Remediation of cadmium-polluted weakly alkaline dryland soils using iron and manganese oxides for immobilized wheat uptake

Cadmium (Cd) contamination of arable soils seriously threatens the safety of food products and sustainable development of agriculture. Iron/manganese oxide-based amendments have been widely applied to remediate heavy metal polluted paddy soils. However, little is known about the performance and mechanism of single iron/manganese oxides and iron-manganese composite oxides in reducing Cd accumulation in wheat plants from dryland soils. Here, iron oxide (α-FeOOH), manganese oxide (δ-MnO2) and iron-manganese composite oxides (birnessite and weakly crystalline iron oxide) were prepared and used the addition of 0.1%, 0.5% and 1.0% to decrease the Cd accumulation in wheat plants cultivated in weakly alkaline soil through pot experiments. The results showed that iron and manganese oxides could reduce the content of available Cd by transforming acid-extractable Cd to reducible Cd and improve the antagonistic effect of wheat rhizosphere Cd with Fe and Mn in weakly alkaline soils, thereby significantly reducing Cd accumulation in wheat plants. The amendment performance followed the order of manganese oxide > iron-manganese composite oxides > iron oxide. The application of 1.0% manganese oxide decreased the available Cd in soils from 0.41 to 0.21 mg kg−1, and correspondingly decreased Cd content in wheat roots, straw and grains by 56.2%, 49.9% and 70.0%, respectively. More importantly, the Cd content in grains was decreased to below the limit set by the China's food safety standard (0.10 mg kg−1). The present work reveals the effect of Cd accumulation by iron and manganese oxides in wheat, and develops an efficient approach for amending Cd polluted weakly alkaline soils.