Geochemical speciation, pollution assessment, and source identification of heavy metals in sediment cores of the Cau River basin, Hai Duong province, Vietnam†

Abstract

Heavy metal contamination in sediment has caused severe threats to the aquatic ecosystem and public health worldwide. Networks of rivers and their tributaries serve as dynamic habitats for these potentially harmful metals through aqueous–sedimentary equilibrium shifts. Hence, determining the distinct chemical forms of a given heavy metal in sediment is crucial for evaluating its bio-lability and toxicity. This study demonstrates the geochemical speciation using a sequential extraction procedure to fractionate individual phases (exchangeable, carbonate, Fe–Mn oxide, organic, and residual) of nine heavy metals (Cu, Pb, Cd, Zn, Fe, Co, Ni, Mn, and Cr) in the sediment of a river system in Hai Duong, a deltaic province in Vietnam. A quantitative assessment of environmental risk factors (e.g., contamination factor and risk assessment code) and the pseudo-partitioning coefficient between pore water and sediment was conducted to define the pollution levels of heavy metals and their contaminated areas. Furthermore, multivariate analyses facilitate a profound comprehension of the contributions to pollution. Analyses of the extracts from the sequential extraction procedure were performed by inductively coupled plasma-mass spectrometry. The results of sedimentary heavy metal speciation indicate that the critical risks of Cd (15.8–38.4%) and Mn (16.3–53.8%) to the aquatic ecosystem are due to their higher retrieval from the exchangeable fraction. Additionally, an appreciable percentage of Co (26.3–58.0%), Mn (16.8–66.3%), Ni (16.0–53.1%), Pb (6.75–69.7%), and Zn (4.42–45.8%) in the carbonate fraction highlights a strong tendency for co-precipitation or ion exchange of these metals with carbonate minerals. Whilst colloids of Fe–Mn oxides act as efficient scavengers for metals such as Fe, Mn, Zn, and Pb, organic matter forms primarily function in trapping Cu, Pb, Fe, Cr, Co, and Ni. Our findings in the ecological risk evaluations and multivariate analyses indicate that Cr, Ni, and Fe are ascribed to natural lithogenic provenances. In contrast, anthropogenic inputs induce Cd, Mn, Cu, and Pb high-environmental risks.