Medium scale-up synthesis of nanomaghemite as an inhibitor of cadmium uptake in seedlings of Theobroma cacao L

The aim of this research was to show that the absorption of divalent cadmium ions (Cd²⁺) in Theobroma cacao L. seedlings can be controlled and adjusted by remediation with maghemite (γ-Fe₂O₃) nanoparticles (NPs), i.e., leading to the low tolerance limits established by the European Union (EU) in 2014 for Cd in products made from cocoa. The research had two stages; the first one involved the medium scale synthesis of 92 g of 15 nm γ-Fe₂O₃ NPs via room temperature chemical co-precipitation of ferrous salts and their characterizations using diverse physicochemical techniques. In the second stage, and for the first time, three doses of these γ-Fe₂O₃ NPs (1, 2, and 4 g) were applied to the substrate of cocoa seedlings of genotypes ICS 39, ICS 95, and CCN 51 for three months of exposure. A morphological analysis revealed non-significant toxic effects on the seedlings’ height, leaf area, and stem diameter among the genotypes and tested doses. The results of the atomic absorption analysis of the treatment with 4 g of γ-Fe₂O₃ NPs showed a concentration of 0.10 mg kg⁻¹ of Cd²⁺, a limit lower than the control and the recommended value by the 2014 EU regulation. Thus, the transport and/or absorption of the Cd²⁺ toxic metal were significantly inhibited, since up to 74 % of Cd²⁺ uptake in the genotype ICS 95 was experimentally observed. Also, the environmental Fe-dynamics in the seedlings and soils demonstrated no transport of γ-Fe₂O₃ NPs to the seedlings. Thus, the study found that the Cd²⁺ adsorption in remediated soil using γ-Fe₂O₃ NPs involves three steps: electrostatic exchange, Fe-oxide adsorption, and substrate-γ-Fe₂O₃ NPs complexation and precipitation.