Metal speciation in anoxic sediments: when sulfides can be construed as oxides

Metal speciation in aquatic sediments is often characterized using wet chemical sequential extraction techniques. However, these methods are operationally defined and subject to artifacts, particularly when dealing with anoxic sediments, in which metal sulfide precipitates are likely to occur. Using X-ray absorption spectroscopy (XAS) and acid-volatile sulfide (AVS) analysis, we evaluated the effectiveness of one of the most widely used sequential extraction protocols, the Tessier method, at determining Zn and Pb speciation in anoxic wetland sediments. Sequential extraction results significantly underestimated the amount of Zn associated with sulfide phases as compared to the other two approaches. XAS analysis of ZnS amended sediments indicates that the most likely source of this conflict is an early dissolution of amorphous metal sulfide phases during the sequential extraction step corresponding to the extraction of iron and manganese oxides. The reagent mixture used in this step, hydroxylamine hydrochloride-HCl, is widely used in other sequential extraction protocols, including the BCR method, limiting their application to anoxic sediments. For this reason, current sequential extraction techniques should only be used on anoxic sediments with caution, and/or in conjunction with complementary approaches to assess metal speciation.