Effects of red mud application on rice growth and transformation of cadmium forms in Cd-contaminated paddy soils

A pot experiment was conducted to study the effects of application of different amount of red mud on rice yield, bioavailability and forms of cadmium in soils as well as cadmium concentrations in brown rice. The results indicate that the appropriate application of red mud can apparently promote rice growth and increase effective panicle number and grain yield. Compared to the control (RM-0) without red mud application, plant height, effective panicle number and grain yield in the 0.75% red mud (W/W) treatment (RM-3) are increased by 5.02%, 1.12% and 6.93%, respectively. With increase of red mud application, soil pH is increased, the exchangeable cadmium concentration is decreased, while the concentrations of carbonate-bound, iron- and manganese-oxides -bound and residual cadmium are increased, and the organic bound cadmium is not changed obviously. Compared to the RM-0 treatment, the soil exchangeable cadmium in the 1.25% (W/W) red mud treatment (RM-5) is decreased by 31.6% (P0.01), while the carbonate-bound, iron- and manganese-oxides-bound and residual cadmium are increased by 16.3%, 22.5% and 8.7%, respectively (P0.01). The cadmium concentration in brown rice is decreased with increase of red mud application, and the cadmium concentration meets the national food hygiene standards when the application amount is greater than or equal to 0.5% (W/W) red mud. Considering the grain yield, the effect of soil remediation and quality of brown rice comprehensively, the appropriate amount of red mud application is 0.75% (W/W) red mud in the studied cadmium contaminated paddy soil.