Biofilm formation on MgFe-LDH@quartz sand as novel wetland substrate under varied C/N ratios for BDE-47 removal.

Layered double hydroxide (LDH)-coated substrates could enhance the removal of various wastewater-born pollutants. However, research on biofilms attached to LDH-coatings and their synergistic purification effects on strongly hydrophobic persistent organic pollutants (POPs) remains limited. This study aims to investigate biofilm formation on MgFe-LDH@quartz sand and its effectiveness in removing tetrabromodiphenyl ether (BDE-47), an emerging halogenated POP in municipal wastewater. Under different C/N ratios (3, 5, and 10), BDE-47 removal rates ranged from 28.0% to 41.6% after 72 h. The optimal performance was achieved with LDH coating at C/N = 5, when substrate biofilm reached its highest extracelluar polymer substances (EPS) content, dehydrogenase activity and relative hydrophobicity. Moreover, distinct distribution patterns of EPS components' fluorescence peaks were observed in the LDH-coating treatment using three dimensional excitation-emission matrix (3D-EEM). While substrate adsorption was the primary mechanism for BDE-47 removal, accounting for 59.6%-83.4% of the total, biofilm adsorption and degradation contributed a relatively lower amount, ranging from 11.5% to 21.4%, and were more dependent on the C/N ratio. Notably, the maximum carrying capacity of protein predicted by the logistic growth model exhibited a strong positive correlation with the total BDE-47 removal rate (R = 0.82, p < 0.05), highlighting the importance of biofilm extracelluar proteins.