柠檬酸对Re(Ⅶ)在不同压实密度钠基和钙基膨润土中扩散的影响

Effect of Citric Acid and Compacted Density on Adsorption and Diffusion of Re(Ⅶ)in Na^+-and Ca^2+-bentonite

柠檬酸是自然界广泛存在的一种有机酸,可通过络合作用影响放射性核素的迁移。通过贯穿扩散法,研究了柠檬酸对Re(Ⅶ)在压实密度为1.40~1.80 g/cm^3的钠基和钙基膨润土中的扩散行为,得到了有效扩散系数De和岩石容量因子α等扩散系数。结果表明:Re(Ⅶ)在钠基和钙基膨润土中扩散的De和α等均随着压实膨润土密度ρ的增大而减小;与钙基膨润土相比较,Re(Ⅶ)在钠基膨润土中得到的扩散系数较小、吸附分配比较大,表明钠基膨润土的阻滞性能更好;同时加入柠檬酸后Re(Ⅶ)在钠基和钙基膨润土中的扩散系数和吸附分配比K d均增大。实验得到膨润土的零点电位点均小于7.0,表明膨润土呈电负性,不易吸附阴离子形式存在的Re(Ⅶ)。吸附试验表明加入柠檬酸后,钠基和钙基膨润土对Re(Ⅶ)的K d增大,并随着柠檬酸的浓度增加而增大。通过公式拟合得到Re(Ⅶ)在膨润土中扩散的De与有效孔隙率εacc均符合Archie定律,其胶结因子n分别为2.14和2.19。同时得到扩散的表观扩散系数Da与膨润土压实密度均呈指数关系,均符合核素在黏土中扩散的规律。

Citric acid is a type of organic acid widely existing in nature,and can affect the migration of radionuclides via complexation.The diffusion behavior of citric acid to Re(Ⅶ)in Na^+-and Ca^2+-bentonite in compacted densities of(1.40~1.80)g/cm^3 was studied by through-diffusion method,from which the effective diffusion coefficient(De)and rock capacity factor(α)were obtained.The results show that the D e andαof Re(Ⅶ)diffused in Na^+-and Ca^2+-bentonite decreased with increasing density of compacted bentonite.Compared with Ca^2+-bentonite,Re(VII)obtained from Na^+-bentonite has a smaller diffusion coefficient and a larger adsorption distribution,indicating that Na^+-bentonite has a better retarding performance.And the diffusion coefficient and adsorption distribution ratio(Kd)of Re(Ⅶ)in Na^+-and Ca^2+-bentonite increased after citric acid adding.The experimental results showed that the zero potential points of bentonites were all below 7.0,indicating that the bentonite was electronegative and not easy to absorb Re(Ⅶ)in anion form.Adsorption experiments showed that the K d of Na^+-and Ca^2+-bentonite to Re(Ⅶ)increased after citric acid adding,which increased with the increase of citric acid concentration.By formula fitting,the D e value and effective porosity(εacc)of Re(Ⅶ)diffused in bentonites fitted Archie's law very well,from which the cementation factors(n)obtained were 2.14 and 2.19,respectively.In addition,their apparent diffusion coefficients(Da)are exponentially related to the compacted density of bentonite(ρ).All of them accord the rule of nuclide diffusion in clay.