Thermally Regulated Al Coordination in Layered Double Hydroxide-Derived Oxides for Enhanced Nitrate Loading and Slow Release

Layered double hydroxides (LDHs) are promising carriers for slow-release nitrate fertilizers; however, their practical application is constrained by their limited anion loading capacity and the difficulty of achieving slow release. Here, a temperature-regulated calcination-reconstruction strategy was employed to enhance the nitrate storage and control the release behavior of MgAl-LDHs. Commercial MgAl-LDHs were calcined at 400–600 °C to produce layered double oxides (LDOs-x) at different temperatures (x) ranging from 400 to 600 °C, followed by reconstruction in a nitrate solution to obtain nitrate-incorporated LDHs-x. Calcination-induced structural evolution, including Al coordination transformation, structural disorder, and pore reconstruction, governed the reconstruction, incorporation, and release behavior. The reconstructed LDH-450 and LDH-600 achieved significantly enhanced nitrate loading (4.18 and 4.38 wt %, respectively) and exhibited sustained release, with cumulative release rates of 66.9 and 80.3% over 30 days in water, respectively. Soil incubation experiments further demonstrated that these optimized LDHs increased soil available nitrate by up to (4.4–10.4)-fold, while maintaining negligible nitrate leaching over 30 days. 27Al nuclear magnetic resonance revealed the formation of tetrahedrally coordinated Al sites during calcination in all LDOs, with LDO-450 exhibiting the most uniform tetrahedral Al environment and LDO-600 showing highly distorted tetrahedral Al species associated with a more heterogeneous local environment. Density functional theory (DFT) calculations indicated that these structural differences modulated the interaction of nitrate with LDO surfaces. Molecular dynamics simulations revealed that such interaction heterogeneity governs interlayer accessibility and release dynamics. This research provides a mechanistic framework for tuning anion storage and release in LDH-based slow-release fertilizers via thermal reconstruction engineering.