Geobiochemistry characteristics of rare earth elements in soil and ground water: a case study in Baotou, China

Distribution characteristics of REEs in ground water

In this study, ground water samples were collected from 18 ground water monitoring wells around tailings ponds and their chemical characteristics were also having been determined, as showed in Figure S1. Fe, Mn²⁺, Cl⁻, SO₄²⁻, ammonia nitrogen and total hardness showed the same trend and decreased with distance. The ground water environmental quality standard (III Grade, National Standard Bureau of PR China, GB3838-2002, the water quality above III Grade can be used for living and drinking after treatment, but the water quality below III Grade was bad and cannot be used as drinking water source) was used as the evaluation standard. The ratio of the number of wells with Fe, Mn²⁺, Cl⁻, SO₄²⁻, ammonia nitrogen and total hardness exceeding the standard in the total number of wells was 33.33%, 61.11%, 66.67%, 77.78%, 100% and 81.25%, respectively.

In order to study the accumulation of REEs in ground water, the concentration of REEs in 18 ground water samples around the tailings pond were measured. The total REEs concentrations in ground water ranged from 0.0820 to 12.3 μg/L, and rare earth in the ground water accumulated in the southeast of the tailings pond (Fig. 2). In addition, the concentrations of REEs in ground water around the tailings pond decreased in the order of Ce > La > Nd > Pr > Gd > Sm > Dy > Er > Eu > Yb > Tb > Ho > Tm > Lu. Chondrite-normalized REEs patterns for ground waters around the tailings were shown in Fig. 4b and Table 1. The well points have the same normalization pattern with a predominance of LREEs over HREEs.

The distribution patterns of REEs in ground water were characterized by obvious fractionation of LREEs and HREEs with the LREEs/HREEs ratios of 2.77 ~ 25.9, and (La/Yb)_N of 1.445 ~ 50.67. The degree of LREEs fractionation with (La/Sm)_N of 0.5806 ~ 5.216. Most sampling points presented the positive anomaly of Ce and Eu, however, GW1, GW5, GW6, GW9, GW10, GW13 and GW6 were negative anomalies of Ce, while GW1, GW5, GW7 and GW8 were negative anomalies of Eu. Individual anomalies showed differentiation between selected elements (Ce and Eu) and the other REEs (Table 1).

Baotou environmental monitoring station, Inner Mongolia, China detected ground water leakage around the pond, and various degrees of ground water pollution were found with relatively lower metals concentration and higher anionic concentration^21,22,23. Therefore, in addition to REEs, for our ground water correlation analysis we chose to also look at Fe, Mn²⁺, Cl⁻, SO₄²⁻, ammonia nitrogen and some other ions (HCO₃⁻, total hardness). Correlation analysis showed that total hardness (r = 0.541, p < 0.05), Mn²⁺ (r = 0.608, p < 0.01) and ammonia nitrogen (r = 0.626, p < 0.01) were significantly positively correlated with the ΣREE. However, there was no significant correlation between pH, Fe, Cl⁻, SO₄²⁻, HCO₃⁻ and ΣREE. δCe was not related to physical–chemical indexes, but δEu was significantly negatively correlated with the Cl⁻ (r = − 0.505, p < 0.05), SO₄²⁻ (r = − 0.559, p < 0.05) and total hardness (r = − 0.483, p < 0.05), but showed no correlation to other physical–chemical indexes (Table 3).

Distribution characteristics of REEs in soil

In soil, the pH value of soil varied from 8.00 to 8.87, with an average value of 8.53. Therefore, the soil samples were all alkaline. The variation ranges of SO₄²⁻ and Cl⁻ content in the soil was 394.120 ~ 2007.73 mg/L and 50.250 ~ 586.70 mg/L, with an average value of 765.42 mg/L and 193.42 mg/L, respectively. Generally, they decreased with the increase of the distance from the tailings pond. The contents of Na⁺, K⁺, Ca²⁺ and Mg²⁺ ranged from 103.49 to 219.73 mg/L, 35.890 to 118.99 mg/L, 140.62 to 474.85 mg/L, and 40.150 to 155.16 mg/L, respectively. The mean values were 137.24 mg/L, 72.120 mg/L, 244.56 mg/L, and 66.000 mg/L. The contents of Na⁺, K⁺, Ca²⁺ and Mg²⁺ showed no significant change trends (Figure S2).

The contents of REEs in the soil of most sampling sites around tailings ponds were significantly higher than the geometric average values in Inner Mongolia. The total amount of REEs in the soil around the tailings pond ranges from 157.580 to 18,543.4 mg/kg, with an average of 2,769.95 mg/kg, The soil around the rare earth tailings pond has accumulated a lot of REEs, the content of REEs was higher the closer to the tailings pond. REEs accumulated in the south and southeast of the tailings pond (Fig. 3).

According to the standardized distribution pattern diagram of chondrite of REEs in soil (Fig. 4b), it could be seen that the soil around the tailings pond was rich in LREEs, and at the same time, normalized REEs pattern also showed that the content of REEs was higher the closer to the tailings pond (such as the normalized curve of point GW5, GW7 and GW8 near the tailings pond was slightly higher than other well points). And after the normalization treatment, the range of LREEs/HREEs, (La/Yb)_N, (La/Sm)_N in the soil samples were 7.350 ~ 31.96, 7.070 ~ 332.1 and 2.306 ~ 11.83 respectively, all more than 1, therefore the samples are relatively enriched with LREEs than HREEs, which was consistent with the observation results of the standardized distribution pattern diagram of chondrites of REEs in the soil (Table 2). According to the calculation formula of positive and negative outliers of Eu and Ce, it was concluded that Eu presented negative anomaly, and a slightly negative Ce anomaly (except for S6 and S9) indicated that Ce, Eu were a slight deficit in soil (Table 3).

Correlation analysis was conducted between REEs and various chemical properties in soil, as showed in Table 4. The result indicated that REEs and Ca²⁺, SO₄²⁻ were significantly positively correlated (r = 0.621, 0.807, p < 0.01), furthermore, REEs showed positive correlation with Cl⁻ (r = 0.541, p < 0.05). Na⁺, K⁺, Mg²⁺ were negatively correlated with La, Ce and ΣREE (r₁ = − 0.141, − 0.141, − 0.141; r₂ = − 0.364, − 0.360, − 0.365; r₃ = − 0.255, − 0.247, − 0.251; p < 0.05), however, it did not reach a significant level. δCe also was not related to chemical indexes, but δEu was significantly negatively correlated with the Ca²⁺ (r = − 0.637, p < 0.01), SO₄²⁻ (r = − 0.658, p < 0.01) respectively.

Microbial community diversity in typical ground water and soil samples around tailings ponds

On account of the concentration of REEs in soil and ground water decreased gradually in the south side of the tailings pond along the direction of ground water flow. Therefore, this study selected the ground water and soil samples in this direction for the analysis of microbial community. A total of 229,802 and 477,145 high-quality bacterial 16S rRNA gene sequences were obtained from six ground water samples (GW1, GW3, GW10, GW13, GW15, GW17) and seven soil samples (S1, S8, S10, S11, S13, S14, S15), respectively. The Shannon index representing the bacterial alpha diversity were shown in Table S1. The higher the Shannon index, the higher the biodiversity. The change of bacterial community diversity in soil and groundwater was not significant, but it tended to higher with the increase of distance (except GW17).

At the phylum level, the dominant bacteria in the ground water samples mainly include Actinobateria, Proteobacteria, Bacteroidetes, Acidobacteria and Chlorobi (Fig. 5a), while in soil samples it is mainly Actinobacteria, Proteobacteria, Chloroflexi, Acidobacteria, Gemmatimonadetes (Fig. 5c). In the upper reaches, there were single species of microbial community in the ground water, while in the lower reaches, there were greater Microbial diversity, and the dominant bacterial of Acidobacteria appeared. That with the distance from the tailings pond farther, the dominant bacterial community of phyla in the soil changed from Proteobacteria to Actinobacteria. On the contrary, the abundance of Chloroflexi increased with the distance increased. At the genus level, Bifidobacteria is the dominant bacteria in the upper reaches of tailings ponds, while Reyranella is the dominant bacteria in the lower reaches (Fig. 5b). The number of genus with relative abundance indicated that norank bacterial sequences were more abundant in soil samples according the tailings pond (Fig. 5d).

The correlation between microbial community and environmental factors was analyzed at genus level. In ground water, Pedobacter, Reyranella and Sediminibacterium were significantly negative correlated with the Cl⁻ and SO₄²⁻, however, no microbes were found to be significantly associated with ΣREE (Fig. 6a). In soil, Bacillus was significantly positive correlated with the SO₄²⁻, La, Ce and ΣREE, Blastococcus also was significantly positive correlated with La, Ce and ΣREE (Fig. 6b).

Source: Ecology - nature.com