Cadmium isotope fractionation during leaching with nitrilotriacetic acid

Abstract

This study aimed to determine the leaching mechanism of cadmium (Cd) from contaminated soil by nitrilotriacetic acid (NTA). 114Cd/110Cd isotope technology was adopted as the main characterization method, and experiments were conducted with a typical high-Cd soil to determine the speciation distribution of Cd, isotopic composition, and changes in bonding environment before and after leaching. The speciation distribution of Cd in the soil before leaching in order from most to least was exchangeable Cd, iron‑manganese oxide Cd, residual Cd, carbonate-bound Cd, and organic fraction Cd. The removal efficiency of exchangeable Cd was the highest, followed by iron‑manganese oxide Cd and carbonate-bound Cd. Residual Cd and organic fraction Cd content did not significantly change. Cd(NTA)− and Cd(NTA)24− were the major species of Cd in the leaching reagent, which was enriched in the heavy isotope 114Cd and had a strong bonding environment. The isotope fractionation coefficient α decreased as the proportion of Cd remaining in the soil decreased. Cd speciation distribution had an important influence on the 114Cd/110Cd fractionation degree in the leaching process. The Cd isotope composition in soil (from −0.12‰ ~ 0.04‰ to −0.35‰ ~ −0.18‰ without buffer and − 0.28‰ ~ −0.09‰ with buffer) can reflect the change of Cd speciation distribution, which determines the characteristics of the bonding environment and, in turn, determines the efficiency of NTA leaching to remove soil Cd. As such, isotope fractionation characteristics can be a significant theoretical support related to NTA leaching Cd contaminated soil.