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稀土元素(REE)是指在元素周期表中原子序数为57—71的15个镧系元素. 基于原子质量和有效离子半径,稀土元素通常可分为两类,即轻稀土元素(LREE: La—Eu)和重稀土元素(HREE: Gd—Lu)[1-2]. 稀土元素具有相似且相对稳定的地球化学性质,在成岩、变质、风化、搬运和沉积等过程中具有不易迁移的特点[3-4],因而被广泛应用于内生和外生地质过程中矿物和岩石的形成条件、物质来源、地球化学分异作用以及其气候变化等领域[5-8].
在土壤形成过程中,由于受到溶解、沉淀、氧化还原和络合作用等影响,REE的地球化学行为发生变化,导致REE总量变化、内部元素分馏或元素异常[1,3]. REE地球化学特征可用于判定成土物源[9]、反映风化成土过程[10-11]、揭示土壤发育程度[5,12]、判断土壤氧化还原环境[13]和人类活动对土壤环境的影响等[14-15]. 然而,由于成土母质的多样性、成土环境的复杂性以及人类活动的叠加影响,对于土壤发育过程中REE迁移、富集及分馏的认识还十分有限且存在不少分歧与争议[5,7,16],需要通过更多的调查以揭示土壤中REE的分布和分异规律.
本文以安徽省无为市南部长江冲积物形成的沿江平原为研究区,在构建土壤时间序列的基础上,对比不同成土时间和不同利用方式下REE地球化学特征,以期揭示土壤发育过程和集约化利用对REE的分布和分异的影响.
长江下游冲积平原区土壤稀土元素富集与分馏特征
Enrichment and fractionation of rare earth elements in alluvial plain soils at the lower reaches of the Yangtze River
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摘要: 在长江下游冲积平原区建立了一个跨度约1500 a的土壤时间序列,通过对比稀土元素(REE)含量和分馏的变化,分析其在成土过程中的演变特征与控制因素. 研究结果表明,REE含量经过成土初期的略有下降后,随着时间推移总体上呈增加的趋势. 次生矿物和有机质对REE较强的捕获和吸附固定能力是土壤REE富集的主要原因,磷肥的长期施用可能也是REE在土壤中不断积累的重要原因. 旱作和水旱轮作两种主要农用方式对单个REE含量和总量差异的影响不显著. 土壤中轻、重REE的分馏和Ce、Eu的亏缺也随成土时间推移变化. 水稻种植导致土壤氧化还原电位下降,直接和间接增加了Ce和Eu的亏损.Abstract: A soil chronosequence with a span of about 1500 years was established in the alluvial plain area of the lower reaches of the Yangtze River. By comparing the changes of rare earth element (REE) contents and fractionations, their evolution characteristics and control factors in the pedological process were analyzed. The results show that after a slight decrease in the initial stage of soil formation, the content of REE generally increases with the passage of time. The strong scavenging and fixing ability of secondary minerals and organic matter on REE is suggested as the main reason for the enrichment of REE in soil. The long-term application of phosphorus fertilizer might also lead to the continuous accumulation of REE in soil. Dry farming and paddy dry rotation had no significant effect on the contents of single REE and ∑REE. The fractionation of light and heavy REE and the deficiency of Ce and Eu also changed with the time of soil formation. Rice planting led to the decrease of soil redox potential, which directly and indirectly increased the negative anomalies of Ce and Eu.
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Key words:
- Lower Yangtze Plain /
- soil chronosequence /
- pedological process /
- rare earth elements /
- fractionation.
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表 1 研究区土壤的稀土元素含量(mg·kg− 1)
Table 1. Contents of rare earth elements in soil of the study area(mg·kg− 1)
最小值
Minimum value最大值
Maximum value均值
Average value变异系数/%
Coefficient of variation中国土壤[19]
Soil in China地壳[1]
CrustLa 29.0 45.5 40.9 7.93 39.7 30.0 Ce 57.0 90.9 81.4 8.07 68.4 60.0 Pr 6.71 10.2 9.20 7.76 7.17 8.20 Nd 25.7 38.8 35.0 7.44 26.4 28.0 Sm 4.98 7.55 6.59 7.80 5.22 6.00 Eu 1.01 1.52 1.34 7.49 1.03 1.20 Gd 4.34 6.69 5.80 8.60 4.60 5.40 Tb 0.682 1.02 0.913 7.91 0.630 0.900 Dy 4.02 6.21 5.39 8.46 4.13 3.00 Ho 0.825 1.21 1.08 7.91 0.870 1.20 Er 2.35 3.55 3.06 8.29 2.54 2.80 Tm 0.359 0.520 0.460 7.93 0.370 0.480 Yb 2.23 3.33 2.91 8.05 2.44 3.00 Lu 0.333 0.493 0.431 8.06 0.360 0.500 ∑REE 140 215 194 7.79 163 150 LREE 124 193 174 7.83 148 133 HREE 15.3 23.0 20.0 8.15 15.9 17.3 LREE/HREE 7.94 9.48 8.71 3.52 9.28 7.72 注:中国土壤稀土元素数据参考魏复盛1991年,地壳稀土元素数据参考刘英俊1984年.
Note: Chinese soil rare earth element data refer to Wei Shengfu 1991, crustal rare earth element data refer to Liu Yingjun 1984.表 2 研究区土壤稀土元素特征参数
Table 2. Characteristic parameters of rare earth elements in soil of the study area
LREE/HREE (La/Yb)N (La/Sm)N (Gd/Yb)N δEu δCe 范围 7.94—9.48 8.22—11.7 3.66—4.11 1.42—2.00 0.610—0.680 0.940—1.00 均值 8.71 9.50 3.91 1.61 0.650 0.970 表 3 部分指标在各片区的平均值
Table 3. The average value of some indicators in each area
片区号
Area codepH CaO/% Fe2O3/% MnO/% LREE/HREE δCe δEu 1 7.92 3.83 6.17 0.128 8.90 0.977 0.656 2 7.88 2.71 4.50 0.077 8.82 0.961 0.648 3 7.47 2.13 5.58 0.098 8.81 0.983 0.647 4 7.45 1.87 5.77 0.087 8.68 0.982 0.654 5 7.15 1.25 5.78 0.083 8.38 0.953 0.630 6 6.24 1.06 5.49 0.067 8.41 0.951 0.636 表 4 主成分分析结果
Table 4. Principal component analysis results
指标
Index主成分
Principal componentP1 P2 P3 La 0.94 0.09 0.29 Ce 0.92 0.20 0.26 Pr 0.95 0.06 0.26 Nd 0.93 0.14 0.25 Sm 0.96 0.01 0.22 Eu 0.94 0.15 0.18 Gd 0.97 −0.05 0.14 Tb 0.98 0.07 0.02 Dy 0.99 0.00 −0.09 Ho 0.97 0.08 −0.17 Er 0.97 −0.03 −0.18 Tm 0.95 0.01 −0.24 Yb 0.95 −0.03 −0.27 Lu 0.93 −0.07 −0.29 HREE 0.97 0.05 −0.16 LREE 0.93 0.16 0.27 LREE/HREE −0.15 0.18 0.87 δCe 0.05 0.70 −0.09 δEu −0.41 0.57 −0.14 Al 0.78 0.47 0.03 Si −0.45 −0.83 −0.19 Fe 0.69 0.67 0.13 Mn 0.11 0.86 0.18 Ca −0.55 0.57 0.16 Ti 0.73 0.34 0.27 Zr −0.26 −0.52 −0.22 P −0.17 −0.38 −0.03 黏粒 0.76 0.23 −0.05 OM 0.56 0.10 0.11 pH −0.51 0.54 0.09 -
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