Redox-driven dissolution of bulk cadmium sulfide triggers in-root immobilization as cadmium–phosphate nanocrystals

Cadmium sulfide, a sparingly soluble product of cadmium immobilization in flooded paddy soils, is generally considered environmentally inert. Here, we show that cadmium sulfide transforms under flooded soil–rice conditions, releasing labile cadmium and increasing its bioavailability. In pot experiments, cadmium sulfide increased dissolved and extractable cadmium, altered bulk pore-water geochemistry, activated iron-regulated transporters, enhanced cadmium uptake, and elevated grain cadmium above food-safety limits. Within roots, the remobilized cadmium was partly retained on cell walls and partly immobilized through phosphate-driven intracellular complexation and precipitation, forming amorphous cadmium–phosphorus–oxygen phases and crystalline cadmium–phosphate nanocrystals. Density functional theory calculations indicate that cadmium promotes phosphate-cluster nucleation and growth rather than binding preferentially to organic ligands. These findings reveal a soil-to-root transfer pathway in which phosphate-mediated immobilization provides only a partial sink and does not offset increased cadmium influx, highlighting the long-term food-safety risks of sulfide-based cadmium immobilization in flooded paddy soils. Within rice roots, cadmium is partly immobilized by cell-wall retention and phosphate-driven formation of cadmium-phosphate phases, but this sequestration does not offset the increased cadmium influx, based on sampling analysis in Minhou County, China.