Investigations on Rare Earth Elements in Sediments from South Vellar Estuary along Bay of Bengal and Palk Strait, Tamil Nadu, India

2.1 Study area and geologic context

The Palk Strait, a Bay of Bengal strait, is between southern India and northern Sri Lanka. Its boundary is bounded by the Gulf of Mannar, Pamban Island, Adam’s (Rama’s) Bridge, and Mannar Island. The southwest part of the strait is also known as Palk Bay [25]. The strait is less than 100 m deep, 64 to 135 km wide, and 137 km long. The South Vellar River, a non-perennial river, flows into the Bay of Bengal along with Palk Strait. The river originates as a cluster of small streams in the Kumirakatti forest reserve, which is located about 9 km west of the village of Karumalai after passing through Tovarankurichchi and Karumalai, flowing to the southeast after receiving the additional flow from the Vembar retention basin, which is located south of Kodumbalur. After passing through Avudaiyarkoil, it turns to the east and flows until Mumpalai village in the Manamelkudi taluk, where it creates an estuary and discharges into the sea northeast of Manamelkudi.

There are three main lithostratigraphic units in Tamil Nadu’s southern region, which contains the coastal areas to be visited. Charnockites, granulitic retrograded products, and basic intrusive and Tertiary calcareous sandstone are layered above Precambrian granitic rocks, as are recent coastal deposits. During the Proterozoic, the granulitic terrain was divided into multiple massive shear zones, which caused the granulitic facies assemblages to retrograde. Based on changes in landforms and the presence of lineaments, seven major blocks have been discovered between Kanyakumari and Kodiyakarai on Tamil Nadu’s southeast coast [26]. According to a theory, the seven blocks from the glaring weaknesses in the lineaments are what keep striking slip faults from continuing. In the shallow marine shelf areas along the Tamil Nadu coast, the mean annual rate of sediment deposition in the Gulf of Mannar, Palk Bay, and Sand Heads are 0.01 m, 0.006 m, and 0.003 m, respectively, according to a research by Chandramohan et al. [27]. There are numerous examples of erosional and depositional landforms along Tamil Nadu’s southeast coast. Beach ridges, spits, swales, and estuaries are important features of depositional landforms. In and around Kodiyakarai, Thondi, Manamelkudi, Rameshwaram, Mandapam, and other surrounding areas, extensive erosional features such as marine terraces, sea caves, cliffs, and wave-cut platforms can be observed [28].

2.2 Sampling technique

Runoff from the land, bank erosion and internal channel reworking are all possible sources of fluvial sediments. In the estuary ecosystem, sediment performs a lot of significant roles and serves as the substrate for a huge assortment of organisms. Field sampling is carried out using some scientific procedures (Figure 2). During the monsoon period of July 2022, 12 sediment samples were collected from various parts of the South Vellar estuary (Figure 3). Twelve sediment samples (SS) were obtained in the South Vellar Estuary (Figure 1). Nine samples (SS1-SS9) were collected from the lower estuarine parts using Van Veen grabs on small boat (Figure 4.a). The basic concept of all grab samplers is the same: A specialized bucket or scoop is lowered through the water column used with rope until it meets the bed, at which point, a release mechanism enables sample collection and withdrawal. To minimize disturbance at the sediment surface on contact and to reduce the loss of fine-grained material as the grab is raised through the water column, the grab was lowered and raised gradually and vertically while avoiding free fall. The grab sampler was properly cleansed after collecting to remove any additional material that had become adhered to the equipment. The samples are transferred to plastic bags with labels. Three samples (SS10-12) were collected manually from surface of middle estuary (Figure 4.b). Surface sediments are relatively simple to collect because there is no need to know the exact depth from which the sediment was collected or to preserve sediment structure. Where sampling locations are easily and safely accessible by foot or small boat, samples can be collected by hand using a scoop, allowing for very accurate sampling site location with GPS. All sediment samples were collected by labeled and sealed plastic bags and transported to the laboratory, where they were stored at 4°C until analysis [29, 30, 31].

2.3 Analytical method

The samples are kept in an air-dried environment for further REE extraction. Using a mortar and pestle, the dehydrated samples were crushed to a homogeneous powder. All samples were prepared in a flux mixture containing 100 mg of each sample. The samples were then heated in an inert, heat-resistant graphite crucible at 1050°C, past the melting point of the flux. The crucible was regularly agitated and heated until the sample was completely dissolved in the molten flux. The melt was then poured into a 50 ml aqueous solution of 5% HNO3. This solution required progressive agitation via shaking and magnetic stirring to dissolve the flux with the sample in the acidic solution. Each sample was prepared for plasma quad analysis by taking 1.5 ml of this dilution, adding 0.1 ml of 9.997 mg l1Indium internal standard, and 8.4 ml de ionized water. This procedure produced an aqueous solution with a ~ 3300 whole sediment sample dilution and a concentration of 99.97 μg/l. VG thermal ionization mass spectrometer was used to analyze the extracted samples for rare earth elements [29]. The technology known as Thermal Ionization Mass Spectrometry (TIMS) was primarily created for the analysis of samples of geological sediment. By using isotope dilution analysis, the technique is frequently used to determine rare-earth elements and other specific elements [32]. Nine Faraday cup collectors were installed on a VG Sector 54–30 thermal TIMS (VG Isotopes Ltd., UK) for the isotope dilution analytical procedures to measure rare earth elements.

An important analytical instrument for determining the elemental composition of environmental samples, especially soil and sediments, is energy-dispersive X-ray fluorescence Spectrometer (EDXRF). The instrument used for this experiment is an EDXRF spectrometer, model EX-6600SDD. This technique has a high sample throughput, which is multi-elementary and nondestructive. EDXRF is utilized in the current study for large area screening as it is quick and only requires a little sample amount [33].

2.4 Result and discussion

As with their abundance in the continental crust, REEs are prevalent in estuary sediments, which can shed light on their marine cycling. Additionally, it gives more knowledge on the interactions between sand and water and redox reactions that take place in surface environments [34]. There are two primary categories of rare earth elements: Currently conducted research compares low and high rare earth elements (Table 2). The distribution and quantity of REEs in the estuary sediments in South Vellar Estuary are displayed in Tables 3–5. In comparison with HREEs, LREEs are typically more enriched in all sampling locations of estuary (Figure 5). Based on mean concentration from LREE group, La (37.75 μg/g) is the dominant element, followed by Ce (36.65 μg/g), Pr (21.38 μg/g), Nd (17.17 μg/g), Sm (7.64 μg/g), Gd (5.35 μg/g), and Eu (4.09 μg/g) (Table 3). In addition, Y (34.53 μg/g) is also abundant in HREE followed by Tb (5.68 μg/g), Dy (3.93 μg/g), Er (3.07 μg/g), Ho (2.67 μg/g), Tm (2.11 μg/g), Yb (1.84 μg/g), and Lu (1.45 μg/g) (Table 4). Based on the individual rare earth element, the average concentration of La (37.75 μg/g) and Ce (35.65 μg/g) described in this research was higher than Pichavaram Mangrove (La: 1.43 μg/g; Ce: 1.26 μg/g0 [7] and Cauvery estuary (La: 19.92 μg/g; Ce: 34.98 μg/g) [35], five times lesser than Yamuna river (156.0 μg/g) [36].

Mean concentration of Dy (3.93 μg/g), Ho (2.67 μg/g), and Er (3.07 μg/g) described in this research was higher than Pichavaram Mangrove (Dy: 130 μg/g; Ho: 1.75 μg/g; Er: 1.06 μg/g) [7], and Cauvery estuary (Dy: 3.43 μg/g; Ho: 0.70 μg/g; Er: 2.19 μg/g) [35]. Distribution of yttrium is 3.22–63.48 μg/g in this research is higher than Tagus estuary (2.4–32.0 mg/kg), which is located in Portugal [37]. The determined heavy metal concentration for 12 coastal locations in the South Vellar estuary by EDXRF, as well as the distribution of heavy metals (other elements than REE) in the sediments, is examined (Table 6). When compared to all other heavy metals, Cr is frequently higher concentrated in all estuarine sampling locations (Figure 6). For Cr, the heavy metal concentration ranges from 16.02 to 186.1 μg/g; for Zn, it ranges from 16.36 to 67.26 μg/g; for Ni, it ranges from 12.13 to 42.16 μg/g; for Pb, it ranges from 4.76 to 60.08 μg/g; for Co, it ranges from 4.64 to 21.08 μg/g; and for Cd, it ranges from 0.10 to 1.12 μg/g. Based on the individual heavy metal, the average concentration of Cr (77.75 μg/g) described in this research was lower than Pichavaram Mangrove (141.2 μg/g) [38], East Coast estuaries 318.0 μg/g [39], and Tamirabharani estuary 97.162 μg/g) [40]. Distribution of Ni reported (12.13 to 42.16 μg/g) and mean concentration (24.86 μg/g) in this research is lesser than Pichavaram Mangrove (62.0 μg/g) [38], East Coast estuaries (582.0 μg/g) [39], and Pondicherry estuary (33.51 μg/g) [41], higher than Tamirabharani estuary (11.852 μg/g) [40].

Distribution of Zn reported (16.36 to 67.26 μg/g) and mean concentration (36.07 μg/g) in this research is higher than Cauvery estuary (26.0 μg/g) [42]; and lesser than Pichavaram Mangrove (89.0 μg/g) [38] and East Coast estuaries (125.0 μg/g) [39]. Mean concentration of Pb (19.76 μg/g) in this research is higher than Cauvery estuary (10.0 μg/g) [42], Pichavaram Mangrove (11.2 μg/g), and East Coast estuaries (11.0 μg/g); and lesser than Mahanadi estuary (23.88 μg/g) [43]. Mean concentration of Co (10.34 μg/g) in this research is lesser than Cauvery estuary (64.0 μg/g) and East Coast estuaries (12.0 μg/g). Distribution of Cd reported (0.10 to 1.12 μg/g) and mean concentration (0.42 μg/g) in this research is lesser than Pichavaram Mangrove (6.96 μg/g) and both sites of Uppanur estuary (site 1; 4.99 μg/g & site 2; 4.375 μg/g) [44]. The mean heavy metal concentrations are in the following order: Cr > Zn > Ni > Pb > Co > Cd (Table 6). According to results from other countries around the world, this study’s heavy metal concentration is comparable. Figure 6 shows the distribution of heavy metals in sediment from South Vellar estuarine for all sampling locations.