Rice Root Iron Plaque as a Molecular Sieve: Iron Oxyhydroxide-Mediated Selective Sequestration of Arsenate over Phosphate

Iron oxyhydroxides in rice root iron plaques act as a molecular sieve, enabling rice to selectively uptake phosphorus while sequestering arsenic in paddies; however, the molecular mechanism of this filtration is still unclear. Here, we selected ferrihydrite and goethite, the active components of iron plaques, to elucidate this selective filtration. We demonstrated that under equimolar concentrations, ferrihydrite and goethite exhibited higher affinity for arsenate, substituting 31.36% to 46.99% of preadsorbed phosphate across pH 4.0-8.0. Then, our Raman spectroscopy and DFT calculations revealed that phosphate formed labile monodentate and outer-sphere complexes via charge reorganization and electrostatic interactions, facilitating its ready remobilization into the rhizosphere. In contrast, arsenate formed stable bidentate-binuclear complexes through significant electron redistribution and stronger Fe-As bonds, leading to its strong sequestration on rice root iron plaques. Furthermore, our DFS measurements directly quantified that the binding free energy of arsenate to Fe oxyhydroxides was 14.57-27.01% higher than that of phosphate. Finally, we confirmed that Fe plaques enhanced phosphate uptake while lowering arsenate bioavailability through Fe-As coprecipitation using rice hydroponic experiments. These findings offer valuable insights into phosphorus and arsenic cycling in paddies and suggest potential strategies for efficiently optimizing phosphorus utilization and arsenic sequestration in wetland plants.