Oxidation of Cr（Ⅲ） by manganese oxides may pose a potential threat to environments due to the formation of toxic Cr（Ⅵ） species. At present, it was still unclear whether the extent of Cr（Ⅲ） oxidation and fate of Cr（Ⅵ） would be changed when manganese oxides co-exist with other minerals, the case commonly occurring in soils. This study investigated the influence of goethite and kaolinite on Cr（Ⅲ） oxidation by birnessite under acidic p H condition（p H 3.5）and background electrolyte of 0.01 mol/L Na Cl. Goethite was found not to affect Cr（Ⅲ）oxidation, which was interpreted as the result of overwhelming adsorption of cationic Cr（Ⅲ） onto the negatively-charged birnessite（point of zero charge（PZC） 〈 3.0） rather than the positively-charged goethite（PZC = 8.8）. However, more Cr（Ⅵ） would be retained by the surface with the increase in addition of goethite because of its strong ability on adsorption of Cr（Ⅵ） at low p H. Moreover, either Cr（Ⅲ） oxidation or distribution of the generated Cr（Ⅵ）between the solid and solution phases was not affected by kaolinite（PZC 〈 3.0）, indicating its low affinity for Cr species. Reactions occurring in the present mixed systems were suggested, which could be partly representative of those in the soils and further indicates that the mobility and risk of Cr（Ⅵ） would be decreased if goethite was present.
The environment of estuarine wetlands has been attracting worldwide attention. To study the spatial distribution of pollutants in the tidal flats of the Yangtze Estuary, Southeast China, the Eastern Tidal Flat of Chongming Island (EC) and the Jiuduansha Shoal (JS) of the estuary were selected as the study sites. At each of the two sites, a cross-transect from land to sea was established and topsoil and soil core samples in the cross-transect were collected spatially and seasonally to determine their contents of heavy metals (Cu, Zn, Pb, Cd, Cr, Ni, Mn, and Fe) and grain-size characteristics. The results showed that the heavy metal loads were commonly higher in the soils of nearshore high tidal flats and had a tendency of decreasing from land to sea at both of the study sites. The contents of heavy metals in the soils of the high and medial tidal flats were mostly higher in April and November but lower in July. Corresponding spatial and seasonal variations in grain size of the intertidal soils were also observed at the two study sites. The soils in the nearshore high tidal flats were finer and gradually got coarser seawards; they were relatively finer in April and November but coarser in July. Furthermore, the contents of heavy metals in the intertidal soils of both the sites EC and JS were significantly positively correlated with the clay (<2 μm) and 2-20 μm fractions, but negatively with the sand (>63 μm) and 20-63 μm fractions, which suggested that the heavy metals in the intertidal soils were primarily combined with the fine particulate fraction (<20 μm), especially clay, and hence the spatial and seasonal variations in heavy metals were actually caused by the change of the grain-size characteristics of the intertidal soils due to the different sedimentary environments in the estuary. The results of this study may also contribute to a better understanding of the soil formation and classification in the tidal flats of the Yangtze Estuary.
HU Xue-FengDU YanFENG Jian-WeiFANG Sheng-QiongGAO Xiao-JiangXU Shi-Yuan
