Recovery of phosphorus from animal wastes and synthesis into hydroxyapatite nanoparticles

Abstract

Phosphorus (P) loss from agricultural soils and its adverse impact on the ecosystem have resulted in renewed calls on resource recovery as well as the use of more efficent fertilizers. The major goal of this dissertation research was to recover P from animal wastes (chicken litter, dairy cow, horse, sheep, and swine manures) and fabricate into hydroxyapatite nanoparticles (HANPs, Ca10(PO4)6(OH)2), a potential nanofertilizer. Among various purification methods tested, HANP synthesis did not require a high purity solution. Direct precipitation of HANPs without concentrating P or removing contaminant ions from waste digests was possible for wastes with high P content such as swine manure and chicken litter but compromised the yield and purity. All synthesized HANPs from partially purified P were carbonated apatite in which columnar hydroxyls were replaced by A-type carbonate, which is an ideal candidate for better P release because the presence of carbonate in HANPs enhances the solubility. Synthesized HANPs were of uniform size with constant lattice fringe indicative of relatively high phase purity. Further efforts to enhance the solubility of HANPs through altering the crystal-chemical environment by cation substitutions revealed that the structural incorporation of potassium increased the lattice parameter a, but the increase was high in pure HANPs. The dissolution of HANPs derived from various extents of P purification, structural substitution, phase purity, and thermal treatment showed that P release was highest in cation substituted HANPs and lowest in thermally treated HANPs, suggesting structural modification is the major parameter to tune the P release kinetics. Overall, these results show that the recovery of P from animal wastes to synthesize HANPs is feasible and could contribute to the efforts to minimize P loss from agroecosystem and protect the environment.