Decolorization of dark brown colored coffee effluent using zinc oxide particles: The role of dissolved oxygen in degradation of colored compounds

The degradation of model dark brown colored coffee effluent using photocatalyst zinc oxide (ZnO) has been systematically studied by varying ZnO dosage from 0 to 4000 mg L−1, coffee loading from 0 to 90 mg L−1 and intensity of UV light having the radiation peak at 352 nm from 0 to 18 W(m-lamp length)−1. Almost complete decolorization was achieved after 180 min for the initial coffee concentration of 50 mg L−1 with ZnO dosage of 3000 mg L−1 and three UV lamps. The dissolved oxygen (DO) largely affected the photodecolorization process. Without air sparging or with oxygen supply only through the free-surface, the DO concentration significantly decreased during the initial decolorization process and then increased to the saturated DO concentration after about 80% decolorization was achieved. Under the anoxic condition with nitrogen gas sparging, the efficient color removal was not obtained unlike the decolorization without air sparging or under the oxic condition with air sparging. These findings suggest that the change in DO concentration was controlled by the oxygen consumption for the formation of oxygen adduct intermediates such as organoperoxy radicals. The mineralization rate of model coffee effluent was rather slow as compared with the decolorization rate and it was insignificantly affected by anoxic and oxic conditions. The present results indicate that ZnO photocatalyst has potential for treatment of coffee processing wastewaters. Colored organic compounds (ROH) adsorbed on the surface of ZnO particles are oxidized by OH radicals (OH). The resulting intermediated radicals react with dissolved oxygen and generate oxygenated radical intermediates (ROO). The colored organic compounds and their intermediates are finally mineralized. Numbering corresponds to reactions in text. [Display omitted] •Photodegradation of colored coffee effluent by ZnO was studied.•ZnO photocatalyst was very effective in the treatment of coffee processing wastewater.•The dissolved oxygen decreased and then increased in the decolorization process.•The change in dissolved oxygen was controlled by the formation of oxygen adduct intermediates.