Acid Red 91 is a dye used in the textile industry, and a water pollutant. Removal of Acid Red 91 from water is typically achieved via catalytic degradation, such as the use of ZnO sonocatalysts. Acid Red 91 is also used in the metal-​free, visible-​light-​mediated preparation of Csp2-​H radical mono- and bis-​trifluoromethylation of coumarins. Dyes and metabolites.