Tailoring hydroxyapatite (HA) nanoparticles as a phosphorus (P) fertiliser in soils

Abstract

Hydroxyapatite nanoparticles (HA-NPs) have recently been proposed as a novel phosphorus (P) fertiliser that not only provide increased efficiency through controlled release but also minimise adverse environmental impacts. In the present study, three types of HA-NPs were synthesised with differences in surface charge. The P release dynamics of the three HA-NPs was investigated by incubating the compounds in two P-deficient soils (an Ultisol and a Vertisol) in the laboratory for up to 240 d. Their fertilising effect on sunflower (Helianthus annuus) was assessed by a glasshouse pot experiment using the two soils, their mobility in both soils was examined through a column leaching experiment with sand as a control. The surface charge of the three HA-NPs was +21 (±4.3), 0 (±3.5) and −12 (±2.7) mV in a neutral medium (pH 7) [hereafter defined as HA-NPs(+), HA-NPs(0), and HA-NPs(‒)], with an average particle size of 25.7 nm. In the Ultisol (pH 4.7), the addition of HA- NPs resulted in a smaller initial increase in P availability than triple superphosphate (TSP) due to their lower solubility. However, P availability in the TSP treatment decreased over time due to adsorption of P to the soil constituents, with a concomitant increase in the NaOH-extractable P fraction. In contrast, the release of P from the HA-NPs remained relatively constant over the entire incubation period. After 45 d, the soils amended with negatively charged HA-NPs had higher levels of available P when estimated using diffusive gradients in thin films (DGT-P) compared to both the neutral and the positively charged HA-NPs. In the Vertisol (pH 8.2), although the addition of TSP increased P availability markedly, amendment with the three HA-NPs did not increase P availability due to the low solubility of hydroxyapatite at this high pH. Indeed, for the Vertisol, most of the P added as HA-NPs remained in the Ca-P fraction. For the glasshouse experiment, in the Ultisol (pH 4.7), addition of TSP or any of the three HA-NP compounds increased plant biomass by 6.4-11.6 times, with the HA-NPs(‒) being significantly more effective than the other three P compounds (including TSP), increasing plant fresh biomass 16.5-fold for the shoots and 8-fold for the roots. The rock phosphate (RP) increased plant biomass the least (only by 1.5-fold). In the Vertisol, none of the three HA-NPs compounds or the RP significantly increased plant growth, whereas TSP increased plant fresh biomass by 28 % after 35 d. Through the column experiment, it was also found that although overall mobility of P in both soils is low, the application of HA-NPs(‒) in the Ultisol slightly increased P mobility, moving 5 % of the P to 100 mm depth of the column. The results show that for the acidic Ultisol, HA-NPs with altered surface charge is a competitive form of P fertiliser. However, HA-NPs were not effective in an alkaline soil due to their lower solubility. This information is important in the development of HA-NPs as a novel fertiliser, and it is important to consider the development of HA-NPs with altered surface properties (i.e. particle size, surface charge) to match the soil properties.