Simultaneous photooxidation and photoreduction of phenol and Cr(VI) ions using titania modi ﬁ ed with nanosilica

The photocatalytic process of phenol oxidation and Cr(VI) reduction in the presence of nano-silica modi ﬁ ed titania was carried out. The activity of composites was tested using two different light sources. The photocatalysts with 10% of nanosilica showed the highest activity. The calcination temperature (200–800 o C) signi ﬁ cantly determined the sensitivity of the obtained materials to the light source used. Photocatalysts alternately adsorbed and desorbed Cr(VI) ions from the reaction mixture during irradiation. In the one-component mixture, complete oxidation of phenol was observed using material calcined at 650 o C, after 3 h of UV-VIS irradiation. In the reaction mixture of Cr(VI) and phenol, the highest activity was demonstrated by photocatalyst calcined at 300 o C. The concentration of phenol decreased in proportion to the decrease of chromium ions. The obtained titania-silica composites showed oxidizing properties towards phenol and reductive properties toward Cr(VI) ions.


INTRODUCTION
Titanium(IV) oxide (TiO 2 ) is a photocatalyst commonly used in advanced oxidation processes (AOPs), mainly in the removal of anthropogenic pollutants (e.g.aromatic and carboxylic acids, pesticides, herbicides, phenol derivatives, textile dyes or drug residues) from water 1-4 .TiO 2 is characterized by high photochemical stability and good mechanical strength for most of the reaction conditions.The band gap energy of titania semiconductor (3.2 eV for anatase) corresponds to the range of UVA radiation (300÷388 nm), which is about 4% of solar radiation 5 .Due to the constant increase in energy costs, the use of artifi cial light sources emitting UVA radiation increases the costs of the photocatalytic process.
Obtaining a titanium semiconductor, the photocatalytic activity of which is in the range of visible radiation (>400 nm), would allow the use of the main part of the solar spectrum and/or the use of artifi cial energy sources that are cheaper to produce and operate.To enhance the photocatalytic activity in visible light, titanium(IV) oxide is subjected to various modifi cation processes, e.g. by using secondary oxides, including nanosilica (nano--SiO 2 ) 6, 7 .Studies performed by many research groups have shown that enrichment of titanium dioxide with nanosilica directly prevents agglomeration and growth of TiO 2 particles, improves the thermal stability of the photocatalyst and increases the effi ciency of the photocatalytic process 8, 9 .It was proved that the photocatalytic properties of the TiO 2 -SiO 2 composite depend on the selection of the preparation method and the percentage content of nanosilica in the fi nal material 10, 11 .
The group of impurities subjected to photocatalytic removal from water includes phenol and chromium(VI) compounds.The physical and chemical properties of phenol determine its use in the chemical, petrochemical, biotechnological and food industries.Industrially synthesized phenol is used primarily in the production of phenolic resins, synthetic fi bers and fungicides 12, 13 .Potassium dichromate is often used in the chemical or textile industries as an oxidant 14, 15 .The wide spectrum of phenol and potassium dichromate applications generates a real threat for drinking water contamination with these compounds 16 .In environmental toxicology, the maximum permissible concentration in drinking waters is 0.1 and 0.2 mg/dm 3 , for chromium(VI) ions and phenol, respectively.The European Union (EU) and the World Health Organization (WHO) have recognized phenol and potassium dichromate as carcinogenic and toxic to humans [17][18][19][20] .
In this work, modifi cations of titanium(IV) oxide with nanosilica were carried out.Titania/silica composites were characterized using UV-VIS/DR, FTIR and XRD methods.The effect of silica addition and thermal treatment conditions on photocatalytic performance was examined.The obtained materials were applied in the photocatalytic reaction of phenol and Cr(VI) removal from water.Modifi cation of titania with silica revealed the benefi ts such as better sedimentation properties of obtained material together with activity under VIS light.These facts were not wildly mentioned in the literature before.The process of chromium(VI) removal was found to be accelerated by the presence of phenol in the reaction mixture.The coexistence of the photoreduction/ photooxidation processes in the reaction mixture of both model pollutants signifi cantly accelerated the process.

Synthesis of photocatalysts
The raw titania containing sulfuric acid residue was rinsed with ammonia water followed by distilled water until neutral pH.The obtained slurry was then dried and thus the starting material (s.m.) was obtained.Modifi cations of TiO 2 were performed with the use of nanosilica in the presence of ethanol.Nanosilica (in an amount of 2.5 to 15 wt.%, with respect to TiO 2 ) was added to ethanol and then homogenized in an ultrasonic bath for 60 minutes.Titania was then added to the prepared suspension and the mixture was stirred for 60 minutes using a magnetic stirrer (500 rpm).The obtained material was dried (time 24 hours, temperature 100 o C) and then calcined in a muffl e furnace for 8 hours using temperatures ranging from 200 to 800 o C.
The prepared titania/silica materials were assigned as, for example, Ph/ 10% 300 -Ph is for the photocatalyst, 10% means the amount of silica added to titania (weight %) and 300 indicates the temperature of thermal treatment.Reference materials were prepared without the addition of silica.The course of the photocatalytic process was also investigated for the raw (untreated) TiO 2 .

Photocatalysts characterization
FT/IR-4200 spectrometer (Jasco, Japan) with the equipment for powdered samples (Pike Technologies, USA) was used to record FTIR spectra in the wavelength range 4000-500 cm -1 .
The UV-Vis/DR spectra were measured in a range of 200 to 800 nm using UV-Vis/DR spectrophotometer (Jasco V-650, Japan).Spectralon white diffuse refl ectance standard was used as a reference.
Analysis results of three photocatalysts samples are presented in this paper.These are for the starting material (s.m.); titania modifi ed with 10% of silica calcined at 300 o C and titania modifi ed with 10% of silica calcined at 650 o C since they showed the highest photocatalytic activity under UV-VIS and VIS illumination, respectively.

Photocatalytic reaction
The photocatalytic activity of titania/silica composites was tested in the reaction of the photocatalytic oxidation of phenol and chromium(VI) ions reduction in water.The oxidation state of chromium ions was determined using the colorimetric method with 1,5-diphenylcarbazide.The model solutions of impurities were prepared: phenol (20 mg/dm 3 ), potassium dichromate (10 mg/dm 3 ) and a mixture of K 2 Cr 2 O 7 (10 mg/dm 3 ) and phenol (20 mg/dm 3 ) in a ratio of 1 : 1.The tests were carried out at pH 6.5 ± 0.1.The photocatalyst in the amount of 0.2 g/ dm 3 and the model impurity solution were placed in a glass beaker (500 ml) and stirred using a magnetic stirrer (550 rpm) until the adsorption equilibrium of the substrates on the surface of the photocatalyst was established.Next, the reaction mixtures were illuminated using a UV-VIS (six lamps, 20 W each, Philips) or VIS (LED Osram Parathom 5W) light source -Figure 1.Using the same experimental conditions, tests were performed without the use of a light source, in the dark.
The radiation spectra of applied light sources was measured with a LB-901 radiometer (LAB-EL, Warsaw, Poland).The radiometer was equipped with CMP3 (Kipp & Zonen, The Netherlands) sensor with the spectral range of 300 to 2800 nm and UV sensor PD204AB Cos (Macam Photometrics Ltd., USA) with the spectral range of UV-A and UV-B radiation.
Samples of about 5 ml were periodically drawn from the reactor, centrifuged to remove solid particles and then analyzed with respect to phenol concentration, chromium ions concentration and total organic carbon content.
The decrease in the concentration of the pollutants was the basis for the assessment of the effectiveness of the processes carried out.The degree of degradation of the test substances was calculated as follows: R= [(C 0 -C t )/ C o ] • 100% where: C o -concentration of pollutant in the reaction mixture after adsorption [mg/dm 3 ]; C t -concentration of pollutant in the reaction mixture after the certain time of illumination [mg/dm 3 ].
Changes in the concentration of phenol were observed using V-630 UV-VIS spectrometer (Jasco, Japan) after centrifugation (3000 rpm, 15 min) and separation of the photocatalyst from the reaction mixture.A linear dependence between the concentration of phenol and the absorption peak at 269 nm was observed thus the phenol concentration was monitored at this wavelength 21 .Total organic carbon (TOC) content in the reaction mixture was measured using TOC analyzer (multi N/C 2000, Analytik Jena, Germany) to monitor the mineralization degree during the photooxidation of phenol.Cr(VI) ions were determined according to the diphenylcarbazide colorimetric method at 540 nm 22, 23 .
Experiments were performed in triplicate to ensure reproducibility of the results.

Physicochemical properties of titania-silica materials
Figure 2 presents FTIR spectra of starting material (s.m.) and photocatalysts modifi ed with 10% of silica thermally treated at 300 and 650 o C. The peaks centered at 1627 cm -1 and 3384 cm -1 are characteristic of δ-H 2 O bonding and vibration of hydroxyl groups 24 .The peak at 732 cm -1 is for O-Ti-O bonding in anatase.Peaks at 970 cm -1 and 1100 cm -1 belong respectively to Si-O-Ti and Si-O-Si asymmetric stretching 25, 26 .There is no peak at 2900 cm -1 (C-H ) for Ph/ 10.0% 300 and Ph/ 10.0% 650 o C, which means that all organic matter) from ethanol was removed from the samples after calcination 27, 28 .
UV-VIS/DR spectra of s.m. and the best-acting photocatalysts are presented in Figure 3. Based on the analysis it was found that the starting TiO 2 and the modifi ed photocatalysts absorb UV radiation well.The addition of silica revealed changes in absorption spectra.The photocatalyst containing 10.0% of nano-silica thermally treated at 300 o C showed absorption in the visible light range also.The presence of nano-SiO 2 resulted in the shift of the absorption spectrum maximum to 385 nm for Ph/ 10.0% 300 photocatalyst and 376 nm for Ph/ 10.0% 650, compared to 369 nm observed for the starting material (titania without silica).Table 1 summarizes the bandgap energies E g determined according to the Kubelka-Munk method 29 .
Figure 4 shows the results of XRD analysis.Crystallite size was changed from 9 nm to 18 and 16 nm, respectively for Ph/ 10.0% 300 and Ph/ 10.0% 650 -based on Scherrer formula 30, 31 .An increase in the intensity of anatase refl ections was found, for the nano-SiO 2 modifi ed photocatalysts.The peaks become less broad and sharper showing increasing crystallinity of material with increasing treatment temperature.It can be also seen that rutile phase refl ections appear with the increase of treatment temperature.It is known that during the calcination of titania, at the temperature of about 400 o C amorphous titania starts to transform to anatase and further rising of the temperature above 600 o C anatase transformation to rutile phase occurs 32 .In a hybrid material, like titania and silica together, the silica material stabilizes the different phases of titania.Silica suppresses the transformation of  titania to anatase and anatase to rutile by decreasing the mobility of titania atoms like an anchor.
The specifi c surface area of selected photocatalysts was determined by analyzing the nitrogen adsorption isotherm by the Brunauer-Emmett-Teller (BET) method (ISO 9277:2010, Gemini 2360, V 2.01, Micromeritics) at the P/P0 range of 0.05-0.25 using multipoint BET equation.The average pore width was calculated by the Barrett-Joyner-Halenda (BJH) method.The starting material for the photocatalysts preparation showed a surface area of 231 m 2 /g (Tab.2).The presence of 10% silica caused the surface area decrease to 191 m 2 /g.As can be seen from Table 2, the increase of treatment temperature resulted in gradual decrease of surface area.After calcination at 650 o C, the surface area of the treated material was only 49 m 2 /g.This can be connected with the phase transformation of titania.

Photocatalytic properties of titania-silica materials
The photolytic decomposition of model pollutants was not observed for both UV-Vis and Vis light sources.The control tests were also performed without the use of any light source, in the dark.There were no changes in the impurities concentration in the reaction mixtures.The adsorption equilibrium time was established as 60 minutes for the phenol solution and 120 minutes for the chromium and phenol/chromium mixture solutions.
The obtained TiO 2 /SiO 2 composites showed high photocatalytic activity under UV-VIS light towards phenol decomposition (Figs.5-7).It is well known that the addition of silica to TiO 2 might signifi cantly increase the photocatalytic activity since the Ti-O-Si bonds activate the catalytic centers of composite oxides in the oxidation of organic compounds 32-38 .The calcination temperature and the amount [wt.%] of SiO 2 additive directly affected the effi ciency of phenol oxidation in the reaction mixtures.
The starting material (s.m.) decomposed only about 25% of phenol from the reaction mixture with the starting concentration of 20 mg/dm 3 (Fig. 5).Increase in the calcination temperature revealed the increase in the photocatalytic activity of titania-silica composites.The photocatalyst modifi ed with 10% of SiO 2 and calcined at 650 o C (Ph /10% 650) showed the highest activity decrease and increase in the concentration of chromium(VI) ions was observed depending on the duration of exposure to a given light source (Table 3), however, on every case, the concentration of chromium(VI) was lower than the starting concentration.It can be observed that the presence of silica in materials resulted in higher decrease in Cr(VI) concentration.
In the reaction mixture of phenol (20 mg/dm 3 ) and chromium(VI) (10 mg/dm 3 ) in the ratio 1: 1, a total loss of chromium was observed using 0.2 g/dm 3 of photocatalyst containing 10.0 % of nano-silica and calcined at the temperature of 300 o C. The necessary exposure time was 1 and 3 hours, respectively for the UV-VIS (Fig. 8) and VIS (Fig. 9) light sources.It takes longer illumination with VIS light in contrary to the UV-VIS to obtain the complete removal of chromium.The loss of phenol was similar in both cases (approx.80%) -Fig.8  and 9.The photocatalysts calcined above 450 o C showed no signifi cant activity under VIS light (Fig. 9).
Papadam et al 41 studied the photocatalytic removal of chromium(VI) by TiO 2 at the presence or absence of organic compounds.A defi nite synergic effect was observed between photocatalytic reduction of chromium(VI) and the presence of organics in the reaction mixture.The photocatalytic reduction of chromium and other metal ions occurs via conduction band electrons and the presence of valence band holes scavengers, like organic compounds that can be oxidized, facilitate the process 42 .
The stability and possible reuse of photocatalytic materials are important aspects for its application.The addition of silica to titania increased sedimentation properties.Since the materials could be easily recovered from the reaction mixture, the tests of reusage have been done.Figure 10 presents the results of this experiment.It was found that the application of the spent photocatalyst did not show any signifi cant decrease in activity during the three subsequent tests.

SUMMARY
The doping of TiO 2 with nano-silica results in a signifi cant increase in sedimentation of the photocatalyst particles -a centrifugation process is enough to separate the photocatalyst particles from the solution.Titania modifi ed with nano-SiO 2 shows high UV-VIS photocatalytic activity towards phenol decomposition.Modifi cation of titania using nano-silica gives a material showing photocatalytic properties toward Cr(VI) ions in the reaction mixture of Cr(VI) and phenol in the ratio 1: 1.The coexistence of the photoreduction/photooxidation processes in the reaction mixture of both model pollutants signifi cantly accelerated the process.The visible light source used in the research is an inexpensive commercial product, commonly used in households, and the obtained results prove that the application of such an inexpensive light under UV-VIS light.Exceeding 650 o C, the photocatalytic activity toward phenol decomposition began to decrease.As can be seen from Figure 7, the increasing amount of silica additive resulted in the increasing mineralization of phenol.The complete removal of TOC was obtained after four hours of the experiment.Gobara et al. 39 reported that the photocatalytic decomposition of trichlorophenol on TiO 2 -SiO 2 composite depends on the TiO 2 /SiO 2 ratio.Sirimahachai et al. 34 established that composite oxide shows the best photocatalytic activity at about 50% of SiO 2 content.From the results of Chiara et al. 40 it can be seen that the best activity was at TiO 2 /SiO 2 80/20 ratio.
In these studies, the best photocatalytic activity was observed for the composite oxides containing 10% of SiO 2 .
In the single-component solution of K 2 Cr 2 O 7 (10 mg/ dm 3 ) in the presence of photocatalysts, an alternating source can result in measurable benefi ts in removal of pollutants and treatment of water.

Figure 1 .
Figure 1.The spectrum of UV-VIS and VIS light sources

Figure 5 .
Photocatalytic decomposition [%] of phenol (20 mg/dm 3 ) after 3 hours of exposure to UV-VIS light in the presence of the photocatalyst Ph/10.0%(0.2 g/dm3 ) modifi ed in the temperature range from 200 to 800 o C Figure6presents the results of the experiments applying titania-silica composites with different amounts of nanosilica from 2.5 to 10% of nanomaterial calcined at 650 o C compared to the reference materials and photolysis.Photolysis showed no decomposition of phenol at all.The reference materials: s.m., Ph/ethanol n.c.(titania in ethanol not calcinated), Ph/ethanol 650 (titania in ethanol calcinated at 650 o C without silica), Ph/ 10% n.c.(titania with addition of 10% silica not calcinated) -showed signifi cantly lower photoactivity comparing to the materials with varying amounts of silica calcined at 650 o C. The complete decomposition of phenol took place after three hours of the process for the sample Ph/ 10% 650.

Figure 7 .
Figure 7. Changes in the concentration of TOC during photocatalytic degradation of phenol under irradiation with UV-VIS light in the presence of s.m. and SiO 2 modifi ed photocatalysts

Table 1 .
Band gap energies of starting material (s.m.) and photocatalysts modifi ed with 10% of SiO 2 treated at 300 and 650 °C

Table 3 .
Cr(VI)concentration in the reaction mixture illuminated with UV-VIS and VIS light in the presence of photocatalysts