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Acta Materialia 52 (2004) 5363–5374
www.actamat-journals.com
Facile synthesis of poly(1,8-diaminonaphthalene) microparticleswith a very high silver-ion adsorbability by a chemical
oxidative polymerization
Xin-Gui Li a,b,c,*, Mei-Rong Huang a,*, Sheng-Xian Li a
a Laboratory of Concrete Materials Research, Institute of Materials Chemistry, College of Materials Science & Engineering,
Tongji University, 1239 Siping Road, Shanghai 200092, Chinab Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA
c The Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, China
Received 27 June 2004; received in revised form 28 July 2004; accepted 29 July 2004
Available online 27 August 2004
Abstract
Poly(1,8-diaminonaphthalene) (PDAN) was traditionally synthesized by an electrochemical polymerization that has some limi-
tations such as low productivity and single form of a film. Here we report a relatively large mass synthesis of PDAN micrometer
particles by a chemical oxidation of 1,8-diaminonaphthalene by (NH4)2S2O8 or FeCl3 with high yield. Elemental analysis, IR, and
solid-state high-resolution 13C NMR spectroscopies indicate that the PDAN chain contains imine (AN@C), amine (ANHAC), and
free amine (–NH2) units as linkages between naphthalene rings. A double-stranded ladder or single-stranded structure via the link-
ages is deduced. The structure and Ag+ absorbability of PDAN particles were characterized by laser particle-size analyzer, wide-
angle X-ray diffractometer, IR, and inductively coupled plasma techniques. The Ag+ adsorbability of the particles was examined
and optimized systematically by varying the adsorption time, the dose and size of the particles, the temperature, pH, and concen-
tration of Ag+ solution. The fine particles obtained using (NH4)2S2O8 exhibit high adsorbability by complexation between Ag+ and
amine/imine groups as well as the redox between Ag+ and free –NH2 group. The Ag+ adsorbance reaches 1.92 g/g (PDAN) with
exposure to a solution containing 82 mM Ag+ ion for 24 h at an initial Ag+/PDAN ratio of 103 mmol/g. Total Ag+ adsorbance
was 1.92 times the PDAN weight, remarkably surpassing the largest Ag+ adsorbance of 1.36 g/g (the best activated carbon fiber)
for 30 days. The PDAN particles could be very useful in collection and removal of heavy metallic ions from water effluents.
� 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Poly(1,8-diaminonaphthalene); Oxidative polymerization; Microparticle; Silver recovery; Adsorption
1. Introduction
Polydiaminonaphthalene synthesized from aromatic
diamine, such as 1,5-, 1,8-, or 2,3-diaminonaphthalene,
by an electrochemically or chemically oxidative polym-
erization, is a new type of multifunctional polymer
1359-6454/$30.00 � 2004 Acta Materialia Inc. Published by Elsevier Ltd. A
doi:10.1016/j.actamat.2004.07.042
* Corresponding authors. Tel.: +86 21 65980524; fax: +86 21
65980530.
E-mail addresses: [email protected] (X.-G. Li), huangmeir-
[email protected] (M.-R. Huang).
material following polyaniline and polypyrrole [1]. Be-
sides electroconductivity, electroactivity [2,3], electro-chromism [1], permselectivity [1], and electrocatalysis
[4], the polydiaminonaphthalene exhibits some other
very interesting properties that originate from chemical
reactivities of functional amino groups on the macro-
molecular structure [5–10]. The applications of electro-
conducting polymers to electrochromic devices [1],
chemical and biological sensors [11–13], and artificial
muscle [14], actuators [15] are substantially basedon an important principle that the novel chemical,
ll rights reserved.
5364 X.-G. Li et al. / Acta Materialia 52 (2004) 5363–5374
electrochemical and electrochemomechanical properties
would be significantly changed with its reversible redox
reaction upon electrochemical stimulation. As a new
functionality, polydiaminonaphthalene possesses
chelating properties and/or reduction properties owing
to the electron donating groups (amine and secondaryamino groups) on the polymer chain. It has been dem-
onstrated that poly(1,8-diaminonaphthalene) (PDAN)
is sensitive to heavy metal ions, and able to extract
some heavy metal ions including Ag+, Cu2+, Hg2+,
Pb2+, VO2+, Cr3+ from their dilute solutions at the
ion concentration down to 1 lM via complexation with
amine groups on the polymer [16–19]. Thus the PDAN
film has been found to exhibit a potential applicationas a modified electrode for collecting metal ions for
anodic stripping analysis of trace amount of metal
ions, such as Ag+ [16] and Pb2+ [20]. When the metal
ion concentration is enhanced, PDAN film is also able
to reduce some ions with higher standard reduction
potential involving Ag+, Hg2+, Cr2O2�7 to form neutral
metal (e.g. Ag [3]) or lower chemical bond ions (e.g.
Hg2þ2 [17], Cr3+ [21]). It was recently reported thatpoly(1,5-diaminonaphthalene) film, which did not exhi-
bit the detective ability of trace metal ions [22,23], has
also shown the similar complexation and the reduction
properties with the metal ions in a higher concentration
range of 0.001–0.1 M [24–26].
Obviously, one could take the advantage of the
reactive functionalities of both PDAN and poly(1,5-
diaminonaphthalene) films for collection of preciousmetal ions and for removal of hazardous heavy metal
ions from water effluents without having to use an
external energy source. As a matter of fact, Nasalska
and Skompska [21] have attempted to remove toxic
chromate ion through exposing the PDAN film elec-
trosynthesized on a Pt electrode to acidic aqueous
solution of K2Cr2O7. The Cr6+ was reduced to less
toxic Cr3+, via the spontaneous redox reaction betweenCr6+ species and free NH2 group in the PDAN film
matrix. As a practically used absorbent for collection
and removal of metal ions, however, they should be
abundant in mass; otherwise the treating efficiency will
be greatly lowered. To our best knowledge, electrosyn-
thesis would generally lead to a polydiaminonaphtha-
lene film adhered tightly to working electrode with
limited area in the most circumstances. The excellentadhesivity and dense structure of the film are indeed
of great benefit to the modification of electrode, but
they could make against further polymerization of
residual monomer due to lower electroconductivity of
the film than bare electrode. Therefore a plentiful sup-
ply of the polymer cannot be realized by the electrical
synthesis. Besides, the resulting polymer film has rela-
tively low specific area and then restrictive sites to con-tact with metal ions. It seems that electrosynthesized
polydiaminonaphthalene film does not satisfy the
highly efficient application for the removal or recovery
heavy metal ions from solutions. Unfortunately, other
shape of the polydiaminonaphthalene has not been
found in the literature with the exception of the films
thus far.
Almost all investigations to date have been based onelectrochemically oxidative polymerization of diamino-
naphthalenes. As an alternative way, chemically oxida-
tive polymerization has been successfully employed for
the synthesis of polyaniline and polypyrrole [27–29].
However, the method has never been employed for the
synthesis of polydiaminonaphthalene with the exception
of one report concerning poly(1,5-diaminonaphthalene),
in which the authors addressed primarily themselves to afabrication and performance of a resistive-type humidity
sensors [30].
To explore the possibilities of obtaining the polymer
with high yield and high amine content on polymer
chains so as to facilitate the application of polydiamino-
naphthalene as a sorbent for effectively treating heavy
metal ions, we employed chemically oxidative polymer-
ization to prepare PDAN. We report here preliminaryresults on the macromolecular structure and physico-
chemical properties of the resulting polymer powders,
especially on the adsorption for silver ion in the aqueous
solution. Effects of the polymer structure, adsorbent
dose, the initial metal ion concentration, pH and tem-
perature of the solution, and ultrasonic treatment on
the adsorption amount of silver were studied in detail
for the first time.
2. Experiments
2.1. Chemical oxidative polymerization
1,8-Diaminonaphthalene (DAN), ammonium persul-
fate, ferric chloride, acetonitrile, and silver nitrate ofanalytical reagent grade were commercially obtained
and used as received. PDAN was prepared by a chemi-
cally oxidative polymerization of DAN in acetonitrile/
water solution or pure acetonitrile solution using ammo-
nium persulfate and ferric chloride as oxidants, respec-
tively [31,33–35]. The PDAN formed with ammonium
persulfate as oxidant is designated as PDANS, and the
PDAN with ferric chloride is designated as PDANF. Atypical procedure for preparation of PDANS is as fol-
lows: to a 50 mL of acetonitrile at room temperature
was added 0.791 g (5 mmol) DAN in a 200 mL glass
flask. 1.14 g (5 mmol) ammonium persulfate was dis-
solved separately in 50 mL distilled water to prepare oxi-
dant solution. The monomer solution was then stirred
and treated with the oxidant solution by adding drop-
wise at a rate of one drop (60 lL) every 3 s over30 min (the total monomer/oxidant molar ratio = 1/1).
The resulting polymer precipitates were filtered and
X.-G. Li et al. / Acta Materialia 52 (2004) 5363–5374 5365
washed thoroughly with distilled water to remove the
residual oxidant and water-soluble oligomers. The solid
powders were left to dry in ambient air at 40 �C for
3 days. PDANS of 0.62 g was obtained with the yield
of ca. 78.4%. PDANF of 0.77 g was obtained by a sim-
ilar procedure with the yield of ca. 97.3%. By the way,the oxidative polymerization was followed by the
open-circuit potential profile technique, using a satu-
rated calomel electrode (SCE) as reference electrode
and a Pt electrode as working electrode [31].
2.2. Structure characterization
The infrared (IR) spectra were recorded on NicoletMagna 550 FT-IR Spectrometer made in USA at
2 cm�1 resolution on KBr pellets. The elemental analysis
experiments are carried out on a Carlo Erba 1106 Ele-
ment Analyzer made in UK. Solid-state 13C NMR spec-
tra were obtained on Bruker DSX 300 made in Germany
at 75.39 MHz. The bulk electrical conductivity of the
PDAN was measured by a two-disk method using a
UT 70A multimeter made in China at ambient temper-ature. Wide-angle X-ray diffraction was performed with
a Bruker D8 Advance X-ray Diffractometer made in
Germany with Cu Ka Radiation at a scanning rate of
0.888�/min. Scanning electron micrographs of the gold
coated samples were taken using S-2360N Scanning
Electron Microscopy made in Japan. The size of the
PDAN particles in water was analyzed with an LS230
laser particle size analyzer from Beckman Coulter,Inc., USA.
2.3. Reduction adsorption of Ag+
Adsorption of silver ion from aqueous solution using
PDAN as an adsorbent was performed in a batch exper-
iments. Aqueous solutions (25 mL) containing silver ion
at the concentration from 10�4 to 10�1 M were incu-bated with a given amount of PDAN particle sample
at a given temperature. After desired treatment period,
the PDAN was filtered from the solution, the concentra-
tion of silver ion in the filtrate after adsorption was
measured by molar titration at an initial Ag+ concentra-
tion of higher than 0.01 M or inductively coupled
plasma (ICP) at an initial Ag+ concentration of lower
than 0.01 M. The adsorbed amount of silver on PDANwas calculated according to the following equations:
Q ¼ ðC0 � CÞVM=W ; ð1Þ
q ¼ ½ðC0 � CÞ=C0� � 100%; ð2Þwhere Q is the adsorption capacity (mg/g), q the adsorp-tivity (%), C0 and C silver concentration before and after
adsorption, (M), respectively, V the initial volume of the
Ag+ solution (mL); M the molecular weight of metal
ions (g/mol), and W the weight of the PDAN added (g).
2.4. Mathematical modeling
The kinetic reaction for the sorption of Ag+ on
PDAN was studied, and the rate constant of adsorption
was determined using the Lagergren equation. The
adsorption rate expression of Lagergren is as follows:
� lnð1� F Þ ¼ kt; ð3Þwhere F equals Qt/Qe, Qe the amount adsorbed (mg/g) at
equilibrium, Qt the amount adsorbed (mg/g) at time t
and k the adsorption rate constant (h�1).
Two linearized Langmuir and Freundlich isotherm
adsorption models were applied to describe and analyzeadsorption equilibrium, as listed in the following
equations:
Ce=Qe ¼ Ce=Qm þ 1=ðKQmÞ; ð4Þ
lnQe ¼ lnK3 þ ð1=nÞ lnCe; ð5Þwhere Ce is the equilibrium concentration (M), Qe the
adsorption capacity (mg/g). Qm and K are Langmuir
constants related to the saturated adsorption capacity
and adsorption energy, respectively. K3 is the equilib-
rium constant indicative of adsorption capacity and n
adsorption equilibrium constant. The values of these
constants were evaluated from the intercept and the
slope, respectively, of the linear plots of Ce/Qe vs. Ce,and lnQe vs. lnCe, based on experimental data using
the least-squares method (through a regression analysis).
3. Results and discussion
3.1. Synthesis of PDAN particles
The chemical oxidative polymerization of DAN with
(NH4)2S2O8 or FeCl3 as an oxidant in acetonitrile sim-
ply affords black, uniform, and fine particles as the
products. With slowly and regularly dropping the oxi-
dant solution, the polymerization solution turns black
accompanied by a sudden temperature increase from
15.0 to 19.0 �C for the initial 10 min of the reaction,
as shown in Fig. 1. With the continuous dropwise addi-tion of the oxidant, the solution temperature rises at a
much lower rate, reaches a maximum of 19.7 �C at the
32 min of the reaction, and finally decline to a nearly
constant temperature of 19.0 �C at 100 min. This sug-
gests that the polymerization is exothermic and that
the polymerization rate is not constant but self-accelerated.
It is seen from Fig. 1 that the solution potential de-
creases from 230 to 220 mV vs. SCE with adding the firstseveral drops of the oxidant solution for initial 5 min of
the reaction and then a gradual increase to a maximum
of 305 mV vs. SCE at the initial 33 min of the reaction.
A dramatic potential decrease to 270 mV vs. SCE at
about 39 min appears at almost the same time as the
Fig. 2. FT-IR spectra of DAN and PDAN particles prepared by
chemically oxidative polymerization and the PDAN particles adsorb-
ing silver.
0 20 40 60 80 100
220
240
260
280
300
320
So
luti
on
po
ten
tial
(m
V v
s. S
CE
)
Polymerization time (min)
The end of dropping oxidant
14
16
18
20
So
luti
on
tem
per
atu
re (
oC
)
O
O
Fig. 1. The variation of the potential and temperature of the
polymerization solution with polymerization time at DAN/
(NH4)2S2O8 molar ratio of 1/1 in CH3CN/H2O.
5366 X.-G. Li et al. / Acta Materialia 52 (2004) 5363–5374
appearance of a temperature decrease, and after that,
the potential decreases at a very low rate to 255 mV
vs. SCE. The potential maximum almost superposes
the temperature maximum at the same reaction time,
indicating that the maximal polymerization rate occurs
during the initial 33 min. The variations of the temper-
ature and potential suggest that the polymerization has
two stages, one before and one after the major potentialpeak. During the first stage, the polymerization reaction
is fast because of high rising rate of the potential and
temperature. It is believed that at the end of the first
stage, all the oxidants are consumed. In fact, the first
stage ended at the end of the addition of the oxidant.
During the second stage, the monomers further polym-
erize with the oxidized form of the polymer chain in-
stead of the oxidant.It is seen from Table 1 that the polymerization yield
depends remarkably on the used oxidant. Apparently,
ammonium persulfate with higher standard reduction
potential of 2.01 V provides lower polymerization yield
than FeCl3 with lower reduction potential of 0.77 V,
possibly due to the residue of a small amount of Fe in
the obtained polymer, which is confirmed by ICP and
X-ray fluorescence analyses. The PDANF is not pure be-cause an impurity from the remnant of the oxidants
could not be expelled by this subsequent treatment. This
phenomenon is different from pyrrole/phenetidine copo-
Table 1
Characteristics of DAN monomer and PDAN polymers obtained with (NH
Monomer or
polymer
Polymn. yield
(%)
Solubility (solution color)
NMP DMSO THF
DAN – 100% (DB) 100% (DB) 100% (DB)
PDANS 78.4 71% 61% 55% (DB)
PDANF 97.2 60% 46% 20% (B)
B, brown; DB, dark brown.
lymerization [27]. Because ferric ion could be adsorbed
on the PDANF with special molecular structure to some
extent, it could not be adsorbed on the pyrrole/pheneti-
dine copolymer. Furthermore, the two types of PDAN
particles exhibit diverse molecular structure and proper-
ties, as discussed below.
3.2. Structure analysis of PDAN
It appears that no reports concerning structure anal-
ysis of PDAN obtained by chemical oxidative polymer-
ization have been published until now because the
PDAN particles are not totally soluble in most solvents.
The structure characterization should be performed withthe solid-state techniques including FT-IR, elemental,
solid-state high-resolution 13C NMR, and wide-angle
X-ray diffraction analyses.
As shown in Fig. 2, very pronounced changes of the
IR spectral characteristics are observed before and after
the chemical oxidative polymerization of DAN with two
oxidants. Almost all absorption bands of the PDANF
and PDANS are broader than those of DAN, simply
4)2S2O8 and FeCl3 as oxidant, respectively
Electrical
conductivity (S/cm)
Solid particles
Ethanol Size (lm) Appearance
100% (DB) Insulator – Brown
30% (B) 3.6 · 10�11 3.150 Brownish black
0% 2.5 · 10�7 7.028 Black
Table 2
Elemental analysis and possible structures of two types of PDAN polymers
Polymer PDANF PDANS
C/H/N/total (wt%) 62.12/3.31/11.14/76.57 72.77/4.18/13.37/90.32
Experimental formula C10.0H6.39N1.54 C10.0H6. 90N1.58
Calculated formula A C10H6N2 C10H8N2
Calculated formula B C10.0H5.60N1.60 C10.0H7.20N1.60
X.-G. Li et al. / Acta Materialia 52 (2004) 5363–5374 5367
due to the occurrence of the polymerization. Two
absorption bands at 3300 and 3380 cm�1 due to sym-
metric and asymmetric –NH2 stretching vibrations,
respectively, of monomeric DAN merge into a very
broad band centered at 3390 cm�1 of PDANF and
Fig. 3. Solid-state high-resolution 13C NMR spectra of PDANF and PDA
assignments.
PDANS. A small peak at 3040 cm�1 of DAN shifts to
a lower wave number at 2930 cm�1 due to the CAH
stretching vibration on naphthylene unit in the poly-
mers. Particularly, PDANF exhibits quite different band
shape in 1600–900 cm�1 as compared with PDANS,
NS prepared by a chemically oxidative polymerization with possible
10 20 30 40 50 60 70 80Bragg angle, 2θ (degree)
PDANSPDANFPDANS-Ag
Fig. 4. WAXD patterns of PDAN particles prepared by chemically
oxidative polymerization and the PDANS particles adsorbing silver.
5368 X.-G. Li et al. / Acta Materialia 52 (2004) 5363–5374
indicating their different structure. Thus, the IR spectral
results confirm the presence of free –NH2 groups in the
polymer chain and great structure difference in the
DAN, PDANF, and PDANS.
The macromolecular structure of the polymers has
also been studied by the C/H/N ratio because a con-firmed structure must correspond to a certain C/H/N ra-
tio. Two groups of C/H/N ratios have been calculated
and listed in Table 2 according to two proposed formula
of the PDANF and PDANS. Obviously, the experiment
C/H/N ratio computed by element analysis is very close
to the ratio calculated from formula B. Excrescent
experimental H content of the PDANF must be due to
the water absorbed firmly in the polymer. From these re-sults, it is concluded that a denitrogenation happens
during the oxidative polymerization, something like
the denitrogenation observed during the phenylenedi-
amine copolymerization [28,29].
Solid-state 13C NMR spectra of the PDANF and
PDANS are shown in Fig. 3. The resonance peaks in
the region of 95–160 ppm are assigned to aromatic car-
bons [27]. Although it is difficult to make an exactassignment of the several aromatic carbons, a possible
peak assignment has been given in Fig. 3 based on a cal-
culated chemical shift of two model oligomers by CS
Chem Ultra 8.0, CambridgeSoft Corporation, 2004.
Note that the carbons in the PDAN polymers sometimes
exhibit slightly higher chemical shift than the corre-
sponding carbons in the model oligomers because the
polymers must have higher aromaticity than theoligomers (see Schemes 1 and 2).
Supramolecular structure of the PDAN particles has
been characterized by wide-angle X-ray diffraction tech-
nique. As shown in Fig. 4, both PDANS and PDANF
are partially crystalline with two broad diffraction
peaks, but their peak intensity and position are different.
At the almost same background intensity of X-ray dif-
fraction, the PDANS exhibits a strong peak at a Braggangle of 13.0� and a medium peak at 23.9�, while the
HN
HN
HN
HN
HN
126.3 123.8108.3
134.7
125.4113.6
134.7
108.1126.3
119.3
134.0
119.3
108.1 108.1108.3123.8
120.4120.4
114.7 123.5 123.5 114.7 114.7133.9
125.7133.9
109.0109.0109.0109.4108.4
133.0 133.9125.4133.0
Scheme 1. PDANF model oligomer and its c
NH2
NH
NH2
NH2
N
NH2
NH2
N
134.8 122.3
143.7109.4126.6
119.0
127.5
126.5 123.8
129.4
134.3 114.1
133.6
108.6125.1
127.7
119.3
126.3 108.1136.4
125.8 122.9
144.0
109.1125.3
111.7
141.2
109.6 124.1
120.1
135.0 122.2
143.6
114.5126.1
135.5
119.0
126.6 109.4143.7
136.4 118.3
150.2118.7132.0
119.3
164.6
133.4 133.4
164.6129.0
115.
143.0
119.3
126.3
Scheme 2. PDANS model oligomer and its c
PDANF exhibits a weak peak at 12.5� and a weak broad
peak at 24.3�. These results indicate that the PDANS
possesses higher crystallinity than the PDANF because
the PDANS contains much more free –NH2 groups that
result in stronger interaction between the chains than
the PDANF.
The size and morphology of the PDAN particles have
been studied by laser particle-size analysis and scanning
electron microscopy. It is found that the virgin particles
of PDANF and PDANS in water exhibit number-average diameter of 7.028 and 3.150 lm, respectively.
The particle sizes will become 0.376 and 2.013 lm,
respectively, after the virgin particles were treated ultra-
sonically for about 30 min. That is to say, the virgin
particles of PDANF in water are bigger but looser and
HN
HN
HN
NH2
NH2120.4
108.1
125.7
108.1 108.1127.4
114.8114.7
125.7 127.4
108.1108.1108.1108.1108.1
123.5 123.5 114.7
123.8
125.7133.0133.4
123.8133.9120.4
alculated carbon chemical shift (ppm).
NH2
NH2NH2
NH2
NH
N
NH2
NH2
N
129.0 114.9
134.9
109.4115.3
143.0
119.3
126.3 108.1136.4
114.9
134.9
109.43
108.1136.4
128.9 123.6
127.9125.1115.5
151.2
119.0
126.6 109.4
143.7
126.6 122.3135.4
114.3110.4
140.8
111.7
125.3 109.1144.0
136.4 118.3
150.2118.7132.0
119.3
164.6
133.4 133.4
164.6
alculated carbon chemical shift (ppm).
X.-G. Li et al. / Acta Materialia 52 (2004) 5363–5374 5369
therefore easily disaggregate into much smaller particles
after an ultrasonic treatment. SEM observation indi-
cates that the dried virgin particles of PDANF and
PDANS seem to display irregular shape and size of 3.2
and 1.2 lm, respectively. The smaller particle size ob-
served by SEM than laser particle-size analysis shouldbe attributed to compression because of the exclusion
of water inside the particles during drying [31].
3.3. Properties of PDAN particles
It can be seen from Table 1 that the solubility and
electrical conductivity of the PDAN particles are signif-
icantly influenced by the used oxidant. PDANS alwaysexhibit higher solubility in the chosen solvents but lower
conductivity than PDANF, because the latter polymer
seems to be a wholly ladder structure with higher conju-
H
H
HN
NH
HN
HN
NH
Ag+ Ag(a)
Ag+H
NH2
HN
NH2
NH2
Ag
H
NH2
HN
NH2
NH2
+.
+.
+.Ag
Ag
H
H
HN
NH
HN
HN
NH
(b)
H
NH2
HN
NH2
Ag+
N
H2
H2N
N
H2N
H2N
NH2N
H2N
(c)
(d)
(e)
Fig. 5. Complexation and redox absorptions of silver ions on PDAN part
complexation between Ag+ and PDANF; (b) intrachain complexation betw
PDANS; (d) redox between Ag+ and PDANF; (e) redox between Ag+ and P
gated extent and symmetry than the former polymer.
However, the PDANS particles exhibit more effective
absorption of silver ions, as discussed in the following
section.
3.4. Silver ion adsorption
The PDANF particles absorbing Ag exhibit slightly
different IR spectrum in Fig. 2 as compared with origi-
nal PDANF because of their lower ability to absorb
Ag ions. However, the PDANS particles absorbing silver
exhibit quite different IR spectral and wide-angle X-ray
diffractogram characteristics in Figs. 2 and 4 as com-
pared with original PDANS particles. Three majorbands centered at 3380 cm�1 due to the NAH stretching
vibration, at 1580 cm�1 to characteristic C@C stretching
vibration, and at 1260 cm�1 to the CAN stretching
HN
HN
NH
NH2
NH2
+ Ag+ Ag+
n
N
NH2
NH2
N NH2
NH2
N
NH2
NH2
N NH2
NH2n+.
+.
+.
+.
Ag
Ag
Ag
Ag
HN
HN
NH
NH2
NH2n
+.
+.
Ag
Ag
nAg+
Ag+
Ag+
NH2
N
NH2
NH2
N NH2
NH2
Ag+
H
H2N
H
N
n
n
icles prepared by chemically oxidative polymerization. (a) intrachain
een Ag+ and PDANS; (c) interchain complexation between Ag+ and
DANS.
0 10 20 30 40 500
100
200
300
400
500
600
700
800
PDANS
PDANF
Linear Fit
Ag
+ Ad
sorp
tio
n C
apac
ity
(mg
/g)
Ad
sorp
tivi
ty (
%)
The Adsorption Time (h)
The Adsorption Time (h)
0
4
8
12
16
20
(a)
0
1
2
3
4
5
0 5 10 15 20 25
(b)
- L
n(1
- F
)
Fig. 6. Time dependent adsorption of PDAN (a) and corresponding
Lagergren plots of the adsorption (b) in 25 mL AgNO3 solution at
initial Ag+ concentrations of 82 mM for PDANS and 85 mM for
PDANF, respectively, with PDAN particle dose of 50 mg at pH value
of 5.3 and 25 �C.
20 30 40 50 60 70 80 90 10 0
1000
2000
300
200
500
50
PDAN dose (mg)
Ag
+ Ad
sorp
tio
n C
apac
ity
(mg
/g)
Ad
sorp
tivi
ty (
%)
PDAN dose (mg)
PDANS
PDANF
8
12
16
20
24
28
32
36
(a)
100(b)
10 20 30 40 50
76
80
84
88
92
96
100
104
Fig. 7. Effect of PDAN dose on adsorption of Ag+ in 25 mL AgNO3
solution at initial Ag+ concentrations of 82 mM (a) and 1.0 mM, (b)
respectively, at pH value of 5.3 and 25 �C for the adsorption time of
24 h.
5370 X.-G. Li et al. / Acta Materialia 52 (2004) 5363–5374
vibration have become broader and shift to higher wave
number after adsorption of Ag ion. Other weak bandsbecome further weaker or even disappear. Similarly,
PDANS–Ag particles exhibit much weaker diffraction
peak at 13.0� and almost no peak at 23.9� but four addi-tional much stronger sharp peaks at 38�, 44�, 64�, and77� that, respectively, correspond to the diffraction of
(111), (200), (220), and (311) lattice planes of Ag crys-
tals, confirming the reduction of Ag ion by the PDANS.
These results indicate a real uptake of Ag ions onto thePDANS particles. A great diversity of absorbing Ag ions
for two PDAN particles should originate from the
apparent difference in their molecular chain structure,
as shown in Fig. 3. There are much more free amino
(–NH2) groups in PDANS chains than PDANF chains.
Therefore, the PDANS can provide much more redox
Table 3
Lagergren model equation for PDAN adsorption kinetics of Ag+
Polymer Equation Reaction rate constan
PDANS Qt = 832.4 (1�e�0.1873t) 0.1873
PDANF Qt = 369.1(1�e�0.1492t) 0.1492
sites with Ag ions in Fig. 5 and then much stronger
absorbability of Ag ions, although PDANS and PDANF
seem to possess similar amount of complexation sites.
3.4.1. Adsorption kinetics
Adsorption kinetics is studied to determine the time
required for reaching equilibrium adsorption of Ag+.
Fig. 6 shows the Ag+ adsorption capacity and adsorptiv-
ity profiles versus adsorption time on the PDAN parti-
cles. The adsorption capacity and adsorptivity of Ag+
on PDAN increased non-linearly with the prolongationof the adsorption time. The adsorption process consists
of two steps: a primary rapid step and a secondary slow
step. The rapid step lasts for about 12 h and accounts
for the major part in the total Ag+ adsorption, while
the secondary step contributes to a very small part.
t (h�1) Correlation coefficient Standard deviation
0.9873 0.27812
0.9743 0.31895
X.-G. Li et al. / Acta Materialia 52 (2004) 5363–5374 5371
The faster step of Ag adsorption may be attributed to
the surface adsorption due to the interaction between
Ag+ and free amine groups in PDAN particles; while
the slower step may represent diffusion of metal ions
into inner of PDAN particles over a period. So the
adsorption of Ag+ on PDAN particles was mainly phys-ical and complexation adsorption at the initial few hours
of adsorption and then became redox adsorption domi-
nated with prolongating adsorption time. Note that the
adsorption capacity and adsorptivity of Ag+ on PDANS
are much higher than those on PDANF at the same
adsorption condition because there are much freer
20 30 40 50 60 70 80 90 100
800
1000
1200
1400
1600
1800
2000
Ag
+ Ad
sorp
tio
n C
apac
ity
(mg
/g) Particle size
3.1502.013 m
m
Ad
sorp
tivi
ty (
%)
PDANS particle dose (mg)
16
20
24
28
32
36
40
Fig. 8. Effects of particle size on Ag+ adsorption onto PDANS
particles at different PDANS particle dose by ultrasonic treatment in
25 mL AgNO3 solution at initial Ag+ concentrations of 82 mM at pH
value of 5.3 and 25 �C for the adsorption time of 24 h.
0.00 0.02 0.04 0.06 0.08 0.100
100
200
300
400
500
600
700
800
Ag
+ Ad
sorp
tio
n C
apac
ity
(mg
/g)
The Initial Concentration of Ag+
(M)
10
100
5
50PDANS
PDANF
Ad
sorp
tivi
ty (
%)
Fig. 9. Effect of initial Ag+ concentrations on adsorption of Ag+ on
PDAN particles with PDAN dose of 50 mg in AgNO3 solution of
25 mL at pH 5.3, temperature 25 �C for adsorption time of 24 h.
amine groups in PDANS chains shown in Fig. 5 and
Table 2.
The Lagergren model (Eq. (3)) was used to analyze
the adsorption kinetics in Fig. 6(b). Batch kinetics data
are fitted to model by the least-squares method, and the
Lagergren model equations obtained for Ag+are shownin Table 3. It can be seen from Table 3 that the PDANS
particles exhibit higher adsorption rate of Ag+ in solu-
tion than PDANF particles. It is observed from Fig.
6(b) and Table 3 that PDANS–Ag+ adsorption fits the
Lagergren model based on correlation coefficient and
standard deviation in comparison with PDANF–Ag+
adsorption.
3.4.2. Effect of PDAN dose and particle size
Figs. 7 and 8 show adsorption capacity and adsorp-
tivity of Ag+ ion as a function of PDAN dose. The data
clearly show that PDANS is much more effective than
PDANF for the removal of Ag+ in the solution. As seen
in Fig. 7, the ion adsorption capacity decreases with
increasing PDAN dose while the adsorptivity increases,
because increased PDAN dose must lead to decreasedadsorption capacity but increased adsorptivity at a fixed
initial Ag+ concentration. In other words, both the
0 20 40 60 80 1000
200
400
600
800
1000
1200
1400
1600
1800
(a)
Ag
+ Ad
sorp
tio
n C
apac
ity
(mg
/g)
Initial Ag+
Present Per Gram of PDAN (mmol/g)
PDANSPDANF
ECF with 5652 C/g[36]
ECF with 9540 C/g[36]
10
100
5
50
(b)
Ad
sorp
tivi
ty (
%)
Fig. 10. Ag+ adsorption capacity (a) and adsorptivity (b) onto PDAN
versus initial Ag+ present per gram of PDAN in AgNO3 solution of
25 mL at pH 5.3 and 25 �C for 24 h.
5372 X.-G. Li et al. / Acta Materialia 52 (2004) 5363–5374
highest adsorption capacity and the highest adsorptivity
could not be realized simultaneously. However, it is
found that relatively high adsorption capacity and high
adsorptivity could be realized simultaneously at the
PDAN dose of 20 mg.
Effect of PDANS particle size on Ag+ adsorption isshown in Fig. 8. It is found that both the adsorption
capacity and adsorptivity demonstrate an increase of
6.4–8.4% in the whole range of PDANS doses with low-
ering the particle size from 3.150 to 2.013 lm, because
larger specific surface area of the particles with smaller
size leads to more sufficient adsorption of Ag+ on and
inside the PDANS particles. That is to say, smaller
PDAN particles will exhibit both higher adsorptioncapacity and higher adsorptivity than larger particles.
The largest amount of adsorbed Ag+ in the initial Ag+
present per gram of absorbent from 10 to 100 mmol/g
can reach 1924 mg/g (PDAN) that is even higher than
theoretical silver absorbance [1500 mg/g (PDAN at
n = 2)] through complex and redox functions shown in
Fig. 5(b), (c), and (e). Therefore, additional silver might
be physically adsorbed on/in the PDAN micrometerparticles because of the porosity of the particles in
water. Apparently, the PDANS particles exhibit too
0.00 0.02 0.04 0.06 0.08 0.10
0.5
1.0
1.5
2.0
2.5
(a)
Ce/
Qe
[10
-4M
/(m
g/g
)]
Ce (M)
-5.0 -4.5 -4.0 -3.5 -3.0 -2.5
5.5
6.0
6.5
7.0
PDANS
PDANF
Linear Fit
(b)
lnQ
e (
mg
/g)
LnCe (M)
Fig. 11. Langmuir (a) and Freundlich (b) plots of the adsorption data
reported in Fig. 9 in the concentration range from 0.01 to 0.1 M.
Table 4
Isotherm model equations for Ag+ adsorption on PDAN based on the data
Mathematical model Polymer Equation Corre
Langmuir PDANS Qe = 1670 Ce/(188.7 Ce + 1) 0.999
PDANF Qe = 32770 Ce/(77.99 Ce + 1) 0.999
Freundlich PDANS Qe ¼ e7:197 C1=5:587e 0.990
PDANF Qe ¼ e6:611 C1=3:555e 0.985
much higher silver adsorption capacity than traditional
adsorbents (25–70 mg/g adsorbent) such as active car-
bon [32], iodized cotton [33], thiol cotton fiber [34]. In
fact the silver adsorption capacity of the PDANS parti-
cles still significantly exceeds 701 and 1360 mg/g of Ag+
adsorbed onto the best polyethylene-1,4-dithio-carb-oxyl-piperazine [35] and electrically activated carbon fi-
ber [36], respectively, under the initial Ag+ to the fiber of
100 mmol/g for 30 days. The carbon fiber is recently
considered as the highest performance Ag+ absorbent
[36]. However, the PDANS particles could be a new
strongest Ag+ adsorbent till now.
Note that too small particles such as nanoparticles
might not be beneficial to collection and removal ofAg+ ions because the Ag+-absorbing nanoparticles
could not be easily recovered from the solution.
3.4.3. Effect of the concentration, pH and temperature of
Ag+ solution
The effect of the initial Ag+ ion concentration on the
ion equilibrium adsorption for 24 h at 25 �C has been
investigated. Figs. 7, 9 and 10 reveal that the Ag+
adsorptivity decreases with an increase in the ion con-
centration but the adsorption capacity and equilibrium
from Fig. 11
lation coefficient Standard deviation Qm (mg/g) Qe/Qm (%)
3 1.590 · 10�6 885.0 95.34
8 1.885 · 1 0�6 420.2 89.05
0 0.03267
9 0.04806
300
400
500
600
700
800
900
10 20 30 40 50
Solution Temperature(oC)
Ag
+ Ad
sorp
tio
n C
apac
ity
(mg
/g)
Solution pH2 4 6 8 10
Fig. 12. Effects of temperature at pH 5.3 and pH value at 25 �C on
adsorption of Ag+ for PDANS in 25 mL AgNO3 solution at initial Ag+
concentrations of 82 mM with PDANS dose of 50 mg for 24 h.
X.-G. Li et al. / Acta Materialia 52 (2004) 5363–5374 5373
ion concentration increase. The adsorption can be con-
sidered to be a very fast process at a lower initial ion
concentration and become very slow and saturation lev-
els at a higher initial ion concentration.
To quantitatively establish the relationship between
ion concentration and adsorption process, two mathe-matical models by Langmuir (Eq. (4)) and Freundlich
(Eq. (5)) are used to analyze the adsorption isotherm,
as shown in Fig. 11 and Table 4. Apparently, both
PDANS-Ag+ and PDANF-Ag+ fit Langmuir model bet-
ter because of their higher correlation coefficient and
lower standard deviation.
The effect of pH in a range of 1.5–10.0 on Ag+
adsorption on PDANS particles is illustrated inFig. 12. It is found that the adsorption capacity of
Ag+ ion increases significantly with raising pH and
reaches a substantially stable value after pH 5.3 because
the weakened protonation of free amine groups on the
PDAN with increasing pH in an acidic condition in-
duces the improvement in the Ag+ ion adsorption onto
the PDAN particles. It is observed from the plot of
Ag+ adsorption capacity as a function of adsorptiontemperature onto PDANS particles in Fig. 12 that the
ion adsorption capacity increases slightly with elevating
adsorption temperature from 10 to 50 �C because the
elevation of temperature accelerates the diffusion and
reaction of metal ions in polymer and also results in
the formation of the more free volume inside the poly-
mer particles. Apparently, both high solution tempera-
ture and high pH are advantageous to theenhancement of the adsorption capacity of Ag+ ion.
4. Conclusions
PDAN fine particles have been synthesized success-
fully by a facile chemical oxidation polymerization of
DAN with (NH4)2S2O8 or FeCl3 as oxidant with a high
yield and productivity [37–39]. The structure and prop-
erties of the PDAN obtained depend strongly on the
oxidant used. FeCl3 oxidant will produce a ladder
PDANF chain with less free –NH2 groups, while(NH4)2S2O8 oxidant will produce a linear PDANS chain
with much more free –NH2 groups. Therefore, the
PDANS particles exhibit much stronger silver-ion
adsorbability than the PDANF particles. Silver-ion
adsorbability of the particles can be further maximized
by carefully controlling the adsorption time, the dose
and size of the particles, the temperature and pH of
Ag+ solution. The amount of adsorbed Ag+ can reach1924 mg/g (PDAN) with exposure to a solution contain-
ing 82 mM Ag+ ion for 24 h at an initial ratio of Ag+ to
PDAN weight of 103 mmol/g. It seems that a total
weight of Ag+ adsorbed was 1.92 times the PDAN
weight, higher than theoretical absorbance by complex-
ation and redox functions. This value significantly
exceeds 1360 mg/g of Ag+ adsorbed onto activated car-
bon fiber under the initial Ag+ to the fiber of 100 mmol/
g for 30 days, implying that the PDANS particles could
be a new strongest Ag+ adsorbent by far. Two different
reactions occurred during Ag+ adsorption onto the
PDAN particles, including complexation adsorption be-tween Ag+ ion and amine or imine units as well as the
redox adsorption between Ag+ and free –NH2 group
in the polymer chain. Wide-angle X-ray diffraction of
the PDAN particles adsorbing Ag+ shows that the
Ag+ was reduced to Ag0. The PDAN has presented a
great potential application in recovery and elimination
of noble or heavy metallic ions from wastewater.
Acknowledgements
The project was supported by: (1) the National Nat-
ural Science Foundation of China (20274030) and (2)
the Fund of the Key Laboratory of Molecular Engineer-
ing of Polymers, Fudan University, China. The authors
would thank Prof. Dr. Yu-Liang Yang and Wei Zhang(Fudan University) for their valuable helps.
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