Tracing cadmium in the environment: Advances and perspectives from stable isotope techniques.

Cadmium (Cd) is a highly mobile and toxic trace metal whose environmental behavior is governed by coupled source inputs, geochemical transformations, and biological processes. Traditional assessments based on total concentrations or operationally defined fractionation have provided valuable insights but often fail to resolve these interacting controls. Recent advances in isotope geochemistry provide a powerful framework to overcome these limitations. Stable isotope dilution enables quantitative assessment of bioavailable Cd pools; isotope labeling allows direct tracing of Cd uptake and redistribution in soil-plant and aquatic systems; and Cd isotope fractionation provides mechanistic constraints on source attribution and biogeochemical pathways. This review synthesizes recent progress in these three isotope-based approaches, critically evaluates their strengths and limitations, and highlights emerging perspectives for advancing process-resolved understanding of Cd behavior across environmental compartments.