Process-driven bioavailability of cadmium in acidic paddy soils and its transfer to rice grain: implications for food safety control

Abstract

Cadmium contamination in rice production systems represents a critical challenge for food safety and process sustainability, particularly in regions characterized by acidic soils. From a chemical engineering perspective, cadmium transfer from soil to grain can be understood as a chemically driven transport process controlled by soil pH, metal speciation, and competitive ion interactions. This study evaluated cadmium accumulation in rice cultivated and commercialized in the El Zulia region (Norte de Santander, Colombia) by analyzing its distribution across soil, paddy rice, and polished grain matrices. A composite sampling strategy was applied to representative mechanized rice farms and commercial rice products. Cadmium concentrations were determined by atomic absorption spectrometry, while key soil parameters governing metal mobility, including pH, iron, and manganese content, were characterized. Results revealed cadmium concentrations of up to 0.84 mg·kg⁻¹ in polished rice, exceeding the Codex Alimentarius maximum limit (0.4 mg·kg⁻¹) by more than 100%. Commercial rice samples also surpassed international regulatory thresholds, demonstrating that industrial processing steps such as dehusking and polishing are insufficient to mitigate contamination when cadmium originates at the production stage. The soil exhibited a strongly acidic pH (3.49) and elevated cadmium levels, conditions that promote the predominance of soluble Cd²⁺ species and suppress immobilization mechanisms. Despite high iron availability, no effective antagonistic control of cadmium uptake was observed, indicating that metal competition strategies collapse under extreme acidity. These findings confirm that cadmium contamination in rice is the result of a systemic process imbalance rather than isolated agronomic factors. The study highlights soil pH as the dominant control variable governing cadmium bioavailability and transfer. Effective mitigation therefore requires upstream process control strategies, such as chemical neutralization or immobilization at the soil level, rather than reliance on downstream post-harvest treatments. This process-oriented interpretation provides a technical basis for designing sustainable cadmium mitigation strategies in acidic rice production systems.