Transport of hydroxyapatite nanoparticles coated with polyacrylic acid under unsaturated water flow in soil columns

ABSTRACT Phosphorus (P) nanoparticles were hypothesized to exhibit greater mobility in soils than water-soluble P (WSP) and were therefore proposed to be used as a P fertilizer. Unsaturated transport is the main pathway from the application site to plant roots. Though its importance to fertilizer efficacy, quantitative evaluation of unsaturated transport of P nanoparticles has been overlooked to date. Mobility of spherical nano-hydroxyapatite particles coated with polyacrylic acid (PAA-nHAP) suspension and WSP (a mixture of KH2PO4 and K2HPO4) was evaluated in breakthrough column experiments under unsaturated states using three soils: alkaline sand (sandy-alk), acidic sandy clay loam (sandy-ac) and clayey soils. Next, P retention was determined by total P extraction layer-by-layer from the disassembled soil columns. In all soils, PAA-nHAP exhibited faster transport compared to WSP. In the sandy-alk soil, earlier breakthrough but lower plateau of the final relative P concentration of PAA-nHAP (64.0% vs. 100% of the input P concentration) and consistent low P retention with depth after washing with 10 mM KBr solution for the two sources were observed. In the other two soils, PAA-nHAP displayed greater retention near the inlet and decreased retention with depth. In the sandy-ac soil, no WSP and low final relative concentrations of PAA-nHAP (11.6%) were transported through the soil column. The retention of PAA-nHAP was much lower than that of WSP with depth. In the clayey soil, the breakthrough (relative P concentration >1%) occurred earlier (~35 pore volumes vs. ~45 pore volumes) and the eluted P concentration increased more rapidly (~2.6 times) for PAA-nHAP compared to WSP. The difference between the two sources mainly occurred at the soil surface with higher retention of WSP. Soil properties affected the P retention capacity of the two P sources, but for all soils, P mobility was increased by changing from the common soluble fertilizers to nanoparticles. Adsorption and size exclusion effect are suggested as the major factors affecting nHAP mobility. We suggest that the nHAP transport can be improved by modifying its coating with more negative zeta-potential to decrease coagulation and adopting drip flows with short hydraulic retention time. The design of the nanoparticles needs to take into account soil properties.