土壤-超积累植物体系中稀土元素的空间富集-分异特征与富集机理

Spatial enrichment and fractionation of rare earth elements in soil-hyperaccumulator system and accumulating mechanism

离子吸附型稀土矿是我国南方特色的优质稀土资源,但由于落后的采矿方式和无节制的开采,大量稀土元素(稀土元素+Y,简称稀土元素)被带入环境中,导致稀土矿区周边出现许多环境污染和植被退化问题。乌毛蕨是一种对稀土元素具有较强耐受性和超强富集能力的蕨类超积累植物,能被用于稀土污染土壤或尾矿的生态修复。本研究以离子吸附型矿区表生土壤上生长的稀土超积累植物乌毛蕨及其根际土为研究对象,通过化学消解和ICP-MS方法测定根际土、根表、根部、叶柄、叶片中稀土元素的含量,分析土壤–植物体系中稀土元素的空间分布、富集与分异特征;采用顺序提取法测定土壤中不同化学形态的稀土元素含量,同时利用微区X射线荧光光谱(μ-XRF)与扫描电镜能谱分析技术(SEM-EDS),阐明乌毛蕨对稀土元素的吸收与富集机理。化学测试结果表明,乌毛蕨对稀土元素有较强的富集和地上转运能力,富集系数(BF)和转移系数(TF)分别为2.61和2.85;植物器官富集能力顺序为:叶片(1750μg/g)>根部(512μg/g)>叶柄(56.5μg/g);植株整体稀土元素配分模式具有与根际土相似的富轻土元素和Ce负异常特征,不同的是,植株整体Eu无异常(根际土δEu=0.51);根际土壤中富集离子交换态的轻稀土离子(34%~64%,(La/Yb)N=2.36),与根系表面呈轻微富轻稀土元素((La/Yb)N=1.29)的现象暗示根际大部分轻稀土元素能以离子形式被根系吸收,重稀土元素与有机配体络合而被吸附在根系表面;稀土元素在向上迁移过程中,(La/Yb)N值下降(叶柄=27.63,叶片=17.17),揭示木质部伤流液的重稀土元素可能比轻稀土元素更容易向上迁移;μ-XRF与SEM-EDS结果显示,乌毛蕨地上部的超富集器官(叶片)将稀土元素主要储存在叶缘的表皮层细胞,且能将大部分非生理需要的稀土元素有效区室化。上述认识揭示了在离子吸附型稀土矿区表生环境中,稀土元素从土壤向植物体内迁移并在地上部富集与分异的过程、规律和主要原因,为今后利用稀土超积累植物实现植物采矿或稀土尾矿生态修复提供科学认知基础。

Ion-adsorption rare earth deposits are high-quality rare earth mineral resources of South China.However,due to backward mining methods and uncontrolled mining,large amounts of rare earth elements(lanthanide,REE)have been brought into the environment,resulting in environmental pollution and vegetation degradation problems around rare earth mining areas.Blechnum Orientale is a fern hyperaccumulator with high tolerance and hyperaccumulation capacity of REE,which can be used for ecological restoration of rare earth contaminated soils or tailings.This study focuses on rare earth hyperaccumulation of Blechnum Orientale and the rhizosphere soil collected from the surface layer of an ion-adsorption rare earth deposit.Chemical digestion and ICP-MS methods were used to determine the concentration of REE in the rhizosphere soil,root surface absorption,root,stem,and leaf to analyze the spatial distribution,enrichment,and fractionation of REE.In addition,sequential extraction method was used to determine the REE concentrations of different chemical forms,and the Micro-X rays Fluorescence Spectrum(μ-XRF)and Scanning Electron Microscope-Energy Dispersive Spectrometry(SEM-EDS)were used to study the assimilation and accumulation mechanism of REE in the plant.The chemical analysis results indicate that Blechnum Orientale has significantly high REE accumulation and transportation ability,with a bioaccumulation factor(BF)of 2.61 and translocation factor(TF)of 2.85.The sequence of accumulation ability of plant organs is leaf(1750μg/g)>root(512μg/g)>stem(56.5μg/g).The whole plant shows REE patterns similar to those of the rhizosphere soil;both exhibit LREE-enriched REE patterns with negative Ce anomalies,except that those of the former show no Eu anomalies,whereas those of the latter show significant negative Eu anomalies(δEu=0.51).A major proportion of REE in the rhizosphere soil is ion exchangeable(34%-64%),which is characterized by LREE enrichment((La/Yb)N=2.36).However,REE absorption of the root surface is slightly less LREE enriched((La/Yb)N=1.29).Therefore,it is likely that the root can preferentially assimilate HREE ions in the rhizosphere soil through the surface absorption of organic-HREE complexes.The stem-to-leaf decrease in the(La/Yb)N value(stem=27.63,leave=17.17)reveals a preferential upward migration of HREE in xylem sap.Moreover,the results ofμ-XRF and SEM-EDS analyses showed that most REE are preserved in the epidermis cells of the leaf margins and could effectively compartmentalize most of the non-physiologically needed REE.This study revealed that the accumulation and fractionation processes,mechanisms of migration,enrichment and differentiation of REE from the rhizosphere soil to different parts of plants in the epigenetic environment of ionic adsorption rare earth deposits.These results provide a scientific basis for the future use of REE hyperaccumulators in phytomining or ecological restoration of rare earth tailings.