Fractionation and Distribution of Rare Earth Elements in Marine Sediment and Bioavailability in Avicennia marina in Central Red Sea Mangrove Ecosystems

Abstract

Rare earth element fractionation and distribution in the coastal ecosystem have been of significant concern and are recognized worldwide as emerging micro-pollutants. However, unlike other metals such as trace elements, little is known about their uptake by aquatic plants such as the mangrove Avicennia marina, especially in the central Red Sea. We investigated the fractionation of rare earth elements in six mangrove ecosystems in the central Red Sea and bioavailability in mangrove A. marina. The concentrations of rare earth elements, sediment grain sizes, multi-elemental ratios, geo-accumulation index (Igeo) and bioconcentration factor (BCF) vary significantly (p < 0.05) across the six mangrove ecosystems. Higher concentrations of rare earth elements were recorded at Al Lith (LT) (101.53 mg/kg) and South Jeddah (SJ) (73.38 mg/kg) mangrove ecosystems. However, multi-elemental ratio R(M/L) reveals positive values. In contrast, multi-elemental ratio R(H/M) reveals negative values corresponding to fractionation patterns enriched with medium rare earth elements and heavy rare earth elements depletion across the six mangrove ecosystems. BCF values for rare earth elements were <1, but Lutetium (0.32) had the highest BCF among the rare earth elements, suggesting an efficient accumulation of Lutetium than any other rare earth elements. The scale of Igeo revealed strong contamination (4 ≤ Igeo ≥ 5) of sediment with Lanthanum, Cerium, Praseodynium, Samarium, Godolinium, Holmium, Erbium, Ytterbium, and moderate contamination with Thulium, Terbium, and Dysprosium (1 ≤ Igeo ≤ 3). Principal component analysis showed that clay silt sediment grain size influences rare earth element concentrations in the central Red Sea. Our results provide new evidence for rare earth element fractionation and accumulation in sediment and the potential use of mangrove A. marina for rare earth element monitoring in mangrove ecosystems in the central Red Sea.

Rare Earth Elements and Bioavailability in Northern and Southern Central Red Sea Mangroves, Saudi Arabia

“…In another study on the Egyptian coast of the Red Sea [ 30 ], the ∑ REE (47.55 mg/kg) reported was about 1/2 of the ∑ REE (112.54 mg/kg) in the Jazan mangrove ecosystem. The depletion of HREEs relative to the lighter ones observed in this study is similar to previous observations elsewhere on the assessment of REEs in mangroves and mangrove soil profiles [ 11 , 24 , 31 ]. HREE depletion in benthic sediment could be as a result of their greater tendency towards the formation of soluble carbonates in a stable state and complexes of organic forms with ligands than the lighter REEs (LREE and MREE) [ 32 ].…”

“…Fine-grain particles have large surface area as one of their properties that can influence an increase in element sorption [ 23 , 24 ]. As such, it is reasonable to hypothesize that there exists a positive correlation between clay silt sediment particles and concentrations of REEs.…”

“…The use of REE fractionation in the determination of detailed LREE, MREE, and HREE enrichment is vital in the assessment of REEs in an ecosystem, even though direct computation using the sum of the concentrations is measured initially. It is important to note that (La/Yb)n, (Sm/La)n, and (Yb/Sm)n fractions of Post-Archean Australian Shale (PAAS) normalized values were used as a model for REE fractionation and represent LREEs, MREEs, and HREEs, respectively [ 12 , 24 ]. Higher fractions of PAAS normalized (Sm/La)n were recorded in Al Wajah (1.96) and Jazan (2.17) compared to (Yb/Sm)n, and the average (Sm/La)n (2.07) being higher than the average (Yb/Sm)n (1.08) is an indication of the predominance of lighter REEs (LREEs and MREEs) over HREEs in sediments, and more precisely, the levels of lighter REEs (LREEs and MREEs) in Jazan are higher than those of the Al Wajah mangrove.…”

“…Bioaccumulation of metals in plants due to the interaction between plants and sediment is commonly evaluated using Bio-Concentration Factor (BCF) [ 19 , 24 ]. The values for BCFs for all the REES determined in this study were less than 1 (BCFs < 1); this either signifies hypo-accumulation of the REEs by A. marina mangrove or the role of an effective mechanism that is involved in detoxification or exclusion of chemicals in A. marina [ 19 , 47 ].…”

Fate and Trophic Transfer of Rare Earth Elements in a Tropical Estuarine Food Web

“…Mandal et al reported average concentrations of ∑REE in mangrove leaves of 0.58 ± 0.2 mg kg –1 in the Indian Sundarban. Higher values (3.40 ± 0.44 mg kg –1 ) have been found in mangroves in the Red Sea . As observed by the LREE/HREE ratio, the fractionation of REE was similar among different species (i.e., from 36.4 ± 0.38 to 47.1 ± 2.57 for Rhizophora mangle and Avicennia sp ., respectively; Table S8).…”