According to grain size analyses the sample classified as silt, showing the dominance of fine texture. The sediment pH (pH of 8.14) was slightly alkaline which might be due to the presence of carbonate shells and the discharge of industrial effluents into the estuary. Percentage LOI was 27% in sediment. The high OM in the sediment is related to the high organic content of bituminous limestone and organisms remains and also may be due to the high level of petrochemical effluents. The CEC in the sediment was 35 meq/100 g. The high CEC can be related to the high organic content and the sediment fine texture (between 2-75 μm in diameter). Total Al, Fe, and Mn contents were 3.19%, 2.07%, and 377 mg kg -1, respectively.
As REEs effectively sorb onto sediment Al/Fe/Mn oxides, organic matter, and clay minerals in neutral/ basic waters (Mandal et al., 2019; Mihajlovic and Rinklebe, 2018) and the sorption capacity is a function of cation exchange capacity (Edahbi et al., 2018) the studied sediment expected to have a high capability for REEs sorption.
Table 4
Comparison of REEs concentration (mg kg -1) in the sediment sample with other catchment areas and mean world sediments.
|
Ligurian Sea
|
Ipojuca River
|
Poyang Lake
|
Thailand Gulf
|
Mean world sediments
|
Musa estuary
|
La
|
4.95
|
29.86
|
60.90
|
22.00
|
28.30
|
62.00 (18%)
|
Ce
|
12.10
|
59.39
|
101.60
|
45.00
|
58.90
|
151.00 (44%)
|
Pr
|
1.76
|
11.47
|
12.70
|
4.70
|
6.52
|
19.66 (5.8%)
|
Nd
|
8.01
|
21.63
|
45.40
|
18.00
|
24.90
|
71.60 (21%)
|
Sm
|
2.58
|
4.27
|
9.40
|
3.50
|
4.23
|
11.78 (3.5%)
|
Eu
|
0.63
|
0.52
|
1.40
|
0.50
|
0.86
|
1.44 (0.4%)
|
Gd
|
2.92
|
3.03
|
7.80
|
2.60
|
3.61
|
10.17 (3.0%)
|
Tb
|
0.44
|
0.64
|
1.00
|
0.40
|
0.60
|
1.26 (0.4%)
|
Dy
|
2.31
|
0.88
|
5.50
|
2.20
|
3.61
|
6.17 (1.8%)
|
Er
|
0.98
|
0.58
|
3.00
|
1.10
|
2.19
|
2.96 (0.9%)
|
Tm
|
0.13
|
0.02
|
0.60
|
0.20
|
0.31
|
0.36 (0.1%)
|
Yb
|
0.76
|
0.41
|
3.20
|
1.20
|
2.14
|
1.70 (0.5%)
|
Lu
|
0.10
|
0.06
|
0.50
|
0.20
|
0.33
|
0.28 (0.1%)
|
∑ REE
|
37.67
|
132.76
|
253.00
|
101.60
|
136.50
|
340.38
|
∑ LREE
|
30.03
|
127.14
|
231.40
|
93.70
|
123.71
|
317.48
|
∑ HREE
|
7.64
|
5.62
|
21.60
|
7.90
|
12.79
|
22.90
|
∑ LREE / ∑ HREE
|
3.93
|
22.62
|
10.71
|
11.86
|
9.67
|
13.86
|
Reference
|
(Consani et al., 2020)
|
(da Silva et al., 2018)
|
(Wang et al., 2019)
|
(Kritsananuwat et al., 2015)
|
(Taylor and McLennan, 2001)
|
Present study
|
LREE: light rare earth elements, HREE: heavy rare earth elements, REE: total rare earth elements
|
The LREEs content were much higher than that of HREEs in the sample (Table 4). More specifically the LREE/HREE ratio was 13.86. This LREEs enrichment pattern is found to occur in other catchment areas and the mean world sediments. This could be associated with that in the fluvial environments, the enrichment of LREEs has been related to high adsorption on clay minerals, whereas HREEs are reported to form stable soluble complexes (da Silva et al., 2018).
According to Table 4 Ce, Nd, and La were the abundant REEs, accounting for 44%, 21%, and 18% of the total REEs concentration, respectively. Among the HREEs, Gd had the highest concentration. These results are in agreement with those concentrations measured in other areas and the mean world sediments.
3.3 REEs enrichment in sediment by EF index
The enrichment factor (EF) of REEs was used for evaluating the effect of anthropogenic sources. The obtained EF values for various REEs were between minimal enrichment and significant enrichment. The maximum EF value belongs to Nd (EF = 6.59) and the minimum EF value is seen for Lu (EF = 1.44).
According to Fig. 3, Lu, Tm, and Yb are classified as minimal enrichment with EF values of 1.44, 1.93, and 1.46, respectively which suggests they may be due to crustal materials or natural weathering processes in sediment. Er, Eu, Gd, and Tb exhibit a moderate enrichment. For Ce, Dy, La, Nd, Pr, and Sm the enrichment factor is significant (5 < EF < 20) indicating the effect of anthropogenic sources in the area.
3.4 Dynamics of pH, DOC, and Fe-Mn under pre-set redox conditions
With Eh changes pH fluctuated. During oxic and anoxic incubations, pH decreased from the initial condition (8.08) to 6.66 under the anoxic condition and 6.80 under oxic condition (Fig. 4). The low pH in anoxic conditions might be related to the presence of CO2 and organic acids from microbial activities and decomposing OM (Shaheen et al., 2020, 2018). This probability is approved by the increase of DOC under anoxic/acidic conditions. Oxic condition also causes the reduction of pH, which could be related to the presence of protons (H+) (Mihajlovic et al., 2017; Zhu et al., 2018). Therefore, various mechanisms might have contributed to pH dynamics in the current experiment.
DOC increased from the initial value (18.93 mg/l) to 704.84 mg/l in the anoxic experiment and 425 mg/l in the oxic experiment (Fig. 5). During anoxic conditions, DOC increased which might have several possible explanations. The reductive dissolution of Fe-Mn oxyhydroxides made by anoxic conditions may dissolve organic carbon bound to Fe-Mn oxyhydroxides, production of dissolved organic metabolites during microbial activity, and reductive fermentation and hydrolysis of complex organic matter to DOC (Shaheen et al., 2016; Shaheen and Rinklebe, 2017). A decrease in DOC along with the increase in Eh is commonly observed (e.g., Beckers et al., 2019; Han et al., 2019). However, in this study during oxic experiment, the increase in Eh was accompanied by an increase in DOC value. The observed increase in DOC could be interpreted as being a result of organic matter desorption from solid phases (Grybos et al., 2009, 2007).
The lowest concentrations of Fe and Mn in sediment were measured in the anoxic experiment and they were highest in the oxic experiment (Fig. 6). Consequently, at high Eh the soluble Fe and Mn concentrations decreased, and conversely, at low Eh they increased. An increase in soluble Fe and Mn concentrations with decreases in redox potential suggests that reductive dissolution of Fe and Mn (hydr)oxides have occurred (Leyden et al., 2016). In addition, it might be due to the decrease in pH (Shaheen et al., 2017). High redox status would cause the precipitation of Fe3+, Mn3+, and Mn4+ oxides and a decrease in the concentration of soluble Fe and Mn (Antić-Mladenović et al., 2017a, 2017b).
3.5 Release dynamics of REEs under pre-set redox conditions
REEs released from sediment in both oxic and anoxic experiments expect for Ce in oxic condition (Fig. 7). The release of REEs under oxic conditions can be due to a decline in pH and an increase in DOC. Dissolution of Fe-Mn oxyhydroxides, low pH conditions, and increase in dissolved organic carbon (DOC) under anoxic conditions cause the mobilization of REEs to increase. Ce adsorbed to sediment in oxic condition. This is because the Ce4+ is formed and Ce4+ is removed from the solution to the sediment by scavenging onto sediment compounds (Suja et al., 2017). Similar observations were reported by other researchers (Cao et al., 2001; Davranche et al., 2011; Grybos et al., 2007; Mihajlovic et al., 2017).
The average amount of REEs mobilization in oxic condition was 39.40% and it was 27.86% in anoxic condition. All REEs released to a greater extent in oxic conditions than anoxic conditions except for La, which was released equally in both cases (16.1%). Lu and Sm are the most mobile elements while Ce has the lowest amount of release (Fig. 7). Since HREEs form soluble complexes more easily, they had higher mobility than LREEs (Duncan and Shaw, 2003; Mihajlovic and Rinklebe, 2018).
In practice, REEs release from sediments may be expected whenever these sediments are subjected to oxygen in air, e.g. under dredging operations and sediment reclamation in the open air (such as land farming) or when they are subjected to oxygen-free conditions in the bottom of the aquatic environment due to the accumulation of organic matter.