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

Hypothesis: Layered double hydroxide (LDH) loaded with orthophosphate (PO) are suggested as slow-release P fertilizers. However, PO-LDHs have a low maximal P content, related to high charge HPO/PO 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, PO), which has a lower molar charge/P ratio than its monomer.

Experiments: Adsorption capacities were compared between PO and PO 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 POLDHs, POLDHs, and corresponding soluble fertilizers to compare their P release and P solubility (CaCl extract).

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