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We investigated local structures around cadmium (Cd) sorbed on montmorillonite at various pH values by EXAFS (extend X-ray absorption fine structure). We found two sorption types, an outer-sphere complex and a surface precipitate. The outer-sphere complex was mainly formed at a lower pH which had an identical structure to aqueous Cd 2þ , while the surface precipitate was found at high pH which had a one-layerCadmium (Cd) is a toxic heavy metal that results in significant pollution of many soil and groundwater systems. Furtherworse, Cd is known to bioaccumulate. Since the mobility and bioavailability of Cd in polluted systems are strongly related to its sorption structures, it is important to understand the sorption structures on clay minerals which are main soil components. Montmorillonite is a dioctahedral smectite mineral that is prevalent in soil environments and has a high surface area and cationexchange capacity (CEC). Over past several decades, macroscopic investigation on the sorption mechanisms of metals on montmorillonite presented many thermodynamic models such as ''surface complexation models'' to describe metal sorption reactions at the solid-water interface.1 The models explained the pH-dependent sorption properties on the montmorillonite. 2-4Outer-sphere complex is mainly formed on permanent charge sites on the interlayer of montmorillonite at low pH values while inner-sphere complex formation occurs at high pH values, where the metals are fixed to deprotonated surface hydroxides on the edge of montmorillonite. However, we have to determine the molecular structures of the sorbed species directly in order to reveal the real sorption mechanism. Recent studies using EXAFS (extend X-ray absorption fine structure) have shown that heavy metals are sorbed on clay and oxide surfaces accompanied by the formation of multinuclear or polynuclear surface complexes (i.e., surface precipitate) much more frequently than previously expected based on the thermodynamical models. 5-9 These microscopic studies have promoted our understanding of the real sorption mechanism. EXAFS analyses of Cd species on the montmorillonite, however, have never been reported in spite of its importance for soil pollution mainly because the K-edge EXAFS requires X-rays with higher energy than 23 keV. We carried out K-edge Cd EXAFS measurements for Cd sorbed on montmorillonite in a vertical Wiggler beam line in SPring-8 and Photon Factory beam line, which provide a high energy X-ray. EXAFS provides only averaged local structure information for all coexisting metal species having the same absorption edge. We varied the pH values in order to change the sorption species continuously and applied a regression analysis to quantitatively evaluate the composition of each sorption structure.Cd was sorbed on Na-montmorillonite (Kunipia-F, Kunimine Industries Co.) using a batch technique at several different pH values. Cd(NO 3 ) 2 (0.002 mol L À1 ) and NaNO 3 (0.004 mol L À1 ) solutions were applied and the reaction time was 24 h (295 AE 1 K). The p...

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In order to better understand the influence of microtopography on the accumulation of radiocesium in a flood channel, we surveyed a secondary tributary of the Abukuma River, the watershed of which was highly contaminated with radiocesium. Fifteen surface soil samples were collected using 5 cm soil core samplers, and their radiocesium concentration, dry density, and grain size distribution were analyzed. Furthermore, the elevation of the soil sampling points was surveyed using a real-time kinematic global positioning system (RTK-GPS). The radiocesium concentrations ranged from 1.46×10 3 to 3.43×10 4 Bq·kg -1. Signifi cant positive and negative correlations were observed between elevation and radiocesium concentration, and between elevation and dry density, respectively. In addition, the proportion of the fine particle size fraction positively correlated with elevation. One possible reason for this observation is as follows: A large amount of radiocesium in particulate form runs off into the river from the watershed when the river swells and the fl ood channel is inundated. The sedimentation/accumulation of suspended particulate materials is facilitated on a riverbed with a higher elevation for the inundated area. On the other land, particulate materials rich in radiocesium tends to be fl ushed away from a riverbed at a lower elevation owing to a higher chance of contact with river water.

Desorption of Cd adsorbed on Montmorillonite (Mt), Pyrophyllite (Py), Hydroalminum montmorillonite (HyA-Mt) is observed by means of QMS. The desorption peak temperatures of Cd are 730°C from Mt, 820°C from Py and beyond 900°C from HyA Mt, respectively. Cadmium absorbs on the side surface of the octahedral sheet for Mt and Py. However the desorption peak temperature of Cd from Py is higher than Mt because of the structure of Py, which is more thermaly stable than the structure of Mt. For HyA-Mt, Cd adsorbs mainly on the Al(OH) 3 layer. Desorption peak temperature of Cd from Al(OH) 3 layer is higher than that from the side surface of the crystal.