Raša estuarine sediments are enriched in a number of trace elements, either toxic or nutritional. Calcium is one of the important elements which fall under the latter category. The association of calcium with the mineralogical framework would be desirable to develop efficient isolation methodologies. Therefore, we decided to perform a systematic analysis of its occurrence and distribution in the abovementioned area in order to evaluate potential ways to utilize the soil in a productive manner.
3.1 Characterization of soils and sediments
The results from the proximate and ultimate analysis of the soil sediments showed change with respect to depth. The average mineral matter content in all the acquired samples was 70.6%; the average moisture content was 3.5% while the average volatile matter content was around 24.8% (Table 1). Interestingly, the last two depth layers (20 – 25 cm and 25 – 30 cm) demonstrated relatively higher carbon content as compared to the content in samples collected at shallower depths (25-28% compared to 5-8% carbon). This could be indicative of a possible intrusion of coaly mass into the sedimentary body (Fiket 2021). Soil samples collected from 25- 35 cm also revealed higher content of both hydrogen (~2%) and sulphur (~5%). Since coal samples from Raša Bay are known to be enriched with organic sulphur, the concomitant increase in both carbonaceous matter and sulphur content could be justifiably correlated.
Table 1 Results of proximate and ultimate analysis of soil samples from various depths (reported in percentage)
Depth/cm
|
Moisture
|
Volatile
|
Ash
|
C
|
H
|
N
|
S
|
0 – 2
|
3.34
|
24.9
|
72.4
|
8.00
|
1.04
|
0.23
|
0.46
|
2 – 3
|
3.09
|
24.4
|
72.5
|
7.39
|
0.91
|
0.24
|
0.35
|
3 – 4
|
3.83
|
25.7
|
71.0
|
8.49
|
1.00
|
0.30
|
0.66
|
4 – 5
|
3.93
|
25.2
|
71.6
|
8.12
|
0.99
|
0.30
|
0.84
|
5 – 6
|
3.54
|
24.4
|
72.2
|
7.73
|
0.91
|
0.25
|
0.79
|
6 – 7
|
3.53
|
24.3
|
72.8
|
7.18
|
0.85
|
0.24
|
0.77
|
7 – 8
|
3.31
|
23.9
|
73.4
|
7.02
|
0.82
|
0.23
|
0.67
|
8 – 9
|
3.21
|
22.4
|
74.5
|
6.06
|
0.74
|
0.18
|
0.63
|
9 – 10
|
3.12
|
22.1
|
74.7
|
5.84
|
0.75
|
0.18
|
0.71
|
10 – 15
|
3.77
|
21.3
|
74.8
|
5.27
|
0.69
|
0.15
|
0.49
|
15 – 20
|
3.29
|
21.7
|
75.0
|
5.43
|
0.68
|
0.18
|
0.80
|
20 – 25
|
3.94
|
22.4
|
73.6
|
6.28
|
0.80
|
0.23
|
0.98
|
25 – 30
|
3.38
|
32.4
|
54.3
|
27.99
|
2.24
|
0.61
|
5.05
|
30 – 35
|
3.38
|
32.4
|
55.6
|
24.85
|
2.13
|
0.56
|
4.92
|
Average
|
3.5
|
24.8
|
70.6
|
9.7
|
1.0
|
0.3
|
1.3
|
The distribution of the major elements (as determined by ICP-OES) in the chosen soil sediment followed the order of Si>Ca>Al>Fe> Mg~ Na~ K) as shown in Table 2. No particular trend was observed with respect to depth apart from the noteworthy increase in carbonaceous matter in the range of 25-35 cm. The average concentrations of the two major elements, silicon and calcium in the depth profile were 22% and 14%, respectively. The results obtained for the chemical composition using ICP-OES (Table 2) tallied well with SEM-energy dispersive X-ray analysis data (Fig. 4). Both the important alkali metals (Na and K) manifest a similar range of concentration (~1%) throughout the depth. Nevertheless, the presence of sodium chloride was omnipresent in all the depths, the XRD data for the representative sample is shown in Fig. 6a, which confirms a possible intrusion of sea water in the chosen location (Sondi et al., 2008). Geochemical evolution owing to the mineralization and salinization, affects the chemical content of the groundwater and the associated soil sediment (Plantak et al., 2021). The presence of high concentration of calcium throughout the entire depth was noteworthy, which was expected to be in the form of carbonate minerals (Table 2, Fig. 4). Calcium carbonate has three polymorphs, out of which calcite is thermodynamically most stable (Fig. 6a, Singh et. al., 2016). The dissolution of calcite was speculated to be the reason behind elevated concentration of Ca2+ and HCO3− ions in the estuarine soil sediment under investigation (Plantak et al., 2021). According to prior literature (Ferdo 2013; Mihalić Arbanas et al., 2017), the clastic source rocks are primarily composed of carbonates along with a significant portion of quartz and clay minerals. Likewise, the primary mineral phases detected in the soil sediment were that of calcite (CaCO3), dolomite [CaMg(CO3)2] along with sporadic distribution of quartz (SiO2) and magnetite (Fe2O3) (Fig. 6a). Such a mineralogical profiling is quite a typical scenario for sedimentation in the estuary where the river flow is dominant and the influence of waves and the tide is relatively low leading to long surface floating duration of clay particles (Plantak et al., 2021).
Table 2 Depth wise analysis results of the major elements for the selected location obtained by ICP-MS
(reported in percentage)
Depth/cm
|
Si
|
Al
|
Fe
|
Ca
|
K
|
Mg
|
Na
|
0 – 2
|
21.5
|
6.20
|
2.14
|
15.25
|
0.97
|
1.20
|
1.23
|
2 – 3
|
23.5
|
6.38
|
2.27
|
14.78
|
1.00
|
1.13
|
1.07
|
3 – 4
|
23.5
|
5.84
|
1.86
|
14.81
|
1.06
|
1.24
|
1.23
|
4 – 5
|
19.6
|
6.75
|
1.94
|
14.70
|
1.10
|
1.22
|
1.25
|
5 – 6
|
18.7
|
6.52
|
2.31
|
14.40
|
1.03
|
1.18
|
1.19
|
6 – 7
|
20.7
|
7.11
|
2.65
|
15.15
|
1.13
|
1.23
|
1.20
|
7 – 8
|
23.6
|
6.49
|
2.78
|
14.37
|
1.09
|
1.17
|
1.18
|
8 – 9
|
21.5
|
5.67
|
1.88
|
13.47
|
1.07
|
1.09
|
1.07
|
9 – 10
|
23.6
|
5.99
|
2.32
|
13.75
|
1.12
|
1.13
|
1.12
|
10 – 15
|
24.1
|
6.46
|
2.65
|
14.69
|
1.21
|
1.20
|
1.10
|
15 – 20
|
23.7
|
6.87
|
2.59
|
15.53
|
1.21
|
1.23
|
1.19
|
20 – 25
|
24.4
|
6.39
|
2.84
|
15.49
|
1.23
|
1.32
|
1.35
|
25 – 30
|
21.3
|
5.67
|
2.91
|
9.15
|
0.89
|
0.94
|
0.88
|
30 – 35
|
22.8
|
5.45
|
1.76
|
9.69
|
0.76
|
0.90
|
0.91
|
Average
|
22.3
|
6.27
|
2.35
|
13.95
|
1.06
|
1.16
|
1.14
|
3.2 Sequential Leaching
Sequential leaching is an important diagnostic tool in order to delineate the association profile of elements with the host material. In the present study, our aim was to investigate the modes of occurrence of the elements whose concentrations were assessed to be significant in the chosen area, for example, Na, K, Ca, Mg, Si, Al and Fe. Therefore, a five step sequential leaching was performed combining BCR and Tessier methods (Park 2021). The results for sequential leaching with respect to individual elements are illustrated in Fig. 5. In the first two steps, the water soluble/ion exchangeable elements are typically detected. Chemical analysis showed that only Na (38%) followed by minor quantity of K (7%) were present in exchangeable form, thus substantiating the presence of ionic Na+ in the soil sample (as NaCl). XRD patterns revealed complete removal of both calcite and sodium chloride after the first two steps (Fig. 6b) supporting high solubility of calcite in water (Segnit et al., 1962). The next important step was to determine those elements which were either carbonate bound or favors complexation in the presence of organic acid (0.3M tartaric acid) 28% of Ca and 30% of Mg got leached out in this step indicative of their association with carbonate, which in this case is expected to be mineral dolomite (CaMg(CO3)2) (Fig. 6a). Therefore, after treating with organic acid in step 3, the characteristic peaks of dolomite got exclusively eliminated (Fig. 6c). The remaining Ca (58%) and Mg (25%) got removed in the next stage where the residue was treated with mineral acid (1M HCl), suggestive of them being strongly associated with the clay matrix. Apart from the alkaline earth elements, noticeable proportion of Fe (46%) got leached out at the acid soluble stage. This is indicative of a faster dissolution of magnetite mineral (Fe3O4) in presence of HCl which could have resulted due to the formation of surface complexes of Fe-Cl, expediting the process of dissolution (Sidhu et. al. 1981). It was obvious that most of the Si (90%) and Al (65%) were confined to the residual fraction (last step of sequential leaching) which could be the consequence of the high stability of Si2+ and Al3+ coupled with their augmented potential to be enriched in clay minerals.
3.3 Leaching of Calcium
The representative sample which was prepared by mixing a fixed quantity from each depth homogenously, was used for optimization of leaching parameters such as optimal acid concentration, pulp density, duration and temperature. The results obtained are shown in Fig. 7. The acid concentration was varied over a wide range up until a maximum concentration of 1M. However, optimum leaching of calcium was achieved using a much lower acid concentration (0.1 M HCl) within a short duration (30 min) which led to the conclusion that calcium was primarily present in ionic form in the chosen Croatian soil sediment. The optimal temperature and liquid to solid ratio for calcium leaching was found to be 50 °C and 10:1 (mL: g), respectively. Nevertheless, for both parameters only minimal change could be observed within the complete experimental range (25-95 °C and 5:1-50:1).
3.4 Selective precipitation of Calcium
Selective precipitation of calcium (Ca2+) from the leached solution was attempted with ammonium oxalate. Calcium oxalate, a mineral of low solubility (0.67mg/100mL at 13 °C) and lower stability constant (log Ks=3 at 25 °C), selectively precipitated out as white precipitate at room temperature instantaneously (Walczyk et al. 2008). The oxalate thus obtained was directly taken forward for calcination at 950 °C for 1 h. The calcined product (CaO) was obtained in 85% yield which was thoroughly characterized using ED-XRF and SEM-EDX (Fig. 8 and Table 3). The co-precipitation of other elemental impurities (Si, Al, Fe, Na and Mg) in the calcined product were found to be negligible (<1%) (Table 3). Micrographs of the final product and the accompanied EDAX profile clearly showed the calcium enrichment (98.7% CaO) with a textural resemblance to flake shaped morphology (Fig. 8, Khine et. al. 2022).
Table 3 Comparative mineralogical composition of the calcined product after selective precipitation compared to the raw material as determined by ED-XRF analysis
Major oxide/%F
|
SiO2
|
Al2O3
|
Fe2O3
|
MgO
|
CaO
|
Na2O
|
Raw
|
51.57
|
12.21
|
3.82
|
2.75
|
16.67
|
1.57
|
Calcined Product
|
0.19
|
0.12
|
0.23
|
0.06
|
98.71
|
0.23
|
3.5 Trace elements
The presence of trace elements plays a pivotal role in either providing nutrients to the soil health or could prove detrimental towards growth of plants and in turn enter the ecological system. Apart from the major elements, some of the environmentally sensitive (Sr, Zn, Cr, V) and economically important trace elements (Ga, Sc) were detected in the soil sediment sample in noticeable quantities (Table 4). Only those trace elements (TEs) whose concentration was detected to be ≥ 40 ppm in the raw soil sediment in the chosen location. The individual concentrations ranges were as follows: Sr (315 – 405 ppm), Zn (100 – 144 ppm), Cr (184 – 315 ppm), V (87 – 168 ppm), Ga (93-186 ppm), and Sc (43 – 70 ppm) (Table 4). While Sc finds major applications in the aerospace and automotive sectors (Williams-Jones and Vasyukova, 2018), Ga is extensively used in integrated circuits and technologically advanced electronic devices (Lu et al. 2017). The presence of significantly higher quantities of hazardous TEs, such as Sr (315-405 ppm), Cr (185-316 ppm) and Zn (100-145 ppm) confirmed the intrusion of residues from coal mining activities (Zhang et al. 2023; Medunić et al. 2018). With respect to association, vanadium was speculated to be associated with iron oxides, which gets released by natural weathering followed by adsorbed by the clay minerals (Berrow et al. 1978). There was no particular trend observed (Table 4) for either of the trace elements with respect to depth indicative of non-uniform intrusions of Raša coal wastes into the soil sediments in the chosen location .
Table 4 Concentrations of trace elements which were detected in the selected soil sediment obtained
by ICP-OES (reported in ppm)
Depth (cm)
|
Sr
|
Zn
|
Ga
|
Cr
|
Sc
|
V
|
|
0 – 2
|
374.2
|
141.7
|
94.9
|
247.3
|
69.5
|
89.6
|
|
2 – 3
|
354.7
|
142.2
|
105.6
|
268.1
|
64.9
|
88.0
|
|
3 – 4
|
370.9
|
143.9
|
113.5
|
300.9
|
67.3
|
97.7
|
|
4 – 5
|
394.2
|
132.3
|
95.8
|
315.9
|
66.2
|
98.5
|
|
5-6
|
374.8
|
136.1
|
101.4
|
292.1
|
66.7
|
90.7
|
|
6-7
|
389.1
|
129.7
|
128.3
|
249.6
|
69.7
|
96.2
|
|
7-8
|
358.0
|
131.2
|
112.1
|
277.4
|
65.6
|
95.6
|
|
8-9
|
315.5
|
137.4
|
115.6
|
257.0
|
62.6
|
87.0
|
|
9-10
|
330.3
|
139.8
|
113.8
|
270.0
|
64.4
|
93.2
|
|
10-15
|
404.1
|
131.9
|
172.8
|
254.4
|
66.6
|
95.2
|
|
15-20
|
386.5
|
145.5
|
185.2
|
270.4
|
72.0
|
99.9
|
|
20-25
|
370.3
|
155.0
|
172.2
|
254.0
|
70.3
|
101.9
|
|
25-30
|
325.7
|
100.1
|
110.8
|
186.7
|
43.8
|
168.2
|
|
30-35
|
316.4
|
104.8
|
93.6
|
184.1
|
43.7
|
137.3
|
3.6 Correlation studies of major oxides and selected trace elements
It is a well known that geochemical phenomena occurring in nature, which includes the trace element concentrations in soil, are correlated. They are primarily caused by the physico-chemical properties of elements during the geochemical process (Romic et al. 2012). Soil properties are typically inter-correlated, in particular that of soil matrix characteristics (clay minerals) with important oxides such as iron, aluminum, calcium, titanium and alkali metal oxides. Growing interest in speciation in recent times has resulted from the fact that the quantification of a specific element fails to provide satisfactory information about its environmental behavior (such as toxicity and migration leading to geographical accumulation). The results obtained in the present study revealed important correlations of selected trace metals with the major oxides (Table 5). All the important correlations, possessing high correlation coefficient (r>0.7) have been highlighted in the table. We were curious to know the specific elemental associations of CaO in the soil matrix owing to its abundance in the area of investigation (~15%). CaO appeared to have the strongest correlation with MgO (r=0.95, as in dolomite) followed by Na2O (r=0.88). The TEs with moderate environmental concern (Sr, Zn and Cr) demonstrated noticeable correlation (r=0.7 – 0.9) with both calcium oxide and magnesium oxide (Table 5), thus indicating a possible inclusion of these elements in the dolomite phase in addition to being a part of the coal waste. Scandium, which possesses high economic value, is present in appreciable quantities (43-70 ppm) in the chosen soil. Interestingly, it showed prominently high correlation (r=0.7-1.0) with all the major oxides (Al2O3, CaO, MgO, K2O and Na2O) implying their association with clay minerals. Elevated levels of scandium could be correlated with karst soils commonly found in Greece, Croatia and Slovenia (Mico et al. 2003). Although Sc is typically held in ferromagnesian minerals (Halkoaho 2020), absence of linear correlation with Fe suggests it not to hold true in the present situation. Nevertheless, its association with siderophile element such as Cr is well known in prior literature (Paul et al., 2019).
Table 5 Statistical correlation of major oxides and selected trace elements
|
SiO2
|
Al2O3
|
Fe2O3
|
CaO
|
K2O
|
MgO
|
Na2O
|
TiO2
|
Sr
|
Zn
|
Ga
|
Cr
|
Sc
|
V
|
SiO2
|
1.00
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Al2O3
|
-0.15
|
1.00
|
|
|
|
|
|
|
|
|
|
|
|
|
Fe2O3
|
0.25
|
0.42
|
1.00
|
|
|
|
|
|
|
|
|
|
|
|
CaO
|
0.13
|
0.72
|
0.08
|
1.00
|
|
|
|
|
|
|
|
|
|
|
K2O
|
0.29
|
0.67
|
0.43
|
0.79
|
1.00
|
|
|
|
|
|
|
|
|
|
MgO
|
0.12
|
0.72
|
0.22
|
0.95
|
0.85
|
1.00
|
|
|
|
|
|
|
|
|
Na2O
|
0.02
|
0.62
|
0.09
|
0.88
|
0.72
|
0.95
|
1.00
|
|
|
|
|
|
|
|
TiO2
|
0.01
|
0.01
|
0.18
|
-0.16
|
0.24
|
-0.14
|
-0.12
|
1.00
|
|
|
|
|
|
|
Sr
|
-0.07
|
0.82
|
0.26
|
0.73
|
0.64
|
0.78
|
0.67
|
-0.19
|
1.00
|
|
|
|
|
|
Zn
|
0.28
|
0.43
|
0.00
|
0.91
|
0.73
|
0.86
|
0.83
|
-0.14
|
0.45
|
1.00
|
|
|
|
|
Ga
|
0.55
|
0.40
|
0.56
|
0.39
|
0.74
|
0.48
|
0.30
|
0.37
|
0.44
|
0.40
|
1.00
|
|
|
|
Cr
|
-0.12
|
0.52
|
-0.19
|
0.78
|
0.60
|
0.75
|
0.73
|
-0.15
|
0.56
|
0.71
|
0.03
|
1.00
|
|
|
Sc
|
0.10
|
0.72
|
0.12
|
0.99
|
0.82
|
0.95
|
0.88
|
-0.08
|
0.71
|
0.90
|
0.42
|
0.77
|
1.00
|
|
V
|
-0.05
|
-0.48
|
0.21
|
-0.88
|
-0.59
|
-0.72
|
-0.69
|
0.05
|
-0.45
|
-0.83
|
-0.12
|
-0.77
|
-0.87
|
1.00
|