Photocatalytic degradation of phenol as a model pollutant by ...

Indian Journal of Chemistry Vol. 44A, November 2005, pp. 2262-2265

Photocatalytic degradation of phenol as a model pollutant by immobilized Ti02 Salljay R Thakarea. * & N S Bhaveb "Departme nt of Chemistry, Science College, Congress Nagar, Nagpur 400 012. India Email: sa njay thakare@ya hoo.co. uk hDepartment of Chemistry. Nagpur University Campus, Nagpur 440 009, India Email: profnsbhave @yahoo.com

Received 4 Novelllber 2004; revised 29 Allgllsf 2005 Ti0 2 has been immobilized in poly vinyl alcohol polymer ne twork and successfu ll y used fo r th e degradation of phenol. The pre pared PVA - TiO~ composite is co mpl e tely insoluble in water. Th e di ffuse reflectance stud y shows that the prepared polymer compos ite show s absorption in vi s ibl e reg io n as compared to th e bare titanium dioxide. The immobili zed TiO~ shows remarkabl e deg radati o n activity in the presence of UV as wel l as visibl e li ght. [n the presence of vis ibl e light, after 5 hours irradiation, 72% degradat io n of phenol takes place wh ile in the presence of UV li g ht and 5 hours irradiation, 89% deg radation of phenol takes place. The support material (po lyvinyl a lcoho l) is not degraded durin g the photocatalysis reaction and the net photocatalytic efficiency does not decrease after five run s. Diffuse retlectance study shows that th e polymer composite is stabl e during the photocatalysis reaction for a short time. 7

IPC Code: Int. CI. B01J2 1106; C02F I /58

Rapid industrialization and urbanization have given rise to several complex environmental issues. Our environment is contaminated mainly by anthropogenic sources and as a result the concentration level of po llutants has increased remarkably. A number of technologies are available for remedial action such as adsorption by GAC (Granular Activated Carbon), air stripping and advanced oxidation technology. Among the advanced oxidation technologies, heterogeneous photocatalysis is one of the advanced and promising techniques for the demineralization of the organic and inorganic pollutants in water as well as in air)". However, heterogeneous photocatalysis for water treatment is still in the research stage for efficient application . One of the inherent probl e ms is adequate immobilizat ion of the catalyst to suppress the expensive filtration or centrifugation and extension of catalys t absorption to the visible zone. Other problems are effic iency o f cata lysis, conversion time and problems regardin g e ffici e nt appli cati on. We report herein the preli min ary results n photocatal ytic degradation of

phenol as a model pollutant by immobilized Ti0 2 by using UV light as well as visible light. [n thi s note, we report a method for the preparation of TiOrPYA polymer composite and its characteristics. The prepared polymer composite of titanium dioxide absorbs the light in the visible zone and use of the polymer composite for the photocatalytic degradation of organic pollu tant in water reduces the expensive filtration after the photocatalytic treatment.

Experimental Ti0 2 (Qualigen fine chemical, Anatase) was heated at 200°C in the oven and then used for the further immobilization in the polyvinyl alcohol (PV A) polymer network. All the chemicals were of A.R . grade and were used without further purification. Doubl y di stilled water was used for the preparation of all solution required for the experiment. Synthesis of PVA- Ti0 2 polymer composite

According to literature, there is a possibility of chemical interaction between the surface of the titanium dioxide and organic molecule 4 as well as the possibility of chemical interaction of hydroxy group of polyvinyl alcohol with the otherS. We have prepared a polymer composite of tita niu m dioxide with polyvinyl alcohol in such a way that there is a possibility of chemical interaction between the titanium dioxide and polyvinyl alcohol. Aqueous solution of polyvinyl alcohol and slurry of Ti0 2 were prepared in doubly distilled water. Equimolor concentrations of aqueous solutions of Ti0 2 and polyvinyl alcohol were mixed in a 500 ml Borosil glass beaker and stirred vigorously for 15 min . The desired pH of the resultant solution was adjusted with 0.1 N H2S04 . Then the mixture was refluxed for 6 h and the desired amount of co' .:entrated am moni a so lution was added to get the resultant TiOr PY A polymer composite. TiOr PV A po lymer was separated from the res ultant sol ution by means of filtration and washed with doubly disti ll ed water severa l times till the sol uti on was free fro m ammonia. Ti0 2 was thoroughly washed w ith doubly di stilled water to ensure co mpl e te removal of free T i0 2 partic les, dried at 60°C in a n oven and the n used for the photocatalysis ex periment.

NOTES

Photolysis study

Photolysis of aqueous I mmol phenol (50 ml) solution containing 100 mg of Ti0 2 immobilized in the PY A polymer network (PV A- Ti0 2) was carried out in a circular glass reactor (designed and fabricated in our laboratory). A high-pressure mercury lamp (125 W Philips, HPL) was used as the UY light source while halogen lamp (SOW1l2Y /360, Philips Essential) was used as a visible light source. During the photolysis experiment, air was bubbled through the solution continuously as a source of oxygen. Aliquots of the reaction mixture were withdrawn and phenol concentration was determined by means of colorimetric method (Amax = 270 nm) using Shimadzu UY160A UY -vi sible spectrophotometer. Formation of CO 2 was monitored by conductivity measurement of barium hydroxide so lution 6.7 by conductivity meter. SEM measurements were carried out using SEM (Philips 515) instrument. Diffuse reflectance spectra for PYA-Ti0 2 and bare Ti0 2 were recorded on a UYvisible spectrophotometer (G8C-lOe) equipped with an integrating sphere attachment. Weight loss measurement was carried on electronic balance. Polymer composite (0.5 g) was taken and immersed in water for 24 h. After the stipulated time, polymer composite was removed and dried at 60°C in an oven and weighed. After the light in·adiation, the same procedure was followed . Weights before and after irradiati on gave its weight loss due to the li ght effect.

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Result and discussion The SEM images of the polymer composite are shown in Fig. 1 (a-d). The SEM images of the polymer composite indicate that the polyvinyl alcohol encapsulates the titanium dioxide particles. Only a very small amount of titanium dioxide is on the surface. The diffuse reflectance spectrum of bare titanium dioxide (supplied by Qualigen) shows the absorption in the UY region similar to the other titani um dioxide. It does not show any absorption in the visible region having wavelength greater that 400 nm. The diffu se reflectance spectrum of th e po lyme~ composite shows the absorption in visible region and its absorption extends up to 550 nm. The diffuse reflectance studies clearly indicate that the prepared polymer composite absorbs the light in the visible region showing that the prepared polymer composite can be used in the presence of so lar light for photocatalysis without using UY li ght. Phenol is a refractory and common compound in industrial waste, and there is a great interest in developing a new type of chemical treatment for phenol. Several reports S- 13 are avai lable on photocatalytic degradation of phenol using Ti0 2 indicating th at for the deg radation , Ti0 2 and UY li ght are necessary while to take advantage of so lar light, the metal impregnation or sensiti zation by dyes is a alternative. To ensure the photocatalytic degradation efficiency of prepared PY A-Ti0 2 polymer composite, a study was under-

Fig. I (a-d)- SEM images of the prepared polymer composite of PYA and Ti0 2 al various mag ni fication s.

INDIAN J CHEM, SEC A, NOVEMBER 2005

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taken with phenol as a model pollutant. The results are shown in Fig. 2. Photocatalytic formation of CO 2 with 100 mg of PY A-Ti0 2 and 50 ml of 1 mmol phenol solution after 5 h irradiation with UY light is 5 mmol and with visible li ght, it is 4.2 mmol. Percentage degradation shown in Fig. 2 is considered for the concentration of phenol as well as for intermediates formed during the photocatalysis reaction. This was perhaps due to the total degradation and not due to the other intermediates. No direct photolysis reaction was taking place by illumination of visible light only and with bare Ti0 2 or PV A-Ti0 2 polymer composite, while 0.5% degradation of phenol was observed in the presence of UV light only. This clearly indicates that for the effective degradation both catalyst and light are necessary . Figure I clearly indicates that the prepared PY A-Ti0 2 polymer compos ite acts as a photocatalyst in presence of UV light as well in presence of visible light. In presence of UV li ght, 89% degradation of phenol was observed while in presence of visible light 72% degradation of phenol was observed. In presence of UV light; the % degradation is more than the visible light, probably due to the direct photolysis occurring only in the presence of UV light only as well as due to the free Ti0 2 particle which was on the surface of polymer composite. According to the diffuse reflectance study, it is clear that bare Ti0 2 particles absorb the light with wavelength less than 400 nm. This indicates that for the activation of Ti0 2 particles, wavelength required is less than 400 nm which was not emitted by visible light source. This was confirmed by diffraction method by prism recorded on film. ' 1415 an d presen t 0 b serConsl'd enng earli er reports' vation it is clear that the photocatalytic degradation of phenol by PY A- Ti0 2 is mainly due to a hydroxy radical attack which may be formed by adsorbed H 20 1oo~----,------------------------~ 80

i·E40 eo

2 1,

~

uv

2, visible

y -

20

or adsorbed OH- species on the surface of Ti0 2 . The hydroxy radical attack photocatalytic mechanism is represented as shown in Scheme I. It is known that, polyvinyl alcohol which is used as a support material for Ti0 2, is a sac ri ficial electron l 6 18 donor (it is labile for the photocatal ytic degradation reaction). Special attention was given to this point in the present work by undertaking a sim ple weight measurement of immobilized Ti0 2 in polyvinyl alcohol polymer network before and after being used as photocatalyst for the degradation of phenol. These experiments were repeated five times and it was found that there is a maximum weight reduction of 0.3 %. The value is so small that it could easily be due to mass balance fluctuations. Also, it was observed that photocatalytic degradati on efficiency for phenol as pollutant did not decrease after five runs in presence of visible as well as UV light. Also, after photocatalysis, no free T i0 2 was observed in solution as well as no positive test was observed for titanium. During the photocatalysi s reaction, weight loss was not observed but some structural changes may b e POSSI. bl e 19'-?O , l'0 anaIyze th e structural changes in the polymer composite, a diffuse reflectance spectrum of a polymer composite was studied after the photocatalysis reaction in presence of UY light after 5 h. No change in the spectrum was observed up to the seventh run. After eight runs (i.e., after 40 hours) change in the diffuse reflectance spectrum was observed and this change was more pronounced after the tenth run (i.e., after 50 hours). The diffuse reflectance study shows that the prepared polymer composite undergoes degradation only after very long irradiation. The above observations clearly indicate that polymer support used in this study is not undergoing the photocatalytic degradation for the short time irradiation. According to earlier reports l S. 2 1 polyvinyl alcohol undergoes the photocatalytic degradation in water in presence of Ti0 2 and UY light. Photocatalytic degradation of polyvinyl alcohol in water is followed by abstraction of hydrogen by hydroxyl radical, a scisVisiblelUV

PYA-Ti0 2

..

PYA-Ti0 2 (e-Cb , h+vb)

E>Eg

O

0

100

200 Tlme(m)

300

400

Fig. 2-Photocatalytic degradation of phenol by prepared PV ATi02 polymer composite. [I mrnol phenol; 100 mg PVA- Ti0 2].

PYA-Ti0 2 (h+ vb )+OH 2 - t PYA-Ti0 2+HO·+H+ PYA-Ti02(h+ vb)+HO- - t PYA-Ti02+HO· HO· +Dads(pollutant) ---7 Doxid(pollutant) Scheme 1

NOTES

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soring of several carbon-carbon bonds which lead to the formation of short chain compounds and then complete mineralisation into CO:;. Considering the above observations and possibility of interaction of surface hydroxy group of Ti0 2 , it is assumed that during synthesis there may be some interaction between surface hydroxyl group of Ti0 2 or surface of Ti0 2 with hydroxy group of polyvinyl alcohol which is used as support. As a result hydrogen of polyvinyl alcohol is not available for the abstraction by hydroxyl radical which is generated during photocatalysis reaction and hence photocatalytic degradation of polymer network is not observed instantly in the present study.

References

The present study clearly demonstrates the ability of immobilized Ti0 2 in polyvinyl alcohol polymer network as a photocatalyst for the degradation of phenol. It can be used to study the mechanistic of degradation of organic pollutants.

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Acknowledgement The authors wish to thanks the Head, Department of Chemistry, Nagpur University, Nagpur, for the use of facilities. One of the authors, SRT, thanks UGC (Govt. of India) for financial support under minor research scheme F.47-50/2002(WRO).

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