采用室内水培和静态吸附实验, 研究了水生植物少根紫萍(Landoltia punctata)活体和干粉对水体中U(Ⅵ)的吸附能力, 并对作用过程和机理进行了初步分析。 结果表明: 常温下少根紫萍2.5 g·L-1(FW)活体和1.25 g·L-1(DW)干粉在pH 5下对5 mg·L-1U(Ⅵ)溶液的去除率分别可达78.70%和95.55%。 活体和干粉对U(Ⅵ)的吸附率随pH升高先增大后减小, 在pH 4～5时达到最大, 并随投加量的增加而增大; 随U(Ⅵ)初始浓度增加先增大后减小; 在作用5 min时, 活体和干粉对水体中U(Ⅵ)的吸附率分别为13.90%和79.97%, 在24 h时吸附率均达90%以上, 吸附逐渐趋于平衡。 当U(Ⅵ)初始浓度增加至250 mg·L-1, 活体和干粉对U(Ⅵ)的吸附量分别达到4.05 mg·g-1(FW)和131.76 mg·g-1(DW), 相比Langmuir模型, Freundlich吸附等温方程能较好地描述少根紫萍对U(Ⅵ)的吸附行为, 吸附过程符合准二级吸附动力学方程, r均在0.99以上。 FTIR分析结果表明: 少根紫萍表面含有羟基、 羧基、 氨基、 磷酸基等多种活性基团; SEM-EDS表明少根紫萍活体与水体中U(Ⅵ)作用48 h后, 大量片状无机磷酸铀晶体在其根系表面生成, 结晶主要由P, O, U元素组成, 不含C, 其中P和U的质量百分比分别为8.76%和82.53%, 原子百分比分别为25.19%和30.89%, 而对照组P的质量百分比和原子百分比仅为0.24%和0.11%, 干粉未观察到类似晶体存在。 XPS分析结果表明: 活体吸附后, 部分U(Ⅵ)被还原为U(Ⅳ), 而干粉吸附的铀主要以U(Ⅵ)形式存在。 由此推断, 少根紫萍干粉对U(Ⅵ)的吸附主要通过静电吸引, 离子交换, 络合配位等方式实现; 活体对U(Ⅵ)吸附的同时还存在还原和矿化的过程, 在U(Ⅵ)胁迫下活体根系表面会分解释放出无机磷酸根, 与吸附的U(Ⅵ)及部分被还原的U(Ⅳ)结合矿化为难溶的氢铀云母。

The biosorption and biomineralization characteristics of uranium by the duckweed Landoltia punctata was investigated in aqueous solutions enriched with 1 to 250 mg·L-1 of U(Ⅵ) supplied as uranyl nitrate ［UO2(NO3)2·6H2O］. The maximum uranium removal for the plant cultivar occurred at pH 4～5 of solution and their uranium removal efficiencies exceeded 90% after 24 h. In kinetics studies, the dried powder of duckweed can finished nearly 80% adsorption within 5 min, the batch adsorption equilibrium can be reached within 24 h for the living and dried powder of duckweed, Both for the living and dried powder of duckweed, the experimental data were well fitted by the pseudo-second-order rate model with the degree of fitting (r) higher than 0.99. The adsorption isotherms could be better described by the Freundlich model than the Langmuir model. In addition, Fourier transform infrared spectroscopy (FTIR) revealed that the surface of Landoltia punctata possess many active groups such as hydroxyl, carboxyl, phosphate and amide groups, the hydroxyl, amino groups involved in adsorption of U(Ⅵ) by living and dried powder of Landoltia punctata, and the phosphate groups also participated in the adsorption behavior of U(Ⅵ) by the living Landoltia punctata. The living Landoltia punctata reduction part of U(Ⅵ) to U(Ⅳ) was observed by XPS analysis. SEM and energy dispersive X-ray spectroscopy (EDS) of duckweed from 10～200 mg·L-1 uranium treatments indeed showed root surface of living Landoltia punctata formed a significant portion of U precipitates with nanometer sized schistose structures that consisted primarily U and P, not containing C. Inorganic phosphate was released by the root cells of Landoltia punctata during the experiments providing ligands for formation of insoluble U(Ⅵ) and U(Ⅳ) phosphates. The distinct uranium peaks in the EDS spectra of the cluster on the root surface can be observed after biosorption and the uranium and phosphorus mass ratio of the cluster spot was measured to be 82.5% and 8.76% of the total component weight, respectively, and the atomic percentage of 30.89% and 25.19%, respectively. It is worth noting that the phosphorus mass ratio and the atomic rate of the control group is only 0.24% and 0.11%, respectively. But there was no similar crystals observed on the surface of dried powder of Landoltia punctata after biosorption. The present work suggests that living and dried powder of Landoltia punctata can remove more than 90% U(Ⅵ) from solution simultaneously precipitated together with phosphate by the living Landoltia punctata, and the dried powder of Landoltia punctata adsorption U(Ⅵ) is mainly through the effect of electrostatic attraction, ion exchange and complexation coordination, etc. Here, for the first time, the presence of U immobilization mechanisms within one aquatic plant is reported using Landoltia punctata.