Powdered activated carbon enhanced synchronous manganese oxidation coupled with denitrification and heavy metal immobilization by Zoogloea sp. MFQ7: Promoting mechanisms and performance.

Co-contamination bynitrate-nitrogen (NO3--N)and heavy metals (HMs) can impair biological nitrogen removal. This study investigated whether powdered activated carbon (PAC) derived from coconut shell, applied at a trace dose, could enhance denitrification, manganese oxidation, and metal immobilization by Zoogloea sp. MFQ7 under HM stress. Under the selected conditions of pH 7.0, a carbon-to-nitrogen ratio of 1.5, and an initial Mn(II) concentration of 10.0 mg L-1, strain MFQ7 removed 92.89 % of NO3--N and 89.12 % of Mn(II). Addition of 0.8 mg L-1 PAC increased these removal efficiencies to 97.02 % and 97.11 %, respectively, while nitrite remained below 0.01 mg L-1. Under combined zinc (Zn(II)), copper (Cu(II)), and nickel (Ni(II)) stress, PAC maintained NO3--N and Mn(II) removal efficiencies at 75.93 % and 69.00 %, respectively, and achieved Zn(II), Cu(II), and Ni(II) immobilization efficiencies of 79.22 %, 78.65 %, and 71.34 %, respectively. PAC also increased electron transport system activity to a level 9.66 % above that of the unstressed control and helped preserve a matrix of extracellular polymeric substances (EPS) rich in proteins. Analyses of the solid phase showed that PAC introduced additional carbonaceous interfaces containing oxygen functional groups, while EPS, biogenic Mn precipitates, and metal carbonate phases contributed to metal immobilization. Overall, trace PAC derived from coconut shell alleviated inhibition caused by HMs through a combination of physiological protection and immobilization in the solid phase, supporting its exploratory application as an amendment derived from waste for complex wastewater treatment at the batch scale.