3.6
Experimental procedures3.6.1 Optimization of experimental parameters
To investigate the effects of SnO2 and different concentrations of dopants on the photocatalytic activity of SnO2 and its dopants, phenol and some of its intermediates were chosen as test contaminants. The study of the photodegradation of phenol and its byproducts in water was carried out at 25 °C using a water-cooled cylindrical glass batch photoreactor of 250 mL volume as described above. Based on different factors that need to be examined in this thesis, the following section briefly discusses external parameters such as follows: catalyst loading, light intensity, reaction time, phenol concentration, sampling time, sample volume, and pH of the reaction medium.
3.6.1.1 Catalyst loading
The amount of catalyst powder (photocatalyst) was kept at 65 mg as a mass in a 50.00 mL volume container (volumetric flask). It is important to find out the optimum loading of the catalyst to avoid excess catalyst using. Experiments on the catalyst loading have been carried out in the range of 65, 75 and 100 mg for 50.00 mL of solution to find out the best catalyst loading. The photocatalytic degradation of phenol exhibited best results from 65-75% at 150 min irradiation time, an increase in the loading amount above 75%
did not give better removal rate and resulted in a decrease in the activity of the catalyst.
A result why 65mg /50 0 mL catalyst dosage was used in all photocatalytic reactions.
Other scientists reported that when TiO2 overdosed, it decreased the light penetration to TiO2 surface because the number of active sites on TiO2 surface become almost constant [37]. In another work, also described that above the optimum of TiO2 loading phenol degradation decreased [377].
3.6.2 Pollutant concentration
The initial concentration of the model pollutant of 100 ppm (phenol) was prepared as follows: exactly 0.1000 g of phenol was weighted and quantitatively transferred into a 1000.0 mL volumetric flask and brought to the calibration curve with milli-Q-pore H2O.
Different solutions of 50, 25, 10 and 5 ppm were prepared by serial dilution of a 100 ppm initial concentration of phenol. These standards were prepared to measure the optimum concentration at which the model pollutant phenol solution might degrade rapidly, which means that the phenol concentration in the reactor would work competently. The photocatalytic degradation of phenol has been carried out at different initial concentrations starting from 5-50 ppm. Each standard was separately mixed in the reactor, while the other parameters were kept constant: such as catalyst loading (65 mg/50.00 mL), light intensity (8 W mercury lamp, 300 W xenon lamp and sunlight), reaction time (2-3 h), sampling time, (12-13), sample volume (250.00 mL), and pH of the reaction medium (5.7). The results showed an increase in the degradation of phenol at 10 ppm, but a slight decrease in the catalytic efficiency above 15 ppm. The best results gained were at 10 ppm, here with all the pollutant concentrations were taken at 10 ppm.
10 ppm phenol concentration was used for photocatalysis, an increase in the initial phenol concentration resulted in a decrease in the rates.
3.6.3 Effect of the pH of the solution
The pH of the solution was adjusted by either adding sodium hydroxide (NaOH) or sulphuric acid (H2SO4). The preparation of the acid and the base solutions was done as follows: about 0.6 mL of 18 M of H2SO4 was transferred into a 100.00 mL volumetric flask and made up to mark with Milli-Q-purified H2O to prepare of 0.1 M. In the same way 0.1 M of NaOH was prepared by weighing about 0.4 g of NaOH into 100.00 mL volumetric flask and made up to mark with Milli-Q-purified H2O. Exactly 250.00 mL of 10 ppm phenol solution was poured in 500 mL beaker in which a pH electrode was immersed. The pH of phenol was adjusted between values (3-8) and was fixed to the desired value by dropping amounts of H2SO2 or NaOH drop wise using a Pasteur pipette.
During this procedure the solution was continuously stirred, but slowly using a small magnetic bar with the pH electrode inside the solution to adjust the pH value required to take care not to break the electrode. The prepared solution was kept in the freezer for further use. It is known that SnO2 is an acidic oxide so phenol in that media will still remain weak acidic.
3.6 Experimental procedures 103 10 ppm phenol solution was used to carry out photocatalysis experiments. Before each test, the mixture was kept in the dark for 30 min under constant magnetic stirring to ensure that adsorption-desorption equilibrium was reached before illumination. The sample was then taken out at the end of the dark adsorption period, prior to turning the light on or before exposing to sunlight. When the lamp turned on, the irradiation time varied from 2-3 h; aliquots of the suspensions were collected and removed from the reactor every 15 or 30 min, inlet air flow 4 L/min.
3.6.4 Sampling from the reactor
At the beginning of the photocatalysis 10 ppm of phenol concentration was prepared in 250.00 mL volumetric flask and put into the reactor while 20 mL aliquots of the suspensions were collected and removed from the reactor every 15 min about 12-13 samples each batch were enough to analyse by all the analytical instruments. The experiment was performed at the following experimental parameters (65 mg/50.00 mL), light intensity (8 W mercury lamp, 300 W xenon lamp and sunlight), reaction time (2-3 h), phenol concentration (10 ppm) sampling time, (12-13), sample volume (250.00 mL), and pH of the reaction medium (5.7) at 25 °C. The photocatalyst particles were removed from the samples first by centrifugation (4,000 rpm, 5 min) using an Eppendorf centrifuge (system model 5810R) and then filtered through a 0.2 m Millipore filter.
Furthermore, the UV-Vis absorbance of phenol solutions sampled was recorded every 15 min. the unknown concentration of phenol solutions and its byproducts were calculated using the calibration curve.
To compare the photocatalytic activity of 10 ppm phenol with the other intermediates, organic chemicals such as 10 ppm HQ, 10 ppm BQ, 10 ppm Res, 10 ppm Cat, 10 ppm AA and 10 ppm 2-P were prepared in the same way exactly like phenol. First, preparing a stock in 1000.0 mL and then preparing the rest of the standards into 250.00 mL volumetric flask and used them in the same way as done for phenol photocatalytical experiments