Oxidized Plastic Residues Accelerate Phosphorus Immobilization under Neutral to Alkaline Conditions via Enhanced Surface-Induced Nucleation

The accumulation of plastic residues (PRs) and their oxidized counterparts (OPRs) poses a potential influence on phosphorus (P) geochemical cycling and availability. Bioavailable phosphorus can be immobilized through its transformation into insoluble calcium phosphates (CaPs). The influence of PRs and OPRs on CaPs formation remains poorly understood. To address this issue, we selected polyethylene (PE), polypropylene (PP), and poly(vinyl chloride) (PVC), along with their oxidized counterparts, as representative PRs and OPRs, respectively. The in situ atomic force microscopy (AFM) revealed that OPRs significantly promoted the nucleation of CaPs at the solid-solution interface. This effect is attributed to enhanced binding between CaP nuclei and PR surfaces following oxidation, which lowers the interfacial free energy and reduces the energy barrier for CaPs nucleation. In addition, real-time confocal Raman microscopy and high-resolution transmission electron microscopy (HRTEM) showed that OPRs promoted the phase transition from amorphous calcium phosphates (ACP) to hydroxyapatite (HAP), a highly insoluble and biounavailable form. These changes are consistent with potential phosphorus immobilization under neutral to alkaline, Ca-rich conditions. Preliminary pot and field explorations suggested that OPRs could influence phosphorus immobilization by accelerating the formation of Ca 10 P (apatite, Ca 10 (PO 4 ) 6 (OH) 2 ). Our findings point to a potential mechanism by which OPRs may facilitate phosphorus immobilization via enhancing surface-induced nucleation. Further research is needed to assess the relevance of this mechanism in complex soil environments; nonetheless, these findings underscore the need for continued investigation of plastic pollution in agricultural systems.