Gabriel Billon

Chemical speciation of sulfur compounds in surface sediments from three bays (Fresnaye, Seine and Authie) in northern France, and identification of some factors controlling their generation

The determination of total reduced sulfur, elemental sulfur, sulfide, thiosulfate and sulfite in porewaters--which were previously extracted from surface sediments collected in three bays, i.e. Seine, Fresnaye and Authie, located in northern France--has been undertaken using different electroanalytical techniques (linear sweep cathodic stripping voltammetry (LSCSV), square wave cathodic stripping voltammetry (SWCSV) and differential pulse cathodic stripping voltammetry (DPCSV)) at a static mercury drop electrode. Furthermore, the analyses of sulfur solids present in these sediments have been performed by sequential extraction procedures. Overall, the description of speciation of dissolved sulfur compounds and reduced sulfur solids has shown that the availability of sulfate and reactive iron, the sedimentation rate and probably even the nature and content of organic matter are important factors for controlling the sulfidisation and pyritisation processes involved in the different sedimentary systems studied. Thus, in the Seine-bay sediments, it has been found that reactive iron scavenges the most part of the generated sulfide as a result of a particularly activated sulfate reduction by bacterial activities; as a consequence, this precipitation limits the accumulation in the porewaters of reduced sulfur compounds such as elemental sulfur and polysulfides which ought to be generated in the redox boundary (where the oxydants, viz. oxygen, nitrate and metal oxides exist abundantly) through a partial oxidation of the H(2)S and HS(-) species. This process is further accentuated by a sedimentation rate measured in the Seine estuary. Conversely, in the Authie bay and, to a lesser extent, in the Fresnaye bay the contribution of the sedimentation rate and/or reactive iron scavenging to the sedimentary sulfur processes is weaker. This explains the increase of the built-up of dissolved reduced sulfur in the interstitial waters as well as the existence of elemental sulfur and polysulfides that permit the conversion of FeS into FeS(2).

Chemistry of metal sulfides in anoxic sediments

Using sequential extraction of solid sulfides, the determination of acid volatile sulfides (AVS) and chromium reducible sulfurs (CRS) in anoxic sediments from the Authie Bay (in northern France) has been undertaken because of the importance of the sediments as sinks for iron, sulfur and trace metals and as possible sources of n pollution when reduced sediments are mixed with oxic waters (as a result of a sediment remobilization induced by physical disturbances such as tidal currents and dredgings), and subsequently oxidized. Chemical analysis of solutions recovered after sequential leaching of sediments with 1 M HCl, 1 M HF and concentrated n HNO3 has enabled us to obtain profiles, s. sediment depth, of trace metals associated with pyrite. Porewater concentration profiles s. depth have been determined for several cations (Ca2+, Cd2+, Cu2+, n Fe2+, Mg2+, Mn2+, Na+, Pb2+, Sr2+ and Zn2+) and anions (CO32−, PO43−, SO42− and S2−). Using the chemical equilibrium modeling program MINEQL+ with these analytical data, thermodynamic calculations have given n information about the possibility of precipitation of discrete metal sulfide phases (FeS as greigite and amorphous FeS; ZnS, PbS, CuS and CdS), and coprecipitation with adsorption on solid FeS to produce solid solutions with iron sulfides. The degree of trace metal pyritization, DTMP, has been determined for these metals and compared to the degree of pyritization, DOP. The findings suggest that in Authie-bay sediments Mn is well pyritized; whereas Zn, Cu, Ni and above all Cd are weakly pyritized (MnZn≃Cu>NiCd). These observations seem to be intimately related to the existence of the discrete/separate solid phases n CuS, CdS and ZnS, as predicted by thermodynamic calculations. Finally, analysis of crude sediments, heavy minerals n and pyrite extracted by a heavy liquid density separation method, has been performed with a Raman microprobe to gain information about the geochemical and mineralogical characteristics of these sediments. The efficiency of sequential leachings of sediments (which were used for sedimentary pyrite recovery/attack and analysis of pyritic Fe and n trace metal) has also been evaluated by these techniques.

Artefacts in the speciation of sulfides in anoxic sediments

The determination of sulfides and trace metals in sediment is nhandicapped by preparation techniques which can alter the chemical nspeciation of the different elements before the analysis. In this work, nsulfide species such as AVS (acid volatile sulfides) and CRS (chromium nreducible sulfides) were determined in anoxic sediment and their ntransformation, their loss and their effect on metal speciation were nevaluated. The experiments were concentrated on the determinands which are nmost commonly used or dedicated to the determination of sulfides and nmetals, with emphasis on artefacts potentially introduced during sediment npreparation such as sample storage, drying or oxygen contamination.

Fractionation of anthropogenic lead and zinc in Deûle River sediments

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 Deule 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 Deule 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.

On the chemical properties of sedimentary sulfur in estuarine environments

The composition of iron, sulfur and organic matter and their distributions with depth have been used as sensitive indicators of diagenetic processes occurring in anoxic sediments derived from the Authie and Seine bays in northern France. The contents of acid volatile sulfurs (AVS), chromium reduced sulfurs (CRS), total organic carbon and reactive Fe present in the sedimentary solid phase have been reported. These investigations have revealed that degree of pyritization (DOP) values remain nearly constant in the two sites, suggesting that pyrite formation is limited by the reactivity/availability of iron in the bulk sediment; whereas neither AVS or organic matter are important determinants of the degree of pyritization. The elemental composition of humic acids extracted from these anoxic sediments has also been examined with X-ray photoelectron spectroscopy (XPS). Diagenetic enrichment of sedimentary organic matter with sulfur occurs nwith depth in Authie Bay sediments, as evidenced by the XPS ratios S/C, whereas a depletion takes place with depth in Seine Bay sediments. Such a phenomenon results mainly from the availability of inorganic reduced sulfurs generated through bacterial sulfate reduction along the sedimentary column and, to a lesser extent, from the absence or weak re-supply of “active organic matter”.

Distribution coefficient and redox behaviour of uranium in Authie Bay (northern France)

Some mechanistic aspects associated with uranium release/immobilization and sedimentation in Authie Bay are presented in the present work. For this purpose, U contents in estuarine oxic waters, porewaters and sediment solids are determined. These analytical data allow us to appraise the partitioning of this metal between the liquid phase and the particulate matter/sedimentary material by calculating its distribution coefficient. Our findings further reveal that the distribution coefficient varies significantly with depth probably in response to the microbial activities in these sediments. This is confirmed by our studies on the geochemical behaviour of Fe and Mn in Authie Bay sediments. Finally, studies on the thermodynamic characteristics of sedimentary U in Authie Bay are undertaken in order to select possible U water–mineral equilibria that could involve in this environment, and to help define conditions of sedimentary U bioreduction.

The chemical behavior of sedimentary uranium in Authie Bay (France)

A general study on uranium present at trace levels in anoxic sediments derived from Authie Bay (in northern France) has been undertaken. For that purpose, concentrations of various uranium species in pore waters and recovered solutions (after mineralization of sediments) were determined by ICP-AES and ICP-MS. To access the extent of early diagenesis occurring in these sediments, reduced solid sulfur species were determined after their conversion into H2S gas following sequential extraction procedures. Our preliminary findings reveal that dissolved U(VI) precipitates rapidly with depth in pore waters certainly in the form of insoluble U(IV). Under stronger reducing conditions, new aqueous species [mostly and ] in which uranium is in oxidation state IV are formed in the pore water. Valuable interpretation of these particular properties of sedimentary U has necessitated a global examination of sediment biochemistry because of the influence of bacterial activities on the chemistry of Fe, Mn, S and more particularly U by metal-reducing bacteria.