CHAPTER6 Silver nanoparticles as a catalyst for...
Transcript of CHAPTER6 Silver nanoparticles as a catalyst for...
CHAPTER6
Silver nanoparticles as a catalyst for degradation of hazardous textile dye
• Abstract
6.1 Introduction
6.2 Experimental
6.2.1 Materials
6.2.2 Apparatus
6.2.3 Preparation of silver (Ag) nanoparticles
6.2.4 Batch degradation
6.2.5 Analysis of degraded products
6.3 Results and discussion
6.3.1 Characterisation
6.3.2 Kinetics of dye degradation
6.3.3 Identification of degraded product
6.3.4 Catalytic degradation
6.3.5 Degradation Pathway
6.4 Conclusion
• Figures and Tables
• References
Chapter 6 Silver nanoparticles
ABSTRACT
The degradation of five different categories of dyes in aqueous solutions was
investigated using silver nanoparticles as catalyst. Silver nanoparticles were prepared by
citrate reduction method and was characterized by Dynamic light scattering (DLS)
Transmission electron microscopy (TEM) and UV-Vis spectrophotometry. Red 5B
(R5B), Red 6BX (R6X), Acid orange 7 (A07), Orange 2R (02R) and Patent blue (PB)
were degraded by batch experiments with silver nanoparticles as nanocatalyst, which
showed complete degradation within 15 min of reaction time. The process was studied by
monitoring the simultaneous decrease in the height of absorbance peak of dye solution
and increase in the height or shifting of plasmon peak corresponding to silver
nanoparticles. The results show that silver nanoparticles are an efficient catalyst for
degradation of dyes and their efficiency depends on surfactant and its nature. The
analysis of the degraded products by chemical oxygen demand (COD), FT-IR
transmission, Mass spectrometry (MS) and Ion chromatography (IC) revealed that the
degradation mechanism proceeds through a reductive cleavage of the azo linkage
resulting in the formation of amines, sulphates, C02 and H20.
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6.1 INTRODUCTION
Industrial development is pervasively connected with the disposal of a large
number of various toxic pollutants that are harmful to the environment and hazardous to
human health. It is difficult to degrade the pollutants by natural means. Synthetic textile
dyes and other industrial dyestuffs constitute the largest group of chemicals produced in
the world. Dye effluents discharged in rivers contaminate water and surrounding
environment. Aquatic animal and the biological processes in river get affected due to this
contamination. Thus the removal of dyes from industrial effluents is of great significance
in connection with environmental and human health safety. During the past decade, much
attention has been paid to investigate the degradation of dye pollutants with different
methods like photo catalytic degradation [1, 2], oxidative processes [3], biodegradations
[ 4, 5] etc. But these methods are time consuming and conditions are critical.
Photocatalytic degradation using titanium dioxide is also a technically viable clean-up
process for dyes [1, 2] but problems like low quantum yield and requirement ofUV light
hinder its widespread acceptance as a practical remediation technology. Therefore, a
rapid and easy method for degradation is desirable.
Synthesis of metal nanoparticles, their characterization and application to
different fields have become an important trend in the modem investigation.
Nanoparticles have unique chemical, electronic, magnetic and optical properties due to
their small size and high surface area [7-9]. Nanoparticles have found uses in many
applications like catalysis, drug delivery system, optoelectronics, magnetic devices etc
[10-12]. For example, due to specific optical properties, metal nanoparticles show
presence of strong absorbance band in the visible region, which is not found for bulk
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metals. This absorbance band arises due to plasmon oscillation modes of conduction
electrons [ 13-15]. Due to this plasmon band optical properties of metal particles,
particularly of gold, silver and copper nanoparticles, have received considerable attention
[ 16-18]. Secondly metal nanoparticles, because of their larger surface area to volume
ratio and size dependent reactivity are found to be promising adsorbents and catalysts.
Silver nanoparticles have shown to be amazingly active and selective catalyst for various
important reactions [19-23].
Several physico-chemical and bio degradation techniques have been reported for
the degradation of dyes like catalytic degradation [ 1, 2], photo oxidative processes [3],
biodegradations [4, 5] etc. However, the time for the degradation by most of these
methods are higher. Biodegradation is an environmentally friendly and cost competitive
alternative but the conventional aerobic treatments have been proved ineffective while
highly toxic aromatic amines can be formed by reductive fission under anaerobic
conditions. Photo oxidative processes take much higher time with very low
concentration. Such degradant are not suitable for the large scale degradation treatment
purpose, especially as some ofthem can work only at limited pH value [4].
In the present investigation Ag nanoparticles were chosen for the first time to
study their catalytic reductive degradation capacities in presence of cationic and anionic
surfactants. Red 5B (R5B), Red 6BX (R6X), Patent blue (PB), Acid orange (AO) and
Orange 2R (02R) dyes, used extensively in textile industries, were chosen as model
compounds for batch degradation experiments. UV-Visible, FT-IR and GC-MS and IC
analysis of dye degradation directs the surface chemistry and drives the mechanistic
pathway of degradation. The research was targeted towards the study of ( 1) the efficiency
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of the nanocatalst for the degradation of dye pollutants; (2) the interaction between the
dye and nanocatalyst in the aqueous systems, and (3) the mechanism of catalytic
enhancement of degradation of dye pollutants.
6.2 EXPERIMENTAL
6.2.1 Materials
AgN03 (crystal extra pure) was purchased from Merck. Sodium dodecyl sulphate
(SDS), cetyl trimethyl ammonium bromide (CTAB), sodium citrate and sodium
borohydride were obtained from Thomas Baker. Red 5B (R5B), Red 6BX (R6BX), Acid
orange (AO), Orange 2R (02R) and Patent blue (PB) dyes obtained from shree Uma dye
chemicals, Ahmedabad, India were used without further purification. All other chemicals
used were of analytical reagent grade. Deionized and doubly distilled water was used
throughout the study. The pH of the solution was adjusted with either dilute HCl or
NaOH. The structures ofthe dyes are given in Fig. 1.
6.2.2 Apparatus
The morphology and s1ze of the nanoparticles were estimated usmg the
transmission electron microscope (TEM) (PHILIPS TECNAI) operated at an accelerating
voltage of 120 kV and a magnification of 85000X. Samples were loaded on carbon
coated grids before being introduced into the vacuum chamber. The size distribution of
Ag nanoparticles was studied using Dynamic light scattering (DLS) (Nanotrac NPA
150/250). DLS analysis was done by dipping a probe of DLS in the dispersed Ag
nanoparticles. [Descreptin of DLS will be added from the reported paper later] UV-vis
absorption spectra were acquired on a Jasco V-570 UV-vis spectrometer. IR spectra
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Chapter 6 Silver nanoparticles
were measured with a Broker Tensor 27 FT-IR spectrometer. The degraded products
identification were done by Perkin-Elmer GC-MS and Dionex IC.
6.2.3 Preparation of silver (Ag) nanoparticles
The procedure for the preparation of Ag nanoparticles was essentially the same as
those developed by Turkevich [24] with difference only in the molar ratio of AgN03 to
sodium citrate. All glassware was thoroughly cleaned with freshly prepared 3: 1
HC1/HN03 (aqua regia) and rinsed thoroughly with deionized water prior to use. To
synthesize Ag nanoparticles in water, in a round bottom flask equipped with a condenser,
100 mL of 0.1 mM AgN03 was brought to a boil with vigorous stirring, and 10 mL of
17.0 mM sodium citrate (Aldrich) was rapidly added to the vortex of the solution. The
yellow solution was cooled to room temperature (25° C) by stirring for another 15 min.
The solution was then filtered through a 0.45 ~filter (Millipore, Nylon membrane).
6.2.4 Batch degradation
All the five dye stock solutions (1.0 x 104 M) were prepared in deionized water
and degradation experiments were performed in an open batch system at neutral pH and
room temperature. Sodium borohydride (1.0 x 10-3M) and SDS (1.0 x 10-2 M) surfactant
solutions were prepared by dissolving the requisite amount in deionized water. In a
typical experiment 100 ml (1.0 x 104 M) dye solution was mixed with 5.0 ml Ag
nanoparticles and 5.0 ml of surfactant SDS in 250 ml conical flask with stirring. To the
above mixture, 5.0 ml of sodium borohydride solution was added. The degradation of all
five dyes was completed within around 15 min.
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6.2.5 Analysis of degraded products
The degraded products of the dye samples were analysed using UV-Vis, FT-IR, GC-MS,
IC and by measuring the chemical oxygen demand (COD) of the samples.
UV-Visible
The rate of degradation was monitored by measuring the absorbance of the
solution from 190 to 800 nm with a UV-Vis spectrophotometer.
FT-IR
The degraded product was analyzed using FT-IR from 4000 to 400 cm-1• TheFT
IR spectra of the dye samples before and after degradation were then compared.
GC-MS
The final degraded product was identified by in Perkin-Elmer GC-MS Clams 500.
Perkin-Elmer GC-MS Clams 500 equipped with DB 1 column, size 30 m X 0.32 mm,
0.25 11m (5° C/min) was used for the analysis.
Ion chromatography
Operating conditions of Dionex ICS 1000 were as follows: column, IonPac® AS-11
HC (4 x 250 mm); suppressor conductivity ASRS®ULTRA II 4-mm anion self- regenerating
suppressor (ASRS); eluent, 20 mM sodium hydroxide (Sigma-Aldrich); column temperature
25±2° C; flow rate 1.5 mllmin; injection volume, 25 11l; System backpressure 1920 psi; and
background conductivity: 22 !J.S.
Chemical oxygen demand
The chemical oxygen demand (COD) of the dye (50 mL of 1.0 x 104 M) was
measured directly, without removal of the silver nanoparticles and surfactant, after time
intervals using the titrimetric method [25]. 1 g of mercury sulphate was added to 50 mL
1.0 x 104 M dye solution, followed by 80 mL of a solution of silver sulphate in sulfuric
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acid. Then 10 mL of 0.00833 M potassium dichromate solution was added and refluxed
for 15 min. After cooling, added 1 mL of ferroin indicator and titrated with 0.025 M
ammonium iron sulfate solution. The end point is marked by the change of color from
blue to green. Calculated the COD using the equation given below.
COD= A-B x 0.2 x 20 mgL-1
Here, A is dye sample reading and B is blank titration reading.
6.3 RESULTS AND DISCUSSION
6.3.1 Characterisation
The TEM image of Ag nanoparticles with the size calibration is shown in Fig.2.
The particle size of Ag nanoparticles is in the range of 50-70 nm. Fig. 3 shows dynamic
light scattering histogram of Ag nanoparticles. The mean particle diameter obtained from
DLS graphs is 60.6 nm which supports the TEM results. The plasmon peak of Ag
nanoparticles was observed at 422 nm which is shown in Fig. 4.
6.3.2 Kinetics of dye degradation
The absorption spectra of an aqueous solution of all five dyes recorded is shown
in Fig. 5 (topmost). The molar absorptivity of dyes at the 1.0 x 104 M concentration
studied, which are measurably high (Table 1) even for dilute solutions and hence
photometric method was adopted for the investigation. Aqueous solutions of the dyes
were degraded within a period of 15 min, which can be inferred from the reappearance of
the color of the nanoparticles (Fig. 6). The present method of degradation is faster and
efficient than biodegradation using Comamonas sp. UVS bacteria, which take 125 h for
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the degradation of Red 5B dye [5]. The absorption band of the dye disappeared
irreversibly (Fig. 5), indicating that the chromophoric structure of the dye was destroyed.
After the addition of the nanocatalyst, the dye solution was stirred for 15 min to
ensure that the equilibrium adsorption/desorption of the dye on the catalyst was attained.
The corresponding concentration of the dye (as measured by UV spectrophotometer and
the concentration evaluated using Beer-Lambert's law) was taken as the initial
concentration (1.0 x 104 M) of the dye for all the catalyzed reactions. Fig. 7 shows the
decrease in dye concentration with the time. From the results it can be concluded that
dyes degraded within 10-12 min which is faster than the previously reported methods [5].
Fig. 8 shows the variation of concentration profile of various dyes, at different
initial concentrations, in silver nanoparticles catalyzed system. The initial rates of the
reaction were determined by extrapolating the tangent (based on the linear fit of the first
four points) of the concentration profile back to initial conditions. The slopes (Table 2) of
logC vs time calculated were nearly constant, indicating the accuracy of the initial rate of
degradation in this study.
6.3.3 Identification of degraded products
The degraded products of the dye samples were identified using FT-IR,
GC-MS, IC and by measuring the chemical oxygen demand (COD) of the samples.
FT-IR
The variation of the IR spectra during the course of the nanocatalytic degradation
of dye is illustrated in Fig 10. The assignments for the principal bands in the IR spectra of
dyes (before degradation): around 1500-1400 cm-1 correspond to aromatic ring and C
aryl bond vibrations; 650 cm-1 are caused by vibrations in the S04-2
• During the
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degradation process, bands of vibrations characteristic of the carbon-nitrogen bond, the
C-aryl bond, and nearly all of the aromatic skeletal vibrations decreased and disappeared.
The S04 .z absorption band was retained in the degraded product. Meanwhile, some new
IR absorption bands appeared in the region 900-650 cm·1 indicating a non-aromatic
structure of the band [25].
GC-MS
Gas chromatography-mass Spectrophotometry (GC-MS) was used to analyze the
degraded final products formed in the direct nanocatalytic degradation of the dyes. The
MS features confirm the major components of the all five dye degraded and fragmented.
Final products of the dyes are described in Fig. 11. Acid orange dye does not give any
final degraded product which depicts that complete mineralization of the dye.
Ion chromatography
The IC analysis of the degraded dye shows presence of S04•2 (Fig. 12) with two
overlapped chromatogram ofS04"2 peak at retention time 3.83 min. Since SDS is used as
a surfactant, the dye sample for degradation was also injected to IC before the addition of
nanoparticles and an increase in the peak height of So4•2 (gray peak) was observed due to
destruction of dyes.
Chemical oxygen demand
Fig. 9 shows the temporal changes of COD in the degradation of the dye, which
reflect the mineralization extent of the organics in the total dispersions (both in the bulk
and on the nanoparticles surface). The decrease in COD values of 1.0 x 104 M of dye
(100 mL) containing silver nanoparticles and surfactant also shows two different stages
before and after 5 min, 90.0 % of total COD was reduced before 10 min, indicating that
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all the dyes investigated mainly underwent the destruction of the chromophore structure
within 5 min, and subsequent mineralization of the fragments occurred after 5 min.
6.3.4 Catalytic degradation
The degradation was very slow and insignificant in the absence of catalyst. The
characteristic absorption band of dye was not affected even after 20 min. However, when
Ag nanopaticles was added, the color of the dye started fading very fast which was
slowed down after a period of 5 min. This may be because of the tendency of Ag
nanoparticles to agglomerate, decreasing their surface area. It is also possible that in
presence ofNaBH4, the surface of Ag nanoparticles becomes vulnerable to oxidation and
their catalytic activity decreases [22]. The degradation was faster in presence of a
surfactant. The surfactant stabilizes the nanoparticles and keeps them dispersed, so that
large surface area of catalyst is available to the reactants. Moreover, the surfactant also
dissolves the oxide layer and makes the surface clear, and hence the degradation process
is not affected. We avoided the use of surfactant while synthesizing the nanoparticles
because presence of surfactant during the formation of nanoparticles influences the
particle size and shape [26-27].
To study the effect of pH, the degradation of all the dyes was carried out in a wide
range of pH from 2-10 [Fig. 12]. It was observed that the degradation is slower at pH<
7.0 in all the cases, while the complete degradation resulted within 10 min in basic
medium (pH 7- 10). The pH study ofR5B is shown in Fig. 12 as a representative case.
The influence of nature of surfactant on dye degradation process was studied and
it was found that the degradation was much faster when SDS was used as surfactant, with
CT AB the degradation becomes very slow (2 h) which may be due to the cationic nature
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of the surfactant SDS is an anionic surfactant and when dye and surfactant molecules are
of similar charges, repulsive interaction will result and thus surfactant molecule will not
hinder the approach of dye molecule to the catalyst surface, which ultimately will result
in faster degradation.
6.3.5 Degradation Pathway
The process of degradation can be explained by electron transfer mechanism. The
reductant molecules and dye molecules probably get adsorbed on the large surface of
nanoparticles with out affecting their activity because of the presence of surfactant [Fig.
13]. The electrode potential of Ag/Ag+ is +0.80 V. When reducing agent NaBH4 is
adsorbed on the nanoparticles, its reductive potential decreases, as NaBH4 is a strong
nucleophile. On the other hand, when dye molecules get adsorbed on nanoparticles, their
reduction potential increase, as the molecules are electrophilic in nature and hence, when
both the species are adsorbed on nanoparticles they become more -ve for NaB~
molecules and more +ve for dye molecule. The electron transfer takes place from
reducing agent to dye molecule via metal nanoparticles, and result in the destruction of
the dye chromophore structure to form small species such as acetamide, S04-2, C02, H20.
6.4 CONCLUSION
This study demonstrates the reductive degradation of azo dyes usmg Ag
nanoparticles as catalyst in aqueous solution. The rapid degradation of the dye solution
was monitored spectrophotometrically and was found to fit the first order kinetic model.
The degradation was more efficient at higher pH (> 7.0). With increasing acidity of dye
solution the degradation rate decreased. FT-IR, MS and IC analysis of degradation
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products revealed the reductive cleavage of the azo linkage to produced amines, amides,
S04-2
, C02 and H20, which are in very small amount and less hazardous than original
dye. Ag nanoparticle was successfully applied as catalyst along with surfactant SDS.
More significantly, Ag nanoparticles showed fast and rapid degradation with sustained
reactivity compared to other degradation technique like biodegradation, and making them
potential candidates for dye degradation technologies.
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Figures and Tables
(a)
(b)
(d)
OH
(e)
OH
Fig. 1 Structure of (a) Red 5B; (b) Red 6BX; (c) Acid orange; (d) Orange 2R; (e) Patent Blue
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Chapter 6 Silver nanoparticles
Fig. 2 Transmission Electron Microscope image of silver nanoparticles
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Chapter 6 Silver nanoparticles
1.0
0.0
0.8
0.7
8 ij
0~
-e 0.0 0
~ 0.4
0.3
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300 ..., 000 000 700
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Fig. 4 UV-vis absorbace spectra shows Plasmon peak (422 nm) of silver nanaoparticles
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00~~~~~~~~~~~~--~~ 300 400 500 600
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Figure 5. UV-visible spectral changes of (A) Red 58; (B) Red 6BX; (C) Acid orange; (D) Orange
2R; (E) Patent Blue dye (1.0 x 10-4M), the topmost to bottom show decreasing the concentration
of dye.
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Chapter 6 Silver nanopartic/es
Table 1: The molar absorptivity of dyes at the 1.0 x 10-4 M concentration
Dye Amax (nm) € (mor em·)
R5B 510 5020
R6BX 508 6950
AO 484 4750
02R 480 4880
PB 658 3120
____ R6BX
Figure 6. Reappearance of original silver nanoparticles color after degradation of dye.
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Chapter 6
0.00012
0.0001
l 0.00008 , .... 0 u 0.00006 c: 8 0.00004
0.00002
0
0 5 10
nme(min)
15
Silver nanoparticles
~RSB
-R6BX
~AO
_._02R
--11-PB
Figure 7. Dye concentration changes as a function of time (Concentration of dye: 0.0001 M)
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Chapter 6
0.00012
CD 0.0001 X! 'S 0.00008 c 0
~-~ 0.00006
" 0.00004 c
s 0,00002
'S c 0
~ " c
s 0.00002
0 2 4 6
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8 10 12
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6
time{min)
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8 10 12 14
Silver nanoparticles
6
lime(min)
6 8
time(min)
8 10 12
10 12 14
Figure 8. Concentration profiles of (a) R58; (b) R6BX (c) 02R (d) OA and (e) PB when
degraded with the initial concentration of (•) 1.0 x 104 M (•) 0.5 x 104 M(.&) 1.0 x 10·5 M.
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Chapter 6 Silver nanoparticles
Table 2 slopes of dyes at different concentration (loge vs time )
Dye 1.0 X 1004 M 0.5 X 1004 M 1.0 X 10"5 M
R5B -6.544 -6.643 -6.643
R6BX -9.698 -9.698 -9.698
02R -6.092 -6.093 -6.092
AO -12.490 -12.490 -12.491
PB -8.569 -8.569 -8.568
60
50
..... 40 .
..J -+-R5B C)
E 30 __._R6BX Q 0 -a-AO () 20
-o2R 10 -&-PB
0
0 5 10 15
Time(min)
Figure 9. Variation in COD during the course of direct degradation of dye (1.0 X 104 M, 50 ml)
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Chapter6 Silver nanoparticles
(a)
R5B
(b)
AO (b)
02R (b)
PB (b)
3XO 2fO) Z1Q) 1800
Wavelength (nm)
Figure 9. Variation of IR spectra in the degradation of dyes (a) before degradation and (b) after
degradation
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Chapter 6 Silver nanoparticles
11:10 st
Ill,) CHaCOt1H2
I ~
~-:t
fO
n :0
~ 10 ;., fO ;0
to
co "' CH(ttH2~
\to ~
CICH2ttH2 Jl I ~
ao •> ll •• .~
:0
avo , .. ... ········•··············· .................. , ...... .
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110
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:0
0
·~
Figure 10. Mass spectra of mineralized component of degraded products (from GC-MS
spectroscopy)
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Chapter6 Silver nanoparticles
9
8
7
6
5
"' 4 E
3
2
1
0
-1
lime(min)
Figure 11. Effect on sulphate ion peak before and after degradation of dye (1.0 x 104
M, 100 ml)
0.6
0.5 (I)
0.4 (,)
c: co -+-pH2 .Q 0.3 .. 0 -a-pH4 fn .Q 0.2 <( ...,..pH6
0.1 -*-PH7-10
0
0 10 20 30
Time (minute)
Figure 12. Variation of pH (2-10); 1.0 X 104 M R5B dye concentration.
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Chapter 6 Silver nanoparticles
Dye•
t Degraded dye
Surfactant layer
Destruction of Chromofore Structure
Final Products
Acetamide(R5B)
Methanetrlamlne {R6BX and 02R)
Dlethyl amine (PB)
Figure 13. Degradation Pathway of dye
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Chapter 6 Silver nanoparticles
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Chapter 6 Silver nanoparticles
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