*3.4.4. Effect of photocatalyst concentration*

Photocatalyst concentration is a crucial parameter that has been widely studied for photocatalytic processes. The optimum photocatalyst concentrations usually vary between 0.15 g l−1 and 8 g l−1 for different photocatalyst systems and photoreactors. A large difference in optimum photocatalyst concentration (0.15–2.5 g l−1) was reported even for the same photocatalyst (DP25). Chen and Ray [15] expressed the photocatalytic degradation rate as follows:

$$r\_l = K \left[ \mathbf{1} - \exp\left(-\epsilon \beta \mathcal{C}\_{photoat} \mathcal{H}\right) \right] \tag{8}$$

$$\text{againin, } K = \frac{\mathbf{k}\_1 A f(\mathbf{C}\_l) \mathbf{t}\_0^\theta}{\epsilon \theta} \tag{9}$$

where *k*<sup>1</sup> is a proportionality constant, *A* is the illuminated area of the photoreactor window, *C*photocat is the photocatalyst concentration, *ε* is the light absorption coefficient of the photocatalyst, *I*<sup>0</sup> is the incident light intensity, and *β* is a constant. They reported *β* values of 0.84, 0.72, and 0.82 for the degradation of 4-NP, 4-CP, and phenol, respectively.

### *3.4.5. Effect of electron acceptor*

Photocatalytic degradation reaction requires the use of electron acceptor to reduce the charge carrier recombination. Oxygen is the most common electron acceptor because of its availability, higher solubility, and nontoxic nature. The partial pressure of oxygen is adjusted by mixing the oxygen stream with nitrogen stream by maintaining the total flow rate of gas at a constant value. The photocatalytic reaction of phenols will terminate if sufficient oxygen is not available in the solution [15]. Chen and Ray [14] showed the improvement of 4-NP photodegradation rate with increasing oxygen partial pressure. The photodegradation rate constant reached approximately 70% of its maximum value at oxygen partial pressure of 0.2 atm. The effect of oxygen partial pressure on the photodegradation of 4-NP is described by a noncompetitive Langmuir kinetic equation as follows:

$$k\_{\rho} \propto \frac{K\_{o\_{\uparrow}} p\_{o\_{\downarrow}}}{1 + K\_{o\_{\uparrow}} P\_{o\_{\uparrow}}} \tag{10}$$

where *kp* is the kinetic constant for 4-NP degradation, *KO*2 is the adsorption constant of dissolved oxygen on photocatalyst, and *pO*2 is the partial pressure of dissolved oxygen.
