Aging mechanism of copper added to bentonite
Aging mechanism of copper added to bentonite
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DOI：
10.1016/j.geoderma.2008.08.003
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发表时间：
2008-09
期刊：
影响因子：
6.1
通讯作者：
S. Zhou;Minggang Xu;Yibing Ma;Shibao Chen;D. Wei
中科院分区：
文献类型：
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作者：
S. Zhou;Minggang Xu;Yibing Ma;Shibao Chen;D. Wei
由于老化过程，土壤中金属的可萃取性、生物有效性和毒性随着时间的推移而下降。这些过程对土壤中金属的生态风险评估有重要影响。本研究采用连续萃取法研究了膨润土中铜的老化过程以及温度和pH对老化过程的影响；利用X射线衍射仪研究了不同pH条件下浸铜膨润土层间距的变化。连续提取的结果表明，最不稳定的铜形态(即铜)所占的比例。水溶态、硝酸铵浸出态和乙二胺四乙酸二乙酯萃取态的铜含量随培养时间的延长而降低，而L−1HCl浸提物和惰性组分(L−1HCl浸提态和残渣态)则表现出相反的趋势。添加铜的老化是一个缓慢的过程(一年以上)，土壤固体表面上易提取的铜物种逐渐转变为难提取的铜形态，并可能扩散到膨润土层间。实验数据符合抛物线扩散方程(R2=0.690-0.943，p<；0.0001)，表观扩散速率系数D/R2为6.43-20.0×10-−6d−-1，扩散活化能Ea为38.3-56.1kJ-−-1，表明微孔扩散可能是铜在膨润土中老化的主要机理。X-射线衍射结果表明，膨润土的层间距随pH的升高而减小，从pH 4.42的1.51 nm和pH 6.40的5.51、1.37 nm减小到7.68的1.26 nm。结果表明，在低pH(？lt；5.5)时，Cu2+是主要的扩散离子，而在较高的pH时，CuOH+是主要的扩散离子。在长期老化过程中，膨润土层间的铜离子可能会通过脱水扩散到粘土矿物的六角形空腔中。
The extractability, bioavailability and toxicity of metals in soils decline with time owing to aging processes. The processes have important impact on the ecological risk assessment of metals in soils. In this study, a sequential extraction procedure was used to investigate the aging processes of copper added to bentonite and the effects of temperature and pH on the aging processes; X-ray diffraction technique was used to study the changes of basal spacing of bentonite saturated with Cu at different pH. The results from the sequential extraction demonstrated that the proportions of the most labile Cu fractions (viz. water-soluble, NH4NO3extractable and EDTA extractable Cu) to total added Cu decreased with increasing incubation time, whereas the potentially labile fraction extracted by 0.5 mol L−1HCl and inert fractions (6 mol L−1HCl extractable Cu and residual Cu) exhibited the opposite trend. Also the aging of added Cu was a slow process (more than one year) where the easily extractable Cu species on soil solid surfaces gradually transformed into less extractable Cu forms which possibly diffused into interlayer of bentonite. The aging experimental data were well fitted using a parabolic diffusion equation (R2=0.690–0.943, p<0.0001) with apparent diffusion rate coefficients (D/r2) of 6.43–20.0×10−6d−1and activation energy of diffusion (Ea) of 38.3–56.1 kJ mol−1, indicating that micropore diffusion could be the main mechanism of aging processes of Cu added to bentonite. The results of X-ray diffraction showed that the basal spacing of bentonite decreased with increasing pH, which was from 1.51 nm at pH 4.42 and 5.51, 1.37 nm at pH 6.40, to 1.26 nm at pH 7.68. It was proposed that Cu2+was dominant diffusive ions at low pH (<5.5), while CuOH+became dominant at relatively high pH. During long-term aging, copper ions in interlayer of bentonite might diffuse into hexagonal cavities of clay mineral by dehydration.