Pore size distributions and degree of anisotropy (pore channel orientation) affect the mechanical properties, fragmentation, and dissolution kinetics of granular composites. Especially in porous solids such as granular urea fertilizers, uncontrolled dissolution leads to soil and water contamination from nutrient leaching. In this work, a dry compaction method and a bilayer granulation process were compared to investigate their ability to control the nutrient release from granular urea fertilizers. Urea and binary mixtures of urea and a xanthan and konjac gum binder were dry compacted. The compacts were then milled to specific particle sizes producing core granules. The core granules were drum granulated along with a known percent of fines and water to produce bilayer granules. The drum granulation parameters were optimized based on the granules’ particle size distributions and color analysis. Dissolution and porosity distributions of core and bilayer granules were investigated and compared to market urea granules. Core granules with or without a binder compacted at 100 MPa had smaller pore size distributions compared with core granules compacted at 50 MPa, bilayer granules, and market urea granules. Core granules compacted at 100 MPa showed a significant decrease in the dissolution rate compared to less dense systems, such as core granules at 50 MPa, bilayer, and market urea granules. The addition of gums as binder caused an additional delay in the dissolution rate, indicating that formulation design is essential in controlling nutrient release. Less porous structures, i.e., higher compaction pressure to produce core granules, along with binder addition resulted in delayed dissolution rates due to binder migration and gelation at the solid-liquid interface. These results show that densification and formulation design can help control the release of urea from granular fertilizers with the potential of reducing leaching losses and pollution.

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