Distributions of dissolved rare earth elements and anthropogenic gadolinium in the Yeongil Bay, Korea

1. Introduction

Rare Earth Elements (REEs) are composed of 15 elements of the lanthanum group (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) with atomic numbers from 57 to 71. REEs behave similarly in water environments because of trivalent oxidation in seawater, except for cerium (Ce⁴⁺) and europium (Eu²⁺) (Goldberg et al., 1963; Elderfield and Greaves, 1982). Recently, REEs have been used in several high-tech industries such as steel mills, wind turbines, electrical car engines, petroleum refining, mobile phones, displays, hi-capacity batteries etc. (Kulaksız and Bau, 2013; Massari and Ruberti, 2013). Thus, over the past few decades, there has been increase in demand for these goods, resulting in the artificial discharge of REEs into the aquatic environments.

Among the REEs, gadolinium (Gd) is used as a paramagnetic contrast-enhancing agent (e.g., Gd-(DTPA)^2-) for magnetic resonance imaging (MRI) (Bau and Dulski, 1996a; Künnemeyer et al., 2009). Because of the high stability of these Gd compounds, they pass through the human body and the sewage system or wastewater treatment plant (WWTP) (Kulaksız and Bau, 2007), and are released into aquatic environments (or marine environment through the estuary), resulting in positive Gd anomaly which are defined by the ratio of the normalized measured concentration to theoretical concentration. Therefore, positive Gd anomalies have been utilized to trace the origins of anthropogenic substances (Bau and Dulski, 1996a; Möller et al., 2000; Hatje et al., 2016; Kim, T. et al., 2020). Positive Gd anomaly was first reported by (Bau and Dulski, 1996a), and previous studies demonstrated the enrichment of REEs with positive Gd anomalies in urbanized aquatic systems that include estuaries, bays, lakes, and rivers in the past few decades (Möller et al., 2000; Nozaki et al., 2000; Elbaz-Poulichet et al., 2002; Knappe et al., 2005; Kulaksız and Bau, 2007; Hatje et al., 2016; Song et al., 2017; Kim, I. et al., 2020; Kim, T. et al., 2020; Lim et al., 2023).

However, only a few studies have investigated ananthropogenic Gd (Gd_anth) on aquatic environments in Korea (Song et al., 2017; Kim, I. et al., 2020; Kim, T. et al., 2020; Lim et al., 2023). Thus, in this study, we collected water samples from the Yeongil Bay, Korea (YB) to investigate the distributions of dissolved REEs and Gd_anth. This study region was selected based on the high discharge probability of anthropogenic REEs into aquatic environment due to the surrounding highly urbanized areas. Furthermore, using Gd anomalies, concentrations of Gd_anth have been evaluated quantitatively.

2. Materials and methods

2.1 Study region

The YB, which is surrounded by Pohang (~0.5 million population) locating on the eastern coast of Korea, has semi-closed topographical characteristics that are indented in the land direction (Fig. 1). YB is 10 km wide and 12 km long with an area of approximately 120 km². The annual amount of freshwater discharge from the Hyeongsan River, which is discharged into YB, is approximately 6 × 10⁸ m³ (Lee et al., 2003). YB is considered to be one of the Korea’s most industrialized regions because of the presence of several industrialized complexes including large-scale steel mill complexes. Therefore, approximately 300,000 m³ d^-1 of industrial and municipal wastewater discharge through the Hyeongsan River or directly into the bay (Koh et al., 2006).

Fig. 1.

Map showing the Yeongil Bay (YB). The triangles indicate sampling sites.

3. Results and discussions

3.1 Distribution of dissolved REEs concentrations

Water temperatures and salinity obtained in YB ranged from 11.2 to 15.7 ℃ and 21.4 to 34.7, respectively. Even though salinity values increased from the inner bay to costal sea, the high salinities (21.4-30.6) at riverside sampling sites (from B1 to B3) indicate that seawater can easily enter the river through the bay (Table 1).

Table 1.

The concentrations of dissolved REEs (pM) with temperature, salinity, Gdanomaly (Gd/Gd*), and estimated concentrations of anthropogenic Gd (Gdanth) in the Yeongil Bay.

The concentration of dissolved REEs in YB varied by one to two orders of magnitude, for example from 20.8 to 102.8 pM for Nd and from 0.8 to 8.6 pM for Lu. Significantly high concentrations dissolved Gd (244.9 ± 100.6 pM, n = 3) were observed at riverside sampling sites (from B1 to B3), which were about 18 times higher than those at other sampling sites (13.7 ± 8.4 pM, n = 24). In general, the concentration of dissolved REEs showed decreasing trends from the riverside to outer bay (Table 1). In addition, the dissolved REEs concentrations showed negative correlations with salinity (Fig. 2). Previous studies showed the dissolved REEs losses in the low-salinity region of estuaries by salt-induced coagulation of river-borne colloids and adsorption onto particles and sediments in the order of light REEs (LREEs; from La to Nd) > middle REEs (MREEs; from Sm to Dy) > heavy REEs (HREEs; from Ho to Lu) (Sholkovitz, 1993; Kulaksız and Bau, 2007). In this study, however, significant losses of dissolved REEs were not observed (Fig. 2). This could be partly due to lack of data from the low-salinity region (0.0-21.4).

Fig. 2.

Plots of dissolved REEs concentrations versus salinity in the Yeongil Bay.

3.2 Shale-normalized REEs patterns

The post-Archean average Australian shale (PAAS)-normalized REEs patterns (McLennan, 1989) are presented in Fig. 3 to eliminate the Oddo–Harkins effect in natural abundances, allowing easy comparison with other REEs data (Migaszewski and Gałuszka, 2015). Generally, all sampling sites show HREEs enrichment relative to LREEs. This pattern could reflect the preferential scavenging of LREEs from river water in the low-salinity region via coagulation, aggregation, and sedimentation of the river colloids as freshwater mixes with seawater (Sholkovitz, 1993; Kulaksız and Bau, 2007). The patterns along the YB could be represented by two different pools: (1) riverside sampling sites (from B1 to B3), where show significantly high Gd concentrations (244.9 ± 100.6 pM, n = 3), (2) bay sampling sites (from C0 to C9) where relatively low Gd concentrations were obtained (13.7 ± 8.4 pM, n = 24) (Table 1; Fig. 3). Bay sampling sites showed patterns similar to those observed in outermost sampling sites (C9) rather than riverside sampling sites, indicating that most of sampling sites are influenced by seawater rather than river water as indicating by salinity.

Fig. 3.

Post-Archean Australian Shale (PAAS) normalized patterns of dissolved REEs obtained the in Yeongil Bay.

3.3 Gd anomaly and anthropogenic Gd

In this study, positive Gd anomalies were clearly observed at riverside sampling sites (from B1 to B3) (Fig. 3). The Gd anomaly (Gd/Gd*) was calculated using the following equation (Bau and Dulski, 1996a):

$\[ \mathrm{G}\mathrm{d}/{\mathrm{G}\mathrm{d}}^{\mathrm{*}}={\mathrm{G}\mathrm{d}}_{\mathrm{S}\mathrm{N}}/\left(0.33\times {\mathrm{S}\mathrm{m}}_{\mathrm{S}\mathrm{N}}+0.67\times {\mathrm{T}\mathrm{b}}_{\mathrm{S}\mathrm{N}}\right) \]$

(1)

where subscript SN indicates PAAS normalized value of the REEs concentration, and superscript * denotes the geogenic background interpolated between neighboring elements. Regardless of normalization, the occurrence of a minor positive Gd anomaly (< 1.4) is frequently observed due to "tetrad effect", resulting from the half-filled 4f electron in its orbital shell (e.g., Bau and Dulski, 1996b; Alibo and Nozaki, 1999).

In this study, throughout the YB, calculated Gd anomaly (Gd/Gd*) values range from 1.0 to 11.0 (1.9 ± 2.42, n = 27) (Table 1). We observed distinctive anthropogenic Gd anomalies (8.2 ± 3.1, n = 3) at the riverside sampling sites (from B1 to B3). The major cause of these anomalies can be attributed to the significant input of refractory Gd-based contrast agents, such as Gd-DTPA, originating from MRI examinations (Bau and Dulski, 1996a; Möller et al., 2000; Bau et al., 2006). These Gd anomalies are comparable to those previously reported in Han River (4.3 ± 2.5, n = 6) and Elbe, Ems, and Weser River (4.8 ± 0.9, n = 3) (Kulaksız and Bau, 2007; Song et al., 2017), while those are relatively low compared to those in Wupper, Spree, and Havel River (56.1 ± 49.9, n = 3) and Rhine River (22.5 ± 29.3, n = 14) (Bau and Dulski, 1996a; Kulaksız and Bau, 2011).

To investigate the influence of Gd_anth in YB, Gd_anth concentrations were calculated as follows (Kulaksız and Bau, 2013):

$\[ {\mathrm{G}\mathrm{d}}^{\mathrm{*}}={\mathrm{G}\mathrm{d}}_{\mathrm{S}\mathrm{N}}\mathrm{*}\times \left[{\mathrm{G}\mathrm{d}}_{\mathrm{P}\mathrm{A}\mathrm{A}\mathrm{S}}\right] \]$

(2)

$\[ {\mathrm{G}\mathrm{d}}_{\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{h}\mathrm{ }}={\mathrm{G}\mathrm{d}}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{e}\mathrm{d}\mathrm{ }}-{\mathrm{G}\mathrm{d}}^{\mathrm{*}} \]$

(3)

where Gd_measured indicates the measured dissolved Gd concentration.

In the YB, the Gd_anth concentrations suddenly decreased toward the outer bay. The calculated Gd_anth concentrations varied by two orders magnitude, which ranged from 0.2 to 298.3 (25.7 ± 74.2 pM, n = 27) (Table 1). High concentrations of Gd_anth (215.5 ± 104.1 pM, n = 3) were obtained at the riverside sampling sites (from B1 to B3). This concentration level is significantly high compared to that obtained in the Han River (80 ± 65.1 pM, n = 6) (Song et al., 2017), the Suyeong River (103.6 ± 56.0 pM, n = 36) (Lim et al., 2023), and the San Francisco Bay (33.8 ± 40.0 pM, n = 8) (Hatje et al., 2016). On the other hand, significantly low concentrations of Gd_anth (2.0 ± 2.3 pM, n = 24) were calculated at bay sampling sites (from C0 to C9) (Table 1). Therefore, these results clearly suggest that Gd_anth from the Hyeongsan River were already diluted by mixing with seawater in YB.

In this study, a negative correlation (R² = 0.96) between Gd_anth and salinity was observed (Fig. 4). Previous study in the Suyeong River also showed this negative correlation and suggested that Gd_anth would be conservatively mixed owing to their high stability (Lim et al., 2023). If we assume that Gd_anth in YB is conservatively mixed with seawater, the Gd_anth could be estimated by extrapolation from the linear relationship between Gd_anth and salinity. Therefore, Gd_anth at zero salinity could be estimated as 869 pM (Fig. 4), which is significantly high compared to previous studies as we referred above. Otherwise, Gd_anth could be discharged from a certain source that is located near riverside sampling sites, where salinity ranges from 21.4 to 30.6 (Table 1). Owing to the limited data, our Gd_anth estimation in YB is only a rough estimate. Nonetheless, our estimation suggests that high concentrations of Gd_anth in YB will be a major concern in the future.

Fig. 4.

Gdanth concentrations as the function of salinities. The dashed black line indicates conservative mixing trend.

4. Conclusion

We investigated dissolved REEs distribution along the YB, which is surrounded by highly urbanized region. The concentrations of dissolved REEs were gradually decreased from riverside to outer bay and generally showed inverse correlations with salinity. The PAAS- normalized REEs patterns show HREEs enrichment relative to LREEs, reflecting the preferential scavenging of LREEs. Positive Gd anomalies were clearly observed at riverside sampling sites (from B1 to B3), suggesting the significant input of Gd_anth from the Hyeongsan River. Calculated Gd_anth concentrations showed a sudden decrease toward the outer bay, suggesting that Gd_anth from Hyeongsan River were already diluted by mixing with seawater in YB. Even though data are limited, our results suggest that high concentration level of Gd_anth in YB could be a major concern. Therefore, mid- and/or long-term monitoring of Gd_anth is required to further solidify this conclusion.

Acknowledgments

This work was supported by a Research Grant of Pukyong National University (2023). We are thankful to the anonymous reviewers for their useful comments that helped to improve the manuscript.

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