UV Light-Assisted Degradation of Methyl Orange, Methylene [611770]

UV Light-Assisted Degradation of Methyl Orange, Methylene
Blue, Phenol, Salicylic Acid, and Rhodamine B: Photolysis
Versus Photocatalyis
Anca Peter &Anca Mihaly-Cozmuta &Camelia Nicula &
Leonard Mihaly-Cozmuta &Agnieszka Jastrz ębska &
Andrzej Olszyna &Lucian Baia
Received: 18 October 2016 /Accepted: 14 December 2016 /Published online: 27 December 2016
#Springer International Publishing Switzerland 2016
Abstract Methyl orange (MO), methylene blue (MB),
phenol (F), salicylic acid (SA), and rhodamine B (ROD)
were used as substrates during the photodegradation
experiments in the absence and in the presence of nano-structured Ag/titania-sili ca. The catalyst was character-
ized by scanning electron microscopy (SEM), scanningtransmission electron microscope high-angle annulardark field (STEM-HAADF), st ereological analysis, ni-
trogen adsorption-desorption, and X-ray photoelectron
spectroscopy (XPS) measurements. The results were
fitted on pseudo-first and pseudo-second kinetic ordermodels. The film diffusion was also determined. The
photolysis degrades MO and F to a greater extent than
the photocatalysis. The degradation of SA occurred at thesame rate either by photolysis or by photocatalysis. MB
was best removed by photocatalysis. With regard to the
photocatalysis, the highest ra tes of film diffusion were
obtained for MB, F, and ROD, meaning that these mol-ecules crossed the film to arrive at the catalyst surfacemore rapidly than the others. For MO and MB, the results
followed the pseudo-first-order kinetic model while for
SA, F, and ROD, the pseudo-second-order kinetic modelwas more appropriate.
Keywords Methyl orange .Methylene blue .Phenol .
Salicylic acid .Rhodamine B .Photolysis/Photocatalysis
1 Introduction
Dyes are widely used in the textile, rubber, paper, plas-
tic, and cosmetic industries (Soltani and Entezari 2013 ).
Due to the discharge of synthetic dye-containing waste-
water, the effective treatment of dye and organic sub-
stances containing wastewater has been intensely stud-
ied (Kim and Kan 2015 ). Conventional biological and
chemical oxidation methods are not efficient for the
degradation of dyes and organic compounds, due to
their complex aromatic structure and recalcitrant nature.Water Air Soil Pollut (2017) 228: 41
DOI 10.1007/s11270-016-3226-z
A. Peter (*) :A. Mihaly-Cozmuta :C. Nicula :
L. Mihaly-Cozmuta
Department of Chemistry and Biology, Technical University ClujNapoca, Victor Babes 76, 430083 Baia Mare, Romania
e-mail: [anonimizat]
A. Mihaly-Cozmuta
e-mail: [anonimizat]
C. Nicula
e-mail: [anonimizat]
L. Mihaly-Cozmuta
e-mail: [anonimizat]
A. Jastrz ębska:A. Olszyna
Faculty of Materials Science and Engineering, Warsaw University
of Technology, Woloska st. 141, 02-507 Warsaw, Poland
A. Jastrz ębska
e-mail: [anonimizat]. Olszyna
e-mail: [anonimizat]
L. Baia
Faculty of Physics and Institute for Interdisciplinarity Research on
Bio-Nano-Sciences, Babes-Bolyai University, M. Kogalniceanu 1,400084 Cluj-Napoca, Romaniae-mail: [anonimizat]

Adsorption using activated carbon and low-cost adsor-
bents is simple and efficient for the removal of dyeswhile requiring expensive regeneration or generating
significant amounts of solid waste (Soltani and
Entezari 2013 ;K i ma n dK a n 2015 ; Sivakumar et al.
2013 ;C r i n i 2006 ).
The dyes under consideration in this study are
rhodamine B (ROD), salicylic acid (SA), phenol (F),methyl orange (MO), and methylene blue (MB). ROD
is a basic red dye of the xanthene class, potentially toxic
and carcinogenic for use as colorant in foodstuffs (RaviChandra et al. 2015 ). SA was found in wastewaters
having possible effects on health and the environmentwith long time exposure (Vilhunen et al. 2009 ). F is an
important environmental pollutant because of its toxic
effect, ubiquitous presence, and carcinogenic character
towards life in the aquatic environment (Naeem and
Ouyang 2013 ). MO and MB dyes used in the textile
industry are released as effluents in the environment,
representing a considerable source of non-aesthetic
pollution since the presence of small amounts of dyes
(below 1 ppm) is clearly visible (Kim and Kan 2015 ;
Mohamed et al. 2013 ).
Advanced oxidation processes have been used to
degrade natural and synthetic dyes efficiently. Inparticular, photocatalytical oxidation can be conve-
niently applied towards the degradation of dyes and
organic pollutants using only light, a catalyst, and air(Mohamed et al. 2013). The tests about the degrada-
tion rate of methyl orange were mentioned in manystudies (Li et al. 2006; Guettaï and Ait Amar 2005a,
b). The photodegradation efficiency of phenol is sig-
nificantly enhanced by such catalysts as CeO
2-TiO 2/
SiO 2,a sL ie ta l .( 2015) have demonstrated. A good
compromise in terms of the crystallinity, the size of
crystallites, and the speci fic surface area of anastase
induces an improved photocatalytic activity of TiO 2
nanomaterials for the degradation of phenol, as Turkiet al. ( 2015) have demonstrated. Ivanova et al. ( 2015)
have prepared single titanate sheets, appearing to be
an excellent precursor for self-organized TiO
2nano-
tubes, which displays excellent photocatalytic activ-ity in the photodegradation of phenol. The fastestdegradation of SA occurred in the presence of cata-
lysts in both the anatase and rutile phases, according
to Vilhunen et al. ( 2009). In the absence of a TiO
2/
SiO 2photocatalyst and under UV-irradiation (photol-
ysis), the rate of degradation of SA increases as theamount of hydrogen peroxide increases, as proved byAdán et al. ( 2006 ). The tests to degrade rhodamine in
t h ep r e s e n c eo faT i O
2-based catalyst were quoted as
being successful in many studies (Ravi Chandra et al.
2015;L ie ta l . 2014 ; Bokhale et al. 2014).
By photolysis, reactive radi cals are directly and rap-
idly generated, within photoreaction processes embrac-ing the whole solution. The advantages of photocatalysis
are the generation of multi-radicals and the electrostaticaffinity between the molecule and the catalyst surface.
The disadvantages are the generation of multi-step radi-
cals, the energy needed to cross the film at the catalystsurface, and the limitation of photoreactions occurring at
the surface (Soltani and Entezari 2013).
The photocatalytical degradation of the five dyes
over a titania-based catalyst was already investigatedin the literature, but the novelty of this study con-
sists in comparing the photochemical behavior of the
f i v ec o m p o u n d si ns o l u t i o na sw e l la sa tt h ec a t a l y s t
surface and in establishing the processes that governthe photo-assisted degradation. The variation of the
degradation efficiency as a function of the chemical
structure and surface charge of the organic moleculewas also discussed.
2 Material and Methods
2.1 MaterialsTetraisopropoxide orthotit anate (IPT) (purity 98%),
tetraethyl orthosilicate (TEOS) (purity 98%), and sodi-
um tetrahydroboride (NaBH
4) (purity 98%) were pur-
chased from Merck, Germany. Nitric acid (reagent foranalysis) and MO (purity 99%) were purchased fromLachner, Czech Republic, anhydrous ethanol (reagent
for analysis) was purchased from Chemical Company,
Romania, and silver nitrate (purity 99%) was purchasedfrom Silal Trading Romania. MB (purity 99.5%) and F
(purity 99.5%) were purchased from Sharlau, Spain. SA
(purity 99%) was purchased from Utchim, Romania,and ROD (purity 99.5%) from Loba, Austria.
2.2 Preparation and Characterization of the CatalystThe catalyst was prepared by the sol-gel route. The
detailed method was described in our previous study(Peter et al. 2015a ). IPT and TEOS were hydrolyzed
with ultrapure water, in the presence of nitric acid andanhydrous ethanol. The molar ratios of the reactants41 Page 2 of 12 Water Air Soil Pollut (2017) 228: 41

were as follows: [IPT]/[TEOS] = 2, [IPT]/[ethanol]
= 0.05, [IPT]/[water] = 0.17, and [IPT]/[nitric acid]= 6.27. The gel was allowed to age within 4 weeks.
The obtained titania-silica gels were immersed for
10 min, in 0.001 M NaBH
4solution at 4 °C, and the
0.005 M silver nitrate solution was added under
continuous stirring. The volume ratio of the two
solutions was 15. The mixture was then filtered andwashed five times with anhydrous ethanol and finally
was kept in ethanol for 24 h. The gel was dried in air in a
Binder oven at 80 °C for 24 h and thermal treated at500 °C for 2 h in a Carbolite furnace. Finally, gray
particles of Ag/titania-silica xerogel were obtained.
The morphology was examined using SEM —LEO
1530, Zeiss, USA, operating at an accelerating voltageof 2.0 kV , as well as SEM S-3500N, Hitachi, USA,
operating at an accelerating voltage of 15.0 kV . The
stereological analysis was performed in order to mea-
sure the size of a particle or an agglomerate by using aMicroMeter v.086b computer program.
The scanning transmission electron microscope
high-angle annular dark field (STEM-HAADF) re-sults were recorded by a TEM (PHILIPS CM 20,
UK), in high-angle annular dark field. The phase
composition was determined by the electron diffrac-tion method (EDX).
The specific surface area of the catalyst was ex-
amined based on the isotherm of the physical nitro-gen sorption, using a Quadrasorb-SI device
(Quantachrome Instruments, USA). Before the mea-
surements, the sample was degassed at a temperatureof 140 °C for 5 h. The nitrogen adsorption-
desorption was measured at a temperature of
−195.8 °C. The specific surface area was determined
by applying the Brunauer, Emmett, Teller (BET)
model and the obtained value was 140 m
2/g.
X-ray photoelectron spectroscopy (XPS) measure-
ment was performed on a SPECS PHOIBOS 150MCD instrument, with monochromatized AlK
αradia-
tion (1486.69 eV) at 14 kV and 20 mA, and a pressure
lower than 10 –9 mbar, in order to establish the content
of the Ag particles (found to be 0.53 wt.%) on thetitania-silica surface.
The point of zero charge of Ag-titania-silica compos-
ite was determined by our research team and the resultsare presented in reference (Jastrzezbska et al. 2015 ).
The results of X-ray diffraction, FTIR, and
adsorption-reflectance spectroscopy were detaileddiscussed in our previous study (Peter et al. 2015a ).2.3 Removal of the Organic Compounds by Photolysisand Photocatalysis Over Ag-Titania-Silica,Respectively
The removal of the organic compounds by photolysis
was carried out in a UV reactor system with recirculation
equipped with medium pressure Hg lamp (150 W), UV
light range 350– 400 nm. The scheme of the UV reactor is
included in our previous study (Peter et al. 2015b ). The
circulation rate of 400 mL solution was 0.875 mL/min.The aqueous solutions (100 μM) of the five organic
compounds such as methyl orange (MO), methylene blue
(MB), phenol (F), salicylic acid (SA), and rhodamine B
(ROD) were prepared. The pH of the initial solutions was7.3 for MO, 5.5 for MB, 6.7 for P, 5.4 for SA, and 6.2 for
ROD. The working temperature was ∼20 °C, assured by
the cold water flow used for UV lamp cooling. Theremoval of the organic compound was monitored using
a Perkin Elmer, Lambda 35 Spectrophotometer, byassessing the intensity of the electronic absorption band
located at 470 nm for MO (Li et al. 2006), at 664 nm for
MB (Soltani and Entezari 2013), at 270 nm for P (Li et al.
2015), at 297 nm for SA (Vilhunen et al. 2009), and at
524 nm for ROD (Ravi Chandra et al. 2015). The
adsorption-desorption equilibrium was reached (in dark)
before the photocatalytic investigation.
The photocatalytic experiments were conducted un-
der conditions identical to photolysis, but in the pres-ence of 0.15 g of catalyst.
The degradation efficiency (%) was determined
using (Eq. 1):
D
e¼c0−ct ðȚ
c0x100 ð1Ț
where c0is the concentration of solution ( μM) at time
t=0a n d ctis the concentration of solution ( μM) at time t.
2.4 Data Modeling
It is well known that photocatalytic oxidation processes
follow the pseudo-first-order kinetics, usually describedthrough the Langmuir-Hinshelwood model. At small
(mM) concentrations, C< <1, the equation can be sim-
plified to the apparent rate order, as presented in Eq. 2
(Kabra et al. 2004 ):
lnC0
C¼kappt ð2ȚWater Air Soil Pollut (2017) 228: 41 Page 3 of 12 41

where C0is the initial concentration of organic com-
pound solution, Cis the concentration of organic com-
pound at time tof irradiation, and kappis the apparent
rate constant of the reaction.
The parameter kappis given by the slope of the graph
of ln ( C0/C)v e r s u s t,a n d C0is the initial concentration
of the solution. Consequently under the same condition,
the initial degradation rate could be written in the fol-lowing form (Eq. 3):
r
0¼kappC0 ð3Ț
The results for SA, F, and ROD were fitted also on the
pseudo-second-order kinetic model written as follows:
dqt
dt¼k2qe−qt ðȚ2ð4Ț
where k2(gμmol−1min−1) is the rate constant of second-
order model, qtis the photoreduction capacity at time t
(μmol g−1), and qeis the photoreduction capacity at
equilibrium ( μmol g−1). For boundary conditions ( t=0
totandqt=0t o qe), the equation becomes
t
qt¼1
k2⋅q2
eț1
qe⋅t: ð5Ț
The plot of t/qtversus tshould give a straight line if
the pseudo-second-order kinetic model is applicable and
qeandk2can be determined from the slope and intercept
of the plot, respectively.
The capacity of photoreduction at time t(qt) was
determined with the following formula:
qt¼C⋅V
mð6Ț
where Vis the solution volume (L) and mis the catalyst
mass (g). In order to apply Eq. 6also for photolysis, the
catalyst mass was considered 10−10(g).
The coefficient of diffusion through the liquid film
was determined in the photocatalytic process, by using
the following formula (Inglezakis et al. 2007 ):
ln 1−αtðȚ ¼ −D
r2−3⋅D⋅c
r⋅δ⋅cex⋅t ð7Ț
where αtis the fraction of molecule bounded at time t
(dimensionless), Dis the diffusion coefficient (nm2/s),r
is the mean radius of the particle (29 nm), candcexare
the concentration of organic compound in solution ( μM)
and, respectively, the concentration of degraded organic
compound on TiO 2(μM), and δis the thickness of thefilm, considered 10−5m. The relative diffusion coeffi-
cient ( D) was calculated from the slope of ln(1 −αt)=
f(t), by considering c/cex= 1. The mean radius was cal-
culated from the mean diameter of the particle obtainedin the stereological analysis.
For the sake of higher accuracy in order to establish
which model fits best experimental data, the normalizedstandard deviation ( Δq) was calculated using Eq. 8(Lin
and Juang 2002 ):
Δq%ðȚ ¼ 100ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Xn
i¼1qexp−qcalc
qexp"#2
n−1vuuuut
ð8Ț
where q
expis the experimental capacity of photoreduc-
tion, qcalis the calculated capacity of photoreduction,
andnis the number of data points.
3R e s u l t s
The morphology of the Ag-titania-silica catalyst is illus-
trated in Figs. 1and2. The scanning electron microsco-
py (SEM) image (Fig. 1) revealed the presence of glob-
ular structures either isolated or in agglomerations, butno proof about the presence of silver. The isolatedparticles have 58 nm diameter, while the agglomerations
have 174 nm diameter.
The EDX results (Fig. 2) showed signals associated
with the presence of Si, Ti, O, C, and Ag. The contentsof these elements in the four selected points, as well as
the mean value, are presented in Table 1. The values for
each element in the four points are close, suggesting the
Fig. 1 SEM image of Ag-titania-silica catalyst41 Page 4 of 12 Water Air Soil Pollut (2017) 228: 41

homogeneous composition of the composite. The mean
composition of Ag (wt.%) is 0.55 in the whole compos-
ite and, according to the XPS results, the content on the
surface is 0.53 wt.%. This suggests that the Ag particles
are dispersed on the surface rather than in the bulk of thecomposite. The detailed discussion of the XPS results of
Ag/TiO
2-SiO 2was performed in our previous study
(Peter et al. 2015a ).
The photodegradation profiles of the five organic
compounds during photocatalysis and photolysis are
illustrated in Figs. 3(a, b) and 4(a, b), respectively.
Figures 3(a) and 4(a) present the profiles obtained after
120 min of process, while Figs. 3(b) and 4(b) illustrate
the variation of the organic compounds after 30 min.The degree to which the components were degraded by
a 30-min photocatalysis process decreases in the order
ROD, SA, F, MO, and MB, as revealed in Fig. 3(b). Theamount of degraded MO and MB, respectively, was
roughly the same, after 30 min of photocatalysis. The
degradation went gradually slower in the order F, ROD,
MB, and MO after 120 min of photocatalysis. After
120 min of degradation, ROD and MB were removedin the same proportion (Fig. 3(a)).
After 30 min of photolysis (Fig. 4(b)), the degradation
was highest for ROD and was gradually smaller for SAand MB. After 120 min of photolysis, F was completely
degraded, while the process has degraded only 95.7% of
ROD, 92.5% of SA, and 45.7% of MO. Only 26.3% ofMB was degraded by photolysis; thus, MB being degrad-
ed to the smallest extent after both 30 and 120 min of
photodegradation. The parameters obtained by fitting theresults in the pseudo-first kinetic order equation (Tables 2
and3) endorse the profiles represented in Figs. 3(a, b)
and4(a, b). The averages of the normalized standard
Fig. 2 STEM-HAADF and EDX for Ag-titania-silica catalystWater Air Soil Pollut (2017) 228: 41 Page 5 of 12 41

deviation ( Δq) for the degradation of molecules after
120 min of photolysis (Table 2) were found to be in
order: 5.32% for MO < 6.89% for MB < 43.19% for
SA < 56.77% for F < 66.16% for ROD. The square cor-
relation coefficients ( R2) can be correlated with the nor-
malized standard deviation, meaning that the values of R2
close to 1 (0.999 for MO, 0.991 for MB) were obtainedfor the lowest Δq. This correspondence between Δqand
R
2was confirmed also by other studies (Günay et al.
2007;A r g u n 2008) and demonstrates that the results for
MB and MO were fitted on the pseudo-first kinetic order
model and the decrease of the concentration of organiccompounds was almost linear. Li et al. ( 2006)o b s e r v e da
similar trend about the degradation of MO on theTiO
2/Cactivated catalyst.
In the photolysis process, the highest value of the initial
rate ( r0) was determined for ROD, being followed by F,
MO, SA, and MB. After 30 min of photolysis, the com-
pounds were different in terms of degradation efficiency
(De) exactly according to the pattern they were different in
terms of the initial rate ( r0) .T h ei n i t i a lr a t ew a sh i g h e s tf o r
F and was gradually smaller for ROD, SA, MO, and,respectively, MB after 120 min of photolysis.
Table 3summarizes the results obtained by fitting the
results of photocatalysis on the pseudo-first-order kinet-ic model. The values of R
2are close to 1 for MO and
MB; thus, the decrease in the concentration of these
molecules was almost linear. The normalized standard
deviation was low for MB and MO and high for F, SA,
and ROD, suggesting that this kinetic model can be
applied for MB and MO.
According to Figs. 3(a, b) and 4(a, b), the variation
profiles for F, SA, and ROD were not linear; thus, theresults were fitted on the pseudo-second kinetic ordermodel (Table 4). The normalized standard deviations
(Δq) were lower and the square correlation coefficients
(R
2) were higher than those reported in Tables 2and3.
This demonstrates that the pseudo-second kinetic order
model fits best experimental results.
F and ROD were degraded by photocatalysis 4 times,
12 times, and, respectively, 12 times faster than MO,MB, and SA. This was due to the different molecular
mass and surface charge of the molecules. F has thelowest molecular mass (94.11 g/mol), which corre-
sponds to the highest rate of diffusion through the film
at the catalyst surface (19.99 × 10
−6nm2/min) as com-
pared with the other organic molecules. The initial rate
of degradation for MO and SA was low as a result of
their high molecular mass.
Regarding the behavior of ROD during
photocatalysis, the initial rate of degradation was rela-tively high ( ∼43%) and the diffusion coefficient was
also high (18.73 nm
2/min × 106), even if its molecular
mass was high (479.02 g/mol). The increase in thedegradation efficiency from 30 to 120 min was verylow ( ∼4%). Theoretically, a Bfatty ^molecule (high mo-
lecular mass) crosses slowly through the diffusion filmat the catalyst surface; thus, the degradation rate must beslow. In reality, there are some exceptions and MB and
ROD can be considered as being some of these excep-
tions, fact explained by the different surface charge ofTable 1 Results of the EDX analysis of the four points shown in
STEM-HAADF image (Fig. 2)
Element Content (wt.%) Content (at.%) Error (wt.%)
Point 1
Ti 45.21 24.53 2.11
O 30.42 49.51 5.94
C 3.14 6.81 0.61
Si 20.45 19.02 2.57
Ag 0.60 0.14 2.64
Point 2
Ti 43.58 23.21 2.45
O 35.24 56.31 4.89
C 1.58 3.37 0.45
Si 18.62 17.00 2.13
Ag 0.55 0.13 3.42
Point 3
Ti 47.52 25.95 3.54
O 31.52 51.63 5.14
C 2.45 5.35 1.15
Si 18.14 16.98 2.09
Ag 0.5 0.12 1.45
Point 4
Ti 48.53 27.41 3.07
O 30.04 50.90 4.75
C 1.21 2.73 0.75
Si 19.45 18.83 2.23
Ag 0.54 0.14 0.97
Mean
Ti 46.21 25.28 –
O 31.81 52.09 –
C2 . 1 0 4 . 5 7 –
Si 19.17 17.96 –
Ag 0.55 0.13 –41 Page 6 of 12 Water Air Soil Pollut (2017) 228: 41

the molecules. The MB and ROD molecules are posi-
tively charged and the TiO 2-SiO 2surface is negatively
charged (Dufour et al. 2015 ); thus, these two molecules
have a high electrostatic affinity to the catalyst surface ascompared with the other molecules.
4 Discussion
The general mechanism for the degradation of or-
ganic molecules was illustrated in Fig. 5.T h eU Vlight energy needed to split the water molecule,
during photolysis, is ∼2.76 eV. The gap energy of
the Ag/titania-silica is reported in our previous study(Peter et al. 2015a ) as being 2.82 eV. The energy of
the UV light used in the experiments is in the range
3.11 and 3.55 eV; thus, the described processes are
possible under the working conditions.
In solution, the organic molecules are surrounded
by water molecules, forming the hydration film. Inorder to react with the photogenerated radicals, thehydrated organic molecules must suffer dehydration.
Fig. 3 Experimental pseudo-first
kinetic order curves for the
degradation of the organic
compounds during thephotocatalysis for 120 min ( a)
and for 30 min ( b)
Fig. 4 Experimental pseudo-first
kinetic order kinetic curves for the
degradation of the organic
compounds during photolysisprocess for 120 min ( a) and for
30 min ( b)Water Air Soil Pollut (2017) 228: 41 Page 7 of 12 41

In photolysis, the water molecule was cleaved, under
UV light, in hydrogen and hydroxyl radicals
(Papoutsakis et al. 2015). In photocatalysis, as UV
light activates the TiO 2, holes and electrons are
generated in the valence band (VB) and the conduc-tion band (CB). The electrons were, subsequently,activated by transfer on Ag nanoparticles, which
reduce the oxygen molecule when superoxide radi-cals were formed. Holes in VB oxidize hydroxylgroups and water, thus forming hydroxyl and perox-
ide radicals. The radicals decompose the dehydrated
organic molecules until final non-toxic products.
As the results showed, the degradation efficiency
f o rM O ,F ,a n dR O Dw a sh i g h e rb yp h o t o l y s i st h a nb yphotocatalysis. The MO molecule, in its quinoidstructure, is reduced in the presence of radicals to
theMO
−, which are splitted in sulfanilic acid and
N,N-dimethyl-p-phenilene-diamine. While being
attacked by those radicals, these two intermediateswere, subsequently, degraded (Wakimoto et al.2015). The low efficiency obtained in the presence
of the catalyst was explained by the fact that Ag-titania-silica catalyst possesses relatively better sta-
bility and low absolute value of zeta potential in
neutral environment, results reported in our previousstudy (Jastrzezbska et al. 2015). This means that the
catalyst surface is negatively charged at the workingpH as well as MO and F molecules (Table 5). This
generates a reduced attraction between these mole-
cules and the catalyst surface. Moreover, MO is
surrounded by water molecules that must cross thefilm at the catalyst in order to arrive at the catalyst
surface (Dufour et al. 2015). The rate of this process
was diminished because of the high volume of the
hydrated molecule, fact confirmed by the low film
diffusion rate (Table 2). Thus, in order to be adsorbed
onto TiO
2surface, the MO molecule must be
dehydrated, which is an energy-consuming process.Table 2 Parameters determined by fitting the results obtained
during the degradation of the organic compounds in photolysis,
at 30 and 120 min of degradation, in the pseudo-first kinetic ordermodel (k app-apparent rate constant for pseudo-first order kinetic
model, R-correlation coefficient, r 0-initial degradation rate, D e-
degradation efficiency, Δq – normalized standard deviation)
r0(μM/min) 30 min 120 min
kapp(min−1) R2De(%) kapp(min−1) R2De(%) Δq(%)
MO 0.754 0.0071 0.999 19.2 0.0049 0.984 45.7 5.326
MB 0.025 0.0005 0.992 1.5 0.0024 0.858 26.3 6.893F 1.232 0.0060 0.984 16.5 0.0200 0.912 100 56.777
SA 0.126 0.0013 0.987 32.2 0.0195 0.899 92.5 43.193
ROD 1.369 0.0015 0.985 37.3 0.0264 0.976 95.7 66.167
Table 3 Parameters determined by fitting the results obtained
during the degradation of the organic compounds by
photocatalysis, at 30 and 120 min of degradation, in the pseudo-first kinetic model (k
app-apparent rate constant for the pseudo-firstorder kinetic model, R-correlation coefficient, r 0-initial
degradation rate, D e-degradation efficiency, D-diffusion
coefficient, Δq – normalized standard deviation)
r0(μM/min) 30 min 120 min
kapp(min−1)R2De(%) kapp(min−1)R2De(%) Diffusion film Δq(%)
D(nm2/min) × 106R2
MO 0.461 0.0038 0.999 10.7 0.0034 0.999 34 1.65 0.981 0.570
MB 0.167 0.0033 0.999 9.5 0.0052 0.981 46.6 19.74 0.987 5.476
F 1.987 0.0090 0.987 23.7 0.0171 0.952 87.6 19.99 0.951 30.894
SA 0.112 0.0109 0.995 27.9 0.0205 0.903 92.9 1.53 0.942 50.897
ROD 1.710 0.0190 0.870 43.5 0.0030 0.446 46.2 18.73 0.934 31.11741 Page 8 of 12 Water Air Soil Pollut (2017) 228: 41

On the other hand, in photocatalysis, all these photo-
induced reactions (electron transfer, radical attack)
are limited to the surface of the catalyst, while, in
the photolysis, they occur in the whole solution. Ac-cording to Turki et al. (2015), the intermediates gen-
erated by radical attack (generated by UV-assisted
activation of titania-based catalyst) on the F moleculewere hydroquinone, 1,4-benzoquinone, and carbox-
ylic acids (C4 –C2), resulted from the cleavage of the
aromatic ring. In photocatalysis, the low degradation
efficiency as compared with photolysis was given by
the fact that supplementary reactions were needed in
order to generate these radicals. In photolysis, thehydroxyl radicals were already generated. The limita-
tion of the catalytic reactions at the surface of thecatalyst is another reason for the low efficiency of
photocatalysis. The ROD molecule was oxidized by
hydroxyl radicals Chen et al. 2015). The degradation
efficiency in the first 30 min was higher by
photocatalysis than by photolysis due to the affinity
of TiO
2surface for the positively charged ROD (fact
demonstrated also by the high diffusion rate) and
because of the more intense attack of the three types
of photo-generated radicals. The degradation process
by photocatalysis slows however down after 30 min,while the rate of degradation by photolysis grows
stronger than photocatalysis, possibly because some
intermediates agglomerate on the surface of the cata-lyst, which subsequently reduce the adsorption of new
molecules on the surface.Table 4 Parameters determined by fitting the results obtained during the degradation of F, SA, and ROD after 120 min, in the pseudo-
second kinetic model
Photocatalysis Photolysis
k2×1 013(gμmol−1min−1) R2Δq(%) k2×1 013(gμmol−1min−1) R2Δq(%)
F 0.00717 0.999 2.460 0.00466 0.984 61.782
SA 1.49 0.996 2.729 1.05 0.992 12.424
ROD 458.3 0.999 3.805 0.0757 0.991 30.492
k2apparent rate constant for the pseudo-second-order kinetic model, Rcorrelation coefficient, Δqnormalized standard deviation
Fig. 5 General mechanism of degradation by photolysis and respectively by photocatalysisWater Air Soil Pollut (2017) 228: 41 Page 9 of 12 41

The SA molecule was also oxidized by the hy-
droxyl radicals, thus generating a salicylate radical,
which eliminates step by step, molecules of carbon
dioxide until the final products (carbon dioxide and
water) were formed (Peter et al. 2014 ). The degra-
dation efficiency was similar in both processes ap-
plied. The low photocatalytic rate is explained by
the relatively better stability and lower absolutevalue of zeta potential in neutral environment (re-sults reported in our previous study (Jastrzezbskaet al. 2015 )), meaning that the catalyst surface is
negatively charged as well as SA. This reduces theadsorption of the SA on the catalyst surface. Inphotocatalysis, even if the oxidation was assured
by the three types of radicals, all these reactions
were limited on the catalys t surface. Additionally,Table 5 Structure of the investigated organic molecules
Organic molecules Structure
Methyl orange (MO)
Methylene blue (MB)
Salicylic acid (SA)
Phenol (F)
Rhodamine (ROD)41 Page 10 of 12 Water Air Soil Pollut (2017) 228: 41

the low diffusion rate of the SA molecules through
the film at the catalyst surface also reduced thedegradation efficiency.
The oxygen molecule (in photolysis) and the
peroxide radical (in photocatalysis) have oxidizedMB until the final formation of carbon dioxide,
nitrate, sulfate, protons, and water (Soltani and
Entezari 2013 ). In photocatalysis, MB was attacked
also by holes, thus inducing the destabilization of
the electronic balance in the molecule, which leads
to an easy cleavage of the aromatic ring. The higherdegradation efficiency obt ained by photocatalysis as
compared with photolysis was explained by theelectrostatic affinity of the positively charged MBto the negative surface of TiO
2,as well as by a
more intense and efficient oxidation of the MB inthe presence of active holes generated by the UV-
induced excitation of TiO
2.
5 Conclusion
Methyl orange, methylene blue, phenol, salicylic ac-
id, and rhodamine B were photodegraded over nano-
structured Ag-doped titania-silica catalyst. For com-parison, photodegradation experiments in the absence
of catalyst were realized. The degradation efficiency
of methyl orange and phenol was higher by photolysisthan by photocatalysis. The amount of rhodamine B
degraded by photocatalysis was higher than by pho-
tolysis, after the first 30 min. The degradation rate ofmethylene blue was higher by photocatalysis than by
photolysis due to the affinity of the positively charged
molecule for the negative surface of the catalyst, factdemonstrated by the high diffusion rate. The degra-
dation of SA occurred at the same rate either by
photolysis or by photocatalysis. With regard to the
photocatalysis, the highest rates of film diffusion were
obtained for MB, F, and ROD, meaning that thesemolecules crossed the film to arrive at the catalyst
surface more rapidly than the others. For MO and
MB, the results followed the pseudo-first-order kinet-ic model while for SA, F, and ROD, the pseudo-
second-order kinetic model was more appropriate.
Acknowledgements We have conducted the research work
within the framework of the SMARTPACK project, program
MNT-ERANET, contract no. 7-065/26.09.2012, financed by
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