Suppressive effects of magnesium oxide materials on cadmium uptake and accumulation into rice grains: I: Characteristics of magnesium oxide materials for cadmium sorption

Abstract

The objective of this study is to assess the applicability of a commercial magnesium oxide (MgO) and a composite material containing MgO and natural minerals (‘MgO–SH–A’) as the soil amendments for suppression of cadmium (Cd) uptake and accumulation into rice grains. Firstly, the mineralogical and physicochemical properties, soil neutralizing capacities and Cd sorption characteristics of these materials were investigated. Both materials were strongly alkaline and possessed large surface areas. The X-ray diffraction pattern of MgO–SH–A indicated the presence of MgO and a magnesium-silicate mineral (antigorite) as the main components. MgO–SH–A showed a milder soil neutralizing capacity as compared to commercial MgO. The sorptions of Cd on commercial MgO and MgO–SH–A both fitted Langmuir isotherm. The maximum Cd sorption capacity of commercial MgO (46.8 mmol g −1 DW) was higher than that of MgO–SH–A (5.87 mmol g −1 DW), although the latter material showed higher affinity to Cd as compared to the former one. The dominant reaction involved in the Cd sorptions was suggested to be precipitation of Cd(OH) 2 on the material surface. About 40% of Cd sorbed on MgO–SH–A was resistant to desorption by 0.1 M HCl, implying that this portion was strongly retained on the material surface.