Distribution and Speciation of Arsenic and Iron around Iron Mottle and Rice Root in Paddy Soil.

Successive flooding and drying for rice cultivation induces reductive dissolution and precipitation of Fe minerals in paddy soil. As well as Fe, mobility of arsenic in soil is strongly affected by soil redox condition. Even when the soil matrix is under reductive condition, soil in the rhizosphere is oxic because oxygen is supplied through aerenchyma of rice stem and root. The gradient of redox condition from rhizosphere to soil matrix would affect the behavior of As and Fe. The purpose of this study is to investigate the effect of redox gradient around rice root on the distribution and speciation of As and Fe in paddy soil.    

Soil core samples (5 cmφ×30 cm) were taken from paddy field, embedded in epoxy resin and made into100 mm-thick sections. Micro-scale distribution patterns of As and Fe around iron mottle and root was determined by micro-beam X-ray fluorescence. Arsenic K-edge m-XANES and m-EXAFS were measured on spots where As accumulation were found. Iron mineral phases were identified by m-XRD.      

Roots were surrounded by orange or brown mottles.  Some mottles did not contain obvious roots; perhaps these roots had decomposed.  Arsenic was concentrated in the inner layer of iron mottle and root surface. As concentration was lower in the outer-layer of iron mottle than that in the inner-layer, though their Fe concentrations were similar. Iron coatings around roots tended to contain more As than found in the Fe aggregates associated with soil component. In the spots accumulating As, the major As species was As(V). The ratio of As(III) to As(V) increased with distance from the root surface. The inner layer of the iron mottle was composed of goethite or poorly-crystalline iron oxyhydroxide, whereas lepidocrocite was found in the outer layer. Compared to lepidocrocite, goethite and poorly-crystalline iron oxyhydroxide have higher adsorption capacity of As(V). Our results suggested that the inner layer of iron mottle and root surface tended to accumulate more As than in the outer-layer or Fe (oxy)hydroxides in soil matrix. This was partly because Fe minerals which have higher affinity for As was present, as well as higher ratio of As(V).