Photocatalytic degradation enhancement in pickering emulsions stabilized by solid particles of bare TiO2

Article abstract of DOI:10.1021/acs.langmuir.8b03806

Pickering emulsions provide a new way to enhance the efficiency of photocatalytic degradation of water-insoluble pollutants. Indeed, the semiconductor solid particles dually act as the photocatalyst and stabilizer of the emulsion droplets whose size dramatically affects the photocatalytic reaction. The present work aims at the validation of this concept by using bare TiO₂ without any surface modification. Nanostructured TiO₂ has been prepared by a simple sol-gel process and characterized by X-ray diffraction, specific surface area analysis, scanning electron microscopy, and diffuse reflectance spectroscopy. The emulsions were prepared by using 1-methylnaphthalene (1-MN) as a model organic contaminant scarcely soluble in water and bare TiO₂ as the photocatalyst/stabilizer. The emulsions have been characterized by electrical conductivity, optical microscopy, and light-scattering analyses. The photocatalytic degradation of 1-MN was 50 times faster in stable Pickering emulsions with respect to the case of biphasic liquid systems containing TiO₂. This finding allows us to propose Pickering emulsions stabilized by TiO₂ nanoparticles as an effective and novel way to intensify the photocatalytic degradation of water-insoluble organic pollutants.

Full text of DOI:10.1021/acs.langmuir.8b03806

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge
Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage
Photocatalytic degradation enhancement in Pickering
emulsions stabilized by solid particles of bare TiO2
Nidhal Fessi, Mohamed Faouzi Nsib, Yves Chevalier, Chantal Guillard, Frederic
Dappozze, Ammar Houas, Leonardo Palmisano, and Francesco Parrino
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03806 • Publication Date (Web): 18 Jan 2019
Downloaded from http://pubs.acs.org on January 20, 2019
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Photocatalytic degradation enhancement in Pickering emulsions stabilized
by solid particles of bare TiO2
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Nidhal Fessi,a,b Mohamed Faouzi Nsib,a,c Yves Chevalier,*b Chantal Guillard,d
Frédéric Dappozze,d Ammar Houas,a Leonardo Palmisano,e
Francesco Parrino*f
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a Unité de Recherche Catalyse et Matériaux pour l'Environnement et les Procédés URCMEP (UR11ES85), Faculté
des Sciences de Gabès, University of Gabès, Campus Universitaire, Cité Erriadh, 6072 Gabès, Tunisia.
Laboratoire d’Automatique et de Génie des Procédés, CNRS, UMR 5007, University Claude Bernard Lyon 1,
b
43 bd 11 Novembre, 69622 Villeurbanne, France.
c Higher School of Sciences and Technology, University of Sousse, Rue Tahar Ben Achour, 4003 Sousse, Tunisia
d Université Lyon, Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR 5256
CNRS, Université Claude Bernard Lyon1, 2 av Albert Einstein, 69626 Villeurbanne, France.
e
Dipartimento di Energia, Ingegneria dell’Informazione e Modelli Matematici (DEIM), University of Palermo,
Viale delle Scienze Ed. 6, 90128 Palermo, Italy.
f Department of Industrial Engineering, University of Trento, Via Sommarive 9, 38123 Trento, Italy.
*Corresponding authors: francesco.parrino@unitn.it; yves.chevalier@univ-lyon1.fr
ABSTRACT
Pickering emulsions provide a new way to enhance the efficiency of photocatalytic degradation
of water-insoluble pollutants. Indeed, the semiconductor solid particles dually act as
photocatalyst and stabilizer of the emulsion droplets whose size dramatically affects the
photocatalytic reaction. The present work aims at the validation of this concept by using bare
TiO2 without any surface modification. Nanostructured TiO2 has been prepared by a simple sol-
gel process and characterized by X-ray diffraction, specific surface area analysis, scanning
electron microscopy, and diffuse reflectance spectroscopy. The emulsions were prepared by
using 1-methylnaphthalene (1-MN) as a model organic contaminant scarcely soluble in water
and bare TiO2 as photocatalyst/stabilizer. The emulsions have been characterized by electrical
conductivity, optical microscopy and light scattering analyses. The photocatalytic degradation
of 1-MN resulted 50 times faster in stable Pickering emulsions with respect to the case of
biphasic liquid systems containing TiO2. This finding allows to propose Pickering emulsions
stabilized by TiO2 nanoparticles as an effective and novel way to intensify the photocatalytic
degradation of water insoluble organic pollutants.
Keywords: Pickering emulsion, TiO2, 1-methylnaphthalene, photocatalysis.
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Introduction
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Plenty of non-biodegradable aromatic organic compounds, such as solvents, grease, oil and
various hydrocarbons, exist in wastewater downstream to various industrial processes
(production of cosmetics, oil refinery and petrochemicals, mechanical industry etc.) [1]. These
pollutants are significant sources of environmental contamination. In fact, in addition to their
possible intrinsic toxicity, they generally spread as insoluble liquid layer floating on the surface
of water, decreasing atmospheric oxygen transfer to water thus dramatically affecting the
natural biological processes and the quality of water. Treating this kind of wastewater is a hard
task. Indeed, various factors (e.g. temperature, presence of surfactants etc.) influence the
existence of these compounds as separate phases or as emulsions (oil-in-water), so the treatment
process should be characterized by high flexibility and efficiency. Physical processes such as
gravity separation, flotation, emulsion breaking or incineration [2,3] are commonly used to
separate large amounts of oil. However, separation is a very time demanding step and the
efficiency results often inadequate due to the possible formation of emulsions. Furthermore,
physical treatments do not afford pollutant mineralization but only separation, thus maintaining
the problem of their disposal.
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TiO2 photocatalysis has proved an extremely promising alternative for the treatment of water
due to its green and sustainable features [5-7]. Formation of highly oxidizing active oxygen
species through light induced redox reactions at the surface of the semiconductor allowed the
degradation of almost every organic substrate with few exceptions. However, despite the large
number of publications on the application of photocatalysis to degrade soluble pollutants in
water [8-11], there are few reports dealing with poorly soluble organic pollutants [12-14]. In
fact, hydrophobic organic pollutants poorly adsorb onto the hydrophilic surface of the
photocatalyst, so the reaction results kinetically hindered. This problem has been faced by
performing the treatment in non-polar green solvents such as dimethyl carbonate [15], or by
coupling photocatalysis with some physical processes such as adsorption on activated carbon
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[16]. The approach hereby presented consists in using the solid photocatalyst particles as
emulsion stabilizers. This reduces the oily droplet sizes and increases the contact between the
photocatalyst and the pollutant. In other words, the system is composed of small and stable
TiO2-surrounded hydrophobic droplets acting as microreactors in which the photocatalytic
degradation can efficiently take place due to the enhanced active sites availability.
Two immiscible liquids may give rise to an emulsion if finely dispersed one into the other by
means of external energy, either in the form of mechanical agitation or sonication. Emulsions
stabilized by adsorbed solid particles are called “Pickering emulsions” [17, 18]. Different types
of emulsions stabilized by solid particles may be obtained: (i) oil-in-water (o/w), (ii) water-in-
oil (w/o), (iii) oil between two layers of water (w/o/w), or (iv) water between two layers of oil
(o/w/o) [17-21].
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Pickering emulsions retain the basic properties of classical emulsions stabilized by surfactants
(emulsifiers), so a Pickering emulsion behaves as a classical emulsion in most of applications
[17]. However, there are specific properties of Pickering emulsions. In particular, type and
stability depend upon the particle wetting ability by aqueous and oil phases [22-24].
Stabilization by solid particles requires that they adsorb at the oil-water interface. For this
reason, partial wetting of the solid particles by oil and water is a compulsory requirement.
Hydrophilic particles can stabilize o/w emulsions whereas more hydrophobic particles can
stabilize the w/o ones. The free energy of a solid particle bound to the oil-water interface is
lower than the free energy of particles fully immersed in water. Therefore, the free energy
related to the transfer of the particles from the aqueous phase to the interface is larger than that
of thermal motion, and the binding of the particles to the interface is favored [17]. The resulting
high resistance against coalescence is a major benefit of the stabilization by solid particles [25-
30].
Pickering emulsions are currently investigated for their use in the fields of pharmaceutical
products [31, 32], cosmetics [33], paper-making [34], and food preparation [35, 36]. Since the
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paper published in 2010 by Crossley at al. [37] showing the possibility of performing
heterogeneous catalytic reactions in Pickering emulsions stabilized by the catalyst, several
examples of catalytic reactions in Pickering emulsions have been reported [38-45]. However,
there are only few reports on heterogeneous photocatalysis for the degradation of organic
contaminants in Pickering emulsions [12-14]. ZnO and TiO2 have been considered in these
applications [12,46] but, to the best of our knowledge, only surface-modified semiconductors
have been used so far [13, 14].
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Notably, recent applications extending the concept of catalysis in Pickering emulsions are
attracting interest within the scientific community. For instance, it is possible to modify the
surface of the nanoparticles stabilizing the Pickering emulsions with shutter molecules
covalently grafted (phase-boundary catalysis) or with supramolecular hierarchical structures
(colloidal tectonics) able to tune the surface hydrophilicity and to generate porous amphiphilic
architectures where organic molecules can be accommodated in reversible and predictable
ways. These novel approaches offer the advantage of a more tunable and flexible system to
formulate emulsions for specific applications such as the synthesis of fine chemicals [47,48].
However, complexity and time demanding research process put a strain on these systems when
used for environmental larger scale applications where simple, scalable and robust systems are
preferred. Furthermore, the presence of organic modifiers makes their stability questionable
under irradiation, due to the presence of highly oxidizing radicals. In this paper, o/w Pickering
emulsions of 1-methylnaphthalene (1-MN) in water were stabilized by bare TiO2 without any
surface modification. 1-MN is a toxic, carcinogenic, and mutagenic polycyclic aromatic
hydrocarbon (PAH) [49,50] which is almost insoluble in water.
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Experimental
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Synthesis of TiO2 nanoparticles
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The TiO2 nanoparticles were synthesized using a simple sol-gel method elsewhere reported
[51]. Briefly, 19 mL of titanium tetraisopropoxide (Ti[OCH(CH3)2]4, > 97%, Alfa Aesar) were
dissolved in 4 mL of methanol (MeOH, 99.9%, Aldrich) and the solution was sonicated in an
ultrasonic bath (Elma, T460/H, 35 kHz and 170 W). The hydrolysis process was then performed
by adding dropwise 74 mL of deionized water under reflux and magnetic stirring. The obtained
white gel was filtered, washed several times using ethanol and deionized water, and dried at
100°C for 18 h. The obtained powder was calcined at 400°C for 4 h and finally ground.
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Characterization
X-ray diffraction patterns were recorded at room temperature by an Ital Structures APD 2000
powder diffractometer using the Cu Kα radiation and a 2θ scan rate of 2°C·min-1. The mean
crystallite size D (nm) of the photocatalysts was estimated from the width of diffraction peaks
using the Debye-Scherrer equation:
D = 0.9 λ / β cosθ
where λ is the wavelength of the X-ray radiation (0.154 nm), β is the resolution-corrected width
at half height and θ is the diffraction angle.
UV-vis absorption spectra of the samples were recorded in the range 200-800 nm using a
Shimadzu UV-2401PC spectrophotometer. BaSO4 was used as the reference.
The surface charging upon pH variation and the isoelectric point were determined by means of
electrophoresis using a NanoZS instrument (Malvern, UK).
The BET specific surface area was determined by using a Micrometrics TRISTAR 3000
instrument.
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The morphology of the photocatalyst was examined by scanning electron microscopy (SEM)
using a Philips XL30 ESEM operating at 30 kV acceleration on samples upon which a thin
layer of gold has been deposited.
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The particle size was determined by transmission electron microscopy (TEM) using a JEOL
JEM 12-20 operating at 120 kV acceleration. The samples were prepared by dispersing the
powders in water and depositing and drying a drop of suspension onto a thin Formvar film
supported on a Cu grid.
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The size distribution of TiO2 suspensions was determined by dynamic light scattering using a
Zetasizer NanoZS instrument (Malvern, UK).
Droplets size measurements of o/w emulsions were carried out by means of small-angle light
scattering (Mastersizer 3000 instrument, Malvern, UK). The refractive indices of the optical
model used for data processing were 1.332 for water, 1.615 for 1-MN, and zero for the
imaginary part of the refractive index of 1-MN.
Emulsion droplets were observed by optical microscopy using a Leica DMLM (Germany)
microscope equipped with a video camera. The concentrated emulsions were diluted and spread
between glass plates for microscopic observation in transmission mode and image analysis. The
size distribution was determined by image analysis using the “Analysis” software.
Preparation and characterization of Pickering emulsions
Pickering emulsions were prepared with different volume fraction of 1-methylnaphtalene (1-
MN: C11H10, 95% Aldrich, USA) according to the following procedure: TiO2 was firstly
dispersed in water with an Ultra-Turrax T25 rotor-stator device equipped with a S25N18G shaft
(IKA, Germany) rotating at 22000 rpm during 5 minutes. The same disaggregation degree was
obtained with an ultrasound disperser Sonics VibraCell at 500 W operating during 3 minutes,
as described below. Thereafter, 1-MN was added and the mixture was dispersed under the same
conditions. O/w emulsions of different compositions (oil and TiO2 contents) were studied
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depending on the type of experiment, either physicochemical characterization or assessment of
photocatalytic activity. A scan of oil contents from 10% to 80% at constant TiO2 concentration
(1%) was done in order to investigate emulsion type. The influence of TiO2 concentration on
stability and droplet size was studied for oil contents of 10% and 20%, which were higher than
that of photocatalysis experiments but lower than the usual 50% oil content of most physico-
chemical investigations. Such conditions kept the asymmetry of oil and water contents
pertaining to photocatalysis and allowed a variation of TiO2 concentration in a wide range.
Photocatalysis experiments were performed with fairly dilute compositions (0.01 % oil and 1%
TiO2) for UV light can penetrate the medium over long enough depth.
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Photocatalytic experiments
The photocatalytic experiments were carried out using a cylindrical Pyrex batch photoreactor
(diameter 10 cm) equipped with a magnetic stirrer rotating at 500 rpm and containing 160 mL
of 1-MN/TiO2/water emulsion with 0.01% v/v of 1-MN and 1% w/w of TiO2. The photoreactor
was illuminated by an LLP lamp UVA (8 mW·cm-2) placed horizontally below it. A circulating
water bath was used to control the temperature at 298 K. The UV irradiation and the mechanical
stirring during the photocatalytic runs did not influence the stability of the emulsion. Samples
were collected at pre-specified time intervals and mixed with acetonitrile/water (50% v/v) in
order to break emulsions and dissolve 1-MN. Thereafter, the samples were filtered through a
Millipore 0.45 μm PTFE filter before analysis.
Analytical methods
The evolution of the 1-MN concentration during the photocatalytic experiments in emulsion
was followed by using a HPLC system model 1290 Infinity. This apparatus was equipped with
a binary pump (Agilent Technologies, G4220B), an autosampler (Agilent Technologies, 1290
Sampler), a DAD (Agilent Technologies, G4212B) and FID detector (Agilent Technologies,
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G1321B). The injected sample volume was 50 μL and a HAP column (Agilent: 4.6×100 mm,
1.8 μm) was used for all of the analyses. Ultrapure water (solvent A) and acetonitrile (solvent
B) were used as the mobile phase with a flow rate of 1 mL·min-1 as it follows: 0 min to 10 min
40% B, 10 min to 19 min 90% B, 19 min to 20 min 40% B. The resulting data were collected
and processed using Agilent MassHunter software.
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Gas chromatography coupled with mass spectrometry analyses were performed on a Hewlett
Packard 6890/5973 GC-MS system, with electron impact ionization at 70 eV, He carrier gas,
30 m × 0.25 mm Zerbon ZB-Wax, 100% polyethylene glycol capillary column. Analyses were
carried out in both SCAN and SIM (selective ion monitoring) modes. The starting temperature
was 60 °C for 10 min; it was then linearly increased up to 220 °C at 5 °C·min-1, and held at
220 °C for 30 min.
Results
Structural and morphological characterization of bare TiO2
The X-ray diffraction pattern of the synthesized TiO2 is shown in Figure 1.
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2q (Degree)
Figure 1. X-ray diffraction pattern of synthesized pristine TiO2 nanoparticles.
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The signals are characteristic of TiO2 in the anatase form (JCPDS-ICDD card: 21-1272). In
particular, the intense diffraction peaks at 25.28°, 37.81°, 47.99°, 53.95°, 55.0°, 62.9° and 75.0°
correspond to the (101), (004), (200), (105), (211), (204) and (215) planes, respectively. The
mean crystallite size of the TiO2 nanoparticles, calculated using Debye-Scherrer equation, was
about 10 nm.
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Nitrogen adsorption and desorption isotherms (Figure 2) are of type IV, suggesting the presence
of a mesoporous structure. At high relative pressures (from 0.45 to 0.80) the isotherm exhibits
hysteresis of H2 type, indicating the existence of ink-bottle type pores with narrow necks and
wide bodies. The specific surface area calculated by using BET method was 103 m2·g-1.
The pore size distribution (inset in Figure 2), calculated according to the BJH equation, is rather
narrow indicating homogeneity of the material with an average pore diameter of about 5 nm.
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Pore diameter (nm)
Figure 2. Nitrogen adsorption/desorption isotherms of TiO2. Inset reports the pore size distribution.
Scanning and transmission electron microscopy images are shown in Figures 3A and 3B,
respectively. The TiO2 nanoparticles are present as aggregates of quite uniform primary
spherical nanoparticles whose mean size is close to the value calculated by means of the Debye-
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Scherrer equation. Notably, Figure 3A suggests that the interstices between the TiO2 particles
in the aggregates can contribute to the mesoporosity as given evidence by the physisorption
measurements, although it cannot be excluded the presence of mesopores with a small neck and
enlarged body in the single particles.
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B
A
50 nm
Figure 3. Scanning (A) and transmission (B) electron microscopy images of the synthesized TiO2.
Figure 4 shows the Tauc plot along with the UV-vis absorption spectrum (inset) of the prepared
TiO2. The obtained band gap of the synthesized TiO2 was 3.22 eV.
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250 300 350 400 450 500 550 600
Wavelength (nm)
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hν (eV)
Figure 4. Tauc plot of the synthesized TiO2 obtained by considering the sample an indirect semiconductor. Inset:
diffuse reflectance spectra of TiO2 in absorbance mode.
Zeta potential measurements (Figure 5) allowed to determine the isoelectric point of the
synthesized TiO2 and to investigate the stability of the nanoparticles as a function of pH.
Figure 5. Zeta potential of TiO2 in aqueous suspensions at different pH values.
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The point of zero charge was at pH = 6.2. Therefore, the TiO2 surface charge is negative at pH
> 6.2 and positive at pH < 6.2 according to the protonation and deprotonation of Ti-OH surface
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groups given by Eqs. (1) and (2), respectively:
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Ti-OH + H+ → Ti-OH2
Ti-OH → TiO- + H+
(1)
(2)
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Characterization of 1-MN/TiO2/water Pickering emulsions
The emulsification process is an operating parameter strongly influencing the stability and the
droplets size. The objective of this study is to prepare emulsions with small droplet size having
a high contact area between the organic pollutant and the photocatalyst particles used as
stabilizers. The emulsification process was carried out in two steps. TiO2 was first dispersed in
water in order to allow the maximum particles disaggregation as described in the experimental
part. Thereafter, 1-MN was dispersed into the TiO2 suspension following the same procedure.
The dimension of the aggregates prior to addition of 1-MN, was measured by means of dynamic
light scattering both after the Ultra-Turrax dispersion and after sonication to compare the effect
of the two dispersion methods. TiO2 nanoparticles (mean diameter = 14 nm) in water
aggregated in lumps whose diameter ranged from 600 nm up to 2 µm. On the other hand,
sonication (3 minutes) and dispersion with Ultra-Turrax (5 minutes) reduced the aggregates
diameter to ca. 300 nm, as shown in Figure 6. Similar value could be retrieved by means of
optical microscopy observations (inset of Figure 6). Prolonging both the treatments for longer
times did not result in further decrease of the nanoparticles aggregates size. Therefore, only
dispersion by means of Ultra-Turrax has been used to formulate the emulsion in the following
tests.
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Figure 6. Particle size distribution and transmission optical microscopy image (inset) of TiO2 nanoparticles
dispersed in water by using the Ultra-Turrax disperser.
Thereafter, the emulsification process was carried out by dropwise adding 20% v/v 1-MN to
the above described 1% w/w TiO2 dispersion, by using the Ultra-Turrax disperser for further 5
minutes at the same conditions. The resulting system (Figure 7A) was comprised of a clear
aqueous solution supernatant over a stable Pickering emulsion layer. This sedimentation of the
emulsion phase is ascribable to the higher density of 1-MN (1.029 g·cm-3) with respect to water.
Notably, it would be possible to take advantage of the higher density of the emulsion droplets
with respect to the aqueous phase. In principle, in fact, a continuous treatment process on large
scale can be proposed where the photodegradation reaction of settled droplets would take place
at the bottom of the reactor, while the supernatant aqueous phase would be continuously
separated by floatation.
The emulsion was stable for at least three months and the droplets size of 1-MN in water
stabilized by TiO2 nanoparticles did not change significantly. These results were confirmed by
optical microscopy observation of the emulsion layer which showed that droplets of 1-MN were
well dispersed and stabilized in the continuous water phase (Figure 7B).
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Figure 7. (A) Image of the system obtained after preparation procedure of the emulsion described in the
experimental part: aqueous solution (supernatant), Pickering emulsion (underlying layer). (B) Optical microscopy
image of the Pickering emulsion.
These results give evidence that unmodified TiO2 nanoparticles can stabilize Pickering
emulsions. The amounts of pollutant (1-MN) and TiO2 nanoparticles are the two crucial factors
influencing the emulsion type and the droplet size as discussed below.
Identification of the emulsion type
Figure 8 reports conductivity measurements of a sequence of emulsions containing different
volume fractions of 1-MN as the oil phase (ϕoil) stabilized by 1% w/w TiO2.
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w/o
Figure 8. Phase inversion of 1-MN in water emulsions stabilized by TiO2 nanoparticles.
Two types of emulsions were found: (i) w/o type, where the continuous phase is oil (low
conductivity) at ϕoil > 50-60% v/v, and (ii) o/w type, where the continuous phase is water
(higher conductivity) at ϕoil < 50-60% v/v.
This change, known as “phase inversion”, is usually accompanied by large changes of emulsion
properties, such as viscosity, droplet size, and conductivity. A layer of pure oil was slowly
released to the bottom of the emulsions containing ϕoil > 60 % v/v. Therefore, 1-MN/TiO2/water
Pickering emulsions formulated with ϕoil higher than 60 % v/v were not stable. Notably, only
o/w emulsions can be efficiently stabilized due to the hydrophilic nature of the surface of TiO2
nanoparticles.
Size of emulsion droplets
Droplets size is an important parameter for the characterization of emulsions. Furthermore, it
should also affect the photocatalytic reaction rate, since small droplets have a large contact area
between the photocatalyst, oil and water phases. The droplets size is influenced by factors such
as the fractions of solid particles and the amount of oil.
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The amount of the photocatalyst in Pickering emulsions plays a crucial role in determining the
droplets size and the stability against coalescence. Figure 9 shows the dependence of the
droplets size on the amount of TiO2 used as the emulsion stabilizer.
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TiO2 amount (% w/w)
Figure 9. Mean droplet diameter of the 1-MN/TiO2/water Pickering emulsion (10 % v/v 1-MN) as a function of
the TiO2 amount.
In the presence of 0.1-0.4% TiO2, emulsions were not stable and coalesced immediately after
preparation (results not shown). Efficient stabilization of Pickering emulsions was obtained
above a minimum content of TiO2 particles equal to 0.5% w/w. By increasing the TiO2 amount
from 0.5 up to 8% w/w, the droplets size firstly decreased and then reached a “plateau” at ca.
12.5 µm for TiO2 amounts higher than ca. 3% w/w. This “plateau” may be explained by a lack
of efficiency in the emulsification process. In fact, the emulsification with Ultra-Turrax
disperser was unable to decrease the droplets size of the pollutant down to the value which
would be obtained by extrapolation of the trend in the decreasing part. For this reason, in the
“plateau” regime an excess of TiO2 nanoparticles dispersed in the aqueous phase could be
observed.
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The decrease of droplets size with respect to the concentration of TiO2 corresponds to the
increase of the interfacial area. Indeed, larger TiO2 content allowed the stabilization of larger
contact areas (smaller droplets). This is in qualitative accordance with the geometrical
relationship [13] R = A/m, where R is the droplet radius, m is the mass of solid particles and A
is a constant that depends on the volume of the dispersed phase.
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Also, the amount of oil is a crucial parameter. Larger oil contents require larger amount of
particles to ensure the full coverage of droplets keeping constant their size. The effect of the
oil-to-water ratio on the droplets size of Pickering emulsions was studied for a range of volume
fractions of 1-MN up to 20% v/v. Results are shown in Figure 10.
10% v/v 1-MN
20% v/v 1-MN
1% v/v 1-MN
0.1% v/v 1-MN
Droplets size (μm)
Figure 10. Size distribution of emulsion droplets prepared with different volume fractions of 1-MN in the presence
of 1% w/w TiO2, along with the corresponding optical microscopy images.
By keeping constant the amount of TiO2 dispersed (1% w/w), the droplets size increased upon
increasing the oil fraction.
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It has been stated that increasing the oil content and decreasing the particles concentration had
similar effects on droplet size because the area-to-volume ratio of a spherical droplet is A/V =
3/R [52, 53]. This geometry-based relationship was not followed in the present case since the
droplet sizes at low 1-MN contents were lower than the limiting size reached for 10% 1-MN
(Figure 9). For a volume fraction of 1-MN equal to 0.1% v/v, the mean droplet size was quite
small (5 µm) and Pickering emulsions were stable. This size is close to the smallest size
dispersion with an Ultra-Turrax can achieve. Therefore, this amount was selected for the
photocatalytic degradation experiments.
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Photocatalytic degradation of 1-MN
The objective of this study was the intensification of the photocatalytic degradation of 1-MN in
Pickering emulsions. The small pollutant droplets surrounded by the photocatalyst
nanoparticles and dispersed in the emulsion act as microreactors in which the degradation
reaction is intensified due to the larger oil-water interfacial area and the maximized contact
between the photocatalyst and the pollutant. Notably, the emulsion was stable during the
photocatalytic run as no 1-MN phase separation was noted under UV light irradiation. As a
proof of concept, two irradiated systems have been considered: (i) 1-MN/water emulsion
stabilized by TiO2 nanoparticles and (ii) not emulsified 1-MN/water mixture in the presence of
TiO2. Results are shown in Figure 11.
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Time (h)
Figure 11. Photocatalytic degradation of 0.1 % v/v 1-MN. (black line) Blank test in the absence of catalyst; (red
line) non-emulsified system in presence of 1% w/w TiO2; (blue line) Pickering emulsion system stabilized by
1% w/w TiO2.
The overall concentration of 1-MN progressively decreased during the photodegradation
process for the two systems. However, the photocatalytic degradation in the emulsified system
was significantly faster and reached 95% after 24 h of UV irradiation, whilst only 25% of the
initial 1-MN could be degraded after the same time in the non-emulsified system. By taking
into account the photoreactor volume V = 160 mL, the volume fraction of 1-MN ϕoil = 0.1% v/v,
and the 1-MN droplet size D = 5 µm, the total 1-MN/water interfacial area in the emulsified
system is:
ꢀ = ꢁ ꢂ ꢃ » 0.02 m2
(3)
ꢄꢅꢆ
ꢇ
On the other hand, the droplets size in the non-emulsified system (only gently stirred) is of the
order 1-2 mm, giving A values ranging between 5×10-5 and 10-4 m2, which is 50 to 100 times
lower than the value obtained in the Pickering emulsion. In the non-emulsified system, 1-MN
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photodegradation takes place within the aqueous phase saturated with 1-MN (solubility of 1-
MN in water at 20°C is 25 ppm), while the large drops of 1-MN act as reservoirs slowly
releasing the pollutant to the aqueous phase in which, therefore, the 1-MN concentration can be
considered constant. For this reason, the rate of the photodegradation reaction is also constant,
thus justifying the linear decay observed in the initial part of Figure 11 for the non-emulsified
system.
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The kinetics of photodegradation in the case of Pickering emulsion was 50 times faster than in
the non-emulsified system. This order of magnitude reflects the difference in the dimension of
droplets between the two systems, thus confirming that droplets size is the main parameter
affecting the photocatalytic degradation in Pickering emulsion. The toxicity of oxidation
products and intermediates is also a matter of concern. Consequently, understanding chemical
transformations during the photocatalytic degradation of 1-MN and identifying the resulting
products are extremely important points. Notably, the TiO2 induced photodegradation of
naphthalene [54] and its substituted derivatives [55] in diluted systems has been thoroughly
investigated in the relevant literature. For this reason, a deep mechanistic discussion is
unnecessary and out of the aims of the present report. On the other hand, some findings
specifically related to the peculiar system under investigation are worth to be mentioned.
The photo-oxidation products were detected by GC-MS analysis and identified by computer-
assisted comparison with a mass spectral database. As previously reported, 1-MN first
undergoes oxidation of the methyl group, yielding successive oxidation products, such as the
corresponding alcohol (1-naphthalenemethanol) and aldehyde (1-naphthalenecarboxaldehyde).
Further OH radical mediated oxidation affords cleavage of carbon-carbon bonds in the
naphthalene structure, yielding light by-products containing only one variously substituted
aromatic ring. On the other hand, heavier compounds obtained by addition of radical
intermediates could be also detected in the present system. In particular, non-negligible amounts
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of 4-(1,3-dimethyl-2-naphthyl)-1,2-benzenediol and naphthalene-1-carboxylic acid 4-formyl-
phenyl ester (Figures 12A and B, respectively) were identified.
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Figure 12. Structures of two addition products identified by means of GC-MS analysis during photocatalytic
degradation of 1-MN in Pickering emulsion: (A) 4-(1,3-dimethyl-2-naphthyl)-1,2-benzenediol; (B) naphthalene-
1-carboxylic acid 4-formyl-phenyl ester.
The presence of these compounds may be justified by considering that the photocatalytic
degradation takes place at the surface of the 1-MN droplets acting as microreactors. Therefore,
the probability of addition of the produced intermediate radicals is higher in the present system
than in previously reported cases where 1-MN degradation occurred in acetonitrile or in low
concentrated aqueous solutions.
Conclusions
Unmodified TiO2 nanoparticles were prepared by a simple sol-gel method and used for the first
time as stabilizers for 1-MN/water Pickering emulsions. Operational parameters affecting the
emulsion droplets size, such as the fraction of solid particles, the amount of 1-MN, and the
emulsification process were studied and discussed. In particular, the emulsion showed phase
inversion from oil in water to water in oil at 1-MN fractions higher than 40% v/v, and the
stabilization of the emulsion occurred above a minimum content of TiO2 particles of 0.5% w/w.
Small drop size improved the contact area between the organic contaminant and the
photocatalyst thus intensifying 50 times the photocatalytic degradation of 1-MN with respect
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to the non-emulsified system. Degradation of 1-MN in Pickering emulsion produced non-
negligible amounts of addition compounds which were never reported before for the same
reaction carried out in diluted naphthalene solutions.
9
Pickering emulsions can effectively be proposed for the treatment of wastewater containing
non-soluble organic contaminants, due to the high efficiency of the process and to the
advantages deriving from the use of stable, cheap and abundant unmodified TiO2 nanoparticles.
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Photocatalysis
TiO2
1-Methylnaphthalene
TiO2
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