In situ iron (Fe) oxide synthesis is an effective chemical precipitation technique for heavy metal stabilization in contaminated soils. However, surface-adsorbed metals remain vulnerable to remobilization under changing environmental conditions. Aqueous Cd stabilization experiments demonstrated that repetitive synthesis progressively reduced the surface-adsorbed Cd fraction from 68.7 ± 1.0% (1st synthesis) to 53.9 ± 2.6% (2nd synthesis) and 23.5 ± 0.8% (3rd synthesis), while single synthesis with equivalent Fe showed no such reduction. Aberration-corrected scanning transmission electron microscopy (Cs-TEM) with energy dispersive spectroscopy (EDS) line scanning revealed that surface-adsorbed Cd migrated from edge sites to bulk sites during repetitive synthesis, confirming physical encapsulation by newly formed Fe oxide layers. Fast Fourier Transform (FFT) analysis indicated lattice distortion in Fe oxide structures, suggesting simultaneous incorporation mechanisms. Stabilization of Cd in soil experiments revealed that stabilization efficiency strongly depended on the Fe/Cd molar ratio in soil solution, with ratios >400 achieving 99% reduction in Cd leachability through single synthesis via discrete Fe oxide formation, while lower ratios (Fe/Cd = 63) required repetitive synthesis to achieve enhanced stabilization through progressive encapsulation mechanisms. Field demonstration validated the practical applicability, achieving complete suppression of Cd leaching in most soil depths after two synthesis cycles. This study establishes repetitive in situ Fe oxide synthesis as a viable remediation strategy for Cd-contaminated soils.

Keywords: In situ Fe oxide repetitive synthesis, Surface adsorption, Sequestration, Incorporation, Encapsulation, Field demonstration