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Metals, particularly Zn, Cu, Fe, and Mn are distributed amongst several geochemical forms, influencing their solubility, mobility and toxicity in soil-water systems. In this study, Sundarbans mangrove forest soils were analyzed by sequential fractionation to understand the distribution of these metals into exchangeable (Exch), organically bound (OrgB), Mn-oxide bound (MnOB), amorphous Fe-oxide bound (AFeOB), crystalline Fe-oxide bound (CrFeOB) and residual (Res) fractions. Soil Fe was predominately present in the residual fraction (55–73%), and manganese was primarily bounded in residual (29–58%) and MnOB (5–31%) fractions. The Res, CrFeOB and AFeOB fractions of Cu account for more than 80% of the total Cu. Zinc was mostly concentrated in the residual fraction and varied between 57 to 78% of the total Zn. Principal Component Analysis (PCA) and correlation studies revealed that exchangeable and organically bound fractions of these metals were significantly correlated, and their concentrations were highly dependent on soil pH, soil texture, organic carbon and redox potential (p < 0.05). Despite low total concentration, ratio of secondary phase and primary phase (RSP) and risk assessment code (RAC) studies showed that Cu (RSP is 1–2.64 and RAC is 1–2.89) and Mn (RSP is 1–2.42 and RAC is 1–16.66) were at low to moderate contamination risk and Fe maintained no contamination (RSP and RAC < 1) in almost all the soils. In contrast, Zn showed low contamination risk in some areas. This contamination can cause adverse effects on the plants and soil-dwelling organisms of Sundarbans mangrove forest due to higher availability. [ABSTRACT FROM AUTHOR]

At present, there are growing concerns over the increasing release of trace metals in the Sundarbans mangrove areas in Bangladesh due to nearby shipbreaking and metallurgical industries, untreated waste discharge, navigation activities, and other natural processes that deposit trace metals into soils. The current study investigated the spatial distribution, contamination level, and ecotoxicity of eight trace metals (Fe, Mn, Cu, Zn, Pb, Cd, Cr, Ni) in Sundarbans soils. Results revealed that all the trace metals except Cr were present in higher concentrations compared to Earth's shale and/or upper continental crust. Principal component analysis and Pearson correlation showed strong positive correlations (p < 0.05) between Fe, Mn, Cu, and Zn; Ni with Mn and Cr. There were significant associations (p < 0.05) of % clay and total organic carbon (TOC) with Pb-Ni-Cr and negative correlations of pH with all the trace metals. The hierarchical cluster analysis grouped Pb, Ni, and Cd into one distinct cluster, suggesting they are derived from the same sources, possibly from anthropogenic activities. Geo accumulation index (I-geo), enrichment factor (EF), contamination factor (CF), and spatial distribution showed moderately polluted soils with Ni, Pb, and Cd (EF = 3–7.4, CF = 1–2.8, I-geo = 0–0.9) and low pollution by Zn, Cu, Fe, and Mn (EF < 3, CF < 1, I-geo < 0). The ecological risk index (RI) revealed that S-4 (RI = 114.02) and S-5 (RI = 100.04) belonged to moderate risk, and other areas posed a low risk (RI < 95). The individual contribution of Cd (25.9–73.7%), Pb (9.2–29.1%), and Ni (9.6–26.4%) to RI emphasized these metals were the foremost concern in the Sundarbans mangroves due to their long persistence time and high toxicity, even if they were present in low concentrations. [ABSTRACT FROM AUTHOR]