*3.4.3. Effect of initial concentration of phenols*

As the effect of phenol concentration is of importance in the process of treatment of phenolic wastewater, it is necessary to investigate its dependence. Different concentration profiles can be seen during phenol degradation at different initial concentrations. The degradation rates at same concentration with different initial concentrations are different. However, all the concentration profiles could be correlated with an exponential function as follows [14]:

$$\mathbf{C} = \mathbf{C}\_0 \exp\left(-kt\right) \tag{7}$$

where *C*0 and *C*, respectively, are the initial concentration and concentration of phenol at time t and k is the apparent rate constant. Chen and Ray [14] studied the photocatalytic degradation of 4-nitrophenol (4-NP) with varying initial concentration between 10 and 120 ppm. The degradation rate mainly followed a pseudo-first-order kinetics with respect to the initial concentration 4-NP.
