Fulvic acid promotes amorphous FeAsO₄ formation and arsenic immobilization under high sulfate during...

Microbe-pyrite/arsenopyrite (M-Py/APy) interactions in acid mine drainage (AMD) are the primary driver of arsenic (As) release into the environment. Fulvic acid (FuA), with strong Fe/As affinity, actively participates in Fe(III)-secondary mineral transformations and As adsorption, thus influencing As immobilization. Understanding the behavior of FuA-mediated As in M-Py/APy interactions is vital, but the mechanisms remain unclear. Herein, we integrate quartz crystal microbalance with dissipation and multiple microanalytical techniques to elucidate the way surface-adsorbed FuA mediates microbial biofilm evolution and secondary product transformation in A. ferrooxidans-Py/APy interactions, thereby affecting As immobilization. FuA covers the mineral surface and forms an interfacial layer between the mineral and solution, which suppresses the direct colonization of A. ferrooxidans by 84% on mineral surface and slightly inhibits mineral dissolution accompanied by Fe(II) indirect bio-oxidation and As(III) oxidation, forming amorphous ferric arsenate (AFeA) in the mineral-FuA interfacial space. Notably, FuA blocks the adsorption of SO₄^2-_(aq) species to AFeA, reducing SO₄^2--mediated driving of AFeA dissolution and conversion, thereby increasing AFeA accumulation as As immobilization increases from 6.2% to 42.2%. This study reveals the key role of FuA in M-Py/APy interactions, offering novel mechanistic insights into the mechanisms by which FuA regulates As biogeochemistry.