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Formation of persistent transformation products during MnO2-Biological degradation of Alizarin Red S in DHS reactors
Roslan Noorain1, Junya Hirota2, Hiromi Kambara3
1Department of Civil and Environmental Engineering, Graduate School of Advanced Science and Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima, 739-8527, Japan; Section of Environmental Engineering Technology, Malaysia Institute of Chemical & Bioengineering Technology, University Kuala Lumpur, Lot 1988, Kawasan Perindustrian Bandar Vendor, Taboh Naning, 78000, Alor Gajah, Melaka, Malaysia.
Abstract:
Anthraquinone dyes such as Alizarin Red S (ARS) are resistant to conventional biological wastewater treatment due to their stable aromatic structures. This study investigated ARS degradation using combined abiotic oxidation by MnO2 and biological treatment in Downflow Hanging Sponge (DHS) reactors. Five reactor configurations were operated to evaluate the individual and combined contributions of MnO2 oxidation and microbial activity. Biological treatment alone resulted in negligible ARS removal, whereas abiotic MnO2 rapidly decolorized the dye but achieved only partial mineralization. The combined MnO2-biological system achieved high removal performance under the tested conditions, with >95% decolorization and approximately 50-55% total organic carbon (TOC) removal, while sequential systems also demonstrated effective removal under different treatment configurations. Liquid chromatography-mass spectrometry (LC-MS) analysis detected multiple transformation products formed during ARS degradation, indicating that MnO2 likely initiated oxidative transformation of the anthraquinone structure, followed by microbial transformation of the resulting intermediates. However, several persistent transformation products were detected across the tested systems, indicating incomplete mineralization. These findings suggest that persistent intermediates may represent a key limitation in MnO2-biological treatment of anthraquinone dyes.
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