Geochemical Characteristics and Factors of Transfer and Accumulation of Rare Earth Elements in Rock-Soil-Tea of the Mengku Tea Region in Yunnan Province, China

Abstract

Rare earth elements (REEs) in tea are usually determined by the soil, but their transfer characteristics and influencing factors have been rarely studied and reported. In order to determine the transfer and accumulation characteristics of REEs, rock, soil, and tea samples were collected in the Mengku tea region. Levels of 15 REEs (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) in samples were determined by inductively coupled plasma atomic emission spectrometry (ICP-MS). The results showed that light rare earth elements (LREEs) were easily enriched in rock, soil, and tea. The average total concentration of REEs in investigated rocks was 199 mg·kg⁻¹, which was lower than the element abundance of China’s continental crust but higher than the standard value of element abundance in the upper continental crust (UCC). The average total REE concentration in the investigated soil was 225 mg·kg⁻¹, which was lower than the background value of soil in Yunnan Province. The chondrite-normalized levels of Ce and Eu in rocks and Eu in soil showed negative anomalies, while Ce levels in soil showed slightly positive anomalies. The total amount of REEs in tea was relatively low and the chondrite-normalized values of Eu in tea showed significantly positive anomalies. Eu anomalies in tea were closely correlated with soil pH (pH < 5, negative anomalies; pH > 5, positive anomalies). K, Na, Ca, and Mg were the main factors affecting the transfer of REEs. LREEs were mainly enriched in tea through Na⁺ channels. Mg and K affected the differentiation and enrichment of heavy rare earth elements (HREEs) in tea. Calcium showed a significantly positive correlation with Eu in tea. The human health risk assessment showed that the risk of drinking tea to the human body was far below the reference value. Ce, Y, and La in tea were the main elements that affected non-carcinogenic risk and carcinogenic risk.

1. Introduction

The rare earth elements (REEs) are a group of 17 elements, including the lanthanide elements, scandium (Sc), and yttrium (Y). REEs can be divided into two groups: LREEs (La~Eu) and HREEs (Gd~Lu) [1]. REEs are widely used as additives in fertilizers. An appropriate amount of REEs can greatly improve the yield and quality of crops [2] by helping plant growth and enhancing disease resistance, plant photosynthesis, and plant seed germination [2,3,4,5]. REEs are thus an important part of the food chain closely related to human health and the economy [6]. However, when REEs exceed a certain content in plants, their toxicity can not only to inhibit plant growth [5] but also cause diseases and damage to the brain, liver, lung, kidney, and other organs [7]. Therefore, it is necessary to understand the geochemical behavior and accumulation of REEs in different matrices in order to put forward scientific and reasonable management measures [8].

The characteristics of REEs in tea are closely related to those of the soil. Previous studies have shown that REEs are obtained from soil by tea through absorption and accumulation [9]. The chondrite-normalized distribution pattern of REEs in Tie Guan Yin tea was similar to that of REEs in the soil [10]. Approximately 2.23% of REEs were transferred from the soil to tea in the study of an organic tea garden in Guangdong [11]. The uptake of REEs by tea is mainly related to the concentration and/or speciation forms in soil [12]. While the physical and chemical properties of soil are determined by the parent material [13], there is a certain degree of correlation between the elemental contents of rock, soil, and plants [14]. REEs are always enriched in plants growing in areas with high REE levels in bedrock [15]. The elements in tea have a significantly positive correlation with those in rock and soil [16].

As one of the most popular beverages, tea is consumed in many countries [17]. The health risk assessment of REEs in tea has become a hot topic in recent years [18]. Understanding the geochemical characteristics of REEs in tea is important to ensure the quality of tea and human health. At present, few studies have investigated the transfer and accumulation characteristics of REEs in tea. Therefore, rock, soil, and tea samples were collected in tea gardens in Yunnan Province, China. This paper aimed to reveal the accumulation characteristics and influencing factors of REEs in tea by comparing the distribution and transfer of REEs in rock, soil, and tea and assessing the human health risk. This work can provide a scientific basis for the protection and development of the tea industry.

2. Materials and Methods

2.1. Study Area

Yunnan big-leaf tea is one of the typical plateau crops. The Mengku tea area, Shuangjiang County, Lincang City, Yunnan Province, is a high-quality production area of big-leaf tea. The area has a subtropical mountain monsoon climate with distinct dry and wet seasons. The average annual temperature of the study area was 18 °C, with an average annual precipitation of 1800 mm. The altitude is between 1600 and 2000 m. Five tea gardens were selected in this study (Figure 1).

Metamorphic rocks from the Paleozoic metamorphic strata, including the Lancang Group series strata A and B, and Triassic acid granite intrusion outcrops widely appear in the study area. The main lithology of strata A is schist, granulite, and garnet schist. Strata B is quartz schist, garnet schist, granulite, hornblende anorthosite, and garnet rock (rich in iron) [19]. The soil types are mainly yellow-brown soil and red soil. The Nanpo, Laozhai, Dijie, and Bawai sites are all located on metamorphic rocks, while the Nuowu site is located on granite stratum.

2.2. Samples Collection and Pretreatment

The rock, soil, and tea samples were collected at the end of November 2021. Fifteen sampling locations were selected based on the stratum of five tea gardens. Two soil profiles samples were collected in each tea garden. Bw01, Dj01, Lz01, Np01, and Nw01 were the profile samples of five tea gardens. Only rocks and soil were collected from other sampling points for comparison Therefore, 5 tea samples, 26 soil samples, and 12 rock samples were collected in total.

D, C, B, and A were used to represent the rock, deep soil, topsoil, and humus layers, respectively. A large hole approximately 3 m deep was dug next to the tea tree. The rocks were dug out by geological hammer from the bottom of the hole. The soil was sampled along the side of the hole with a shovel. The depths of the deep soil, topsoil, and humus layers were 2~2.5, 1~1.5, and approximately 0.5 m, respectively. Tea samples of one bud and two leaves were collected from the tea tree near the hole.

The tea samples were washed with distilled water in the laboratory, air-dried to constant weight, and weighed after grinding with an agate mortar. In order to avoid contamination with metal elements, a plastic shovel was used to collect the soil samples. Each sample was approximately 1.5 kg of a mixture of 3–5 sub samples. The samples were placed in a clean, cool, and ventilated location and dried to constant weight. The soil samples were passed through a 2 mm nylon sieve to remove debris such as stones and grass roots. Then, the samples were sealed in plastic bags. A quarter of the samples were ground with an agate mortar and passed through a 0.1 mm nylon sieve for element and physicochemical property analysis. The rock samples were washed with distilled water, dried, and ground to <2 mm for further chemical analysis.

2.3. Sample Test

REEs in tea samples were tested in accordance with the technical requirements for ecological and geochemical evaluation. A 0.2~0.5 g sample of tea was digested with HCl (ρ = 1.19 g/mL), HNO₃ (ρ = 1.42 g/mL), and HClO₄ (ρ = 1.68 g/mL) in a vessel at 120 °C in a microwave oven for 5 h under high pressure. After digestion, the solution was transferred to a 10 mL tube for dilution. Samples were analyzed using an inductively coupled plasma mass spectrometer (ICP-MS, Thermo Fisher Scientific, Waltham, MA, USA). GBW100106 (GSB-7) biological component reference materials were used for quality control. The detection limit of La, Ce, Eu, and Y was 0.02 mg∙kg⁻¹. The detection limit of Pr, Nd, Sm, Gd, and Tb was 0.01 mg∙kg⁻¹. The detection limit of Ho, Er, and Yb was 0.005 mg∙kg⁻¹. The detection limit of Tm and Lu was 0.003 mg∙kg⁻¹.

Soil and rock were measured according to the requirements of the regional geochemical sample analysis method. GBW07385 (GSS-29) soil reference material was used to monitor the quality of the results. Approximately 0.2 g of rock/soil sample was placed in a 50 mL tetrafluoroethylene beaker, to which 5 mL of HNO₃, 2 mL of HF, 1 mL of H₂SO₄, and 5 mL aqua regia were added. Samples were digested on an electric heating plate. Then, samples were transferred and diluted into 10 mL tubes and analyzed by ICP-MS. The detection limit of La and Yb was 0.05 mg∙kg⁻¹. The detection limit of Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, and Y was 0.01 mg∙kg⁻¹. The detection limits of Ce, Er, and Lu were 0.05, 0.02, and 0.1 mg/kg, respectively.

Major elements were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES). The detection limits of Ca, Mg, K, and Na in tea were 100, 95, 90, and 100 mg∙kg⁻¹, respectively. GBW100106 (GSB-7) biological component reference material was used for quality control. The detection limit of Ca, Mg, K, and Na in soil and rock was 100 mg∙kg⁻¹. GBW07385 (GSS-29) soil reference material was used for quality control. Soil pH was determined using the potential method. GpH-5 soil reference material was used for quality control. Organic samples were processed according to the standard process of NY/T 1121.6-2006.

During chemical analysis, quality assurance and quality control (QA/QC) agreement was applied to ensure the reliability and accuracy of the results. The relative standard deviation (RSD) of repeated measurement was less than 5%. High-quality deionized water was used to clean glassware, prepare standards, and dilute samples.

2.4. Bioaccumulation/Transport Factor

The bioaccumulation factor (BF) is the ratio of REE concentration in plant to that in soil. It reflects the ability of plants to accumulate REEs from soil [20]. The transport factor (TF) is used to express the transfer ability of rare earth elements from rock to soil. The expression formulae are as follows:

$$BF=\frac{REEs in tea}{REEs in soil}$$

(1)

$$TF=\frac{REEs in soil}{REEs in rock}$$

(2)

2.5. Health Risk Assessment Methods

The risk of drinking tea to the human body mainly occurs by the oral route. The content of REEs in tea was assessed using the health exposure risk assessment method. All assessment indicators were based on average daily intake (ADI) (mg/kg-day) [21,22].

$$ADI=\frac{C\times IR\times EF\times ED}{BW\times AT}$$

(3)

where $C$ is the content of REEs in tea (mg/kg); $IR$ is the average daily intake of tea, assuming 11.4 g/day [23]; $EF$ represents exposure frequency, assuming 365 days/year; $ED$ is the duration of exposure, assuming 57 years [24]; $AT$ is the time period, which can be calculated as:$AT=ED\times 365$ days; and $BW$ is the body weight of the exposed population. In this study, the adult weight was assumed to be 61.75 kg [25].

The hazard quotient (HQ) is used to evaluate the non-carcinogenic risk of REEs in tea. The total non-carcinogenic risk (THQ) is the sum of the HQ values of all REEs.

$$H{Q}_i=\frac{AD{I}_i}{Rf{D}_i}$$

(4)

$$THQ={\sum }_{i=1}^{n}H{Q}_i$$

(5)

where $Rf{D}_i$ is the reference oral dose of specific ith REEs (mg/kg/day). The US EPA Integrated Risk Information System (IRIS) gives the values of La (2 × 10⁻²), Pr (5 × 10⁻¹), Nd (5 × 10⁻¹), Sm (5 × 10⁻¹), Eu (2 × 10⁻³), Lu (9 × 10⁻⁴), and Y (4 × 10⁻³) (Weber, 2012). According to the standards of the US Environmental Protection Agency, if the HQ and THQ are higher than 1, exposed persons are considered to be adversely affected. If HQ and THQ are below 1, the REEs in tea are considered to be safe for human health.

The cancer slope factor (SF) is used to assess the risk of carcinogenesis. Among them, $Ris{k}_i$ is the carcinogenic health risk of the ith rare earth element.

$$Ris{k}_i=AD{I}_i\times S{F}_i$$

(6)

$$Ris{k}_{Total}={{\displaystyle \sum }}_{i=1}^{n}Ris{k}_i$$

(7)

According to the US EPA Integrated Risk Information System (IRIS), the cancer slope factor SF (kg/day/mg) is selected as La (1.10 × 10⁻¹¹), Ce (3.52 × 10⁻¹¹), Pr (7.92 × 10⁻¹²), Nd (5.44 × 10⁻¹³), Sm (3.74 × 10⁻¹¹), Eu (1.03 × 10⁻¹¹), Gd (1.52 × 10⁻¹²), Tb (4.88 × 10⁻¹²), Dy (4.14 × 10⁻¹³), Ho (9.21 × 10⁻¹²), Er (2.53 × 10⁻¹²), Tm (6.99 × 10⁻¹³), Yb (4.00 × 10⁻¹²), Lu (3.53 × 10⁻¹²), and Y (1.81 × 10⁻¹¹) [26]. $Ris{k}_{Total}$ is the sum of the $Ris{k}_i$ values of all REEs. The carcinogenic risk of $Ris{k}_i$ or $Ris{k}_{Total}$ is classified as: no significant risk (<10⁻⁶); acceptable/tolerable (10⁻⁶~10⁻⁴); unacceptable (>10⁻⁴) [27].

2.6. Statistical Analysis

Microsoft Excel 2019 and SPSS 26.0 were used for data analysis, statistics, and basic graphics. Corelkit was used to draw the chondrite-normalized REE distribution patterns. CorelDRAW 2020 was used for further drawing all graphics in the manuscript.

3. Results and Discussion

3.1. Geochemical Characteristics of REEs in Tea, Soil, and Rock

3.1.1. Levels of REEs in Tea, Soil, and Rock

The mean total amounts of rare earth elements (∑REE) and light rare earth elements (∑LREE) in the rock samples from the study area were 199 and 175 mg·kg⁻¹, respectively (Table 1). These values were lower than the REE abundance of the continental crust in China [28] but higher than that of the UCC [29]. The average content of heavy rare earth elements (∑HREE) was 24 mg∙kg⁻¹, which was lower than that of the UCC and the rare earth element abundance of China’s continental crust. As shown in Table 1, the Nw03 sample had the highest value of ∑HREE (62 mg·kg⁻¹). Other rock samples had similar values of ∑HREE, with an average value of 20 mg·kg⁻¹. According to previous studies, the ∑HREE value in igneous rock samples was higher than that in metamorphic rock samples [30]. The Nw03 sample was fine-grained biotite granodiorite, belonging to igneous rocks. Therefore, ∑LREE/∑HREE in the Nw03 sample was much lower than that in other rock samples (Table 1). The average ∑LREE/∑HREE value in all samples was 8.6, which implied a significant fractionation of LREEs and HREEs [31]. The (La/Sm)_N values were 3.0~4.3, with an average value of 3.8, reflecting strong differentiation of LREEs in the rock samples [32,33]. The (Gd/Yb)_N values were 1.6~6.8, with an average value of 4.1 [34]. Rock samples showed slight Ce negative abnormalities (δCe mean = 0.9) and Eu showed negative anomalies of moderate intensity (δEu mean = 0.7). The δEu was negatively related to HREE content [35]. Eu tends to be enriched in the crystalline phase [36]. Only the Nw03 sample was located on S-type granite with a high degree of crystallization differentiation. This was consistent with the minimum δEu (0.5) in the Nw03 sample.

The average values of ∑REE and ∑LREE in the soil samples were 225 and 190 mg·kg⁻¹, respectively (Table 1). These values were lower than the background value of Yunnan soil but higher than the background value of Chinese soil [37]. The average value of ∑HREE was 35 mg·kg⁻¹, which was slightly lower than the background values of Yunnan soil and Chinese soil. The value of ∑HREE in the Nw03 sample developed from igneous rock was significantly higher than that of metamorphic rock. The range of ∑LREE/∑HREE values was 1.7~12.3, with an average value of 7.7. This result was similar to that of the rock samples. The average values of (La/Sm)_N and (Gd/Yb)_N were 3.3 and 3.2, respectively. These values reflected that there were significant differences between LREEs and HREEs in the soil samples. δCe was 0.6~2.3, with an average value of 1.1. Generally, Ce showed a slightly positive anomaly and an enriched state. δEu was 0.5~0.8, with an average value of 0.7. The overall performance of Eu showed a negative anomaly and loss.

Table 1. REE content in rock and soil samples (mg∙kg⁻¹), pH, organic matter (OM), and types of soil samples (g∙kg⁻¹).

Rock	$\sum \mathit{R}\mathit{E}\mathit{E}$	$\sum \mathit{L}\mathit{R}\mathit{E}\mathit{E}$	$\sum \mathit{H}\mathit{R}\mathit{E}\mathit{E}$	$\frac{\sum \mathit{L}\mathit{R}\mathit{E}\mathit{E}}{\sum \mathit{H}\mathit{R}\mathit{E}\mathit{E}}$	$\left(\frac{\mathit{L}\mathit{a}}{\mathit{S}\mathit{m}}\right){\mathit{N}}^{*}$	$\left(\frac{\mathit{G}\mathit{d}}{\mathit{Y}\mathit{b}}\right){\mathit{N}}^{*}$	${\mathit{\delta }\mathit{C}\mathit{e}}^{*}$	${\mathit{\delta }\mathit{E}\mathit{u}}^{*}$	pH	OM	Soil Type
Bw01-D	309	280	30	9.5	3.9	4.7	1.1	0.6	—	—	—
Bw03	194	176	18	9.9	4.1	3.2	1.0	0.7	—	—	—
Bw04	276	255	21	12.1	3.8	6.8	1.0	0.6	—	—	—
Dj01-D	208	195	13	14.6	4.0	6.1	1.0	0.7	—	—	—
Dj02	153	141	13	11.0	3.7	4.4	1.0	0.7	—	—	—
Lz01-D	177	160	17	9.6	4.3	4.2	0.9	0.8	—	—	—
Lz02-D	160	145	15	9.7	4.0	4.0	0.8	0.6	—	—	—
Lz03	122	97	24	4.0	3.5	3.6	0.6	1.0	—	—	—
Lz04	198	179	19	9.5	4.0	5.0	0.9	0.8	—	—	—
Np01-D	238	209	29	7.2	3.3	3.8	1.0	0.6	—	—	—
Nw01-D	132	106	26	4.0	3.4	1.6	0.7	0.6	—	—	—
Nw03	225	162	62	2.6	3.0	1.6	0.9	0.5	—	—	—
Mean	199	175	24	8.6	3.8	4.1	0.9	0.7	—	—	—
China a	235	185	49	3.8	—	—	—	—	—	—	—
UCC b	169	134	35	3.8	—	—	—	—	—	—	—
Soil
Bw01-B	170	149	21	7.2	3.4	3.0	1.2	0.6	5.69	8.6	Alfisols
Bw01-C	161	141	20	7.2	3.6	3.5	1.1	0.6	5.42	3.4	Ultisols
Bw02-B	157	140	17	8.3	3.8	2.8	1.2	0.7	6.06	5.9	Alfisols
Bw02-C	189	170	19	9.1	3.9	3.0	1.2	0.7	5.91	3.45	Ultisols
Bw02-D	180	158	22	7.2	3.7	2.6	1.1	0.7	5.76	2.6	Ultisols
Dj01-B	151	137	14	10.0	3.7	3.2	1.2	0.8	5.34	9.2	Alfisols
Dj01-C	208	189	19	10.1	3.7	4.5	0.9	0.8	5.70	5.0	Ultisols
Lz01-A	189	168	21	8.0	3.5	4.1	1.0	0.7	5.49	24.8	Mollisols
Lz01-B	204	180	24	7.5	3.4	4.1	1.0	0.7	5.49	30.1	Alfisols
Lz01-C	211	190	21	9.2	3.7	4.4	1.0	0.7	5.88	8.5	Ultisols
Lz02-A	145	128	17	7.6	3.3	2.7	1.4	0.7	5.12	49.1	Mollisols
Lz02-B	204	186	18	10.4	3.1	2.2	2.3	0.7	5.32	8.4	Alfisols
Lz02-C	175	155	20	7.9	3.7	2.7	1.2	0.7	5.84	2.6	Ultisols
Np01-B	212	196	16	12.3	3.8	4.9	1.0	0.6	5.07	2.6	Alfisols
Np01-C	159	146	13	11.5	3.9	4.3	1.1	0.7	5.50	1.6	Ultisols
Np02-B	171	153	18	8.5	3.5	2.8	1.2	0.7	4.98	18.0	Alfisols
Np02-C	181	165	16	10.5	3.9	4.6	1.0	0.7	5.12	10.9	Ultisols
Np02-D	183	166	17	9.9	3.9	4.8	1.0	0.7	5.23	3.5	Ultisols
Nw01-A	304	252	52	4.9	2.6	2.2	1.4	0.6	5.65	15.2	Mollisols
Nw01-B	365	293	72	4.1	2.7	2.0	1.2	0.6	5.24	8.6	Alfisols
Nw01-C	327	263	21	4.1	2.7	2.0	1.2	0.6	5.45	8.4	Ultisols
Nw02-A	287	252	34	7.3	2.2	4.0	1.1	0.6	4.90	12.8	Mollisols
Nw02-B	295	249	45	5.5	1.9	3.3	1.1	0.6	5.04	12.2	Alfisols
Nw02-C	406	279	127	2.2	2.8	1.3	1.1	0.7	5.94	1.2	Ultisols
Nw02-D	400	253	147	1.7	2.6	1.7	0.6	0.5	5.93	1.3	Ultisols
Mean	225	190	35	7.7	3.3	3.2	1.1	0.7	—	—	—
Yunnan soil c	248	195	53	3.8	—	—	—	—	—	—	—
China soil c	187	148	39	3.8	—	—	—	—	—	—	—

Note: *: values were standardized to REEs in chondrite [38]; a: Element abundance of continental crust in China [28]; b: Element abundance of upper continental crust [29]; c: Soil background values of Yunnan and China [37]. $\sum REE$ = (La~Lu + Y), $\sum LREE$ = (La~Eu), $\sum HREE$ = (Gd~Y), $(\sum LREE)/(\sum HREE)$ is the ratio of both. ${\left(La/Sm\right)}_N$ is the value of chondrite-normalized La/Sm, it reflects the degree of fractionation between LREEs. ${\left(Gd/Yb\right)}_N$ is the value of chondrite-normalized Gd/Yb, it reflects the degree of fractionation between HREEs. $\delta Ce$: anomalous value of Ce. $\delta Eu$: anomalous value of Eu.“—”: indicates that references are not given.

Table 1. REE content in rock and soil samples (mg∙kg⁻¹), pH, organic matter (OM), and types of soil samples (g∙kg⁻¹).

Rock	$\sum \mathit{R}\mathit{E}\mathit{E}$	$\sum \mathit{L}\mathit{R}\mathit{E}\mathit{E}$	$\sum \mathit{H}\mathit{R}\mathit{E}\mathit{E}$	$\frac{\sum \mathit{L}\mathit{R}\mathit{E}\mathit{E}}{\sum \mathit{H}\mathit{R}\mathit{E}\mathit{E}}$	$\left(\frac{\mathit{L}\mathit{a}}{\mathit{S}\mathit{m}}\right){\mathit{N}}^{*}$	$\left(\frac{\mathit{G}\mathit{d}}{\mathit{Y}\mathit{b}}\right){\mathit{N}}^{*}$	${\mathit{\delta }\mathit{C}\mathit{e}}^{*}$	${\mathit{\delta }\mathit{E}\mathit{u}}^{*}$	pH	OM	Soil Type
Bw01-D	309	280	30	9.5	3.9	4.7	1.1	0.6	—	—	—
Bw03	194	176	18	9.9	4.1	3.2	1.0	0.7	—	—	—
Bw04	276	255	21	12.1	3.8	6.8	1.0	0.6	—	—	—
Dj01-D	208	195	13	14.6	4.0	6.1	1.0	0.7	—	—	—
Dj02	153	141	13	11.0	3.7	4.4	1.0	0.7	—	—	—
Lz01-D	177	160	17	9.6	4.3	4.2	0.9	0.8	—	—	—
Lz02-D	160	145	15	9.7	4.0	4.0	0.8	0.6	—	—	—
Lz03	122	97	24	4.0	3.5	3.6	0.6	1.0	—	—	—
Lz04	198	179	19	9.5	4.0	5.0	0.9	0.8	—	—	—
Np01-D	238	209	29	7.2	3.3	3.8	1.0	0.6	—	—	—
Nw01-D	132	106	26	4.0	3.4	1.6	0.7	0.6	—	—	—
Nw03	225	162	62	2.6	3.0	1.6	0.9	0.5	—	—	—
Mean	199	175	24	8.6	3.8	4.1	0.9	0.7	—	—	—
China a	235	185	49	3.8	—	—	—	—	—	—	—
UCC b	169	134	35	3.8	—	—	—	—	—	—	—
Soil
Bw01-B	170	149	21	7.2	3.4	3.0	1.2	0.6	5.69	8.6	Alfisols
Bw01-C	161	141	20	7.2	3.6	3.5	1.1	0.6	5.42	3.4	Ultisols
Bw02-B	157	140	17	8.3	3.8	2.8	1.2	0.7	6.06	5.9	Alfisols
Bw02-C	189	170	19	9.1	3.9	3.0	1.2	0.7	5.91	3.45	Ultisols
Bw02-D	180	158	22	7.2	3.7	2.6	1.1	0.7	5.76	2.6	Ultisols
Dj01-B	151	137	14	10.0	3.7	3.2	1.2	0.8	5.34	9.2	Alfisols
Dj01-C	208	189	19	10.1	3.7	4.5	0.9	0.8	5.70	5.0	Ultisols
Lz01-A	189	168	21	8.0	3.5	4.1	1.0	0.7	5.49	24.8	Mollisols
Lz01-B	204	180	24	7.5	3.4	4.1	1.0	0.7	5.49	30.1	Alfisols
Lz01-C	211	190	21	9.2	3.7	4.4	1.0	0.7	5.88	8.5	Ultisols
Lz02-A	145	128	17	7.6	3.3	2.7	1.4	0.7	5.12	49.1	Mollisols
Lz02-B	204	186	18	10.4	3.1	2.2	2.3	0.7	5.32	8.4	Alfisols
Lz02-C	175	155	20	7.9	3.7	2.7	1.2	0.7	5.84	2.6	Ultisols
Np01-B	212	196	16	12.3	3.8	4.9	1.0	0.6	5.07	2.6	Alfisols
Np01-C	159	146	13	11.5	3.9	4.3	1.1	0.7	5.50	1.6	Ultisols
Np02-B	171	153	18	8.5	3.5	2.8	1.2	0.7	4.98	18.0	Alfisols
Np02-C	181	165	16	10.5	3.9	4.6	1.0	0.7	5.12	10.9	Ultisols
Np02-D	183	166	17	9.9	3.9	4.8	1.0	0.7	5.23	3.5	Ultisols
Nw01-A	304	252	52	4.9	2.6	2.2	1.4	0.6	5.65	15.2	Mollisols
Nw01-B	365	293	72	4.1	2.7	2.0	1.2	0.6	5.24	8.6	Alfisols
Nw01-C	327	263	21	4.1	2.7	2.0	1.2	0.6	5.45	8.4	Ultisols
Nw02-A	287	252	34	7.3	2.2	4.0	1.1	0.6	4.90	12.8	Mollisols
Nw02-B	295	249	45	5.5	1.9	3.3	1.1	0.6	5.04	12.2	Alfisols
Nw02-C	406	279	127	2.2	2.8	1.3	1.1	0.7	5.94	1.2	Ultisols
Nw02-D	400	253	147	1.7	2.6	1.7	0.6	0.5	5.93	1.3	Ultisols
Mean	225	190	35	7.7	3.3	3.2	1.1	0.7	—	—	—
Yunnan soil c	248	195	53	3.8	—	—	—	—	—	—	—
China soil c	187	148	39	3.8	—	—	—	—	—	—	—

Note: *: values were standardized to REEs in chondrite [38]; a: Element abundance of continental crust in China [28]; b: Element abundance of upper continental crust [29]; c: Soil background values of Yunnan and China [37]. $\sum REE$ = (La~Lu + Y), $\sum LREE$ = (La~Eu), $\sum HREE$ = (Gd~Y), $(\sum LREE)/(\sum HREE)$ is the ratio of both. ${\left(La/Sm\right)}_N$ is the value of chondrite-normalized La/Sm, it reflects the degree of fractionation between LREEs. ${\left(Gd/Yb\right)}_N$ is the value of chondrite-normalized Gd/Yb, it reflects the degree of fractionation between HREEs. $\delta Ce$: anomalous value of Ce. $\delta Eu$: anomalous value of Eu.“—”: indicates that references are not given.

The total content of REEs in tea samples was relatively low, ranging from 0.407 to 0.924 mg∙kg⁻¹ (

Table 2. Content of REEs and major elements in tea samples (mg∙kg⁻¹).

Elements	Bw	Dj	Lz	Np	Nw	Mean
Na	45.340	50.590	80.490	64.460	49.160	58.008
Mg	2584	2074	2308	2387	3302	2531
K	15,420	11,050	13,830	12,880	16,040	13,844
Ca	4357	4536	4647	4265	4352	4429
La	0.059	0.081	0.231	0.157	0.207	0.147
Ce	0.137	0.162	0.171	0.343	0.188	0.200
Pr	0.017	0.017	0.041	0.036	0.037	0.030
Nd	0.050	0.064	0.156	0.139	0.137	0.109
Sm	0.015	0.013	0.030	0.029	0.031	0.024
Eu	0.014	0.012	0.017	0.012	0.015	0.014
Gd	0.014	0.015	0.033	0.023	0.031	0.023
Tb	0.005	0.003	0.005	0.003	0.005	0.004
Dy	0.013	0.015	0.025	0.017	0.031	0.020
Ho	0.005	0.003	0.005	0.003	0.006	0.004
Er	0.008	0.010	0.014	0.009	0.018	0.012
Tm	0.004	0.002	0.002	0.001	0.003	0.002
Yb	0.008	0.009	0.011	0.009	0.016	0.011
Lu	0.003	0.002	0.002	0.001	0.002	0.002
Y	0.057	0.092	0.149	0.080	0.198	0.115
∑REE	0.407	0.499	0.891	0.862	0.924	0.717
∑LREEs	0.290	0.349	0.646	0.715	0.615	0.523
∑HREEs	0.117	0.150	0.245	0.147	0.310	0.194
∑LREEs/∑HREEs	2.487	2.327	2.634	4.881	1.984	2.863

). The values of ∑LREE and ∑HREE ranged from 0.290 to 0.715 mg∙kg⁻¹ and 0.117 to 0.310 mg∙kg⁻¹, respectively. The values of ∑LREE/∑HREE were higher than 1, indicating an enrichment of LREEs in tea. These results were similar to those in previous studies [39,40]. Ce, La, Nd, and Y were dominant REEs in tea. LREEs were enriched in tea due to fractionation by plants [41]. The ∑HREE and ∑REE values of tea grown on igneous rock are slightly higher than those grown on metamorphic rock.

3.1.2. Distribution Patterns of REEs in Rock, Soil, and Tea Samples

The distribution patterns of chondrite-normalized REEs in rock and soil samples from the study area is shown in Figure 2. The distribution curve was inclined to the right, which was the type of light rare earth enrichment. The soil REE distribution curves were basically consistent with the rock distribution curves. LREEs were richer than HREEs. The REE content of soil from igneous rock was higher than that from metamorphic rock. This implied element transfer between rock and soil to a certain extent.

Ce showed slightly negative anomalies in rock samples, but slightly positive anomalies in soil samples. This was related to the geochemical behavior caused by the oxidation-reduction reaction of Ce [42]. Ce³⁺ is easily oxidized to Ce⁴⁺. Weathering facilitates the hydrolysis of Ce⁴⁺, where it remains in situ. With increasing plantation time of tea, the soil will gradually acidify [43,44]. In an acidic soil environment, H⁺ will promote the complexation of Ce⁴⁺, resulting in enrichment of Ce in soil [42,45]. Eu in the soil inherits the negative anomalous characteristics in bedrock. In a humid and warm environment, Eu³⁺ is reduced to Eu²⁺ with strong activity. This is different from the trivalent cations of other REEs. Eu²⁺ is leached out, resulting in the loss of Eu in soil [46,47].

The distribution patterns of chondrite-normalized REEs in tea samples from the study area and previous studies are shown in Figure 3 [10,48,49,50]. The distribution curves of REEs in tea were the same as those in rock and soil. Eu showed significantly positive anomalies in tea samples, in opposition to those in soil and rock samples. According to previous studies, divalent Eu could have a significantly different geochemical behavior from other REEs [51,52]. Positive Eu anomalies in plants were usually explained as the result of the reduction of Eu³⁺ to Eu²⁺ in a reducing environment and preferential uptake of Eu²⁺ through the roots of plants [53].

The pH value of surface soil in Fujian was approximately 4.27 [54], while the soil pH in Guizhou ranged from 3.92 to 5.45, with an average value of 4.49 [55]. Comparatively, the average pH values of soil were 5.5 in Guangxi [56] and 5.64 in Sichuan [57,58]. It was found that tea grown in soil pH < 5 generally showed negative Eu anomalies, whereas positive Eu anomalies were found in tea grown in soil pH > 5. Soil pH could affect the fractionation of Eu in tea.

The pH value of soil samples ranged from 4.90 to 6.06 in this study, with an average value of 5.46. Levels of chondrite-normalized Eu in tea increased linearly with soil pH (Figure 4a). Similar to the distribution of Eu, positive anomalies were shown in the chondrite partition map of tea (Figure 3). Additionally, the ratio of Eu content in tea to that in soil increased with increasing soil pH (Figure 4b). These results suggested that soil pH promoted Eu uptake by tea. This process might be related to the improvement of Eu absorption by Ca, according to the results of previous experiments [59,60]. As is well known, Ca can be leached and washed away under lower soil pH. Therefore, the soil pH value was one of the important factors affecting Eu anomalies in tea.

3.2. Transfer of REEs in Rock-Soil-Tea

The transport factor could characterize the behavior of REEs in rock-soil-tea. As shown in Figure 5, with increasing LREE concentration (in tea and/or soil), the bioaccumulation factor and transport factor of the LREEs increased logarithmically (Figure 5a,d). The higher the content, the larger the factor. The transport factor of LREE from rock to soil increased with increasing bioaccumulation factor (Figure 5g). Research has shown that LREEs are easily enriched in plants. Moreover, during rock weathering, LREEs are preferentially adsorbed by clay and enriched in soil due to the large radii of LREE ions and their strong adsorption capacities [61]. Therefore, Figure 5a,d,g supported LREE fractionation in soil to tea, as shown in Table 1 and Table 2 above.

The transfer process of HREEs was more complex than that of LREEs. The concentration of HREEs in tea changed significantly at 0.2 mg·kg⁻¹. The bioaccumulation factor of HREEs gradually increased below 0.2 mg·kg⁻¹ and decreased above 0.2 mg·kg⁻¹ (Figure 5b). The absorption capacity of REEs by tea would be enhanced in an environment with low REE content but inhibited under high soil REE content. There are hormones in the plant to regulate the concentrations of elements so that the enrichment of REEs can be maintained within a certain range. With increasing soil HREE concentrations, the transport factor of HREEs decreased, which may be related to the immobilization of HREEs during metamorphism [62]. With increasing HREE bioaccumulation factor, the HREE transport factor increased logarithmically (Figure 5h).

Eu is one of the most active rare earth elements [59]. There are many factors affecting the absorption of Eu in tea, such as soil pH, Ca, and the regulation of hormones in tea [5,53,59]. Under the effects of multiple factors, there was no significant correlation between the Eu concentration in tea and the Eu transfer coefficient from soil to tea. The transfer coefficient of Eu from rock to soil had a logarithmic relationship with the content of Eu in soil.

Most plants discriminate between cations by their ionic radius and charge during uptake and transfer within plant tissues [63,64]. The absorption of REEs is mediated by Na⁺, Ca²⁺, and K⁺ channels in plant cells [65]. LREEs and Ca²⁺ have similar ion radii and physical and chemical properties in plants [60,66]. Eu can replace Ca in plants and/or replace Ca at cation binding sites in soil [59,67]. This also explains the preferential accumulation of LREEs (Table 2) [68,69]. This study found that Na and LREEs had similar biological characteristics (

Table 3. Principal component analysis between major elements and REEs in tea.

	Principal Component
Elements	1	2	3	4
Ca	0.113	0.070	−0.167	0.977
K	0.174	0.299	0.908	−0.236
Mg	0.071	0.677	0.588	−0.437
Na	0.869	−0.166	−0.248	0.394
La	0.844	0.524	−0.006	0.111
Ce	0.499	−0.171	−0.483	−0.698
Pr	0.933	0.351	−0.028	−0.078
Nd	0.916	0.374	−0.132	−0.066
Sm	0.881	0.419	0.023	−0.218
Eu	0.573	0.284	0.508	0.577
Gd	0.819	0.557	0.098	0.099
Tb	0.236	0.264	0.923	0.154
Dy	0.496	0.845	0.182	0.089
Ho	0.062	0.643	0.758	0.092
Er	0.293	0.930	0.195	0.103
Tm	−0.381	−0.099	0.919	0.036
Yb	0.240	0.959	0.152	−0.002
Lu	−0.334	0.047	0.941	−0.005
Y	0.387	0.900	0.126	0.157
Cumulative variance contribution rate	32.13%	61.16%	87.71%	100.00%

Note: 1: First principal component, elements with larger values were Na and LREEs, explaining 32.13% of the experimental data. 2: Second principal component, explaining 29.03% of the data. 3: Third principal component, explaining 26.55% of the data. The values of Mg, K, and HREEs were large and distributed in principal components 2 and 3. 4: Fourth principal component, elements with larger values were Ca and Eu, explaining 12.29% of the data.

, principal component 1). Mg and K were related to the differentiation of HREEs (principal components 2 and 3). The competition between Ca and Eu was an important part of tea growth (principal component 4).

Soil weathering and plant metabolism are important processes that affect the transfer of REEs from soil to tea. There was a significantly negative correlation between soil K and tea Dy in the study area (

Table 4. Correlation analysis of soil pH, major elements, organic matter, and REEs in tea.

Elements	pH	K	Ca	Mg	Na	Organic Matter
La	−0.466	−0.785	0.618	0.853	0.634	0.610
Ce	−0.678	0.072	−0.495	−0.193	−0.136	−0.454
Pr	−0.511	−0.724	0.462	0.709	0.506	0.464
Nd	−0.563	−0.669	0.439	0.692	0.489	0.446
Sm	−0.597	−0.769	0.357	0.718	0.591	0.346
Eu	0.270	−0.765	0.946 *	0.787	0.472	0.919 *
Gd	−0.436	−0.850	0.634	0.893 *	0.694	0.615
Tb	0.314	−0.809	0.560	0.621	0.568	0.486
Dy	−0.488	−0.886 *	0.566	0.982 **	0.923 *	0.516
Ho	0.020	−0.846	0.475	0.774	0.836	0.384
Er	−0.470	−0.830	0.505	0.957 *	0.964 **	0.444
Tm	0.616	−0.244	0.082	0.002	0.124	0.012
Yb	−0.557	−0.796	0.391	0.917 *	0.978 **	0.328
Lu	0.504	−0.372	0.115	0.138	0.281	0.034
Y	−0.478	−0.824	0.566	0.973 **	0.926 *	0.515

Note: **: Significant correlation at 0.01 level (two tailed). *: Significant correlation at the 0.05 level (two tailed).

). Previous experiments showed that the K⁺ content decreased and the Dy concentration increased with the release of H⁺ in plant roots during the process of soybean growth [70], but there was a significantly positive correlation between Ca and Eu. Ca promoted the accumulation of Eu. Organic matter also promoted the increase in Eu content under the action of Ca [59]. A significantly positive correlation occurred between Mg, Na, and HREEs in tea.

3.3. Assessment of Potential Risk to Human Health through Tea Drinking

The last step of the migration of rare earth elements in the food chain is entering the human body by oral intake, such as in drinking tea. The human health risk assessment of tea samples can quantitatively evaluate the safety of tea drinking for the human body. China and other countries all over the world have not yet formulated health guidance value for REEs. The daily allowable intake of rare earth oxides is 4.2 mg [71], which was the most common guidance value used in the current study to assess the health risk of REE exposure. If the study weight was 61.75 kg, the ADI value of the reference standard was 7.75 × 10⁻⁴ mg/kg·day. The maximum and minimum ADI values of REEs in this study were 2.09 × 10⁻⁴ and 7.31 × 10⁻⁵ mg/kg·day, respectively (

Table 5. Acceptable daily intake of REEs (ADI) (mg/kg (body weight)∙day), non-carcinogenic risk (HQ) and carcinogenic risk (Risk).

	ADI			HQ			Risk
	Min	Max	Mean	Min	Max	Mean	Min	Max	Mean
La	1.08 × 10−5	4.26 × 10−5	2.71 ×10−5	5.41 × 10−4	2.13 × 10−3	1.36 × 10−3	1.19 × 10−16	4.69 ×10−16	2.98 × 10−16
Ce	2.52 × 10−5	6.33 × 10−5	3.69 ×10−5	—	—	—	8.88 × 10−16	2.23 × 10−15	1.30 × 10−15
Pr	3.12 × 10−6	7.62 × 10−6	5.48 ×10−6	6.24 × 10−6	1.52 × 10−5	1.10 × 10−5	2.47 × 10−17	6.04 × 10−17	4.34 × 10−17
Nd	9.19 × 10−6	2.87 × 10−5	2.01 ×10−5	1.84 × 10−5	5.75 × 10−5	4.03 × 10−5	5.00 × 10−18	1.56 × 10−17	1.10 × 10−17
Sm	2.49 × 10−6	5.69 × 10−6	4.36 × 10−6	4.98 × 10−6	1.14 × 10−5	8.73 × 10−6	9.32 × 10−17	2.13 × 10−16	1.63 × 10−16
Eu	2.14 × 10−6	3.21 × 10−6	2.54 × 10−6	1.07 × 10−3	1.61 × 10−3	1.27 × 10−3	2.21 × 10−17	3.31 × 10−17	2.62 × 10−17
Gd	2.57 × 10−6	6.07 × 10−6	4.29 × 10−6	—	—	—	3.90 × 10−18	9.23 × 10−18	6.53 × 10−18
Tb	4.98 × 10−7	9.60 × 10−7	7.75 × 10−7	—	—	—	2.43 × 10−18	4.68 × 10−18	3.78 × 10−18
Dy	2.36 × 10−6	5.69 × 10−6	3.70 × 10−6	—	—	—	9.78 × 10−19	2.35 × 10−18	1.53 × 10−18
Ho	5.72 × 10−7	1.13 × 10−6	8.16 × 10−7	—	—	—	5.27 × 10−18	1.04 × 10−17	7.52 × 10−18
Er	1.55 × 10−6	3.32 × 10−6	2.19 × 10−6	—	—	—	3.92 × 10−18	8.41 × 10−18	5.53 × 10−18
Tm	2.58 × 10−7	6.65 × 10−7	4.14 × 10−7	—	—	—	1.81 × 10−19	4.65 × 10−19	2.89 × 10−19
Yb	1.40 × 10−6	2.88 × 10−6	1.93 × 10−6	—	—	—	5.61 × 10−18	1.15 × 10−17	7.71 × 10−18
Lu	2.58 × 10−7	5.72 × 10−7	3.88 × 10−7	2.87 × 10−4	6.36 × 10−4	4.31 × 10−4	9.12 × 10−19	2.02 × 10−18	1.37 × 10−18
Y	1.06 × 10−5	3.65 × 10−5	2.13 × 10−5	2.65 × 10−3	9.12 × 10−3	5.31 × 10−3	1.92 × 10−16	6.61 × 10−16	3.85 × 10−16
Total∑	7.31 × 10−5	2.09 × 10−4	1.32 × 10−4	4.58 × 10−3	1.36 × 10−2	8.43 × 10−3	1.37 × 10−15	3.73 × 10−15	2.26 × 10−15

). These values were far below the reference value and no health risk was found. The size order of ADI values was: Ce > La > Y > Nd > Pr > Sm > Gd > Dy > Eu > Er > Yb > Ho >Tb > Tm > Lu. Approximately 27.90% of Ce, 20.50% of La, and 16.06% of Y elements entered the human body through tea, which was within the safe range. According to the statistics, the average ADI values of REEs absorbed by tea drinkers in China, southern China, and northern China were 6.2 × 10⁻⁵, 7.7 × 10⁻⁵, and 4.8 × 10⁻⁵ mg/kg·day, respectively [72].

The HQ values of REEs were far less than 1 (Table 5), indicating no carcinogenic risk. The HQ values of Y and La accounted for the largest proportion, accounting for approximately 63.02% and 16.08% of the total HQ value, respectively (Figure 6a). The bioavailability of Y is the highest among REEs, but excessively high concentrations will have a negative impact on plant growth [73,74]. Reference doses (RfD) of Ce, Gd, Tb, Dy, Ho, Er, Tm, and Yb have not been officially reported. Thus, the information for assessing carcinogenic risk is insufficient [75]. The Risk and Risk_Total values of REEs in all tea samples were much less than 10⁻⁶ (Table 5), suggesting that long-term tea drinking will not cause cancer-related diseases. The proportions of element risk values were: Ce (57.49%) > Y (17.02%) > La (13.20%) > Sm (7.22%) > Pr (1.92%) > Eu (1.16%) > Nd (0.48%) > Yb (0.34%) > Ho (0.33%) > Gd (0.29%) > Er (0.24%) > Tb (0.17%) > Dy (0.07%) > Lu (0.06%) > Tm (0.01%) (Figure 6b). La and Ce generally appear at high levels in the mesophyll and leaves of plants [76] and are easily accumulated in crop plants [77]. Therefore, Ce, La, and Y in plants could have effects on human health [78].

4. Conclusions

The total content of REEs in tea samples was low. Ce, La, Nd, and Y were dominant REEs in tea. LREEs were more easily enriched than HREEs in tea. This study found that the soil pH affected the Eu content in tea. K, Na, Ca, and Mg were the main factors affecting the migration of REEs in tea. K was negatively correlated with REEs, whereas Na, Ca, and Mg were positively correlated with REEs. LREEs was considered to enter tea mainly through the Na⁺ channel. Mg and K jointly affected HREEs in tea. Ca can significantly promote the enrichment and transfer of Eu. The human health risk assessment proved the safety of tea in the study area. Ce, Y, and La in tea were the main elements that affected non-carcinogenic risk and carcinogenic risk. Tea gardens should pay attention to timely control.

The geological background affects the transfer and accumulation of REEs in soil and plants. Rock provides the initial content of REEs in soil and tea. The soil obviously inherited the characteristics of LREE enrichment from rock in this study. The characteristics of REEs in tea were both similar and different from those in rock and soil. Understanding the effects of the geological background on tea growth is essential for scientific planting and economic benefits.