The variation of REE (rare earth elements) patterns in soil-grown plants: a new proxy for the source of rare earth elements and silicon in plants

Rare earth elements (REEs) in five species of soil-grown plants (Taxodium japonicum, Populus sieboldii, Sasa nipponica, Thea sinensis and Vicia villosa) and in the soil on which each plant grew were determined with an inductively coupled plasma mass spectrometer (ICP-MS) in order to observe the variation in the distribution of REEs and to elucidate their source in soil-grown plants. The plant samples were divided into root (secondary root and main root), trunk (stem) and leaf; the soils into water soluble (soil_{soluble fraction}), HCl and HNO₃ soluble (soil_{non-silicate fraction}) and HF soluble (soil_{silicate fraction}). The REE abundances of samples were compared using REE patterns where the abundances were normalized to those of a chondrite and plotted on a logarithmic scale against the atomic number. All the plants showed similar REE patterns independent of species and location, and a W-shape variation (W-type tetrad effect) and abundance depletion of cerium (negative Ce anomaly) were found in each REE patterns of plants, more conspicuous tetrad effect being observed in HREE (heavier rare earth elements) region than in LREE (lighter rare earth elements) region. The overall variation of REE patterns of each secondary root was not similar to that of soil_{soluble fraction}, but similar to that of soil_{silicate fraction} except for the tetrad effect and Ce anomaly. The REE patterns can be interpreted by the idea that plants of different species take in REEs and Si from different parts in the soil. The results of this study seem to imply that Sasa nipponica and Vicia villosa take in free REEs and Si rather directly from silicate in the soil, and that a majority of REEs and Si in Taxodium japonicum and Thea sinensis are originated from the soluble fraction in the soil.     

Accumulation of rare earth elements in maize plants (Zea mays L.) after application of mixtures of rare earth elements and lanthanum

Rare earth elements are applied in China to improve crop production, and the distribution patterns of individual rare earth elements in native plants have widely been reported. But our knowledge is still limited about the dose-dependent accumulation of individual rare earth elements in agricultural crops after application of rare earth elements. Effects of lanthanum and mixtures of rare earth elements were studied in pot experiments on the accumulation of individual rare earth elements in maize plants. All plant samples were divided into plant tops and roots. On addition of mixtures of rare earth elements and lanthanum to the soil, a significant dose-dependent accumulation of individual rare earth element(s) was found in the roots and in the plant tops. Application of mixtures of rare earth elements at >10 mg kg^–1 soil, resulted in a significant increase in contents of light rare earth elements in the roots, and at a dose of 50 mg kg^–1 soil, a similar phenomenon was found in the plant tops. When mixtures of rare earth elements were replaced by lanthanum alone, at a dose higher than 10 mg La kg^–1 soil, a significant increase in La content occurred in the roots and in the plant tops. The content ratio of La to Ce in maize plants appeared to increase as the application doses of rare earth element(s) increased. At a highest dose (50 mg kg^–1soil), the transport of the absorbed La from the roots to the plant tops might be substantially reduced after treatment with lanthanum alone, compared with mixtures of rare earth elements. Increasing the application doses of rare earth element(s) appeared to cause a positive Gd and negative Ce anomaly in the roots and in the plant tops, and the anomaly was more obvious in the plant tops than in the roots. The results indicated that the Gd and Ce anomaly in corns might be considered as important parameters for the safety assessment of agricultural application of rare earth elements.     

Fractionations of rare earth elements in plants and their conceptive model

Fractionations of rare earth elements (REEs) and their mechanisms in soybean were studied through application of exogenous mixed REEs under hydroponic conditions. Significant enrichment of middle REEs (MREEs) and heavy REEs (HREEs) was observed in plant roots and leaves respectively, with slight fractionation between light REEs (LREEs) and HREEs in stems. Moreover, the tetrad effect was observed in these organs. Investigations into REE speciation in roots and in the xylem sap using X-ray absorption spectroscopy (XAS) and nanometer-sized TiO2 adsorption techniques, associated with other controlled experiments, demonstrated that REE fractionations should be dominated by fixation mechanism in roots caused by cell wall absorption and phosphate precipitation, and by the combined effects of fixation mechanism and transport mechanism in aboveground parts caused by solution complexation by intrinsic organic ligands. A conceptive model was established for REE fractionations in plants based on the above studies.     

Fractionations of rare earth elements in plants and their conceptive model

Fractionations of rare earth elements (REEs) and their mechanisms in soybean were studied through application of exogenous mixed REEs under hydroponic conditions. Significant enrichment of middle REEs (MREEs) and heavy REEs (HREEs) was observed in plant roots and leaves respectively, with slight fractionation between light REEs (LREEs) and HREEs in stems. Moreover, the tetrad effect was observed in these organs. Investigations into REE speciation in roots and in the xylem sap using X-ray absorption spectroscopy (XAS) and nanometer-sized TiO₂ adsorption techniques, associated with other controlled experiments, demonstrated that REE fractionations should be dominated by fixation mechanism in roots caused by cell wall absorption and phosphate precipitation, and by the combined effects of fixation mechanism and transport mechanism in aboveground parts caused by solution complexation by intrinsic organic ligands. A conceptive model was established for REE fractionations in plants based on the above studies.     

稀土元素对农田生态系统的影响研究进展

稀土矿的开采和冶炼、稀土农用等导致农田土壤稀土元素含量不断积累,对农田生态系统结构和功能稳定产生严重的影响。综述了近20年来国内外农田生态系统稀土元素的主要来源、分配和输出,土壤和植物中稀土元素的测定方法,稀土元素对农田生态系统中植物、微生物、动物以及人类健康影响的研究进展。探讨了农田生态系统稀土元素的毒性评价和稀土污染土壤的修复措施。最后提出开展稀土元素对农田生态系统影响研究还需要加强的一些问题。     

有机配体对稀土元素在小麦体内积累和分异的影响

采用营养液培养和添加外源混合稀土等方法,研究了有机配体柠檬酸、EDTA和DTPA对稀土元素在小麦的根和叶中积累与分异的影响。结果表明,低浓度有机配体对小麦根和叶中的稀土元素,尤其是轻稀土元素的积累有轻微的促进作用,随浓度的升高则表现出显著的降低作用。有机配体对重稀土的作用比轻稀土强,使根和叶中稀土元素的分布曲线向重稀土相对亏缺的方向发展。3种配体对轻、重稀土分异的作用强度为:EDTA>DTPA>柠檬酸。通过VM INTEQ计算表明,在EDTA和DTPA作用下小麦叶中稀土元素的积累与轻、重稀土的分异主要由溶液中呈自由离子态稀土元素的含量和组成控制;柠檬酸作用下小麦叶中稀土元素的变化与自由离子态稀土的含量和组成关系较弱,推测REE-柠檬酸络合物可被小麦直接吸收并运转到小麦的叶中。     

南方稀土采矿恢复地土壤稀土元素含量及植物吸收特征

通过野外取样调查和室内ICP-MS测定,研究了福建省长汀县稀土矿治理地土壤和主要植物中稀土元素的含量、分布以及转移特征.结果表明:稀土矿治理地的土壤养分含量较低;土壤中稀土元素的含量为507.40～841.37 mg· kg-1,高于对照地土壤的含量.稀土矿治理地土壤中稀土元素主要为交换态,其含量占总量的61％～98％.稀土矿治理地土壤中稀土元素从原来偏单一的交换态转变为多种形态共存,其中有机态含量显著升高.植物根、茎、叶中稀土元素含量分别为40.27～986.01、5.14 ～ 206.58、6.81 ～ 2364.51mg·kg-1.稀土元素在植物各器官中含量水平除芒萁为叶＞根＞茎,其他植物均表现为根＞叶＞茎.根据不同植物吸收和积累稀土元素的差异,可将矿区治理地植物分为富集型和根部囤积型植物.芒萁属于富集型植物,桉树、高羊茅、宽叶雀稗、木荷和油茶属于根部囤积型植物.     

稀土元素对红霉素发酵的影响

通过对红霉素发酵培养基中添加稀土元素的研究,确定了几种能对发酵效价有提高作用的稀土元素及其浓度,当其中镧La^3＋、钕Nd^3＋和铈Ce^4＋离子浓度分别为50mg／L、50mg/L和100mg／L时对提高红霉素效价水平最显著,提高了32％、25％和25％,并且对改善红霉素组分也有明显作用,红霉素A组分相对百分含量分别提高18．9％、32．7％和34．4％,红霉素B组分分别减少24．1％、58．6％和62．1％。     

稀土元素在赣南非稀土矿区和不同稀土矿区土壤-铁芒萁(Dicranopteris linearis)系统中的分布、累积和迁移

ICP-M法测定了江西赣南地区非稀土矿区和4处不同稀土矿区内,土壤-铁芒萁系统中15个稀土元素的含量,并对稀土元素在土壤剖面层及铁芒萁植物体内的分布、迁移特征进行了研究.结果表明,稀土元素总量在土壤剖面层的底土层含量最高,但表土层铈相对富集.稀土元素在铁芒萁植物体内的分布规律是叶、根＞茎＞叶柄.铁芒萁根中稀土元素的丰度与其母土表土层,尤其是母土表土层可溶态稀土元素的分布模式基本相似.稀土元素在铁芒萁体内的迁移过程中,发生了明显的分馏作用,茎、叶柄、叶中的重稀土相对贫乏.     

稀土元素在小麦体内分配行为的研究

采用水培,土培试验及中子活化分析技术,在作物生长效应曲线研究的基础上,系统地研究了稀土远征顷作物体内的含量、吸收、分布和转移等行为。所获结果表明,名稀土元素在作物体现人的分配行为受生物的内外因素与稀土来源、自身特征和元素间关系的影响,是作物稀土元素分配行为已有研究成果的重要补充与深化,并为土壤施用稀土元素提供促进一步的科学依据。     

低剂量混合稀土积累对黄褐土微生物主要类群的生态效应

采用田间小区试验和室内低剂量模拟叠加试验相结合的方法,研究低剂量混合稀土在黄褐土中积累对土壤微生物主要类群的生态效应．结果表明,低剂量稀土的持续积累对土壤细菌、放线菌产生刺激、抑制、再刺激的交替作用;对真菌也产生类似的作用,但抑制作用不显著,而刺激作用持续、明显．混合稀土对3类土壤微生物数量抑制程度顺序为：细菌>放线菌>霉菌．稀土积累至150mg·kg^-1时,土壤各类微生物的种群结构均发生显著的改变,耐稀土微生物数量大幅度增加,细菌中的G^-细菌、链霉菌的白孢类群、真菌中青霉分别成为优势种群．对低浓度稀土积累的田问土壤微生物学参数模拟计算结果表明,稀土对土壤细菌、放线菌和真菌的EC50(半抑制浓度)值分别为24．1、41．6～73．8和55．3～150．1mg·kg^-1,30mg·kg^-1值可以初步确定为稀土在黄褐土中积累的安全临界值．     

Rare earth elements (REEs) in PM10 and associated health risk from the polymetallic mining region of Nandan County,China

A total of 26 samples were collected from Dachang, Chehe and Liuzhai in Nandan County, China, in order to investigate the concentrations of rare earth elements (REEs) and associated health risk in particulate matter. The concentrations of REEs in Dachang (23.54 ng/m³), Chehe (20.29 ng/m³) were significantly higher than those in Liuzhai (8.1 ng/m³). The light rare earth elements (LREEs) account for 87.08%, 87.09%, and 86.17% of the total REE burden in PM10 at Dachang, Chehe, and Liuzhai, respectively, indicating that the distribution pattern of REEs in PM10 was characterized by the obvious fractionation of LREEs. Enrichment factor calculation indicated that EFs for La, Ce, Nd, Pr, and Nd in PM10 from Dachang and Chehe were greater than 2, indicating moderate enrichment. Source identification indicated that REEs in PM10 from Dacheng and Chehe originated from anthropogenic activities such as mining and smelting activities, while REEs in PM10 from Liuzhai were associated with natural sources, like soil erosion. Noncarcinogenic and carcinogenic risks associated with the exposure of REEs in PM10 were negligible based on the health risk assessment models. However, greater noncarcinogenic risk for children was found in studied areas compared with the adults.     

Rare earth elements in soil and plant systems - A review

The rare earth elements (REEs) form a chemically uniform group and include yttrium (Y), lanthanum (La) and the lanthanides cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Their average abundance in the Earth’s crust range from 66μg g⁻¹ in Ce to 0.5μg g⁻¹ in Tm and ?0.1μg g⁻¹ in Pm. Recent great improvements in more routine analytical technique, the use of REEs as fertilisers, at least in East Asian agriculture, and the importance of these elements as indicators in both pedological and physiological processes and reactions have contributed to an increased interest in these previously less considered elements in environmental sciences. This review of recent and current literature deals with REEs in primary and secondary soil minerals, concentrations in surface soils, factors influencing adsorption, solubility and transport in soils, including weathering and transformations of REE minerals, and vertical distribution in soil profiles. Reviewed and discussed are also concentrations, distribution and localisation of REEs in plants and plant organs, soil-plant relationships and interactions, effects on plant growth and crop production and their importance in plant physiology and biochemistry. The REEs are found, usually several elements together, as phosphates, carbonates and silicate minerals finely dispersed especially in magmatic and metamorphic rocks. REE concentrations in surface soils of humid climates, such as the A(E)-horizons of Podzols and Laterites, are usually lower than in the parent material, due to higher weathering and leaching rates than of the average soil constituents. Some fractionation may occur due to the formation of more element-specific secondary minerals. Transfer from soil to plant is usually low, but extreme accumulators are found, e.g., among several species of ferns. Roots have generally higher concentrations than shoots. Possible uptakemechanisms of REEs are discussed. Uptake is positively, though often weakly, correlated with soil acidity and easily soluble concentrations of the elements, but rarely well related to their total concentrations in the soil. Under certain conditions, low concentrations of at least some REEs seem to favour plant growth and productivity, but the physiological mechanisms are still not well understood. Some considerations concerning the boundary between essential and non-essential micro nutrients are discussed.     

Contents and the biogeochemical characteristics of rare earth elements in wheat seeds

Contents of fifteen rare earth elements (REEs) in the seeds ofsixty breeds of wheat collected from seed bank were measured byinductively coupled plasma mass spectrometry (ICP-MS). The distributionpatterns of contents of REEs in wheat seeds (n = 58) wereobserved and compared with their average level in soils (n =364). Differences among regions and between spring and winter wheat weretested. Comparison with literature data was also made. The results showthat the content of REEs in wheat seed ranges between10^–11 g · g^–1 and10^–8 g · g^–1, 3–4 orderof magnitudes lower than that in soils. The distribution patterns arethat light REEs enriched and the Eu-anomaly is weak, similar to the soilcase. No obvious differences were found among different regions andbreeds. The data obtained in this study represent the contents of thefifteen REEs in wheat seeds.     

Bioaccessibility and health risk assessment of rare earth elements in Porphyra seaweed species

In this study, the content and bioaccessibility of rare earth elements (REEs) in Porphyra spp. Samples before and after thermal treatments were investigated. Additionally, the risks of REEs exposure to human health were assessed. REEs content significantly reduced after thermal processing (P < 0.01), and the removal rate of REEs was approximately 30%. Thermal treatment increased REEs bioaccessibility from 44% to 64.34%. The concentration and bioaccessibility of Ce, La, Y, Nd were high in raw and thermally treated Porphyra samples, and there was no correlation between REEs content and bioaccessibility. Based on the following parameters: highest content of REEs in the studied seaweed samples (13.45 mg/kg), the highest daily seafood consumption (44.9 g/day), and the highest bioaccessibility (64.34%), the ratio of the calculated daily intake (DI) to daily allowable intake by diet (DAI_diet) of REEs did not exceed the reference value in rare earth mining areas or under extreme conditions. The DI via seafood consumption would be exceeded when the content of REEs in the seafood sample is greater than 15.77 mg/kg. In this study, the concentration of REEs did not exceed 15.77 mg/kg in any sample. Thus, the human health risks of REEs associated with seafood are low.     

Distribution of rare earth elements in seaweed: implication of two different sources of rare earth elements and silicon in seaweed

Rare earth elements (REEs) and Si in five species of seaweed, ambient surface seawaters, and suspended solid particles in the seawaters were determined separately. Inductively coupled plasma mass spectrometry (ICP-MS) was used for REEs and inductively coupled plasma emission spectrometry (ICP-ES) was used for Si in order to evaluate REEs as a tracer in seaweeds and to understand the source of inorganic elements, especially Si, in seaweeds. Two different REE patterns, one similar to that of the seawater solution and another resembling that of suspended particles, were observed in seaweeds, and the variation of REE patterns seems to show a clear dependence on the abundance of Si. The REE pattern and Si concentration seem to vary depending on the division: green and red algae showed REE patterns similar to that of suspended particles, but brown algae showed patterns closer to that of seawater solutions and relatively lower Si concentration. The possibility of contamination from silicate particles on the surface of seaweeds was ruled out for several reasons. Silicate particles, not dissolved silicate, have been identified as the direct source of REEs and Si in plants ( Fu et al. 1998 ), and seaweeds are no exception. We have to consider that seaweeds can take up Si from suspended particles through their blade or branches. From the appearance of tetrad-effect-like variation of REEs, Si is assumed to enter a dissolved state just before the particles are taken up. From the results of a sonication experiment, REEs, once taken up as silicate particles, seem to be separated from Si in the thallus.     

Effects of citric acid and the siderophore desferrioxamine B (DFO-B) on the mobility of germanium and rare earth elements in soil and uptake in Phalaris arundinacea

Effects of citric acid and desferrioxamine B (DFO-B) on the availability of Ge and selected rare earth elements (REEs) (La, Nd, Gd, Er) to Phalaris arundinacea were investigated. A soil dissolution experiment was conducted to elucidate the effect of citric acid and DFO-B at different concentrations (1 and 10 mmol L^?1 citric acid) on the release of Ge and REEs from soil. In a greenhouse, plants of P. arundinacea were cultivated on soil and on sand cultures to investigate the effects of citric acid and DFO-B on the uptake of Ge and REEs by the plants. Addition of 10 mmol L^?1 citric acid significantly enhanced desorption of Ge and REEs from soil and uptake into soil-grown plants. Applying DFO-B enhanced the dissolution and the uptake of REEs, while no effect on Ge was observed. In sand cultures, the presence of citric acid and DFO-B significantly decreased the uptake of Ge and REEs, indicating a discrimination of the formed complexes during uptake. This study clearly indicates that citric acid and the microbial siderophore DFO-B may enhance phytoextraction of Ge and REEs due to the formation of soluble complexes that increase the migration of elements in the rhizosphere.     

Rare earth elements,aluminium and silicon distribution in the fern Dicranopteris linearis revealed by μPIXE Maia analysis

BackgroundThe fern Dicranopteris linearis is a hyperaccumulator of rare earth elements (REEs), aluminium (Al) and silicon (Si). However, the physiological mechanisms of tissue-level tolerance of high concentrations of REE and Al, and possible interactions with Si, are currently incompletely known.MethodsA particle-induced X-ray emission (μPIXE) microprobe with the Maia detector, scanning electron microscopy with energy-dispersive spectroscopy and chemical speciation modelling were used to decipher the localization and biochemistry of REEs, Al and Si in D. linearis during uptake, translocation and sequestration processes.ResultsIn the roots >80 % of REEs and Al were in apoplastic fractions, among which the REEs were most significantly co-localized with Si and phosphorus (P) in the epidermis. In the xylem sap, REEs were nearly 100 % present as REEH₃SiO₄²⁺, without significant differences between the REEs, while 24–45 % of Al was present as Al-citrate and only 1.7–16 % Al was present as AlH₃SiO₄²⁺. In the pinnules, REEs were mainly concentrated in necrotic lesions and in the epidermis, and REEs and Al were possibly co-deposited within phytoliths (SiO₂). Different REEs had similar spatial localizations in the epidermis and exodermis of roots, the necrosis, veins and epidermis of pinnae of D. linearis.ConclusionsWe posit that Si plays a critical role in REE and Al tolerance within the root apoplast, transport within the vascular bundle and sequestration within the blade of D. linearis.