TiO2 nanoparticles for photocatalytic

Fe3O4/SiO2/TiO2 nanoparticles for photocatalytic degradation of 2chlorophenol in simulated wastewater Jamshaid Rashid, M. A. Barakat, Y. Ruzmanova & A. Chianese

Environmental Science and Pollution Research ISSN 0944-1344 Volume 22 Number 4 Environ Sci Pollut Res (2015) 22:3149-3157 DOI 10.1007/s11356-014-3598-9

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Author's personal copy Environ Sci Pollut Res (2015) 22:3149–3157 DOI 10.1007/s11356-014-3598-9

RESEARCH ARTICLE

Fe3O4/SiO2/TiO2 nanoparticles for photocatalytic degradation of 2-chlorophenol in simulated wastewater Jamshaid Rashid & M. A. Barakat & Y. Ruzmanova & A. Chianese

Received: 5 June 2014 / Accepted: 11 September 2014 / Published online: 21 September 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Photocatalysis has emerged as an advance and environmental-friendly process for breakdown of organic contaminants in wastewater. This work reports facile synthesis and characterization of stable magnetic core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles and their effectiveness for photocatalysis. The surface morphology, crystal structure, and chemical properties of the photocatalyst were characterized by using scanning electron microscope (SEM), energydispersive X-ray (EDX), X-ray diffraction (XRD), and nitrogen physisorption. Stability of synthesized nanoparticles in aqueous medium was tested by leaching test. The photocatalytic degradation of 2-chlorophenol was investigated and reaction parameters for best catalyst performance were optimized. Catalyst dose of 0.5 g/L under optimized conditions

Responsible editor: Philippe Garrigues J. Rashid : M. A. Barakat Department of Environmental Sciences, Faculty of Meteorology and Environment, King Abdulaziz University (KAU), Jeddah, Saudi Arabia J. Rashid e-mail: [email protected] M. A. Barakat e-mail: [email protected] M. A. Barakat Central Metallurgical R & D Institute, Cairo 11421, Helwan, Egypt M. A. Barakat Center of Excellence in Environmental Studies (CEES), King Abdulaziz University (KAU), Jeddah, Saudi Arabia Y. Ruzmanova : A. Chianese (*) Department of Chemical Material Environmental Engineering, Sapienza University of Rome, Rome, Italy e-mail: [email protected] Y. Ruzmanova e-mail: [email protected]

produced complete degradation of 25 mg/L 2-chlorophenol (2-CP) within 130 min of 100-W ultraviolet (UV) irradiation while 97.2 % degradation of 50 mg/L 2-CP was achieved within 3 h. The rate of photocatalytic degradation was determined by considering pseudo first-order kinetics and Hugul’s kinetic equations. The Hugul’s kinetics was found to provide a better interpretation of the experimental results than the generally adopted pseudo first-order reaction kinetics. Keywords Photocatalysis . Magnetic . Core-shell-shell nanoparticles . Separation . 2-chlorophenol . Wastewater

Introduction Water pollution has become a major problem in recent years. The biological, physical, and physicochemical water treatments are often ineffective or environmentally incompatible (Xu et al. 2013). Heterogeneous photocatalysis is one of the most effective methods for degradation of organic pollutants to innocuous substances such as H2O and CO2, or other species in wastewater (Luo et al. 2013). Among the various heterogeneous photocatalysts investigated, TiO2 has been most commonly used for wastewater treatment, being an inexpensive and nontoxic material known for its high stability and photoactivity (Tu et al. 2013). The main drawback of the use of TiO2 nanoparticles for environmental applications is their recovery from the purified water stream. The use of magnetic core TiO2 nanoparticles offers a solution to this problem (Chalasani and Vasudevan 2013). The application of an external magnetic field to such materials provides an easy way for removing and recycling the photocatalyst (Ye et al. 2010). Addition of an SiO2 intermediate layer between the Fe3O4 core and the TiO2 shell weakens the adverse influence of Fe 3 O 4 on the photocatalysis of TiO 2 . However, very few studies are available on the synthesis

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of core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles and their photocatalytic properties. Gao et al. (2003) reported magnetically separable TiO2/ SiO2/Fe3O4 photocatalyst prepared by the solid-phase synthesis method, which showed good photocatalytic activity and could be separated easily from the solution by application of a magnetic field. Santra et al. (2001) used the microemulsion method for the preparation of core-shell-shell Fe3O4/SiO2/ TiO2 nanoparticles which was a lengthy process and involved the use of several types of surfactants. Gad-Allah et al. (2009) reported the preparation of Fe3O4/SiO2/TiO2 nanocomposites in the form of patches and not discrete nanoparticles therefore exhibiting reduced surface and photocatalytic properties. Similarly, Abramson et al. (2009) also reported preparation of core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles of a few tens of nanometers dimensions. In this work, the synthesis procedure and characterization of stable core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles is presented and the effectiveness of such nanoparticles for photocatalysis is also reported. 2-chlorophenol (2CP) was adopted as a model of the organic pollutants present in wastewater. The choice of 2-CP was made due to the fact that phenol and its derivatives are commonly encountered organic pollutants in industrial wastewater which cause severe environmental problems without proper treatment (Lin et al. 2011). Eleven common phenols and some phenolic compounds are included in the US Environmental Protection Agency priority pollutants list and the European Union Water Framework Directive list of priority substances (EPA 2014; Directive 2008). Many works in the literature deal with the photooxidation of chlorophenolic compounds and the kinetic studies of the involved degradation processes. Hugul et al. (2006) proposed an interesting theoretical model for the degradation pathway expressing the rate as a linear function of the concentrations of chlorophenol and catalyst. In the present study, a facile three-stage synthesis method was adopted for preparation of core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles. The synthesized nanoparticles were evaluated for their photocatalytic properties in terms of degradation of 2CP in synthetic wastewater and easy post operation recovery by application of an external magnetic field. The study also attempts to provide a kinetic interpretation of the experimental results by comparing the pseudo first-order kinetics and Hugul’s equation.

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(Sigma Aldrich), 98 %; 2-propanol (Fluka); hydrogen peroxide (Sigma Aldrich), ≥35 %; and titanium tetraisopropoxide (TTIP) (Sigma Aldrich), 97 %. For the photodegradation experiments, 2-chlorophenol (2-CP) (Merck) standard grade was utilized as a pollutant. A water-cooled 100-W highpressure mercury lamp (Hanovia 608A36, ACE Glass, NJ, USA) was used as an irradiation source with a spectral irradiance of 260 W/m2 and spectral range from 228 to 420 nm at a distance of 1 m from the light source, according to information provided by the manufacturer. Synthesis of core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles The core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles were prepared by three steps. Firstly, Fe3O4 magnetic nanoparticles were synthesized using a spinning disk reactor (SDR). Then, core-shell Fe3O4/SiO2 nanoparticles were prepared by Stober method, and, finally, a TiO2 sol-gel material was used for the external coating of the Fe3O4/SiO2 nanoparticles. More details on the production procedure are reported in Ruzmanova et al. (2014). Characterization of the synthesized nanoparticles Size distributions of the Fe3O4 core nanoparticles and the core-shell-shell Fe3O4/SiO2/TiO2 were measured by using of the dynamic light scattering (DLS) technique (a dynamicsized instrument model Plus 90 supplied by Brookhaven). Surface morphology of the synthesized nanomaterial was investigated by scanning electron microscope (SEM) (AURIGA supplied by Zeiss) coupled with an energydispersive spectrometer. X-ray diffraction (XRD) analysis of the core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles was obtained by using a diffractometer Seifert PAD VI provided with a molybdenum tube and LiF monochromator on the diffracted beam. Brunauer-Emmett-Teller (BET) measurement of the nanoparticles was performed by nitrogen adsorption by using a Quantachrome NovaWin instrument. Acid leaching test was carried out to check the effectiveness of the silica-coating insulation: 0.1 g of Fe3O4/SiO2 nanoparticles was dispersed in 50 mL of aqueous solution of HCl (37 %) and the suspension was kept under stirring. Every 15 min, samples of solution were withdrawn, centrifuged, and ultraviolet (UV)-visible spectra of supernatants were measured.

Experimental Photocatalytic experiments Materials The chemicals used in the study include the following: iron (III) chloride (Sigma Aldrich), >97 %; sodium sulfite (Sigma Aldrich), 98 %; ammonium hydroxide solution (Sigma Aldrich), 33 %; hydrochloric acid (J.T. Baker), 36–38 %; ethanol (Carlo Erba), 96 %; tetraethyl orthosilicate (TEOS)

Photocatalytic degradation of aqueous 2-CP as a model pollutant was performed in a cylindrical Pyrex reactor (1-L capacity). A quartz immersion tube harboring the ultraviolet (UV) light source was inserted inside the reactor vertically. For a typical experiment, fixed dose of the photocatalyst was added to 1 L of aqueous solution of 2-CP provided with

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constant air circulation to ensure aerobic conditions required for photocatalysis. Constant supply of O2 has been reported to be essential for photocatalytic oxidation of organic compounds since it prevents surface recombination of photogenerated electron hole (e−/h+) pairs (Naeem and Feng 2009). To establish adsorption equilibrium of 2-CP on photocatalyst surface, the reaction contents were initially kept in the dark for 30 min, followed by exposure to UV irradiation over a certain period of time. After every 30 min, 5 mL of liquid samples were withdrawn to analyze the residual 2-CP concentration by UV-visible spectrophotometer (HACH LANGE DR6000) at 270 nm. The degradation efficiency was calculated, after analysis, using the following relationship: Degradation % ¼

Co − Ct Co

100

ð1Þ

where Co and Ct denote initial and residual concentration of 2CP (mg/L), respectively, at irradiation time (t). To monitor the pH change impact on photocatalytic efficiency, solution pH was maintained from 3 to 9 in different sets of experiments. Under optimized pH conditions, effect of other controlling parameters like initial concentration of 2-CP, catalyst dose, and catalyst reusability on photocatalytic efficiency was also examined.

Results and discussion Characterization of materials The size distribution results of the produced core magnetic nanoparticles is reported in Fig. 1. The average size of the magnetic core is equal to 24.0 nm, whereas one of the produced core magnetic-shell-shell nanoparticles was equal to 70.2 nm. The presence of particles in the right part of the d i s t r i bu t i o n i s d ue t o t h e a g gl om er at i o n o f t h e nanoparticles. The strong tendency of nanoparticles toward agglomeration is confirmed by the images of the nanoparticles acquired by means of SEM (Fig. 2). In the micrograph-produced nanoparticles, less than 100 nm are clearly detectable together with some agglomerates. By the energy-dispersive X-ray (EDX) microanalysis, the weight percentages of iron, silica, and titanium in the nanoparticles resulted equal to 9.6, 1.9, and 35.1, respectively (Fig. 3). On the basis of these percentages, the thickness of the two shells were estimated more or less equivalent to the diameter of the nanoparticles core, in particular the silica layer thickness has been estimated to be equal to 22 nm.

Fig. 1 The core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles size distribution by DLS measurement

The XRD results (Fig. 4) exhibit the presence of the crystalline form of magnetite, SiO2, and TiO2. The latter compound was preferentially present as an anatase phase. In order to ensure enhanced photocatalytic activity and stability of the core-shell-shell nanomaterials, it is necessary to avoid the transfer of electrons between external shell of TiO2 and the magnetic core. The results of the leaching test presented in Fig. 5 clearly indicate that no dissolution of magnetite was ensured by presence of the silica shell. Finally, the BET surface area measurement was carried out to evaluate the average surface area of synthesized photocatalyst. The surface area was found to be 105.8 m2/g which corresponds to the previously determined average particle size by dynamic light scattering (DLS) measurement.

Fig. 2 SEM image of the core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles

Author's personal copy 3152 Fig. 3 EDX spectrum of the Fe3O4/SiO2/TiO2 nanoparticles

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3.0

2.5

2.0

1.5

Ti Fe C O

Si

Ti

Fe

1.0

0.5

0.0 1

Photocatalysis Solution pH plays a vital role in determining the rate of a photocatalytic process by influencing the catalyst surface interaction with the pollutant molecules and generation of hydroxyl-free radicals. The influence of pH over Fe2O3/ SiO2/TiO2-catalyzed photodegradation of 2-CP is provided in Fig. 6. Point of zero charge (pHPZC) of Fe2O3/SiO2/TiO2 was measured by salt addition method to be 5.8. Surface of the photocatalyst is positively charged at pHpHPZC bearing TiOH+, TiOH, and TiO− species, respectively (Lin et al. 2011). Solution pH therefore controls the adsorption of the pollutant molecules on the catalyst surface based on their dissociation constants, which in turn influences the rate of photocatalysis. Decrease in degradation efficiency from 97.2 to 32.4 % was observed when pH of the solution was changed

Fig. 4 XRD spectra of magnetic core nanoparticles

2

3

4

5 keV

6

7

8

9

10

from 3 to 6. Maximum photocatalytic efficiency at pH 3 may be the result of electrostatic binding of 2-CP molecules with a positively charged surface of TiO2 particles. Electrostatic repulsion between negatively charged TiO2 surface and phenolate ions may be responsible for low degradation efficiency at pH 9. These results were confirmed by other studies in the literature, reporting that photodegradation of phenols is favored by acidic pH values (Barakat et al. 2013; Shaban et al. 2013; Anju et al. 2012). Photocatalyst dose effect on 2-CP degradation was studied over a range of 0.25 to 1 g/L of Fe2O3/SiO2/TiO2 at pH 3. Increase in photocatalyst dose from 0.25 to 0.5 g/L showed a significant increase in 2-CP degradation efficiency (Fig. 7). Such increase in degradation efficiency can be attributed to increase in catalyst active sites with increase in catalyst dose. Further increase in catalyst dose to 1 g/L however showed decline in degradation efficiency due to catalyst


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for the apparent decrease in photocatalytic efficiency with increasing 2-CP concentration (Sin et al. 2012). The used catalysts were easily separated by decanting the residual solution after putting a small magnet under the reaction flask which made the particles cling together and facilitated catalyst removal. The separated catalysts were washed with distilled water and were subjected to reuse without further treatment after drying at 40 °C for 3 h. Under optimized conditions mentioned above, the catalyst remained stable and retained up to 60 % efficiency after three reuse cycles (Fig. 9). The change in photocatalytic efficiency after first use can be attributed to catalyst agglomeration which results in the decrease in number of catalyst active sites due to layer by layer stacking of catalyst molecules. Fig. 5 Dissolution kinetics of magnetite nanoparticles and magnetic core-silica shell nanoparticles

agglomeration, which becomes significant with increase in catalyst dose, and reduced light penetration throughout the solution (Sin et al. 2012). The rate of photocatalytic degradation is greatly affected by the initial concentration of the target pollutant. To assess the effect of initial pollutant concentration on photocatalytic efficiency, a wide range of 2-CP concentration from 25 to 100 mg/L was subjected to photocatalysis at optimum pH and catalyst dose. As shown in Fig. 8, 25 mg/L of 2-Cp was completely degraded in 130 min of irradiation. The degradation efficiency for 3 h of irradiation period decreased from 97.8 % for 50 mg/L up to 50 % when the initial 2-CP concentration was increased to 100 mg/L. While total number of active sites on the photocatalyst surface and irradiation time remained the same, increase of 2-CP concentration causes a smaller probability of 2-CP molecules reacting with the active sites. Furthermore, increased absorption of photons by 2-CP molecules compared to TiO2 surface may also be responsible Fig. 6 Effect of solution pH on photocatalytic efficiency (inset pHpzc) (catalyst dose=0.5 g/L, 2CP=50 mg/L, UV irradiation= 100 W, irradiation time=3 h)

Kinetics of the 2-CP degradation The two most popular kinetic equations describing the photodegradation reaction assisted by a semiconductor catalyst such as TiO2 include the first-order rate kinetics and the Langmuir-Hinshelwood equation. However, when the concentration of the organic compound is very low, this latter kinetics can be represented by a pseudo first-order expression (Asenjo et al. 2013). Therefore, the concentration of the organic compound throughout the batch reaction process can be determined by the integration of differential material balance, where the reaction rate follows first-order equation, that is the following: −

dC ðt Þ ¼ k c C ðt Þ dt

ð2Þ

The initial condition of the differential Eq. 2 is C=Co for t=0.


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Fig. 7 Effect of catalyst dose on photocatalytic efficiency (2-Cp concentration=50 mg/L, solution pH=3, UV irradiation=100 W, irradiation time=3 h)

90

0.5g/L

1.0 g/L

0.25 g/L

80

Eﬃciency %

70 60 50 40 30 20 10 0 0

30

60

90

120

150

180

Time (min)

The numerical integration can be easily accomplished by the Euler numerical integration method. More sophisticated algorithms as Runge-Kutta are not required in this case for the very low evolution of the integration function with time. Usually, the kinetic interpretation of the experimental results in photocatalysis fields is carried out by adopting an analytical integration of Eq. 2, which leads to the following logarithmic expression: C0 ln ¼ kc t C

ð3Þ

In this case, the kinetic constant is obtained by the minimization between the experimental and calculated values of ln(Co/C) and not by the minimization of the deviation between the experimental and calculated values of concentration. This approach is not accurate; moreover, it cannot be adopted when the kinetic expression deviates from the first order, and for such reasons, in this work, the numerical integration of the

differential material balance has been adopted to predict the concentration of the 2-CP during its degradation. A quite different kinetics was proposed by Hugul et al. (2006) in a work concerning the degradation of three examined chlorophenols which is represented as follows:

−

dC ðt Þ ¼ kc C ðt Þ þ k½TiO2 surface dt

Equation 4 accounts for the effect of the activated sites of the TiO2, which play a specific role for the conversion of the adsorbed chlorophenol. In the present work, the interpretation of the experimental results has been attempted by assuming a pseudo first-order kinetics and the kinetics proposed by Hugul et al. (2006). For each run, the obtained experimental data were compared with the values calculated by means of the differential integration of Eqs. 2 and 4. The kinetic parameters were derived by the minimization of the mean square deviations between the 25ppm

Fig. 8 Effect of initial 2-CP concentration on photocatalytic efficiency (catalyst 0.5 g/L, UV=100 W, pH=3, irradiation time=3 h)

ð4Þ

50ppm

100ppm

100 90

Degradaon %

80 70 60 50 40 30 20 10 0 0

30

60

90

Time (min)

120

150

180


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100

Fig. 9 Effect of catalyst reuse on photodegradation efficiency (2-CP concentration=50 mg/L, pH=3, UV irradiation=100 W, irradiation time=3 h)

90

Fresh Catalyst

2nd use

3rd use

80 Degradaon %

70 60 50 40 30 20 10 0 0

experimental and the calculated data. In Fig. 10, it is reported a comparison between three series of experimental data obtained operating at pH 3, 6, and 9 for an initial concentration of 2CP of 50 mg/L and the concentration curves predicted by adopting the pseudo first-order kinetics. The values of kinetic coefficients obtained by the mean square deviation minimization by adopting the pseudo first-order kinetics are reported in Table 1. In Fig. 10 and Table 1, the pseudo first order appears to adequately interpret the kinetics of 2-CP photocatalytic degradation when the degradation efficiency is quite far from 100 %, but it is not the case when the degradation percentage is close to 100, i.e., at pH 3 and 0.5 g/L catalyst dose. Otherwise, if the kinetics of Hugul et al. (2006) is adopted, the predicted curves compare much better with the experimental data and the mean square deviations are generally of one order of magnitude less. Figure 11 shows the variation of the predicted and the experimental values of the 2-CP molar

30

60

90 Time (min)

120

150

180

concentrations with an irradiation time of 3h for 2-CP initial concentration of 25, 50, and 100 mg/L. It is clear that the Hugul’s equation allows a very good simulation of the 2-CP degradation. The values of the two kinetic coefficients of the Hugul’s equation are reported in Table 2. It is useful to notice that the second contribution of the Hugul’s kinetics derives from the 2-CP adsorbed on the TiO2 surface activated because of all the radicals and holes formed by the collision of one photon and one molecule of photocatalyst. At pH 9, there is an electrostatic repulsion between negatively charged TiO2 surface and phenolate ions giving rise to a strong reduction of 2CP adsorbed molecules on the activated titanium dioxide surface. As a consequence, the role played by the TiO2 sites, represented by the value of the kinetic parameter k[TiO2]sur is very low with respect to the values achieved at pH 3 and pH 6 (see Table 2). On the contrary, the kinetics coefficient does not Table 1 Effects of catalyst dose, solution pH, and initial 2-CP concentration on the photocatalytic activity of Fe2O3/SiO2/TiO2 Variable parameter Catalyst loading (g/L)a

Solution pHb

Initial 2-CP concentration (mg/L)c

a

0.25 0.5 1.0 3 6 9 25 50 100

kc (per min)

R2

0.0026 0.0075 0.0033 0.0075 0.030 0.0044 0.013 0.0011 0.0030

0.754 0.754 0.968 0.754 0.962 0.932 0.919 0.793 0.932

2-CP=50 mg/L, pH=3, irradiation time=3 h, UV irradiation=100 W

b

Fig. 10 Comparison between experimental data and predicted curves for a pseudo first-order kinetics (2-CP concentration=50 mg/L, solution pH=3, UV irradiation=100 W, irradiation time=3 h)

2-CP=50 mg/L, catalyst dose=0.5 g/L, irradiation time=3 h, UV irradiation=100 W c

Catalyst dose=0.5 g/L, irradiation time=3 h, pH=3, UV irradiation= 100 W


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Fig. 11 Comparison between the experimental data and the curves predicted by means of the Hugul’s equation (catalyst=0.5 g/L, UV= 100 W, pH=3, irradiation time=3 h)

change too much since it depends on the activation of 2-CP in solution which is not affected by the pH value.

Conclusion Magnetic core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles were synthesized by facile three-step synthesis producing stable and highly photocatalytically active material. The material’s characterization confirmed that the average nanoparticle size of the magnetic core was 24.0 nm, whereas the produced core magnetic-shell-shell nanoparticles were up to 70.2 nm in diameter. The XRD results confirmed the presence Table 2 Kinetic coefficients (Hugul’s equation) for effects of catalyst dose, solution pH, and initial 2-CP concentration on the photocatalytic activity of Fe2O3/SiO2/TiO2 kc (per min) k[TiO2]sur ×10−9 R2 (mol/L/min)

Variable parameter Catalyst loading (g/L)a

0.25 0.0014

0.5 1.0 b Solution pH 3 6 9 Initial 2-CP concentration 25 (mg/L)c 50 100 a

0.0031 0.002 0.0031 0.002 0.0033 0.0031 0.0031 0.00086

4.40

0.981

9.40 4.00 9.40 10.0 1.0 9.40 9.40 12.0

0.928 0.947 0.991 0.981 0.984 0.971 0.981 0.993

2-CP=50 mg/L, pH=3, irradiation time=3 h, UV irradiation=100 W

b

2-CP=50 mg/L, catalyst dose=0.5 g/L, irradiation time=3 h, UV irradiation=100 W c

Catalyst dose=0.5 g/L, irradiation time=3 h, pH=3, UV irradiation= 100 W

of crystalline magnetite, SiO2, and TiO2 (anatase phase). The results of leaching test proved the stability of the nanoparticles in aqueous medium. The synthesized nanoparticles showed high photocatalytic activity toward 2-CP model pollutant degradation. The photocatalytic process was found to be pH dependent, and under optimized conditions of catalyst dose (0.5 mg/L) and pollutant concentration (50 mg/L) and 3-h irradiation time, highest 2-CP removal percentage was achieved at pH 3. Catalyst was recovered after use by placing a magnet under the reactor surface and simple decantation. The core-shell-shell Fe3O4/SiO2/TiO2 nanoparticles retained their high photocatalytic activity for multiple uses except for an initial one-time decrease due to catalyst agglomeration. The kinetic investigation on the experimental results performed by adopting a pseudo first-order kinetics did not lead to satisfactory results, in particular when the 2-CP degradation was almost complete. On the contrary, these latter experimental data were very well predicted when the kinetic equation proposed by Hugul et al. (2006) was applied. This kinetics assumes two driving forces of the reaction: the reagent concentration and the activated catalytic sites. This work thus confirms that at low pH, it is possible to achieve a complete degradation of the 2-CP because of the crucial role of the activated TiO2 surface.

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