Direct Nanoscale Imaging Reveals the Mechanism by Which Organic Acids Dissolve Vivianite through Proton and Ligand...

The coprecipitation of iron (Fe) and phosphorus (P) in natural environments limits their bioavailability. Plant root-secreted organic acids can dissolve Fe-P precipitates, but the molecular mechanism underlying mobilizing biogenic elements from highly insoluble inorganic minerals remains poorly understood. Here, we investigated vivianite (Fe₃(PO₄)₂·8H₂O) dissolution by organic acids (oxalic acid (OA), citric acid (CA), and 2'-dehydroxymugineic acid (DMA)) at three different pH values (4.0, 6.0, and 8.0). With increasing pH, the vivianite dissolution efficiency by OA and CA was decreased while that by DMA was increased, indicating various dissolution mechanisms of different organic acids. Under acidic conditions, weak ligand OA (HC₂O₄^- > C₂O₄^2- at pH 4.0 and C₂O₄^2- at pH 6.0) dissolved vivianite through the H⁺ effect to form irregular pits, but under alkaline condition (pH 8.0), the completely deprotonated OA was insufficient to dissolve vivianite. At pH 4.0, CA (H₂Cit^- > HCit₂^- > H₃Cit) dissolved vivianite to form irregular pits through a proton-promoted mechanism, while at pH 6.0 (HCit^2- > Cit^3-) and pH 8.0 (Cit^3-), CA dissolved vivianite to form near-rhombohedral pits through a ligand-promoted mechanism. At three pH values ((H₀)DMA^3- > (H₁)DMA^2- at pH 4.0, (H₀)DMA^3- at pH 6.0, and (H₀)DMA^3- and one deprotonated imino at pH 8.0), strong ligand DMA dissolved vivianite to form near-rhombohedral pits via ligand-promoted mechanisms. Raman spectroscopy showed that the deprotonated carboxyl groups (COO^-) and imino groups were bound to Fe on the vivianite (010) face. The surface free energy of vivianite coated with OA decreased from 29.32 mJ m^-2 to 24.23 mJ m^-2 and then to 13.47 mJ m^-2 with increasing pH, and that coated with CA resulted in a similar pH-dependent vivianite surface free-energy decrease while that coated with DMA increased the vivianite surface free energy from 31.92 mJ m^-2 to 39.26 mJ m^-2 and then to 49.93 mJ m^-2. Density functional theory (DFT)-based calculations confirmed these findings. Our findings provide insight into the mechanism by which organic acids dissolved vivianite through proton and ligand effects.