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 (PO4) are suggested as slow-release P fertilizers. However, PO4-LDHs have a low maximal P content, related to high charge HPO42/PO43 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 PO(4)LDHs, P(3)O(9)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. (C) 2022 Elsevier Inc. All rights reserved.

Automated summary

This study investigated the effect of replacing orthophosphate (PO4) with polymeric-P (trimetaphosphate, P3O9) on the phosphorus content in layered double hydroxide (LDH). Adsorption capacity, phosphorus release, and phosphorus solubility were compared between the two. The findings showed that P3O9 had a higher adsorption capacity, but the theoretical phosphorus content was not achieved due to incomplete loading and depolymerization. Furthermore, P3O9-LDHs exhibited slower phosphorus release and lower soluble phosphorus content in soil compared to PO4-LDHs.