Barraza et al. 2019 — Cadmium isotope fractionation in Ecuadorean soil-cacao system
Barraza et al. conducted a pilot study of Cd isotope fractionation across the soil-to-plant pathway in three cacao farms in Ecuador, one of the highest-Cd cacao-producing regions globally. Using multi-collector ICP-MS (MC-ICP-MS) to measure stable Cd isotope ratios (δ¹¹⁴/¹¹⁰Cd) in soil, cacao leaves, pod husks, and beans, the study demonstrates that Cd is isotopically fractionated during uptake and translocation in cacao, with lighter isotopes preferentially retained in beans relative to leaves. This isotopic fingerprinting approach can be used to trace the geogenic or anthropogenic source of Cd accumulating in cacao beans — particularly relevant for distinguishing volcanic soil Cd (geogenic) from phosphate fertilizer Cd (anthropogenic) as the source driving high Cd in Ecuadorean high-altitude cacao.
Key numbers
- 3 cacao farms in Ecuador (pilot study — small n)
- Bean Cd concentrations: range approximately 0.5–2.8 mg/kg dry weight across the 3 farms (values estimated from figures; exact table values require re-reading)
- Soil Cd: 0.3–1.5 mg/kg across farm types
- Isotope fractionation: lighter ¹¹⁰Cd enriched in beans vs. heavier isotopes preferentially in leaves
- δ¹¹⁴/¹¹⁰Cd in beans: approximately −0.2 to +0.3‰ relative to NIST SRM 3108
Methods (brief)
MC-ICP-MS for Cd isotope ratios; total Cd by quadrupole ICP-MS; 3 farms, Ecuador; plant tissues (leaves, pod husk, beans) and topsoil (0–30 cm); pilot study design.
Implications
Certification: Isotope tracing is a high-value forensic tool for supply chain audits — it can distinguish sources of Cd contamination and, in principle, identify falsely-labeled origin claims. Relevant to HMT&C source verification discussions. Courses: Mechanistic insight into why Ecuador high-altitude cacao has such high Cd — volcanic parent material plus possible phosphate fertilizer Cd amplification; isotopes can now distinguish these. App: Ecuador high-altitude provenance remains the highest Cd risk for cocoa-bean-derived ingredients. This study provides the geochemical framing.