Iron redox cycling coupled to transformation and immobilization of heavy metals: Implications for paddy rice safety in the red soil of South China

Red soil is an important soil resource, which bears substantial implication for sustainable development of agriculture and healthy growth of economy. However, the red soil in China has been deteriorating in recent years and facing many threats, such as soil erosion, acidification, and pollution. Among these, contamination of heavy metals, particularly arsenic and cadmium pollution in paddy soils of the red soil regions, has become a major environmental concern. In this paper, we reviewed recent publications on iron redox cycling and its coupling to the fate of heavy metals and metalloids. The most exciting findings on the iron biogeochemistry processes include dissimilatory iron reduction, Fe(II) oxidation, and Fe²⁺-catalyzed recrystallization of iron (hydro)oxides, all of which contribute to the immobilization of heavy metals. Although these findings are mainly based on laboratory experiments, they provide guidance for exploring innovative remediation strategies for controlling pollution of heavy metals in paddy soils of the red soil regions. We also summarized how the iron redox cycling may be affected by other biogeochemical processes or active constituents, such as the nitrogen cycling, the sulfur cycling and humic substances. It appears that the mechanisms underlying the interactions among these multiple components and processes are not sufficiently understood and may require further studies. Finally, future research needs pertaining to iron redox cycling coupled to the fate of heavy metals are suggested. The results summarized in this review may provide insights for solving the heavy metal pollution of paddy soils in the red soil regions.