Effect of pH

During the phenol photodegradation formation of several byproducts occur which also lower the pH solution [47]. Many organic compounds are acidic nature when dissolved in solution, in addition, to the surface of the catalyst is also acidic. This behaviour let the pH to put a significant influence on the oxidation potential and determines the charges of the pollutant. In this study the pH is controlled before the irradiation and is not adjusted during the experiment. The effect of changing the pH from (3-8) on the photodegradation of phenol is shown in Figure 28. It is vital that the reaction is done under stable catalyst conditions. Phenol photodegradation showed stable results in the acidic media, no attempts were made to prepare phenol concentrations at higher basic media. It was noticed that all the SnO2 Nps were insoluble at all pH ranges tested.

Figure 26, shows each 10 ppm phenol standard was adjusted with pH (3, 4, 5, 6, 7 and 8) was separately mixed in the reactor, while the other parameters were kept constant such as catalyst loading of (65 mg/50.00 mL) by using (SnO2/Ce 0.6 wt. %), reaction

4.4 Parameters affecting phenol photodegradation in an aqueous solution dispersion of SnO2

137 time (2-3 h), sampling time (12-13), sample volume (250.00 mL) and inlet air flow 4 L/min.

Figure 28: Effect of pH adjusted value on phenol photodegradation rate, (65 mg/50.00 mL) of (SnO2/Ce 0.6 wt. %), under UV light irradiation, reaction time (2-3 h), sampling time (12-13), sample volume (250.00 mL) and inlet air flow 4 L/min.

Setting the pH of phenol solution at 3 enhanced the degradation rate to 84% but then decreased to 60% at the time of 150 min at pH 4. An increment was noticed in the phenol photodegradation rate in pH from 5-6, this increment reached to 89% at the same irradiation time. Acidic contaminant such as phenol can create an attraction between photocatalyst and phenol molecules, which can facilitate the adsorption of the phenol molecule on the SnO2 surface resulting in the enhancement of phenol degradation [398].

SnO2 may be acidic in nature and therefore the pH effect needs to be considered. The pH solution may affect the surface of the photocatalyst and increase the ^•OH and enhance its adsorption.

In addition, acidic conditions were more preferable for production of hydrogen peroxide which increase OHˉ ions and the amount of ^•OH also increase. When the ^•OH increase,

the chance for ^•OH to react with phenol molecule increase and the result is increase in the photodegradation efficiency is noticed. In addition, more pH values (basic) initiate the formation of carbonate ions, these ions are known to be scavengers OHˉ ions which decrease the photodegradation rate at alkaline values [53, 377]. Many scientists used lower pH ranges and were found to be beneficial for the degradation of phenolic compounds [399, 400]. Enhanced degradation of phenol under slightly acidic condition has been continuously done by scientists motivated in this subject [400]. On the other hand, if the pH of the reaction mixture falls under a certain threshold, the rate of the reaction mixture decreases considerably.

Figure 29: Effect of pH changes as a function of time for (65 mg/50.00 mL) SnO2/I 1.0 wt. % Nps under UV light irradiation, reaction time (3 h), sampling time (12-13), sample volume (250.00 mL) and inlet air flow 4 L/min.

The effect of initial pH 5.7 of 10 ppm phenol solution with the (65 mg/50.00 mL) SnO2/I 1.0 wt. % catalyst on the photodegradation of phenol was examined within 3 h reaction time as shown in Figure 29. In the beginning the pH decreased to a minimum pH 4.5, later raised up to values equal to or slightly higher than the initial pH, because more acidic byproducts were being formed and resulted. The phenol photodegradation depended on the formed intermediates and on the compounds mineralized until it rose again to reach pH 7. Many scientists noticed the same behaviour [262], as they shown that organic compounds can be degraded at medium pH and in acidic ranges [208, 399].

4.4 Parameters affecting phenol photodegradation in an aqueous solution dispersion of SnO2

139 It is known that the pH optimal degradation falls in between pKapH, due to the increase in the electrostatic interaction between the anions of the weak acids and the cations catalyst surface other researchers reported that the photodegradation of chlorophenoxyacetic acid was increased when the pH of the reaction mixture was higher than its pKa, but below the pH of the point of zero charge (pHzpc) of the nanocomposite.

Optimum electrostatic interaction of the deprotonated acid with the positively charged surface of the photocatalyst with higher reaction rates would be continued [401]. When an organic compound is neutral or protonated, its pH is lower than its pKa value, however, the pH of the solution is greater than its pKa value, the compound deionized or deprotonated and exists as negatively charge species [402]. The pKa value is different for every organic dye and the point of zero charge varies for different photocatalysts, so careful control of the reaction mixture is needed and the settings must be optimized [403]

The formation of acidic binary hydroxyl compounds, such as Cat, BQ, HQ, during the photocatalytic degradation of phenol were considered to be the main reason for the pH cycle change. The ^•OH formed by redox reaction oxidized the conjugate structure of the phenyl ring, yielded carboxylic acids formed via ring cleavage to produce malonic acid, then short chain organic acids such as maleic, oxalic, acetic and formic acids, which lowered the solution pH [47]. Finally, upon continuous oxidation of the carboxylic acids these intermediates are consumed, returning the pH values to normal as initiated.

Consequently, a cycle of pH changes occurred and the TOC is finally eliminated by the conversion of the intermediates to CO2 and H2O [404].