Influence of acids, bases and surfactants on the photocatalytic degradation of a model dye rhodamine B

Article abstract of DOI:10.1016/j.molliq.2017.04.063

Photocatalytic materials and additional components, especially promoters, play a pivotal role in the success of photocatalysed environmental remediation processes. To understand the influence of acid promoters in the absence and presence of surfactants, photocatalytic degradation of a model dye rhodamine B (RhB) was studied by UV-photocatalysis (254?nm UV source) with the subsequent addition of each of 0.01?mol?dm^??3 of nitric acid (HNO₃), sulphuric acid (H₂SO₄), hydrochloric (HCl), citric acid and acetic acid. Furthermore, the effect of 0.01?mol?dm^??3 of sodium hydroxide (NaOH) and sodium carbonate (Na₂CO₃), as well as surfactants, including the cationic cetrimonium bromide (CTAB), anionic sodium dodecylsulphate (SDS) and non-ionic surfactant tween 80, were analyzed using a UV–Visible spectrophotometer. The k_obs. values for photodegradation of RhB under UV-irradiation was heavily dependent on type of acid and base. Pre-micellar, micellar and post-micellar concentrations of surfactants were also analyzed for their effects on the photodegradation of RhB. Concentrations at and above the critical micelle concentration (CMC) were found to suppress the photocatalytic degradation of RhB in the presence of a promoter (HNO₃). Micellar solubilization is responsible for the changes in the photocatalytic activity. Nonionic micelles are unable to protect the dye over longer times due to autooxidation process. The resulting degradation products of RhB by [rad]OH were identified by LC-MS and GC–MS analysis and accordingly a schematic degradation pathway was proposed.

Full text of DOI:10.1016/j.molliq.2017.04.063

Accepted Manuscript
Influence of acids, bases and surfactants on the photocatalytic
degradation of a model dye rhodamine B
Asad M. Khan, Abid Mehmood, Murtaza Sayed, Muhammad F.
Nazar, Bushra Ismail, Rafaqat A. Khan, Hameed Ullah, Hafiz M.
Abdur Rehman, Athar Y. Khan, Abdur R. Khan
PII:
DOI:
Reference:
S0167-7322(17)30509-3
doi: 10.1016/j.molliq.2017.04.063
MOLLIQ 7219
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Accepted date:
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29 March 2017
15 April 2017
Please cite this article as: Asad M. Khan, Abid Mehmood, Murtaza Sayed, Muhammad F.
Nazar, Bushra Ismail, Rafaqat A. Khan, Hameed Ullah, Hafiz M. Abdur Rehman, Athar
Y. Khan, Abdur R. Khan , Influence of acids, bases and surfactants on the photocatalytic
degradation of a model dye rhodamine B. The address for the corresponding author was
captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi:
10.1016/j.molliq.2017.04.063
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ACCEPTED MANUSCRIPT
Influence of Acids, Bases and Surfactants on the Photocatalytic Degradation of a Model
Dye Rhodamine B
Asad M. Khana*, Abid Mehmooda, Murtaza Sayedb, Muhammad F. Nazarc, Bushra Ismaila,
Rafaqat A. Khana, Hameed Ullahd, Hafiz M. Abdur Rehmane, Athar Y. Khane, Abdur R.
Khana.
aDepartment of Chemistry, COMSATS Institute of Information Technology, Abbottabad-
22060, Pakistan
bNational Center of Excellence in Physical Chemistry, University of Peshawar, Peshawar,
Pakistan
cDepartment of Chemistry, University of Gujrat, Hafiz Hayat Campus, Gujrat Pakistan
dDepartment of Chemistry, Hazara University, Dodhial, Mansehra, Pakistan
eDepartment of Chemistry, Forman Christian College, 54600-Lahore, Pakistan
Email: *amk.qau@gmail.com
Tel: +92(0)992-383591-6 Ext 568
Fax: +92(0)992 - 383441
Abstract
Photocatalytic materials and additional components, especially promoters, play a pivotal role
in the success of photocatalysed environmental remediation processes. To understand the
influence of acid promoters in the absence and presence of surfactants, photocatalytic
degradation of a model dye rhodamine B (RhB) was studied by UV-photocatalysis (254 nm
UV source) with the subsequent addition of each of 0.01mol dm-3 of nitric acid (HNO3),
sulphuric acid (H2SO4), hydrochloric (HCl), citric acid and acetic acid. Furthermore, the
effect of 0.01 mol dm-3 of sodium hydroxide (NaOH) and sodium carbonate (Na2CO3), as
well as surfactants, including the cationic cetrimonium bromide (CTAB), anionic sodium
dodecylsulphate (SDS) and non-ionic surfactant tween 80, were analyzed using a UV-Visible
spectrophotometer. The kobs values for photodegardation of RhB under UV-irradiation was
heavily dependent on type of acid and base. Pre-micellar, micellar and post-micellar
concentrations of surfactants were also analyzed for their effects on the photodegradation of
RhB. Concentrations at and above the critical micelle concentration (CMC) were found to
suppress the photocatalytic degradation of RhB in the presence of a promoter (HNO3).
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Micellar solubilization is responsible for the changes in the photocatalytic activity. Nonionic
micelles are unable to protect the dye over longer times due to autooxidation process. The
•
resulting degradation products of RhB by OH were identified by LC-MS and GC-MS
analysis and accordingly a schematic degradation pathway was proposed.
Keywords Surfactants; Micelles; Photodegradation; Solubilization; Encapsulations;
UV/Visible
Introduction
Different methods are currently applied for the removal of organic pollutants from waste
water, including coagulation, adsorption, membrane processes, and advance oxidation
processes (AOPs) [1-4]. AOPs are more effective for the degradation of dyes [5-7], i.e., to
convert them to simpler, non-toxic compounds. The mechanism of AOPs includes photolysis,
photo-ozonation, photo catalysis etc., [1, 6-9]. Photocatalysis is widely used for its
applications in the degradation of various toxins and pollutants, across a range of
environmental remediation processes. Photocatalysis is also an important approach in dealing
with the environmental problems relating to various “emerging pollutants” or “contaminants
of emerging concern” [8, 10] such as pharmaceuticals, which are drained in to the
environment and are not properly monitored [11]. With the passage of time, these types of
pollutants are becoming varied and difficult to handle, and while the introduction
nanotechnology is, on the one hand, helping scientists to revolutionize old scientific concepts
such as photo degradation of pollutants, on the other hand, it is causing environmental
pollution of a different kind [12]. This new paradigm of complicated pollutants and their
associated problems requires researchers to improve their in depth understanding of the
photocatalytic processes. In this manner, the role of promoters and inhibitors are very
important.
Recently, scientists have been concerned with how surfactants can affect photocatalytic
reactions [13-16], due to their ubiquity waste waters. Surfactants are currently used to create
textiles, employed in dye houses, and leather tanning industries, as well as in the oil and
pharmaceutical arena. In some photocatalytic processes, surfactants are used for facilitating
the adsorption of target pollutants upon photocatalytic materials e.g., TiO2. The pollutants
having hampered solubility have shown enhanced degradation rates under the influence of the
surfactant effect, which helped in improving the interaction of pollutants with the surface of
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catalyst [17, 18]. It has also been observed that surfactants can suppress the degradation by
some photocatalytic materials, which has been attributed to the dye/surfactant interactions
and the possibility of encapsulating dyes inside the micelles [16, 18, 19] e.g., Fenton process
for the degradation of Rhodamine B (RhB) was subjected to the influence of surfactants by
Dar et al. and they found that sodium dodecyl sulfate (SDS) can completely inhibit the
Fenton driven degradation of RhB [20]. However, there is a lack of information available that
reports upon the impact of surfactant in the absence of any photocatalytic materials or the
impact of acids and bases on the photocatalytic degradation under UV photo-reactors.
In this context, we have tried to experimentally explore the role of acids, base and surfactants
in the photocatalysis of a model dye RhB, which belongs to triphenyl methane family, and is
shown in Figure 1(a). Figure 1 also shows the structures of surfactants used in this study.
Figure 1: (a) Rhodamine B, (b) Sodium dodecylsulphate, (c) Cetyltrimethylammonium
bromide (d) tween 80
RhB is a synthetic organic dye and has wide range of applications in textile industries, for
dyeing leather, paper and cotton etc, [21]. It is widely studied and characterized [22, 23],
having established photophysical properties, which qualifies its use as model dye. Certain
combinations of acids, bases and surfactants are tried and results reported in this research
article. This research will contribute in the better understanding of the photocatalysis based
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experimental designs for the degradation of dye waste from different industries, which also
contain surfactants and acids as their default effluents.
Experimental
Materials and reagents
RhB was obtained from Fluka UK, acetic acid from VWR International S.A.S France,
Sodium hydroxide (NaOH) , Nitric acid (HNO3) and Sulfuric acid (H2SO4) Tween 80 from
Sigma Aldrich, Citric acid mono hydrate from BDH laboratory supplier England, Sodium
carbonate (Na2CO3) (anhydrous) from Merck Germany, HCl (37%) from Schar lab Spain,
Sodium dodecylsulphate (SDS) (>99% pure) Fluka, Cetyltrimethylammonium bromide
(CTAB) (>99% pure) Daejung chemicals Korea, from suppliers in Pakistan. A double beam
UV visible spectrophotometer, model UV-1602 (Bio technology Medical Services, Canada)
and a UV photo reactor fitted with a UV source (254 nm) was used. All analysis were
performed at room temperature.
Spectrophotometry Methodology
A stock (5.22×10-4 mol dm-3) solution of RhB was prepared by dissolving 6.25mg of RhB in
distilled water and diluting up to 25ml.From the stock solution, 0.6 mL of RhB was added to
45mL of distilled water and 5mL of 0.1 mol dm-3 HNO3 solution was added to it. The
mixture was exposed to UV-irradiation under a UV-lamp of 254 nm wavelength and
absorbance was recorded at different time interval (0, 3 to 30 min) using above mention
UV/Vis spectrophotometer. All experiments were conducted in the same way except for,
where required, nitric acid was replaced by hydrochloric acid, sulfuric acid, acetic acid, citric
acid, sodium carbonate and sodium hydroxide, and absorbance was recorded at different time
interval (0, 10, 20 up to 100 min).
A Second set of experiments were performed in the same way above. However, SDS, tween-
80 and CTAB at a different range of concentration i.e., pre-micellar, micellar and post-
micellar were additionally added for the purpose of the evaluation of their impact on the
degradation process of RhB in the presence of surfactant.
Degradation Products Analysis
The degradation products of RhB were identified by LC-MS (Agilent, USA) in positive
ionization mode (ESI +). The chromatographic mobile phase was set at a flow rate of 1 mL
min−1 with composition of methanol-water (60:40 V/V). The samples were separated on a
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chromatographic column of Beta Basic-18 column (150 × 2.1 mm). The degradation
products of RhB were also analyzed on GC-MS a Polaris Q Ion Trap Mass Spectrometer
(GC-MS, Thermo Finnigen, USA) equipped with a CTC CombiPAL autosampler for SPME
(solid phase micro extraction). The MS was operated with 70 eV electron impact (EI) mode
and positive ion mode.
Result and Discussion
RhB shows absorption maxima (max) at 554 nm in aqueous solution [24] which was verified
as shown in figure 2 (a). max at 554 nm ( 98309 cm-1 mol-1 dm3), was used to find a linear
correlation between absorbance and concentration of RhB as is shown in figure 2 (b).
Figure 2: (a) Visible spectra of aqueous solution of the different concentrations of rhodamine
B and (b) a linear calibration plot for Rhodamine B at room temperature at a fix wave length
of 554 nm.
There was a regular increase in absorbance with respect to the concentration of the dye as is
shown by the arrow pointing from lower to the higher concentrations of dye (figure 1a) while
the corresponding absorbance is given on the y-axis. Afterwards, RhB was subjected to the
effect of various acids and bases during the process of photocatalysis.
Effect of Acids and Bases
To check the effect of acids and bases on photo-degradation of RhB in aqueous media, 3 ppm
solution of RhB was checked under UV radiation in the presence and absence of various
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acids (HNO3, H2SO4, HCl, acetic acid, citric acid) and bases (NaOH, Na2CO3). Figure 3 (a)
& (b) shows the % degradation of RhB at different time intervals under the influence of UV
light and also in the presence of tested acids and bases respectively. In addition, figure 3(b)
also shows a comparison of efficient acid degrader of RhB with bases. Later on, acids and
bases are also compared with KNO3 in figure 4.
Figure 3: (a) Degradation of rhodamine B (6.26×10-6 mol dm-3) under UV light in presence
0.01mol dm-3 each of (i) nitric acid (ii) citric acid (iii) sulfuric acid (iv) hydrochloric acid (v)
acetic acid and figure 3(b) is comparison of (i) nitric acid and (ii) sodium hydroxide (iii)
sodium carbonate.
Figure 3(a) shows that HNO3 was able to degrade the target dye completely in less than 10
min and therefore was most efficient among the used promoters. Citric acid degraded RhB up
to 96 % at 70 minutes. At UV photolysis time of ca. 70 minutes, sulfuric acid, hydrochloric
acid and acetic acid, degraded RhB up to the extents of 88, 78 and 58 % respectively. The
bases NaOH and Na2CO3, when compared with acids, as given in figure 3(b), has shown
significantly low contribution towards the degradation of RhB and were only able to degrade
up to the extents of 36 and 38 % respectively at UV-photolysis time of 70 min.
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Table 1: Kinetic parameters for the degradation of RhB in the presence of studied promoters
Promoter(0.01M) kobs (10-2) /min
t1/2 /min
32.4
2.14
HNO3
3.71
18.7
H2SO4
2.37
1.24
5.10
0.76
0.83
29.2
55.9
13.6
91.2
83.5
HCl
Acetic Acid
Citric Acid
NaOH
Na2CO3
0.51
135.9
UV Effect
The values of various observed rate constants (kobs.) for the promoters observed for the
photodegradation of RhB are recorded in table 1. The values of kobs. followed the order of:
HNO3>Citric Acid>H2SO4>HCl>Acetic acid>Na2CO3≈NaOH≈UV alone.
The highest kobs. value for the degradation of RhB in the presence of 0.01 mol dm-3 HNO3 can
be attributed to the provision of H+ ions by HNO3 in aqueous media as given in equation 1 :
e-aq + H+  H
k = 2.31010 M1 sec1
1
These H+ are strongly scavenging eaq— from the aqueous media and converts them to
[25,

H
H
26]. These produced
then react with target contaminant RhB causing its degradation.
Thus, it can also be concluded that H• radicals has significant role in the degradation of RhB.
The second highest kobs. value was found for citric acid. The high kobs. value for citric acid can
be attributed to its acidic nature, which is already explained in the preceding discussion.
However, its kobs. value is lower when compared to HNO3. This can be related to the high
•OH radical scavenging property as shown by equation 2: [25], which is considered as main
oxidative specie involved in the degradation of wide range of organic pollutants.
citric acid + OH•  product
k = 5107 M1 sec1
2
For the photodegradation of RhB in the presence of 0.01 mol dm-3 H2SO4, kobs. value was
found to be 3.71×10-2 min-1. This lower kobs. value in the presence of H2SO4 when compared
2−
•
to citric acid can be correlated to the production of SO4 , which scavenges OH from the
RhB containing aqueous solution as shown by following equation [27]:
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SO42− + •OH → SO4•− + OH−
k = 3.5 × 105 M−1sec−1
3
The addition of HCl (0.01 mol dm-3) inhibited the degradation of RhB to the greater extent
and the value of observed rate constant (kobs.) reduced to 2.37 × 10-2 min-1. This can be
attributed to the fact that HCl induces Cl‒ ions to the media. These Cl‒ ions are strong
scavengers of •OH [28], which is considered to be main specie involved in the degradation of
Rhb, as shown by equation 4.
Cl− + •OH → ClOH−
k = 3.0 ×109 M−1sec−1
4
•
The second order rate constant of acetic acid (CH3COOH, 0.01 mol dm-3) with OH is of
•
smaller magnitude when compared to Cl‒ with OH [28], yet it suppressed the degradation
rate of RhB (kobs. = 1.24 × 10-2 min‒1) higher than that in the presence of Cl‒. This can be due
to production of •CH2CO2H in case of acetic acid; these •CH2CO2H in addition to CH3COO−
are responsible for greater inhibition in the degradation rate of RhB.
The addition of NaOH (0.01 mol dm-3) and Na2CO3 (0.01 mol dm-3) alternatively to the
aqueous solution of RhB (6.26×10-6 mol dm-3) shifts the pH of the media from pH = 6.5 to
•
‒
pH ≈ 12. At this pH, all the available OH are consumed by OH ions [29] as shown by
equation 5.
OH− + •OH → H2O + O•−
k = 1.3 × 1010 M−1sec−1
5
RhB has absorption wavelength (λmax) of 554 nm, and thus is resistant to degradation by UV-
lamp at 254 nm, explaining the lowest apparent rate constant (kapp = 0.51 × 10‒2 min-1) solely
by UV irradiation.
Figure 3(a) shows that HNO3 can decolorize the target dye in shorter time and to the highest
possible extent. Due to this reason, the effect of HNO3 was studied more carefully and was
also compared with KNO3, which is just a salt. This can be clearly observed in figure 4.
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Figure 4: Degradation of rhodamine B (a) with only HNO3(0.01 mol dm-3) and no UV was
applied (b) with UV light used but no HNO3(c) with HNO3 in the presence of UV light (d)
KNO3 (0.01 mol dm-3) with UV light.
Initially, as is clear from the figure 4, RhB was left with HNO3, without applying any UV
source, which shown no effect on the colour of RhB and hence there was no degradation
observed. Afterwards, RhB solution was kept in UV light for about 90 min and that degraded
the dye up to ≈40%. However, addition of HNO3 speeded up the process and completed the
degradation of dye in less than 10 min time. This conclusively asserted the important role of
HNO3 in the degradation of target dye RhB.
To understand the role of acid, this observation can be compared with the result from figure
3. In figure 3(a), we used nitric acid (% degradation ≈100%, kobs. = 32.4 min-1), citric acid (%
degradation ≈96%, kobs. = 5.10 min-1), sulfuric acid (% degradation ≈88%, kobs.=3.71 min-1),
hydrochloric acid (% degradation ≈78%, kobs.= 2.37 min-1) and acetic acid (% degradation
≈58%, kobs.= 1.24). Whereas, as shown in figure 4(d), KNO3 degraded to the extent of 97%
NO3
and the time of degradation was also comparable to that of HNO3, which implies that
NO3
as
an important role to play in the degradation of RhB dye. The effect of
when compared to any other acid from figure 4(a).
was prominent
It was further observed that the results from the “effect of base” were comparable with the
degradation due to “UV-photolysis alone”, when used without any other additional
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component, which implies that base has no role or doesn’t affect much the outcome of
photodegradation of RhB.
Dependency of the degradation of RhB on the concentration of HNO3 was also checked.
Three different concentrations of HNO3 i.e., 1×10-2 mol dm-3, 1×10-3 mol dm-3 and 1 ×10-4
mol dm-3 of HNO3 were used as shown in figure 5.
Figure 5: Degradation of rhodamine B with different conc. of nitric acid (a) 1×10-2 mol dm-
3(b) 1×10-3 mol dm-3 (c) 1×10-4 mol dm-3 (d) just UV light without addition of HNO3
Figure 5 shows that the degradation rate of RhB is dependent upon the concentration of nitric
acid. As the concentration of acid decrease, the extent and rate of degradation of RhB also
decreased within the same window of time. Figure 5 also shows that relatively lower
concentration of nitric acid (10-4 mol dm-3) is very slow (44% degradation at 70 min) at
degrading the target dye, while the higher concentration of 10-3 M shows 88% degradation at
70 min time.
Effect of surfactants on the degradation of RhB in acidic media
Anionic surfactant
Figure. 6 shows that the concentration of surfactant (SDS) effects the rate of degradation. It is
clear from the above graph that pre-micellar concentration of SDS have no significant effect
upon the degradation of dye. At the approximate critical micelle concentration of SDS, the
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degradation of the target dye is taking place and the degradation proceed up to 94 % in the
observed time period.
Figure 6: Degradation of Rhodamine B (6.26×10-6 mol dm-3) in presence of (a) UV-
irradiation alone (b) nitric acid (0.01 mol dm-3) and UV light (c) SDS (1×10-3 mol dm-3) in
the presence of UV-irradiation and nitric acid (d) SDS(8.1×10-3 mol dm-3) in the presence of
UV-irradiation and nitric acid (e) SDS (4×10-2 mol dm-3) in the presence of UV-irradiation
and nitric acid concentration.
In case of post-micellar concentration of SDS, the degradation of dye is further slowed down,
when compared to the aqueous dye solution just in the presence of HNO3. The degradation
proceeds up to the extent of 87% compared to 100% due to the presence of nitric acid in the
same time period. This can be correlated with the micellar solubilization of RhB by the
surfactant micelles. Tajalli and co-workers reported that anionic surfactants can solubilize
Xanthene dyes such as RhB and Rhodamine 6G in their micelles and also suppress
aggregation of dyes due to the steric hindrances in the post micellar concentrations [13].
There is also a clear indication of electrostatic interaction[30] between surfactant monomers
and the dye molecules as shown in figure 7. As the concentration of surfactant was increased,
a red shift in max value (figure 7(b)) was observed. The shifting of max value stopped at 563
nm, and at a concentration of SDS, which was a little below the literature value of critical
micelle concentration of SDS i.e., 8.1×10-3 mol dm-3 [31]. A hypochromic shift was also
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observed which was reversed after further increase in the concentration of anionic surfactant
SDS.
Figure 7: (a) the UV-Visible pattern of 6.26×10-6 mol dm-3 RhB with the variation in the
concentration of SDS. The inset shows the magnified image of the encircled peak of RhB and
demonstrate the shift in max; (b) Gradual Red shift (■) with the increase in the concentration
of SDS along with the hypochromic shift (▲)
There was no change in max value in the post micellar concentration region of SDS and it
remains constant at 563 nm. RhB molecules encapsulated per micelle (n) were determined
using the absorbance values at 563 nm in the post-micellar region of SDS concentrations. The
equations used [32] were as follows:
n  Cm / M
6
where Cm is the concentration of solubilized target dye RhB, M represents the concentration
of micelles. To determine M, following equation was utilized:
[SDS]T CMC
M 
7
Nagg
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where [SDS]T is the total concentration of anionic surfactant SDS and CMC is the “critical
micelle concentration” of SDS. Nagg is the aggregation of number of SDS in this particular
case. The same set of equations can be employed for other surfactants by replacing SDS by
the surfactant for which the same values are measured. For SDS, the literature CMC value is
8.1×10-3 mol dm-3 and Nagg used was 80 [31].
Cm can be determined by using equation 8 as follows:
A  A
 m
Cm 
8
Here
A
corresponds to the absorbance of dye solution at max when no surfactant is present.
A is the absorbance of solubilized dye on the max. The results obtained for various
surfactants are reported in table 2.
Table 2: Number of RhB per micelle of SDS, Tween 80 and CTAB
RhB ×10-6
(mol dm-3)
6.26
Ao
Cm ×10-6
(mol dm-3)
6.26
M ×10-7
(mol dm-3)
425
n

(dm3/mol.cm)
0.673
0.664
0.664
108987
0.15
10
9
SDS
6.26
104473
6.26
6.33
Tween 80
CTAB
6.26
103035
6.26
6.67
The values of Nagg and CMC for tween 80 was 60 [33] and 1×10-5 mol dm3 [34] while for
CTAB the values were 9.2×10-4 mol dm3 and 120 respectively [31].
The low “n” value for SDS, which was almost ≈0.15 dye molecule per micelle is due to the
presence of electrostatic interactions of RhB as shown in figure 7, with the surface of
micelle[35] and also due to the larger size of the RhB. The micellar concentration for SDS is
also many fold higher for SDS when compared to the other two surfactants, which implies
that number of micelles formed are larger in number when compared to the dye molecules
which are needed to be solubilized by the available micelles. We can assume that one dye
molecule is solubilized by a micelle however, due the larger number of available micelles, the
number of dye molecules encapsulated per micelle is smaller number than expected. The
literature also supports the “local solubilization of Xanthene dyes[13]” by the micelles of
ionic surfactants as was discussed earlier. This observation is also consistent with Carlotti et
al., where they show the protective performance of SDS against photolysis of salicylic acid
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under UV lamp due to the solubilization of drug by micelle [36]. However, salicylic acid is
smaller molecule when compared to RhB and have repulsive electrostatic interactions with
the micellar surface due to the formation of negatively charged ion when in solution form.
This inhibit any attractions of salicylic acid with the negatively charged surface of the SDS
micelle and hence possibly allow this molecule to be solubilized in the core of the micelle
rather than to stay on the surface of the micelle. This is demonstrated in the case of two other
surfactants i.e. tween 80 and CTAB and RhB dye.
Non-ionic surfactant
Tween80, a nonionic surfactant, was also checked for its influence on the degradation
behavior of RhB under UV light in the presence of HNO3. The results are given in figure 8,
which shows the effect of the presence of tween-80 on the photodegradation of RhB in the
presence of 0.01 mol dm-3 HNO3. From figure 8, it is clear that premicellar and micellar
concentrations allow 100% degradation of RhB within 40 minutes of the start of degradation
under UV light. However, it initially slows down the process of degradation when compared
with the degradation process in the absence of surfactant. Post micellar concentration of
Tween80 significantly affect the degradation process and only allow the dye, degradation up
to the extent of 92% that also after 80 minutes.
Figure 8: Degradation of Rhodamine B in presence of (a) UV light (b) nitric acid (0.01 mol
dm-3) and UV light (c) Tween80, 2×10-6 mol dm-3 (d) Tween80, 1.2×10-5 mol dm-3 (e)
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Tween80, 1×10-3 mol dm-3, c,d and e are in the presence of UV light and nitric acid(0.01 mol
dm-3).
This result is in agreement with the protecting behavior demonstrated by Tween 20 in the
case of cinamic acid as is demonstrated by Ugazio et al. They shown that cinamic acid is
partitioned in to the micelle of the surfactant, which in turn protect its degaradation.
Furthermore, they argue that at possibly higher acidic concentration, the extent of degradation
is relatively higher even in the presence of micellar concentrations [37].
In this study, we also observed a dramatic lowering of photodegaradation rate of RhB for the
postmicellar concentration of tween 80, when compared to the other two concentrations,
which finally reach above 90% degaradation of RhB in the given time period. Tween 80
shows a larger “n” value (table 2) when compared to SDS. The initial impact upon the
reduction of degradation rate of RhB is also larger. This can be attributed to the miceller
solubilization of dye, which is also supported by “n” value of 10.
The reason could be the absence of electrostatic interactions and a rather complicated and
relatively bigger structure of this surfactant compared to the other two contenders. The “n”
value for tween 80 is also higher when compared to SDS. The possibility of •OH to penetrate
inside core of the micelle is hampered by the non-availability of suitable electron rich specie,
which may allow the •OH to reach the core. Hence, we can assume two possibilities here i.e.,
(a) since the interior of non-ionic surfactant is relatively wetter so we can visualize a
•
situation, where RhB is moving in to and out of the micelles which may allow the OH
radical to oxidize the RhB molecules, when available outside the micelle or near the surface
of micelle (b) the structure of the tween 80 micelle is loose enough to allow the production of
•OH inside the micelle but at relatively slower rate.
Finally, auto oxidation in polysorbates over extended time periods is well known.
Furthermore the simultaneous presence of a UV source and acidic conditions may also
•
facilitate the production of OH radicals within the micelles, which may in turn degrade
encapsulated RhB [38]. This explanation is in agreement with Ashraf et al., who shown that
addition of a nonionic surfactants to anionic surfactant system, promote the degradation of
•
RhB due to the formation of OH radicals [20]. This argument by Ashraf et al., is also
supported by the possibilities of the formation of mixed micelles of ionic and non-ionic
surfactants [39].
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Cationic surfactant
Effect of CTAB, which is a cationic surfactant, on the degradation of RhB in the presence of
HNO3 has been shown in figure 8. In case of pre-micellar and micellar concentrations of the
aforementioned surfactant, 99.4% degradation of RhB occurs at 80 minutes time. However,
in the post micellar concentration of CTAB, only 91% of dye degrades within 80 minutes
with 90% of dye degraded at 50 minutes.
Figure 9: Degradation of Rhodamine B in the presence of (a) UV-irradaition (b) nitric acid
(0.01 mol dm-3) in the presence of UV-irradiation (c) CTAB (1×10-4 mol dm-3) in the
presence of UV- irradiation and nitric acid (d) CTAB (1.0×10-3 M) in the presence of UV-
irradiation and nitric acid (e) CTAB (1×10-2 mol dm-3) in the presence of UV-irradiation and
nitric acid (0.01 mol dm-3).
This shows that CTAB allow degradation initially and afterwards it keep protecting the
remaining content of the dye because no significant change is noted in the remaining 30 min.
There is no upward trend in figure 9 (e), which may conclude that the remaining dye can
degrade if it is allowed more time under the same conditions. Clearly, this is not the case if
we closely look for the trend in the post micellar concentrations in figure 7&8 (e).
In general, a slowing down of the degradation process is very obvious. Even low
concentrations of surfactants initially show impact upon the rate of degradation. Typically,
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higher concentrations of surfactants lead to incomplete degradation of dyes even in the
presence of acids. This can be attributed to the “dye-surfactant interactions”, which may
possibly lead to (a) adsorption of dye on the micelle surface or (b) inclusion of dye in the
palisade layer of the micelle or (c) encapsulation of dye in the micelle core. All the three
processes can lead to a change in degradation process of dye.
In case of RhB as target contaminant, and HNO3 as promoter, hydroxyl radical is supposed to
be responsible for the degradation of target. However, once the dye relocate itself in one of
the positions mentioned earlier, which are related to the micelles formed by the added
surfactants, the transport of hydroxyl radicals to reach the relocated dye become important. It
can be suspected that the dye molecules present in the palisade layer or especially present in
the core of the micelle will be difficult to reach for the hydroxyl radical which are responsible
for the dye degradation. Our explanation is in agreement to the work reported by Wang and
co-workers [16].
The free dye is therefore rapidly degraded while the encapsulated dye remains safe as is
shown in the case of SDS and CTAB. However, in case of tween 80, surfactant itself is
available to produce hydroxyl radicals over longer period of time [38, 40] and this can be
relative faster in the presence of a UV-irradiations (254 nm) which explains the upward trend
in case of tween 80 although the encapsulation capacity will be larger for tween 80 (n=10,
Table 2), due to larger size of micelle (C17 chain) when compared to SDS or CTAB.
Similarly, CTAB (C16) micelle will have larger diameter as compared to SDS (C12) and will
accommodate more dye molecule (n = 9) as also given in table 2; although it may not allow
the dye molecules to stay in the palisade layer due to the electrostatic repulsions.
Identification of degradation products and proposed degradation pathway
For a mature treatment technology, the knowledge about the degradation products (DPs)
is also of great concern. For this purpose, the DPs of RhB in the presence of nitric acid (0.01
mol dm-3) were identified by LC-MS and GC-MS and are summarized in table 3. The DP at
m/z value of 415 can be suggested as de-ethylated product of RhB. Both direct photolysis of
•
RhB and attack by the OH are responsible for such type of elimination reactions [41].
Further elimination at N-substituent of RhB molecule gives N-de-ethylated DP with m/z of
387. Similar, DP have also been reported by Zhong and co-workers [42]. Subsequent
eliminations and substitution reactions by •OH results in the formation of DP with m/z value
of 122. Depending on the nature of target molecule, •OH can undergo three different types of
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competing reactions with organic pollutants such as: (i) addition, (ii) hydrogen abstraction
(H-abstraction), and (iii) electron abstraction (e-abstraction) [43]. The DP with m/z of 138 is
resulted from the reaction of •OH via two steps, i.e., H-abstraction (1st step) and •OH addition
(2nd step) as shown below;
A second hydroxylated DP with m/z of 154 is identified by GC-MS. Similar (mon- and
multiple) hydroxylated DPs are reported in literature for degradation of organic contaminants
•
•
by OH based Advanced Oxidation Processes [44, 45]. Further attack of OH causes
destruction of conjugated structure of RhB causing DP with m/z of 132, which indicates the
near mineralization of RhB molecule. Scheme 1 depicts a possible degradation pathway of
RhB molecule by •OH.
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Scheme 1 Possible degradation pathway for degradation of RhB molecule as identified by
LC-MS and GC-MS.
Generally, radical reactions involve chain reactions that causes complications in the detailed
mechanistic study of any target contaminant and results in the formation of diverse class of
degradation products from organic contaminants. Therefore, the mechanistic pathways of
elementary reactions that involve the generation of a particular degradation product need
further insight into the mechanistic study with more sophisticated instrumentation.
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Table 3: Detected DPs of RhB in the presence of nitric acid (0.01 mol dm-3) by LC-MS and
GC-MS
Used
Identified DP
name
m/z
Identified DP structure
Equipment
N, N- diethyl -N-
ethyl rhodamine
OR
415
LC-MS
N, N-
diethyl
rhodamine
OR
387
LC-MS
N-ethyl-N-
ethyl
rhodamine
OR
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Benzoic
122
138
GC-MS
acid
1-hydroxy
benzoic
acid
GC-MS
1,4-
hydroxyl
benzoic
acid
154
GC-MS
GC-MS
succinic
acid
132
Conclusions
RhB (0.15mg/50ml) is completely decolorized in 25 minutes under UV-irradiations (254 nm)
with dilute nitric acid (0.01 mol dm-3). The efficient photodegradation of RhB in the presence
•
•
of HNO3 could be attributed to the dual attack of OH and H radicals on RhB molecule.
Citric acid also degrades RhB up to 100%. It is observed from results that after degradation
of RhB, no peak was found in the range of 200 to 800 nm. Additionally, degradation process
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is influenced by all types of surfactants up to some extent. Post-micellar concentrations of
surfactants can slow down the degradation process due to the dye surfactant interactions and
preponderantly due to the “local solubilization” [13] of dyes in the micelles of the surfactant.
Higher concentrations of surfactants i.e., the post-micellar concentrations are more efficient
at suppressing the initial photodegradation of solubilized compounds, which also agree with
literature [37]. However, for HNO3, the protective impact of micelles is weakened for longer
intervals of time and a substantial photodegradation is noted in all the three surfactants.
From the discussion, it is evident that the presence of surfactants in waste water can affect the
fate of treatment procedures used for the removal/degradation of dyes. Conventional
processes used for the degradation of dye waste may not necessarily follow the same
pathways as in the absence of surfactants. Waste containing surfactants can be efficiently
treated if the material used for the photodegradation of target chemical have proven
synergistic influence due to the presence of surfactant. Mixing surfactants, i.e., non-ionic
surfactant with ionic surfactants can also be beneficial in some cases [20] as the nonionic
•
surfactants can be a mean of producing OH [38], which are required for the degradation of
RhB. Finally, degradation products of RhB were identified and a possible degradation
pathway was proposed. Similar strategies can be proposed for increasing the efficiency of
photodegradation processes. These findings will also be useful to increase the understanding
of miceller solubilization processes.
Acknowledgement
AMK is thankful to HEC Pakistan for funding this research under HEC-NRPU Project Grant
No. 4271/R&D/HEC/14 and HEC-SRG Grant No. IPFP/HRD/HEC/2014/1612.
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Graphical abstract
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Influence of Acids, Bases and Surfactants on the Photocatalytic Degradation of a Model
Dye Rhodamine B
Asad M. Khana*, Abid Mehmooda, Murtaza Sayedb, Muhammad F. Nazarc, Bushra Ismaila,
Rafaqat A. Khana, Hameed Ullahd, Hafiz M. Abdur Rehmane, Athar Y. Khane, Abdur R.
Khana.
aDepartment of Chemistry, COMSATS Institute of Information Technology, Abbottabad-
22060, Pakistan
bNational Center of Excellence in Physical Chemistry, University of Peshawar, KP, Pakistan
cDepartment of Chemistry, University of Gujrat, Hafiz Hayat Campus, Gujrat Pakistan
dDepartment of Chemistry, Hazara University, Dodhial, Mansehra, Pakistan
eDepartment of Chemistry, Forman Christian College, 54600-Lahore, Pakistan
*amk.qau@gmail.com
Highlights
 Micellar encapsulation of dyes protect from degradation under UV light and acidic
conditions.
 Longer time protection is provided by the micelles of ionic surfactants.
 Micelles of nonionic surfactants can solubilize more but protect less.
28