# Photocatalytic Degradation of Polynitrophenols on  Various Commercial Suspended or Deposited Titania  Catalysts Using Artificial and Solar Light

Authors: Hinda Lachheb; Ammar Houas; Jean-Marie Herrmann
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

Phenol (PH) and three polynitrophenols (4-nitrophenol (PNP), 2,4-dinitrophenol (DNP), 2,4,6-trinitrophenol (TNP)) were photocatalytically degraded by using titania under either artificial or solar light. These four reactions were chosen as test reactions to compare the efficiencies of two suspended commercial titania photocatalysts (Degussa P-25 and Millennium PC-500). It appears that P-25 has a higher initial efficiency in all nitrophenol disappearance reactions. However, for the overall degradation rate, measured by the chemical oxygen demand (COD) disappearance, the performance of PC-500 was similar to that of P25. This was attributed to a favorable textural effect since PC-500 has a much higher surface area which facilitates the readsorption of intermediates. PC-500 was subsequently supported on a special photoinert paper provided
by Ahlstrom Company (38-Pont Evèque, France). The influence of the silica binder used for sticking titania particles on the paper fibers was put in evidence as an inhibitor of the coulombic adsorption of anionic species, especially 2,4,6-trinitrophenol, because of the low pzc of silica. Once validated, this supported photocatalyst was introduced in an autonomous solar pilot photoreactor identical to the several prototypes built in the European AQUACAT program. It was demonstrated that the purification of water could be efficiently obtained in a larger scale without any final tedious filtration.


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## Body

## 1. Introduction

The  photocatalytic  degradation of toxic compounds  in
aqueous medium  provides a new opportunity for the treatment of urban and
agricultural wastewater. Nitrophenols, for example, are released into the
environment because of their wide use in syntheses of dyes, pesticides,
plasticizers, and solvents. In addition, they are formed as degradation
products of many chemicals such as herbicides (see for instance herbicides
nitrofen [1] and parathion [2]) when they are exposed to sunlight [3]. The
photocatalytic degradation rate of the different nitrophenols depends on various
parameters, such as temperature [4],
pH [4, 5], initial concentration of
the pollutant [4–6], and light intensity. Various studies have shown that
the number and positions of substituent in some aromatic compounds are widely
linked to their photocatalytic reactivity [7–11].Advanced oxidation processes (AOPs) have been proposed
as an alternative for the treatment of wastewater containing nitrophenols. A
lot of studies focused on these processes [12–16] have pointed out that,
although the reacting systems were different [17, 18], they are all characterized
by the same chemical feature, that is, the production of OH° radicals, which as
the second best oxidant after fluorine, is the active species responsible for the
destruction of organic pollutants [19, 20].The objective of this study was the validation of the
performances of two commercial catalysts using the photocatalytic degradation of
polynitrophenols, (4-nitrophenol (PNP), 2,4-dinitrophenol (DNP), and
2,4,6-trinitrophenol (TNP) or picric acid (TNP)) as test reactions. Such
degradations are now well known
[4–6, 17, 18, 21]. These reactions were performed in
presence of powder or of supported TiO2. In the last case, we used
Millennium PC500 titania coated on an Ahlstrom nonwoven paper to validate its
performances. Subsequently, this deposited photocatalyst was used in an
“AQUACAT” pilot solar photoreactor to clean and reuse contaminated water in semiarid
areas. This is in line with the pioneer works performed at Plataforma Solar de
Almeria (PSA) on many pollutants [22–25].


## 2. Experimental

### 2.1. Materials and Reagents

Titanium dioxide
Degussa P-25 was used as the reference photocatalyst. It is constituted of 80%
anatase and 20% rutile and its BET surface area is 50 m2/g, corresponding
to a mean particle size of ca.
30 nm.  Millennium PC500 titanium dioxide
(100% anatase, specific surface area of 320 m2/g) was
also used either as a powder in slurry or supported on a nonwoven paper (paper
Ahlstrom 1048), offerred
by Mr. J. Dussaud from Ahlstrom
Company.  The area loading in TiO2 was 18 g/m2. Supported titania has been tested by using either UV
artificial light in laboratory experiments or solar light for outside
experiments in South Tunisia . The phenolic
compounds were purchased
from Aldrich (38-Ville D'Abeau, France) and used as received
without further purification.  Their solutions were prepared using
twice-distilled water. Their physicochemical characteristics are given in Table 1.Table 1
Physicochemical characteristics of polynitrophenols.
CompoundMw (g/mol)λmax (nm)ε  (L·mg−1·cm−1)pKapHPhenol94.112700.01539.206.74-nitrophenol (PNP)139.113170.06237.165.82,4-dinitrophenol (DNP)184.112600.05604.005.32,4,6-trinitrophenol (TNP)229.113560.04610.804.0


### 2.2. Apparatus

A one-liter reactor
was used in the laboratory experiments. It is equipped with a plunging tube in
which a Philips HPK 125 W UV-lamp was placed vertically. To avoid the heating of
the solution, water was circulated through a cylindrical jacket, made of Pyrex and located around
the plunging tube. The photon flux of the UV irradiation reaching the exposed
inner part of the reactor atλ>290 nm was measured to be around  6·10−6 Einstein (mol of photons) per second. Another photoreactor was employed for
solar experiments. It is an autonomous pilot scale prototype developed by the
European Research Project AQUACAT using TiO2 supported on Ahlstrom
paper 1048 (Figure 1). This new photoreactor was employed for solar experiments
and consists of one sun collector, one water tank, and one electrical
circulating pump connected with a photovoltaic panel. The reactor itself
consists of glass tubes connected in series and placed on a fixed platform
inclined at a 37° angle corresponding to the local latitude for collecting a
maximum solar irradiation around noon. The water circulating flow was fixed at
12 L/min. The total volume (VT) of the water treated was 20 L and
divided in 12 L in the irradiated glass tubes and 8 L in the tank.This pilot plantis a plug-flow reactor connected to a tank with
recirculation.  This ensemble corresponds
to a batch reactor. During experiments, the temperature of the effluent varied
in the range of 35–40°C, that is, close to the optimum for photocatalysis
[19, 20].Figure 1
Photograph of the autonomous AQUACAT
solar pilot prototype.


### 2.3. Procedure

#### 2.3.1. Laboratory Experiments

One liter of
aqueous solution of each phenolic compound (C0=30 mg/L) was
introduced into the lab reactor and an amount of 500 mg of
TiO2 (P25
or PC500) was added to get a final concentration of 0.5 g/L. The same
photoreactor was used with PC500-TiO2 photocatalyst supported on
Ahlstrom paper 1048. A sheet of this titania supporting paper was deposited on
a glass cylinder of 12.5 cm in diameter and 12 cm high. The corresponding
concentration of TiO2 was about 850 mg/L. The degradation reactions
were carried out at 293 K and at natural pH values (6.7, 5.8, 5.3, and 4.0, for
phenol, PNP, DNP, and TNP, resp.). The suspension was first stirred in
the dark for 90 minutes before irradiation in order to reach the adsorption
equilibrium before starting the photocatalytic reaction.


#### 2.3.2. Solar Experiments

The total volume (VT) treated was 20 L and
the water flow rate was fixed at 12 L/min. The suspension was first stirred in
the dark for 90 minutes before irradiation to reach equilibrated adsorption.
Stirring was obtained by recirculating the total volume of 20 L at a flow rate
of 12 L/min obtained by the electrical pump run by the photovoltaic panel. As
for laboratory experiments, the initial concentration of phenols was fixed at
30 mg/L.


### 2.4. Analyses

Before analysis, the aqueous samples were filtered through a 0.45μm
membrane filter to remove TiO2 agglomerates in suspension. A
Schimadzu-1700 UV-vis spectrophotometer was used to determine the concentration
of the nitrophenols and to follow their kinetic disappearance. Calibration
plots based on Beer-Lambert’s law were established to relate the absorbance to
the concentration. The molar extinction coefficients for phenol, 4-nitrophenol
(PNP), 2,4-dinitrophenol (DNP), and 2,4,6-trinitrophenol (TNP) were estimated
at 270, 317, 260, and 356 nm, respectively. Chemical oxygen demand (COD) was
measured using the acidic dichromate method with a bioblock COD analyzer.


## 2.1. Materials and Reagents

Titanium dioxide
Degussa P-25 was used as the reference photocatalyst. It is constituted of 80%
anatase and 20% rutile and its BET surface area is 50 m2/g, corresponding
to a mean particle size of ca.
30 nm.  Millennium PC500 titanium dioxide
(100% anatase, specific surface area of 320 m2/g) was
also used either as a powder in slurry or supported on a nonwoven paper (paper
Ahlstrom 1048), offerred
by Mr. J. Dussaud from Ahlstrom
Company.  The area loading in TiO2 was 18 g/m2. Supported titania has been tested by using either UV
artificial light in laboratory experiments or solar light for outside
experiments in South Tunisia . The phenolic
compounds were purchased
from Aldrich (38-Ville D'Abeau, France) and used as received
without further purification.  Their solutions were prepared using
twice-distilled water. Their physicochemical characteristics are given in Table 1.Table 1
Physicochemical characteristics of polynitrophenols.
CompoundMw (g/mol)λmax (nm)ε  (L·mg−1·cm−1)pKapHPhenol94.112700.01539.206.74-nitrophenol (PNP)139.113170.06237.165.82,4-dinitrophenol (DNP)184.112600.05604.005.32,4,6-trinitrophenol (TNP)229.113560.04610.804.0


## 2.2. Apparatus

A one-liter reactor
was used in the laboratory experiments. It is equipped with a plunging tube in
which a Philips HPK 125 W UV-lamp was placed vertically. To avoid the heating of
the solution, water was circulated through a cylindrical jacket, made of Pyrex and located around
the plunging tube. The photon flux of the UV irradiation reaching the exposed
inner part of the reactor atλ>290 nm was measured to be around  6·10−6 Einstein (mol of photons) per second. Another photoreactor was employed for
solar experiments. It is an autonomous pilot scale prototype developed by the
European Research Project AQUACAT using TiO2 supported on Ahlstrom
paper 1048 (Figure 1). This new photoreactor was employed for solar experiments
and consists of one sun collector, one water tank, and one electrical
circulating pump connected with a photovoltaic panel. The reactor itself
consists of glass tubes connected in series and placed on a fixed platform
inclined at a 37° angle corresponding to the local latitude for collecting a
maximum solar irradiation around noon. The water circulating flow was fixed at
12 L/min. The total volume (VT) of the water treated was 20 L and
divided in 12 L in the irradiated glass tubes and 8 L in the tank.This pilot plantis a plug-flow reactor connected to a tank with
recirculation.  This ensemble corresponds
to a batch reactor. During experiments, the temperature of the effluent varied
in the range of 35–40°C, that is, close to the optimum for photocatalysis
[19, 20].Figure 1
Photograph of the autonomous AQUACAT
solar pilot prototype.


## 2.3. Procedure

### 2.3.1. Laboratory Experiments

One liter of
aqueous solution of each phenolic compound (C0=30 mg/L) was
introduced into the lab reactor and an amount of 500 mg of
TiO2 (P25
or PC500) was added to get a final concentration of 0.5 g/L. The same
photoreactor was used with PC500-TiO2 photocatalyst supported on
Ahlstrom paper 1048. A sheet of this titania supporting paper was deposited on
a glass cylinder of 12.5 cm in diameter and 12 cm high. The corresponding
concentration of TiO2 was about 850 mg/L. The degradation reactions
were carried out at 293 K and at natural pH values (6.7, 5.8, 5.3, and 4.0, for
phenol, PNP, DNP, and TNP, resp.). The suspension was first stirred in
the dark for 90 minutes before irradiation in order to reach the adsorption
equilibrium before starting the photocatalytic reaction.


### 2.3.2. Solar Experiments

The total volume (VT) treated was 20 L and
the water flow rate was fixed at 12 L/min. The suspension was first stirred in
the dark for 90 minutes before irradiation to reach equilibrated adsorption.
Stirring was obtained by recirculating the total volume of 20 L at a flow rate
of 12 L/min obtained by the electrical pump run by the photovoltaic panel. As
for laboratory experiments, the initial concentration of phenols was fixed at
30 mg/L.


## 2.3.1. Laboratory Experiments

One liter of
aqueous solution of each phenolic compound (C0=30 mg/L) was
introduced into the lab reactor and an amount of 500 mg of
TiO2 (P25
or PC500) was added to get a final concentration of 0.5 g/L. The same
photoreactor was used with PC500-TiO2 photocatalyst supported on
Ahlstrom paper 1048. A sheet of this titania supporting paper was deposited on
a glass cylinder of 12.5 cm in diameter and 12 cm high. The corresponding
concentration of TiO2 was about 850 mg/L. The degradation reactions
were carried out at 293 K and at natural pH values (6.7, 5.8, 5.3, and 4.0, for
phenol, PNP, DNP, and TNP, resp.). The suspension was first stirred in
the dark for 90 minutes before irradiation in order to reach the adsorption
equilibrium before starting the photocatalytic reaction.


## 2.3.2. Solar Experiments

The total volume (VT) treated was 20 L and
the water flow rate was fixed at 12 L/min. The suspension was first stirred in
the dark for 90 minutes before irradiation to reach equilibrated adsorption.
Stirring was obtained by recirculating the total volume of 20 L at a flow rate
of 12 L/min obtained by the electrical pump run by the photovoltaic panel. As
for laboratory experiments, the initial concentration of phenols was fixed at
30 mg/L.


## 2.4. Analyses

Before analysis, the aqueous samples were filtered through a 0.45μm
membrane filter to remove TiO2 agglomerates in suspension. A
Schimadzu-1700 UV-vis spectrophotometer was used to determine the concentration
of the nitrophenols and to follow their kinetic disappearance. Calibration
plots based on Beer-Lambert’s law were established to relate the absorbance to
the concentration. The molar extinction coefficients for phenol, 4-nitrophenol
(PNP), 2,4-dinitrophenol (DNP), and 2,4,6-trinitrophenol (TNP) were estimated
at 270, 317, 260, and 356 nm, respectively. Chemical oxygen demand (COD) was
measured using the acidic dichromate method with a bioblock COD analyzer.


## 3. Results and discussion

### 3.1. Adsorption of Phenolic Compounds on Titania

The adsorption of
the different phenolic compounds is studied at room temperature (20±2°C) and at natural pH. The adsorption kinetic curves for the four
phenolic compounds (phenol, PNP, DNP, and TNP) on different catalysts are
represented in Figure 2. The initial concentration is fixed at 30 mg/L. For each
phenolic compound, whatever the catalyst employed, the steady state of
adsorption is reached within 90 minutes. Therefore, this time has been selected
as the duration of the dark period previously to irradiation.  The degradation initiates at the equilibrium
of adsorption. The maximum absorbed quantities  qmax and the
adsorption constants Kads of phenols were  determined at different
initial  concentrations varying between 5 and 30 mg/L using the linear transform
of the Langmuir equation, (1)Ceqe=1Kqmax+Ceqmax,
where  Ce  is the equilibrium concentration of the phenolic
compounds and qe is the quantity of the adsorbed phenolic compounds
at TiO2 surface. The linearity of the curves clearly indicates that
the Langmuir isotherm is correctly observed, implying a monolayer adsorption
model. The calculated values are listed in Table 2.Table 2
Langmuir parameters
of phenolic compound adsorption.
ReactantsDegussa P25PC500PC500/Paper 1048qmax  (mg/g)Kads
(L/mg)nads
(mol/nm2)qmax
(mg/g)Kads
(L/mg)nads
(mol/nm2)qmax
(mg/g)Kads
(L/mg)Phenol4.870.0310.626.270.1360.12510.440.2904-nitrophenol
(PNP)10.250.0460.889.110.2900.1208.320.2172,4-dinitrophenol (DNP)15.520.0251.0011.730.1100.1206.602.7802,4,6-trinitrophenol (TNP)18.930.0201.0015.860.0210.1305.667.00nads: area
density of adsorbed phenol molecules (in molecules/nm2).Kinetics of
adsorption of phenolic compounds in presence of different catalysts (a). Degussa P25, (b) Millennium PC-500, and (c) Millennium PC-500 TiO2 deposited on Ahlstrom paper 1048. Experimental conditions: C0 = 30 mg/L,
[P25] = [PC500] = 2.5 g/L, V = 100 mL, natural pH.
(a)(b)(c)The following
general observations can be made. For both unsupported Degussa P25 and
Millennium PC-500 photocatalysts, the maximum quantities of adsorbed phenolic
compounds increase in the following order:(2)Phenol<PNP<DNP<TNP.This is in line with an ionic type of adsorption. Because of their
decreasing pKa’s and of the resulting pH’s of the solutions, the surface of
titania samples gets positively charged according to
(3)Ti−OH+H+⇄Ti−OH2+. This favors the coulombic attraction and the related adsorbed
quantities of the nitrophenols.By contrast, the variations ofqmax are opposite for
PC-500/paper 1048. This is due to the presence of the amorphous silica binder.
Since the pzc of silica is very low in comparison with that of titania (56),
there is a coulombic repulsion of the anionic polynitrophenols by the
negatively charged together
(silica binder   +     titania catalyst). Surprisingly,
there is a proportional variation between  qmax and the number  n   of nitrosubstituents
as illustrated in Figure 3.Figure 3
Linear variations between the
maximum quantity of adsorbed polynitrophenolsqmax and the number of NO2 substituent.


### 3.2. Photocatalytic Degradation of The Polynitrophenols

The disappearance
rates of nitrophenolic compounds in contact with the three UV-irradiated
photocatalysts are given in Figure4. Starting with an initial concentration of
30 mg/L, the total disappearance of all nitrophenolic compounds is reached in
between 2 and 7 hours.Kinetics of the photocatalytic degradation of phenols in
presence of different catalysis ((a)
P25, (b) PC500, and (c) supported TiO2 (paper 1048)). In the insert, first-order
linear transforms Ln C0/C  =    f(t). Experimental
conditions: C0 = 30 mg/L,
[P25] = [PC500] = 0.5 g/L, V = 1 L,
natural pH.
(a)(b)(c)All reactions followed an apparent first-order verified
by the linear transforms Ln(C0/C) = f(t) illustrated in the insert in Figure 5. The slopes give the apparent rate
constants listed in Table 3. The initial rates of disappearance in ppm/min are
in the following order: TNP>DNP>PNP>phenol. It appears that PC500
is less efficient for the photocatalytic disappearance of nitrophenols than
P25. This also observed for specific activities, that is, rates per g of
catalyst (see Table 3). For PC500 supported on Ahlstrom paper 1048, the order
is different—phenol<PNP<DNP<TNP.Table 3
Kinetic data of the
photocatalytic degradation of nitrophenols.
P25PC500Paper 1048Kapp·102  (mn-1)Kapp/g·102    (mn−1·g−1)ro (ppm/min)Kapp·102 (mn-1)Kapp/g·102 (mn−1·g−1)ro (ppm/min)Kapp·102 (mn-1)Kapp·102 (mn−1·g−1)ro (ppm/min)PH0.561.120.0930.531.060.130.921.080.11PNP1.002.000.1840.731.460.160.670.780.10DNP1.182.360.2400.961.920.180.480.560.06TNP1.983.960.2981.282.560.240.330.380.05Figure 5
Linear variations of the rate
constants with the number of NO2 substituents.All these differences can be explained by considerations
about adsorption. The rates of polynitrophenol disappearance vary in parallel
with the quantities of adsorbed pollutants as indicated by the corresponding
parameters indicated in Tables2 and 3. In addition,  this is confirmed by the
parallel proportional variations of either the quantities of polynitrophenols
adsorbed (Figure 3) or of the rate constants as function of the number of NO2 groups (Figure 5). This confirms the
Langmuir-Hinshelwood mechanism involved in these reactions.


### 3.3. Kinetics of Cod Disappearance

The kinetics of the total mineralization of nitrophenols
has been determined by following the disappearance of the chemical oxygen
demand (COD). The kinetic isotherms are given in Figure6. For all
nitrophenols, COD reached 0 within 8 hours. The initial values of COD are in
agreement with the chemical formulas and with the stoichiometries of all nitrophenols
degradation reaction, TNP having the smallest COD.Kinetics of COD disappearance of
nitrophenols in presence of different catalysts ((a) Degussa P25, (b)
Millennium PC-500, and (c) PC-500 supported on Ahlstrom paper 1048).
Experimental conditions:
C0 = 30 mg/L, [P25] = [PC500] = 0.5 g/L, V = 1 L, natural pH.
(a)(b)(c)Although titania Degussa P-25 has the highest activity for
nitrophenols disappearance, it clearly appears from Table4 that PC-500 has an
important efficiency in their total mineralization. This can be explained by a
favorable textural effect. Since total degradation involves many consecutive
reactions before reaching CO2, the readsorption of all the numerous
intermediates is favored by catalyst having large surface areas, which is the
case of PC-500.This advantage can be transposed to PC-500 supported on paper
Ahlstrom 1048, as seen from data in Table 4. This underlines the interest of
using supported catalyst to decontaminate wastewater by avoiding the tedious
final filtration step, which strongly increases the overall cost of this
potential treatment.Table 4
Initial COD values in mgO2/L for nitrophenols having the same initial ponderal concentration (30 mg/L). Apparent first-order rate constant Kapp and corresponding specific one Kapp/gcat.
COD initial (mg/L)P25PC500Paper 1048Kapp  (mn-1)Kapp/g  (mn−1·g−1)Kapp (mn-1)Kapp/g (mn−1·g−1)Kapp (mn-1)Kapp/g (mn−1·g−1)Phenol (PH)740.501.01.152.300.800.944-nitrophenol (PNP)500.801.61.733.461.121.312,4-dinitrophenol (DNP)451.282.52.194.381.661.952,4,6-trinitrophenol (TNP)301.843.73.146.283.624.25


### 3.4. Kinetics of Ph Variations during The Degradation of Phenols

According to the
stoichiometry of the total oxidation reaction,(4)(NO2)x−C6H(5−x)−OH+7O2→6CO2+xNO3−+xH++(3−x)H2O, the pH of the reaction medium is expected to decrease except for
phenol (x=0). This is actually observed with its temporal variations given in Figure 7. Therefore, the kinetics of pH variations is a good indicator of nitrophenols
degradation or, at least, of their denitration. However, pH in the
UV-irradiated titania slurry is a rather important and complex parameter since
it governs (i) water dissociation equilibrium, (ii) the surface charge of
titania with respect to its pzc, and (iii) the ionization state of the organic
reactants and of their intermediates. In addition, it also depends on the
degradation of other pollutants present in the water to be treated.
Nevertheless, in a batch photoreactor, its steady-state final value could be an
easy and cheap indicator of the final reaching of total mineralization.Figure 7
Kinetics of pH variations during the photocatalytic degradation of nitrophenols. Experimental conditions:
C0 = 30 mg/L, [P25] = [PC500] = 0.5 g/L, V = 1 L


### 3.5. Solar Experiments in The Autonomous Pilot Plant

The polynitrophenol
degradation reactions have been transposed to the solar pilot photoreactor and
used to determine the efficiency of supported PC500
TiO2 using solar
energy in South Tunisia. This pilot plantis a plug-flow reactor connected to a tank with recirculation. This
ensemble corresponds to a batch reactor. Because of the design of the reactor
and of the varying solar radiant flux, a direct comparison with the results
obtained in laboratory would not be correct. Only basic fundamental features
are comparable for laboratory and pilot results. Actually, all reactions
followed an apparent first-order kinetics, confirmed by the linear transforms
Ln C0/C =  f(t)  in the
inserts in Figure 8. The rate constants of the disappearance of nitrophenols
are in a similar order of reactivity, PNP>DNP>phenol>TNP. The
kinetic parameters of COD disappearance, which are illustrated by Figure 9 and
listed in Table 5 show that TNP has the higher rate of COD disappearance as in
laboratory experiments.Table 5
Solar degradation and mineralization kinetics (Kapp,  r0) of phenolic compounds.
Kapp·102 (mn-1)r0 (mol/L·min)DegradationMineralizationDegradationMineralizationPhenol0.170.150.0680.0664-nitrophenol (PNP)0.420.230.1050.0832,4-dinitrophenol (DNP)0.280.450.0800.1082,4,6-trinitrophenol (TNP)0.130.670.0550.150Figure 8
Experiments with pilot plant using solar energy, phenols disappearance. In the insert: first-order linear transforms
Ln C0/C =  f(t)   of  nitrophenols degradation. Experimental conditions:
C0 = 30 mg/L, V = 20 L, TiO2 supported on Ahlstrom paper 1048, natural pH.Figure 9
Experiments with pilot plant using solar energy, Kinetics of COD disappearance, in the insert: first-order linear transforms
Ln (COD0)/COD = f(t)   of nitrophenols mineralization. Experimental conditions:
C0 = 30 mg/L, V = 20 L, supported TiO2 (paper 1048), natural pH.


## 3.1. Adsorption of Phenolic Compounds on Titania

The adsorption of
the different phenolic compounds is studied at room temperature (20±2°C) and at natural pH. The adsorption kinetic curves for the four
phenolic compounds (phenol, PNP, DNP, and TNP) on different catalysts are
represented in Figure 2. The initial concentration is fixed at 30 mg/L. For each
phenolic compound, whatever the catalyst employed, the steady state of
adsorption is reached within 90 minutes. Therefore, this time has been selected
as the duration of the dark period previously to irradiation.  The degradation initiates at the equilibrium
of adsorption. The maximum absorbed quantities  qmax and the
adsorption constants Kads of phenols were  determined at different
initial  concentrations varying between 5 and 30 mg/L using the linear transform
of the Langmuir equation, (1)Ceqe=1Kqmax+Ceqmax,
where  Ce  is the equilibrium concentration of the phenolic
compounds and qe is the quantity of the adsorbed phenolic compounds
at TiO2 surface. The linearity of the curves clearly indicates that
the Langmuir isotherm is correctly observed, implying a monolayer adsorption
model. The calculated values are listed in Table 2.Table 2
Langmuir parameters
of phenolic compound adsorption.
ReactantsDegussa P25PC500PC500/Paper 1048qmax  (mg/g)Kads
(L/mg)nads
(mol/nm2)qmax
(mg/g)Kads
(L/mg)nads
(mol/nm2)qmax
(mg/g)Kads
(L/mg)Phenol4.870.0310.626.270.1360.12510.440.2904-nitrophenol
(PNP)10.250.0460.889.110.2900.1208.320.2172,4-dinitrophenol (DNP)15.520.0251.0011.730.1100.1206.602.7802,4,6-trinitrophenol (TNP)18.930.0201.0015.860.0210.1305.667.00nads: area
density of adsorbed phenol molecules (in molecules/nm2).Kinetics of
adsorption of phenolic compounds in presence of different catalysts (a). Degussa P25, (b) Millennium PC-500, and (c) Millennium PC-500 TiO2 deposited on Ahlstrom paper 1048. Experimental conditions: C0 = 30 mg/L,
[P25] = [PC500] = 2.5 g/L, V = 100 mL, natural pH.
(a)(b)(c)The following
general observations can be made. For both unsupported Degussa P25 and
Millennium PC-500 photocatalysts, the maximum quantities of adsorbed phenolic
compounds increase in the following order:(2)Phenol<PNP<DNP<TNP.This is in line with an ionic type of adsorption. Because of their
decreasing pKa’s and of the resulting pH’s of the solutions, the surface of
titania samples gets positively charged according to
(3)Ti−OH+H+⇄Ti−OH2+. This favors the coulombic attraction and the related adsorbed
quantities of the nitrophenols.By contrast, the variations ofqmax are opposite for
PC-500/paper 1048. This is due to the presence of the amorphous silica binder.
Since the pzc of silica is very low in comparison with that of titania (56),
there is a coulombic repulsion of the anionic polynitrophenols by the
negatively charged together
(silica binder   +     titania catalyst). Surprisingly,
there is a proportional variation between  qmax and the number  n   of nitrosubstituents
as illustrated in Figure 3.Figure 3
Linear variations between the
maximum quantity of adsorbed polynitrophenolsqmax and the number of NO2 substituent.


## 3.2. Photocatalytic Degradation of The Polynitrophenols

The disappearance
rates of nitrophenolic compounds in contact with the three UV-irradiated
photocatalysts are given in Figure4. Starting with an initial concentration of
30 mg/L, the total disappearance of all nitrophenolic compounds is reached in
between 2 and 7 hours.Kinetics of the photocatalytic degradation of phenols in
presence of different catalysis ((a)
P25, (b) PC500, and (c) supported TiO2 (paper 1048)). In the insert, first-order
linear transforms Ln C0/C  =    f(t). Experimental
conditions: C0 = 30 mg/L,
[P25] = [PC500] = 0.5 g/L, V = 1 L,
natural pH.
(a)(b)(c)All reactions followed an apparent first-order verified
by the linear transforms Ln(C0/C) = f(t) illustrated in the insert in Figure 5. The slopes give the apparent rate
constants listed in Table 3. The initial rates of disappearance in ppm/min are
in the following order: TNP>DNP>PNP>phenol. It appears that PC500
is less efficient for the photocatalytic disappearance of nitrophenols than
P25. This also observed for specific activities, that is, rates per g of
catalyst (see Table 3). For PC500 supported on Ahlstrom paper 1048, the order
is different—phenol<PNP<DNP<TNP.Table 3
Kinetic data of the
photocatalytic degradation of nitrophenols.
P25PC500Paper 1048Kapp·102  (mn-1)Kapp/g·102    (mn−1·g−1)ro (ppm/min)Kapp·102 (mn-1)Kapp/g·102 (mn−1·g−1)ro (ppm/min)Kapp·102 (mn-1)Kapp·102 (mn−1·g−1)ro (ppm/min)PH0.561.120.0930.531.060.130.921.080.11PNP1.002.000.1840.731.460.160.670.780.10DNP1.182.360.2400.961.920.180.480.560.06TNP1.983.960.2981.282.560.240.330.380.05Figure 5
Linear variations of the rate
constants with the number of NO2 substituents.All these differences can be explained by considerations
about adsorption. The rates of polynitrophenol disappearance vary in parallel
with the quantities of adsorbed pollutants as indicated by the corresponding
parameters indicated in Tables2 and 3. In addition,  this is confirmed by the
parallel proportional variations of either the quantities of polynitrophenols
adsorbed (Figure 3) or of the rate constants as function of the number of NO2 groups (Figure 5). This confirms the
Langmuir-Hinshelwood mechanism involved in these reactions.


## 3.3. Kinetics of Cod Disappearance

The kinetics of the total mineralization of nitrophenols
has been determined by following the disappearance of the chemical oxygen
demand (COD). The kinetic isotherms are given in Figure6. For all
nitrophenols, COD reached 0 within 8 hours. The initial values of COD are in
agreement with the chemical formulas and with the stoichiometries of all nitrophenols
degradation reaction, TNP having the smallest COD.Kinetics of COD disappearance of
nitrophenols in presence of different catalysts ((a) Degussa P25, (b)
Millennium PC-500, and (c) PC-500 supported on Ahlstrom paper 1048).
Experimental conditions:
C0 = 30 mg/L, [P25] = [PC500] = 0.5 g/L, V = 1 L, natural pH.
(a)(b)(c)Although titania Degussa P-25 has the highest activity for
nitrophenols disappearance, it clearly appears from Table4 that PC-500 has an
important efficiency in their total mineralization. This can be explained by a
favorable textural effect. Since total degradation involves many consecutive
reactions before reaching CO2, the readsorption of all the numerous
intermediates is favored by catalyst having large surface areas, which is the
case of PC-500.This advantage can be transposed to PC-500 supported on paper
Ahlstrom 1048, as seen from data in Table 4. This underlines the interest of
using supported catalyst to decontaminate wastewater by avoiding the tedious
final filtration step, which strongly increases the overall cost of this
potential treatment.Table 4
Initial COD values in mgO2/L for nitrophenols having the same initial ponderal concentration (30 mg/L). Apparent first-order rate constant Kapp and corresponding specific one Kapp/gcat.
COD initial (mg/L)P25PC500Paper 1048Kapp  (mn-1)Kapp/g  (mn−1·g−1)Kapp (mn-1)Kapp/g (mn−1·g−1)Kapp (mn-1)Kapp/g (mn−1·g−1)Phenol (PH)740.501.01.152.300.800.944-nitrophenol (PNP)500.801.61.733.461.121.312,4-dinitrophenol (DNP)451.282.52.194.381.661.952,4,6-trinitrophenol (TNP)301.843.73.146.283.624.25


## 3.4. Kinetics of Ph Variations during The Degradation of Phenols

According to the
stoichiometry of the total oxidation reaction,(4)(NO2)x−C6H(5−x)−OH+7O2→6CO2+xNO3−+xH++(3−x)H2O, the pH of the reaction medium is expected to decrease except for
phenol (x=0). This is actually observed with its temporal variations given in Figure 7. Therefore, the kinetics of pH variations is a good indicator of nitrophenols
degradation or, at least, of their denitration. However, pH in the
UV-irradiated titania slurry is a rather important and complex parameter since
it governs (i) water dissociation equilibrium, (ii) the surface charge of
titania with respect to its pzc, and (iii) the ionization state of the organic
reactants and of their intermediates. In addition, it also depends on the
degradation of other pollutants present in the water to be treated.
Nevertheless, in a batch photoreactor, its steady-state final value could be an
easy and cheap indicator of the final reaching of total mineralization.Figure 7
Kinetics of pH variations during the photocatalytic degradation of nitrophenols. Experimental conditions:
C0 = 30 mg/L, [P25] = [PC500] = 0.5 g/L, V = 1 L


## 3.5. Solar Experiments in The Autonomous Pilot Plant

The polynitrophenol
degradation reactions have been transposed to the solar pilot photoreactor and
used to determine the efficiency of supported PC500
TiO2 using solar
energy in South Tunisia. This pilot plantis a plug-flow reactor connected to a tank with recirculation. This
ensemble corresponds to a batch reactor. Because of the design of the reactor
and of the varying solar radiant flux, a direct comparison with the results
obtained in laboratory would not be correct. Only basic fundamental features
are comparable for laboratory and pilot results. Actually, all reactions
followed an apparent first-order kinetics, confirmed by the linear transforms
Ln C0/C =  f(t)  in the
inserts in Figure 8. The rate constants of the disappearance of nitrophenols
are in a similar order of reactivity, PNP>DNP>phenol>TNP. The
kinetic parameters of COD disappearance, which are illustrated by Figure 9 and
listed in Table 5 show that TNP has the higher rate of COD disappearance as in
laboratory experiments.Table 5
Solar degradation and mineralization kinetics (Kapp,  r0) of phenolic compounds.
Kapp·102 (mn-1)r0 (mol/L·min)DegradationMineralizationDegradationMineralizationPhenol0.170.150.0680.0664-nitrophenol (PNP)0.420.230.1050.0832,4-dinitrophenol (DNP)0.280.450.0800.1082,4,6-trinitrophenol (TNP)0.130.670.0550.150Figure 8
Experiments with pilot plant using solar energy, phenols disappearance. In the insert: first-order linear transforms
Ln C0/C =  f(t)   of  nitrophenols degradation. Experimental conditions:
C0 = 30 mg/L, V = 20 L, TiO2 supported on Ahlstrom paper 1048, natural pH.Figure 9
Experiments with pilot plant using solar energy, Kinetics of COD disappearance, in the insert: first-order linear transforms
Ln (COD0)/COD = f(t)   of nitrophenols mineralization. Experimental conditions:
C0 = 30 mg/L, V = 20 L, supported TiO2 (paper 1048), natural pH.


## 4. Conclusions

The photocatalytic
degradation reactions of phenol and of mono-, di-, and trinitrophenol, chosen
as test reactions to compare the efficiencies of two suspended commercial
titania photocatalysts (Degussa P-25 and Millennium PC500), underlined the
difficulty and complexity in objectively comparing two solids. If Degussa P-25
appeared easily as that having the higher initial efficiency in all nitrophenol
disappearance reactions (as mostly observed in literature), however, for the
overall degradation rate, measured by the chemical oxygen demand (COD)
disappearance, the performances of PC-500 were similar to its own ones. This
was attributed to a favorable textural effect since PC-500 has a much higher
surface area,  which facilitates the readsorption of the very large number of
intermediates. When PC-500 was subsequently supported on a special photoinert
paper provided by Ahlstrom Company, the presence of the silica binder, used for
sticking titania particles on the paper fibers, perturbs several key parameters, principally the natural pH,  because of the low pzc of silica. It acts as an
inhibitor of the coulombic adsorption of anionic species, especially, 2,4,6-trinitrophenol.
However, despite this negative influence, the supported photocatalyst when
introduced in an autonomous AQUACAT solar pilot photoreactor clearly
demonstrates that it can achieve purification of water with a reasonable
efficiency, very compatible with the use of solar energy at a larger scale. In
particular, it avoids the final tedious filtration necessary (i) to provide
clean water and (ii) to recover and recycle the catalysts. Since filtration is
declared as redhibitory by the chemical engineering point of view, this
stresses the usefulness of such  a material.Scheme 1



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*Source: 497895-2008-03-19.xml*