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... Humic acid was adsorbed with rare earth ions in a ternary complexation form [15]. In particular, the coordination stability constant and capacity of cerium and yttrium with humic acid were much higher than those of other rare earth elements, resulting in the abnormal phenomenon in the organic binding state [34,38]. Meanwhile, soluble rare earth ions were adsorbed by organic matter and fixed in organic phase, thus the content of these rare earth ions was small [34]. ...
To reveal the regularity of variation in the rare earth occurrence states of weathered crust elution-deposited rare earth ores, ore samples from different weathering crust layers were obtained by performing the sequential extraction procedure. The order of rare earth contents firmly obeyed the following sequence: the weathered layer > humic layer > partly weathered layer. The occurrence states of rare earth elements were mainly the ion exchange state, carbonate bound state, iron–manganese oxide state, organic binding state and residual state. The proportions of rare earth elements found in the rare earth ion exchange state of the weathered layer, humic layer and partly weathered layer were 78.55%, 73.53% and 53.88%, respectively. The light rare earth elements (LREEs) found in the rare earth ion exchange state were enriched in the upper part of the weathering crust, while the heavy rare earth elements (HREEs) were enriched in the lower part. There were also obvious negative anomalies in the content of cerium in the ion exchange state. The content of rare earth elements found in the carbonate bound state was small, and the rare earth partition pattern was basically consistent with that of the ion exchange state, which had little effect on the differentiation of the rare earth elements. The iron–manganese oxide state was mainly enriched with cerium, and the content of cerium increased with the depth of the weathering crust. The iron–manganese oxide state was the main factor causing the phenomenon of the anomaly in the cerium content. Meanwhile, the iron oxides in the iron–manganese oxide state were mainly hematite and goethite. The organic binding state mainly beneficiated yttrium and cerium by complexation and certain adsorption. The content of elements found in the rare earth residual state was related to the degree of weathering and reflected the release sequence of rare earth elements in the mineralization process. Clarifying the rare earth occurrence states is conducive to better revealing the metallogenic regularity of weathered crust elution-deposited rare earth ores. In addition, the results can provide a valuable reference for expanding the available rare earth resources and the efficient comprehensive utilization of rare earth ore.
... Furthermore, microorganisms that produce organic acids can lower the pH and enhance the mobility of REEs in macro-aggregates (Marsac et al. 2013). In general, from the perspective of ecological restoration, the distribution characteristics of REEs aggregates at different periods reflect their migratory process in aggregates (Chen et al. 1997;Tyler 2004). Higher organic matter concentration in aggregates may enhance the selective adsorption capacity of colloids to individual REEs. ...
Rare earth elements (REEs) are widely applied in high-tech fields. However, their increasing presence in the food chain poses significant risks to human health. At present, little is known about the effects of organic matter on the distribution of ion-adsorbed REEs in soil aggregates during ecological restoration. Red soil derived from coarse-grained granite in Southern China is both prone to ecosystem degradation from soil erosion and rich in REEs. Understanding the distribution of REEs in soil aggregates undergoing ecological restoration is helpful to formulate effective measures for controlling the environmental migration of REEs. Four sites that had undergone different durations/degrees of ecological restoration were selected in the areas to analyze. REEs concentration of six different aggregates sizes (<0.25, 0.25−0.5, 0.5−1, 1−2, 2−5, and >5 mm) were analyzed and the enrichment coefficients were calculated in 4 sample sites of severe-degraded ecosystem in Changting County, Fujian Province, Southern China. The results showed that the total rare earth elements (TREEs) concentration in the aggregates increased from 213 mg kg−1 to 528 mg kg−1 with the extension of the ecological restoration time. At the initial stages of ecological restoration, there was no significant difference in the TREEs concentration among the six aggregates sizes. However, in the middle and late stages of restoration, the concentration of TREEs increased significantly with the decrease of aggregate size. The concentration of individual REEs showed three changing trends with sizes of aggregates during ecological restoration, respectively: 1) no obvious regular change (S1), 2) a V-shaped change trend (S2), and 3) increasing concentration with the decrease of aggregate size (S3 and S4). Ce and Eu showed a positive and negative anomaly in the soil aggregates, respectively. Moreover, the light rare earth elements (LREEs) were enriched, while the heavy rare earth elements (HREEs) were depleted during the initial stages, and the HREEs were enriched during the middle and late stages of restoration. The correlation coefficient between organic matter and REEs in aggregates was generally low; however, LREEs showed a stronger correlation with organic matter than that of HREEs during the initial stages of ecological restoration. The correlation between organic matter and HREEs gradually increased and even exceeded that of LREEs with on-going ecological restoration. The distribution of REEs concentration in degraded soil aggregates in Southern China showed obvious variability with the ecological restoration time.
... First, significant positive anomalies exist in the top of the profile, which may be related to the strong oxidation of surface tillage soil under tilling. Some studies suggested that abundant organic matter (humic acid) on the surface may also contribute to the formation of positive Ce anomalies on the surface, as the complexation of humic acid with Ce is significantly stronger than that of other REEs [54]. According to the fluctuation process of Ce in the TG profile and its negative anomaly, the weathered TG limestone profile experienced several relatively reductive climatic environments. ...
Research on weathered crusts on carbonate rock is essential for paleoenvironmental studies in karst areas. Terra rossa, widely distributed in tropical karst areas, has not been studied in terms of its material sources and geochemistry. Two typical terra rossa profiles on dolomite (SC profile located at Sang Cai, Hoa Binh province) and limestone (TG profile located at Tong Gia, Lao Cai province) in Northern Vietnam were selected to examine the geochemical characteristics and the evolutionary processes of rare earth elements (REEs). Chondrite and bedrock normalized patterns indicated that these two profiles are in situ weathering crusts, meaning they are the residual material remaining after chemical weathering of the lower carbonate rocks. The average value of total REE in the SC profile is 381.19 ppm, which is 30 times higher than the bedrock. In the TG profile, the value is 386.26 ppm, 13 times higher than the bedrock. Compared with the profiles in nearby subtropical areas in Southeast China, the REE enrichment coefficients of terra rossa in Northern Vietnam are much higher. The REE depletion was also different between the SC and TG profiles. The light and heavy REE fractionations in the SC profile are higher than in the TG profile. Paleoclimate inversion analysis shows that the SC profile experienced a stable oxidation condition, whereas the TG profile was subjected to several reducing environments since a weathering crust formed.
... , the retrograde solubility of monazite under acid conditions and possibility for high activities of carbonate and organic ligands (Wood, 1993; Chen et al., 1998) means there is high potential for REE mobility during weathering. Ceitner and Xiong (2008) concluded from a model soil solution that La 3+ transport would be dominated by oxalate, sulphate and carbonate species alongside the free ion, although low solubilities were predicted on the basis of monazite as the primary phase in the pH range 5.5 to 8.5. ...
The rare earth elements are unusual when defining giant-sized ore deposits, as resources are often quoted as total rare earth oxide, but the importance of a deposit may be related to the grade for individual, or a limited group of, the elements. Taking the total REE resource, only one currently known deposit (Bayan Obo) would class as giant (>1.7×107 tonnes contained metal), but a range of others classify as large (>1.7×106 tonnes). With the exception of unclassified resource estimates from the Olympic Dam IOCG deposit, all of these deposits are related to alkaline igneous activity – either carbonatites or agpaitic nepheline syenites. The total resource in these deposits must relate to the scale of the primary igneous source, but the grade is a complex function of igneous source, magmatic crystallisation, hydrothermal modification and supergene enrichment during weathering. Isotopic data suggest that the sources conducive to the formation of large REE deposits are developed in subcontinental lithospheric mantle, enriched in trace elements either by plume activity, or by previous subduction. The reactivation of such enriched mantle domains in relatively restricted geographical areas may have played a role in the formation of some of the largest deposits (e.g. Bayan Obo). Hydrothermal activity involving fluids from magmatic to meteoric sources may result in the redistribution of the REE and increases in grade, depending on primary mineralogy and the availability of ligands. Weathering and supergene enrichment of carbonatite has played a role in the formation of the highest grade deposits at Mount Weld (Australia) and Tomtor (Russia). For the individual REE with the current highest economic value (Nd and the HREE), the boundaries for the large and giant size classes are 2 orders of magnitude lower, and deposits enriched in these metals (agpaitic systems, ion absorption deposits) may have significant economic impact in the near future.
... Meteoric water percolating through a layer of humus-rich topsoil could become more acidic and corrosive and can carry a variety of cations, including REE (Nesbitt, 1979). Organic acids, including amino acids, fulvic acids, etc., have been found to be abundant in the upper parts of the weathering profiles developed on granitic rocks in South China (Chen et al., 1997). Taunton et al. (2000) demonstrated that microorganisms might accelerate the dissolution of apatite and enhance the dissolution of preformed secondary phosphates by changing the saturation states through microbial uptake of phosphorous. ...
Mineralization with exchangeable rare earth element (REE) and yttrium (MEX-REY) has been recognized in the weathering profiles in South China since the early 1970's. This type of REY mineralization occurs in weathering profiles of parent rocks ranging in composition from granite to acidic volcanic rocks and lamprophyre. The majority of the known resources occurring in the weathering profiles of granitic rocks. Total resources of this type of REY amount to millions tons of rare earth oxides, and therefore represent one of the most important types of rare earth resources in China, particularly for heavy rare earth elements (HREE) and yttrium.Accumulation of REY in the weathering profiles of granitic rocks is strongly controlled by the resistance to weathering of the principal REY-bearing accessory minerals in the parent rocks; only a limited proportion of total REY (
To understand the characteristics and evolution of REE during the process of the ultramafic laterization under different climate conditions, two outcrops Kolonodale in Indonesia and Yuanjiang in China are chosen for comparision. It is found that the contents of REE from the laterite crusts are higher than those from the bed rocks in both places (enrichment factor being 44.21 and 236.19 respectively). The indices of differentiation between the LREE and HREE decrease with profile downward toward, and the indice of Ce anomaly shows a shift from the positive Ce anomaly in the upper segment to negative Ce anomaly in the lower part. The difference between the two profiles lies in the distribution of the highest REE enriched segment. The laterite layer represent the most REE enriched for the Yuanjiang whereas the saprolite layer for the Kolonodale. The evaluation of the mass balance shows remarkable migration and differentiation of REE in the ultramafic laterization process, which were constrained effectively by the pH environmentand organic matter (O. M.). The results indicate that climate have had great influence on the geochemical evolution of REE during the ultramafic laterization. Under the rainforest climate condition, the REE from the Kolonodale originates mainly from the basal rocks and has experienced intensive redistribution during the laterization; whereas the REE from the Yuanjiang has a mixed source stemming from both the parent rock and aeolian sediment, and it has been through only slight redistribution during the laterization.
The three kinds of soil formed by the weathering of the basalt in Yema village, Weining county of Guizhou province, the carbonate rock in Guiyang district and the phosphorite in the REE-bearing phosphate mining area in Zhijin county of Guizhou province were all rich in REE. The contents of LREE in these three soil were higher than that of HREE, which was caused by the different properties of LREE and HREE complexes. Since light REE could be easily absorbed by kaolinite grains, LREE was fully enriched in the process of desorption, migration and re-absorption, which formed a typical LREE soil. The chondrite normalized distribution models of those three types of REE-enriched soil are basically similar, which belong to the right deviation type, namely, rich in light rare earth element but short of heavy rare earth element, and showing Ce negative anomalies. Eu showed intense negative anomalies in the REE in the soil formed by the weathering of the sedimentary parent rocks (such as carbonate rock and phosphorite), while no pronounced Eu anomaly was found in the REE in the soil formed by the weathering of volcanic rocks (such as basalt). It is clear that though there were intense migration and enrichment of REE during the weathering process of rocks, no obvious differentiation of REE occurred and some features of the parent rocks were retained. That is why REE is of value in analyzing the source of materials.
A new type of rare earth elements (REEs) deposit was discovered from the gaolinite mudstone in the weathering crust of Permian basalt, Bijie region, western Guizhou, China. It contained ΣRE2O3 0.065%–1.086%. This type of REEs deposit was widely distributed with steady horizon and thickness of 3–4 m. The ore-bearing weathering crust (kaolinite) of the three discovered REEs deposits belonged to the third episode of the Emeishan basalt eruption. The new type of REEs deposit was suggested that basalt (tuff) weathering could lead to the enrichment of the rare earth elements. Therefore, it is of important economic significance to explore REEs deposits in the weathering crust of basalt (tuffs) in Yunnan, Guizhou, and Sichuan Provinces.
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