Enhanced cadmium immobilization by phosphorylated nano zero valent iron: Characterization, water chemistry, and mechanism

Cadmium (Cd) pollution poses significant risks to ecosystems and human health, necessitating urgent management and remediation efforts. Nano zero-valent iron (nZVI) is a cost-effective material for heavy metal immobilization, but its efficiency in removing Cd requires enhancement. Here, we modified nZVI via phosphorylation (P-nZVI), resulting in an increase in Cd removal from 54.5 % (nZVI) to 96.7 % (P-nZVI), with an optimal P/Fe molar ratio of 0.75. The Cd removal kinetics were better described by the pseudo-second-order model. Compared to nZVI, P-nZVI demonstrated consistent performance across pH 4.0–8.0 and resilience to interfering ions. Characterization results indicated that dominant immobilization mechanisms for P-nZVI shifted to replacement, surface complexation, and coprecipitation, rather than reduction mechanisms observed with nZVI. Phosphorylation introduced FeP₂ and FeP 4 phases to facilitate the replacement reactions. After reaction, the formation of stable cadmium phosphide (Cd₃P₂ and Cd₂P₂O₇) precipitates were identified by XRD and HR-TEM. These findings establish phosphorylated nZVI as a promising strategy for cadmium immobilization in aquatic environment.