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: lixingui@tongji.edu.cn (X.-G. Li), huangmeir-

ong@tongji.edu.cn (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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