Equilibrium Cd isotopic fractionation between Cd(OH)_2(s), apatite, adsorbed Cd²⁺, and Cd²⁺_(aq): Potential application of δ¹¹⁴Cd in evaluating the effectiveness of Cd-contamination remediation

Abstract

Exacerbation of Cd contamination in soil is exerting a threat to food safety. Recently, Cd isotope geochemistry has been suggested for tracing the fate of this heavy metal in soil systems. However, its utility as a tool for evaluating the effectiveness of pollution remediation efforts has not been carefully considered. In the present work, first principle calculations based on density functional theory were employed to probe the most stable geometries of Cd-containing species including aqueous Cd²⁺, Cd(OH)_2(s), Cd²⁺ adsorbed on kaolinite edge surfaces, and Cd²⁺ incorporated into apatite lattice sites. Meanwhile, equilibrium Cd isotopic fractionations during conversions of free Cd²⁺ to immobile species were precisely quantified. At room temperature, equilibrium Cd isotopic fractionations between Cd(OH)_2(s), Cd²⁺ adsorbed on kaolinite edge surfaces, Cd²⁺ incorporated into apatite lattice sites (Ca1 and Ca2 sites), and aqueous Cd²⁺ were 0.58–0.64‰, –0.13‰, –0.88‰, and –0.72‰, respectively. During an in situ remediation, with the aid of these isotopic fractionation factors, we predicted a gradual decreasing trend in δ¹¹⁴Cd in soil with decreased fractions of mobile species (free Cd²⁺, exchangeable Cd²⁺, and non-specific adsorbed Cd²⁺). These results may provide a theoretical framework for future observations in field experiments.