Chemistry of metal sulfides in anoxic sediments

Using sequential extraction of solid sulfides, the determination of acid vo latile sulfides (AVS) and chromium reducible sulfurs (CRS) in anoxic sedime nts 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 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 oxidiz ed. Chemical analysis of solutions recovered after sequential leaching of s ediments with 1 M HCl, 1 M HF and concentrated HNO3 has enabled us to obtai n profiles, vs. sediment depth, of trace metals associated with pyrite. Por ewater concentration profiles vs. depth have been determined for several ca tions (Ca2+, Cd2+, Cu2+, Fe2+, Mg2+, Mn2+, Na+, Pb2+, Sr2+ and Zn2+) and an ions (CO32-, PO43-, SO42- and S2-). Using the chemical equilibrium modeling program MINEQL(+) with these analytical data, thermodynamic calculations h ave given information about the possibility of precipitation of discrete me tal sulfide phases (FeS as greigite and amorphous FeS; ZnS, PbS, CuS and Cd S), and coprecipitation with adsorption on solid FeS to produce solid solut ions 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 pyritiz ed; whereas Zn, Cu, Ni and above all Cd are weakly pyritized (Mn >> Zn simi lar or equal to Cu > Ni >> Cd). These observations seem to be intimately re lated to the existence of the discrete/separate solid phases CuS, CdS and Z nS, as predicted by thermodynamic calculations. Finally, analysis of crude sediments, heavy minerals and pyrite extracted by a heavy liquid density se paration method, has been performed with a Raman microprobe to gain informa tion about the geochemical and mineralogical characteristics of these sedim ents. The efficiency of sequential leachings of sediments (which were used for sedimentary pyrite recovery/attack and analysis of pyritic Fe and trace metal) has also been evaluated by these techniques.