5.1. The influence of the photocatalyst dose

In general, the increase of the photocatalyst dose promotes higher photoreduction efficiency, and then it declines when the photocatalyst dose is further increased. Such trend can be explained in terms of the number of active sites available for photocatalytic reactions. The larger number of the active sites is available in as the dose of the photocatalyst increases that would generate more numbers of electrons and so higher photoreduction effectiveness. However, the large amount of catalyst may result in agglomeration form with larger particle size [40] that may provide smaller surface area. The agglomeration can also induce light scattering that reduces the light contacting with TiO2 surface.

The smaller surface area can reduce the number of active sites and so decreases the electron number provided. Consequently lower photoreduction is obtained. Another reason of the decrease in the photoreduction can be attributed to the increase in the turbidity of suspension due to the large amount of photocatalyst. This leads to the inhibition of photon absorption by the photocatalyst. As an effect, the lesser photoinduced electrons can be provided, causing the photoreduction decreased [23]. From several studies, the optimum photocatalyst dose was reported to be 2 g/L (Ag(I)) [8], 1.6 g/L (Cr(VI)) [20], 0.1 g/L (Cu(II)) [25], and 2 g/L (Hg(I)) [27].
