Heavy Metals in Estuarine Sediments: Chemical Bioavailability and Toxicity Assessment

This study focused on heavy metals (HM) (Cr, Ni, Zn, Cu and Pb) along with Fe and Mn detected in surface sediments from the River Cai—Nha Trang Bay estuarine system (South China Sea). The Adverse Effect Index (AEI) together with Toxic Units (TUs) were used to assess the potential heavy metal toxicity in the sediments. The selective single-step extraction procedure was applied to determine the chemical forms of HM in order to assess their potential bioavailability. Among HM, only Ni and Pb may be associated with considerable adverse effects on biota (AEI > 1) and high potential toxicity (contributing up to 30–40% of the sum of toxic units (∑TUs)). The metal form study revealed that Ni is low-labile and mainly occurs in the residual phase, while the high levels of labile (weak-acid-soluble), amorphous (oxalate-soluble) and organically bound (pyrophosphate-soluble) Pb fractions (30%, 8% and 11% of the total content in sediment, on average, respectively) indicate Pb contamination arising from the Cai River discharge. HMs were arranged in the following decreasing order of their selective (single-step) extractability and, consequently, chemical bioavailability in the studied sediments: Mn ≫Pb ≫Fe > Zn > Cu > Ni > Cr.