The Degradation of Reactive Black Wastewater by Fe / Cu Co-doped TiO 2

The organic contaminant, REACTIVE Black 5 (RB5) was degraded by Fe/Cu co-doped TiO2 nano-particles under mercury lamp. In the present study, the combined effects of Fe/Cu doping amount, calcination temperature and calcination time on catalyst activity were investigated, and through quadratic regression general rotation design, the optimal condition for preparing the catalyst was obtained as follows: calcination time, 3.55h; calcination temperature, 453 ; Cu doping amount, 1.89%; Fe doping amount, 1.6 °C 2%.


Test design (Table
1.2 Material preparation 17 mL TiO 2 was added to 50 mL ethanol solution, and 10 mL acetic acid solution followed, which we called as A; Different amount of ferric sulfate and copper sulfate was added to 10 mL water and mixed, which we called as B. B was slowly added to A along with magnetic stirring, agitating after addition, and then the resultant solution was dried at 80 oven.The obtained powder was grinded to pieces, and calcined in muffle furnace, and Fe/Cu ℃ co-doped TiO 2 was obtained.

Photocatalytic degradation of RB5
After preparing 10 mg/L RB5 solution, the absorbance of the solution with different concentration was measured by 721B spectrophotometer at the wavelength of 600 nm, and working curve was depicted.The prepared TiO 2 was added to RB5 solution, and photocatalytic degradation experiments were undertaken under the condition of UV irradiation and magnetic stirring.The absorbance of the solution was measured every 5 min, and the concentration curve along with time variation was depicted.

Initial tests on the experimental condition
2.1.1Effects of catalyst addition on the decolorization rate Experimental conditions were as follows: light source, a 150W ultraviolet light; initial concentration of RB5, 30 mg/L, pH, 10.67; calcination temperature, 450 ; calcination time, 3h; TiO ℃ 2 , not doped.As seen from Figure 1, decolorization effects of RB5 were quite low without the addition of catalyst.When catalyst concentration reached to 2 g/L, the degradation rate attained the maximum level which was higher than that when catalyst concentration reached to 1 g/L or 3 g/L.When the concentration was 1 g/L, less amount of catalyst couldn't make full use of ultraviolet rays generated by UV light; when the concentration was 3g/L, the collisions among small particles in too much amount of catalyst increased, and the generated energy could not be well used.Therefore, we selected 2 g/L in the following experiments.

Effects of pH value on the decolorization rate
Experimental conditions were as follows: light source, a 30 W ultraviolet light; the initial concentration of RB5, 30 mg/L; calcination temperature, 450 ; calcination time, 3h; not doped TiO ℃ 2 , 2 g/L.As seen from Figure 2, acidic conditions (PH = 3.26) resulted in the worst treatment effects, neutral conditions (PH = 7.08) followed, while alkaline conditions brought in the best results.Under alkaline conditions, a large amount of -OH existed in the solutions, •OH was more conducive to produce, and the removal of RB5 primarily depended on the large amount of •OH generated in the solution; the reason might be related to the properties of the solution itself when acidity and alkalinity reduced.We select pH 10.67 in the following experiments.

Experimental design result
Based on the preliminary exploration, we continued the following tests.We used c = c 0 e -kt to fit with the variation curve of concentration against time, and found that c 0 in each treatment was the same, namely initial concentration of the solution.Consequently, we used k value to represent the effects of each treatment.Results were listed in Table 2 in detail.
After calculation, regression equation was obtained as follows: After the obtained equation was substituted into treatment conditions, it was found that serial number 18, 20, 21 and 24 varied greatly from the equation.Therefore, we discarded these treatments.After variance analysis, the lack of fit was not significant, and regression relationship was greatly significant, which showed that the quadratic regression equation fitted very well.

Verification tests
We applied the above-mentioned optimal experimental conditions to prepare the catalyst, and added 2 g/L catalyst into RB5 with the pH value of 10.67 to react.Reaction results were depicted in Figure 3.
After fitting, c = 29.45e -2.70t was obtained (where the unit of t was h), and closer to the obtained k max , which simultaneously showed that the quadratic regression equation fitted very well.

Effects of Cu and Fe doped amount on the catalyst
Compared to pure TiO 2 , co-doped metal ions resulted in better effects than single doped metal ions.When Cu and Fe doped amount reached to 1.89% and 1.62%, mineralization effects were the best.Proper amount of Cu and Fe co-doping could enhance the catalytic activity of TiO 2 , which was determined by the natural properties of TiO 2 and doped ions.When TiO 2 semiconductor was lighted by photons higher than or equal to the forbidden gap energy, it would be excited to generate electron (e -) hole (h + ) pairs.e -and h + was capable of oxidizing and reducing the substances absorbed on the surface.e -and h + pairs had a very shot life span, and if there was no suitable e -or h + trapping agent, the energy would consumed by recombination within a few microseconds (Asahi, 2001, 269-271).Proper amount of Cu 2+ and Fe 3+ could generate e -capture traps; when co-doped Cu and Fe exceed certain level, the average distance among traps decreased, the recombination rate of e -and h + increased, which reduced the quantum efficiency of photons on one hand, resulted in the reduction of the carrier number that migrated to the surface of TiO 2 on the other hand, and thus declined the photocatalytic activity of catalyst.

Effects of calcination temperature on catalyst
When calcination temperature was 453 , the catalytic activity attained the maximum level.Because when ℃ temperature was lower than 450 , TiO ℃ 2 crystallization failed to complete, samples still contained more amorphous TiO 2 , not beneficial to the producing of the above-mentioned e -and h + , and therefore the catalytic activity decreased.With the increasing of temperature, the transformation from amorphous to anatase increased while anatase TiO 2 had higher photocatalytic activity, and therefore photocatalytic activity increased gradually.When activation temperature was above 450 , TiO ℃ 2 would change from anatase to rutile, particle size would increase with the increasing of temperature.Its internal porosity and specific surface reduced, recombination rate between e -and h + increased, and therefore the catalytic activity reduced.

Effects of calcination time on catalyst
With the increasing of calcination time, the catalytic activity also increased gradually.When calcination time was 3.55 h, the catalytic activity attained the maximum level.With the continuous increasing of time, the catalytic activity began to decline.When calcination time was too short, dry gel failed to discard the water and alcohol absorbed in the surface completely, co-doped TiO 2 was not able to transform into the required crystal form, and therefore the catalytic activity was not high.However, when calcination time was too long, the particles of co-doped TiO 2 agglomerated, made the average particle size and specific surface of photocatalyst increase and reduce, respectively, which led to the reduction of catalytic activity.

Conclusions
1 The degradation process of RB5 by TiO 2 catalysis and UV conformed to the first-order kinetic equation.
2 Preparing Cu/Fe co-doped TiO 2 photocatalyst by sol-gel method, the degradation of RB5 was investigated, and the impact of each factor on the first-order kinetics, viz.k value was obtained.The optimum preparation condition was obtained as follows: calcination time, 3.55 h; calcination temperature, 453 ; Cu ℃ doping amount, 1.89%; Fe doping amount, 1.62%.