for phenol and
cresol adsorption
[23]
and Gupta et al. used ash for phenol and p-nitrophenol
adsorption,
[24]
and they have reported 20 h and 24 h adsorption rate, respect-
ively. The equilibrium adsorption of chlorophenol using aluminosilicate has
been reported as 48 h
[25]
and for chloro- and nitrophenols using activated
carbon has been reported as 2 weeks.
[26]
There are several parameters that
determine the adsorption rate such as stirring rate in the aqueous phase, struc-
tural properties of sorbent-like porosity, surface area, amount of sorbent,
adsorbate properties, initial concentration of phenolic species, and existence
of other species. Therefore, it is too difficult to compare the adsorption rates
reported. However, the adsorption rate obtained with the IP2 microbeads
produced by us seem to be very satisfactory.
Comparison of Initial Concentration of 4-Nitrophenol for IP1 and IP2
Adsorption of 4-nitrophenol compounds from aqueous solutions was
investigated in batch experiments. As can be seen in Fig. 2, the amount of
adsorbed 4-nitrophenol per unit mass of the polymer increased with the initial
concentration of the phenolic compounds. In order to reach the “saturation,”
the initial phenolic compounds concentrations were increased, and it was
Figure 2. The 4-nitrophenol adsorption capacities of these IP1 and IP2 at 258C and
pH 3.0.
Say et al.3478
observed that the amount of adsorption was increased with the initial phenolic
concentration. The maximum adsorption capacities of these IP1 and IP2 were
25.9 mg/g(187mmol/g) and 27.6 mg/g(198.3mmol/g) for 4-nitrophenol,
respectively.
IP2-MIPs, which were prepared by suspension polymerization and based
on surface imprinting, have high porosity and surface area, so there is better
accessibility of the active site. Because of this, higher adsorption capacity
were observed than IP1, which was prepared via bulk polymerization.
In the literature studies different adsorbents were used with a wide range
of adsorption capacities for phenolic compounds. Gupta et al. showed a
60 mmol/g adsorption capacity for phenol and p-nitrophenol with a bagasse
fly ash.
[24]
Ravi et al. reported adsorption capacities betwen 3.2 and
4.4 mmol/g with activated carbon for phenol and cresol isomers.
[23]
Tewari
and Kamaluddin studied the removal of aminophenol and o-nitrophenol by
copper, zinc, molybdenum, and chromium ferrocynanides, and they found
the adsorption capacities in the range of 107 – 256mmol/g for nitrophenols.
[27]
The adsorption capacity of nitrophenol on the poly(HEMA) (PHEMA)
microbeads with Alkali Blue 6B attached was found to be 112.6 mmol/gby
Denizli et al.
[7]
. However, note that the adsorption capacities that we achieved
are comparable with the values reported in previous publications.
Effects of pH
The pH, which is the most critical parameter that effects the adsorption
capacity, was determined for different pH values ranging from 2.0– 9.0, using
a50.0mg/L 4-nitrophenol. The effect of pH on adsorption of 4-nitrophenol
is shown in Fig. 3. It was observed that, the phenolics binding capacity
decreased with increasing pH. IP1 and IP2 exhibited a low affinity in basic
conditions (pH 6.0). However, there is no significant decrease in the equili-
brium adsorption capacity within the pH range 2.0–6.0, the highest adsorption
of 4-nitrophenol occured at pH 2.0 for both IP1 and IP2.
Imprinting Efficiency to Adsorption
Binding ability to 4-nitrophenol, phenol, 2-nitrophenol, cresol, chlorophenol
of IP1 and IP2 are given in Table 1. As seen from Table 1, the IPB values of
imprinted-IP1 for 4-nitrophenol with respect to phenol, 2-nitrophenol, cresol,
chlorophenol is higher than the IPB values of nonimprinted IP1. The same was
observed for IP2, and 4-nitrophenol is adsorbed more efficiently than other
phenolics. Imprinted IP2 has higher adsorption selectivity than nonimprinted
4-Nitrophenol IP Prepared via Bulk and Suspension Polymerization 3479
IP2 for 4-nitrophenol. It can be obviously seen that imprinted IP1 and IP2
polymers are more selective than nonimprinted polymers because the
imprinted polymers, are memorized in the structures of the 4-nitrophenol
templates and so they have suitable size and shape of cavity for 4-nitrophenol.
IP2 has higher IPB value than IP1 for 4-nitrophenol with respect to other
phenolic compounds. As it was mentioned before, this may be because of
surface imprinting of IP2, which has high porosity and surface area (the
specific surface area of IP1 and IP2 were found to be 130.3 and 207.6 m
2
/g,
respectively) and better accessibility of the active site. The template can
Figure 3. The effect of pH on the adsorption of 4-nitrophenol on the microbeads at
258C and initial compounds of 50mg/L.
Table 1. Binding ability of 4-nitrophenol imprinted polymers IP1 and IP2 toward
phenolics.
Phenolics
IP1 IP2
Imp.
(mg/g)
Non-Imp.
(mg/g) IPB
Imp.
(mg/g)
Non-Imp.
(mg/g) IPB
4-Nitrophenol 21.2 9.1 133 24.8 10.3 141
Chlorophenol 17.3 9.4 84 13.5 10.4 30
2-Nitrophenol 15.5 8.7 78 11.7 9.1 29
Cresol 16.1 9.3 73 12.4 9.1 15
Phenol 13.2 7.4 24 9.8 8.6 14
Say et al.3480
easily not react with the cavity when the bulk polymerization technique was
applied.
K
ass
and the N values for the specific interaction between the template
imprinted polymer and the template itself were determined by Scatchard’s
plots using both IP1 and IP’ polymers. K
ass
and the N values can be estimated
as 4.7 10
3
mol
21
2 L and 1.08 mmol/g for IP1 and 5.3 10
3
mol
21
L and
1.24 mmol/g for IP2, respectively, from the slope and the intercept in Fig. 4.
Comparing IP1 with IP2, higher K
ass
and the N values were obtained in the IP2
imprinted system. This can be explained again for the same reason, IP2 has
high porosity, surface area, and the active site, and so has high-adsorption
capacity.
CONCLUSION
The development of an effective technology for the removal of phenolic
compounds from wastewater is important because of its carcinogenic property
and pollutant effect. For this purpose, using an adsorptive resin for the removal
of phenolic compounds could be a method of choice.
Figure 4. Scatchard’s plot of 4-nitrophenol rebinding by the imprinted polymers;
Concentration of polymer: 25 mg/25 mL.
4-Nitrophenol IP Prepared via Bulk and Suspension Polymerization 3481
In this study, a new polymer with memory was prepared for the selective
removal of phenolic compounds from wastewater. By considering this
purpose, 4-nitrophenol imprinted polymers were synthesized using MAAP
monomer, applying both bulk (IP1) and suspension (IP2) polymerization tech-
niques. The effects of these polymers and polymerization techniques on
4-nitrophenol and other phenolics adsorption were investigated. IP2-MIPs,
which were prepared by suspension polymerization and based on surface
imprinting, have high porosity and surface area, and so have better accessi-
bility of the active site. Because of this, higher adsorption capacity was
observed than IP1, which was prepared via bulk polymerization. The
maximum adsorption capacities of these IP1 and IP2 were 25.9 and
27.6 mg/g for 4-nitrophenol, respectively. IPB values of IP1 and IP2 for
phenol, 2-nitrophenol, cresol, chlorophenol showed that both IP1 and IP2
have significant selectivity to 4-nitrophenol, and it was observed that IP2
has the higher binding capacity compared to IP1. The same results were
also obtained using Scatchard’s plot.
ACKNOWLEDGMENTS
The authors thank the Turkish Science and Technology Research Council
(TUBITAK-MISAG-235) for financial support to carry out this research work.
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Received December 2003
Accepted June 2004
Say et al.3484
