In-depth characterization of phosphate intercalated Mg Al Layered double hydroxides and study of the PO4 release properties†

Abstract

Slow-release fertilizers (SRFs) form the core of innovative strategies in sustainable agriculture. Layered Double Hydroxides (LDH), known for their high capacity to sequester plant nutrients, especially phosphate, are emerging as promising candidates for SRF synthesis. The phosphate release properties of MgAl LDH (with a targeted Mg/Al ratio of 2.0) intercalated with HPO₄²⁻ anions were assessed in various aqueous environments. A comprehensive analysis, including in-depth chemical and structural characterizations (ICP-OES, XRD, PDF, ²⁷Al NMR, ³¹P NMR, FTIR, SEM) of the as-prepared phase unveiled a more intricate composition than anticipated for a pure or ideal Mg₂Al-HPO₄ LDH, encompassing an excess of intercalated phosphate in conjunction with K⁺. Beyond the intercalated phosphate, solid state ³¹P NMR speciation identified multiple H_xPO₄^(−3+x) environments, indicating a portion of the phosphate reacting with intralayer Mg²⁺ to form K-struvite. Additionally, some phosphates were adsorbed onto the surface of amorphous aluminum hydroxide, a side phase formed during MgAl coprecipitation. The phosphate release demonstrated rapid kinetics, occurring within 6 days. Moreover, the released phosphate increased significantly when reducing the Solid/Liquid (S/L) ratio (58%) and further increasing in the presence of carbonate ions (90%). The released phosphate varied from 12% to 90% under different release conditions, transitioning from water to a 3.33 mM NaHCO₃ aqueous solution at a low S/L ratio (from 20 mg LDH per mL to 0.02 mg LDH per mL). The simultaneous release of K⁺, Mg²⁺, Al³⁺ indicated the complete dissolution of the K-struvite and partial dissolution of phosphate intercalated MgAl LDH. These results enhanced our understanding of the mechanism governing phosphate release from MgAl LDH, paving the way for potential phosphate recovery by LDH or for the development of LDH-based SRFs.