Functional effects of manganese and iron oxides on the dynamics of trace elements in soils with a special focus on arsenic and cadmium: A review

The dynamics and availability of arsenic (As), cadmium (Cd), and other trace elements in soils are of great concern to soil scientists. Owing to the human toxicity of As and Cd, the Codex Alimentarius Commission set maximum permissible concentrations of these elements in foodstuffs, including rice which is a major source of human intake of As and Cd, especially in Asia. Therefore, attenuation of As and Cd concentrations in crops, especially rice, is important for both human health and agroindustry.Manganese (Mn) and iron (Fe) oxides have great capacity to sorb trace elements. Adsorption of cationic elements increases with increasing pH, whereas adsorption of arsenate [As(V)] shows the opposite trend. Adsorption of cationic elements by Mn oxides is generally much greater than adsorption by Fe oxides, whereas Fe oxides have a much higher adsorption capacity for As than Mn oxides do. Trace elements can co-precipitate with Mn/Fe oxides and are subsequently released from or incorporated into solid phases with aging depending on the natures of elements and on the surrounding conditions. Moreover, Mn/Fe oxides, especially Mn oxides, oxidize arsenite [As(III)] to As(V), which is less soluble. Sorbed elements solubilize under reducing conditions through reductive dissolution of Mn/Fe oxides. In addition to the transformation of As from As(V) to As(III), the release of As from Fe oxides is responsible for the increase in dissolved As concentration that accompanies flooding. In contrast, Cd precipitates as barely soluble sulfides under strong reducing conditions. This difference may explain why the concentrations of As and Cd in rice grown with different water management show opposite trends. Iron plaque, an Fe oxide precipitate that forms on rice roots, may attenuate As concentrations in rice tissues by fixing and sequestering As, although some studies have questioned this possibility.There have been many attempts to use Mn/Fe oxides (or materials containing them) to attenuate the soil solubility of trace elements and/or reduce their uptake by crops. Application of Mn/Fe oxides has useful effects in most cases; in particular, application of Fe oxides or Fe oxide-containing materials immobilizes As in soils. Therefore, application of Mn/Fe oxides combined with flooded cultivation may attenuate the concentrations of both As and Cd in rice. However, additional studies are required because only a few studies have focused on the effects of application of Mn/Fe oxides on As mobility in anaerobic soils and consequent uptake by rice plants.