Enhancing the phosphorus content of layered double hydroxide fertilizers by intercalating polymeric phosphate instead of orthophosphate: A feasibility study

Abstract

Hypothesis: Layered double hydroxide (LDH) loaded with orthophosphate (PO₄) are suggested as slow-release P fertilizers. However, PO₄-LDHs have a low maximal P content, related to high charge HPO₄^2-/PO₄^3- anions occupying the anion exchange capacity (AEC) of LDHs. We postulate that the P content of LDHs can be enhanced by exchanging them with polymeric-P (i.e. trimetaphosphate, P₃O₉), which has a lower molar charge/P ratio than its monomer.

Experiments: Adsorption capacities were compared between PO₄ and P₃O₉ for as-synthesized and calcined MgAl LDHs with Mg/Al ratio of 2, 3, or 4; the P-LDHs were characterized (XRD, FTIR). Dialysis and soil incubation experiments were performed with PO₄^-LDHs, P₃O₉^-LDHs, and corresponding soluble fertilizers to compare their P release and P solubility (CaCl₂ extract).

Findings: The P adsorption capacities were 1.25-1.60 fold larger for P₃O₉ compared to PO₄, yet the high theoretical P contents with P₃O₉ were not achieved (incomplete loading, P₃O₉ depolymerization). P₃O₉-Mg₃Al released polymeric-P whereas P₃O₉-Mg₂Al released depolymerized PO₄, and P release from P₃O₉-LDHs was slower than that of PO₄-LDHs. With soil incubation, soluble P from P₃O₉-LDH was initially lower but later converged to that of PO₄-LDH as result of continued hydrolysis, yet did not exceed that of the soluble P₃O₉ and PO₄ fertilizers.

Keywords: Anion exchange; Anionic clays; Calcination; Layered double hydroxide (LDH); Phosphorus; Polyphosphate; Slow-release fertilizer; Trimetaphosphate.