4. Textile dyes are often toxic and non-biodegradable. Wastewater containing these dyes needs to be treated before it can be released into the environment. One technique that has been explored for this purpose is photocatalytic degradation. Catalyst particles are added to the colored wastewater and the slurry mixture is exposed to UV light, which initiates reactions that break down the organic dye compounds. A recent article [Kansal, S.K., Sing, M., Sud, D. "Studies on photodegradation of two commercial dyes in aqueous phase using different photocatalysts," (2007) Journal of Hazardous Materials, 141 (3), 581-590.] studied the performance of different photocatalysts under a range of conditions. Find this article and answer the following questions: a) What type of reactor did they use for their experiments? b) Which catalyst had the highest efficiency (TiO2, ZnO, SnO2, ZnS, CdS)? see Figures 5 and 6 c) Are acidic, neutral, or basic conditions better for decolorization? Figure 10 d) What concentration of catalyst is needed? Figure 9 e) What irradiation time under solar light is needed to reach 80% conversion (as measured by COD reduction)? Table 2

Fig: 1