5.4. The influence of the initial concentration

The initial concentration of the reducible metal ions is also important to be paid of attention in the photoreduction. In general, the relationship of the initial concentration is as follows: in the increase of the initial concentration, the metal ions in the photoreduction are enhanced. When the amount of the reduced ions is compared/relative to the initial concentration, it is usually observed that the photoreduction percentage decreases as the initial concentration increases. With the low initial concentration, the small number of the reducible ions is present with abundant electrons that can be reduced completely or 100%. By increasing the initial concentration of the metal ion, the number of the metal ions to be reduced is also increased, but the percentage is usually decreased. The optimum initial concentrations are reported to be <sup>4</sup> <sup>10</sup><sup>3</sup> mole/L for Ag (I) [8], 3 <sup>10</sup><sup>3</sup> mole/L for Cu(II) [24], 10 mg/L for Cr(VI) [23], 100 mg/L for Hg(II) [28], and the CO2/H2O ratio is 1/2 [66].
