Red mud amendments reduce cadmium mobility in paddy soil and limit cadmium accumulation in rice grains: A mechanistic investigation

Abstract

Background and aims

Red mud, an alkaline byproduct of aluminum smelting, can serve as a soil amendment to mitigate soil cadmium contamination. Rice uptakes cadmium mainly during the grain-filling stage after soils are drained. During the flooding-drainage process, substantial fluctuations in redox potential occur, and the effectiveness of red mud amendment under these conditions remains unresolved. The aim is to test the effects of red mud on soil cadmium mobility after soil drainage and subsequent rice cadmium accumulation.

Methods

Soil cadmium fractions and release kinetics were assessed by sequential extraction and stirred-flow experiments followed by soil microcosm incubation with slightly cadmium-contaminated paddy soil (0.52 mg Cd kg⁻¹), respectively. Rice cadmium accumulation was assessed in a potted rice experiment.

Results

Incorporating up to 2% red mud enhanced the proportions of cadmium bound to iron and manganese oxides from 12.5–28.1% to 25.1–44.0%, thus effectively reducing the mobility of cadmium in paddy soil after soil drainage. As a result, cadmium contents were decreased by 72.5–76.9% and 84.7–91.2% in the rice grains and straw, respectively.

Conclusions

Applying 2% red mud enhanced the soil iron and manganese oxide-cadmium proportions after soil drainage, decreasing the soil cadmium release maximum and rate, limiting cadmium uptake and subsequent accumulation into the rice grain to a level permissible for human ingestion. The stability of iron minerals impacts soil cadmium adsorption. These findings provide a scientific basis for using red mud in paddy soil cadmium remediation.