Differential blocking effects of Fe0 nanoplates on rice accumulation of typical essential and non-essential heavy metal elements in paddy fields

Abstract

This study investigated how nanoscale zero-valent iron (nZVI) affects the transfer of essential and non-essential heavy metals (HMs) from soil to rice in two paddy fields with varying Cd, Cu, and Zn pollution levels. Rice plants were cultivated with conventional field managements. 100 mg kg⁻¹ Fe⁰ nanoplates were injected in rhizosphere soil at pre-sowing (P0), tillering (T1), jointing (J2), flowering (F3), and grain-filling (GF4) stages, respectively. Among them, the GF4 treatment performed the best, decreasing rice grain contents of Cd, Cu, and Zn (by 66.4%, 20.0%, and 24.8%, respectively) to the required safe levels. This reduction was attributed to significant inhibitions (25.9–49.4% for Cd, 52.4–61.2% for Cu, and 30.0–47.8% for Zn) in the soil-to-root transfers of these metals. Interestingly, essential and non-essential HMs exhibited different transfer patterns under the GF4 treatment. The root-to-stem transfer of Cd was also significantly inhibited (by 29.3–39.8%, p < 0.05) and its stem-to-grain transfer remained largely unchanged (p > 0.05) under the GF4 treatment, while the root-to-stem transfers of Cu and Zn kept relatively constant (p > 0.05) and their stem-to-grain transfers all increased (by 22.0–173.3%, p < 0.05), which fortified the essential trace elements in grain, causing the better performance of Fe⁰ nanoplates in Cd blocking. Additionally, the immobilization of soil HMs by Fe⁰ nanoplates persisted for approximately 60 days, contributing to the sustained efficacy of the GF4 treatment. These findings highlight the potential of nZVI, particularly applied at the grain-filling stage, to effectively mitigate HM accumulation in rice grains and improve crop safety in polluted environments.