The Role of Sulfides in the Speciation of Nickel in Anoxic Sediments.

摘要

The formation of nickel sulfide (NiS) in sediments has been assumed based on thermodynamic considerations (e.g., solubility products), but the presence of nickel sulfide in natural systems remains elusive. In this work, direct evidence of the formation of NiS in anoxic sediments is provided for the first time, using the direct method X-ray Absorption Spectroscopy (XAS). Though NiS precipitates in the sediments, it is prone to rapid oxidation, leading to the remobilization of Ni2+ into pore waters, where it can form other metal phases. This can have direct impact on nickel bioavailability and toxicity. XAS is also used here to aid in the development of a 2-step chemical extraction method to assess the complete pool of nickel sulfides in sediments.;The first step in this chemical extraction consists in performing an acid leaching following the Simultaneously Extracted Metals over Acid Volatile Sulfide (SEM/AVS) method, which extracts the reactive fraction of nickel from the sediments. This reactive fraction may be composed of a mixture of various nickel compounds, even in sulfide-rich sediments. The second step consists of an oxidative extraction, which leaches the remaining fraction of nickel from the residual of the first extraction. The majority of the nickel in this remaining fraction is associated with sulfide phases.;A portion of this work was used in a USGS Interlaboratory Study, which showed that the SEM_Ni/AVS method can give reproducible results if a consistent protocol is followed, in contrast to previous studies where no protocol guidelines were required and results were irreproducible. However, I also show that SEM_Ni/AVS does not provide the complete picture of nickel speciation in sediments.;Nickel speciation was studied by XAS spectral decomposition, an effective approach to probe multiple metal phases in sediments. I demonstrate that background removal of XAS spectra is a crucial step that needs to be performed systematically throughout unknown samples and known reference standards. This is in contrast to traditional XAS data reduction done for structural studies of pure compounds. In addition, an approach is provided to verify whether a XAS library of reference standards forms a unique mathematical basis set.