In situ Fe oxide synthesis for field-scale cadmium stabilization in heterogeneous vadose zone soils: Roles of Fe/Cd stoichiometry and repetitive synthesis.

In situ iron (Fe) oxide synthesis is an established approach, while the field-scale application to the cadmium (Cd) contamination in the vadose zone soils beneath an operating facility remains to be verified. The single and repetitive synthesis was examined in sand (SandCd), Cd-spiked soil (SoilCd), field soils (SoilField; Soils A-E), and a horizontal injection well demonstration. In the SandCd, the transition from surface adsorption to sequestration occurred between the Fe/CdSand molar ratios of 30 and 60, with the sequestered fraction increasing from 8.74% to 73.2%. In the SoilField, the Cd speciation and the Fe availability superseded the stoichiometric Fe/Cd control. The leachability was reduced by at least 99.2-99.8% where the ion-exchangeable Cd prevailed and remained at the untreated level where the Fe and Mn oxide-bound Cd dominated. The repetitive synthesis increased the sequestered fraction from 9.25% to 31.1% in the SoilCd, while the SoilField exhibited three responses reflecting the soil pH buffering capacity, cation exchange capacity, and clay content. At the field scale, the single synthesis stabilized the Cd where the reagent delivery zone, with the down-gradient groundwater at background levels, while the repetitive synthesis redistributed the Cd across the treatment zone. The reagent delivery governed the field-scale outcome.