The release and transformation mechanisms of heavy metals from the vanadium-titanium magnetite tailings: Role of organic acids and light exposure

The long-term accumulation of vanadium-titanium (V-Ti) magnetite tailings has caused severe soil and groundwater pollution through metal leaching, which is exacerbated by light-driven redox reactions and organic acid-mediated mineral dissolution. However, the mechanisms of metal leaching from tailings and their subsequent transformation under the combined effects of light and natural organic acids remain unclear. This study investigated the release and transformation mechanisms of heavy metals from V-Ti magnetite tailings under three organic acids (citric, oxalic, and acetic) and light exposure. Results showed that in dark conditions, citric acid (CA) had the strongest metal dissolution capacity for Fe, V, Mn, and Al, followed by oxalic acid (OA) and acetic acid (AA), achieving 2.5-5.2 times higher release than inorganic acids. CA promoted metal release through stable tricarboxylate complexes, while OA dominated in acidic conditions via Fe(II)-oxalate radical formation but declined at pH > 5 due to iron oxalate precipitation. AA, with its monocarboxylate structure, had limited complexation capacity, achieving only 20-30% of CA’s leaching efficiency, primarily weakening the mineral surface through proton attack. Under light exposure, Fe(II) release increased by 200-500% via ligand-to-metal charge transfer (LMCT), triggering Eh fluctuations (>300   mV), rapid pH elevation (ΔpH = 4.0 in 2   h), and colloidal nanoparticle formation (209.6   nm, ζ = -49.9   mV). Elevated temperatures accelerated initial leaching but enhanced metal immobilization through hydrolysis. The study demonstrated that organic acids and light exposure are key drivers of heavy metal release from V-Ti magnetite tailings, with light playing a crucial role in catalyzing these processes.