Fractionation characteristics of rare earth elements (REEs) linked with secondary Fe,Mn, and Al minerals in soils

Soil secondary minerals are important scavengers of rare earth elements (REEs) in soils and thus affect geochemical behavior and occurrence of REEs. The fractionation of REEs is a common geochemical phenomenon in soils but has received little attention, especially fractionation induced by secondary minerals. In this study, REEs (La to Lu and Y) associated with soil-abundant secondary minerals Fe-, Al-, and Mn-oxides in 196 soil samples were investigated to explore the fractionation and anomalies of REEs related to the minerals. The results show right-inclined chondrite-normalized REE patterns for La–Lu in soils subjected to total soil digestion and partial soil extraction. Light REEs (LREEs) enrichment features were negatively correlated with a Eu anomaly and positively correlated with a Ce anomaly. The fractionation between LREEs and heavy REEs (HREEs) was attributed to the high adsorption affinity of LREEs to secondary minerals and the preferred activation/leaching of HREEs. The substantial fractions of REEs in soils extracted by oxalate and Dithionite-Citrate-Bicarbonate buffer solutions were labile (10 %–30 %), which were similar to the mass fraction of Fe (10 %–20 %). Furthermore, Eu was found to be more mobile than the other REEs in the soils, whereas Ce was less mobile. These results add to our understanding of the distribution and geochemical behavior of REEs in soils, and also help to deduce the conditions of soil formation from REE fractionation.     

Arsenic mobility in weathered gold mine tailings under a low-organic soil cover

Historical gold mining operations in Nova Scotia, Canada, resulted in numerous deposits of publicly accessible, arsenic (As)-rich mine waste that has weathered in situ for 75–150 years, resulting in a wide range of As-bearing secondary minerals. The geochemical heterogeneity of this mine waste creates a challenge for identifying a single remediation approach that will limit As mobility. A 30-cm-thick, low-organic content soil cover was evaluated in a laboratory leaching experiment where, to simulate natural conditions, the equivalent of 2 years of synthetic rainwater was leached through each column and two dry seasons were incorporated into the leaching protocol. Each column was a stratigraphic representation of the four major tailings types found at the historical Montague and Goldenville gold mine districts: hardpan tailings, oxic tailings, wetland tailings, and high Ca tailings. Hardpan tailings released acidic, As-rich waters (max 12 mg/L) under the soil cover but this acidity was buffered by surrounding oxic tailings. Leachate from the oxic tailings was circumneutral, with average As concentrations between 4.4 and 9.7 mg/L throughout the experiment. The presence of carbonates in the high Ca tailings resulted in near-neutral to weakly alkaline leachate pH values and average As concentrations between 2.1 and 6.1 mg/L. Oxidation of sulfides in the wetland tailings led to acidic leachate over time and a decrease in As concentrations to values that were generally less than 1 mg/L. This study shows that the use of a low-organic content soil cover does not create reducing conditions that would destabilize oxidized, As-bearing secondary phases in these tailings. However, oxygen penetration through the cover during dry seasons would continue to release As to tailings pore waters via sulfide oxidation reactions.     

Using an integrated method for the determination of environmental TCLP arsenic for sulphide-rich mine tailing remediation in Ghana,West Africa

Toxicity characteristic leaching procedure (TCLP) is a versatile short-term leaching protocol used to estimate the release of toxic metals from waste prior to disposal to a repository. This paper uses the integrated TCLP leachate data from tailings, tailings dam monitoring borehole data, and acidity ratio (AR) range of sulphide-rich ores to simulate the environmental TCLP As in tailings at the AngloGold Ashanti Obuasi mine in Ghana. The aim was to incorporate long-term leaching characteristics of tailings to minimise the risk of TCLP As test failure. The mean As concentration and pH value are 2.26 mg/l and 5.7 in TCLP leachate, 0.35 mg/l and 6.7 in monitoring boreholes, and?<?0.01 mg/l and 5.7 in control boreholes, respectively. The evaluation of the TCLP As data using a one-sample t test performed at 80% confidence interval has the upper confidence limit (UCL) of 2.41 mg/l; this value which constitutes the short-term characterised environmental TCLP As is below the USEPA criterion of 5 mg/l and, therefore, qualifies the waste as safe for disposal. Alternatively, TCLP leachate, borehole and AR data were integrated to simulate the long-term environmental TCLP As of 2.40 mg/l and pH value of 5.7, and As concentration and pH value of 0.01 mg/l and 6.7 in monitoring boreholes, respectively. Such laboratory simulations of TCLP As leaching aimed at achieving 0.01 mg/l in field monitoring data would provide a more robust predictive value for environmental management decision making due to long-term considerations.     

Heavy metals and rare earth elements (REEs) in soil from the Nam Co Basin,Tibetan Plateau

Twenty-eight soil samples collected from the Nam Co Basin, Tibetan Plateau, have been analyzed for heavy metals (Cd, Cr, Ni, Cu, Zn, Pb and Mn), arsenic (As) and rare earth elements (REEs). In addition, for establishing the basic physio-chemical characteristics of the soil, pH, total organic concentration, electrical conductivity, and effective cation exchange capacity were measured. The results indicate that soil in the Nam Co Basin is still at an early alkaline weathering stage (pH = 7.94). Mean concentrations of heavy metals and As decreased as follows: Mn > Cr > Zn > Pb > Ni > Cu > As > Cd. The values of Cd, Cr, As, Pb and REEs in soil from the Nam Co Basin are higher relative to averaged background values over China. The chondrite-normalized diagrams of REEs suggest high REE(ΣLa → Eu)—enrichment, HREE(ΣGd → Lu)—depletion and Eu—depletion. Vertical profiles indicate that both heavy metals (except As) and REEs primarily exist in the surface soil. However, heavy metals and REEs vary in the surface soil between the southern and the northern bank of Nam Co. These differences are controlled mainly by parent materials.     

The study of soil acidification of paddy field influenced by acid mine drainage

The acidification of paddy fields was studied in Guizhou Province, China. Affected by acid mine drainage, the pH value of irrigation water was 2.9 with the concentrations of iron and aluminium above 40 mg/L. Based on the pH(H₂O) of topsoil, the paddy fields studied were classified spatially into three zones, the natural zone (pH value from 6.2 to 5.5), the acidified zone (pH value from 5.5 to 4.5), and the seriously acidified zone (pH value from 4.5 to 3.2), respectively. Comparing to the natural zone, the buffering processes for acidification of paddy soil were discussed by considering the changes of calcium, magnesium, potassium and aluminium in soils. The Ca, Mg and K were leached from the soil by the decomposition of carbonate and kaolinite. The leaching of Mg became less with the enrichment of iron in topsoil layer. When the soil pH was below 5.0, aluminium was leached from soil because of the dissolution of alumino silicate minerals. In addition, the hydrolysis of iron and aluminium in soil provided more protons to promote the soil acidification. Furthermore, the buffer capacity of paddy soil was discussed by the results of buffer experiment, based on which the pH buffer curve was drawn and the empirical formula for calculating the acidification rate was developed. Because pH buffer capacity of soil is about 2.78 cmol_c/kg pH for the pH(H₂O) value above 5.0, it is estimated that only another 50 years are needed for the pH(H₂O) of the paddy soil decrease to 3.5 in the acidified zone if the acid water is used for irrigation continuously.     

石灰土盐基离子迁移对模拟酸雨的响应

为揭示酸雨条件下石灰土盐基离子迁移特征及其缓冲性能,对石灰土进行了室内模拟酸雨淋溶试验,探讨酸雨酸度、土层厚度、土表覆被对石灰土酸化/石灰土抗酸性以及盐基离子迁移的影响。结果表明:（1）在pH=3.5、4.5、5.5的酸雨淋溶下,石灰土盐基离子迁移量差异不显著,对酸雨具有极强的缓冲能力。期间,Ca2+与Mg2+释放呈显著线性相关。（2）土壤厚度对石灰土抗酸性有极显著影响,土壤越厚,对酸雨的抗性越大,反之越小。（3）土厚比1∶2.5∶5的石灰土盐基离子淋失状况为K+淋失量1∶1.43∶2.06,Ca2+淋失量为1∶1.63∶3.13,Mg2+淋失量为1∶1.64∶3.15,表明土厚的增加能大大降低酸雨对盐基的淋溶。（4）石灰土土表覆盖不同,其盐基离子迁移淋溶量差异明显,但并不对土壤酸化效应产生显著影响。      

Interaction of organic acids and pH on multi-heavy metal extraction from alkaline and acid mine soils

Vegetation at mining sites can produce increased heavy metal leaching by the organic acids and protons originating from root secretion and litter degradation. Batch experiments were conducted to investigate the effects of organic acids and pH on the extraction of Pb, Cd, Zn and Cu from an alkaline mine soil (sampled from a mining site of Chenzhou City, Hunan Province) and an acid mine soil (sampled from a mining site of Daxin county, Guangxi Province). The results showed that in the presence of organic acids (acetic, oxalic, malic, fumaric, tartaric and citric acids) at pH 7, the extraction of Pb, Cd, Zn and Cu from the acid mine soil was much higher than that from the alkaline mine soil, in which only citric acid with higher concentration was capable of extracting some heavy metals. Citric acid had the strongest ability in extracting heavy metals, followed by oxalic acid. Heavy metal extraction dramatically decreased with increasing pH. Moreover, at low pH, oxalic acid promoted the risk of Cu leaching; at high pH, the leaching of Pb, Zn, Cd and Cu was enhanced by both oxalic and citric acids. This indicated that those plants, which can produce substantial citric acid or oxalic acid by root secretion and litter degradation, should not be selected for the revegetation of mining sites.     

模拟酸雨及施磷对水稻土中铅的淋溶特性影响

采用pH=3.0、pH=4.5与对照pH=5.6的3种模拟酸雨淋溶土柱的方法,研究酸雨及含磷酸雨淋溶下合肥市郊水稻土中铅的释放特征与规律。结果表明:经过相当于1 980 mm降水量的淋溶后,铅释放量总体上表现为随着酸雨强度增加累积淋溶量也增加的现象。酸雨作用下,施磷量越多土壤中铅的淋失总量越大。在酸雨及施磷条件下,大兴地区黄褐土中Pb较义城地区水稻土中易于淋失。     

Reducing leachable petroleum hydrocarbon concentration in weathered fuel oil contaminated soil by chemical oxidation with hydrogen peroxide

Number 6 fuel oil is one of the most used energy sources for electricity generation. However, leaks can contaminate soil and also groundwater due to leaching. At old sites, the oil may have low toxicity but still contaminate groundwater with foul-tasting compounds even at low concentrations. The purpose of this study was to evaluate the feasibility of applying H₂O₂ to reduce the leaching potential of a fuel oil contaminated soil. A silt-loam soil was collected from a contaminated thermal-electric plant with a hydrocarbon concentration of 3.2% in soil producing 4.3 mg/l in leachate. Hydrogen peroxide was applied (0.1, 0.2, 0.3, 0.6, 1.2% dry weight basis), and petroleum hydrocarbons were measured in soil and leachate pre- and post-treatment (72 h). At first, the soil and leachate concentrations diminished linearly (24.4 and 27.3% in soil and leachate, respectively). This was followed by a phase in which the concentration in leachate diminished greatly (75.8%) although the concentration in soil was reduced only moderately (15.1%). Overall, hydrocarbons in leachates were reduced 82.4% even though concentrations in soil were only reduced 35.8%. Correlation analysis showed that at only 1.0% w/w H₂O₂ a concentration of petroleum hydrocarbons in leachate safe for human consumption (≤ 1 mg/l) could be obtained even with a final hydrocarbon concentration in soil > 2%. Thus, this study presents an alternative strategy for remediation of fuel oil contaminated soils in urban environments that protects water sources by focusing on contamination in leachates, without spending extra financial resources to reduce the hydrocarbon concentration in low-toxicity soil.     

Lead enrichment, adsorption and speciation in urban garden soils under long-term wastewater irrigation in northern Nigeria

Unsafe lead (Pb) concentrations in leafy vegetables raised in urban and peri-urban agricultural production systems have been reported across cities in Northern Nigeria, even though Pb concentrations in soils are within regulatory safe levels. This study examined the soil enrichment, adsorption and chemical species of Pb in urban garden fields irrigated with untreated wastewater at three industrial locations in Kano, northern Nigeria. Total Pb in the soil profiles ranged from 9 to 91 mg kg^?1 and decreased rapidly from the surface to the subsurface layer, but attaining nearly constant concentration at depth ≥1.2 m in the profiles. The potentially labile Pb maintained fairly constant concentration with depth up to 0.9 m, but decreased fairly rapidly with depth thereafter. There was a significant Pb enrichment of the soils, extending up to 30–60 cm depth in the soil profiles. The adsorption of Pb by the soils increased drastically with pH, and attained maximum adsorption at pH ≥ 7.0 in the surface layer, and at pH ≥ 6 in the subsurface layer. The surface soils adsorbed between 85 and 97 % of added Pb at pH ≤ 5. Free Pb²⁺ activities in soil solution accounted for between 46 and 87 % at pH 5–7 of total dissolved Pb (Pb_T). The quantifiable chemical species of Pb in solution consisted mainly of PbOH⁺, PbSO ₄ ^· , PbCl⁺ and PbOH ₂ ^· which accounted for between 0.9 and 26 % of Pb_T in soil solution at pH ≥ 5.0, but declining to between 0.1 and 2.1 % at pH ≥ 7.5. There was no apparent equilibrium between Pb²⁺ activities and known Pb-compounds in the soils. It was concluded from the data that reports of excess Pb concentrations in leafy vegetables raised in these soils are consistent with high free Pb²⁺ activities maintained in soil solution by these predominantly sandy-textured soils.     

The effects of hardpan layers on the water chemistry from the leaching of pyrrhotite-rich tailings material

Column leaching experiments were used to determine the effects of an iron-rich hardpan layer, on the rate of tailings oxidation and the composition of leachate waters, from the Renison Bell tailings dams in western Tasmania, Australia. One-meter-long PVC columns, filled with tailings, cover material (Cassiterite Flotation Tailings) and hardpan samples from the tailings dams, were leached over a period of 14 weeks. Under dry cover conditions, when hardpan was present, the solute loads peaked at 21–49 days (Fe at 2,294 ppm and SO ₄ ^2- at 4,700 ppm), and stabilised at much lower concentrations after 9 weeks. In contrast, the solute loads steadily increased over time in the column where hardpan was absent (SO ₄ ^2- from 1,800 to 3,100 ppm, and Fe from 407 to 1,692 ppm). Under saturated cover conditions, the solute concentrations in the leachate also increased with time (SO ₄ ^2-from 1,900 to 17,000 ppm, and Fe from 480 to 8,500 ppm). The presence of a hardpan layer between the reactive tailings and cover material has been found to improve leachate water chemistry and lessen the rate of sulphide oxidation.     

Geochemical survey of rare earth elements (REEs) in the concealed ore body of Hongcheon,Korea

Because of the risk of diminishing supplies of rare earth elements (REEs) worldwide due to China’s dominance over REE supply, the necessity of developing domestic resources of REE has been realized in other countries. To explore new ore bodies, a geochemical survey was conducted at one existing carbonatite REE deposit in the Hongcheon area of Korea. Proper sampling strategies and baseline data for the interpretation of the results were determined through a pilot study conducted in the area. Enrichment in the concentration of light REE (LREE) over that of heavy REE, which is typical in carbonatite-type deposits, was observed in stream sediments and heavy mineral samples collected during the geochemical survey. Maximum concentrations of LREE were 2,299 and 27,798 mg/kg for stream sediments and heavy minerals, respectively. Among LREEs, La and Ce are the dominant components of all REEs, comprising approximately 68 % of mean concentrations. Considering the distribution pattern of La + Ce contents and the associations with the existing outcropping ore bodies, the zone of prospective REE mineralization was determined to be in the south-western part of the area. A detailed follow-up soil survey of the zone found even higher concentrations of La and Ce (2,450 and 3,100 mg/kg, respectively), and suggested the possible extension of the existing ore bodies. Likewise, a systematic geochemical survey for REE is feasible for locating concealed ore bodies in the area, where the mineralization is mostly covered with soil, and rock outcrops are scarce.     

石灰土对硫酸型酸雨缓冲过程模拟及碳汇效应研究

硫酸型酸雨沉降至地表经石灰土缓冲后,参与碳酸盐岩溶蚀及对岩溶碳汇的影响尚不明确,严重制约了我国岩溶碳汇效应的准确评估。本研究通过设置不同土层厚度条件下pH 4.5的硫酸型酸雨淋滤实验,以明确石灰土对硫酸型酸雨的缓冲过程及关键控制因素。结果表明：石灰土对酸雨的缓冲作用主要发生在表层(10 cm),淋出液中Ca2+、Mg2+、${\rm{HCO}}_3^{-}$含量在淋溶初期均表现快速降低,当淋溶量(土壤水达饱和后)为1 020 mL时Ca2+、Mg2+、${\rm{HCO}}_{{3}}^{-}$淋失量趋于稳定,稳定淋失量分别为20 mg·L−1、6 mg·L−1、40 mg·L−1。淋出液中被酸雨H+交换出的Ca2+、Mg2+仅占很小一部分,土壤水溶性Ca2+、Mg2+是淋出液中Ca2+、Mg2+的主要部分,开放系统中,大气和土壤CO2溶于降雨形成H2CO3不仅增加碳汇,且H2CO3解离产生的H+交换土壤中交换态Ca2+、Mg2+,造成Ca2+、Mg2+的淋失量不容忽视。不同厚度石灰土中交换性钙镁可缓冲酸雨容量均大于土壤碳酸钙矿物可缓冲容量,前者是后者的1.17~1.59倍。相同酸度、同一降雨量(土壤水达饱和后)下土壤盐基离子参与酸雨缓冲产生的碳汇量约为碳酸盐矿物风化缓冲产生碳汇量的2.1倍,不同厚度(≥10 cm)石灰土产生的碳汇量大致相等。根据本次实验及桂林市降雨数据计算,桂林市质纯石灰岩风化残积土壤区(土层厚度≥10 cm),土壤盐基离子参与酸雨缓冲每年可产生0.59 ~0.93 mol·m−2的碳汇通量。     

Experimental Modeling of Cyanobacterial Bloom in a Thermokarst Lake: Fate of Organic Carbon,Trace Metal,and Carbon Sequestration Potential

Thermokarst lakes, formed during permafrost thaw in Western Siberia Plain over past tens to hundreds years, cover overall territory close to million km² and may represent significant source of CO₂ and CH₄ to the atmosphere. These acidic (3 < pH < 6) and humic [10 < dissolved organic carbon (DOC) < 50 mg/L] lakes are essentially inhabited by heterotrophic bacterioplankton with rare phytoplankton bloom occurring during warm periods. In order to understand possible effects of phytoplankton bloom on thermokarst lake hydrochemistry under climate warming scenario, we cultured pure cyanobacterium (Gloeocapsa sp.) and native cyanobacterial associate separated from the natural lake water. As substrates, sterilized thermokarst lake water and peat leachate from western Siberia were used. In these laboratory microcosm experiments which lasted 10 days, we monitored daily pH, biomass, DOC, and 40 major and trace elements. Despite significant variation of pH (4 to ~10.5) and biomass (a factor of 3–5), very few dissolved elements responded to massive cyanobacterial growth. The DOC varied within a factor of 1.2–1.5, exhibiting slow increase due to exometabolite production in thermokarst lake water and an initial decrease due to photodegradation in peat leachate. Elements appreciably affected by photosynthesis in both lake water and peat leachate substrates were P, Zn, Mn, and, in a lesser degree, Cd, K, Rb, Sr, Ba, Cr, Al, and U. While P, K (Rb), Mn, and Zn removal from solution during cell growth could be linked to biological demand by cyanobacteria, the adsorption of Cd, Sr, Ba, Al, Cr, U on the cell surface in response to the pH rise is most likely. Many other trace elements did not exhibit any significant evolution of the concentration during 10-day experiment either due to their strong complexation with allochthonous organic matter and essentially organic/organo-mineral colloidal status (Fe, Ni, Co, Cu, Pb, REEs, Ti, Zr, Hf, Th) or due to the lack of element interaction with cyanobacterial cells, via both adsorption and intracellular uptake (B, Si, V, Mo, As, Sb, Cs). Therefore, possible intensification of cyanobacterial bloom in thermokarst lakes caused by leaching of thawing peat will likely affect only few macronutrients and micronutrients such as P, K, Mn, and Zn, while the majority of trace elements bound to allochthonous DOC in the form of organic and organo-mineral colloids will not be affected by cyanobacterial biomass production and pH rise due to photosynthesis. Cyanobacterial bloom in organic-rich (20 mg DOC/L) thermokarst lakes exhibited significant potential of carbon sequestration from the atmosphere, which is more than an order of magnitude higher than the CO₂ evasion due to heterotrophic plankton respiration of allochthonous DOC.     

Effects of antecedent soil moisture on losses of rare earth elements and phosphorus in runoff

The effects of antecedent soil moisture on losses of rare earth elements (REEs) and phosphorus (P) in runoff were evaluated through a simulated rainfall experiment at the rate of 83?mm?h^?1. The results revealed that antecedent moisture was a significant factor affecting the amounts of REE and P losses. It was observed that most REEs and P transported with sediments in the runoff. There was a significant correlation between the total amounts of REE losses and those of P losses in the runoff. The optimal antecedent moisture for minimization of losses for REEs ranged from 10.19 to 11.17% and it was 10.91% for P.     

Experimental investigation on shear strength and permeability of a deeply dewatered sewage sludge for use in landfill covers

This paper presents an experimental study on a deeply dewatered sewage sludge produced by using a new technique of membrane filter press. The experiments involve measurements of sludge composition, basic physical properties, shear strength, water permeability, and leaching toxicity. The measurements of shear strength and permeability were also performed on the sludge specimens soaked in a low acid leachate or distilled water for 1 and 2 months. This is to investigate the influence of chemical change in pore fluid as a result of rainfall infiltration or leachate seepage at landfills. Comparison tests were also carried out on silty clay that is commonly used for landfill cover material. The experimental results show that the deeply dewatered sludge contains 66 % organic content and 85 % water content (dry mass basis). The undrained shear strength of the sludge is >25 kPa even after 2-month soaking in the leachate and distilled water, meeting the requirement of the Chinese standard [CJ/T249-2007, Disposal of sludge from municipal wastewater treatment plant: sludge quality for co-landfilling. Ministry of Building and Construction, P.R. China (in Chinese), 2007]. The measured cohesion and friction angle for the sludge are >20 kPa and 22.3°, respectively. The soaking of sludge specimens in either leachate or distilled water resulted in an increase in frictional angle by several degrees. The water permeability for the sludge ranges from 0.68 × 10^?8 to 1.3 × 10^?8 cm/s, and permeability after 2-month soaking is less than the minimum requirement for the barrier layer of landfill covers (i.e., 1.0 × 10^?7 cm/s). The concentrations of heavy metals leaching from the dewatered sludge are lower than the limit values of leaching toxicity for the wastewater discharge standard of China. The experimental results indicate that deeply dewatered sludge can be used as an alternative material for the barrier layer of landfill covers.     

成都经济区农业土壤Cd元素的解吸动力学

对不同酸雨条件下水稻土Cd释放的研究发现,吸附态Cd释放的过程可以分为快反应和慢反应2个阶段。采用常见的动力学方程和本文创建的反三角函数方程,对淋滤实验数据进行了拟合。结果表明,反三角函数方程对多种酸雨条件下水稻土吸附态Cd释放过程的拟合度最佳。此外,多项式方程也能较好地描述这一过程。     

成都经济区农业土壤Cd元素的解吸动力学

对不同酸雨条件下水稻土Cd释放的研究发现,吸附态Cd释放的过程可以分为快反应和慢反应2个阶段。采用常见的动力学方程和本文创建的反三角函数方程,对淋滤实验数据进行了拟合。结果表明,反三角函数方程对多种酸雨条件下水稻土吸附态Cd释放过程的拟合度最佳。此外,多项式方程也能较好地描述这一过程。     

Geochemistry of rare earth elements in the mainstream of the Yangtze River,China

Water, suspended matter, and sediment samples were taken from 8 locations along the Yangtze River in 1992. The concentration and speciation (exchangeable, bound to carbonates, bound to Fe–Mn oxides, bound to organic matter, and residual forms) of rare earth elements (La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu) were determined by instrumental neutron activation analysis (INAA).The contents of the soluble fraction of REEs in the river are low, and REEs mainly reside in particulate form. In the particles, the chondrite-normalized distribution patterns show significant LREE enrichment and Eu-depletion. While normalized to shales, both sediments and suspended matter samples show relative LREE enrichment and HREE depletion. REEs are relatively enriched in fine-grained fractions of the sediments.The speciation characteristics of REEs in the sediments and suspended matter are very similar. The amount of the five forms follows the order: residual>>bound to organic matter∼bound to Fe–Mn oxides>bound to carbonates>>exchangeable. About 65 to 85% of REEs in the particles exist in the residual form, and the exchangeable form is very low. High proportions of residual REEs reveal that REEs in sediments and suspended matter are controlled by their abundances in the earth's crust. Carbonate, Fe–Mn oxide and organic fractions of REEs in sediments account for 2.4–6.9%, 5.2–11.1%, and 7.3–14.0% of the total contents respectively. They are similar to those in the suspended matter. This shows that carbonates, Fe–Mn oxides and organic matter play important roles during the particle-water interaction processes. By normalization to shales, the 3 forms of REEs follow convex shapes according to atomic number with middle REE (Sm, Eu, and Tb) enrichment, while light REE and heavy REE are depleted.     

Ligand extraction of rare earth elements from aquifer sediments: Implications for rare earth element complexation with organic matter in natural waters

The ability of organic matter as well as carbonate ions to extract rare earth elements (REEs) from sandy sediments of a Coastal Plain aquifer was investigated for unpurified organic matter from different sources (i.e., Mississippi River natural organic matter, Aldrich humic acid, Nordic aquatic fulvic acid, Suwannee River fulvic acid, and Suwannee River natural organic matter) and for extraction solutions containing weak (i.e., CH₃COO⁻) or strong (i.e., ) ligands. The experimental results indicate that, in the absence of strong REE complexing ligands in solution, the amount of REEs released from the sand is small and the fractionation pattern of the released REEs appears to be controlled by the surface stability constants for REE sorption with Fe(III) oxides/oxyhydroxides. In the presence of strong solution complexing ligands, however, the amount and the fractionation pattern of the released REEs reflect the strength and variation of the stability constants of the dominant aqueous REE species across the REE series. The varying amount of REEs extracted by the different organic matter employed in the experiments indicates that organic matter from different sources has different complexing capacity for REEs. However, the fractionation pattern of REEs extracted by the various organic matter used in our experiments is remarkable consistent, being independent of the source and the concentration of organic matter used, as well as solution pH. Because natural aquifer sand and unpurified organic matter were used in our experiments, our experimental conditions are more broadly similar to natural systems than many previous laboratory experiments of REE-humic complexation that employed purified humic substances. Our results suggest that the REE loading effect on REE-humic complexation is negligible in natural waters as more abundant metal cations (e.g., Fe, Al) out-compete REEs for strong binding sites on organic matter. More specifically, our results indicate that REE complexation with organic matter in natural waters is dominated by REE binding to weak sites on dissolved organic matter, which subsequently leads to a middle REE (MREE: Sm-Ho)-enriched fractionation pattern. The experiments also indicate that carbonate ions may effectively compete with fulvic acid in binding with dissolved REEs, but cannot out compete humic acids for REEs. Therefore, in natural waters where low molecular weight (LMW) dissolved organic carbon (DOC) is the predominant form of DOC (e.g., lower Mississippi River water), REEs occur as “truly” dissolved species by complexing with carbonate ions as well as FA, resulting in heavy REE (HREE: Er-Lu)-enriched shale-normalized fractionation patterns. Whereas, in natural terrestrial waters where REE speciation is dominated by organic complexes with high molecular weight DOC (e.g., “colloidal” HA), only MREE-enriched fractionation patterns will be observed because the more abundant, weak sites preferentially complex MREEs relative to HREEs and light REEs (LREEs: La-Nd).