Distribution and source determination of rare earth elements in sediment collected from the continental shelf off Hainan Island, China

Contents of rare earth elements (REEs), major elements, and the total organic carbon (TOC) were determined for 152 surface sediment samples collected from the continental shelf off Hainan Island (CSHI). From high to low, the average contents of REEs were as follows: Ce > La > Nd > Pr > Sm > Gd > Dy > Er > Yb > Eu > Ho > Tb > Tm > Tm. The LREEs in the south are more abundant than in the north, which is shown by the higher LREE/HREE values in south than in the north. This resulted higher values for the LREE/HREE ratio in the south than in the north. The mean enrichment factor (EF) could be arranged from highest to lowest as follows: Tm > Sm > Pr > Er > La > Lu > Ce > Tb > Eu > Nd > Yb > Gd > Ho > Dy. The EF indicates that pollution as a result of human activity was more serious in the southeast of the study area than in the north. The factors affecting the REE concentrations in this area include naturally occurring minerals and industrial pollution. Based on the spatial variation of upper continental crust (UCC)-normalized REE concentrations, the CSHI was classified into three geochemical provinces. The sediment of province I was controlled by the Red and Pearl rivers. The composition of the province II is mainly controlled by the Red River and the Pearl River, although some sediments have originated from the South China Sea Island. Province III sediments mainly originated from sources on Hainan Island.


Introduction
Rare earth elements (REEs), a group of related lanthanides (McLennan et al. 1980;Balaram 2019), have been used as indicators of geochemical processes. Examples of these geochemical processes include the chemical evolution of the crust and mantle (Cai et al. 2021a), tectonic evolution (Chen et al. 2020), surficial weathering (Hannigan et al. 2010;Qiao et al. 2020), and for identifying the source of material in different environmental systems (Delgado et al. 2012;Khan et al. 2016;da Silva et al. 2018;Li et al. 2018;Oral et al. 2019). Actually, the high content of REEs in soil, sediment, and water has been proved to be harmful to human and animal health (Adeel et al. 2019). In studies of marine sediments, REEs have been used as important indicators for sediment source tracking (Wang et al. 2014), climate change (Hatano et al. 2020), and environmental assessment (González et al., 2014;Herrmann et al., 2016;Renjan et al. 2017;Trifuoggi et al. 2018). REEs are not used as indicators of environmental pollution as often as heavy metals (e.g., Cu, Pb, Zn, Cr, Co, Ni, Cd, and Hg). However, REEs are widely used in industry, which will change their content in sediments and increased the risk of environmental pollution (Trifuoggi et al. 2018;Adeel et al. 2019;Morin-Crini et al. 2021). The distribution, transport, and generation of REEs in continental shelf surface sediments have started to attract the attention of researchers (Wu et al. 2020).
Hainan Island is located in the northwestern reaches of the South China Sea (SCS). It is the second largest island in China, with an area of more than 30000 km 2 (Fig. 1a). The concentrations of contaminants in the surface sediments off the island used to be relatively low, but in recent years, the rapid development of industry, agriculture, domestic sewage discharge, and tourism has been associated with a decrease in environmental quality (Mo et al., 2019;Cai et al. 2021b). Most previous studies of the chemical composition of the marine sediments have had a heavy metal focus and only considered part of the continental shelf off Hainan Island (CSHI) (Hu et al. 2013;Xu et al. 2015;Zhao et al. 2020;Cai et al. 2021b). No studies have considered the concentration and distribution of REEs in the sediments off the island. This has limited the assessment of environmental change in this area.
In this study, the concentrations of REEs, major elements, and TOC in surface sediments collected from the CSHI were determined. As the distribution of REEs in the surface sediments has never been reported before, the data can be used as a baseline for future research. The aims of this paper were as follows: (i) to identify the distribution of REEs in surface sediments collected from the CSHI; (ii) to calculate the enrichment factor (EF) value of REEs and its spatial variation; (iii) to determine whether the enrichment of REE is related to human activities; and (iv) to identify the potential sources of REEs in surface sediments collected from the CSHI. The purpose of this study is to provide a scientific basis for environmental management of the CSHI, with particular reference to the distribution of REEs in marine sediments. In addition, we hope that the results presented in this study can contribute to an understanding of the distribution, sources, and associated risks of REEs in continental shelf environments in other parts of the world.

Regional setting
The study area is located in the southwestern part of continental shelf of the northern reaches of the SCS (Fig. 1b) Fig. 1 Location of the study area and sampling sites western part of the study area is the Beibu Gulf, and the eastern part is the shelf of the northern SCS, while the southern part is the western slope (Cao et al. 2019;Xu et al. 2015). The isobaths offshore are aligned parallel to the coastline. The water depth ranges from 20 to 200 m. The water depth in the western part of the study area changes gradually, while that in the east drops sharply (Huang et al., 2013). Previous studies of the chemical composition of surface sediments in this area were restricted to heavy metals (Hu et al. 2013;Xu et al. 2015;Zhao et al. 2020;Cai et al. 2021b). As their conclusions were limited to the spatial variation of heavy metal elements, their data cannot be used to provide a comprehensive baseline of environmental conditions for the CSHI.

Analytical methods
In this study, a grab was used to obtain 152 marine sediment samples from the CSHI in August 2019 (Fig. 1c). The work was carried out by the China Geological Survey (project no. DD20190627). The pretreatment of samples was described by Cai et al. (2021b). The determination of REE concentrations was carried out by inductively coupled plasma-mass spectrometry (ICP-MS, ThermoFisher IRIS Intrepid II XSP, USA) at the Geological and Environmental Institute of China University of Geosciences (GEICUG). The mixed standard solution including trace elements and rare earth elements was used to configure the working curve, and the standard sample (GBW07333) was used as the internal standard to correct the instrument signal deviation. The determination of major element (SiO 2 , TiO 2 , Al 2 O 3 , Fe 2 O 3 , MnO, MgO, CaO, Na 2 O, K 2 O, and P 2 O 5 ) concentrations was calculated by the test curves of China national first-class reference materials by X-ray fluorescence spectrometer (XRF, Bruker S4 PIONEER, German) at GEICUG. The methods used for quality control and assurance have been described in Wang et al. (2015). In order to determine the total organic carbon (TOC) content, the sample was pretreated as follows: firstly, the sample was soaked in 40 mL 4 mol/L HCl for 48 h to remove the inorganic carbon, and then the acidified sample was repeatedly washed with ultrapure water to pH 7, and finally the sample was freeze-dried. Every 0.05 to 40 mg samples were used to determine the TOC with an element analyzer (Vario EL-III) at GEICUG. A total of 30 parallel samples were tested, one of which was randomly selected from every five samples. The measurement errors of major elements, trace elements, and TOC were within 5% RSD, 3% RSD, and 5% RSD for repeated sediment analysis. This work was carried out at the GEICUG. The statistical analyses were carried out by using IBM SPSS statistics software (version 25). ArcMap 10.2 was used to produce maps, while Origin 2021 was used to produce the graphs.

Descriptive statistics for the concentrations of major elements and REEs
The descriptive statistics for the concentrations of major elements and REEs in sediments collected from the CSHI have been presented in Supplementary Table 1. The average concentrations (as a percentage) of SiO 2 , Al 2 O 3 , Fe 2 O 3 , CaO, MgO, K 2 O, Na 2 O, MnO, P 2 O 5 , and TiO 2 were 60.48%, 11.80%, 4.68%, 5.51%, 2.02%, 2.16%, 1.62%, 0.06%, 0.10%, and 0.62%, respectively. In addition, the average concentration of TOC was 0.58%. The average concentrations (as μg/g) of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu were 35.82 μg/g, 70.83 μg/g, 8.25 μg/g, 28.85 μg/g, 5.79 μg/g, 1.09 μg/g, 4.12 μg/g, 0.78 μg/g, 3.66 μg/g, 0.79 μg/g, 2.65 μg/g, 0.40 μg/g, 2.19 μg/g, and 0.36 μg/g, respectively. The total concentration of REEs (ΣREE) was between 44.21 and 221.68 μg/g (mean of 165.57 μg/g). The concentration of light REEs (LREEs, mean of 150.62 μg/g) was higher than for the heavy REEs (HREE, mean of 14.95 μg/g) in the sediment samples. Wilding (1985) divided coefficient of variation (CV) into three degrees of variation, namely, low variation (CV < 0.16), moderate variation (0.16 < CV < 0.36), and high variation (CV > 0.36). The variation coefficients of La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu displayed a moderate degree of variation, namely, 0.22, 0.22, 0.22, 0.24, 0.20, 0.24, 0.33, 0.33, 0.29, 0.32, 0.31, respectively (Supplementary Table 1). The result showed that Gd had a high degree of variation, with a variation coefficient of 0.44. In conclusion, all the REEs of the CSHI show moderate to high variability, indicating that they may be affected by human activities.  Fig. 3. The concentrations of REEs in the north of study area were higher than in the south. The LREEs in the south are more abundant than in the north, which is shown by the higher LREE/HREE values in south than in the north.  Fig. 3 Spatial distribution of REEs in surface sediments of the CSHI were similar to those reported for the Yellow Sea (Mi et al. 2020), Bohai Bay (Zhang and Gao 2015), and East China Sea (Mi et al. 2020). It is worth noting that the continental shelf of China has a higher background value of REEs and is more likely to reach the critical value of pollution because of human activities.

Enrichment factor of REEs
The enrichment factor (EF) of REEs in surface sediments can be used to evaluate the degree of anthropogenic pollution (N'Guessan et al. 2009). Al, Fe, and Ti are often used as reference elements due to their chemical stability (N'Guessan et al. 2009). In this study, the concentration of Ti vs. REEs showed a low coefficient of variation (0.27) and a high correlation (0.73), so Ti was used as a reference value to calculate the EF. The EF values of REEs in sediments were calculated by using formula (1).
(Ci/Ti) sample was the ratio of REEs and Ti concentrations in the sample, while (Ci/Ti) background was the ratio of REEs and Ti concentrations in the upper continental crust. Sutherland (2000) assigned levels of pollution to one of five classes according to the EF value as follows: (i) EF < 2 (no or minimal pollution); (ii) 2 ≤ EF < 5 (moderate level of pollution); (iii) 5 ≤ EF < 20 (significant level of pollution); (iv) 20 ≤ EF < 40 (very high level of pollution); and (v) EF > 40 (extremely high level of pollution).
The calculated EF values for REEs for this study have been presented in Supplementary Table 3 (Fig. 4), which indicates that all samples could be assigned to the no or minimal pollution class (Sutherland 2000). However, the maximum values for individual elements indicate different levels of pollution. The maximum EF values of Pr and Nd were 7.80 and 6.26, respectively, consistent with the significant pollution class described above. The maximum EF values of the other rare earth elements are between 2 to 5, which would indicate moderate levels of pollution. The distribution of EF for REEs in the CSHI has been showed in the Fig. 5. There is no pollution by REEs in the northern reaches of the CSHI. However, the EF values of REEs in the southeast of the shelf near Hainan Island are the highest, which could be attributed to human activity. This may be related to the close proximity of this area to Wenchang City, a city with a range of industries and port facility.

The correlation analysis between major elements and REEs in the sediment
The correlations between major elements and REEs are shown in Fig. 6. The depth of color in the graph is based on the strength of the correlational relationship between the variables. REEs are correlated with TiO 2 (always ≥ 0.66), indicating that REEs are hosted in rutile. There is no correlation between TOC and REEs, indicating that the content of organic matter has no effect on the distribution of REEs in sediments. It is well known that acidic mine drainage can form small-scale (colloidal) "iron hydroxide" deposits (Ayora et al. 2016). These hydroxides can enrich REEs by precipitation and adsorption (Barcelos et al. 2018) and then enter the sediments through the so-called salting out process (Sholkovitz and Szymczak 2000;Kulaksiz and Bau 2007). This also explains the high correlation between Fe 2 O 3 and REEs (always ≥ 0.44). There is also a high correlation between Al 2 O 3 and REEs (always ≥ 0.41), while REEs may also occur in clay minerals. There is a significant correlation among the REEs, which indicates that the factors controlling the spatial variation in the concentrations in the sediments should be the same . In general, the main factors affecting the distribution of REEs in this area are the composition of naturally occurring minerals and industrial pollution.

Sediment source discrimination
It is generally acknowledged that the movement of rare earth elements in the natural environment is mainly determined by the activities of the earth's surface, such as the weathering of parent materials from the crust through hydrodynamic, geochemical, and biological processes (Adeel et al. 2019). The upper continental crust (UCC)normalized REE concentrations are often used to identify the source of sediments (Hossain et al. 2010;Wu et al. 2020). The results of UCC-normalized REE concentrations are showed in Fig. 7. The sediments of the CSHI can be divided into three provinces.
Province I is the northeastern area of the CSHI near the mouth of the Pearl River. The Pearl River, the largest river in southern China, discharges 84.30 Mt of suspended particulate matter into the SCS each year (Cao et al. 2019). The UCC-normalized REE patterns for sediments from province I are similar to those for the Pearl River and Red River (Fig. 7a). This indicates that the northern part of the   This would also explain the high REEs concentrations in the north and the Qiongzhou Strait (Fig. 3). Province II is in the northwestern part of the Hainan Island shelf, with the Red River in the north. The spatial variation in UCCnormalized REE concentrations for province II is similar to those for the Red River and the Pearl River, but also slightly affected by sediments from Hainan Island (Fig.  7b). It also explains the higher REE concentrations in this area, which is located close to the mouth of the Red River, Qiongzhou Strait, and Hainan Island (Fig. 3). The spatial variations of UCC-normalized REE concentrations in province III are different from the other two provinces, which indicates the importance of Hainan Island as a sediment source for province III (Fig. 7c). This is consistent with the distribution of REE from high to low in the south of CHSI (Fig. 3). In summary, the spatial variation of REEs in province I was determined by close proximity to the mouths of the Red and Pearl rivers, whereas the concentrations in sediments in provinces II and III could be attributed to a mixed contribution from three sources (Fig. 7d).

Conclusions
(  19 μg/g, and 0.36 μg/g, respectively. The ΣREE ranges from 44.21 to 221.68 μg/g. (2) The EF value of REEs is the highest southeast of Hainan Island, which indicates that elevated levels of REEs in this area were affected by human activities.
(3) Based on the spatial variation of the UCC-normalized REE concentrations, the CSHI was classified into three geochemical provinces. The sediment of province I is controlled by close proximity to the mouths of the Red and Pearl rivers, while the origin of sediment in provinces II and III could be attributed to three sources and hence is of a more mixed composition.  (Clift et al. 2008), soil of Hainan Island (Wang et al.,2011)