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

## TL;DR

Researchers tested nanoparticles to reduce cadmium in cocoa soils and beans, finding partial success in immobilizing and removing the toxic metal.

## Contribution

A novel in-field prototype produced multicomponent nanoparticles using rainwater and commercial chemicals to address cadmium contamination in cocoa-growing regions.

## Key findings

- Field application of nanoparticles achieved 22% cadmium immobilization and 20% removal during cocoa bean fermentation.
- Nanoparticles made in the field had an average size of 76.6 nm and formed chain-like aggregates.
- Fixed-bed column tests showed higher immobilization (80%) compared to field results.

## Abstract

Some areas of tropical soils where cocoa grows contain high cadmium (Cd) concentrations. The cocoa plant's need for nutrition causes the reticular system to uptake the toxic metal, translocate it, and accumulate it in roots, stems, and other edible parts such as cocoa beans and shells, threatening the health of cocoa consumers. To cope with this difficulty, different treatments have been applied to cadmium-contaminated soils, but they showed limited success. In this study, we prepared multicomponent nanoparticles (MCNPs) to treat cocoa soils in fixed-bed columns and field tests. Also, MCNPs were mixed with two varieties of cocoa beans, CNN51 and Fino de Aromain, during the fermentation, aiming to capture the cadmium. Our field investigations began by collecting soil samples from three Ecuadorian cocoa-producing farms to determine their physicochemical properties, cadmium, and iron contents. A few weeks later, a home-built prototype was installed in a cocoa plantation to fabricate the nanomaterials using commercial-grade chemicals and rainwater. Scanning Electron Microscope (SEM) images showed MCNPs with an average size of 65.21 nm and the formation of chain-like aggregates. In contrast, MCNPs size was 76.6 nm, measured with Dispersed Light Scattering (DLS). The chemical composition of MCNPs was 90.7 % Fe0 and 1.81 % sulfur (S), analyzed by energy dispersal X-ray (EDX) and confirmed by X-ray diffraction (XRD) spectrometer measurements. Regarding the treatments, for the fixed-bed column tests, 0.28–1.08 L/kg was dosed into the soil, while for field treatments, 250, 500, and 835 mL/min MCNPs were injected in soil areas of 5 m2 surrounding each cocoa tree (0.5–4.5 mg MCNPs/kg soil) at depths between 0 and 5 cm. Test results revealed that the procedure applied in the field did not reproduce the metal's immobilization (∼22 %) as in the fixed-bed columns (∼80 %). Moreover, cadmium removal was ∼20 % and ∼75 % after treating CCN51 and Fino de Aroma cocoa beans with 0.112 kg MCNPs/kg in the fermentation on-site and 0.034 kg MCNPs/kg in the laboratory. In this study, the size and chain-like aggregates of MCNPs notably influenced the field treatments as they precluded the filtration of MCNPs downwards. However, immobilizing cadmium could be more effective in soils with higher concentrations of the metal and multicomponent nanoparticles of smaller size, expanding our research scope and offering a practical solution to pollution issues in cocoa-growing regions.

•The home-built and field-operated prototype can supply large-scale multicomponent nanoparticles to immobilize cadmium in on-site applications.•Preparing in-field nanoparticles using rainwater and commercial-grade chemicals yielded nanoelements with average diameters of 76.6 nm and lengths of a few microns.•The injection of multicomponent nanoparticles in parcels of cocoa-producing soils and during the fermentation of cocoa beans resulted in 22 % immobilization and 20 % removal of cadmium.

The home-built and field-operated prototype can supply large-scale multicomponent nanoparticles to immobilize cadmium in on-site applications.

Preparing in-field nanoparticles using rainwater and commercial-grade chemicals yielded nanoelements with average diameters of 76.6 nm and lengths of a few microns.

The injection of multicomponent nanoparticles in parcels of cocoa-producing soils and during the fermentation of cocoa beans resulted in 22 % immobilization and 20 % removal of cadmium.