Nanoparticles for treatment of cadmium-contaminated cocoa-growing soils and beans: Performance on metal immobilization and removal

Overview

This study evaluated the use of multicomponent nanoparticles (MCNPs) composed of a zero-valent iron (Fe0) core coated with iron sulfide (FeS) as a treatment for cadmium-contaminated cocoa-growing soils and for cadmium removal from cocoa beans during fermentation. The work was conducted in Ecuador using soils and beans from three farms in the Manabi and Pichincha provinces. A transportable field prototype was designed and built for large-scale nanoparticle fabrication on cocoa plantations.

This is a soil and bean treatment study, not a finished-food contamination survey. The paper reports no data on cadmium in processed cocoa products, chocolate, or consumer-facing goods. Its relevance to the wiki is as context for uptake and remediation pathways: it characterizes baseline cadmium in Ecuadorian cocoa soils (0.45 to 2.32 mg/kg) and in unprocessed cocoa beans (0.45 to 1.7 mg/kg), and it quantifies the performance gap between laboratory-scale and field-scale nanoparticle treatments for cadmium immobilization and removal.

The central finding is that MCNPs achieved approximately 80% cadmium immobilization in laboratory fixed-bed columns but only approximately 22% in field plots, and approximately 69 to 81% cadmium removal from beans in laboratory fermentation but only approximately 20% in field fermentation. The authors attribute the lab-to-field gap to larger nanoparticle size when fabricated on-site (76.6 nm vs 17.2 nm in the lab), chain-like aggregation that prevented soil infiltration, heterogeneous clayey soil texture, and intensive rainfall at the field site.

Key numbers

Baseline soil cadmium

All soils exceed the mean cadmium (Cd) concentration in non-polluted European agricultural soils (0.3 mg/kg) and the median soil Cd in the USA (0.2 mg/kg).

Baseline bean cadmium

• Fino de Aroma (Flavio Alfaro, lab fermentation): 1.7 mg/kg
• CCN51 (Flavio Alfaro, lab fermentation): 1.3 mg/kg
• Fino de Aroma (Santa Lucia, field fermentation): 0.46 mg/kg
• CCN51 (Santa Lucia, field fermentation): 0.45 mg/kg

EU maximum limit for Cd in ground cocoa: 0.6 mg/kg. Codex Alimentarius limit for chocolate with >70% cocoa: 1.0 mg/kg.

Cadmium immobilization in soils (fixed-bed columns, 14 mL of 400 mg/L MCNPs)

• S001 exchangeable fraction (F1): decreased from 26.7% to 5.2% (80.5% immobilization)
• S001 carbonate-bound fraction (F2): decreased from 22.2% to 4.2% (approximately 81% immobilization)
• S002 exchangeable fraction (F1): decreased from 27.4% to 5.1% (approximately 81% immobilization)
• S002 carbonate-bound fraction (F2): decreased from 21.7% to 4.3% (80.2% immobilization)
• Residual fraction (F5) increased: 94.4% for S001 and 95.8% for S002

Cadmium immobilization in field plots (Santa Lucia, 835 mL/min, 5 m² plots)

• After 1 day: approximately 22% immobilization (F1 decreased from 5.48% to 3.50%)
• After 15 days: immobilization decreased to 12%

Cadmium removal from beans during laboratory fermentation

Optimal laboratory dose: 3 mL MCNPs per 100 g beans (30 mL/kg), p < 0.05 for both varieties.

Cadmium removal from beans during field fermentation (Santa Lucia)

• Fino de Aroma at 100 mL MCNPs/kg beans: approximately 20% removal
• CCN51 at 100 mL MCNPs/kg beans: approximately 19% removal
• Lower doses (40 and 60 mL/kg) produced lower removal percentages (8 to 15%)

Nanoparticle characterization

• Field-prepared MCNPs: 76.6 ± 27.11 nm (DLS), 65.21 ± 16.12 nm (SEM); Fe0 90.7%, S 1.81% (EDX)
• Lab-prepared MCNPs: 17.2 nm average diameter; Fe 61.50%, S 2.48%, C 5.91%, O 30.11% (EDX)
• Aggregate size at dropper outlet during field tests: 166 ± 111 nm

Evidence fitness

EF-4, Context only. This paper is a soil remediation and bean treatment experiment, not a food contamination survey. It provides no occurrence data for cadmium in finished cocoa products or consumer goods. The baseline bean Cd values (0.45 to 1.7 mg/kg) are useful as contextual data points for Ecuadorian cocoa, but they come from a small, purposively selected set of farms chosen for their Cd contamination and cannot be treated as representative of the Ecuadorian cocoa supply. The study’s primary contribution to the wiki is documenting the performance and limitations of nanoparticle-based remediation at field scale, which informs the mitigation literature rather than the contamination profile.

Methods

Analytical instruments: Atomic absorption spectrometry with graphite furnace (PerkinElmer AAnalyst 800) and ICP-OES (ThermoScientific iCAP 7400) for cadmium and iron in soils and beans.

Sample digestion: Soils were subjected to Tessier sequential extraction (five fractions: exchangeable, carbonate-bound, Fe/Mn oxide-bound, organic/sulfide-bound, residual) or aqua regia digestion. Cocoa beans were digested with 6 mL of 1M HNO₃ and 2 mL of 1M HClO₄ at 140 °C for 180 min.

LOD: 0.002 mg/kg for Cd in soils; 0.01 mg/kg for Cd in cocoa beans.

LOQ: Not reported.

CRM/recovery: Certified reference material Metals in Soil RTC SQC001-30G ([CdCl₂] = 0.025 to 0.1%) used for soil QC. Bean QC used locally prepared spiked standards (50, 250, and 500 µg/kg Cd); recovery percentages were calculated but specific values are not reported in the main text.

Statistical analysis: ANOVA with Shapiro-Wilk normality test; Tukey (parametric) and Kruskal-Wallis/Dunn (non-parametric) post-hoc tests; significance level α = 0.05.

Nanoparticle characterization: DLS (HORIBA LB-550), FEG-SEM (Tescan Mira3), TEM (Tecnai G2 Spirit TWIN), EDX (Bruker Nano GmbH Quantax), AFM, XRD (Empyrean PANalytical).

Implications

Certification: This study does not supply occurrence data usable for setting or calibrating HMTc cocoa thresholds. Its value is as mitigation context: it documents a substantial gap between laboratory and field efficacy for nanoparticle soil treatment (approximately 80% vs approximately 22% Cd immobilization) and for bean treatment during fermentation (approximately 69 to 81% vs approximately 20% removal). This reinforces the broader pattern in the cocoa Cd mitigation literature that laboratory results overestimate field performance, and that supply-chain screening remains more reliable than post-harvest remediation for achieving regulatory compliance.

Courses: The lab-to-field performance gap is a strong teaching case for the “mitigation options” module on cocoa cadmium. The nanoparticle aggregation problem (76.6 nm field vs 17.2 nm lab, with 166 nm aggregates at the dropper outlet) illustrates why scalability claims from bench-scale experiments require field validation. The comparison table (Table 9 in the paper) placing MCNPs alongside biochar, lime, and compost treatments is useful for contextualizing the full range of soil amendment strategies.

Provenance notes

Published in Heliyon (Elsevier), available online 2 December 2024, print volume 11 (2025). Licensed under CC BY 4.0 (confirmed in the article footer: “This is an open access article under the CC BY license”). Funded by SENESCYT Ecuador (Grant PIC-18-INE-ESPE-004). No competing interests declared by the authors. Deposited into this wiki on 2026-11-10; DOI: 10.1016/j.heliyon.2024.e40519; PMCID: PMC11889560.

Related evidence

• Cocoa
• Cadmium
• Mitigation of Cadmium Accumulation in Cocoa Beans Using Arbuscular Mycorrhizal Fungi, Biochar and Callisia repens (AMF, biochar, and Callisia repens mitigation for cocoa Cd in Peru)
• Induction of rhizobium japonicum in the fermentative mass of two varieties of cacao (Theobroma Cacao L.) as a strategy for the decrease of cadmium (fermentation-based Cd reduction)
• ClimaLOCA project: Fostering innovations for cadmium reduction in cocoa beans in Latin America (Cd in cocoa systems)
• Cd and lead (Pb) in cocoa beans: Occurrence and effects of chocolate processing (Cd and Pb in cocoa processing)

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