Overview
A field study of common bean (Phaseolus vulgaris) and yellow corn (Zea mays) grown in Cotopaxi Province, Ecuador, under varying volcanic ash exposure conditions, finds no detectable accumulation of the principal toxic heavy metals in bean grain. Lead, cadmium, arsenic, and mercury were all at or below the instrument detection limit of approximately 0.01 mg/kg (10 ppb) on a dry weight basis under both volcanic ash exposure and non-exposure conditions. The study used ICP-OES on 0.5 g finely ground dry plant material digested with concentrated nitric acid and hydrogen peroxide in a microwave digestion system. Elemental analysis was outsourced to the Thermo Fisher Scientific 7400 Duo model spectrometer with yttrium internal standard at 5 ppm.
Key numbers
Phaseolus vulgaris grain, dry weight basis, by ICP-OES:
- Lead (Pb): not detected (below detection limit of approximately 0.01 mg/kg = 10 ppb dry wt)
- Cadmium (Cd): not detected (below detection limit of approximately 0.01 mg/kg = 10 ppb dry wt)
- Arsenic (As): not detected (below detection limit of approximately 0.01 mg/kg = 10 ppb dry wt)
- Mercury (Hg): not detected (below detection limit of approximately 0.01 mg/kg = 10 ppb dry wt)
The authors state: “No significant heavy metal accumulation was detected, indicating that the benefits of volcanic ash application can be harnessed without associated toxicity risks.” The conclusions confirm: “these metals were not detectable.”
For context, the same paper reports Figure 4 bar data for bean samples. Elements detectable above 0.01 mg/kg included: aluminum (6.22 mg/kg), bismuth (1.1 mg/kg), cobalt (5.06 mg/kg), chromium (6.52 mg/kg), copper (8.64 mg/kg), iron (68.30 mg/kg), boron (32.30 mg/kg), manganese, zinc, calcium (1005.83 mg/kg), magnesium (1910.48 mg/kg), sodium (119.11 mg/kg), and potassium (14,370.87 mg/kg). These are essential macrominerals and microminerals, not toxic heavy metals for HMI purposes.
The authors note that the corn sample showed Pb < 0.01 mg/kg, below the Codex STAN 193-1995 limit of 1 mg/kg for canned sweet corn, and state that “all the values identified in our samples are lower than those registered in CODEX STAN 193-1995 for different cereals and legumes.”
Methods (brief)
Field experiment: Phaseolus vulgaris and Zea mays grown in Cotopaxi Province under volcanic ash exposure (Stages 1 and 2) and in absence of ash. Mature fruits harvested and ground to flour; 0.5 g dry sample digested with concentrated HNO3, H2O2, and ultrapure water in a microwave digestion system. Resultant solutions filtered and diluted to 10 mL. ICP-OES analysis on Thermo Fisher Scientific 7400 Duo with yttrium internal standard. Calibration with multielement ICP Mix 33 standard (0.01 to 7.5 mg/L). Data visualization restricted to elements with concentrations greater than 0.001 mg/kg. No LOD/LOQ values reported individually per element; detection limit characterized as approximately 0.01 mg/kg from context. No certified reference material or recovery data reported. n = 3 per condition (triplicate measurements). Two-factor ANOVA, p < 0.05.
Limitation: Small sample size (triplicate per condition), single field season, single geographic site. ICP-OES is less sensitive than ICP-MS for trace toxic metals; stated detection limit of approximately 0.01 mg/kg (10 ppb) means values below this threshold are reported as non-detect rather than zero. The finding of non-detect is consistent with the broader literature showing commercial bean grain at low-single-digit ppb for Pb and Cd in non-contaminated settings.
Implications
The non-detectable result for toxic heavy metals in P. vulgaris grain at a volcanic soil site is contextually useful: volcanic ash from Cotopaxi contains arsenic, cadmium, lead, and mercury (acknowledged in the introduction), yet these did not accumulate detectably in grain under the conditions studied. This supports the general observation that most commercial bean growing regions produce grain with toxic heavy metals well below regulatory thresholds, though the analytical sensitivity here (ICP-OES at approximately 10 ppb detection limit) would not resolve differences in the low-ppb range that ICP-MS methods can detect.
For kidney beans specifically: this study covers P. vulgaris, the same species as most commercial kidney beans, under realistic field conditions in a volcanic soil environment. The absence of detectable toxic metals supports commodity-level context, though a single Ecuadorian site under volcanic ash conditions is not representative of global commercial production.
Related evidence
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