Natural speciation of nickel at the micrometer scale in serpentine (ultramafic) topsoils using microfocused X-ray fluorescence, diffraction, and absorption

Abstract

Serpentine soils and ultramafic laterites develop over ultramafic bedrock and are important geological materials from environmental, geochemical, and industrial standpoints. They have naturally elevated concentrations of trace metals, such as Ni, Cr, and Co, and also high levels of Fe and Mg. Minerals host these trace metals and influence metal mobility. Ni in particular is an important trace metal in these soils, and the objective of this research was to use microscale (µ) techniques to identify naturally occurring minerals that contain Ni and Ni correlations with other trace metals, such as Fe, Mn, and Cr. Synchrotron based µ-XRF, µ-XRD, and µ-XAS were used. Ni was often located in the octahedral layer of serpentine minerals, such as lizardite, and in other layered phyllosilicate minerals with similar octahedral structure, such as chlorite group minerals including clinochlore and chamosite. Ni was also present in goethite, hematite, magnetite, and ferrihydrite. Goethite was present with lizardite and antigorite on the micrometer scale. Lizardite integrated both Ni and Mn simultaneously in its octahedral layer. Enstatite, pargasite, chamosite, phlogopite, and forsterite incorporated various amounts of Ni and Fe over the micrometer spatial scale. Ni content increased six to seven times within the same 500 µm µ-XRD transect on chamosite and phlogopite. Data are shown down to an 8 µm spatial scale. Ni was not associated with chromite or zincochromite particles. Ni often correlated with Fe and Mn, and generally did not correlate with Cr, Zn, Ca, or K in µ-XRF maps. A split shoulder feature in the µ-XAS data at 8400 eV (3.7 Å^-1 in k-space) is highly correlated (94% of averaged LCF results) to Ni located in the octahedral sheet of layered phyllosilicate minerals, such as serpentine and chlorite-group minerals. A comparison of bulk-XAS LCF to averaged µ-XAS LCF results showed good representation of the bulk soil via the µ-XAS technique for two of the three soils. In the locations analyzed by µ-XAS, average Ni speciation was dominated by layered phyllosilicate and serpentine minerals (76%), iron oxides (18%), and manganese oxides (9%). In the locations analyzed by µ-XRD, average Ni speciation was dominated by layered phyllosilicate, serpentine, and ultramafic-related minerals (71%) and iron oxides (17%), illustrating the complementary nature of these two methods.

Keywords: EXAFS; Laterite; Nickel; Serpentine; Soil chemistry; Trace metal; Ultramafic; XRD.

Fig. 1

Ni distribution in forsterite. High-resolution photograph delineating the area of analysis (a); averaged µ-XRD spectra from points A–F (b); µ-XRF map (c); and the µ-EXAFS data obtained at the smaller white circle in the map along with Ni K-edge EXAFS of San Carlos Olivine [64] for comparison. This figure a–d was selected from Additional file 1: Figure S22 (“s20unt” region 4); the combination of microfocused techniques highlights the benefits of using multiple tools to analyze the same sample location. Here, the µ-EXAFS and µ-XRD spectra both indicate that Ni is located in forsterite, which is an olivine-series mineral

Fig. 2

A compilation of all µ-XAS spectra. Normalized Ni K-edge µ-XANES spectra, 13 total (a); a close-up view of the first large oscillation close to the end of the XANES region (b); Ni K-edge µ-EXAFS spectra, 8 total (c). The spectra are shown in order of appearance in Additional file 1 and progress numerically and alphabetically with respect to sample name and regions of interest. All spectra are color coded to remain the same throughout the manuscript and Additional file 1