土壤中有机质镉的同位素组成特征:来自第一性原理计算的约束

Isotope Composition Characteristics of Cadmium in Soil Organic Matter:Constraints from First Principles Calculations

基于密度泛函理论(DFT)的第一性原理计算方法来精确量化有机质表面配合物(OSCs)和金属-有机框架(MOFs)形成过程中产生的镉(Cd)同位素平衡分馏。计算结果表明,25℃下OSCs和水溶液之间的Cd同位素平衡分馏较小且无明显的选择性,Δ114/110 Cd_(OSCs-Cd(aq))=-0.34‰~0.02‰。相反,Cd^(2+)一旦从OSCs转换为MOFs时,Cd同位素将产生极大的分馏,Δ114/110 Cd_(MOFs-Cd(aq))=-1.92‰~-0.29‰。此外,Cd的第一配位层中出现的N和S原子越多,MOFs与水溶液之间的Cd同位素平衡分馏越大。计算结果范围覆盖了实验测定的表层土壤Cd同位素分馏,表明土壤有机质中Cd的不同赋存状态控制着其同位素组成。最后,这些分馏数据丰富了Cd同位素数据库,便于学者们定量地使用Cd同位素手段来限定Cd的迁移转化过程。

In this study,first principles calculations based on density functional theory(DFT)were used to accurately quantify the equilibrium Cd isotope fractionations produced during the formation of organic surface complexes(OSCs)and metal-organic frameworks(MOFs).The results showed that the equilibrium Cd isotope fractionations between OSCs and the aqueous solution at 25℃were small and insensitive,i.e.,Δ114/110 Cd_(OSCs-Cd(aq))=-0.34‰to 0.02‰.In contrast,the formation of MOFs produced significant Cd isotope fractionations,i.e.,Δ114/110 Cd_(MOFs-Cd(aq))=-1.92‰ to-0.29‰.Additionally,the more N and S atoms appear in the first coordination layer of Cd,the greater the equilibrium fractionation of Cd isotopes between MOFs and aqueous solutions.Our calculation results cover the experimental determination of Cd isotope fractionation in surface soil,indicating that the different occurrence states of Cd in soil organic matter control its isotopic composition.Finally,these fractionation data enrich the Cd isotope database,making it easier for scholars to quantitatively use Cd isotope methods to limit the migration and transformation process of Cd.