Comparative Study of Rhodamine B Treatment: Assessing of Efficiency Processes and Ecotoxicity of By-Products

In this work, a comparative study between two processes was performed—biodegradation and photocatalysis, as an advanced oxidation process—to discover which one is more efficient to degrade Rhodamine B, a synthetic dye widely used in the textile and food industries. The advantage of this study is that it correlates treatment efficiency with the ecotoxicity of the by-products resulting from the treatments. Since the COVID-19 pandemic, it has been difficult to use activated sludge because of the risk factor of COVID-19 infection. Therefore, biodegradation tests were conducted with the yeast Saccharomyces cerevisiae in this study. For the photocatalysis assays, TiO2 doped with 5 per cent Cerium was used as a catalyst under UV light irradiation. S. cerevisiae cannot reduce RhB by biodegradation. However, a 13 per cent biosorption was observed with an uptake capacity of 4.2 mg g−1 dry matter of S. cerevisiae cultivated in the presence of 5 mg L−1 of RhB after 150 min. At a 5 mg L−1 of RhB concentration, the 6 h photocatalysis treatment led to 55% color removal and 8.6% COT reduction. The biodegradability of the photocatalyzed solution increased since the BOD5/COD ratio raised from 0.10 to 0.42. In the presence of glucose as a source of carbon, yeast can grow on the by-products generated by photocatalysis. The phytotoxicity of RhB in solution was measured using the germination index (GI) of watercress seeds. The GI decreases by 75% for an RhB solution of 100 mg L−1 compared to the control sample. The by-products of the photocatalytic treatment, using crustaceans Daphnia magna and conducted with solutions of Rhodamine B, induced a decrease of 24% in the GI. Lethality test. After 3 or 6 h of treatment, no increase in immobilization or mortality of D. magna was observed compared to the negative control.