1.State Key Laboratory of Soil and Sustainable Agriculture,Institute of Soil Science,Chinese Academy of Sciences;2.Institute of Soil Science Chinese Academy of Sciences
【Objective】The contents of background chromium (Cr) and manganese (Mn) oxides in latosols are high. The dissolution of Cr and subsequent oxidation of Cr(III) in the soils may occur to produce Cr(VI) with high toxicity, thus, posing a threat to the surrounding environment and humans. Therefore, this study aimed to study the dissolution of Cr and the oxidation of Cr(III) in latosols collected from Yunnan, Hainan, and Guangdong Provinces to understand the ecological and environmental risks associated with chromium in latosols.【Method】The dissolution of Cr was investigated using batch experiment , while both batch and incubation experiments were used to investigate the oxidation of Cr(III) in the soils. The concentration of Cr in extractants was determined by an atomic absorption spectrometer. Also, sodium hydrosulfite-sodium citrate-sodium bicarbonate (DCB) extraction and electron probe scanning were used to examine the forms of Cr in the soils.【Result】The results of DCB extraction and electron probe scanning showed that Cr in latosols was mainly combined with iron oxides and silicates. Acidification by hydrochloric acid and complexation by citric acid promoted the dissolution of Cr from the soils. However, even in 1 mol?L-1 hydrochloric acid, only 3.68% and 3.54% of total Cr was dissolved from the latosols of Hainan 3 and Guangdong 9, respectively, suggesting that Cr in latosols is stable and presents a low environmental risk. The oxidation of Cr(III) was observed in the soils during a 42-day incubation experiment. Based on the net production of Mn(II), Cr(VI) content, and organic matter content, it is probable that the Cr(VI) generated in the soils might be reduced to Cr(III) again by soil organic matter. The results of the batch experiment showed that the exogenous Cr(III) was oxidized to Cr(VI) in the latosols with organic matters removed, and the amount of Cr(III) oxidized was determined by the content of easily reducible Mn(III/IV) oxides in the soils. With the increase of suspension pH, the oxidized amount of Cr(III) increased firstly, reached the largest amount at pH 4.5 and then decreased. At this pH, only 1.48% of Cr(III) added was oxidized to Cr(VI). Although exogenous Cr(III) can be oxidized to Cr(VI) by soil Mn(III/IV) oxides in the latosols, the percentage of Cr(III) oxidized to Cr(VI) in the latosols was small, indicating that the risk of exogenous Cr(III) oxidization in the soils was very low.【Conclusion】Despite the high contents of background Cr and easily reducible Mn(III/IV) oxides in the latosols, the dissolution of Cr and oxidation of Cr(III) were difficult. This shows that under the experimental conditions, it was not easy to convert Cr(III) to Cr(VI) through oxidation reactions, therefore, the risk of background Cr in latosols is low.