Modulating Structure and Sustained-Nutrient Release in Bimetallic MOFs via Metal-Ion Competition: A Study on...

Bimetallic metal-organic frameworks (MOFs) are promising for controlled-release fertilizers (CRFs), yet the deliberate use of metal-ion competition to tune their properties is a nascent and powerful strategy. This study presents one of the first systematic investigations into harnessing competitive coordination, specifically between Fe^{3 +} and Zn^{2 +} with a tannic acid ligand, to modulate the structure and sustained-release performance of bimetallic MOFs. A monometallic Fe-MOFs (Compound 1) and a bimetallic Fe/Zn-MOFs (Compound 2) were synthesized. While their compositions were analogous, the dominance of Fe^{3 +} during coordination, due to its hard acid character and stronger chelation tendency, dictated the framework formation, leading to distinct properties for Compound 2, including smaller crystal sizes and altered morphology. These structural modifications resulted in an enhanced nutrient release profile for the bimetallic system, achieving a higher cumulative releaseparticularly in acidic conditionswhile sustaining effective release for 30 d in various soils. Release kinetics was best described by diffusion-based models (Higuchi and Ritger-Peppas). In pot experiments with bok choy, Compound 2 significantly improved plant growth and nutrient uptake, outperforming both the monometallic MOF and the conventional fertilizer. These findings conclusively demonstrate that metal-ion competition is a critical, underexplored lever for rationally designing efficient bimetallic MOFs-based CRFs, providing a foundation for their development in sustainable agriculture.