DISCUSSIONCharacterization of the photocatalyst

The crystallinity of the catalyst was examined by XRD technique. The diffraction lines at 29=25 corresponded to TiO2 is observed in TiO2-clinoptilolite but with lower intensity, (Nikazar et al. 2006). Compared to the XRD pattern of clinoptilolite, no major changes was observed in the line intensity and position of TiO2-clinoptilolite indicating that the zeolite framework remained unchanged after TiO2 loading. The average particle size of TiO2 was determined from XRD patterns of the samples according to the Scherrer,s equation:
D = K X / p cos9

Where D is average particle size (nm), X is wavelength of the radiation,  is the Braggs angle and p is the full width at half maximum (radian). The average particle size for TiO2 was obtained 6.3 nm.

The absorption peak observed in 3000-3500 cm-1 range is assigned to the stretching modes of OH bonds and related to free water. The peak near 558 cm-1 is attributed to the stretching vibration of Ti-O bonds. The peak at ~ 1626 cm-1 is attributed to the adsorbed water. In the infrared spectrum of TiO2-clinoptilolite, the bands are mostly similar to those of clinoptilolite. The band covers a range from 945 to 950 cm-1 corresponding to the stretching vibration of Ti-O-Si and Ti-O-Al. The stretching vibration of O-H (3632 cm-1), corresponding to the surface -OH, become weaker after TiO2 loading, indicating that a certain amount of -OH has been demolished by loading of TiO2 (Nikazar et al. 2007; Hayashi and Nasayama, 2008).

The average particle diameter of TiO2 obtained by this technique was about 6.29 nm and very near to the value calculated by Scherrers equation (Su et al. 2004).

Photodegradation Experiments

Effect of irradiation time

To investigate the effect of irradiation time, 10 mL of benzothiophene solution (100 mg/L) and 0.2 g photocatalyst were mixed and placed in dark room under UV irradiation for different reaction times. Degradation was increased with the prolonged reaction time up to 5 h and then leveled off beyond this time. As the irradiation time increased more free radicals are produced and causing degradation of benzothiophene until the concentration of benzothiophene on the photocatalyst surface is reduced.

Effect of photocatalyst amount

Increasing photocatalyst amount accelerated charge transfer and consequently accelerating photocatalytic oxidation of, benzothiophene while an excessive amount of photocatalysts would partly shield the UV light source. Thus, the photocatalyst amount had an optimal value of 0.4 g.

Effect of TiO2 content

In a series of test, 0.4 g of photocatalyst with different TiO2 content (5-20%) was added to 10 mL of n-hexane solution of benzothiophene (200 mg/L). The samples were placed in dark room under UV irradiation for 5h. The mixture was filtered and the amount of remaining benzothiophene was measured in the filtrate. It was observed that degradation increased as TiO2 content of photocatalyst increased up to 15% and then remained constant. The optimized value was between 15-20% wt of TiO2.

Kinetics of the reaction

Reaction kinetics is of great importance in explaining the reaction mechanism. The rate constant for the apparent consumption of BT was obtained from the pseudo first-order equation:
-lnCt/C°=kpt (2)

Where Ct and C° are respectively the concentrations of the compound at time zero and time t(s), and kp is the first-order rate constant. When -ln (Ct/C°) was plotted against t, a straight line was fitted to the data, and correlation factor of (R= 0.961) was obtained, suggesting that the reaction follows first order kinetics.

Identification and adsorption of degradation products by P-zeolite and clinoptilolite

Among these compounds; benzothiophene, 1,2 di (thiophene-2-eil) ethan and 2- pentylthiophene are sulfur containing materials and are to be removed from the solution. The removal efficiency of two zeolites; clinoptilolite and P-zeolite was examined for the removal of these compounds. To study the removal efficiency, 50 mL of the solution was mixed with 0.3 g of P-zeolite or clinoptilolite. The mixture was shaken for 24 hours at room temperature and the solid was separated by centrifugation. The ion chromatogram of the solution was obtained under similar conditions applied for degradation products. The remaining compounds were identified from the ion chromatograms. By comparing these remaining compounds with those, it may be concluded that:

– Two sulfur compounds of 1, 2 di ( thiophene- 2- eil) ethan and 2-pentylthiophene are completely removed by clin„p„l whereas diphenilene sulfide is not fully removed. Thus, under experimental conditions clinoptilolite as an adsorbent removed most of sulfur containing compounds, but desulfurization is still incomplete.
– In the same way by comparing the remaining compounds after adsorption by P-zeolite with those in table 1, it is concluded that all sulfur compounds including benzothiophene are fully removed.

Reuseability of the photocatalyst

It is important to use the catalysts in which active species loaded on zeolite to be reusable without significant loss of activity and with minimal need for regeneration. In order to investigate the reusability of the catalyst, desulfurization experiments were performed under similar conditions for four cycles. After each step, the photocatalyst was washed with n-hexane solution and calcined at 450°C for 2 hours. It was concluded that after 4 cycle 76% of desulfurization efficiency was remained. The full efficiency of the photocatalyst was recovered by adding calculated amount of new catalyst to the mixture.