(544gg) Photocatalytic Degradation of Acid Violet 7 Dye Using a Composite of ZnO/Ppy in Annular Continuos Reactor

PHOTOCATALYTIC DEGRADATION OF ACID VIOLET 7 DYE
USING A COMPOSITE OF ZnO/PPy IN ANNULAR CONTINUOS REACTOR

Diego Alexander González Casamachin.¹, Javier Rivera De la Rosa¹*, Carlos Javier Lucio
Ortiz¹, Víctor Manuel Ovando Medina², Nancy Elizabeth
Dávila Guzmán¹, David Alejando de Haro del Río¹, Diana
Bustos Martínez¹, Gerardo Antonio Flores Escamilla¹ and  Francisco
José Morales Leal¹.

¹Universidad Autónoma de Nuevo León, Facultad de
Ciencias Químicas, Av Universidad s/n, Cd. Universitaria, San Nicolás de Los
Garza. N. L. C.P 64450 (México)

²Universidad Autónoma de San Luis de Potosí, Ingeniería
Química, Coordinación Académica Región Altiplano (COARA), Universidad Autónoma
de San Luis Potosí, Carretera a Cedral KM 5+600, San José de las Trojes,
Matehuala, SLP 78700, (México).

*javier.riverad@uanl.edu.mx

Introduction

Nowadays, the textile
industries discharge large amount of dyes during the dyeing process. In that
sense, about 20% of the dyes in the world enter the textile
wastewater during this process. The amount of discarded dye by industrial use
depends on the kind of dye, which can range between 2% for basic dyes and 50%
for azo dyes [1]. Accordingly, organic contaminants such as dyes are considered
an important group of synthetic dyes, since they are xenobiotic chemical
compounds characterized by the presence of one or more azo groups (-N=N-). In
response to the need for water decontamination, different researchers have
developed photocatalytic nanomaterials for the degradation of contaminants
present in the aqueous phase. Therefore, ZnO has been considered a low cost
photocatalyst with physics, operational capacity and availability [2]. However,
due to its band-gap (3.25 eV) [2], ZnO is photoactive only under irradiation of
UV light (265 <λ <370 nm), which is dangerous and
expensive. In this sense, some research has been focused on the design of
active photocatalysts in visible light. This is achieved by reducing the band-gap
of semiconductors, by performing different doping with metals. In this sense,
polypropyl (PPy) has both electrical and optical properties such as high light
absorption coefficients, high mobility of charge carriers and excellent
stability, which provides photocatalytic activity. In the present work, the
ZnO/PPy compound was synthesized and applied as photocatalyst in the
elimination of the violet acid dye 7 (AV 7) in an annular continuous reactor.

Method and materials

The composite of ZnO/PPy was
synthesized as follows: First, ZnO nanoparticles were prepared by precipitation
method with sodium hydroxide and then, the polymerization of pyrrole onto ZnO
nanoparticles was carried out following the methodology reported in [3]. In
that sense, zinc chloride (8.17 g) and sodium dodecyl sulfate (14.40 g) were
mixed in 150 ml of water under magnetic stirring during 5 minutes, and then the
sodium hydroxide was added slowly. The mixture was stirred for 1.5 hours and
then placed in a drying oven at 85 ° C for 5 hours. Finally, the precipitate
was washed several times with water and dried at 60 ° C for 24 h to obtain
purified ZnO nanoparticles (white powder). Subsequently, the synthesized composite
was tested in the photodegradation of acid violet 7 (AV7) dye under visible
light irradiation in an annular continuous reactor using a 15 Volt LED lamp as
a visible source light. The mass-transfer was evaluated in photodegradation
testing with a three-resistance model [4].

Results

The previously synthesized ZnO nanoparticles were characterized
by UV-Vis analysis with diffuse reflectance to determine its bandgap. In Figure
1 it can be seen that the maximum absorption of the ZnO nanoparticles is 385.38
nm, then the bangap was calculated from the inflection point method obtaining a
value of 3.22 eV, which coincides with the bandgap reported in the literature
for ZnO (3.25 eV) (2). On the other hand, the FTIR of the ZnO nanoparticles,
polypyrrole and the ZnO / PPy composite was performed as shown in Figure 2,
finding that in the region of 3388 cm^-1 there is the OH interaction
of stretching attributed to the physisorbed water or hidroxyl species in all
samples, while in the region of 1500 cm^-1 for the ZnO/PPy composite
is the interaction C=C and C-C of stretching attributed to the polipyrrol ring.
Near to 890 cm^-1 for the composite is assigned of   Zn-O of
stretching, which indicates that ZnO nanoparticles were well coated with PPy. To
evaluate the mass transfer the Thiele modulus was calculated and reactions
rates were obtained via the Langmuir–Hinshelwood (L-H). For the elimination of
AV7, the Thiele modulus at 25, 35 and 40°C were 5.03×10^-3, 6.51×10^-3
and 4.48×10^-3 respectively. As shown in Figure 3 the reaction
occurred in the kinetically controlled regime. Figure 4 shows the experimental
surface reaction data for AV7 degradation as a function of the inlet
concentration of AV7 in a photocatalytic continuous annular reactor at three
temperatures using ZnO/PPy composite deposited on the interior wall surfaces of
cylinders.

The L-H model was fitted to the reaction data using
the Microcal™ Origin® version 6.0 software package, which allows for parameter
estimation. The nonlinear least-squares fitting routine based on the
Levenberg–Marquardt algorithm was employed (See Figure 4). The model
considers the reversible adsorption of AV7 and the reaction rate on the
surface, which is the controlling step. The activation energy and thermodynamic
properties of constant of adsorption of L-H model for ZnO/PPy were E_a:
12,566 cal/mol, ΔH°_ads:
-24,774.509 cal/mol and ΔS°_ads: -79.120 cal/mol°K. The low
activation-energy values (less than 10,000 cal/mol) indicate that the process
is carried out more in the regime of transport phenomena rather than an
entirely reactive process. As result, the AV7 molecules are adsorbed strongly
in active sites on a surface but with some mobility (according the  ΔH°_ads
and ΔS°_ads) then the water is oxidized by the holes (h+) to
produce •OH, and the reaction occurs on the surface.

Figure 1.
UV-Vis-NIR a) Polypirrol (PPy); b) ZnO/PPy; c) ZnO

Figure 3. Overall resistance
and its three individual resistances as function of the Thiele modulus and the
three controlling regimens in annular continuos reactor

Figure 2. FTIR of:
a) Polypirrol (PPy); b) ZnO/PPy; c) ZnO.

Figure 4. Surface reaction
rates for AV7 elimination in annular continuos reactor at three different
temperatures. The continues lines are the model L-H fitting.

Conclusions.

• Using the FTIR it was observed that ZnO nanoparticles were totally immersed into a PPy matrix.
• By means of the UV-Vis-NIR method, a 3.22 eV bangap was obtained for the ZnO. Besides, due to the presence of PPy semiconducting in the ZnO/PPy composite, the catalyst was reactive under visible light.
• The model of three resistance of mass transfer left to conclude the regimen of control was the kinetic one.
• An L-H model was fitted at three different temperatures and the values of the parameters were interpreted according to the reaction that was carried out on the surface.

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