The stress of Cr (VI) on the microalgal-bacterial granular sludge in wastewater treatment

• Non-aeration MBGS achieved a high removal efficiency of municipal wastewater. • The effect of Cr (VI)<10 mg/L on municipal treatment by MBGS was insignificant. • The inhibition of Cr (VI) for phosphate-P was more severe than for COD and ammonium-N. • The functional groups played a key role in good Cr (VI) binding capacity for EPS. • The good performance of MBGS benefited from the symbiosis of microalgae and bacteria. This study systematically investigated the effect of hexavalent chromium [Cr (VI)] on the performance of microalgal-bacterial granular sludge (MBGS) in treating wastewater and elucidated its intrinsic defensive mechanisms against Cr (VI) stress. Results demonstrated as Cr(VI) concentration gradually increased from 0 to 10 mg/L over 30-day period, the MBGS system maintained efficient pollutant removal, achieving rates of 87% for chemical oxygen demand (COD), 63% for ammonium nitrogen (NH+ 4-N), and 69% for phosphate phosphorus(PO3- 4-P). Bioreduction and adsorption were identified as the primary pathways for Cr (VI) removal, supported by a high Cr accumulation capacity of 7.49 mg/g VSS. The Fourier transform infrared spectroscopy (FTIR) analysis revealed that Cr ions could bind to extracellular polymeric substances (EPS) of MBGS via functional groups including C-O, P=O, C-H and O-H, establishing a primary biochemical barrier. MBGS adapted to Cr(VI) toxicity by reconfiguring EPS components, specifically the protein-to-polysaccharide ratio, and shifting microbial community structures. Although Proteobacteria abundance declined from 89.16% to 53.18%, the relative stability of Cyanobacteria indicates that the microalgal component, alongside EPS modulation, plays a decisive role in maintaining MBGS resilience against heavy metal stress.