Fractionation of anthropogenic lead and zinc in Deûle River sediments

Abstract

Environmental context. Metal contamination from smelting plants can have significant environmental and geochemical impacts on surrounding river systems, where large amounts of ores, dusts and slag are often discharged. Pollution levels in a river in northern France in the vicinity of a plant that had been producing zinc and lead have been measured. The authors assessed and identified the forms and phases of these metals in the polluted sediments, in order to assess the ability of these metals to pass into water when physicochemical changes (pH, redox potential) occur in the medium, for instance, as a result of dredging and barge traffic. Abstract. The degradation of a fluvial environment, the Deûle River in northern France, with metals has been examined. Sites of environmentally significant sediment metal contamination were identified near a former smelting plant (Metaleurop) that produced lead and zinc. The chemical fractionation of sedimentary lead and zinc was carried out by using a four-stage sequential procedure in the polluted sediments. Chemical treatments were performed on these sediments with increasingly strong phase-specific reagents and under controlled thermal conditions; the recovered solutions were subsequently analysed using inductively coupled plasma–atomic emission spectroscopy (ICP-AES). The partitioning of lead and zinc in Deûle River sediment samples was further compared with those found in less contaminated sites upstream and downstream from the former Metaleurop factory. Analytical data showed the extent of industrial pollution in this sediment, particularly, the implication of: (i) anthropogenic lead and zinc on the easily extractable fraction; (ii) smelter inputs containing of sulfidic ores on the sulfide/organic fraction; and (iii) smelter dust, slags and possibly ores derived from oxides in the reducible fraction. Overall, in polluted water, sediment-bound lead and zinc were found to be associated with all the sedimentary phases (the average mass percentages of lead and zinc in the exchangeable ions/carbonate fraction were respectively: 12% and 23%; in Fe and Mn oxides and hydroxides: 48% and 35%; in sulfides and organics: 33% and 29%; and in clays and aluminosilicates: 7% and 14%). Using X-ray diffraction, heavy minerals that were previously separated from sediments by decantation with gravity were shown to consist mostly of galena (PbS), wurtzite (ZnS), and pyrite (FeS2), showing the importance of sulfides in this sedimentary material. Using environmental scanning electron microscopy with energy dispersive X-ray spectroscopy (ESEM/EDS), sediments were found to be highly heterogeneous assemblages or aggregates, but with some isolated crystals that were identified. Detailed ESEM/EDS analyses (with imaging) have enabled us to demonstrate the existence of numerous lead and zinc phases that agree well with X-ray diffraction results and sequential extraction data.