Sepiolite-supported nanoscale zero-valent iron alleviates Cd&As accumulation in rice by reducing Cd&As bioavailability in paddy soil and promoting Cd&As sequestration in iron plaque

Abstract

Cadmium and arsenic (Cd&As) co-contaminated paddy soil is the main reason for excessive Cd&As in rice. However, the opposite chemical behaviors of Cd&As posed a challenge to the simultaneous remediation of Cd&As and the reduction of Cd&As content in rice. The synthesized sepiolite-supported nanoscale zero-valent iron (S-nZVI) exhibits exceptional efficacy in Cd&As adsorption. The effects of S-nZVI on Cd&As availability in soil and Cd&As accumulation in rice need to be further investigated. This study applied different amounts of S-nZVI (0.1%−0.6%) to paddy soils for rice cultivation. The findings revealed that the Cd&As concentrations in rice grains reduced by 46.18%− 68.13% and 24.26%− 57.16%, respectively, under 0.1%− 0.6% S-nZVI application. S-nZVI significantly decreased Cd&As concentrations (Cd: 8.23%−86.70%, As: 5.69%−82.03%) in pore water and transformed Cd&As chemical speciation to immobile fractions, reducing the Cd&As availability to rice. Moreover, S-nZVI facilitated iron plaque formation and Cd&As sequestration in iron plaques, inhibiting the absorption of Cd&As by rice roots. Notably, the lower application of S-nZVI (0.1%−0.2%) improved soil pH, more effectively inhibited rice uptake of soil Cd, and increased rice yield. The increased soil pH, decreased Eh, and surface complexation with iron oxyhydroxide were responsible for Cd immobilization, while the formation of inner-sphere chelates with iron oxyhydroxide predominated As immobilization. These findings suggest S-nZVI has the potential to be an effective material for remediating Cd&As co-contaminated paddy soil and alleviating Cd&As accumulation in rice.