Distribution and forms of cadmium on the rhizosphere ofBrassica junceain Cd-contaminated soils and implications for phytoremediation

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Abstract
To investigate the chemical and positional availability of soil cadmium (Cd) absorbed by plants, the distribution and forms of Cd in the rhizosphere were analyzed by growing Brassica juncea L. in three artificially Cd-contaminated soils (two Fluvisols with 21.7 and 7.4 mg Cd kg−1 and an Andosol with 25.1 mg Cd kg−1 designated as FH, FL, and AH, respactively) using the rhizobox method. Mtar a 25-d growth period, plant samples were harrn vested to determine the total amount of Cd absorbed, Soil samples were also collected at a distance of every 2-mm from the root-accumulating central compartment (C.C.) for the determination of the Cd concentration in the water-soluble, exchangeable, inorganically and organically bound fractions using a sequential extraction method. The amount of Cd absorbed by the plants was 168, 31, and 12 µg pot−1 for the FH, FL, and AH soils, respactively, suggesting that the efficiency of Cd removal or phytoremediation in the Fluvisol was more than 10 times higher than that in the Andosol. In accordance with the plant uptake, a significant decrease in the content of soil Cd was observed in both inorganically and organi cally bound fractions in the Fluvisol and mostly in the organically bound fraction in the Andosol. The depletion was limited to a volume extending less than 10 mm from the C.C., and about 60% of the amount of Cd depleted was observed within 2 mm of the C.C., irrespective of the soils examined. The effective recovery rates were, therefore, 6.0, 3.2, and 0.6% for FH, FL, and AH, respectively, if only the soil within 10 mm from the C.C. was taken into account. The amounts of water-soluble and exchangeable Cd, in contrast, showed some increase, especially in the FH soil, suggesting a possible accumulation of readily available Cd in the rhizosphere in certain soil-plant combinations. These results suggest the importance of 1) selection of soil, 2) increase in the volume of the rhizosphere, and 3) minimization of the possible leaching loss of Cd, when efficient and environmentally sound phytoremediation is to be accomplished for Cd-contaminated soils.