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2007_6_Nr6_EEMJ

Caliman et al.

Caliman et al. /Environmental Engineering and Management Journal 6 (2007), 6, 483-489 intermediates is observed in the first minutes of irradiation. In the second case, the results concerning the degradation degree are not very much different, as data from the Table 2 reveal. absorbanta (609 nm) Table 1. Values of the percent of H 2 O 2 remained unconsumed at the final of the process 0,8 0,7 0,6 0,5 0,4 0,3 0,2 H 2 O 2 initial concentration mg L -1 % H 2 O 2 in solution at the end of the process mg L -1 10 - 25 - 50 - 100 32 200 66 400 74.5 0,5 g L -1 TiO 2 P-25 0,5 g L -1 TiO 2 P-25 + 100 mg L -1 H 2 O 2 0,5 g L -1 TiONa 0,5 g L -1 TiONa + 100 mg L -1 H 2 O 2 The efficiencies of the dark adsorption process (15 minutes) on 0,5 g L -1 P-25, respectively photocatalitic process, after 30 minutes of irradiation of 40 mg L -1 Alcian Blue 8 GX, in the presence of FeCl 3 are exhibited in Fig. 5. As one can see, the removal of color was mainly achieved in the dark period (70%), followed by a small variation of the process efficiency after irradiation, especially for the higher amounts of iron salt (>28 mg L -1 ). dark adsorption efficiency % 100 80 60 40 20 0 0 7 14 21 28 35 42 49 56 63 FeCl 3 concentration, mg L -1 (a) 0,1 0,0 -20 0 20 40 60 80 durata iradierii, min 100 (b) Fig. 4. Influence of optimum concentration of H 2 O 2 (100 mg L -1 ) on photocatalytic degradation of 40 mg L -1 Alcian Blue upon two different catalysts: TiONa and P-25 (concentration of catalysts: 0.5 g L -1 ) Table 2. Comparison between degradation rates in the case of heterogeneous photocatalytic degradation of 40 mg L -1 Alcian Blue 8 GX on two types of catalysts (TiONa and TiO 2 P-25 in the presence and, respectively, absence of H 2 O 2 Concentration of catalyst or catalyst + H 2 O 2 Reaction rate (mg L -1 min -1 ) Correlation coefficient R 0.5 g L -1 TiO 2 P-25 0.62831 0.99396 0.5 g L -1 TiO 2 P-25 + 100 1.16706 0.99913 mg L -1 H 2 O 2 0.5 g L -1 TiONa 0.62502 0.97736 0.5 g L -1 TiONa + 100 mg L -1 H 2 O 2 0.39774 0.95963 3.2. Study concerning dark adsorption and photocatalytic degradation of Alcian Blue 8 GX on TiO 2 P-25 in the presence of FeCl 3 Addition of FeCl 3 should have a positive effect on photocatalytic degradation of the organic compounds owing to generation of HO• in aqueous solutions with the participation of ferric ions and the products of their hydrolysis, although there were reported also cases of a detrimental action of the iron salt (Baran et al., 2003). photodegradation efficiency % 80 60 40 20 0 0 7 14 21 28 35 42 49 56 63 FeCl 3 concentration, mg L -1 Fig. 5. Influence of FeCl 3 concentration upon dark adsorption (a) and photocatalytic degradation (b) efficiencies of 40 mg L -1 Alcian Blue 8 GX (0.5 g L -1 P-25, pH = 3.7) Considering the fact that the removal of color was observed mainly for the dark adsorption period, the study of the dark adsorption process appeared necessary in order to assess the influence of the iron salt upon this process. Thus, experiments concerning the measurement of the absorbance after 15 minutes of dark adsorption of 20 mg L -1 dye in the presence of different concentrations of catalyst P-25 only (Fig. 6a) and also in the presence of 0.1 g L -1 catalyst and different amounts of FeCl 3 (Fig. 6b) were conducted. The values regarding the percents of color removal after dark adsorption were calculated with the following expression: 486 4

Study concerning the oxidizing agents on heterogeneous photocatalytic degradation A0 − A % = 100 (3) A0 where: A 0 = absorbance of dye solution before adding catalyst, A = absorbance of dye solution after a certain time t. The calculated values are presented in Table 3 for 1 minute and 15 minutes, respectively, of dark adsorption. is added in system, a competition must occur between its molecules and that of the dye for the active sites of the catalyst one can conclude that color removal may be the result not of adsorption but of precipitation of the formed insoluble complex formed by the dye with iron. Table 3. Dark adsorption efficiency (percent removal of color) at different concentration of TiO 2 P-25 and FeCl 3 , respectively abs/abs 0 (609 nm) 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,02 g L -1 TiO 2 P-25 0,05 g L -1 TiO 2 P-25 0,1 g L -1 TiO 2 P-25 0,2 g L -1 TiO 2 P-25 Duration of dark adsorption of 20 mg L -1 AB 8 GX % Removal of color at different catalyst concentration (g L -1 ) on P-25 (min) 0.02 0.05 0.1 0.2 1 33.78 34.28 37.83 41.66 15 45.94 36 40.54 37.77 Duration of dark adsorption of 20 mg L -1 AB 8 GX on 0.1 g L -1 P-25 (min) % Removal of color at different concentration of FeCl 3 (mg L -1 ) 14 28 56 1 69.69 66.66 70.83 15 78.84 78.18 75 0,1 0,0 0 5 10 15 20 25 30 dark adsorption time, min In the case of solution with very high amount of FeCl 3 (112 mg L -1 ), a smaller decrease of color removal (75%) was achieved after 15 minutes dark adsorption, while a higher one was observed after first minutes of irradiation (Fig. 7). abs/abs0 (597 nm) 1,0 0,1 g L -1 TiO 2 P-25 0,8 0,6 0,4 0,2 0,0 14 g L -1 Fe 3+ , pH = 3,5 28 g L -1 Fe 3+ , pH = 3,55 56 g L -1 Fe 3+ , pH = 3,45 0 2 4 6 8 10 12 14 16 dark adsorption time, min Fig. 6. Effect of concentrations of catalyst and iron salt, respectively, on dark adsorption of 20 mg/L Alcian Blue 8 GX: abs/abs 0 vs irradiation time at various concentrations of TiO 2 P-25 (0.02-0.2 g L -1 ) (a) and abs/abs 0 vs irradiation time at various concentrations of FeCl 3 (14-56 mg L -1 ) (b) In the absence of FeCl 3 , it is obvious that the adsorption is very fast at the beginning, in the first minute being removed around 34-42 % of the dye color, after this time, the process becoming almost constant, with slow variation of dye concentration. Thus, after 15 minutes, which was the period of dark pre-equilibration of the dye solution in all the experiments, percents of color removal ranged between 36–46% were achieved. The same trend is also observed for the case of FeCl 3 presence in system, but the percent removal of color was higher (75-79%). Due to the fact that, generally, when FeCl 3 absorbance (597 nm) 0,7 0,6 0,5 0,4 0,3 0,2 0,1 -20 -10 0 10 20 30 irradiation time, min Fig. 7. Absorbance versus irradiation time for degradation of 40 mg L -1 Alcian Blue 8 GX on 0.5 g L -1 P- 25 in the system containing very high amounts of FeCl 3 (112 mg L -1 ) In order to check this behavior, another experiment was done with the system containing 112 mg L -1 FeCl 3 that was subjected one hour to dark adsorption and one hour to irradiation (Fig. 8). One may observe that a significant decrease of the absorbance occurs in the first 20 minutes, remaining almost constant after this period and decreasing quite high again under the first 10 minutes of irradiation. It should be noticed that, while for 7-56 mg L -1 FeCl 3 most of the dye color was removed in 15 minutes of dark adsorption, in the case of using very high amount of iron(III) salt (112 mg L -1 ), removal of color of the solution in the adsorption process is smaller, as Figs. 7 and 8 show. The lower decolorization in the latter case may owe to the excess of Fe 3+ and colored products resulted from its 487

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