This chapter focuses on the synthesis and characterization of photocatalysts that absorb in the visible part of the solar spectrum and simultaneously improve the photocatalytic detoxification properties in this spectral region. The absorption of photons by semiconductors leads to the formation of an equal number of positive and negative charge carriers. The chapter describes dichloroacetic acid was used as a model compound in all experiments. It shows that the photocatalytic detoxification of polluted aquifers constitutes a technique that bears a considerable potential for solar as well as for artificial illuminations. It also shows that the novel fixed-bed flow reactor. The chapter describes that the novel fixed-bed flow reactor presents a useful alternative for solar applications of the photocatalytic cleanup of polluted aquifers. Since during the normal use periodical variations of the temperature in the reactor are also encountered, a systematic study of the light intensity and the temperature dependence of photocatalytic mineralizations is required.