Speciation of selected trace metals (Fe, Mn, Cu and Zn) with depth in the sediments of Sundarban mangroves: India and Bangladesh

Abstract

The study was conducted to highlight the distribution of trace metals in the world’s largest mangrove ecosystem, Sundarban, in India and Bangladesh. A speciation technique was used to document the bioavailability and environmental hazards associated with operationally defined chemical forms of trace metals. Seven sediment cores were selected to study the speciation of selected trace metals (Fe, Mn, Cu and Zn) in the sedimentary mangrove environment. A modified European Community Bureau of Reference (BCR) three-step sequential extraction procedure was used to target chemical forms. Texture analysis showed that the hydrodynamic conditions prevailing in the region are high in energy that leads to the deposition of sediment fractions in the following order: silt > clay > sand. The fine-grained sediments have the potential for binding trace metals. The trace metal content was higher in the fine-grained fraction that was dominant in the Indian Sundarban (IS) than Bangladesh Sundarban (BS). The ranges of concentrations of four trace metals were as follows: 38,760–52,829 μg g−1 for Fe, 424–770 μg g−1 for Mn, 36–82 μg g−1 for Cu and 55–83 μg g−1 for Zn in IS, while in BS, the ranges were 29,081–45,025 μg g−1 for Fe, 342–792 μg g−1 for Mn, 12–45 μg g−1 for Cu and 29–75 μg g−1 for Zn. Speciation suggested the dominance of Mn in the acid-soluble fraction (>68 % IS; >52 % BS) while Fe (>83 % IS; >80 % BS), Cu (>56 % IS; >60 % BS) and Zn (>77 % IS; >80 % BS) dominated in the residual fraction. The speciation pattern showed that due to low organic carbon (OC) content (<0.99 % IS; <1.52 % BS), the Fe–Mn oxyhydroxide fraction (fraction 2) acts as a scavenging phase for Fe, Cu and Zn. The Risk Assessment Code (RAC) showed moderate to very high risk with respect to Cu and Mn, whereas Fe and Zn were in the low-risk category suggesting an anthropogenic influence with respect to Cu and Mn. The dominance of Cu and Mn in the non-residual fraction suggests anthropogenic sources, such as port-related activities and sewage runoff. The study highlights the dominance of anthropogenic activity and lack of freshwater flow in IS compared to BS, which affects the speciation profile of trace metals in the mangrove sediments.